625478012A
Technology Transfer
Sludge Treatment
and Disposal
Sludge Treatment
This document has not been
submitted to NTIS, therefore it
should be retained.
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EPA-625/4-78-012
October 1978
Sludge Treatment and Disposal
Sludge Treatment
Volume 1
Environmental Research Information Center
Cincinnati, Ohio 45268
CL
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NOTICE
The mention of trade names or commercial products in this publication is for illustration
purposes, and does not constitute endorsement or recommendation for use by the U.S.
Environmental Protection Agency.
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Acknowledgments
This seminar publication contains material prepared for the U.S. Envi-
ronmental Protection Agency Technology Transfer Program. It has been
presented at Technology Transfer Design Seminars held at various loca-
tions throughout the United States. The information in this publication was
prepared by the following:
Volume I, Sludge Treatment
Introduction. Donald J. Ehreth, Office of Air, Land and Water Use, EPA,
Washington, D.C.; and Dr. Joseph B. Farrell, Municipal Environmental
Research Laboratory and Dr. J. E. Smith, Jr., Environmental Research
Information Center, EPA, Cincinnati, Ohio.
Lime Stabilization of Wastewater Treatment Plant Sludges. Richard F.
Noland and James D. Edwards, Burgess & Niple, Limited, Columbus,
Ohio.
Anaerobic Digestion of Municipal Wastewater Sludges. N. A. Mignone,
Envirex Inc., Waukesha, Wis.
Aerobic Digestion of Municipal Wastewater Sludges. N. A. Mignone,
Envirex Inc., Waukesha, Wis.
Thermal Treatment for Sludge Conditioning. Dr. G. M. Wesner, Gulp/
Wesner/Culp, Santa Ana, Calif.
Thickening of Sludge. Richard F. Noland and Ronald B. Dickerson,
Burgess & Niple, Limited, Columbus, Ohio.
Developments in Dewatering Wastewater Sludges. J. R. Harrison, Con-
sulting Environmental Engineer, Hockessin, Del.
Volume II, Sludge Disposal
Incineration-Pyrolysis of Wastewater Treatment Plant Sludges. Ronald B.
Sieger and Patrick M. Maroney, Brown and Caldwell, Walnut Creek,
Calif.
Sewage Sludge Composting. Dr. G. M. Wesner, Culp/Wesner/Culp,
Santa Ana, Calif.
Principles and Design Criteria for Sewage Sludge Application on Land.
Dr. L. E. Sommers, Purdue University, West Lafayette, Ind.; R. C.
Fehrmann, H. L. Selznick and C. E. Pound, Metcalf and Eddy, Palo
Alto, Calif.
Sludge Landfilling. James J. Walsh and Wayne M. Coppel, SCS Engi-
neers, Reston, Va.
In addition, G. Kenneth Dotson and Dr. J. A. Ryan, Municipal Environ-
mental Research Laboratory, EPA, Cincinnati, ONo; and Dr. Ronald Lofy,
SCS Engineers, Long Beach, Calif., assisted in preparation of the "Land
Utilization" section of "Principles and Design Criteria for Sewage Sludge
Application on Land" chapter. David Sussman, Office of Solid Waste,
EPA, Washington, D.C., was a contributor to the "Incineration-Pyrolysis of
Wastewater Treatment Plant Sludges" chapter. Donald J. Ehreth, Office of
Air, Land and Water Use, Robert K. Bastian, Office of Water Program
Operations, and Bruce Weddle, Office of Solid Waste, EPA, Washington,
D.C., provided technical review of Volume II. Dr. J. E. Smith, Jr., Environ-
mental Research Information Center, EPA, Cincinnati, Ohio, provided over-
all direction, guidance in preparation, as well as review of the document.
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Contents
Page
Volume 1
Introduction vii
Chapter 1. Lime Stabilization of Wastewater Treatment Plant
Sludges 1
Chapter 2. Anaerobic Digestion and Design of Municipal Wastewater
Sludges 35
Chapters. Aerobic Digestion and Design of Municipal Wastewater
Sludges 57
Chapter 4. Thermal Treatment for Sludge Conditioning 69
Chapter 5. Thickening of Sludge 79
ChapterG. Review of Developments in Dewatering Wastewater
Sludges 101
Volume 2
Chapter 7. Incineration-Pyrolysis of Wastewater Treatment Plant
Sludges 1
Chapter 8. Sewage Sludge Composting 35
Chapter 9. Principles and Design Criteria for Sewage Sludge Applica-
tion on Land 57
Chapter 10. Sludge Landfilling 113
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Introduction
Of the utilization and disposal options available for
sludge, each has its own specific set of environmental
problems. In order to implement any policy, resolution to
a number of problems that presently inhibit sludge man-
agement must be developed. These problems can be
summarized and categorized into four general areas:
• Public health issues.
• Technological factors.
• Intermedia issues.
• Social/economic/institutional factors.
This section briefly discusses the seminar publication's
contents, their evolution, and the U.S. Environmental
Protection Agency's (EPA) sludge management research
and development program.
This publication is the culmination of two years of
intensive work and eight seminars that were presented
around the nation in an effort to develop and dissemi-
nate the latest information on the design of sludge treat-
ment and disposal systems. It presents in great detail
technical information for the following sludge treatment
and disposal processes:
Lime Stabilization.
Anaerobic Digestion.
Aerobic Digestion.
Thermal Sludge Conditioning.
Thickening.
Dewatering.
Incineration and Pyrolysis.
Composting.
Land Utilization.
Landfilling.
The discussion of each process includes where possi-
ble a presentation of performance data for existing oper-
ations as well as operation and maintenance experiences
and energy and cost information. Each chapter includes
one or more design examples to illustrate step-by-step,
the philosophy, rationale, and methodology behind the
design of the particular process.
The "Lime Stabilization" discussion gives information
for determining lime requirements as well as a detailed
case history for a 1.0 Mgal/day (.04 m3/s) plant. Com-
parative designs and cost information are presented for
both the "Lime Stabilization" and "Anaerobic Digestion"
processes in 4 (.18 m3/s) and 40 (1.175 m3/s) Mgal/day
plants. A list of installations employing lime stabilization
is included.
Both the "Anaerobic Digestion" and "Aerobic Diges-
tion" chapters thoroughly review the pertinent parame-
ters for such biological processes and include a design
relationship between the percent reduction in volatile
solids and sludge age and digestion temperature. The
"Thickening" chapter provides design examples for two
different plant sizes which include a detailed cost effec-
tive analysis for choosing the alternative techniques of
gravity, dissolved air flotation, centrifugation, and no
thickening. The "Dewatering" discussion includes various
schemes for designing and operating drying beds, the
continuous belt filter presses, and both the plate and
frame and recessed chamber pressure filters. Standard
as well as membrane and diaphragm pressure filters are
discussed. The electric or infrared furnace is explained
in the chapter on "Incineration and Pyrolysis," and per-
formance data are presented. The plant scale partial
pyrolysis (starved air combustion) work done at the Con-
tra Costa County Sanitation District is described in de-
tail. Considerable discussion is devoted to the use of
alternative fuels and energy recovery.
The chapter on "Composting" discusses at length the
work on forced aeration static pile composting at Belts-
ville, Md.; Bangor, Maine; and Durham, N.H. European
developments with mechanical systems are also covered.
Very detailed design information applicable to any size
system is presented in both the "Land Utilization" and
"Landfilling" chapters along with step-by-step examples.
EPA sludge management research and development
program encompasses four major technical areas: proc-
essing and treatment, utilization, disposal, and health and
ecological effects. The primary objective of the program
is to develop new and improved technology and man-
agement schemes which will enable communities to solve
problems associated with the residues or byproducts of
wastewater treatment in a cost effective and environmen-
tally acceptable manner.
The present state-of-the-art provides adequate (but ex-
pensive) capability to dewater sludges. Incineration prac-
tice is well established with exception of the potential
impact of air emissions on health and ecology. However,
coincineration (e.g., sludge plus solid waste) and pyroly-
sis technology is just emerging. Controversy continues
both within and outside the Agency with regard to the
environmental acceptability of applying municipal sludges
to the land. This is especially true for agricultural uses.
Heavy metals (especially cadmium), complex organics,
and microbiological contaminants are the constituents of
primary concern.
Specific examples of technological gaps that presently
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exist are:
• Cost of sludge processing and disposal is a major
factor in wastewater treatment.
• Methods of converting sludge to beneficial byprod-
ucts are in the embryonic stages.
• Limited confidence exists in the efficacy of local
industrial pretreatment programs for metals removal
and methods for monitoring their effectiveness.
• Relative risks associated with land application need
to be established with greater precision.
• Varying climatic and soil conditions as well as vary-
ing sludge composition require evaluation for a vari-
ety of sludges with optimum combinations of soil
and vegetation.
• Methods for removing toxicants at the treatment
plant are in the development stage; application is
impeded because of economics of technology.
PROCESSING AND TREATMENT
Sludge must undergo some processing or treatment to
prepare it for ultimate disposition.
The goal of processing and treatment research and
development is to produce technology alternatives which
can be used to prepare the sludge for application to the
land or for one of the conversion processes so that the
total cost of handling or disposal is minimized.
Implementation of the program is focused on the fol-
lowing objectives:
• Evaluate the efficacy of pretreatment as an option
to minimize toxicants in sludge.
• Characterize the nature of, and the dewatering
properties of, "new" sludges using existing, up-
graded and new technology.
• Develop hardware capable of producing a substan-
tially drier sludge cake.
• Develop and define performance of existing and
new processes for stabilizing sludge (anaerobic di-
gestion, auto thermal thermophilic aerobic digestion,
composting, etc.).
• Investigate ways to minimize energy consumption
while simultaneously maximizing fuel production (ac-
tivated carbon enhancement, solar heating, etc.).
• Determine cost and environmental impact of sludge
processing systems.
• Provide guidance on technology for disinfection (up
through sterilization) of sludge.
CONVERSION PROCESSES
This part of the research program has been divided
between efforts devoted to upgrading conventional incin-
eration and tasks oriented toward development of new
processes.
Current program objectives directed to meeting these
needs include several projects, ongoing and planned to:
• Develop techniques for substitution of more abun-
dant, less costly supplemental fuels such as coal
and solid wastes (incineration and co-incineration).
• Develop processes and hardware for pyrolysis. co-
pyrolysis and starved-air combustion.
• Characterize emissions to determine levels of poten-
tial pollutants (gaseous, liquid, solid) contained in
emissions from sludge conversion facilities.
• Establish the "least cost" approaches to sludge
conversion to the satisfaction of administrators,
technologists and the general public.
• Evaluation of cementation processes and other ben-
eficial use alternatives.
LAND APPLICATION—MANAGEMENT
The objective relating to land application management
is to develop methods and technology to control the
transformation and/or movement of pollutants through
the soil, plants, groundwater, and human food chain.
The function of research and development associated
with the health and ecological area is to analyze, evalu-
ate, and interpret the data for purposes of establishing
safe loading rates.
It is anticipated that accomplishment of the primary
objectives will result in the establishment of management
schemes for a variety of sludges with optimum combina-
tions of soil and vegetation. Practices can then be de-
fined for applying sludge to the land for purposes of
reclaiming marginal or sub-marginal land, determining ag-
ricultural uses for both food and fiber, and landfill dis-
posal.
HEALTH EFFECTS
The difficulty in resolving this issue is that data which
will permit a definitive evaluation and decision regarding
the significance of sludge in the human food chain im-
pact do not exist to the satisfaction of the several sci-
entific disciplines involved. EPA is, therefore, working
cooperatively with other Federal agencies, particularly
USDA and FDA, to develop the information required to
resolve the issue. Information developed by others, nota-
bly universities, State agencies, and municipalities also is
being obtained.
Some current work directed to this issue includes:
• Evaluation of current knowledge of potential health
effects.
• Determine viral contamination of ground and surface
water of a land reclamation site.
• Developing methods for isolating viruses and chemi-
cals.
• Characterize type, quantity and biological persist-
ence of biologicals, trace metals, and other organic
and inorganic substances in the environs of a
sludge disposal site.
• Determine the potential of biologicals, metals, and
organic substances entering the human food chain
when digested sludge is used as a fertilizer.
• Study heavy metal uptake in beef animals grazed
on sludge amended pasture.
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Chapter 1
Lime Stabilization of Wastewater Treatment
Plant Sludges
INTRODUCTION
Sludge constitutes the most significant byproduct of
wastewater treatment; its treatment and disposal is per-
haps the most complex problem which faces both the
designer and operator. Raw sludge contains large quan-
tities of microorganisms, mostly fecal in origin, many of
which are pathogenic and potentially hazardous to hu-
mans. Sludge processing is further complicated by its
variable properties and relatively low solids concentra-
tion. Solutions have long been sought for better stabili-
zation and disposal methods which are reliable and eco-
nomical and able to render sludge either inert or stable.
Lime stabilization has been shown to be an effective
sludge disposal alternative when there is a need to:
A. Provide alternate means of sludge treatment during
the period when existing sludge handling facilities,
e.g., anaerobic or aerobic digesters, are out of
service for cleaning or repair.
B. Supplement existing sludge handling facilities, e.g.,
anaerobic or aerobic digesters, incineration or heat
treatment, due to the loss of fuel supplies or be-
cause of excess sludge quantities above design.
C. Upgrade existing facilities or construct new facilities
to improve odor, bacterial, and pathogenic orga-
nism control.
Lime stabilization has been demonstrated to effectively
eliminate odors. Regrowth of pathogens following lime
stabilization is minimal. Of the organisms studied, only
fecal streptococci have a potential for remaining viable.
Lime stabilized sludges are suitable for application to
agricultural land; however, lime stabilized sludges have
lower soluble phosphate, ammonia nitrogen, total Kjel-
dahl nitrogen, and total solids concentrations than anaer-
obically digested primary/waste activated mixtures from
the same plant.
The purpose of this chapter is to present a review of
stabilization and disinfection of municipal wastewater
treatment plant sludges using lime stabilization, including
specific design considerations. Two design examples in-
corporating lime stabilization into a 4 and 40 Mgal/d
(0.18 and 1.75 m3/s) wastewater treatment plant have
been included to demonstrate the design procedure.
Comparisons of the performance, capital and annual op-
eration and maintenance costs for lime stabilization and
anaerobic digestion were included for each design exam-
ple. To further illustrate the application of lime stabiliza-
tion techniques to small plants and/or facilities in need
of an emergency sludge-handling process, an actual
case history of lime stabilization at a 1 Mgal/d (0.04
m3/s) facility was also included. The case history in-
cludes capital and annual operation and maintenance
costs; chemical, bacterial, and pathological properties;
and land application techniques.
LIME STABILIZATION PROCESS
DESCRIPTION
Background
Historically, lime has been used to treat nuisance con-
ditions resulting from open pit privies and from the
graves of domestic animals. Prior to 1970. there was
only a small amount of quantitative information available
in the literature on the reaction of lime with sludge to
make a more stable material. Since that time, the litera-
ture contains numerous references concerning the effec-
tiveness of lime in reducing microbiological hazards in
water and wastewater.1'3 Information is also available on
the bactericidal value of adding lime to sludge. A report
of operations at the Allentown, Pa., wastewater treat-
ment plants states that conditioning an anaerobically di-
gested sludge with lime to pH 10.2 to 11, vacuum filter-
ing and storing the cake destroyed all odors and patho-
genic enteric bacteria.4 Kampelmacher and Jansen5 re-
ported similar experiences. Evans6 noted that lime addi-
tion to sludge released ammonia and destroyed bacillus
coli and that the sludge cake was a good source of
nitrogen and lime to the land.
Lime stabilization of raw sludges has been conducted
in the laboratory and in full-scale plants. Parrel et al.7
reported, among other results, that lime stabilization of
primary sludges reduced bacterial hazard to a negligible
value, improved vacuum filter performance, and providec
a satisfactory means of stabilizing sludge prior to ulti-
mate disposal.
Paulsrud and Eikum8 reported on the effects of long-
term storage of lime stabilized sludge. Their research
included laboratory investigations of pH and microbial
activity over periods up to 28 days.
Pilot scale work by C. A. Counts et al.9 on lime stabil
zation showed significant reductions in pathogen popula
tions and obnoxious odors when the sludge pH was
greater than 12. Counts conducted growth studies on
greenhouse and outdoor plots which indicated that the
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Table 1-1.—Lime required for stabilization to pH 12 for 30 minutes
Sludge type
Primary sludge0
Waste activated sludge
Septage
Anaerobic
Percent
solids
3-6
1-1 5
1-45
6-7
Average lbsa
Ca(OH)2/lbs
dry solids
012
030
020
019
Range lbsa
Ca(OH)2/lbs
dry solids
006-017
0 21-043
009-0 51
014-025
Totalb
volume
treated
1 36 500
42000
27500
23500
Average
total
solids,
mg/l
43276
13143
27494
55345
Average
initial
pH
67
7 1
73
72
Average
final
PH
127
126
127
124
Numerically equivalent to kg Ca(OH)2 per kg dry solids.
"Multiply gallons x 3.785 to calculate liters.
Includes some portion of waste activated sludge.
disposal of lime stabilized sludge on cropland would
have no detrimental effects.
A research and demonstration contract was awarded
to Burgess & Niple, Ltd. in March 1975 to complete the
design, construction, and operation of full-scale lime sta-
bilization facilities for a 1 Mgal/d (0.4 m3/s) wastewater
treatment plant, including land application of treated
sludges. The contract also included funds for cleaning,
rehabilitation, and operating an existing anaerobic sludge
digester. Concurrent with the research and demonstration
project, a considerable amount of full-scale lime stabili-
zation work was completed by cities in Ohio and Con-
I3.0
I20-
10 . .
IOO- -
9.0-
8D- •
7.0. -
6.0
L^«dA^^A*^±^Al*'l^ii^£*.^_—» ^^^^x^_^_
1»7^K»^»*^K^^^^K^^ ^^^^» ——^^—
AVERAGE
•:W:¥:::::":¥ RANGE OBSERVED
35%PRIMARY SLUDGE
3% PRIMARY SLUDGE
jl / *-S% PRIMARY SLUDGE
3% PRIMARY SLUDGE
3.5% PRIMARY SLUDGE
4% PRIMARY SLUDGE
4.5% PRIMARY SLUDGE
5% PRIMARY SLUDGE
6V.PRIMARY SLUDGE
1,000 2JDOO 3POO 4,000
DOSAGE Ca IOH>2 MG/L
5,000
6.0
1,000 2000 3JOOO 4/300
DOSAGE Ca (OH)2 MG/L
spoo
Figure 1-1.—Combined lime dosage versus pH for all Figure 1-2.—Lime dosage versus pH primary sludge.
sludges.
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necticut. Wastewater treatment plant capacities which
were representative ranged from 1 to 30 Mgal/d (0.04
to 1.31 m3/s). A summary of these results has previously
been reported.10
Lime Requirements
The lime dosage required to exceed pH 12 for at
least 30 minutes was found to be affected by the type
of sludge, its chemical composition, and percent solids.
As an operational procedure, a target pH of 12.5 was
selected to insure that the final pH would be greater
than 12. A summary of the lime dosage required for
various sludges is shown in table 1-1. Of the total
amount of lime which was required, an excess of 0 to
50 percent was added after pH 12 was reached in
order to maintain the pH. Figure 1-1 shows the com-
bined lime dosage versus pH for primary, anaerobically
digested, waste activated, and septage sludges. Figures
1-2 to 1-5 describe the actual lime dosages which were
required for each sludge type.
Table 1-2 compares the Lebanon full-scale test re-
sults, which are described later in the case history, with
the data previously presented by Farrell et al., Counts,
et al., and Paulsrud and Eikum for raw primary sludges.
In general, excellent correlation was achieved.
Counts9 has proposed the following equation for pre-
dicting the lime dosage required for primary and secon-
dary sludges from the Richland, Wash., trickling filter
plant:
Lime dose = 4.2+ 1.6 (TS)
When: Lime dose is expressed in grams Ca(OH)2 per
liter of sludge TS is the total solids fraction in the
sludge.
Table 1-3 compares the values predicted by the
Counts equation to the Lebanon data for raw primary,
waste activated, anaerobically digested, and septage
sludges.
With increasing solids concentrations, the Counts
equation results in lower than actual lime dosages.
pH Versus Time
Previous research has attempted to determine the
magnitude of pH decay versus time and to quantify the
I30r
I20--
no-
IO.O
6.0
2,000 4POO 6,000 8,000 IOPOO
DOSAGE Ca(OH)2 MG/L
3.0 r
I2.0
1,000 ZjOOO 3jOOO 4,000 5.00C
DOSAGE Ca (OH>2 MG/L
Figure 1-3.—Lime dosage versus pH anaerobic digested Figure 1-4.—Lime dosage versus pH waste activated
sludge. sludge.
3
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i3.o r
130
110-
100
CL
90
8.0
6.0
Table 1-3.—Comparison of lime dosages predicted by
the counts equation to actual data at Lebanon, Ohio
-1.5%
-3%
-4%
-4.5%
1,000 3,000 3,000 4,000 5,000
DOSAGE Co (OH)2 MG/L
Figure 1-5.—Lime dosage versus pH septage.
Table 1-2.—Comparison of lime dosages required to
treat raw primary sludge
Investigator
Lime dose,
kg lime/kg sludge
dry solids
Burgess & Niple, Ltd (Lebanon).
Farrell et al
Counts et al
Paulsrud et al
a0.120
b 0.098
c 0.086
a0.125
"Based on pH 12.5 for sludges reported.
bBased on pH 11.5 for sludges reported.
cBased on 4.78% solids.
variables which affect pH decay. Paulsrud8 reported that
negligible pH decay occurred when the sludge mixture
was raised to pH 12 or greater or when the lime dose
was approximately five times the dose to reach pH 11.
In either case, for raw primary sludge, Paulsrud's dose
was in the range of 0.100 to 0.150 kg lime/kg dry
Sludge type
Percent
solids
Actual
lime dose,
kg lime/kg D.S.
Counts'
lime dose,
kg limeykg D.S.
Raw primary
Waste activated.
Anaerobically digested . . .
Septage
4.78
1 37
6.40
2.35
0.120
0.300
0.190
0.200
0.086
0.305
0.065
0.180
solids, which was approximately the dosage used at
Lebanon.
Counts9 hypothesized that pH decay was caused by
the sludge chemical demand which was exerted on the
hydroxide ions supplied in the lime slurry. He further
concluded that the degree of decay probably decreased
as the treated sludge pH increased because of the ex-
tremely large quantities of lime required to elevate the
pH to 12 or above. However, this pH phenomenon is
probably because pH is an exponential function, e.g.,
the amount of OH~ at pH 12 is ten times more than the
amount of OH~ at pH 11.
In the full-scale work at Lebanon, all sludges were
lime stabilized to pH 12 or above and held for at least
30 minutes with the addition of excess lime. All treated
sludges had less than a 2.0 pH unit drop after six
hours. Limed primary sludge was the most stable with
septage being the least stable. During the full-scale pro-
gram, only the pH of limed primary sludge was meas-
ured for a period greater than 24 hours, which showed
a gradual drop to approximately 11.6 after 18 hours
beyond which no further decrease was observed.
The total mixing times from start through the 30 min
contact time at Lebanon were as follows:
Hours
Primary sludge 2.4
Waste activated sludge 1.7
Septic tank sludge 1.5
Anaerobic digested sludge 4.1
Mixing time was a function of lime slurry feed rate
and was not limited by the agitating capacity of the
diffused air system. Mixing time may have been reduced
by increasing the capacity of the lime slurry tank.
To further examine the effects of excess lime addition
above the levels necessary to reach pH 12, a series of
laboratory tests were set up using a standard jar test
apparatus. The tests were made on six one-liter portions
of primary sludge with 2.7 percent total solids. The pH
of each of the samples was increased to 12 by the
addition of 10 percent hydrated lime slurry. One sample
was used as a control. The remaining samples had 30
percent, 60 percent, 90 percent, 120 percent, and 150
percent by weight of the lime dose added to the con-
trol. The samples were mixed continuously for 6 hours
and then again 10 minutes prior to each additional pH
measurement. There was a negligible drop in pH over a
-------
10-day period for those tests where excess lime was
added.
A second laboratory scale test was completed using a
5 gal (19 I) raw primary sludge sample which was lime
stabilized to pH 12.5 and allowed to stand at 18°C.
Samples were withdrawn weekly and analyzed for pH
and bacteria concentration. The results of the pH and
bacteria studies are shown on figures 1-6 and 1-7,
respectively. After 36 days, the pH had dropped to 12.0.
In conclusion, significant pH decay should not occur
once sufficient lime has been added to raise the sludge
pH to 12.5 and maintain that value for at least 30 min.
Odors
Previous work9 stated that the threshold odor number
of raw primary and trickling filter sludges was approxi-
mately 8,000, while that of lime stabilized sludges usually
ranged from 800 to 1,300. By retarding bacterial re-
growth, the deodorizing effect can be prolonged. Fur-
ther, it was concluded that by incorporating the stabi-
lized sludge into the soil, odor potential should not be
significant.
During the full-scale operations at Lebanon, there was
I30p-
e.o
no
O.O
9.0
8.0
60
> I0 20 30 40 50
DAYS
Figure 1-6.—Lime stabilized primary sludge pH versus
time.
an intense odor when raw sludge was first pumped to
the lime stabilization mixing tank, which increased when
diffused air was applied for mixing. As the sludge pH
increased, the sludge odor was masked by the odor of
ammonia which was being air stripped from the sludge.
The ammonia odor was most intense with anaerobically
digested sludge and was strong enough to cause nasal
irritation. As mixing proceeded, the treated sludge ac-
quired a musty humuslike odor, with the exception of
septage which did not have a significant odor reduction
as a result of treatment.
Sludge Characteristics
Several authors have previously attempted to summa-
rize the chemical and bacterial compositions of sewage
sludges.11"13 Recent data on the nutrient concentrations
for various sludges have been reported by Sommers.12
Chemical and pathogenic data on raw and lime stabi-
lized raw primary, waste activated, septage, and anaero-
100,000,0001
io,ooopoo|
ipoopoojl
too poo I
10,000 I
1,000 N.
100
0
100,000,000
lopoopoo
ipoo.ooo
100,000
10,000
1,000
100
0
loopoopoo
iopoo,ooo
IPOO.OOO
100,000
10,000
I POO
100
0
FECAL STREP
ECAL COLIFORM
oTAL COLIFORM
20f
10!
°I
501
40J
301
201
104
t/PS. AERUGINOSA
^SALMONELLA
10
20
30
40
5C
TIME . DAYS
Figure 1-7.—Bacteria concentration versus time labora-
tory regrowth studies.
-------
bically digested sludges from the Lebanon, Ohio full
scale project have been summarized below and are in-
cluded in more detail in the case history.
The addition of lime and mixing by diffused air altered
the chemical characteristics of each sludge. In all sludg-
es, lime stabilization resulted in an increase in alkalinity
and soluble COD and a decrease in soluble phosphate.
Total COD and total phosphate decreased for all sludg-
es except waste activated. Ammonia nitrogen and total
Table 1-4.—Volatile solids concentration of raw and lime
stabilized sludges
Sludge type
Raw sludge
volatile solids,
solids concentration,
mg/l
Lime stabilized sludge
volatile solids,
solids concentration,
mg/l
Privary
Waste
Septage
Anaerobically digested .
732
806
695
49.6
544
542
506
37.5
Table 1-5.—Nitrogen and phosphorus concentrations in
anaerobically digested and lime stabilized sludge
Sludge type
Lime stabilized primary
Lime stabilized waste activated.
Lime stabilized septage
Anaerobic digested
Total
phosphate
as P, mg/l
283
263
134
580
Total
Kjeldahl
nitrogen
as N, mg/l
1,374
1,034
597
2731
Ammonia
nitrogen
as N, mg/l
145
53
84
709
Kjeldahl nitrogen decreased for all sludges except waste
activated.
The volatile solids concentrations of raw and lime sta-
bilized sludges are shown in table 1-4. The actual vola-
tile solids concentrations following lime stabilization are
lower than those which would result only from the addi-
tion of lime. Neutralization, saponification, and hydrolysis
reactions with the lime probably result in the lower vola-
tile solids concentrations.
In terms of the agricultural value, lime stabilized sludg-
es had lower soluble phosphate, ammonia nitrogen, total
Kjeldahl nitrogen, and total solids concentrations than
anaerobically digested primary/waste activated mixtures
from the same plant, as shown in table 1-5. The signifi-
cance of these changes is discussed in the section on
land disposal.
Considerable research has been conducted on the de-
gree of bacterial reduction which can be achieved by
high lime doses.14'15 In general, the degree of pathogen
reduction increased as sludge pH increased with consist-
ently high pathogen reductions occurring only after the
pH reached 12.0. Fecal streptococci appeared to resist
inactivation by lime treatment particularly well in the low-
er pH values; however, at pH 12, these organisms were
also inactivated after 1 hour of contact time.9
In all lime stabilized sludges, Salmonella and Pseudo-
monas aeruginosa concentrations were reduced to near
zero. Fecal and total coliform concentrations were re-
duced greater than 99.99 percent in the primary and
septic sludges. In waste activated sludge, the total and
fecal coliform concentrations decreased 99.9 percent
and 99.94 percent, respectively. The fecal streptococci
kills were as follows: primary sludge, 99.93 percent;
waste activated sludge, 99.41 percent; septic sludge,
99.90 percent; and anaerobic digested, 96.81 percent.
Pathogen concentrations for the lime stabilized sludges
are summarized in table 1-6.
Anaerobic digestion is currently an acceptable method
of sludge stabilization.16 For reference, lime stabilized
sludge pathogen concentrations at Lebanon have been
compared in table 1-6 to those observed for well di-
gested sludge from the same plant.
Table 1-6.—Comparison of bacteria in anaerobic digested versus lime
stabilized sludges
Anaerobically digested . .
Lime stabilized3
Primary
Waste action
Seotaae
Fecal
conform
#/100 ml
1.450X103
4x103
16X103
265
Fecal
streptococci
#/100 ml
27X103
23X103
61 x 1 03
665
Total ,
coliform
#/100 ml
27,800 X103
27.6 X 1 03
212 X103
2,100
Salmonella
#/100 ml
6
b3
3
3
Ps.
aeruginosa
#/100 ml
42
3
13
3
aTo pH equal to or greater than 12.0.
bDetection limit = 3.
-------
Pathogen concentrations in lime stabilized sludges
range from 10 to 1,000 times less than for anaerobically
digested sludge.
A pilot scale experiment was completed in the labora-
tory to determine the viability and regrowth potential of
bacteria in lime stabilized primary sludge over an ex-
tended period of time.
The test was intended to simulate storing stabilized
sludge in a holding tank or lagoon when weather condi-
tions prohibit spreading. In the laboratory test, 5 gal (19
I) of 7 percent raw sludge from the Mill Creek sewage
treatment plant in Cincinnati were lime stabilized to pH
12.0. Lime was added until equivalent to 30 percent of
the weight of the dry solids which resulted in a final pH
of 12.5. The sample was then covered with foil and kept
at room temperature 65° F (18.3°C) for the remainder of
the test. The contents were stirred before samples were
taken for bacterial analysis.
The results, shown on figure 1-7, indicate that a hold-
ing period actually increases the bacteria kill. Salmonella
in the raw sludge totaling 44 per 100 ml were reduced
to the detection limit by lime stabilization. Pseudomonas
aeruginosa totaling 11 per 100 ml in the raw sludge
were reduced to the detection limit by lime stabilization.
The initial fecal coliform count of 3.0xi07was reduced
to 5xi03after lime stabilization, and after 24 hours was
reduced to less than 300. The raw sludge contained
3.8 X108 total conforms, but 24 hours after lime stabiliza-
tion the coliform total was less than 300. The fecal strep
count in the raw sludge was 1.8 x 108 which decreased
to 9.6 X104 after lime stabilization. After 24 hours, the
count was down to 7.0xl03and after 6 days reduced
to less than 300. The count increased to 8 x 105 after 40
days.
20 r
I5--
IO--
10
20
TIME-DAYS
-H-
25
30
35
40
Figure 1-8.—Dewatering characteristics of various
sludges on sand drying beds.
Sludge Dewatering Characteristics
Farrell et al.7 have previously reported on the dewater-
ing characteristics of ferric chloride and alum treated
sludges which were subsequently treated with lime. Trub-
nick and Mueller17 presented, in detail, the procedures to
be followed in conditioning sludge for filtration, using
lime with and without ferric chloride. Sontheimer18 pre-
sented information on the improvements in sludge filtera-
bility produced by lime addition.
Standard sand drying beds, which were located at the
Lebanon, Ohio wastewater treatment plant, were used
for sludge dewatering comparisons. Each bed was 30
ft x 70 ft (9.2 x 21.5 m). For the study, one bed was
partitioned to form two, each 15 ftx70 ft (4.6x21.5 m).
Limed primary sludge was applied to one bed with limed
anaerobically digested sludge being applied to the other
side. A second full-sized bed was used to dewater un-
limed anaerobically digested sludge. The results of the
study are summarized on figure 1-8.
Lime stabilized sludges generally dewatered at a lower
rate than well digested sludges. After 10 days, lime sta-
bilized primary sludge had dewatered to approximately
6.5 percent solids as opposed to 9 percent for lime
stabilized anaerobically digested sludge, and 10 percent
for untreated anaerobically digested sludge.
The anaerobically digested sludge cracked first and
dried more rapidly than either of the lime stabilized
sludges. Initially, both of the lime stabilized sludges mat-
ted, with the digested sludge cracking after approximate-
ly 2 weeks. The lime stabilized primary sludge did not
crack which hindered drying and resulted in the lower
percent solids values.
Land Application
Numerous references are available regarding the appli-
cation of anaerobically digested sludges to agricultural
land.11'12'16'19 The application of sewage sludge on land
has generally been viewed from two standpoints, either
as a rate of application consistent with the utilization of
nutrients in sludge by growing plants (i.e., agricultural
utilization), or as the maximum amount of sludge applied
in a minimum amount of time (i.e., disposal only).
USEPA guidelines16 generally favor the former approach.
The successful operation of a program utilizing the appli
cation of sewage sludge on land is dependent upon a
knowledge of the particular sludge, soil, and crop char-
acteristics.
-------
Organic matter content, fertilizer nutrients, and trace
element concentrations are generally regarded as being
vital to the evaluation of the applicability of land applica-
tion of sewage sludge. The range of nitrogen, phospho-
rus, and potassium concentrations for sewage sludges
have been reported by Brown et al.11
Sommers12 has also summarized fertilizer recommenda-
tions for crops based primarily on the amount of major
nutrients (nitrogen, phosphorus, and potassium) required
by a crop and on the yield desired.
Counts9 conducted greenhouse and test plot studies
for lime stabilized sludges which were designed to pro-
vide information on the response of plants grown in
sludge-soil mixtures ranging in application rate from 5 to
100 tons/acre (11 to 220 Mg/ha). Counts concluded
that sludge addition to poor, e.g., sandy, soils would
increase productivity, and therefore would be beneficial.
The total nitrogen and phosphorous levels in plants
grown in greenhouse pots, which contained sludge-soil
mixtures, were consistently lower than plants which were
grown in control pots. The control set, which contained
only soil with no sludge additions, received optimum
additions of chemical fertilizer during the actual plant
growth phase of the studies. Calcium concentration in
plant tissues from the sludge-soil pots were higher than
those for the controls. The pH values of the various
sludge-soil mixtures were lower after plant growth than
before. Counts attributed the decrease to carbon dioxide
buildup in the soil which resulted from biological activity.
Land application studies at Lebanon, Ohio, were con-
ducted by spreading liquid sludge on agricultural land
and on controlled test plots. Winter wheat, soybeans,
and hay were grown on fields which were in normal
agricultural production. Corn, swiss chard, and soybeans
were grown on 22 test plots, each with an area of
0.021 acre (0.0085 ha). A preliminary report on the re-
sults of the land application studies will be published in
1978.29
Sludge application was accomplished by spreading as
a liquid using a four-wheel drive vehicle which was
equipped with a 600 gal (2.3 m3) tank. The width of
sludge spread per pass was approximately 24 in. (60
cm).
The lime stabilized sludge formed a filamentous mat
1/8-1/4 in (0.3-0.6 cm) thick which, when dry, partly
choked out the wheat. The mat partly deteriorated over
time, but significant portions remained at the time of
harvest. There was no matting on the fields where the
lime stabilized sludge was incorporated into the soil be-
fore planting.
Spontaneous growth of tomatoes was significant in the
fields which had lime stabilized sludge incorporated into
the soil before planting. Seeds were contained in the
sludge and were not sterilized by the lime. These plants
were absent at the site where the sludge was not incor-
porated, even though no herbicide was applied, probably
because of frequent frosts and the lack of sludge incor-
poration into the soil. During the next year's growing
season, an increase in insect concentration was noticed
on the fields which had received lime stabilized sludge.
LIME STABILIZATION DESIGN
CONSIDERATIONS
Overall Design Concepts
Lime and sludge are two of the most difficult materials
to transfer, meter, and treat in any wastewater treatment
plant. For these reasons, design of stabilization facilities
should emphasize simplicity, straightforward piping layout,
ample space for operation and maintenance of equip-
ment, and gravity flow wherever possible. As discussed
in more detail in the following sections, lime transport
should be by auger with the slurry or slaking operations
occurring at the point of use. Lime slurry pumping
should be avoided with transport being by gravity in
open channels. Sludge flow to the tank truck and/or
temporary holding lagoon should also be by gravity if
possible.
Figures 1-9, 1-10, and 1-11 show conceptual designs
for lime stabilization facilities at wastewater treatment
facilities with 1, 5, and 10 Mgal/d (0.04, 0.22 and 0.44
m3/s) throughputs. The 1 Mgal/d (0.04 m3/s) plant, as
shown on figure 1-9, utilizes hydrated lime and a simple
batch mixing tank, with capability to treat all sludges in
less than one shift per day. Treated sludge could be
allowed to settle for several hours before hauling in
order to thicken, and thereby reduce the volume hauled.
Alternately, the sludge holding lagoon could be used for
thickening.
Figure 1-10 shows the conceptual design for lime
stabilization facilities of a 5 Mgal/d (0.22 m3/s) waste-
water treatment facility. Pebble lime is utilized in this
installation. Two sludge mixing tanks are provided, each
with the capacity to treat the total sludge production
from two shifts. During the remaining shift, sludge could
be thickened and hauled to the land disposal site. Alter-
nately, the temporary sludge lagoon could be used for
sludge thickening.
Figure 1-11 shows the conceptual design for lime
stabilization facilities of a 10 Mgal/d (0.44 m3/s) waste-
water treatment plant. A continuous lime treatment tank
with 2 hours detention time is used to raise the sludge
pH to 12. A separate sludge thickening tank is provided
to increase the treated sludge solids content before land
application. Sludge transport is assumed to be by pipe-
line to the land disposal site. A temporary sludge hold-
ing lagoon was assumed to be necessary, and would
also be located at the land disposal site.
Lime Requirements
The quantity of lime which will be required to raise
the pH of municipal wastewater sludges to pH greater
than 12 can be estimated from the data presented in
table 1-1 and from figures 1-2 to 1-5. Lime dosages
have been shown as 100 percent Ca(OH)2and should be
adjusted for the actual type of lime used. Generally, the
lime requirements for primary and/or waste activated
sludge will be in the range of 0.1 to 0.3 Kg 100 percent
Ca(OH)2 per Kg of dry sludge solids. Laboratory jar test-
ing can confirm the dosage required for existing sludges.
-------
w~-
DUST COLLECTOR
FILL
PIPE
TAMK- TRI irkr
TANK TRUCK
LAGOON
SLUDGE FROM LAGOON
Figure 1-9.—Conceptual design for lime stabilization facilities for a
1 Mgal/d (0.04 rrvVs) treatment plant.
DUST COLLECTOR
WATER J TURBINE AGITATORS
TREATED SLUDGE
TO LAGOON ^
TANK TRUCK
LAGOON
SLUDGE FROM LAGOON
Figure 1-10.—Conceptual design for lime stabilization facilities for a
5 Mgal/d (0.22 m3/s) treatment plant.
-------
INFLUENT
SLUDGE
PEBBLE
I LIME
STORAGE
BIN
=0--
•DUST COLLECTOR
^BUILDING
AUGERS
• LIME SLAKERS/FEEDERS
MECHANICAL TURBINE AGITATOR
MIX TANK WITH 2 HOUR
DETENTION TIME
DECANT TO
PRIMARY
INFLUENT
TANK TRUCK
00 00
TREATED SLUDGE
TO LAGOON *-
LAGOON
SLUDGE FROM LAGOON
Figure 1-11.—Conceptual design for lime stabilization facilities for a
10 Mgal/d (0.44 m3/s) treatment plant.
Types of Lime Available
Lime in its various forms, as quicklime and hydrated
lime, is the principal, lowest cost alkali. Lime is a gener-
al term, but by strict definition, it only embraces burned
forms of lime—quicklime, hydrated lime, and hydraulic
lime. The two forms of particular interest to lime stabili-
zation, however, are quicklime and hydrated lime. Not
included are carbonates (limestone or precipitated calci-
um carbonate) that are occasionally but erroneously re-
ferred to as "lime."20
Quicklime
Quicklime is the product resulting from the calcination
of limestone and to a lesser extent shell. It consists
primarily of the oxides of calcium and magnesium. On
the basis of their chemical analyses, quicklimes may be
divided into three classes:
1. High calcium quicklime—containing less than 5 per-
cent magnesium oxide, 85-90 percent CaO
2. Magnesium quicklime—containing 5 to 35 percent
magnesium oxide, 60-90 percent CaO
3. Dolomitic quicklime—containing 35 to 40 percent
magnesium oxide, 55-60 percent CaO
The magnesium quicklime is relatively rare in the Unit-
ed States and, while available in a few localities, is not
generally obtainable.
' Quicklime is available in a number of more or less
standard sizes, as follows:
1. Lump lime—the product with a maximum size of 8
in (20 cm) in diameter down to 2 in (5.1 cm) to 3
in (7.6 cm) produced in vertical kilns.
2. Crushed or pebble lime—the most common form,
which ranges in size from about 2-1/4 in (5.1-0.6
cm), produced in most kiln types.
3. Granular lime—the product obtained from Fluo-Sol-
ids kilns that has a particulate size range of 100
percent passing a #8 sieve and 100 percent re-
tained on a #80 sieve (a dustless product).
4. Ground lime—the product resulting from grinding
the larger sized material and/or passing off the fine
size. A typical size is substantially all passing a #8
sieve and 40 to 60 percent passing a #100 sieve.
5. Pulverized lime—the product resulting from a more
intense grinding that is used to produce ground
lime. A typical size is substantially all passing a
#20 sieve and 85 to 95 percent passing a #100
sieve.
10
-------
6. Pelletized lime—the product made by compressing
quicklime fines into about 1-inch size pellets or
briquettes.
Hydrated Lime
As defined by the American Society for Testing and
Materials, hydrated lime is: "A dry powder obtained by
treating quicklime with water enough to satisfy its chemi-
cal affinity for water under the conditions of its hydra-
tion."
The chemical composition of hydrated lime generally
reflects the composition of the quicklime from which it is
derived. A high calcium quicklime will produce a high
calcium hydrated lime obtaining 72 percent to 74 per-
cent calcium oxide and 23 percent to 24 percent water
in chemical combination with the calcium oxide. A do-
lomitic quicklime will produce a dolomitic hydrate. Under
normal hydrating conditions, the calcium oxide fraction
of the dolomitic quicklime completely hydrates, but gen-
erally only a small portion of the magnesium oxide hyd-
rates (about 5 to 20 percent). The composition of a
normal dolomitic hydrate will be 46 percent to 48 per-
cent calcium oxide, 33 percent to 34 percent magnesium
oxide, and 15 percent to 17 percent water in chemical
combination with the calcium oxide. (With some soft-
burned dolomitic quicklimes, 20 percent to 50 percent of
the MgO will hydrate.)
A "special" or pressure hydrated dolomitic lime is also
available. This lime has almost all (more than 92 per-
cent) of the magnesium oxide hydrated; hence, its water
content is higher and its oxide content lower than the
normal dolomitic hydrate.
Hydrated lime is packed in paper bags weighing 50 Ib
(22.7 kg) net; however, it is also shipped in bulk.
Quicklime is obtainable in either bulk carloads or tank-
er trucks or in 80 Ib (36.4 kg) multiwall paper bags.
Lump, crushed, pebble, or pelletized lime, because of
the large particle sizes, is rarely handled in bags and is
almost universally shipped in bulk. The finer sizes of
quicklime, ground, granular, and pulverized, are readily
handled in either bulk or bags.
Lime Storage and Feeding
Depending on the type of lime, storage and feeding
can be either in bag or bulk. Bagged lime will probably
be more economical for treatment plants less than one
Mgal/d (0.04 m3/s) and for temporary or emergency
feed systems, e.g., when a digester is out of service for
cleaning and repair. In new facilities, bulk storage will
probably be cost effective. Storage facilities should be
constructed such that dry lime is conveyed to the point
of use and then mixed or slaked. Generally, augers are
best for transporting either hydrated or pebble lime. Au-
ger runs should be horizontal or not exceeding an in-
cline of 30°.27
The feeder facilities, i.e., dry feeder and slaking or
slurry tank, should be located adjacent to the stabiliza-
tion mixing tank such that lime slurry can flow by gravity
in open channel troughs to the point of mixing. Pumping
lime slurry should be avoided. Slurry transfer distances
should be kept to a minimum. Access to feeder, slaker
and/or slurry equipment should be adequate for easy
disassembly and maintenance.
Mixing
Lime/sludge mixtures can be mixed either with me-
chanical mixers or with diffused air. The level of agita-
tion should be great enough to keep sludge solids sus-
pended and dispense the lime slurry evenly and rapidly.
The principal difference between the resultant lime stabi-
lized sludges in both cases is that ammonia will be
stripped from the sludge with diffused air mixing. Me-
chanical mixing has been used by previous researchers
for lime stabilization but only on the pilot scale.
With diffused air mixing, adequate ventilation should
be provided to dissipate odors generated during mixing
and stabilization. Coarse bubble diffusers should be used
with air supplies in the range of 150-250 ftVmin per
1,000 ft3 (150-250 rrvVmin per 1,000 m3) of mixing tank
volume. Diffusers should be mounted such that a spiral
roll is established in the mixing tank away from the point
of lime slurry application. Diffusers should be accessible
and piping should be kept against the tank wall to mini-
mize the collection of rags, etc. Adequate piping support
should be provided.
With the design of mechanical mixers, the bulk velocity
(defined as the turbine agitator pumping capacity divided
by the cross sectional area of the mixing vessel) should
be in the range of 15 to 26 ft/min (4.6 to 7.9 m/min).
Impeller Reynolds numbers should exceed 1,000 in order
to achieve a constant power number.21 The mixer should
be specified according to the standard motor horsepow-
er and AGMA gear ratios in order to be commercially
available.
For convenience, table 1-7 was completed which
shows a series of tank and mixer combinations which
should be adequate for mixing sludges up to 10 percent
dry solids, over a range of viscosity, and Reynolds num-
ber combinations which were as follows:
Max. Reynolds number 10,000 at 100 cp sludge vis-
cosity
Max. Reynolds number 1,000 at 1,000 cp sludge vis-
cosity
Table 1-7 can be used to select a mixer horsepower
and standard AGMA gear combination depending on the
volume of sludge to be stabilized. For example, for a
5,000 gal (1.9 m3) tank, any of the mixer-turbine combi-
nations should provide adequate mixing. Increasing tur-
bine diameter and decreasing shaft speed results in a
decrease in horespower requirement as shown.
Additional assumptions were that the bulk fluid velocity
must exceed 26 ft/min (7.9 m/min), impeller Reynolds
number must exceed 1,000, and that power requirements
11
-------
Table 1-7.—Mixer specifications for sludge slurries
Tank
size,
liters
18925
56 775
1 1 3 550
283 875
378 500
Tank Prime mover,
diameter, hp/shaft
meters speed, r/min
2.9 7.5/125
5/84
3/56
4.2 20/100
15/68
10/45
7.5/37
5 3 40/84
30/68
25/56
20/37
7.2 100/100
75/68
60/56
50/45
8.0 125/84
100/68
75/45
Turbine
diameter,
centimeters
81
97
109
114
135
160
170
145
155
168
206
157
188
201
221
183
198
239
ranging from 0.5-1.5 hp/1,000 gal (0.5-1.5 hp per 3,785
I) are necessary. The mixing tank configuration assumed
that the liquid depth equals tank diameter and that baf-
fles with a width of 1/12 the tank diameter were placed
at 90° spacing. Mixing theory and equations which were
used were after Badger,21 Hicks,22 and Fair.23
Raw and Treated Sludge Piping, Pumps,
and Grinder
Sludge piping design should include allowances for
increased friction losses due to the non-Newtonian pro-
perties of sludge. Friction loss calculations should be
based on treated sludge solids concentrations and
should allow for thickening in the mixing tank after stabi-
lization. Pipelines should not be less than 2 in (5.1 cm)
in diameter and should have tees in major runs at each
change in direction to permit rodding, cleaning, and
flushing the lines. Adequate drains should be provided. If
a source of high pressure water is available (either non-
potable or noncross-connected potable), it can be used
to flush and clean lines.
Spare pumps should be provided and mounted such
that they can be disassembled easily. Pump impeller
type materials of construction should be adequate for
the sludge solids concentration and pH.
Sludge grinding equipment should be used to make
the raw sludge homogenous. Sticks, rags, plastic, etc.,
will be broken up prior to lime stabilization to improve
the sludge mixing and flow characteristics and to elimi-
nate unsightly conditions at the land disposal site.
A CASE HISTORY OF LIME
STABILIZATION
Background
Facilities for lime stabilization of sludge were incorpo-
rated into an existing 1.0 Mgal/d (0.04 m3/s) single
stage activated sludge wastewater treatment plant locat-
ed at Lebanon, Ohio. Lebanon has a population of
about 8,000 and is located in southwestern Ohio, 30 mi
(48 km) northeast of Cincinnati. The surrounding area is
gently rolling farmland with a small number of light in-
dustries, nurseries, orchards, and truck farms.
Major unit processes at the wastewater treatment plant
include influent pumping, preaeration, primary clarifica-
tion, conventional activated sludge, and anaerobic sludge
digestion. Average influent BOD5 and suspended solids
concentrations are 180 and 243 mg/l, respectively. The
treatment plant flow schematic is shown on figure 1-12.
Prior to completing the sludge liming system, the exist-
ing anaerobic sludge digester was inoperative and was
being used as a sludge holding tank. The digester pH
was approximately 5.5 to 6.0. Grit and sand accumula-
tions had reduced its effective volume to 40-50 percent
of the total. Waste activated sludge was being returned
to the primary clarifiers and resettled with the primary
sludge. Combined primary/waste activated sludge was
being pumped to the digester and ultimately recycled to
the primary clarifiers via the digester supernatant. Typi-
cal supernatant suspended solids concentrations were in
the range of 30,000 to 40,000 mg/l. When possible,
sludge was withdrawn from the digester and dewatered
on sand drying beds.
USEPA made the decision to utilize lime stabilization
at Lebanon not only as a full scale research and dem-
onstration project, but also as a means of solids han-
dling during the period while the anaerobic digester was
out of service for cleaning and repair.
Revisions to the Existing Wastewater
Treatment Plant
Lime Stabilization
The lime stabilization process was designed to treat
raw primary, waste activated, septic tank, and anaerobi-
cally digested sludges. The liming system was integrated
with the existing treatment plant facilities, as shown on
figure 1-13. Hydrated lime was stored in a bulk storage
bin and was augered into a volumetric feeder. The feed-
er transferred dry lime at a constant rate into a 25 gal
(95 I) slurry tank which discharged an 8-10 percent lime
slurry by gravity into an existing 6,500 gal (25 m3) tank.
The lime slurry and sludge were mixed with diffused air.
A flow schematic for the lime stabilization facilities is
shown on figure 1-14. Design data are shown in table
1-8.
Septage Holding Facilities
Because the Lebanon wastewater treatment plant rou-
tinely accepted septic tank pumpings, a 5,000 gal (18.4
12
-------
PREAERATION
PRIMARY
CLARIFER
AERATION
FINAL
CLARIFIER
_J
FINAL
CLARIFIER
J
AERATION
RETURN SLUDGE
WASTE ACTIVATED SULDGE
SUPERNATANT
REAERATION
TANK
\J"@
SLUDGE
WELL
SUPERNATANT
ANAEROBIC
DIGESTER
CLARIFIER
FINAL
EFFLUENT \
CREEK
Figure 1-12.—Treatment plant flow schematic prior to incorporating lime
stabilization.
VOLUMETRIC FEEDER
LIME SLURRY TANK
WATER
PRIMARY AND/OR
WASTE ACTIVATED
SLUDGE
Figure 1-13.—Treatment plant flow schematic after incorporating lime sta-
bilization.
13
-------
ANAEROBIC
DIGESTED SLUDGE
PRIMARY
SLUDGE
WASTE
ACTIVATED SLUDGE;
-VOLUMETRIC FEEDER
SLUDGE
WELL AND
PUMP
TREATED
SLUDGE
TANK TRUCK FOR
LAND DISPOSAL
CO 00
Figure 1-14.—Lime stabilization process flow diagram.
m3) tank was installed to hold septic tank sludges prior
to lime treatment. The tank was equipped with a transfer
pump which could be used to either feed the lime stabi-
lization process or transfer septage to the primary tank
influent at a controlled rate.
Ultimate Sludge Disposal
Treated sludges were applied to sand drying beds, to
test plots, and to three productive agricultural sites.
Land spreading operations began in early March and
continued through October. The sludge hauling vehicle
was a four-wheel drive truck with a 600 gal (2.3 m3)
tank.
Anaerobic Digester
As previously described, the existing single stage an-
aerobic sludge digester was inoperative and was being
used as a sludge holding tank. The digester and auxilia-
ry equipment were completely renovated and returned to
good operating condition which allowed a comparison of
anaerobic digestion and lime stabilization. The digester
was cleaned, a new boiler and hot water circulating
system were installed, and all necessary repairs were
made to piping, valves, pumps, and electrical equipment.
The anaerobic digester design data are shown in table
1-9.
Operation and Sampling
Raw sludge, e.g., primary, waste activated, septage or
digested sludge, was pumped to the mixing tank where
it was mixed by diffused air. Four coarse bubble diffus-
ers were mounted approximately 1 ft (30.5 cm) above
the top of the tank hopper and 1.25 ft (38 cm) from the
tank wall. This location permitted mixing to roll sludge
up and across the tank at which point lime slurry was
fed. Lime which was used for the stabilization of all
sludges was industrial grade hydrated lime with CaO and
MgO contents of 46.9 percent and 34 percent, respec-
tively. All lime requirements have been converted and
are expressed as 100 percent Ca(OH)2 except as noted.
Samples were taken from the untreated, but thoroughly
mixed, sludge for chemical, pH, bacteria, and parasite
analyses.
After the initial pH determination, the lime slurry addi-
tion was started. Hydrated lime was augered from the
lime storage bin to the volumetric feeder which was
located directly above the sludge mixing tank. The lime
was slurried by the tangential injection of water into a
14
-------
Table 1-8.—Design data for lime stabilization facilities
After 30 minutes, samples were taken for chemical, bac-
teria, and parasite analyses. Air mixing was then discon-
tinued, allowing the limed sludge to concentrate. The
Mixing tank sludge then flowed by gravity to a sludge well from
Total volume 30 m3 (8,000 gal) wnicn it was pumped to the land disposal truck.
working volume 25 m (6.500 gal) Samples of raw and treated Lebanon sludges were
3.05 mX3^66 mx2.38 m (10'x12 X7.8) ^ * egch t| da of tne |ime stabilization
co9ars: Si£ ' operations. Anaerobically dFgested sludge samples were
Number of diffusers 4 taken at the same time and analyzed for use in compari-
Air supply """ 14-34 m3/min (500-1,200 ft3/min) sons of chemical, bacterial, and pathogen properties.
Bulk lime storage Sample preservation and chemical analysis techniques
Total volume 28 m3 (i.ooo ft3) were performed in accordance with procedures as stated
Diameter 2.74 m (9') in "Methods for Chemical Analysis of Water and Wastes,
vibrators 2 ea Syntron v-41 USEPA,"24 and "Standard Methods for the Examination
FIN system Pneumatic of Water and Wastewater.' '25
Discharge system 15 cm (6") dia. auger Salmonella species and Pseudomonas aeruginosa were
Material of construction.. steel . . _ „ determined by EPA staff using the method developed by
voLmethc feeder e"' Kenner and Clark.26 Fecal coliform, total coliform, and
Total volume 028 m3(io cu ft) fecal streptococcus were determined according to meth-
Diameter 71 cm (28") ods specified in Standard Methods for Examination of
Material of construction., steel Water and Wastewater.
Type and manufacturer.. Vibrascrew LBB 28-10
Feed range 45-227 kg/hr (100-500 Ib/hr)
Average feed rate 78 kg/hr (173 Ib/hr)
Lime slurry tank Chemical data for Lebanon, Ohio raw primary, waste
Total volume 94.6 i (25 gal) activated, anaerobically digested, and septage sludges
Diameter....... 0.61 m (2') hgve been summarizecj in table 1-10. Data for each
toM^JSSkT9 parameter include the average and range of the values
Total volume 18.4 m3 (650 ft3) observed.
working volume 15 m3 (4,000 gal) Analyses for heavy metals were conducted on grab
Dimensions 3.66 mxi.92 mx2.62 m samples of raw primary, waste activated, and anaerobi-
O2'x6.3'x8.6') cally digested sludges. These data have been reported
Mixing Coarse bubble in table 1-11 as mg/kg on a dry weight basis and
Number of diffusers 1 include the average and range of values.
Air supply 2.8-8.4 m3/min (100-300 ft3/min) Pathogen data for Lebanon, Ohio raw primary, waste
Transfer pumps activated, anaerobically digested, and septage sludges
Raw and treated sludge. 1 136 I/mm (300 gpm) nave been summarized jn table -,_12. |n general, the
Septage transfer pump... 379 I/mm (100 gpm) ^ ^ jn agreement wjth the va|ues reported by
" Stern,14 with the exception of Salmonella and Pseudomo-
nas aeruginosa, which are lower than the reported val-
Table 1-9.—Anaerobic digester rehabilitation design data ues-
Tank dimensions
Total volume
Actual volatile solids loading..
Hydraulic detention time
Sludge recirculation rate
Boiler capacity
15 m (50') dia. x 6.1 m (20') SWD
1,223 m3 (43,200 ft3)
486 g VSS/m3 (0.03 Ib VSS/ft3)
36 days
757 1/min (200 gpm)
2.53 X108 Joules/hr (240,000 Btu/hr)
25 gal (95 I) slurry tank. The lime solution (8-10 percent
by weight) then flowed by gravity into an open channel
with three feed points into the sludge mixing tank.
The sludge pH was checked every 15 minutes as the
lime slurry was added until the sludge reached a pH of
12, at which time it was held for 30 minutes. During the
30 minute period, lime slurry continued to be added.
Lime Stabilized Sludges
Chemical and bacterial data for lime stabilized sludges
have previously been summarized in the general discus-
sion on lime stabilization. Specific data from the Leba-
non, Ohio full scale project have been summarized in
tables 1-13 and 1-14. Lime stabilized sludges had lower
soluble phosphate, ammonia nitrogen, total Kjeldahl nitro-
gen, and total solids concentrations than anaerobically
digested primary/waste activated mixtures from the same
plant.
In all lime stabilized sludges, Salmonella and Pseudo-
monas aeruginosa concentrations were reduced to near
zero. Fecal and total coliform concentrations were re-
duced greater than 99.99 percent in the primary and
septic sludges. In waste activated sludge, the total and
fecal coliform concentrations decreased 99.99 percent
15
-------
Table 1-10.—Chemical composition of raw sludges at
Lebanon, Ohio
Table 1-11.—Heavy metal concentrations in raw sludges
at Lebanon, Ohio
Parameter
(mg/l)
Alkalinity
Alkalinity range
Total COD
Total COD range
Soluble COD
Soluble COD range
Total phosphate, as P
Total phosphate range, as P
Soluble phosphate, as P
Soluble phosphate range, as P
Total Kjeldahl nitrogen
Total Kjeldahl nitrogen range
Ammonia nitrogen
Ammonia nitrogen range
Total suspended solids
Total suspended solids range
Volatile suspended solids
Volatile suspended solids range ....
Volatile acids
Volatile acids range
Alkalinity
Alkalinity range
Total COD
Soluble COD
Soluble COD range
Total phosphate, as P
Total phosphate range, as P
Soluble phosphate, as P
Soluble phosphate range, as P
Total Kjeldahl nitrogen
Total Kjeldahl nitrogen range
Ammonia nitrogen
Ammonia nitrogen range
Total suspended solids
Total suspended solids range
Volatile suspended solids
Volatile suspended solids range ....
Volatile acids
Volatile acids range
Raw
primary
sludge
1,885
1 ,264-2,820
54,146
36,930-75,210
3,046
2,410-4,090
350
264-^96
69
20-150
1 ,656
1 ,250-2,470
223
19-592
48,700
37,520-65,140
36,100
28,780-43,810
1 ,997
1,368-2,856
3,593
1 ,330-5,000
66,372
39,280-1 90,980
1,011
215-4,460
580
379-862
15
6.9-34.8
2,731
1,530-4,510
709
368-1,250
61,140
48,200-68,720
33,316
27,000-41,000
137
24-248
Waste
activated
sludge
1,265
1,220-1,310
12,810
7,120-19,270
1,043
272-2,430
218
178-259
85
40-119
711
624-860
51
27-85
12,350
9,800-13,860
10,000
7,550-12,040
NA
NA
1,897
1,200-2,690
24,940
10,770—32,480
1,223
1 ,090-1 ,400
172
123-217
25
21 .6-27.9
820
610-1,060
92
68-116
21,120
6,850-44,000
12,600
3,050-30,350
652
560-888
Metal
(mg/kg)
Cadmium, average
Cadmium, range
Total chromium, average . .
Total chromium, range
Copper, average
Copper, range
Lead, average
Lead, range
Mercury, average
Mercury, range
Nickel, average
Nickel, range
Zinc, average
Zinc, range
Table 1-12. — Pathogen
non, Ohio
Parameter
(#/100 ml)
Salmonella average
Salmonella range
Ps. aeruginosa average
Ps. aeruginosa range
Fecal coliform average MF...
Fecal coliform range MF
Fecal coliform average MPN.
Fecal coliform range MPN . . .
Total coliform average MF . . .
Total coliform range MF
Total coliform average MPN .
Total coliform range MPN....
Fecal streptococci average . .
Fecal streptococci range
Parameter
(#/100 ml)
Raw Waste Anaerobic
primary activated digested
sludge sludge sludge
105 388 137
69-141 119-657 73-200
633 592 882
287-979 133-1,050 184-1,580
2,640 1 ,340 4,690
2,590-2,690 670-2,010 4,330-5,050
1,379 1,624 1,597
987-1 ,770 398-2,850 994-2,200
6 46 0.5
0.4-11 0.1-91 0.1-0.9
549 2,109 388
371-727 537-3,680 263-540
4,690 2,221 7,125
4,370-5,010 1,250-3,191 6.910-7,340
data for raw sludges at Leba-
Raw Waste
primary sludge activated sludge
62 6
11-240 3-9
195 5.5 X103
75-440 91-1. 1X104
NA 2.65 X107
NA 2.0X107-3.3X107
8.3 X108 NA
. 1.3X108-3.3X109 NA
NA 8.33 X108
NA 1.66X10*-1.5X109
2.9 X109 NA
. LSXI^-S.SXIO9 NA
3.9X107 1.03X107
. 2.6X107-5.2X107 5X105-2X107
Anaerobically Septage
digested sludge sludge
and 99.47 percent, respectively. The fecal streptococci
kills were as follows: primary sludge, 99.93 percent;
waste activated sludge, 99.41 percent; septic sludge,
99.90 percent; and anaerobic digested sludge, 96.81
percent. Pathogen concentrations in lime stabilized
sludges range from 10 to 1,000 times less than for
anaerobically digested sludges.
Economic Analysis
Lebanon Facilities
As previously described, the anaerobic sludge diges-
tion facilities at Lebanon were essentially inoperable at
16
Salmonella average
Salmonella range
Ps. aeruginosa average
Ps. aeruginosa range
Fecal coliform average MF...
Fecal coliform range MF
Fecal coliform average MPN.
Fecal coliform range MPN ...
Total coliform average MF ...
Total coliform range MF
Total coliform average MPN .
Total coliform range MPN....
Fecal streptococci average ..
Fecal streptococci range
6
3-30
42
3-240
2.6 X105
s^xioM.exio5
1.45X106
2.42 X107
1.3X105-1.8X108
2.78 X107
2.7 X105
6
3-9
754
14-2.1 X107
1.5X107
1.0X107-1.8X107
NA
NA
2.89 X108
1,
NA
NA
6.7X105
3.3X105-1.2X106
-------
Table 1-13.—Chemical composition of lime stabilized sludges at Lebanon, Ohio
Parameter
(mg/l)
Alkalinity
Alkalinity range
Total COD
Total COD range
Soluble COD
Soluble COD range
Total phosphate
Total phosphate range
Soluble phosphate
Soluble phosphate range
Total Kjeldahl nitrogen
Total Kjeldahl nitrogen range
Ammonia nitrogen
Ammonia nitrogen range
Total suspended solids
Total suspended solids range
Volatile suspended solids
Volatile suspended solids range . . .
Raw
primary
sludge
4,313
3 830-5 470
41 180
26 480-60 250
3556
876-6 080
283
164-644
36
17-119
1 374
470-2510
145
81-548
38,370
. . 29,460-44,750
23,480
19,420-26,450
Waste
activated
sludge
5,000
4 400-5 600
14700
10880-20800
1 618
485-3010
263
238-289
25
17-31
1,034
832-1 ,430
64
36-107
10,700
10,745-15,550
7,136
6,364-8,300
Anaerobically
digested
sludge
8,467
2,600-13,200
58,690
27 190-107,060
1,809
807-2,660
381
280-460
29
1 .4-5.0
1,980
1,480-2,360
494
412-570
66,350
46,570-77,900
26,375
21 ,500-29,300
Septage
sludge
3,475
1,910-6,700
17,520
5,660-23,900
1,537
1,000-1,970
134
80-177
2.4
1.4-4.0
597
370-760
110
53-162
23,190
14,250-29,600
11,390
5,780-19,500
Table 1-14.—Pathogen data for lime stabilized sludges at Lebanon, Ohio
Parameter
(#/100 ml)
Salmonella average
Salmonella range
Ps aeruginosa average
Ps. aeruginosa range
Fecal coliform MF average
Fecal coliform MF range
Fecal coliform average MPN...
Fecal coliform range MPN
Total coliform average MF
Total coliform range MF
Total coliform average MPN . . .
Total coliform range MPN
Fecal streptococci average ....
Fecal streptococci range
Raw
primary sludge
a3
a3
a3
a3
NA
NA
5.93 X103
560-1 .7 X104
NA
NA
1.15X105
640-5.4 X105
1.62X104
4.0X103-5.5X104
Waste
activated sludge
a3
a3
a3
a3-26
1.62X104
S.SxlC^-S^XIO4
NA
NA
2.2 X105
3.3X10M.2X105
NA
NA
6.75X103
I.SXIOM.SSXIO3
Anaerobically
digested sludge
a3
a3
a3
a3
3.3 X103
3.3 X103
18
18
NA
NA
18
18
8.6 X103
3.3X103-1.4X104
Septage
sludge
a3
a3
a3
a3
2.65 X102
2 x 102-3.3 X102
NA
NA
2.1 X103
200-4 X103
NA
NA
665
3.3X102-! X103
"Detectable limit = 3.
the start of the lime stabilization project. Funds were
allocated to construct lime stabilization facilities, as well
as to rehabilitate the anaerobic digester. In both cases,
the existing structures, equipment, etc., were utilized to
the maximum extent possible. Table 1-15 includes the
actual amounts paid to contractors, following competitive
bidding, and does not include engineering fees, adminis-
trative costs, etc.
The cost of the lime stabilization facilities was
$29,507.45 compared to $32,134.59 for cleaning and
repair of the anaerobic sludge digester.
Capital Cost of New Facilities
Capital and annual operation and maintenance costs
for lime stabilization and anaerobic sludge digestion fa-
cilities were estimated assuming new construction as a
part of a 1.0 Mgal/d (0.04 m3/s) wastewater treatment
plant with primary clarification and single stage conven-
tional activated sludge treatment processes.
The capital costs for lime stabilization facilities includ-
ed a bulk lime storage bin for hydrated lime, auger,
volumetric feeder and lime slurry tank, sludge mixing and
17
-------
Table 1-15.—Actual cost of digester rehabilitation and
lime stabilization facilities construction
475 Ib/day (215 kg/day)
3 hrs/day
1,000 ft3 (28 m3)
Daily lime requirement as 100
percent Ca(OH)2
Treatment period
~ ~~~Bulk lime storage bin volume min-
Anaerobic digester cleaning imum
Cleaning contractor $5,512.12 Bulk lime storage bin detention 34 days
Temporary sludge lagoon 2,315.20 time
Lime for stabilizing digester contents 514.65 Lime feeder and slurry tank ca- 5-15 ft3/hr (0.14-0.42 m3/hr)
Temporary pump rental 300.30 pacity (spared)
Subtotal digester cleaning 864227 I"/'"6"1 sludge grinder capacity 20° gpm (12'6 "?>
Sludge mixing tank volume 15,000 gal (57 m3)
Anaerobic digester rehabilitation Sludge mixing tank dimensions 14 ftxl4 ftxlO ft SWD (4.3
Electrical equipment, conduit, etc 1,055.56 mx4.3 mx3 m)
Natural gas piping 968.76 Sludge mixer horsepower 15 HP (11.2 kW)
Hot water boiler, piping, pump, heat exchanger repair... 7,472.26 Sludge mixer turbine diameter 53 in (135 cm)
Control room rehabilitation 1,465.00 Turbine speed 68 rpm
Sludge recirculating pump repair 771.00 Sludge transfer pump capacity 400 gpm (25.2 l/s)
Piping and valve rehabilitation 8,587.30 (spared)
Floating cover roof repair 1,014.04 Treated sludge percent solids 4 percent
Repair utilities, drains 211.52 Sludge holding lagoon volume 100,000 ft3 (2,860 m3)
Miscellaneous 1,946.88 Sludge holding lagoon maximum 60 days
Subtotal digester rehabilitation 23 492 32 Detention time
' Treatment building floor area 150 ft2 (14m2)
Lime stabilization process Treatment building construction Brick and block
Electrical equipment, conduit, etc 1,692.00 Instrumentation pH record treated sludge volume
4" sludge crossover1 'pipfTvaives and^ttings ' m9s"" 'MO? 48 CaPital costs for the lime stabilization facilities were
1-1/2" air line and diffusers ' '.'.'.'.'.'.'.'.'.'.'.'. U10.00 based on JulV 1- 1977> bid date- and nave been SUmma
3/4" water lines and hose bibbs 865.00 r'zec' 'n *ab'e 1-16.
Lime bin, auger, vibrators 7,229.44 ^me stabilization operation assumed one man, 2 hours
Volumetric feeder, trough and gate 3,460.00 Per day, 365 days per year, at $6.50 per hour, including
Existing pump repairs 3,399.00 overhead. Maintenance labor and materials assumed 52
Miscellaneous metal 1,200.00 hours per year labor at $6.50 per hour and $800 per
Relocate sanitary service ime 200.00 year for maintenance materials. The total quantity of
Repair utilities 134.00 46.8 percent CaO hydrated lime required was 141 tons
Miscellaneous.. 934.34 (128 Mg) per year at $44.50 per ton ($49.06/Mg)
contractors overhead 1.842.00 Tne total annual cost for lime stabilization, excluding
Subtotal lime stabilization 29,507.45 land application of treated sludge, has been summarized
Septage holding tank in table 1-17.
Septage holding tank and pump 617470 Tne bas's f°r design of a single stage anaerobic
subtotal septage holding tank e!i 74^70 fudge digester for the same treatment plant was as
Total cost for digester cleaning and rehabilitation, TOIIOWS.
lime stabilization, and septage facilities 67,816.74 Daily primary sludge dry solids 1,250 Ib/day (568 kg/day)
production
thickening tank with a mechanical mixer, sludge grinder,
all weather treatment building, electrical and instrumenta-
tion, interconnecting piping and transfer pumps, and 60-
day detention treated sludge holding lagoon. The basis
for design is as follows:
Daily primary sludge dry solids
production
Average primary sludge volume
@ 5 percent solids
Daily waste activated dry solids
production
Average waste activated sludge
volume @ 1.5 percent solids
Average lime dosage required per
unit dry solids
1,250 Ibs/day (568 kg/day)
2,910 gal/day (11 rrrVday)
1,084 Ibs/day (493 kg/day)
8,580 gal/day (32 m3/day)
0.20 kg/kg
Table 1-16.—Capital cost of lime stabilization facilities
for a new 1 Mgal/d (0.04 m3/s) wastewater treatment
plant
Site work, earthwork and yard piping
Lime storage bin and feeders
Treatment tank, pumps, sludge grinders, and building
structure
Electrical and instrumentation
Sludge holding lagoon
$6,OOC
30.00C
52.00C
10.00C
.. 20,00(
Subtotal construction cost 118,000
Engineering
Total capital cost
Amortized cost at 30 yrs., 7% int. (CRF = 0.081).
Annual capital cost per ton dry solids
12.00C
130.00C
10.50C
24.66
18
-------
Table 1-17.—Total annual cost for lime stabilization ex-
cluding land disposal for a 1 Mgal/d (0.04 m3/s) plant
Item
Operating labor
Maintenance labor and materials
Lime
Laboratory
Capital
Total annual cost
Total
annual
cost
$4,700
1,100
6,300
500
10,500
23 100
Annual
cost
per Kkg
dry solids
$12.14
2.84
16.20
1.29
27.11
5958
Annual
cost
per ton
dry solids
$1 1 .03
2.58
14.74
1.17
24.65
54.17
Average primary sludge volume
@ 5 percent solids
Daily waste activated dry solids
production
Average waste activated sludge
volume @ 1.5 percent solids
Daily volatile solids production
Volatile solids loading
Digester hydraulic detention time
Digester gas production
Average volatile solids reduction
Digested sludge dry solids pro-
duction
Digested sludge percent solids
Digester net heat requirement
Mechanical mixer horsepower
Sludge recirculation pumps (2 ea)
2,910 gal/day (11 m3/day)
1,084 Ib/day (493 kg/day)
8,580 gal/day (32 m3/day)
1,634 Ib/day (743 kg/day)
0.05 Ib VSS/ft3/day (0.8
kg/m3/day)
21 days
13 ft3/lb VSS (0.8 m3/kg) feed
50 percent
1,515 Ib/day (690 kg/day)
6 percent
186,000 Btu/hr (54,500 W)
15 HP (11.2 kW)
350 gal/min ea (22 l/s)
Capital cost for the anaerobic sludge digestion facili-
ties, including the control building, structure, floating
cover, heat exchanger, gas safety equipment, pumps,
and interconnecting piping, assuming July 1, 1977, bid
date, and engineering, legal, and administrative costs are
summarized in table 1-18.
Digester operation assumed one man, 1 hour per day,
Table 1-18.—Capital cost for single stage anaerobic
digestion facilities for a 1 Mgal/min (0.04 m3/s) waste-
water treatment plant
Site work, earthwork, yard piping $44,000
Digester 233,000
Control building 133,000
Electrical and instrumentation 47,000
Subtotal construction cost 457,000
Engineering
46,000
Total capital cost 503,000
365 days per year at $6.50 per hour, including over-
head. Maintenance labor and material assumed 52 hours
per year at $6.50 per hour and $1,500 per year for
maintenance materials.
The cost of anaerobic digester operation was offset
by assuming a value of $2.10 per million Btu ($1.99 per
million kJ) for all digester gas produced above the net
digester heat requirement.
The total annual cost for anaerobic sludge digestion,
excluding land application has been summarized in table
1-19.
Both the lime stabilization and anaerobic digestion al-
ternatives were assumed to utilize land application of
treated sludge as a liquid hauled by truck. The capital
cost for a sludge hauling vehicle was assumed to be
$35,000, which was depreciated on a straight line basis
over a 10-year period. Alternatively, a small treatment
plant could utilize an existing vehicle which could be
converted for land application at a somewhat lower cap-
ital cost.
The assumed hauling distance was 3 to 5 miles (5 to
8 km), round trip. Hauling time assumed 10 minutes to
fill, 15 minutes to empty, and 10 minutes driving, or a
total of 35 minutes per round trip. The truck volume was
assumed to be 1,500 gal (5.68 m3) per load. The cost
of truck operations, excluding the driver and deprecia-
tion, was assumed to be $8.50 per operating hour. The
truck driver labor rate was assumed to be $6.50 per
hour, including overhead.
Truck operation time was based on hauling an aver-
age of 6,860 gal (1.812 m3) of lime stabilized sludge,
i.e., five loads and 2,940 gal (0.777 m3) of anaerobically
digested sludge, i.e., two loads per day. The reduced
volume of anaerobically digested sludge resulted from
the volatile solids reduction during digestion and the
higher solids concentration compared to lime stabilized
sludge.
Although it may be possible to obtain the use of farm-
land at no cost, e.g., on a voluntary basis, the land
application economic analysis assumed that land would
Table 1-19.—Total annual cost for single stage anaero-
bic sludge digestion excluding land disposal for a 3,785
rrrVday plant
Amortized cost at 30 yrs, 7% int. (CRF = 0.081).
Annual capital cost per unit feed dry solids
40,700
95.54
Item
Operating labor . . .
Maintenance labor and materials...
Laboratory
Capital
Fuel credit
Total annual cost
annual
cost
$2400
1,800
500
40700
(2 900)
42.500
Annual
cost
per Kkg
dry solids
$620
4.65
1 29
10509
(749)
109.74
Annual
cost
per ton
dry solids
$563
4.23
1 17
9554
(6811
99.76
19
-------
Table 1-20.—Annual cost for land application of lime stabilized and an-
aerobically digested sludges for a 3,785 rrvVday plant
Lime stabilization
Item
Amortized cost of land
Truck depreciation
Truck driver
Truck operation. .
Laboratory
Fertilizer credit .
Land credit . .
Total annual cost
Total
annual
cost
$2,600
3,500
7,100
9,300
500
(3,100)
(2,200)
17,700
Annual
cost
per Kkg
solids
$6.75
9.04
18.35
24.03
1.29
(8.05)
(5.68)
45.73
Annual
cost
per ton
solids
$6.14
8.22
16.67
21.83
1.17
(7.30)
(5.16)
41.57
Anaerobic digestion
Total
annual
cost
$1,700
3,500
2,800
3,600
500
(2,000)
(1 ,400)
8,700
Annual
cost
per Kkg
solids
$4.39
9.04
7.24
9.30
1.29
(8.05)
(3.62)
19.59
Annual
cost
per ton
solids
$3.99
8.22
6.57
8.45
1.17
(7.30)
(3.29)
17.81
be purchased at a cost of $750 per acre ($1850 per
ha). Sludge application rates were assumed to be 10 dry
tons per acre per year (22.4 Mg/ha/year). Land costs
were amortized at 7 percent interest over a 30-year
period.
To offset the land cost, a fertilizer credit of $7.30 per
ton ($8.05 per Mg) of dry sludge solids was assumed.
This rate was arbitrarily assumed to be 50 percent of
the value published by Brown11 based on medium fertiliz-
er market value and low fertilizer content. The reduction
was made to reflect resistance to accepting sludge as
fertilizer. The land cost was further offset by assuming a
return of $50 per acre ($124 ha), either as profit after
farming expenses, or as the rental value of the land.
Capital and annual operation and maintenance costs
for land application of lime stabilized and anaerobically
digested sludges have been summarized in table 1-20.
For each item in table 1-18, the total annual cost was
calculated and divided by the total raw primary plus
waste activated sludge quantity, i.e., 426 tons/year (386
Mg). Anaerobically digested sludge land requirements
were less than for lime stabilized sludge because of the
volatile solids reduction during digestion. Truck driving
and operation costs were similarly less for digested
sludge because of the volatile solids reduction and more
concentrated sludge (6 percent versus 4 percent) which
would be hauled. Fertilizer credit was less for digested
sludge because of the lower amount of dry solids ap-
plied to the land. Land credit was based on the amount
of sludge applied and was, therefore, less for digested
sludge.
The total annual capital and annual operation and
maintenance costs for lime stabilization and single stage
anaerobic sludge digestion, including land application for
a 1 Mgal/day (0.04 m3/s) wastewater treatment plant,
are summarized in table 1-21.
Table 1-21.—Comparison of total annual capital and
annual O. & M. cost for lime stabilization and anaerobic
digestion including land disposal for a 3,785 mVday
plant
Lime stabilization
Facilities
Land application . . .
Total annual cost ..
Total
annual
O. & M.
cost
$23,100
17,700
40,800
Annual
cost per
Kkg dry
solids
$59.58
45.70
105.28
Anaerobic digestio
Total
annual
O. & M.
cost
$42,500
8,700
51,200
Annual
cost pe
Kkg dr
solids
$109.7-
19.5!
129.3:
Lime Stabilization by Others
A considerable amount of lime stabilization work has
occurred in Connecticut. A number of incinerators have
been shut down and replaced by lime stabilization. A
total of 27 plants with capacities from 0.3 to 29 mgd
(0.01 to 1.27 m3/s) are utilizing lime stabilization either
on a full- or part-time basis. The following tabulation anc
comments for nine plants are typical and summarize the
current situation. Lime stabilized sludges are either used
as landfill cover or are composted. These methods have
been satisfactory. Most of the communities have indicat-
ed that they will continue with lime stabilization. Typical
plants in Connecticut which are utilizing lime stabilization
are shown at the top of the facing page.
20
-------
Design
plant Incinerator
size,
mgd Installed Used Hours
Stratford3
West Bridgeport".
Stamford0
Middletownd
Willimatic6
Glastonburg*
Torrington9
Naugatuckh
Enfield1
11
29
20
7
5.5
3.2
7
7
10
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
Yes
No
24
24
No
N/A*
N/A
N/A
N/A
N/A
Used
Yes
Yes
N/A
Yes
Yes
Yes
Yes
Yes
Lime stabilization
Hours Ultimate disposal
8
8
N/A
16
N/A
N/A
N/A
N/A
Landfill cover
Landfill cover
Lagoon
Landfill cover
Land and landfill
Landfill cover
Landfill
Landfill cover
Yes 1/4 of year Yes 3/4 of year Land
*N/A denotes data not available at the time of writing.
"Incinerator abandoned in favor of lime stabilization to pH 12. Composted and used as final cover at landfill.
bStabilized cake used as final cover at landfill.
°Centrifuged with lime sludge. Haul away and lagooned.
dPreviously plagued with odors; now all sludge processed in two shifts, 5 days per week with no odors. Lime
stabilized and final cover at landfill.
"Began lime stabilization in 1973. Screened sludge and leaf material used on parks as fertilizer and final cover
for landfill.
'Final cover for landfill and composted with leaves.
9Lime stabilization used when incinerator out of service.
hLime stabilized sludge used as final cover at two landfills.
'Incineration is used in winter during inclement weather. Lime stabilized sludge stockpiled and spread on corn
land during remainder of year.
LIME STABILIZATION DESIGN EXAMPLES
Statement of Problem
The problem is to provide lime stabilization facilities
for two communities, both of which have existing con-
ventional activated sludge wastewater treatment plants.
The smaller community has existing wastewater treat-
ment facilities capable of treating 4.0 Mgal/day (0.18
m3/s). The facilities consist of screening, grit removal,
primary settling, conventional activated sludge aeration,
final settling, cnlorination, and sludge lagooning. Present
flow to the plant is 3.5 million gallons per day (0.15
m3/s); the 20-year projected flow is 4.0 million gallons
per day (0.18 m3/s). The plant meets its proposed dis-
charge permit requirements, but the city has been or-
dered to abandon the sludge lagoons (which are periodi-
cally flooded by the receiving stream). Sludge disposal
alternatives include the following:
1. Lime stabilization followed by liquid application to
farmland.
2. Anaerobic digestion followed by liquid application to
farmland.
The larger community has existing wastewater treat-
ment facilities capable of treating 30 million gallons per
day (1.31 m3/s). Present flow to the plant is 35 million
gallons per day (1.53 m3/s); the 20-year projected flow
is 40 million gallons per day (1.75 m3/s). The existing
treatment system consists of screening, grit removal, pri-
mary settling, conventional activated sludge aeration, fi-
nal settling, chlorination, aerobic sludge digestion, sludge
dewatering, and landfilling of dried sludge solids. The
existing treatment scheme will meet proposed permit re-
quirements. As a part of the treatment plant expansion
planning and in view of future electric power costs, the
following solids handling alternatives were proposed:
1. Lime stabilization followed by pipeline transportation
to the land application site.
2. Anaerobic digestion followed by mechanical dewa-
tering and land application.
The design logic which will be followed to develop
and evaluate the sludge handling alternatives is summa-
rized on figure 1-15.
Wastewater Characteristics
The wastewater characteristics and removal efficiencies
of the various treatment units are required to determine
the basis for design of the sludge stabilization and ulti-
mate disposal processes. This information may be ac-
quired from plant records or from sampling programs at
the existing facilities. When these data are not available
(such as in the case of new wastewater treatment plants
for new service areas), assumptions based on sound
engineering judgment and previous experience are nec-
essary. For the sake of simplicity, the wastewater char-
acteristics and treatment unit removal efficiencies for the
example plants were assumed to be equal. Raw waste-
water characteristics for the example plants are given in
table 1-22.
Treatment Unit Efficiencies
Both plants in this example will meet their proposed
permit requirements by utilizing the existing treatment
21
-------
1. Establish regulatory constraints for effluent and sludge disposal
2. Determine WWTP influent loads
3. Deterimme WWTP unit process scheme
4. Determine raw sludge loads
5. Establish cost effective constraints and sludge solids concentrations
for ultimate sludge disposal processes
6. Set sludge thickening requirements
7. Select stabilization alternatives
Figure 1-15.—Process alternative design logic.
7a. Develop capital cost
7b. Develop 0 & M requirements and cost
7c. Develop environmental constraints
7d. Evaluate supernatant impact on plant
7e. Evaluate estimated total sludge handling costs
8. Screen alternatives
9. Select final stabilization process
10. Prepare final flow sheets and cost estimates
Table 1-22.—Raw wastewater characteristics
Parameter
Concentration
(mg/l)
BOD5
Suspended solids.
Organic nitrogen...
Ammonia nitrogen .
Phosphorus
Grease
200
240
15
25
10
100
Table 1-23.—Treatment unit efficiencies
Unit
Parameter
Removal efficiency
(percent)
Primary settling
Aeration and final settling . . .
BOD*
SS
BODK
SS
30
65
60
25
processes. Nitrification and phosphorus removal are not
required. Removal efficiencies based on percentages of
the raw "domestic" wastewater characteristics are pre-
sented in table 1-23.
Sludge Characteristics
The characteristics of sludge discharged to the sludge
stabilization facilities may vary considerably depending
on the type and amount of industrial waste treated, the
sludge origin (which particular treatment unit) and the
sludge age. Ideally, samples of sludge would be avail-
able for analysis. The assumed sludge characteristics for
each example plant are as follows:
Sludge type
Thickened raw primary
Thickened waste activated.
Design percent solid
7.0
2.5
Thickening facilities for primary and waste activated
sludge were assumed to be cost effective for both the A
and 40 Mgal/d (0.18 and 1.75 m3/s) wastewater treat-
ment plants. Waste activated sludge production was 0.5
pound of volatile solids per pound of BOD5 reduced.
Preliminary studies have indicated that anaerobic
22
-------
sludge digestion will not be adversely affected by the
inclusion of thickened waste activated sludge.
The sludge quantities for the 4 Mgal/d (0.18 m3/s)
wastewater treatment plant were developed as follows:
Influent BOD5
Influent 4.0 Mgal/d x 8.34X200 mg/l = 6,672 Ib/day
(3,033 kg/day)
Primary removal = 6,672x0.3 = 2,002 Ib/day (910
kg/day)
BOD5 remaining in settled sewage = 4, 670 Ib/day
(2,123 kg/day)
Influent suspended solids
Influent 4x8.34x240 mg/l = 8,006 Ib/day (3,639
kg /day)
Primary removal = 8,006x0.65 = 5,204 Ib/day (2,365
kg /day)
Suspended solids remaining in settled sew-
age =2,802 Ib/day (1,274 kg/day)
Waste activated solids
Biological = 6,672x0.60x0.5 Ib VSS/lb
BOD5 = 2,002 Ib VSS/day
Suspended solids = 8,006x0.25 = 2,002 Ib/day
Total biological solids produced = 4,004 Ib/day
(1,820 kg/day)
Net daily sludge quantities
Primary: 5,204 Ib/day (2,360 kg/day) at 7 percent
following thickening
5'2°4
= 8'740
(33 m'/day)
8.34 X102X 0.07
Waste activated sludge
0.025X48°3044X1.01=19'014
Net sludge produced (5,204 + 4,004) = 9,208 Ib sol-
ids/day (4,185 kg/day)
Volume = (8,740 + 19,01 4) = 27,754 gal/day (105
m3day)
Percent solids = 3.9 percent
Design sludge quantities were developed for the 40
Mgal/d (1.75 m3/s) facility in an identical manner. The
design sludge quantities are summarized as follows:
Primary sludge solids, Ib/day
Primary sludge volume at 7 percent,
gal/day
Biological sludge solids, Ib/day
Biological sludge volume at 2.5 percent,
gal/day
Total sludge solids, Ib/day
Combined sludge volume, gal/day
Combined sludge percent solids
4.0 Mgal/d
WWTP
5,204
8,740
4,004
19,014
9,208
27,754
3.9
40 Mgal/d
WWTP
52,040
87,400
40,040
190,140
92,080
277,540
3.9
For simplicity, the design examples for the 4 and 40
Mgal/d (0.18 and 1.75 m3/s) treatment plants will be
presented separately. Each example will include the de-
sign basis for each alternative stabilization and ultimate
disposal process, final sludge volumes, capital and annu-
al operation and maintenance costs.
Process Alternatives—-4 Mgal/d
(0.18 mVs) WWTP
As previously discussed, process alternatives for the 4
Mgal/d wastewater treatment plant will be as follows:
1. Lime stabilization followed by liquid application to
farmland.
2. Anaerobic digestion followed by liquid application to
farmland.
Lime Stabilization
A flow diagram for the proposed lime stabilization fa-
cilities is shown on figure 1-16. Significant process
equipment includes a bulk lime storage bin for pebble
quicklime, auger, lime slaker and feed slurry tank, sludge
mixing and thickening tank with a mechanical mixer,
sludge grinder, all weather treatment building, electrical
and instrumentation, interconnecting piping and transfer
pumps, and a sludge holding lagoon with 60 days deten-
tion time. The basis for design is as follows:
Total sludge solids
Sludge volume
Raw sludge percent solids
Overall lime dosage required per unit dry
solids, as 100 percent Ca(OH)2
Daily lime requirement as Ca(OH)2
Treatment period
Bulk lime storage bin volume minimum
Bulk lime storage bin detention time
Lime slaker and slurry tank capacity (2
ea)
Influent sludge grinder capacity (spared)
Sludge mixing tank volume
Sludge mixing tank dimensions
Sludge mixer horsepower
Sludge mixer turbine diameter
Turbine speed
Sludge transfer pump capacity (spared)
Treated sludge volume
Treated sludge percent solids
Sludge holding lagoon total volume (4
cells)
Sludge holding lagoon maximum detention
time
Treatment building floor area
Treatment building construction
Instrumentation
9,208 Ib/day (4,185
kg/day)
27,754 gal/day (105
m3/day)
3.9
0.20 Ib/lb
1,826 Ib/day (830
kg/day)
6 hrs/day
1,000 ft3 (28 m3)
34 days
200-300 Ib CaO/hr (91-
136 kg/hr)
200 gal/min (12.6 l/s)
25,000 gal (95 m3)
18 ftxl8 ftxlO ft SWD
(5.5 mX5.5 mx3 m)
15 HP (11.2 kW)
53 in (135 cm)
68 rpm
400 gal/min (25.2 l/s)
24,050 gal (91 m3)
4.5
240,000 ft3 (6,800 m3)
60 days
250 ft2 (23.2 m2)
brick and block
pH record treated sludge
volume
With the exception of the lime storage bin detention
time and pump capacities, the reasons for selecting the
particular design quantities have been discussed in previ-
ous sections. Lime storage bin capacity was based on a
minimum detention time of 30 days to allow capacity for
a standard 20-ton (18 Mg) lime shipment. The pump
capacity was based on convenient transfer times be-
tween units.
23
-------
TREATED
SECONDARY
CHLORINATION
EFFLUENT
TO DISCHARGE
LIQUID SLUDGE
TO LAND APPLICATION
Figure 1-16.—4 Mgal/d (0.18 m3/s) lime stabilization/truck haul and land
application.
Capital costs for the lime stabilization facilities were
based on January 1, 1978, bid date and have been
summarized in table 1-24.
Lime stabilization operation assumed one man, 8 hours
per day, 365 days per year, at $6.50 per hour, including
overhead. Maintenance labor was assumed to be 156
hours per year labor at $6.50 per hour and $2,400 per
year for maintenance materials. The total quantity of 85
percent CaO quicklime required was 297 tons (269 Mg)
per year at $40 per ton ($44/Mg).
The total annual cost for lime stabilization, excluding
land application of treated sludge, has been calculated
as follows and is summarized in table 1-25.
Lime Stabilization Operating Costs
Labor: 8hr/dayx365 day/yrx$6.50/hr = $18,980 say
$19,000
Table 1-24.—Capital costs of lime stabilization facilities
for a 4 Mgal/d wastewater treatment plant
Site work, earthwork, yard piping $26,00
Lime storage bin and feeders 84,00
Treatment tank, pumps, sludge grinders, and building
structure
Electrical and instrumentation
Sludge holding lagoon
Subtotal construction cost
Engineering.
142,00
29,00
.. 54,00
.. 335,00
36,00
Total capital cost 371,00
Amortized cost at 30 yrs., 7 percent int. (CRF = 0.081)
Annual capital cost per ton dry solids
30,10
17.9
24
-------
Table 1-25.—Total annual cost for lime stabilization ex-
cluding land disposal for a 4 Mgal/d plant
Item
Operating labor
Maintenance labor and materials
Lime .
Laboratory . .
Capital . .
Total annual cost
Total
annual
$19,000
. . . 3,400
12,000
1 ,500
30,100
66000
Annual
per ton
dry solids
$11.31
2.02
7.14
0.89
17.91
3927
Maintenance labor: 156 hr/yrx$6.50 = $1,014 say $1,000
Maintenance materials: $2,400/yr lump sum
Lime primary: 5,204 Ib/dayx0.12 Ib Ca(OH)2/lb = 624
Ib/day (283 kg/day)
Waste activated: 4,004 Ib/day x 0.3 Ib Ca(OH)2/lb = 1,201
Ib/day (545 kg/day)
Total lime = (624 + 1,201 ) = 1,825lb Ca(OH)2/day (828
kg/day)
1,825 lb/day/0.85) x 56/74 = 1,625 Ib/day CaO (737
kg/day)
1,625x365/2,000 = 297 ton/yr (269 Mg/yr)
say 300 ton/yr x$40/ton = $12,000/yr
Laboratory: $1,500/yr lump sum
Capital: $371,000X0.081 =$30,100/yr
Both the lime stabilization and anaerobic digestion al-
ternatives were assumed to utilize land application of
treated sludge as a liquid hauled by truck. The capital
cost per sludge hauling vehicle was assumed to be
$35,000, which was depreciated on a straight-line basis
over a 5-year period.
The assumed hauling distance was 3 to 5 miles (5-8
km), round trip. Hauling time assumed 10 minutes to fill,
15 minutes to empty, and 10 minutes driving, or a total
of 35 minutes per round trip. The truck volume was
assumed to be 1,500 gallons (5.7 m3) per load. The cost
of truck operations, excluding the driver and deprecia-
tion, was assumed to be $8.50 per operating hour. The
truck driver labor rate was assumed to be $6.50 per
hour, including overhead.
Truck operation time was based on hauling on a
5-day per week basis, approximately 10 months per
year, which results in the assumed 215 hauling days per
year. The average volume hauled is 40,800 gallons
(154.4 m3) per day. Two trucks were assumed to be
required, with a combined total of 28 loads per day.
Although it may be possible to obtain the use of farm-
land at no cost, e.g., on a voluntary basis, the land
application economic analysis assumed that land would
be purchased at a cost of $750 per acre ($1850/ha).
Sludge application rates were assumed to be 10 dry
tons per acre per year. Land costs were amortized at 7
percent interest over a 30-year period.
To offset the land cost, a fertilizer credit of $7.30 per
ton ($8.05 Mg) of dry sludge solids was assumed. This
rate was arbitrarily assumed to be 50 percent of the
value published by Brown11 based on medium fertilizer
market value and low fertilizer content. The reduction
was made to reflect resistance to accepting sludge as
fertilizer. The land cost was further offset by assuming a
return of $50 per acre ($123/ha), either as profit after
farming expenses or as the rental value of the land.
Capital and annual operation and maintenance costs
for land application of lime stabilized sludge were calcu-
lated as follows and have been summarized in table 1-
26.
Lime Stabilization Land Application Costs
Land: 9,208 Ib solids/day x 365 days/2,000
lb/ton = 1,681 ton/yr (1,525 Mg/yr)
1,681 ton/yr/10 ton/acre = 168 acres (68.0 ha) say
200 (80.9 ha)
200 acres x $750/acre = $150,000
$150,000x0.081 =$12,150/yr say $12,200
Truck depreciation: $35,000 x 2 = $70,000 capital
$70,000/5 yrs = $14,000/yr
Truck driver: 40,800 gal/day/2,571 gal/truck/hr = 15.9
hr/day
say 2 trucks at 8 hr/day
$6.50x2 menxS hr/day = $104/day
$104X215 = $22,360 say $22,400/yr
Truck operation: 2 trucks x 8
hr/day x $8.50/hr = $136.00/day
$136.00x215 = $29,240 say $29,200/yr
Laboratory: $1,500/yr lump sum
Fertilizer credit: 1,681 ton/yrx$7.30/ton = $12,271 say
$12,300/yr
Land credit: 168 acres x$50/acre = $8,400/yr
Table 1-26.—Annual cost for land application of lime
stabilized sludge for a 4 Mgal/d plant
Item
Amortized cost of land
Truck depreciation
Truck driver
Truck operation
Laboratory
Fertilizer credit
Land credit. . . .
Total annual cost
Total
annual
cost
$12,200
14,000
22,400
29,200
1,500
(12300)
(8 400)
58600
Annual
cost
per ton
dry solids
$7.26
8.33
13.33
17.38
0.89
(7.30)
(5.00)
3489
25
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Anaerobic Digestion
A flow diagram for the proposed anaerobic sludge
digestion facilities is shown on figure 1-17. Two-stage
anaerobic digestion was assumed with stabilized sludge
being hauled to farmland. Sludge storage was allowed in
the digester design and no lagoon was included. The
basis for design for the anaerobic digesters for the 4
Mgal/d (.18 m3/s) treatment plant was as follows:
First Stage
Feed solids loading
Feed volume
Feed percent solids
Feed percent volatile solids
Digester dimensions
Digester volume
Mixers
Hydraulic detention time
Loading rate
Digester bulk temperature
Average feed temperature
Volatile solids reduction
Overall total solids reduction
Sludge heaters
9,208 Ib/day (4,185 kg/day)
27,754 gal/day (105 m3/s)
39
65
60 MX25 ft SWD (18.3 mX7.6 m)
529,000 gal (2,002 m3)
2 ea at 3,500 gpm (221 l/s)
19 days
0.085 lb/VSS/ft3/day (1.36
kg/m3/day)
95° F (35° C)
55° F (13°C)
50 percent
32 percent
3 ea at 500,000 Btu/hr (14,650 W)
Second Stage
Digester dimensions
Digester volume
Hydraulic detention time
Digester gas production
Digester gas heat value
Digested sludge dry solids pro-
duction
Digested sludge percent solids
Sludge recirculation pumps (2
ea)
60 ft x 25 ft SWD (18.3 mx7.6 m
529,000 gal (2,002 m3)
19 days
10 ft3/lb VSS (0.6 rrvVkg) feed
500 Btu/ft3 (18,625 kJ/m3)
6,261 Ib/day (2,846 kg/day)
6.5 percent
500 gpm ea (31.5 l/s)
Design conditions were based on the criteria enumer-
ated in Ten States' Standards28 and assumed installation
in the Midwestern United States.
Capital cost for the anaerobic sludge digestion facili-
ties, including the control building, structures, floating
cover, heat exchanger, gas safety equipment, pumps,
and interconnecting piping, assuming January 1, 1978,
bid date, and engineering, legal, and administrative cost:
is summarized in table 1-27.
Digester operation assumed one man, 3 hours per
day, 365 days per year at $6.50 per hour, including
overhead. Maintenance labor and material assumed 416
ACTIVATED
SLUDGE
TREATED
SECONDARY
CHLORINATION
EFFLUENT
TO DISCHARGE
RETURN SLUDGE
IASTE ACTIVATED SLUDGE
RECYCLE SUPERNATANT
THICKENED SLUDGE v
^^-
C
/
0 C
11 STAGE
VNAEROBIC
DIGESTER
— ^
0
LIQUID SLUDGE
TO LAND APPLICATION
TANK TRUCK
Figure 1-17.—4 Mgal/d (0.18 m3/s) anaerobic digestion/truck haul and
land application.
26
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Table 1-27.—Capital cost of two-stage anaerobic diges-
tion facilities for a 4 Mgal/d wastewater treatment plant
Site work, earthwork, yard piping and pumps $151,000
Digesters 675,000
Control building 251,000
Electrical and instrumentation 125,000
Subtotal construction cost 1,202,000
Engineering
107.000
Total capital cost 1,309,000
Amortized cost at 30 yrs., 7 percent int. (CRF = 0.081)
Annual capital cost per unit feed dry solids
106,000
63.08
hours per year at $6.50 per hour and $7,000 per year
for maintenance materials.
The cost of anaerobic digester operation was offset
by assuming a value of $2.70 per million Btu ($2.56 per
million kJ) for all digester gas produced above the net
digester heat requirements.
The total annual cost for anaerobic sludge digestion,
excluding land application was calculated as follows and
has been summarized in table 1-28.
Anaerobic Digester O & M Cost
Operator labor: 3 hr/dayx365
day/yrx$6.50/hr = $7,118/yr say $7,100/yr
Maintenance labor: 416 hr/yrx$6.50/hr = $2,704 say
$2,700/yr
Maintenance materials: $7,000/yr lump sum
Laboratory: $1,500/yr lump sum
Capital $1,309,000X0.081 =$106,000
Fuel credit: 9,208 Ibx 0.65 = 5,985 Ib VSS feed/day
5,985 IbX 10 cf/lb VSS = 59,850 cf/day (1,695
m3/day) gas
59,850 ft3 X 500 Btu/ft3 = 29.9 X106 Btu/day
(31.6xl06kJ/day)
475,000 Btu/hrx24 hr/day/0.5 eff = 22.8xl06
Btu/day (24.1 xi06kJ/day) required for digester
heat
29.9 X106-22.8X106 = 7.1 X106 Btu/day (7.5 X106
kJ/day) excess gas
7.1 X106 X $2.70 XIO^X 365 = $6,997 say $7,000/yr
Land application costs were developed for the anaero-
bic digestion alternative in a manner similar to that pre-
viously described for lime stabilization. Anaerobically di-
gested sludge land requirements were less than for lime
stabilized sludge because of the volatile solids reduction
during digestion. Truck driving and operation costs were
similarly less for digested sludge because of the volatile
solids reduction and more concentrated sludge (6.5 per-
cent versus 4.5 percent) which would be hauled. The
total fertilizer credit was based on $7.30 per ton
($8.05/Mg) of dry solids, but was lower because of the
lower amount of dry solids applied to the land. The total
land credit was less because land requirements were
based on the total amount of sludge solids applied.
Land application costs for the anaerobic digestion alter-
native were calculated in a manner similar to those for
the lime stabilization alternative and are summarized in
table 1-29.
The total annual capital and annual operation and
maintenance costs for lime stabilization and two-stage
anaerobic sludge digestion, including land application for
a 4 Mgal/d (0.18 m3/s) wastewater treatment plant, are
summarized in table 1-30.
Process alternatives—-40 Mgal/d
(1.75 m3/s) WWTP
As previously discussed, process alternatives for the
40 Mgal/d wastewater treatment plant will be as follows:
1. Lime stabilization followed by pipeline transportation
to the land application site.
2. Anaerobic digestion followed by mechanical dewa-
tering and land application.
Table 1-28.—Total annual cost for two-stage anaerobic
sludge digestion excluding land disposal for a 4 Mgal/d
plant
Item
Operating labor
Maintenance labor and materials.
Laboratory
Capital
Fuel credit
Total
annual
cost
Annual
cost
per ton
dry solids
Total annual cost
$7,100
9,700
1,500
106,000
(7,000)
117,300
$4.23
5.77
0.89
63.08
(4.16)
69.81
Table 1-29.—Annual cost for land application of anaero-
bically digested sludges for a 4 Mgal/d plant
Item
Total
annual
cost
Amortized cost of land .
Truck depreciation
Truck driver
Truck operation
Laboratory
Fertilizer credit
Land credit
Annual
cost
per ton
solids
Total annual cost
$8,200
7,000
11,200
14,600
1,500
(8,300)
(5,700)
28,500
$4.88
4.17
6.66
8.69
0.89
(4.94)
(3.39)
16.96
27
-------
Table 1-30.—Comparison of total annual capital and
annual O. & M. cost for lime stabilization and anaerobic
digestion including land disposal for a 4 Mgal/d plant
Lime stabilization Anaerobic digestion
Total Annual
annual cost per
O. & M. ton dry
cost solids
Total Annual
annual cost per
O. & M. ton dry
cost solids
Facilities
Amortized capital
Operating labor
Maintenance labor and
materials
Lime
Laboratory
Fuel credit
Subtotal facilities ...
Land application
Amortized cost of land..
Truck depreciation
Truck drivers
Truck operations
Laboratory
Fertilizer credit
Land credit
Subtotal land appli-
cation
Raw sludge percent solids
Overall lime dosage required per
unit dry solids as 100 percent
Ca(OH)2
Daily lime requirement as Ca(OH)2
Treatment period
Bulk lime storage bin volume min-
imum
Bulk lime storage bin detention
time
Lime slaker & slurry tank capacity
(2 ea)
Influent sludge grinder max. ca-
pacity
Sludge mixing tank volume at 1
hr detention time (2 ea)
Sludge mixing tank dimensions
Sludge mixer horsepower (2 ea)
Sludge mixer turbine diameter
Turbine speed
Sludge thickener dimensions (2
ea)
Thickened sludge volume
Thickened sludge percent solids
Sludge transfer pump capacity (2
ea)
Intermediate pump station pumps
Treatment building floor area
Treatment building construction
Instrumentation
58,600 34.89 28,500 16.96 Lagoon volume at application site
$30,100
19,000
3,400
12,000
1,500
N/A
$17.91
11.31
2.02
7.14
0.89
N/A
$106,000
7,100
9,700
1,500
(7,000)
$63.08
4.23
577
0.89
(4.16)
66,000 39.27 117,300
69.81
12,200
14,000
22,400
29,200
1,500
(12,300)
(8,400)
7.26
8.33
13.33
17.38
0.89
(7.30)
(5.00)
8,200
7,000
1 1 ,200
14,600
1,500
(8,300)
(5,700)
4.88
4.17
6.66
8.69
0.89
(4.94)
(3.39)
Total annual cost facilities
and land application..
124,600 74.16 145,800
86.77
The design logic which will be followed to develop
and evaluate the sludge handling alternatives has previ-
ously been summarized on figure 14. Wastewater char-
acteristics, treatment unit efficiencies, and sludge charac-
teristics have also been previously summarized.
Lime Stabilization
A flow diagram for the proposed lime stabilization fa-
cilities is shown on figure 1-18. Significant process
equipment includes a bulk lime storage bin for pebble
quicklime, augers, lime slakers and feed slurry tanks,
sludge mixing tanks, sludge thickeners, sludge grinders,
all weather treatment building, electrical and instrumenta-
tion, interconnecting piping, and sludge pump stations.
The sludge pipeline was assumed to be 10 miles (1.6
km) long with two intermediate pump stations. One land
application farm site was assumed. A sludge storage
lagoon with 60-days holding capacity was provided at
the land application site.
The basis for design is as follows:
Pipeline length
Pipeline diameter
Pipeline working pressure
Land application trucks
3.9
0.20 Ib/lb dry solids
18,250 Ib/day (8,295 kg/day)
24 hrs/day
2 ea 4,260 ft3 (120.6 m3)
30 days
500-750 CaO/hr
2 ea 200 gpm (12.6 l/s)
12,000 gal (45.4 m3)
10 ftxiO ft SWD (3 mx3 m)
10 HP (7.5 kW)
5 ft (1.5 m)
45 rpm
65 ft dia. X12 ft SWD (19.8
mx3.7 m)
240,500 gal/day (910.3 m3/day)
4.5
250 gpm at 200 psi (15.8 l/s at
14.1 kg km2)
4 ea 250 gpm at 200 psi (15.8
l/s at 14.1 kg/cm2)
600 ft2 (55.7 m2)
brick and block
pH record/control raw sludge vol-
ume treated sludge volume
pipeline pressure control
10,000,000 gal (37,850 m3) (20
cells)
53,000 ft (16,154 m)
6 in (15 cm)
200-250 psig (14.1-17.6 kg/cm2)
12 at 1,500 gal (5.7 m3) ea
Total sludge solids
Sludge volume
92,080 Ib/day (41,855 kg/day)
227,540 gal/day (861.2 m3/day)
The reasons for selecting the particular design quanti-
ties have been discussed in previous sections. Sludge
pump capacities were selected to permit reasonable
pipeline pressure drops and velocities. The sludge lagoon
was divided into several cells to permit convenient with-
drawal of all sludge and to prevent solids accumulation.
Capital costs for the lime stabilization facilities, based
on January 1, 1978, bid date, excluding final sludge
pumping, pipeline, application trucks, lagoon, and land,
are summarized in table 1-31.
Lime stabilization operation assumed two men, three
shifts per day, 365 days per year at $6.50 per hour,
including overhead. Maintenance labor was assumed to
be 1,664 hours per year at $6.50 per hour and $7,500
per year for maintenance materials. The total quantity of
85 percent CaO quicklime required was 2,966 tons
(2691 Mg) per year at $40 per ton ($447Mg).
The total annual cost for lime stabilization, excluding
land application of treated sludge, was calculated in a
manner to that previously shown on the 4 Mgal/d (0.18
m3/s) example and has been summarized in table 1-32.
Ultimate sludge disposal was assumed to be as a
liquid on farmland with truck spreading. The total land
28
-------
TREATED
SECONDARY
CHLORINATION
EFFLUENT
TO DISCHARGE
Figure 1-18.—40 Mgal/d (1.75 m3/s) lime stabilization/pipeline transport
and land application.
Table 1-31.—Capital cost of lime stabilization facilities
for a 40 Mgal/d wastewater treatment plant
Site work, earthwork and yard piping
Lime storage, slakers, and feed
Lime treatment tanks, mixers, grinders and building.
Sludge thickeners
Electrical and instrumentation
Subtotal construction cost
Engineering
Total capital cost
Amortized cost at 30 yrs., 7% int. (CFR = 0.081).
Annual capital cost per unit feed dry solids
90,000
1,077,000
87,200
5.19
Table 1-32.—Total annual cost for lime stabilization ex-
cluding land disposal for a 40 Mgal/d plant
Item
Total
annual
cost
Annual
cost per
ton dry
solids
$95,000
106,000
155,000
529,000
102,000
987,000 Operating labor $114,000
Maintenance labor and materials.
Lime
Power
Laboratory
Total annual cost
18,300
118,600
2,000
4,500
$6.78
1.09
7.06
0.12
0.27
257,400 15.32
29
-------
spreading operation will require 2,000 acres (810 ha).
Land cost was assumed to be $1,250 per acre
($3088/ha) to reflect the more urban setting than the 4
Mgal/d (0.18 m3/s) case. The capital cost per sludge
hauling vehicle was assumed to be $35,000, with 12
being required. The vehicles were depreciated over a 7-
year period. The sludge holding lagoon was located at
the farm site and was sized to hold 60-days sludge
production. The lagoon was partitioned into 500,000 gal-
lon (1,892 m3) cells to permit access and efficient utiliza-
tion of the storage volume.
The assumed hauling time was 10 minutes to fill, 20
minutes to haul, empty and return, for a total of 30
minutes per round trip. The truck volume was assumed
to be 1,500 gallons (5.7 m3) per load. The cost of truck
operations, excluding the driver and depreciation, was
assumed to be $8.50 per operating hour. The truck
driver labor rate was assumed to be $6.50 per hour,
including overhead.
Truck operating time was based on hauling on a 215-
day-per-year schedule, 12 hours per day.
To offset the land cost, a fertilizer credit of $7.30 per
ton ($8.05/Mg) of dry sludge solids was assumed. This
rate was arbitrarily assumed to be 50 percent of the
value published by Brown11 based on medium fertilizer
market value and low fertilizer content. The reduction
was made to reflect resistance to accepting sludge as
fertilizer. The land cost was further offset by assuming a
return of $50 per acre ($124/ha), either as profit after
farming expenses, or as the rental value of the land.
Easements for the sludge pipeline were assumed to
cost $2.50 per foot ($8.20/m). Two intermediate booster
stations were provided to maintain a reasonable pressure
profile along the line. Progressive cavity pumps were
used for both the treatment plant and intermediate pump
stations. Allowance was assumed to permit regular
cleaning of the line by utilizing pipeline "pigs."
Table 1-33.—Capital cost of lime stabilization land appli-
cation facilities for a 40 Mgal/d wastewater treatment
plant
Site work, earthwork
Sludge transfer pumps
Sludge pipeline
Booster station
Sludge lagoon
Electrical and instrumentation.
Subtotal construction cost
Engineering
Total capital cost pipeline, pump stations and
lagoon
Amortized cost at 30 yrs., 7 percent int. (CFR = 0.081)
Annual capital cost per unit feed dry solids
$17,000
45,000
675,000
104,000
569,000
19,000
1,429,000
124.000
1,553,000
125,800
7.49
Table 1-34.—Annual cost for transportation and land
application of lime stabilized sludge for a 40 Mgal/d
plant
Item
Land
Easements
Pipeline, pump stations and
lagoon
Truck depreciation
Truck drivers .
Truck operation
Power
Pipeline operation and mainte-
nance
Laboratory
Fertilizer credit
Land credit
Total annual cost
Capital
cost
$2 500 000
1 32 000
1 553 000
420 000
4 605 000
Total
annual
cost
$202 500
10700
1 25 800
60000
201 200
263 200
35000
17000
4500
(122700)
(84 000)
713200
Annual
cost pe
ton dry
solids
$1205
064
749
357
11 97
1566
208
1 01
027
(730
(500
42 44
Capital costs for the lime stabilization land application
site, based on January 1, 1978, bid date, have been
summarized in table 1-33.
Annual operation and maintenance costs for transpor-
tation and land application of lime stabilized sludge were
calculated in a manner similar to that previously summa-
rized and have been shown in table 1-34.
Anaerobic Digestion
A flow diagram for the proposed anaerobic diges-
tion/vacuum filtration alternative is shown on figure 1-
19. Significant process equipment includes two-stage
standard rate anaerobic sludge digestion, bulk lime and
ferric chloride storage, lime slakers, vacuum filtration,
sludge conveyors, and sludge storage bin. All facilities
were assumed to be housed in an all weather brick-
block type building and included all electrical, instrumen-
tation, interconnecting piping, and sludge pumps. The
existing sludge dewatering equipment was assumed not
to be capable of functioning over the project life and
was replaced. Similarly, the existing filter building and
chemical feed facilities were replaced.
Design data for the anaerobic digester alternative are
as follows:
Primary digesters
Secondary digesters
Vacuum filtration
Vacuum filter loading rate
Lime storage bin
3 ea 110 ftx30 ft (33.5 mX9.1
SWD
3 ea 110 ftx30 ft (33.5 mx9.1
SWD
3 ea at 400 ft2 ea (37.2 m2)
3.5 Ib dry solids/ft2/hr (17.1
kg/m2/hr)
1 ea 4,000 ft3 (113.3 m3)
m)
m)
30
-------
SECONDARY
CHLORINATION
EFFLUENT
TO DISCHARGE *"
-r—WASTE ACTIVATED SLUDGE
THICKENED
SLUDGE
^•-
c
t
o o
•!• STAGE
ANAEROBIC
DIGESTER
(3 EACH)
— ,
35
STAGE
ROBIC
STER
ACH)
•
OIOESTED
4-
>
VACUUM
FILTER
(3 EACH)
TRUCK TO
LAND APPLICATION SITE
-LJL
oo oo
SLUDGE
Figure 1-19.—40 Mgal/d (1.75 rrrVs) anaerobic digestion/vacuum filtration
and land application.
Lime slaker/feeders
Ferric chloride storage tanks
Dewatered sludge storage bin
Filter building
Digester loading—1st stage
Hydraulic detention time—1st
stage
Digester gas production
Digester gas heat value
Volatile solids reduction
Overall solids reduction
Sludge mixers
Digester heat requirement
(primary only)
Gas production
Net gas available
3 at 250-500 Ib CaO/hr (113.6-227.3
kg/hr)
2 ea at 5,000 gal ea (18.9 m3)
1 ea at 2,000 ft3 (56.6 m)
3,000 ft2 (278.7 m2) w/basement
0.07 Ib VSS/ft3/day (1.1 kg/m3/day)
23 days
10 ft3/lb VSS feed (0.6 m3/kg)
500 Btu/ft3 (18,625 kJ/m3)
50 percent
32 percent
4 at 5,000 gpm ea (315.4 l/s)
22.7X107 Btu/day (2.8X106W)
30.0 x 107 Btu/day (3.7 X106 W)
7.3 x 107 Btu/day (0.9 X106 W)
Design conditions were based on the criteria enumer-
ated in Ten States' Standards28 and assumed installation
in the midwest.
Annual capital costs operation and maintenance for
the anaerobic digestion facilities were based on the Jan-
uary 1, 1978 bid date and have been summarized in
table 1-29. Capital costs included the digesters, control
buildings, covers, heat exchangers, gas safety equip-
ment, interconnecting piping, engineering, legal and ad-
ministrative costs. Capital costs are summarized in table
1-35.
Digester operation assumed one man, two shifts per
day, 365 days per year, at $6.50, including overhead.
Maintenance labor and material assumed 4,160 hours
per year at $6.50 per hour and $30,000 per year for
maintenance materials.
The cost of anaerobic digester operation was offset
by assuming a value of $2.70 per million Btu ($2.56 per
million kJ) for all digester gas produced above the net
digester heat requirement. Total annual operation and
maintenance cost for the digestion facilities is summa-
rized in table 1-36.
31
-------
Table 1-35.—Capital cost of two-stage anaerobic diges-
tion facilities for a 40 Mgal/d (1.75 m3/s) wastewater
treatment plant
Table 1-38.—Vacuum filtration capital and annual opera-
tion and maintenance costs for a 40 Mgal/d (1.75 m3/s]
plant
Site work, earthwork, yard piping $688,000
Digesters and control building 7,222,000
Pumping 35,000
Electrical and instrumentation 745,000
Subtotal construction cost
8,690,000
649.000
Total capital cost 9,339,000
Engineering.
Amortized cost at 30 yrs., 7 percent int. (CFR = 0.081)
Annual capital cost per unit feed dry solids
756,500
45.02
Table 1-36.—Total annual cost for two-stage anaerobic
sludge digestion excluding vacuum filtration and land
disposal for a 40 Mgal/d (1.75 m3/s) plant
Item
Operating labor
Maintenance labor and materials
Laboratory
Capital
Fuel credit
Total annual cost
Total
annual
cost
$38 000
. . 57,000
6000
756 500
(71 ,900)
785,600
Annual
cost per
ton dry
solids
$226
3.39
0.36
45.02
(4.28)
46.75
Capital costs for the filtration facilities are summarized
in table 1-37
Vacuum filtration costs were estimated as summarized
in table 1-38.
Land application costs were calculated based on haul-
ing 20 miles (32 km) round trip. A sludge transfer site
Table 1-37.—Capital cost for vacuum filtration facilities
for a 40 Mgal/d (1.75 m3/s) wastewater treatment plant
Site work, earthwork, yard piping.
Chemical storage and feed
Filtration equipment
Filter and chemical building
Sludge loading pad
Electrical and instrumentation
Subtotal construction cost.
Engineering
Total capital cost
Amortized cost at 30 years., 7 percent int. (CFR =
Annual capital cost per unit feed dry solids
< 0.081).
$297,000
177,000
546,000
230,000
78,000
322.000
1,650,000
140,000
1,790,000
145,000
8.63
Item
Total
annual
cost
Annual
cost pe
ton dry
solids
Variable cost
Electric power
Chemicals
Lime
FeCI3
Maintenance materials. ...
Maintenance labor
Laboratory
Subtotal variable cost
Operator labor ...
Supervision
Capital
Subtotal fixed cost
Total annual cost
$7,100
91 400
52 000
. . 7800
25800
6000
190100
47000
15000
145000
207000
397100
$0.42
544
309
046
1 54
036
11 31
280
089
863
1232
2363
was assumed to be located at the land application site.
Sludge transfer trucks were assumed to be equipped
with 8 yd3 (6.1 m3) dump beds. A total of four trucks
were required, operating 8 hours per day, 215 days per
year. The loader and land spreading vehicle were as-
sumed to operate 8 hours per day. Land application
vehicles were assumed to have 17 yd3 (13 m3) capacity.
Sludge application rate assumed 7 dry tons (6.4 Mg) pei
hour, including loading time. The land application vehicle
was depreciated on a straight-line basis over a 7-year
period. Sludge hauling was based on current rental
costs for equipment. Dewatered sludge was assumed to
be 22 percent dry solids.
Anaerobically digested sludge land requirements were
less than for lime stabilized sludge because of the vola-
tile solids reduction during digestion. The fertilizer value
and land rental return credits were taken as previously
described in the 4 Mgal/d (0.18 m3/s) design case.
Table 1-39 summarizes the total land application cost.
To summarize, the total cost for the lime stabilization
and anaerobic digestion alternatives, including ultimate
disposal, is shown in table 1-40.
In the 4 Mgal/d case (0.18 m3/s), the total annual
cost for the lime stabilization alternative is $74.16 per
dry ton ($81.75/Mg) compared to $86.77 per dry ton
($95.65 Mg) for anaerobic digestion. Each of these alter
natives assumed liquid application to farmland, with a 3-
5 mile (5-8 km) round trip hauling distance. With in-
creasing haul distances, lime stabilization will be de-
creasingly cost effective because of the greater volume
of sludge which must be transported.
In the 40 Mgal/d case (1.75 rrrVs), the total annual
cost for lime stabilization alternative is $62.94 per dry
32
-------
Table 1-39.—Annual cost for land application of dewa-
tered anaerobically digested sludges for a 40 Mgal/d
(1.75 m3/s) plant
Item
Amortized cost of land
Truck depreciation (spreader only)
Truck drivers
Truck and loader operation
Laboratory
Fertilizer credit
Land credit
Total annual cost
Total
annual
cost
$202,500
12100
67100
260,600
4,500
(83,400)
(57,000)
406,400
Annual
cost per
ton dry
solids
$12.05
072
3.99
15.51
0.27
(4.96)
(3.39)
24.19
ton ($69.38/Mg) compared to $94.56 per dry ton
($104.23/Mg) for anaerobic digestion. The cost of pipe-
line transportation/land application of the liquid sludge is
$42.44 per dry ton ($46.78/Mg) compared to $47.82 per
dry ton ($52.71 /Mg) for dewatering and land application.
The pipeline alternative also has the disadvantage of
being inflexible for long-term implementation. With the
dewatered sludge and truck hauling system, sites could
be changed with little difficulty.
REFERENCES
1. Riehl, M. L. et al, "Effect of Lime Treated Water on Survival of
Bacteria," Journal American Water Works Assn., 44,466 (1952).
2. Grabow, W. O. K. et al., "The Bactericidal Effect of Lime Floccu-
lation Flotation as a Primary Unit Process in a Multiple System for
the Advanced Purification of Sewage Works Effluent," Water Re-
sources 3, 943 (1969).
3. Buzzell, J. C., Jr., and Sawyer, C. N., "Removal of Algal Nutrients
from Raw Wastewater with Lime," Journal WPCF, 39, R16, 1967.
Table 1-40.—Comparison of total annual capital and annual O. & M. cost
for lime stabilization and anaerobic digestion including land disposal for a
40 Mgal/d (1.75 m3/s) plant
Lime stabilization
Anaerobic digestion
Total
annual
O. & M.
cost
Annual
cost per
ton dry
solids
Total
annual
O. & M.
cost
Annual
cost per
ton dry
solids
Facilities
Amortized capital lime stabilization
Amortized capital digesters
Amortized capital filtration
Operating labor
Maintenance labor and materials
Chemicals
Laboratory
Fuel credit
Power
Subtotal facilities
Land Application
Amortized cost of land, facilities and
easements
Truck depreciation
Truck drivers
Truck operations
Pipeline O. & M
Power
Fertilizer credit
Land credit
Laboratory
Subtotal land application
Total annual cost facilities and land
application
$87,200
N/A
N/A
114,000
18,300
118,600
4,500
N/A
2,000
344,600
339,000
60,000
201,200
263,200
17,000
35,000
(122,700)
(84,000)
4,500
713,200
1 ,057,800
$5.19
N/A
N/A
6.78
1.09
7.06
0.27
N/A
0.12
20.51
20.17
3.57
11.97
15.66
1.01
2.08
(7.30)
(5.00)
0.27
42.43
62.94
N/A
a $756,000
a1 45,000
a1 00,000
a90,600
a 143,400
a12,000
a(71 ,900)
a7,100
a1,182,700
a 202,500
a 12,1 00
a67,100
a 260,600
N/A
N/A
a(83,400)
a(57,000)
a 4,500
3 406,400
a 1,589,1 00
N/A
$45.02
a8.63
"5.95
a5.39
a8.53
a0.71
a(4.28)
a042
"70.37
a 12.05
a0.72
a3.99
a15.51
N/A
N/A
a(4.96)
a(3.39)
a0.27
a24.19
a94.56
"Includes cost for digestion and vacuum filtration.
33
-------
4. "How Safe is Sludge?" Compost Science 10 March-April 1970.
5. Kempelmacher, E. H. and Van Noorle Jansen, L. M., "Reduction
of Bacteria in Sludge Treatment," Journal WPCF 44, 309 (1972).
6. Evans, S. C., "Sludge Treatment at Luton," Journal Indust Sew-
age Purification 5, 381, 1961.
7. Farrell, J. B., Smith, J. E., Hathaway, S. W., "Lime Stabilization of
Primary Sludges," Journal Water Pollution Control Federation, vol.
46, No. 1, January 1974, pp. 113-122.
8. Paulsrud, B and Eikum, A. S., "Lime Stabilization of Sewage
Sludges," Norwegian Institute for Water Research, volume 9, pp.
297-305, 1975.
9. Counts, C A., Shuckrow, A. J., "Lime Stabilized Sludge: Its Sta-
bility and Effect on Agricultural Land," EPA-670/ 2-75-012, April
1975.
10. Noland, R F., Edwards, J. D., "Stabilization and Disinfection of
Wastewater Treatment Plant Sludges," USEPA Technology Trans-
fer Design Seminar Handout, May 1977.
11. Brown, R. E. et al., "Ohio Guide for Land Application of Sewage
Sludge," Ohio Agricultural Research and Development Center,
Wooster, Ohio, 1976.
12. Sommers, L. E., "Principles of Land Application of Sewage
Sludge," USEPA Technology Transfer Design Seminar Handout,
May 1977.
13. Sommers, L. E., et al., "Variable Nature of Chemical Composition
of Sewage Sludges," Journal of Environmental Quality 5:303-306.
14. Stern, Gerald, "Reducing the Infection Potential of Sludge Dispos-
al," presented at Northwest Regional Physical Chemical Wastewater
Treatment Short Course at University of Washington, Seattle, March 25,
1975
15. U.S. Environmental Protection Agency, "Process Design Manual
for Sludge Treatment and Disposal," USEPA Technology Transfer,
1006, Oct 1974
16. U.S. Environmental Protection Agency, "Municipal Sludge Manage-
ment: Environmental Factors," Federal Register, vol. No. 41, No.
108, p. 22533.
17. Trubnick, E. H., Mueller, P. K., "Sludge Dewatering Practice,"
Sewage and Industrial Wastes 30, 1364 (1958).
18. Sontheimer, H., "Effects of Sludge Conditioning with Lime on De-
watering," Proc. 3d Int'l Conference, Water Pollution Research,
Munich, 1966, in Advances in Water Pollution Research.
19. Zenz, D. R., Lynam, B. T., el al., "USEPA Guidelines on Sludge
Utilization and Disposal—A Review of Its Impact Upon Municipal
Wastewater Treatment Agencies," presented at the 48th Annual
WPCF Conference, Miami Beach, Fla., 1975.
20. National Lime Association, "Lime Handling Application and Stor-
age in Treatment Processes Bulletin 213," National Lime Associa-
tion, Washington, D.C., pp. 1-3.
21. Badger and Banchero, "Introduction to Chemical Engineering,"
page 614, McGraw-Hill, 1955.
22. Hicks, R. W. et al., "How to Design Agitators for Desired Process
Response," Chemical Engineering, April 26, 1976, pp. 103-106 ff.
23. Fair, G. M. and Geyer, J. C., "Water Supply and Wastewater
Disposal," John Wiley & Sons, New York, 1956.
24. USEPA, "Methods for Chemical Analysis of Water and Wastes," USEPA,
Technology Transfer 625/6-74-003a, Cincinnati, Ohio, 1974.
25. Standard Methods for Examination of Water and Wastewater, 13th
and 14th Editions, AWWA, APHA, WPCF, American Public Health
Association, Washington, D.C.
26. Kenner and Clark, "Enumeration of Salmonella and Pseudomonas
aeruginosa," Journal WPCF, vol. No. 46., No. 9, September 1974,
pp. 2163-2171.
27. USEPA, "Lime Use in Wastewater Treatment: Design and Cost
Data," Office of Research and Development, U. S. EPA-600/2-75-
038, October 1975.
28. Recommended Standards for Sewage Works, Health Education
Service, Albany, N.Y. 1971, pp. 57-64.
29. U.S. EPA, "Full Scale Demonstration of Lime Stabilization," Office
of Research and Development, U.S. EPA, 600/2-78-171, Cincinnati, Ohio
34
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Chapter 2
Anaerobic Digestion and Design
of Municipal Wastewater Sludges
The claimed advantages of the anaerobic digestion pro-
cess are:1'2
• Low sludge production.
• The production of a useful gas of moderate caloric
value.
• A high kill rate of pathogenic organisms.
• Production of a solids residue suitable for use as a
soil conditioner.
• Low operating cost.
Table 2-1 indicates the kinds of sludges which have
been studied on a full-scale basis.
In the past 50 years municipal wastewater sludge has
changed from simple primary sludge of purely domestic
origin to complex sludge mixtures (primary, secondary,
chemical) of domestic and industrial origins.
At first when design engineers only had to consider a
primary sludge, the developed rules of thumb22 were ade-
quate. As the sludge generated became more complex,
more and more systems failed and the process devel-
oped a "bad reputation." The use of steady state mo-
dels in the 1960's,23~2s dynamic models in the 1970's,26"31
and research into the basic biochemical processes32"35
has led to significant improvements both in the design
Table 2-1.—Type and reference of full-scale studies on
high rate anaerobic digestion of municipal wastewater
sludge
Reference on
mesophilic
Reference on
thermophilic
Primary and lime 3,4
Primary and ferric chloride 5
Primary and alum 5
Primary and trickling filter 7,8
Primary, trickling filter and alum 9
Primary and waste activated 10,11,12 11,13,14
Primary, waste activated and lime 15,16
Primary, waste activated and alum 15,17,18
Primary, waste activated and ferric
chloride 15
Primary, waste activated and sodium
aluminate 17,18
Waste activated only (pilot plant only) 19,20,21 19,20,21
and operation of the process. Still the transfer of data
from the laboratory to the real world can be difficult.
GENERAL PROCESS DESCRIPTION
Anaerobic digestion of municipal wastewater sludge is
a two-step, very complex biochemical process, depend-
ent on many physical (temperature, solids concentration,
degree of mixing, organic loading, detention time) and
chemical (pH, alkalinity, volatile acid level, nutrients, tox-
ic materials) factors. Probably the easiest way to visual-
ize what is taking place is to think in terms of a two-
step process.
In the first step, facultative microorganisms (sometimes
called acid forming bacteria) convert complex organic
waste sludge substrate (proteins, carbohydrates, lipids)
into simple organic fatty acids by hydrolysis and fermen-
tation. The principal end products, with sludge as sub-
strate, are acetic acid, approximately 70 percent, and
propionic acid, about 15 percent.36"38 The microorganisms
involved can function over a wide environmental range
and have doubling times normally measured in hours.
In the second step, strictly anaerobic microorganisms
(sometimes called methane-forming bacteria) convert the
organic acids to methane, carbon dioxide and other
trace gases. The bacteria involved are much more sensi-
tive to environmental factors than step one bacteria and
normally have doubling times measured in days. Because
of this, step two bacteria control the overall process.
Figure 2-1 gives an overview of the entire process.
For a more complete review the reader is referred to
either Kirsch35 or Toerien.32
MESOPHILIC—THERMOPHILIC DIGESTION
Temperature can be considered one of the most im-
portant factors in the anaerobic digestion process. Even
though the total temperature range for operation of the
process is very broad, specific microorganisms often
have relatively narrow temperature ranges in which they
can grow.
For the purpose of classification, the following three
temperature zones of bacterial action will be used
throughout this chapter:
Cryophilic zone Liquid temperature below 10°C (50° F)
Mesophilic zone Liquid temperature between 10°C to 42° (
(50°F to 108°F)
Thermophilic zone Liquid temperature above 42° C (108° F)
35
-------
Raw sludge
Complex
substrate
Carbohydrates,
fats and
proteins
Micro-
+ organisms
"A"
Principally
acid formers
K1 Non-
products
C02, H2O
Stable and
intermediate
degradation
products
Cells
Reactive
products
Organic acids
Cellular and
other inter-
mediate
degradation
products
Micro- K2
organisms ^- CH^
"B"
Methane
fermenters
Other
i- C02 end
products
~H2O, H2S
Cells and stable
degradation
products
Figure 2-1.—Summary of anaerobic digestion process.
39
In the past, the vast majority of lab, pilot, and full-
scale research has been done in the mesophilic range
and some in the thermophilic range. The reason for this
is that thermophilic digestion did not seem economical
because of the higher energy requirements and the gen-
eral feeling that operation at the higher temperature
would be highly unstable. Recently, the literature indi-
cates that there is a renewed interest in thermophilic
digestion40 because of its elimination of pathogens, high
reaction rates and possibly higher gas yields and better
dewaterability.
VOLATILE SOLIDS REDUCTION
One of the main objectives of the anaerobic digestion
process is to reduce the amount of solids that need to
be disposed. This reduction is normally assumed to take
place only in the volatile content of the sludge and it is
probably safe to assume only in the biodegradable vola-
tile fraction of the sludge. Research into the area of the
biodegradable fraction is quite limited but the following
generalities can be used:
1. Approximately 20 to 30 percent of the influent sus-
pended solids of a typical domestic wastewater is
nonvolatile.41 Of the remaining suspended solids that
are volatile, approximately 40 percent are nonbiode-
gradable organics consisting chiefly of lignins, tan-
nins, and other large complex molecules.
2. For waste activated sludges generated from sys-
tems having primary treatment, approximately 20 to
35 percent of the volatile solids produced are non-
biodegradable.42'43
3. For waste activated sludges generated from the
contact-stabilization process (no primaries—all in-
fluent flow into aeration tank), 25 to 35 percent of
the volatile suspended solids are
nonbiodegradable.44
Though it is realized that only the biodegradable frac-
tion can actually be destroyed, all past research and
most of the present day work report on volatile solids
destroyed without making any distinction between biode-
gradable and nonbiodegradable. Because of lack of
data, all reference here to solids destruction will be
based on volatile solids only.
Figures 2-2, 2-3, and 2-4 show the effect of sludge
age and temperature on volatile solids reduction for
three common sludges.
70 -
O
0
UJ
EC
>
I-
LU
O
60
50
40
A A
• PI LOT PLANT43
* PI LOT PLANT44
200 400 600 800 1000 1200 1400 1600 1800
TEMP <°C) X SLUDGE AGE (DAYS)
Figure 2-2.—Volatile solids versus reduction versus tem-
perature x sludge age for anaerobically digested primary
sludge.
UJ
CJ
(T
UJ
D.
70
60
50
40
30 -
/A
10
• FULL SCALE
A PI LOT PLANT47
FULL SCALE
13
1
200 400 600 800 1000 1200 1400 1600
TEMP (°C) X SLUDGE AGE (DAYS)
Figure 2-3.—Volatile solids versus reduction versus tem-
perature X sludge age for anaerobically digested mixture
of primary and waste-activated sludge.
36
-------
z
o
h-
o
O
LU
DC
C/3
1-
z
L1J
O
oc
LU
Q.
60
50
40
30
20
10
-
. A ' • ' '
A A . " •
A • *m m •
" ' •*•
. . • A PILOT PLANT19
* • PI LOT PLANT20
• PI LOT PLANT20
i i I I I i i I I i
200 400 600 800 1000 1200 1400 1600 1800 2000
•
1
2200
Table 2-2.—Concentration—organic loading—time pa-
rameters for several full-scale anaerobic digestion facili-
ties
Figure 2-4.—Volatile solids versus reduction versus tem-
perature x sludge age for anaerobically digested waste-
activated sludge.
Though the data is somewhat scattered, the following
generalizations seem valid.
1. For all three sludges the practical upper limit of
volatile solids reduction seems to be 55 percent.
Though it was noted that approximately 60 percent
of the volatile solids are biodegradable, figures 2-2,
2-3, and 2-4 suggest that practically all the biode-
gradable fraction is being consumed.
2. The data seem to indicate that under the same
design conditions primary sludge will degrade faster
than a mixture of primary and waste activated,
which in turn degrade faster than straight waste
activated. This implies that adjustments must be
made in design depending on the type of sludge to
be processed.
SOLIDS CONCENTRATION—
ORGANIC LOADING—SLUDGE AGE
Considerable capital cost savings could be realized if
the anaerobic digestion process could be operated at
higher organic loadings and shorter detention times than
commonly used today.
There have been several pilot plant studies48"51 which
have been able to operate at levels approaching 4-5
days residence times, organic loadings approaching 0.5
Ib volatile solids/cu ft/day (8.0 kg/m3/day) and solids
concentrations up to 12-15 percent solids. Unfortunately,
pilot plant digesters are ideally mixed and environmental-
ly controlled, and scaling up the results can be difficult.
Nevertheless, over the years there have been several
full-scale facilities which were and still are being oper-
ated successfully at short detention times, high organic
loadings and high solids concentrations. Some of these
plants are listed in table 2-2.
Solids concentration.—It must be remembered that the
solids concentration within the digester affects the vis-
cosity which in turn affects the ability of the mixing
Feed solids
concentration,
percent
6.0
6.6
69
4 6-52
5.0
63
80
Organic load
Ibs VS/ftVday
016
017
0 15-038
0.28
0 13-0 17
03
0 16
015
Hydraulic
retention
time3 days
150
144
117 159
8.0
14
10
165
21 0
Reference
10
52
53
54
12
12
55
56
aAII data based on primary digester only. Digester equipped with
mixing and sludge heating.
equipment (see section on mixing). Also, because of the
solids reduction taking place, the solids concentration
within the digester is less than the feed solids concen-
tration. Though it depends on sludge type, the practica-
ble upper limit on the feed solids concentration is in the
range of 8 to 9 percent. With a properly designed mix-
ing system this will not cause any operational problems
within the digester.
Organic loading rate.—The organic loading rate is a
function of the solids concentration within the digester
and system sludge age. These two parameters are im-
plicit when one speaks of a loading rate of pounds
volatile solids per cubic foot per day. As is shown in
table 2-2 designing a digestion system to operate at
0.15 to 0.20 Ib VS/cu ft/day (2.4 to 3.2 kg/m3/day) is
no problem.
Sludge age.—At present, high-rate (mixed and heated)
primary digesters to recycle concentrated digested solids
are not practiced; therefore, hydraulic residence time
and sludge age are almost synonymous. As noted in
table 2-2, a minimum time of 15 days in the primary
digester is very practicable. It should be remembered
though that this time is also related to sludge type and
tank temperature, as was shown in figures 2-2, 2-3,
and 2-4.
There seems to be an important relationship between
the above design parameters. In a study conducted by
Clark,51 involving solids concentration, organic loading
rate, and sludge age, the curve shown in figure 2-5 was
developed.
The shape of the probable digestion limit curve, i.e.,
higher organic loadings as the sludge age decreases, is
a reflection of the accumulation of various system by-
products which may reach inhibitory concentration levels.
If for a given digester volume and organic loading rate
the sludge age is increased (only possible if influent
37
-------
15
si
st
§3
1.0
0.8
0.6
0.4
0.2
0.0
PROBABLE DIGESTION LIMIT'
10 15 20 25 30
SLUDGE AGE (DAYS)
35 40
Figure 2-5.—Relationship between solids concentration—
organic loading—sludge age limits for anaerobic diges-
tion.51
sludge concentration is increased), then the chance for
potential inhibitory byproduct concentration levels also is
increased.
An engineer designing a high-rate primary digester
might, e.g., determine that an organic loading rate of 0.5
Ib VS/cu ft/day (8.0 kg/m3/day) and a sludge age (hy-
draulic detention time) of 15 days is possible (figure 2-
5, point 1). If the detention time were doubled (point 2)
by doubling the influent solids concentration (volume of
tank stays the same), the digester would fail. If instead,
the tank volume is doubled (point 3) rather than dou-
bling the influent solids concentration, the unit would still
be operating on the failure boundary and nothing would
be gained. As a third alternative, halving the loading
rate by doubling the tank volume (point 4), assuming the
influent solids concentration is halved, would be accept-
able. Finally, if the loading rate is to be maintained at
0.5 Ib VS/cu ft/day (8.0 kg/m3/day), the sludge age
(hydraulic detention time) should be decreased since the
tank volume is fixed to allow a lower influent solids
concentration.
MIXING
Mixing in an anaerobic digester that treats municipal
wastewater sludge of domestic origin is considered to
have the following benefits. (Note: It is assumed that a
favorable environment exists to allow development of an
anaerobic digestion system.)
• It keeps the food supply uniformly dispersed and in
constant contact with the growing cells to promote
maximum utilization of the system.
• It keeps the concentration of biological end prod-
ucts at their lowest value by dispersing them uni-
formly throughout the digester.
• It provides environmental uniformity (temperature,
nutrients, etc.) throughout the digester allowing best
possible cell development.
• It allows fairly fast dispersion of any toxic material
entering the system, thus, possibly minimizing its
effect on the anaerobic process.
• It assists in the prevention of a scum layer buildup
at the top of the digestion tank.
At the present time not many in the environmental
engineering field would dispute the advantages of mixing
in an anaerobic digester; however, problems arise with
such questions as what is adequate mixing, how do you
define mixing, how do you specify mixing, etc.
Before any discussion about mixing can be developed,
some time must be spent discussing what and where
this mixing is to take place.
Defining Mixing
In recent years it has become popular to use the terrr
"complete mix" when discussing biological process reac-
tors. Unfortunately, engineers associate this term on a
time scale as applied to activated sludge systems when
talking about mixing an anaerobic digester.
The term "complete mix" means that the time for
dispersion of the feed stream is short in relation to the
total hydraulic residence time in the reactor. It is also
defined as sufficient mixing so that concentration gra-
dients of chemical and biological ingredients are uniform
for the particular reaction rates that exist in the basin.
Present-day "complete mix" activated sludge systems
have hydraulic residence times of approximately 3 hours
based on plant influent flow. Generally a "turn over
rate" of 15 to 20 minutes is considered sufficient to
achieve "complete mix" conditions within the aeration
basin. This would give a turnover rate to hydraulic de-
tention time ratio of 0.08. Present-day high-rate primary
digesters have hydraulic detention times of 12 to 17
days. This would seem to imply that a "turnover rate"
of about 1 day would provide complete mix conditions
within the system.
Mixing within the anaerobic digestion tank occurs on
two levels: macromixing and micromixing.35 Macromixing
deals with the bulk mass flow within the digester, while
micromixing deals with the degree of intermingling of the
system molecules. In biological theory, "complete mix"
assumes micromixing.
The actual mixing of the sludge within the digester
can be by gas recirculation, mechanical, or a combina-
tion of the two. Malina and Miholites60 describe all
present-day systems.
No matter what type of device is utilized the intent is
to achieve mixing through a pumping action. Because of
this relationship, engineers have come to use the term
horsepower/unit volume as some type of parameter to
define mixing in an anaerobic digester. Unfortunately,
this term by itself has no meaning. For mechanical type
mixers the wide variation in impeller diameters and
speeds can result in similar horsepower but widely differ-
ent pumping capacities. For gas mixing systems, gas
flow, depth, and bubble size can also result in similar
horsepower but widely different pumping capacities.
Probably the best way to evaluate mixing is from the
standpoint of zone influence (figure 2-6): energy is dissi-
pated with movement horizontally away from the energy
source. The loss due to friction between the fluid mole-
38
-------
TOP VIEW
PROFILE VIEW
LIQUID HEIGHT
D., = EFFECTIVE ZONE DIAMETER FOR MICROMIXING.
D2 = EFFECTIVE ZONE DIAMETER FOR MACROMIXING.
Figure 2-6.—Shear-stress relationship for a thixotropic
pseudo plastic material.
cules is a function of liquid density, temperature, and
solids concentration. Within a certain area of the point
source there is sufficient energy to achieve micromixing.
There is also a larger area where bulk flow (macromix-
ing) still takes place even though there is insufficient
energy for micromixing.
Presently, the only published work that could be found
discussing this concept in the sanitary engineering field
was done by the EPA.61'62 This concept indicates that the
width of the micromixing zone in water is no more than
twice the liquid depth, with the liquid depth being a
function of the type of mixing device utilized and not
necessarily the tank liquid depth. It is probably safe to
assume that for thickened, anaerobically digested sludg-
es, the zone of influence for any given energy input is
smaller than for mixing plain water.
CHARACTERISTICS OF AN
ANAEROBIC DIGESTER
The existing trend in wastewater treatment is to re-
move more and more material from the main liquid proc-
essing stream. This is done through the use of secon-
dary biological treatment schemes, chemical addition,
and filters. The sludge produced can vary widely and
change rapidly even on an hour-to-hour basis.
Table 2-3 gives specific gravity and particle size dis-
tribution on two common type sludges: plain primary and
plain waste activated.
There is little data on the rheology of municipal waste-
water sludge and even less on anaerobically digested
sludge.48'59 One of the main problems is that it is ex-
tremely difficult to do such studies correctly.58
Even though the majority of raw wastewater sludges
behaves as a thixotropic (time dependent), pseudo plas-
tic, material (figure 2-7), it may not be correct to as-
sume that the sludge within the anaerobic digester has
Table 2-3.—General characteristics of raw primary and
waste activated sludge57
Primary sludge
Waste activated
sludge
Specific gravity 1.33-1.4 1.01-1.05
Particle size 20% < 1 pm 40% 1-50 ^m
35% 1-100 urn 60% 50-180 fim
45% > 100 turn
Physical appearance Fibrous Slimy, gelatinous
CO
CO
LU
DC
CO
oc
<
in
I
CO
RATE OF STRESS
Figure 2-7.—Schematic of zone of mixing influence for
energy source in fluid with only fixed upper and lower
boundaries.
39
-------
the same general properties. The liquid within the tank is
normally at a higher temperature where there is entrap-
ment of gas bubbles and a general reduction in particle
size, all of which affect fluid viscosity.
At the present time anaerobic digestion tanks have a
ratio of inside tank diameter to average liquid depth of
1:1 to 5:1. This imposes some restrictions on the ability
to develop a mixing regime.
SUPERNATANT
Poor quality anaerobic digester supernatant is a major
operational problem at many municipal wastewater treat-
ment plants. The supernatant will most likely contain
high concentrations of carbonaceous organic materials,
dissolved and suspended solids, nitrogen, phosphorus,
and other materials63 to impose extra loads on other
treatment processes and effluent receiving waters. An
analysis of 20 high-rate, mesophilic, two-stage, anaerobic
digestion systems63 showed a range of supernatant sus-
pended solids from 100 to 32,400 mg/l with an average
value of 5500 mg/l and for BOD5from 100 to 6,000
mg/l with an average value of 875 mg/l. Table 2—4
indicates the effects at one midwestern treatment facility
where anaerobic digester supernatant from a high-rate
system was returned to the plant influent.
Many supernatant treatment alternatives have been
tried,65 some working with a certain degree of success.
The question that really needs to be asked is why even
expect a clean supernatant stream when digesting mixed
primary and waste activated sludges.
The concept of obtaining high quality supernatant de-
veloped during the early days of separate anaerobic
digestion systems. During this time period the only
sludge being digested was primary sludge, which had
excellent settling properties (table 2-3).
Modern-day sludges are much more complex. They
contain not only primary sludge but sludges generated
from secondary treatment, predominately activated sludge
systems. Waste activated sludge tends to have fragile
floe and is difficult to concentrate by gravity thickening.
Because of this, waste activated sludges are thickened
by dissolved air flotation thickeners.
Also present-day, high-rate anaerobic digesters are
mixed. This constant mixing of the sludge tends to re-
duce particle size. At the same time the process itself is
reducing particle size through biological destruction.
Finally anaerobic digestion systems generate gas
throughout the entire tank under a slightly positive pres-
sure (6- to 15-inches (15-38 cm) water column). Thus,
the system becomes supersaturated with digester gas.
When the digested sludge is finally pumped to the
secondary digester, it contains many fines or sludge that
is difficult to gravity thicken and is supersaturated with
gas. The gas is then liberated in the form of small gas
bubbles which tend to attach themselves to the sludge
particles, thus promoting a flotation effect. The combina-
tion of these events is very detrimental to gravity con-
centration. It has been estimated that at least 30 or
more days12 would be required in a secondary digester
to obtain a clear supernatant from high-rate systems
digesting sludges containing waste activated sludge.
In many cases, it would be better to take all digester
contents directly to mechanical dewatering and eliminate
provisions for gravity solids-liquid separation. This would
give a constant, predictable centrate stream having low
suspended solids content.
ENERGY
Energy Production
One of the advantages of anaerobic digestion of mu-
nicipal wastewater sludge is that energy is produced
rather than consumed and could go a long way in meet-
ing energy requirements at wastewater plants.66 One
problem encountered with this energy source is in pre-
dicting how much energy will be produced for any given
plant. This variability in possible production is indicated
in table 2-5.
Figure 2-8 shows temperature effects on anaerobic
digestion. Schwerin71 reviewed the literature and plotted
Table 2-4.—Effect of returning supernatant from high rate anaerobic
digester to plant influent64
Suspended solids
Total phosphorus
To
primaries
Ib/day
15,969
"(36,801)
914
0,304)
To
secondaries
Ib/day
9,501
(15,306)
803
(991)
Final
effluent
Ib/day
2,836
(3,467)
500
(435)
Primary
sludge
Ib/day
13,249
(19,626)
156
(299)
Waste
activated
sludge
Ib/day
9,593
(14,645)
287
(453)
"Data in parentheses were obtained when untreated anaerobic digester supernatant was
discharged to head of plant. Data not in parenthesis were obtained when no supernatant
was discharged to head of plant. Data shown is average for the entire time period of study.
40
-------
Table 2-5.—Cubic feet digester gas produced per
pound of organic matter destroyed
Material
Fats
Scum
Grease
Crude fibers
Protein ...
Carbohydrate
Fat
Insoluble soap
Protein
Percent CH4
Pure compounds67
62-72
70-75
68
45-50
73
Pure compounds39
Ft3 gas/lb digested
18-23
14-16
17
13
12
14.2
24.6
22.3
9.4
Municipal sludges68
"The volume of gas produced per pound of volatile solids digested
is reported as 17 to 18 cu. ft./lb at the larger and better instrumented
plants. Smaller plants report lesser values, sometimes as low as 6 cu.
ft./lb. volatile solids destroyed, but these values are probably due to
poor measurement techniques."
Municipal sludges69
".. .maximum gas production of approximately 11 to 12 cu. ft. of
gas per pound of total solids destroyed."
Municipal sludges70
"In terms of solids digested, the average yield.. .is about 15 cu. ft.
of gas per pound of volatile solids destroyed."
FT3/LBVS ADDED
90
100 110 120
TEMPERATURE -°F
130
140
Figure 2-8.—Effect of digestion temperature on gas pro-
duction based on data from 23 studies.71
reported gas production values as a function of diges-
tion temperature. The results show the potential effect of
digestion temperature on gas production.
Since the basis of all cost analysis depends on the
value of gas produced per mass of solids destroyed,
and since there is no existing data, it is suggested that
a range of 12 to 17 cu ft/lb (0.75-1.06 m3/kg) volatile
solids destroyed be used.
Note: As was noted in the section on Volatile Solids
Reduction, the amount of solids destroyed is a function
of sludge type and solids retention time (figures 2-1,
2-2, and 2-3).
The heating value of the gas can also range from 550
to 650 Btu/cu ft (20,500-24,200 kJ/m3). Based on an
average of 50 plants72 a value of 600 is suggested.
Hazards of Digester Gas
Explosion.—Sludge gas becomes violently explosive in
mixtures of 1 volume gas to 5-15 volumes air. There
are many case histories which have shown just how
violent and explosive it can be.
Burning.—When the ratio of gas to air is higher than
the above values, a "burning mixture is encountered."
Such a mixture is not as dangerous as an explosive
mixture, since it can be extinguished if encountered.
However, sewage plant workers have been seriously
burned by an instantaneous flame "puff."
Toxicity.—Of the minor constituents of sewage gas,
hydrogen sulfide (H2S) is the most important. Table 2-6
shows the effects at various concentrations.
Suffocation.—Man works best and breathes easiest
when the air contains about 21 percent oxygen. Men
breathing air that has as little as 15 percent of oxygen
usually become dizzy, have a rapid heart beat, and suf-
fer from headache.
Though over 30 years old, two publications by Lang-
ford73'74 on gas safety design considerations are still rec-
ommended reading for design engineers. Figure 2-9
shows a schematic of a modern-day gas piping system.75
Digester Gas Utilization
Since digester gas was first used in the United States
in 191576for heating and cooking, the use of digester
gas has increased, decreased in the 1950's and 1960's
because of cheap power alternatives, and presently in-
creasing again because of the energy situation.77 Several
recent publications have described not only operating
experience with conventional utilization methods, power
generation, and heating72'77'79 but also potential new
Table 2-6.—Effects of various concentrations of H2S
Immediate death Greater than 2,000 ppn
Fatal in 30 minutes or less 600 to 1,000 ppm
Severe illness caused 1 /2 to 1 hour 500 to 700 ppm
No severe effects if exposed 1/2 to 1 hour 50 to 100 ppm
41
-------
SERVICE
OR HEATER
DIGESTER
WASTE
FLAME TRAP
PRESSURE CONTROL LINE
LOW PRESSURE
CHECK VALVE
CONTROL PANEL
DIGESTER HEATER AND
HEAT EXCHANGER
WASTE
WASTE
GAS
BURNER
PILOT LINE
COMBINED PRESSURE
RELIEF AND FLAME TRAP
FLAME
CELL
VENTTO OUTSIDE
ATMOSPHERE
V
\
p
PRESSURE GAGE
@
DRIP TRAP
o
GAS METER
Figure 2-9.—Gas piping schematic of a modern anaerobic digestion sys-
tem.75
uses.77 One piece of important operating information
which has come to light is the amount of hydrogen
sulfide permissible for operation of engine generators.72'79
Because of its potential corrosive action early uses of
digester gas as engine fuel tried to keep H2S levels
under 60 grains per 100 ft3 (200 grains/m3).80'81 This was
done by incorporating some type of dry gas scrubber or
wet type bubbling scrubber. Recently a new simple
method82 of removal has been developed.
A recent publication72 describing the operating results
of several plants noted that even though levels of 1,000
to 3,000 mg/l of H2S were in the gas no adverse ef-
fects had been seen on the engines.
Digester Heat Requirements
In calculating digester heat requirements the two par-
ameters of concern are (see figure 2-10):
1. Heat required to raise the temperature of the in-
coming sludge flow to digester operating tempera-
ture.
2. Heat required to maintain the digester operating
temperature (radiation heat loss).
Heat Required for Raw Sludge.—It is often necessary
to raise the temperature of the incoming sludge stream.
The amount of heat required is given by equation 1.
gal of sludge 8.34 Ibs (T2 - T,) 1 day
X X X
day
gal
hrs
(D
where:
Qs = Btu/hr required to raise incoming sludge stream
from temperature T, to T2
T, = temperature of raw sludge stream
T2 = temperature desired within the digestion tank
hrs = length of time raw sludge is pumped through
the heat exchanger.
Heat Required for Heat Losses.—Digesters have radia-
tion heat losses which must be controlled to maintain
digester operating temperatures within ±1°F otherwise
the system could go into thermal shock. The amount of
heat loss depends on the tank shape, materials of con-
struction, and external temperatures.
The general design equation for heat flow through
compound structures is:
Q = U X A X (T2 - T3)
(2)
42
-------
Figure 2-10.—A heater and heat exchanger.
where:
Q = heat loss Btu/hr
A = area of material normal to direction of heat flow
in ft2
T2 = temperature desired within the digestion tank
T3 = temperature outside the digestion tank
1 - (3)
U =
EC,
1 +
Table 2-7.—"U" factors for various anaerobic digestion
tank materials75
Material
U
Fixed steel cover (1/4" plate)
Fixed concrete cover (9" thick)
Floating cover (wood composition)
Concrete wall (12" thick) exposed to air
Concrete wall (12" thick), 1" air space and 4" brick
Concrete wall or floor (12" thick) exposed to wet earth
(10' thick)
Concrete wall or floor (12" thick) exposed to dry earth
(10' thick)
0.91
0.58
0.33
0.86
0.27
0.11
0.06
where:
C, = conductance for a certain thickness of material
Btu/hr-ft2-°F
x, = thickness of material—inches
k. = thermal conductivity of material Btu - (inch)/hr-
ft2-°F
Values of C, and k, can be found in various
handbooks.82
Various values of U for different digester covers, wall
construction, and floor conditions are given in table 2-7.
NUTRIENTS
In general, it is commonly assumed that municipal
wastewater sludge is not nutrient deficient. It has been
extremely difficult to conduct research on optimum nutri-
ent requirements of anaerobic bacteria on sewage
sludge.84 To date, the literature has shown85 that, like
aerobic bacteria, nitrogen and phosphorus are required
in the highest amount (12 and 2 percent, respectively,
based on the weight of biological solids present in the
system). It is suggested that a minumum C:N:P ratio of
100:15:1 be used for design purposes.
Several researchers have also shown that the addition
of certain trace materials, iron86 and sulfur,84 could be
very beneficial to the process.
pH CONSIDERATIONS
As was noted under General Process Description, an-
aerobic digestion is a two-step process consisting of an
"acid forming" and "methane forming" step. During the
first step the production of volatile acid tends to reduce
the pH. The reduction is normally countered by destruc-
tion of volatile acids by methane bacteria and the sub-
sequent production of bicarbonate.
Past research87"89 has shown that the optimum pH val-
ue for methane producing bacteria is in the range of 6.4
to 7.5 and that these bacteria are very sensitive to pH
change. Recent research though90 now seems to indicate
that the pH tolerance of methane producing bacteria is
greater than previously expected. The study also indicat-
ed that high and low pH values were only bacteriostatic
and not bactericidal as previously thought. Because of
the importance of this finding to system control, more
research is needed in this area to verify these results.
pH is related to several different acid-base chemical
equilibria. In the anaerobic digestion process the range
of interest is between 6.0 to 8.0, which for all practical
purposes makes the carbon dioxide-bicarbonate relation-
ship the most important. In an anaerobic digestion sys-
tem the amount of carbon dioxide is dependent only on
the law of partial pressure. Since soluble carbon dioxide
depends primarily on the CO2 gas content and since at
any given time the composition of digester gas is rela-
tively fixed, pH is a function of the bicarbonate concen-
tration as shown in figure 2-11.
This relationship is very important from a process con-
trol standpoint.92 Also, it points out the importance of
43
-------
LIMITS OF
NORMAL
ANAEROBIC
TREATMENT
250
500 1000 2500 5000 10,000
BICARBONATE ALKALINITY (mg/l AS CaC03)
25,000
Figure 2-11.—Relationship between pH and bicarbonate
concentration near 95° F.91
analyzing for bicarbonate alkalinity instead of total alka-
linity as is commonly done today. The relationship be-
tween the two is given in equation 4.
BA = TA - 0.71 VA
(4)
where:
BA
TA
VA
0.71
= bicarbonate alkalinity as mg/l CaCO3
= total alkalinity as mg/l CaCO3 determined by
titration to pH 4.0.
= volatile acids measured as mg/l acetic acid
= a combination of two factors (0.83)(0.85). 0.83
converts volatile acids as acetic to volatile acid
alkalinity CaCO3 and 0.85 from the fact that in
a titration to pH 4.0, about 85 percent of the
acetate has been converted to the acid form.
It has been suggested92 that the only way to increase
digester pH is by the addition of sodium bicarbonate.
Other materials such as caustic soda, soda ash, and
lime cannot increase bicarbonate alkalinity without react-
ing with soluble carbon dioxide, which in turn causes a
partial vacuum within the system. Also above pH 6.3,
lime may react with bicarbonate to form insoluble calci-
um carbonate, thus promoting scale formation or encrus-
tration.
Sodium can be toxic at certain concentrations (see
section on Toxicity—light metal cations), and it is re-
commenced to keep sodium levels below 0.2M (approxi-
mately 4,600 mg/l), which may require dilution of the
digester contents as part of the corrective measures.
TOXICITY
Kugelman and Chin93 have noted that much of the
published data on toxicity in anaerobic digestion systems
is erroneous and misleading because of inadequate ex-
perimental techniques and general lack of understanding.
Therefore, before any discussion of toxicity takes place
a review of several fundamentals must be made.
First of all for any material to be biologically "toxic it
must be in solution. If any substance is not in solution,
it is not possible for it to pass through the cell wall and
therefore cannot affect the organism.
Second, toxicity is a relative term. There are many
organic and inorganic materials which, depending if they
meet condition one above, can be either stimulatory or
toxic. A good example of this is the effect of ammonia
nitrogen on anaerobic digestion—table 2-8.
Acclimation is a third consideration. When potential
toxic materials are slowly increased within the environ-
ment, many biological organisms can rearrange their
metabolic resources, thus overcoming the metabolic
block produced by the toxic material. Under shock load
conditions there is not sufficient time for this rearrange-
ment to take place.
Finally, there is the possibility of antagonism and syn-
ergism. Antagonism is defined as a reduction of the
toxic effect of one substance by the presence of anoth-
er. Synergism is defined as an increase in the toxic
effect of one substance by the presence of another.
This is an important relationship for cation toxicity.
Though there are many potential toxic materials, this
section will only concern itself with the following: volatile
acids; heavy metals; light metal cations; oxygen; sulfides;
and ammonia.
Volatile acids.—Up until the 1960's it was commonly
believed that volatile acid concentrations over 2,000
mg/l were toxic to an anaerobic digester. There was
also considerable controversy on whether or not alkaline
substances should be added to maintain adequate buffer
capacity.
In the early 1960's, McCarty and his coworkers pub-
lished results from very carefully controlled studies in
this area.94'96'97 Their results showed the following:
1. Studies clearly indicate that volatile acids, at least
up to 6,000-8,000 mg/l, were not toxic to methane
bacteria. Therefore, as long as there was adequate
buffer capacity to maintain the system pH in the
range of 6.6 to 7.4, the system would function.
2. Control of pH by the addition of an alkaline materi-
al is a valid procedure as long as the cation of the
Table 2-8.—Effect of ammonia nitrogen on anaerobic
digestion94'95
NH3-N
Effect
50-200 Beneficial
200-1,000 No adverse effects
1,500-3,000 Inhibitory at pH over 7.4-7.6
Above 3,000 Toxic
44
-------
alkaline material does not cause toxicity. It was
found that the addition of sodium, potassium, or
ammonium compounds is detrimental but magnesium
or calcium alkaline compounds are not.
Heavy metals—Heavy metal toxicity has frequently
been cited as the cause of many anaerobic digestion
failures. Even though trace amounts of most heavy met-
als are necessary for maximum biological development,98
the concentrations existing in raw wastewater sludges
could cause potential problems.
Since heavy metals tend to attach themselves to
sludge particles,99'100 even low influent concentrations can
be concentrated significantly in the sludge-handling proc-
ess. Table 2-9—column 2 gives the range of influent
concentrations of some heavy metals. The range is quite
wide with the higher values normally being attributed to
a local industrial polluter.
Column 3 of table 2-9 gives the typical range of
removal that can be expected through a standard sec-
ondary treatment system. Published data seem to indi-
cate that the percent removal, without chemical addition,
is a function of influent concentration. The higher the
influent concentration the higher the percent removal.
Column 4 of table 2-9 shows expected removals with
lime additions at a pH of 11.0. In fact it has been
noted106 that treatment systems which add lime or other
chemical coagulations for phosphate removal can expect
significant amounts of influent heavy metals to also be
removed.
Because of the dependence of inhibition on naturally
occurring reagents, such as carbonate and sulfide, it is
Table 2-9.—Influent concentrations and expected re-
movals of some heavy metals in wastewater treatment
systems
Removal
Heavy metal
Cadmium
Chromium +3 ....
Chromium +6 . . . .
Coooer
Mercury
Nickel
Lead
Zinc
Arsenic
Iron
Manganese
Silver
Cobalt
Barium
Selenium
innueni
concentrations
(mg/l)
<.008-1.142101'104
<.020-5.8101'104
<.020-5.8101'104
< 020-9 6101'104
< 0001 -068101'104
< 1-880101'104
< 05-1 2 2101'104
< 02-1 8 OO101'104
< 002- 0034102
<.1-13104
<.02-95102
<.05-6104
Below detection104
Secondary
treatment,
(percent)
20-45101
40-80101
0-10101
0-70101
20-75101
15-40101
50-90101
35-80101
28-73102
72105
25105
47105
7g105
Lime— pH 11,
(percent)
95103,109
95106
2Q106
gQl03,109
40106
gg103,106
90103,106
70106
gg103
95103,106
96103
75106
Table 2-10.—Total concentration of individual metals
that have been found to cause severe inhibition in
anaerobic digesters107
Concentration of metal
in digester contents
.. i , (dry sludge solids)
Metal
Percent
mM Kg"
CoDDer
Cadmium ...
Zinc
Iron
Chromium +6
Chromium +3 .
0.93
1.08
0.97
9.56
2.20
2.60
150
100
150
1,710
420
500
not possible to define precise total toxic concentrations
for any heavy metal.107 Table 2-10 gives some concen-
trations of individual metals required to cause severe
inhibition. Table 2-11 gives an indication of the differ-
ence between total and soluble concentrations that may
exist in an anaerobic digester.
The problem of heavy metal toxicity may not necessar-
ily be reduced with strict enforcement of industrial point
sources. For example, the normal digestion and excre-
tion of zinc is approximately 10 mg per person.109 Anoth-
er nonprofit source is the paved street. Table 2-12
gives the results of a study on heavy metal pollution
from paved road surfaces of several large cities.109 In
another extensive study,110 based on 9,600 analyzed sam-
ples, it was shown that if all industry in metropolitan
New York had zero discharge, there would only be a 9
percent reduction in copper, 20 percent in chromium, 6
percent in zinc, 16 percent in cadmium, and 62 percent
in nickel.
Except for chromium, heavy metal toxicity in anaerobic
digesters can be prevented or eliminated through precip-
itation with sutfides.108'111"113 Hexavalent chromium is nor-
mally reduced to trivalent chromium which under normal
anaerobic digester pH levels is relatively insoluble and
not very toxic.114
Table 2-11.—Total and soluble heavy metal content of
digesters108
Metal
Total Soluble
concentrations concentrations
(mg/l) (mg/l)
Chromium +6
Copper ...
Nickel .
Zinc
420
196
70
341
30
07
1 6
0.1
45
-------
Table 2-12.—Heavy metal from paved-curb streets109
Metal
Zinc
Copper
Lead. .
Nickel ....
Mercury .
Chromium
Arithmetic
mean
1075
21
68
060
080
.12
Range
10 062-2 1
.020-. 59
.03 -1.85
011-.19
01 9-. 2
0033-.45
Table 2-13.—Stimulatory and inhibitory concentrations of
light metal cations117'118
1Data given in pounds/mile of paved street.
The reason for using sulfide precipitation is the ex-
treme insolubility of heavy metal sulfides.115 Approximately
0.5 mg of sulfide is required to precipitate 1.0 mg of
heavy metal. If insufficient sulfide is not available from
natural sources, then it must be added in the form of
sulfate which is reduced to sulfide under anaerobic con-
ditions.
One potential drawback of using the sulfide saturation
method is the possible production of hydrogen sulfide
gas or sulfuric acid due to excess amounts of sulfide in
the digester. Because of this, it is recommended that
ferrous sulfate be used as a source of sulfide.93 Sulfides
will be produced from the biological breakdown of sul-
fate, and the excess will be held out of solution by the
iron. However, if heavy metals enter the digester, they
will draw the sulfide preferentially from the iron because
iron sulfide is the most soluble heavy metal sulfide.
Two other methods of controlling excess sulfide addi-
tions have been proposed.112-116 One method would be to
continuously analyze the digester gas for hydrogen sul-
fide.103 When there are detectable levels of H2S, sulfate
addition would be terminated; when the level becomes
undetectable, additions would start. A second method116
was the use of a silver-silver sulphide electrode to
measure very low levels of soluble sulphides. The elec-
trode is calibrated in standardized solutions of sodium
sulphide of known value to yield a parameter, pS, de-
fined in a manner similar to pH, as the negative com-
mon logarithm of the divalent sulphide ion concentration.
For example, when S~2 is 10~5M, pS would be 5.
Light metal cations—Only recently93'117'118 has the signif-
icance of the light metal cations (sodium, ammonium,
potassium, magnesium, calcium) in anaerobic digestion
started to be unraveled. Normally, domestic wastewater
sludges have low concentrations of these cations but
significant contributions, enough to cause toxicity, can
come from two sources.
1. Industrial operations.
2. The addition of alkaline material for pH control.
Not only can each of these cations be either stimula-
tory or toxic depending on concentration (table 2-13)
but when combined with each other will produce either
an antagonism or synergism relationship.
Cation
Calcium
Magnesium
Potassium
Sodium
Stimulatory
(mg/l)
100-200
75-150
200-400
1 00-200
Moderately
inhibitory
(mg/l)
2,500-4,500
1 ,000-1 ,500
2,500-4,500
3,500-5 500
Strongly
inhibitory
(mg/l)
8,000
3,000
12,000
8,000
Based on current knowledge whenever inhibition is
being caused by an excess of a certain cation, the
cation can be antagonized by the addition of one or
more of the cations listed in table 2-14.
Oxygen.—Engineers have always been concerned with
air getting into anaerobic digesters since a mixture of
one volume digester gas with 5 to 15 volumes of air is
an explosive mixture.
Many engineers have also expressed concern over the
possibility of oxygen toxicity when using dissolved air
flotation thickeners for sludge thickening. In 1971 Fields
and Agardy119 showed "... that small additions of air (up
to 0.01 volume per volume of digester contents) ap-
proaching one percent by volume, will not significantly
affect anaerobic digester performance." This value is
several magnitudes higher than the amount of air that
would be generated from a dissolved air thickening sys-
tem.
Sulfides.—By itself soluble sulfide concentrations over
200 mg/l are toxic to anaerobic digestion systems.111'120
The soluble sulfide concentration within the digester is a
function of the incoming source of sulfur, the pH, the
rate of gas production, and the amount of heavy metals
to act as complexing agents. The high levels of soluble
sulfide can be reduced by the addition of iron salts, or
gas scrubbing.
Ammonia.—Whenever there are high concentrations of
protein waste, which is possible in some systems with
highly concentrated feed sludges, ammonia toxicity must
be considered.94'118 Ammonia can be in two forms, ammo-
nium iron NH4 + or ammonia gas. Both forms are always
Table 2-14.—Cation antagonists
Inhibiting
cation
Antagonist
cation
Ammonium Potassium
Calcium Sodium, potassium
Magnesium Sodium, potassium
Potassium Sodium, potassium, calcium, ammonium
Sodium Potassium
46
-------
in equilibrium, the concentration of each depending on
pH. Equation 5 shows the relationship.
Table 2-17.—Pathogenic organisms in mesophilic anaer-
obically digested sludge123'124
NH,
NH
(5)
When the pH is 7.2 or lower, equilibrium is shifted
toward the ammonium ion and inhibition is possible at
certain concentrations. At pH values over 7.2, the reac-
tion shifts toward the gas phase which is inhibitory at
low values.
Analysis for ammonia toxicity is done by analyzing the
total ammonia concentration. If the total ammonia con-
centration is between 1,500 to 3,000 mg/l and the pH is
above 7.4-7.6, there are possible inhibitory effects due
to ammonia gas. This can be controlled by the addition
of enough HCI to maintain the pH between 7.0 to 7.2. If
total ammonia levels are over 3,000 mg/l, then the
NH4 + ion will become toxic no matter what pH level.
The only solution is to dilute the incoming waste sludge.
BACTERIAL EFFECTS
Pathogenic organisms in wastewaters consist of bacte-
ria, virus, protozoa, and parasitic worms. Many of these
Table 2-15.—Human enteric pathogens occurring in
wastewater and the diseases associated with the patho-
gens125
Pathogens
Diseases
Vibrio cholera Cholera
Salmonella typhi Typhoid and other enteric fevers
Shigella species Bacterial dysentery
Coliform species Diarrhea
Pseudomonas species Local infection
Infectious hepatitus virus Heptatitis
Poliovirus Poliomyletis
Entamoeba histolytica Amoebic dysentery
Pinworms (eggs) Aseariasis
Tapeworms Tapeworm infestation
Table 2-16.—Pathogenic organisms in sludge123'124
Type
Raw primary
Trickling filter
Raw WAS
Thickened raw
WAS
Salmonella
(No./ 100 ml)
460
62
93
74
2,300
6
9300
Pseudomones
aeruginosa
(No./100 ml)
46X103
195
110X103
1.1 X103
24X103
5.5 X103
2X103
Fecal coliform
(No. X106/100 ml)
11.4
11.5
2.8
2.0
26.5
20
Primary only
WAS only
Mixture
Primary and WAS....
Salmonella
(No./100 ml)
29
7.3
6
Pseudomonas
aeruginos
(No./ 100 ml)
34
103
42
Fecal coliform
s 106
(No./lOO ml)
0.39
0.32
.26
organisms, especially enteric viruses,122 have a strong
tendency to bind themselves to sludge solids.
Table 2-15 lists the human enteric pathogens that
have been found in wastewater sludges along with the
diseases normally associated with them. Table 2-16 lists
some data on bacterial concentrations found in raw
sludges from two studies.123'124
The reduction of pathogenic organisms under meso-
philic, anaerobic digestion has been studied by various
researchers.122'126"129 Though some early research indicat-
ed die off may be due to bactericidal effects,126'127 cur-
rent research supports that die off is strictly related to
natural die off. Data from two studies are given in table
2-17 for mesophilic anaerobically digested sludge.
No reported work on pathogen destruction for thermo-
philic anaerobic digestion could be found.
ACTIVATED CARBON
The first reported studies on the addition of activated
carbon to anaerobic digesters treating municipal waste-
water sludges was in 1935, at Plainfield, N.J.,131 and in
1936 in U.S. Patent 2,059,286.132 At this time the addi-
tion of activated carbon was claimed to have the follow-
ing benefits:
1. Enhanced the rate of digestion.
2. Increased the total amount of gas produced.
3. Produced clear supernatants.
4. Enhanced the drainability of the digested sludge.
5. Increased temperatures within the digester.
6. Gave higher volatile solids reductions.
Until recently no other reported work in this area
could be found. In 1975 Adams133'134 discussed the re-
sults of studies carried out by ICI. In his discussion he
pointed out the following advantages based on full-scale
studies carried out at Cranston, R.I.135 and Norristown,
Pa.136
1. Promoted sludge settling and clear supernatants
due to the high carbon density.
2. Catalyzes the breakdown of sludge solids, thereby
reducing the amount of sludge to be handled.
3. Increase gas production per mass of solids added
plus producing a gas with higher methane content.
4. Can absorb certain substances such as pesticles,
heavy metals, grease, scum, and detergents.
47
-------
5. Reduction in odors.
6. Possible improvement in mechanical dewatering op-
eration at least for vacuum filtration.
Even though several full-scale studies have been con-
ducted, they have not been done scientifically but more
of a general "add some carbon and see what happens"
attitude. Though improved operating results have been
shown, the real mechanism for these results have not
yet been clearly identified. At the present time EPA has
awarded a grant to Batelle to study the effects of acti-
vated carbon addition on anaerobic digesters.
TANK LAYOUT
Essentially four basic types of anaerobic digestion sys-
tems are available to stabilize municipal wastewater
sludges. The four systems are discussed below in order
of their complexity.
Conventional low rate anaerobic digestion.—Figure
2-12 shows what is typically thought of as a convention-
al, low rate, anaerobic digestion system. Essentially, this
system is nothing more than a large storage tank and
no attempt to control the environment or accelerate the
process is made.
Conventional high rate anaerobic digestion.—Figure
2-13 shows what is typically considered a conventional,
high rate, anaerobic digestion system and is the most
commonly used system in the United States today. In
this system attempts are made to control the environ-
ment (through thickening, heating and mixing) and accel-
erate the process. Essentially, all digestion takes place
in the first tank. This tank is normally maintained at
RAW SLUDGE
SUPERNATANT
DIGESTED SLUDGE
NO SUPPLEMENTAL HEATING
NO SUPPLEMENTAL MIXING
95°F (34°C) and mixed with some type of gas mixing
system. Hydraulic detention times are normally 15-25
days. The majority of designs also provide a so-called
"secondary digester" for solids-liquid separation (dotted
line tank in figure 2-13) but this practice is being chal-
lenged as not being useful in many applications and that
going direct to mechanical dewatering can have several
significant advantages.137
Anaerobic contact.—The advantage of sludge recycle
in the anaerobic digestion process has not only been
discussed but applied138"141 in treating high strength
waste and has been indicated to be worthwhile in treat-
ing waste sludges.142 Nevertheless, this process alterna-
tive is rarely considered in municipal anaerobic sludge
digestion.
Figure 2-14 shows a typical schematic of the process.
The essential feature of this system is that positive sepa-
ration through the use of a centrifuge biomass is uti-
lized. Part of this biomass is recycled back to the an-
aerobic digester where it is mixed with the incoming
sludge. This recycling of the sludge thus allows for ade-
quate cell retention to meet kinetic requirements yet
significantly reduces hydraulic detention time.
Phase separation.—As was noted under the general
process section, the anaerobic digestion process con-
sists of two distinct phases. The previous three systems
attempted to do this in one reactor. As early as 1958143
the possible value of actually separating the two proc-
esses was discussed. Work in 1968144 using dialysis se-
paration techniques clearly showed "—that the hydrol-
ysis-acid production sludge is the rate limiting process in
anaerobic digestion of sewage sludge. Furthermore, the
acid formers in a digester must operate at below opti-
mum conditions in order to maintain a healthy population
of methane forming bacteria." During the past several
years considerable research has been conducted in this
area which was summarized by Ghosh145 and has also
led to a patented process.146 Figure 2-15 shows a sche-
RAW ^n«rn _
SLUDGE
^ ex I
1
_.
<;
POSIT
—SOLIDS
SEPAR
IVE
LIQUID
4TION
CLARIMED^LIQUIP
DIGESTED SLUDGE
Figure 2-12.—Schematic of conventional low rate anaer-
obic digestion system.
Figure 2-14.—Schematic of anaerobic contact process.
RAW
^
*-t
1
L.__
-^
1
^"•^ DIGESTED SLUDGE
DIGESTED SLUDGE
Figure 2-13. — Schematic of conventional high rate an-
aerobic digestion system.
ACID DIGESTER
METHANE DIGESTER
RAW
SLUDGE
POSITIVE
-•-SOLID LIOUID-v-EFFLUENT
SEPARATION
Figure 2-15. — Schematic of phase separation anaerobic
digestion of sludge.145
48
-------
matic of this multistage system as conceived by
Ghosh.145
The phase separation process has several potential
benefits when compared to the other processes. These
are:145
1. Capability of maintaining the optimum environment
for each group of digester organisms.
2. Substantial reduction in total reactor volume and
the consequent savings in capital and operating
costs.
3. Higher rates of solids stabilization and increased
production rate and methane content of the final
product gases.
4. Decreased heat requirement and increased thermal
efficiency.
5. Suitable for incorporation into existing treatment
plants with minimum capital investment.
6. Reduction of nitrogen content of the system effluent
by simultaneous liquefaction and denitrification of
waste feeds in the acid digester.
GENERAL OPERATIONAL CONTROL
PROCEDURES
It should be noted that there is no one test or control
parameter that will signify good or bad anaerobic diges-
tion operation. Control or operation of an anaerobic dig-
estion system should be done through a combination of
several analyses, the results plotted as a function of
time. In this way an unbalanced digester would be de-
fined as one which starts to radically deviate from past
norms. Note that the norm at one plant can be failure
conditions at another.
At the present time it is suggested that a minimum of
four different tests be performed on a regular basis. The
four proposed tests are: pH, bicarbonate alkalinity, vola-
tile acids and percent carbon dioxide (CO2) in the di-
gester gas.
pH.—As was discussed under pH Considerations, opti-
mal pH is between 6.4 to 7.5. Unfortunately, the pH test
by itself is not a good control procedure92 because:
1. It is a logarithmic function and is not very sensitive
to large fluctuations in the alkalinity concentration.
For example, a change in alkalinity from 3,600 to
2,200 mg/l would only change the pH from 7.1 to
6.9 which is within the error involved in pH meas-
urement.
2. It does not provide adequate warning. A low pH
only informs the operator that an upset has oc-
curred.
Bicarbonate alkalinity.—The importance of measuring
bicarbonate alkalinity rather than total alkalinity was dis-
cussed in the section entitled "pH Considerations." The
bicarbonate alkalinity and volatile acid test are used
together to develop the ratio of volatile acid to bicar-
bonate alkalinity. In order to insure good operation (that
is good buffering capacity), this ratio should be below
0.7.
Note: A fast, simple method for differentiating bicar-
bonate and volatile acid alkalinity without using distilla-
tion has been developed by DiLallo and Albertson.147
Volatile acids.—By itself this analysis means nothing.
Only when plotted as a function of time or used in
conjunction with the volatile acid-bicarbonate ratio can
impeding operation problems be interpreted early enough
to allow some type of correctional procedures.
Carbon dioxide content.—Under good operation the
CO2 content in digester gas will be between 35-45 per-
cent. As an unbalance condition starts to occur, there
will be an increase in the percentage of CO2 as the
methane producers become incapable of functioning.
When the control parameters indicate an unbalance
condition, the following steps of action have been rec-
ommended:91
1. Maintain pH near neutrality
2. Determine cause of unbalance
3. Correct cause of unbalance
4. Provide pH control until treatment returns to nor-
mal.
Maintaining the pH near neutrality can be done two
ways. The first is to reduce the waste feed. A second
way is through the addition of some type neutralizing
material (see section on pH Considerations and Toxici-
ty—Light Metal Cations).
Determining the cause of unbalance can be difficult.
Some of the easier things to check are hydraulic wash-
out, heat exchanger not capable of providing sufficient
heat, mixing system not operating, sudden change in the
amount of sludge pumped to the digester and extreme
drop in pH. If nothing shows up after the above prelimi-
nary analysis, then testing for ammonia, free sulfides,
heavy metal and light metal concentrations will have to
be made.
Once it has been determined what is causing the
problem, corrective measures can be taken to put the
digester back on line. Depending on the cause of unbal-
ance, the length of time required to bring a digester
back to normal operating condition may take from 2 to
3 days to 4 to 6 months.
BASIC SIZING CRITERIA—ANAEROBIC
DIGESTION SYSTEMS
Operating temperature for optimiz-
ing gas production
Hydraulic detention time (no recy-
cle) of primary digester to
achieve max. volatile solids de-
struction
primary sludge only
primary plus waste activated
sludge
waste activated sludge only
Mesophilic 35-37.8° C (95-100° F)
Thermophihc 54.4-57.2° C (130-
135°F)
8.5-10 days at 35° C
5.5-7 days at 54.4° C
15-17 days at 35° C
11-13 days at 54.4 °C
25-27 days at 35 °C
16-18 days at 54.4 °C
49
-------
Feed solids concentration, degnt- Mixing difficulties start to develop
ted sludge at feed solids concentrations
over 8-9 percent.
Organic loading rate Function of hydraulic detention
time and feed solids concentra-
tion. Many present-day facilities
are operating from 0.15 to 0.25
Ib VS/ft3/day (2.4^1.0
kg/m3/day).
Note: When dealing with anaerobic digestion of waste
activated sludges or mixtures of, should not assume any
solids-liquid separation within secondary digester.
DESIGN PROBLEM
Two designs, a 4 Mgal/d (0.18 m3/s) and 40 Mgal/d
(1.75 m3/s), are evaluated. Influent is typical domestic
wastewater of 200 mg/liter biochemical oxygen demand
(BOD5) and 200 mg/liter suspended solids (SS) with no
heavy industrial contributors. Liquid treatment consists of
grit removal, primary treatment, secondary treatment (ac-
tivated sludge), and chlorination. No chemicals are add-
ed to liquid treatment portion.
Sludge Type and Amount
Every million gallons (3,785 m3) of raw plant influent
will generate approximately 1,000 Ib (453.6 kg) of pri-
mary sludge and 1,000 Ib (453.6 kg) of waste-activated
sludge.148 This can be further broken down as in table
2-18.
Based on table 2-18, the total sludge generated for
the design examples would be 8,000 Ib (3,636 kg) for
the 4-Mgal/d (0.18 m3/s) design and 80,000 Ib (36,364
kg) for the 40 Mgal/d (1.75 m3/s) design.
Temperature
Operating temperature in a high-rate digester would
be:
• 35°C (95° F) for a 4-Mgal/d (0.18 m3/s) design
based on mesophilic conditions
• 54.4°C (130°F) for a 40-Mgal/d (1.75 m3/s) design
based on thermophilic conditions
The coldest ambient air temperature for both designs
is assumed to be 12.2°C (10°F). The coldest raw
Table 2-18.—Breakdown of inert and volatile suspended
solids per mg of plant influent (Ibs)
Inert Inert Biodegradable
Nonvolatile volatile volatile
sludge temperature for both designs is assumed to be
4.5° C (40° F).
Required Hydraulic Residence Tii
Organic Loading—Influent Solids
Concentration for High-Rate Digester
For both designs maximum volatile solids destruction
desired. Figure 2-3 shows that for this particular type
sludge, a practical upper limit of 55 percent volatile
solids destruction is possible and can be obtained in
600 degree-days.
Thickened sludge recycle will not be used in either
design; therefore, sludge age will equal hydraulic resi-
dence time (HRT) in a high-rate digester.
4 Mgal/d design
600°C - days^-35°C = 17 days minimum HRT.
CD
SLUDGE AGE (DAYS)
Figure 2-16.—Relationship between solids concentra-
tion—organic loading—sludge age for anaerobic diges-
tion.
1.0
0.8
0.6
0.4
0.2
0.0
(
—
h^~ PROBABLE DIGESTION LIMIT
L F
I
: I
,
) 5 10
s.
s.
j^
>>
^
^
i i l 1
15 20 25 30 35 40
Primary sludge
Waste-activated sludge
Totals ....
250
300
550
300
210
510
450
490
940
Figure 2-17.—An anaerobic digester floating cover with
a gas mixing system.
50
-------
40 Mgal/d design
600°C - days-5-54.4°C = 11 days minimum HRT.
For both designs, a three-day storage capacity also is
desired. This dictates that floating covers will be utilized
with minimum hydraulic detention time based on when
the cover rests on landing corbels and maximum deten-
tion time based on when the cover is floating at maxi-
mum liquid level.
Figure 2-16 indicates the possible safe range of or-
ganic loading for a given HRT.
The practical upper limit on feed solids concentration
is 8 to 9 percent.
Within the constraints given, the designer has consid-
erable latitude for selection of digester tank volume (see
figure 2-17 for example) and, to a certain point, selec-
tion of necessary thickening equipment. For the designs
given, the following organic loading has been selected:
4 Mgal/d design—0.15 Ibs VS/ftVday
5,800 Ibs VS/day 7.48 gal 1
0.15 Ib VS/ft3/day ft3 17 day minimum
= 17,014 gallons/day
8,000 Ibs solids/day
(17,014 gal/day)(8.34)
X100
= 5.64 percent feed solids required
Minimum tank volume:
17 day x 17,014 gal/day = 289,238 gal
(38,668 ft3).
Maximum tank volume:
20 day x 17,014 gal/day = 340,280 gal
(45,492 ft3).
Use one digester, 45 ft (13.7 m) diam, 5.7 ft (1.7 m)
deep cone, 28.7 ft (8.7 m) side wall depth with 4.3 ft
(1.3 m) cover travel.
40 Mgal/d design—0.20 Ibs VS/ft3/day
58,000 Ibs VS/day 7.48 gal 1
0.2 Ib VS/ft3/day ft3 11 day minimum
= 197,200 gal/day
80,000 Ibs solids/day 1QQ
(197,200 gal/day)(8.34)
= 4.87 percent feed solids required
Minimum tank volume.
11 day x 197,200 gal/day = 2,169,200 gal
(290,000 ft3).
Maximum tank volume.
14 day x 197,200 gal/day = 2,760,800 gal
(369,091 ft3).
Use two digesters, each 95 ft diam, 11.9 ft deep
cone, 24.5 ft side wall depth with 5.6 ft cover travel.
Table 2-19 gives various calculated results for volatile
suspended solids destruction in an anaerobic digester.
Expected Energy Production
Depending on sludge composition (oil, grease, fiber,
protein), gas production can range from 12 to 18 ft3/lb
(0.75-1.12 m3/kg) VS destroyed, with the higher values
indicating high grease content.
Depending on methane content, each cubic foot of
digester gas has an energy value between 550 to 650
Btu (580-685 kJ).
4 Mgal/d design at 55 percent VS destruction.
Ibs VS Cu ft Total cu ft
destroyed produced per produced per
per day Ib VS destroyed day
3,190 12 38,280
15 47,850
18 57,420
Btu
per
cu ft
550
600
650
550
600
650
550
600
650
Total Btu
produced per
dayxW6
21.054
22.960
24.862
26.317
28.710
31.102
31.581
34.452
37.323
Table 2-19.—Various calculated results for volatile suspended solids de-
struction in anaerobic digester
4 Mgal/d
design
40 Mgal/d
design
Lbs volatile suspended solids (VSS) destroyed
per day
Percent of TS destroyed
Percent of biodegradable VS destroyed.
Percent original inlet feed VSS/TS
Percent final VSS/TS
0.55 (2,040+ 3,760) = 3,190
3.190
8,000
3,190,
X100 = 39.9
3,760
5,800
8,000
5,800 - 3,190
X100 =72.5
8,000
X100 = 32.6
31,900
39.9
84.8
72.5
32.6
51
-------
40 Mgal/d design at 55 percent VS destruction.
Would be same as 4 Mgal/d except 10 times greater.
Note: 1 hp-hr = 2,545 Btu; electrical energy conversion
32 to 37 percent.
Sludge Heat Requirements
4 Mgal/d design.—40 hrs/wk.
17,014 gal 7 day.
day wk
1 wk 8.34 Ibs
x-
'40 hrs gal
(95-40) °F
= 1,365,756 Btu/hr
40 Mgal/d design.—12 hrs/day-
197,200 gal
day
x
1
2 units
x
. 8.34 Ibs.
gal
-x
-7 day/wk.
(130-40) °F
12 hrs
= 6,167,430 Btu/hr/unit
For both designs, the designer has selected a floating
cover with wood composition roof, 12-in. (30.5 cm) thick
concrete wall with air space and 4-in. (10.2 cm) brick,
and 12-in. (30.5 cm) thick concrete floor exposed to wet
earth (table 2-20).
Summary of Heat Requirements
Btu/hr 4 Mgal/d 40 Mgal/d
Expected max. winter output per unit 1,491,584 6,738,006
Expected max. winter input per unit (heat
ex. only 80 percent efficient) 1,864,480 8,422,508
Expected total max. winter Btu requirement 14.9 X106 219.3 X106
Expected total min. summer (air at 75° F,
sludge at 50° F) Btu requirement 10.5X106 182.3X106
Expected min. summer input per unit
(heat ex. only'80 percent efficient) 1,482,643 7,224,150
Matching Output With Requirement
4 Mgal/d
Average Max. req. Min, req.
hourly winter summer
production, conditions, conditions,
Btu/hr Btu/hr Btu/hr
Expected'
total Btu
produced per
dayxw6
21.054..
22 960.
24 862 .
26.317..
28.710..
31 102.
31 581.
34.452..
37.323..
877,250
956,666
1,035,916
1,096,541
1,196,250
1,295,916
1,315,875
1,435,500
1,555,125
Table 2-20.—Maximum winter—full tank heat radiation
loss (Btu/hr)
4 Mgal/d 40 Mgal/d
Roof/tank
Wall/tank
Floor/tank
Total
44,61 1
71,162
10,055
125,828
251,925
252,840
65,811
570,576
If^J
RAW
A
•— 1
1
L
1
1
-^ ^•'-DIGESTED SLUDGE
DIGESTED SLUDGE
Figure 2-18.—Schematic of conventional high-rate anaer-
obic digestion system.
digester at lower temperature, (2) increase heat ex-
changer operating time, and (3) provide some type of
gas storage, either a low pressure gas holder (12-24 hr
capacity) or high pressure gas holder (several weeks'
capacity).
An example of a system for heating anaerobic diges-
ters is shown in figure 2-10.
Expected
total Btu
produced per
dayxw6
210.54...
229.60..
248.62...
263.17...
287.10...
311.02 ..
315.81 .
344.52...
373.23...
40 Mgal/d
Average
hourly
production,
Btu/hr
8,772,500
9,566,660
10,359,160
10,965,410
11,962,500
12,959,160
13,158,750
14,355,000
15,551,125
Max. req. Min. req.
winter summer
conditions, conditions.
Btu/hr Btu/hr
8,422,508 7,224,150
Figure 2-18 shows the general system layout pro-
posed for both designs.
1,482,643 SIZING GAS SAFETY EQUIPMENT
1,864,480
Note that the maximum hourly requirement is above
the expected hourly production and that even the mini-
mum just makes it, even though total maximum require-
ments are below minimum total expected gas production.
There are three actions which can be taken: (1) Operate
The objective is to remove moisture and convey di-
gester gas from digester to gas utilization, storage or
flaring device.
Since hourly production fluctuates greatly each day, it
is common to size piping to handle 2.5 times the hourly
average.
52
-------
4 Mgal/d Design
Assume all gas is produced in one digester.
Possible to produce 57,420 ft3/day.
2.5 = 5.963 ff/hr
,h
24 hrs/day
5,963 ftVhr
Ax 3,600 sec/hr
A = Cross sectional area inside pipe
2 in. pipe A = 0.022 ft2 4 in. pipe A = 0.088 ft2
3 in. pipe A = 0.049 ft2 6 in. pipe A = 0.1 96 ft2
With a 4-in. (10.2 cm) pipe, the maximum velocity is
almost 19 ft/sec (5.8 m/s), well in excess of the 11 to
12 ft/sec (3.4-3.7 m/s) recommended for successful
condensate removal. Rather than increase the line size
to 6 in (15.2 cm), it is recommended that oversize accu-
mulators be used.
Gas safety piping specifications are as follows (see
figure 2-9):
1. All gas lines must be tight, sloped (1/4 in./ft)(2.1
cm/m) toward condensate traps and accumulators,
have ample capacity and be protected against
freezing.
2. Lines leading to gas burners or gas engines must
be protected against flashbacks by flame traps.
Trap should be located near point of combustion
with a maximum allowable distance of 30 ft (9.1 m)
from point of gas combustion.
3. Bypasses are needed to permit flexibility of opera-
tion, but flame traps are never bypassed.
4. Total pressure loss through the appurtenances and
gas lines from the digester to use should be only
2.0 in. (5.1 cm) W.C. at maximum gas flow rate.
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Plants" Diesel and Gas Turbine Progress, April (1971).
79. Joseph, J., "Sewage Plant's Engines Operate Economically on
Digester Gas" Diesel and Gas Turbine Progress August (1970)
80. Communications with Cooper - Bessemer Corp., and Worthingti
Pump & Machinery Corp. 1946 - Archives Pacific Flush Tank
Co.
81. Backmeyer, D., and Drautz, K. E., "Municipality and Industry
Combine to Solve Hydrogen Sulfide Problem" Water and Sewai
Works, March (1962).
82. Chemical Engineer, April 3 (1972).
83. Lang, N R., Handbook of Chemistry, 10th edition 1966.
84. Bryant, M. P., et al., "Nutrient Requirements of Methanogenic
Bacteria" Anaerobic Biological Treatment Processes, American
Chemical Society No. 105 published (1971).
85. Speece, R. E., and McCarty, P. L., "Nutrient Requirements and
Biological Solids Accumulation in Anaerobic Digestion" Advance
In Water Pollution Research, vol. II, ed. by W. W. Eckenfelder,
Pergamon Press (1964).
86. Pfeffer, J. T. and White, J. E , "The Role of Iron In Anaerobic
Digestion" Proceedings 19th Purdue Ind. Waste Conference
(1964).
87. Heukelekian, H., and Heinemann, B., "Studies on Methane Pro-
ducing Bacteria" Sewage Works Journal, vol. 11, pp. 426-453,
571-586, 965-970 (1939).
88. Barker, H. A., "Studies Upon the Methane Fermentation Proc-
ess" Proceedings of the National Academy of Science, vol. 29,
p. 184 (1943).
89. Mylroie, R. L., and Hungate, R. E., "Experiments on Methane
Bacteria In Sludge" Canadian Journal of Microbiology, vol. 1, p
55 (1954).
90. Clark, R. H., and Speece, R. F., "The pH Tolerance of Anaero
bic Digestion" Advances In Water Pollution Research, vol. I, ed
by S. H. Jenkins, Pergamon Press (1970).
91. McCarty, P L., "Anaerobic Waste Treatment Fundamentals -
54
-------
Part 2 Environmental Requirements and Control" Public Works,
October, p. 123 (1964).
92. Brovko, N., et al., "Optimizing Gas Production, Methane Content
and Buffer Capacity in Digester Operation" Water and Sewage
Works, July, p. 54 (1977).
93. Kugelman, I. J., and Chin, K. K., "Toxicity, Synergism and An-
tagonism in Anaerobic Waste Treatment Processes" Anaerobic
Biological Treatment Processes, American Chemical Society
#105 published (1971).
94. McCarty, P. L, and McKinney, R. E., "Salt Toxicity in Anaerobic
Treatment" Journal WPCF, vol. 33, p. 399 (1961).
95. Albertson, O. E., "Ammonia Nitrogen and the Anaerobic Environ-
ment" Journal WPCF, vol. 33, p. 978 (1961).
96. McCarty, P. L, and McKinney, R. E., "Volatile Acid Toxicity in
Anaerobic Digestion" Journal WPCF, vol. 33, p. 223 (1961).
97. McCarty, P. L., and Brosseau, M. H., "Toxic Effects of Individual
Volatile Acids in Anaerobic Treatment" Proceedings 18th Purdue
Ind. Waste Conference (1963)
98. Wood, D. K., and Tchobanoglous, G., "Trace Elements in Biolog-
ical Waste Treatment" Journal WPCF, vol. 47, p. 1933 (1975).
99. Oliver, B. G., and Cosgrove, E. G., "The Efficiency of Heavy
Metal Removal by a Conventional Activated Sludge Treatment
Plant" Water Research, vol. 8, p. 864 (1974).
100. Neufeld, R. D., and Hermann, E. R., "Heavy Metal Removal by
Acclimated Activated Sludge" Journal WPCF, vol. 47, p. 311
(1975).
101. Cohen, J. M., "Trace Metal Removal by Wastewater Treatment"
EPA Technology Transfer Newsletter, January (1977).
102. Johnson, W. F., and Hinden, E., "Bioconcentration of Arsenic by
Activated Sludge Biomass" Water and Sewage Works, vol. 119,
October, p. 95 (1972).
103. Argo, D. G., and Clup, G. L., "Heavy Metals Removal in Waste-
water Treatment Processes: Part 1" Water and Sewage Works,
vol. 119, August, p. 62 (1972).
104. Mytelka, A. I., et al., "Heavy Metals in Wastewater and Treat-
ment Plant Effluents" Journal WPCF, vol. 45, No. 9, p. 1859
(1973).
105. Esmond, S. E., and Petrasek, A. C., "Removal of Heavy Metals
by Wastewater Treatment Plants" presented at WWEMA Industri-
al Water and Pollution Conference, Chicago, March (1973).
106. Maruyama, T., et al., "Metal Removal by Physical and Chemical
Treatment Processes" Journal WPCF, vol. 47, p. 962 (1975).
107. Mosey, F. E., "Assessment of the Maximum Concentration of
Heavy Metals in Crude Sludge Which Will Not Inhibit the Anaero-
bic Digestion of Sludge" Water Pollution Control, vol. 75, p. 10
(1976).
108. Barth, E. F., et al., "Interaction of Heavy Metals in Biological
Sewage Treatment Processes" U.S. Department of Health, Edu-
cation, and Welfare, May (1965).
109. Montague, A., "Urban Sludge Disposal or Utilization Alterna-
tives - Socio-Economic Factors" Municipal Sludge Management
and Disposal published by Information Transfer Inc., August
(1975).
110. Klein, L. A., et al., "Sources of Metals in New York City Waste-
water" Journal WPCF, vol. 46, No. 12, p. 2653 (1974).
111. Lawrence, A. W., and McCarty, P. L., "The Role of Sulfide in
Preventing Heavy Metal Toxicity in Anaerobic Treatment" Journal
WPCF, vol. 37, p. 392 (1965).
112. Masselli, J. W., et al., "Sulfide Saturation For Better Digester
Performance" Journal WPCF, vol. 39, p. 1369 (1967).
113. Regan, T. M., and Peters, M. M., "Heavy Metals in Digesters:
Failure and Cure" Journal WPCF, vol. 42, p. 1832 (1970).
114. Moore, W. A., et al., "Effects of Chromium on the Activated
Sludge Process" Journal WPCF, vol. 33, p. 54 (1961).
115. Lang's Handbook of Chemistry (1973).
116. "Inhibition of Anaerobic Digestion by Heavy Metals" abstract
from Water Research, vol. 6, p. 1062 (1972).
117. Kugelman, I. J., and McCarty, P. L., "Cation Toxicity and Stimu-
lation in Anaerobic Waste Treatment—I Slug Feed Studies" Jour-
nal WPCF, vol. 37, p. 97 (1965).
118. Kugelman, I. J., and McCarty, P. L., "Cation Toxicity and Stimu-
lation in Anaerobic Waste Water—II Daily Feed Studies" Pro-
ceedings 19th Purdue Ind. Waste Conference, p. 667 (1965).
119. Fields, M., and Agardy, F. J., "Oxygen Toxicity in Digesters"
Proceedings 26th Purdue Ind. Waste Conference, p. 284 (1971).
120. Lawrence, A. W., and McCarty, P. L., "Effects of Sulfides on
Anaerobic Treatment" Proceedings 19th Purdue Ind. Waste Con-
ference (1964).
121. Process Design Manual for Land Treatment of Municipal Waste-
water EPA Technology Transfer, EPA-625/1-77-008, October
(1977).
122. Ward, R. L., "Inactivation of Enteric Viruses in Wastewater
Sludge" Proceedings 3rd National Conference on Sludge Man-
agement Disposal and Utilization, p. 138, December (1976).
123. Kenner, B. A., et al., "Simultaneous Quantitation of Salmonella
Species and Pseudomonas Aeruginos" USEPA, National Environ-
mental Research Center, Cincinnati, Ohio (1971), PB 213-
706/6BE.
124. "Stabilization and Disinfection of Wastewater Treatment Plant
Sludges" EPA Technology Transfer Sludge Treatment and Dis-
posal Seminar (1977).
125. Love, Gary J., et al., "Potential Health Impacts of Sludge Dis-
posal on the Land" Municipal Sludge Management and Disposal,
published by Information Transfer Inc., August (1975).
126. Lund, E., and Ronne, V., "On The Isolation of Virus From Sew-
age Treatment Plant Sludges" Water Res., vol. 7, p. 863 (1973).
127. Palf, A., "Survival of Enteroviruses During Anaerobic Sludge Di-
gestion" In Advances in Water Pollution Research, Proceedings
6th International Conference, Jerusalem, published by Pergamon
Press, N.Y., p. 99 (1973).
128. McKinney, R. E., et al., "Survival of Salmonella lyphosa During
Anaerobic Digestion" Sewage and Industrial Waste, vol. 30, p.
1469 (1958).
129. Leclerc, H, and Brouzes, P. "Sanitary Aspects of Sludge Treat-
ment" Water Research, vol. 7, p. 355 (1973).
130. Drnevich, R. F., and Smith, J. E., Jr., "Pathogen Reduction In
the Thermophilic Aerobic Digestion Process" Presented at the
48th WPCF Conference, Miami Beach, October (1975).
131. Rudolfs, W., and Trubnick, E. H., "Activated Carbon in Sewage
Treatment" Sewage Works Journal, vol. 7, p. 852 (1935).
132. Statham, N., "Method of Sewage Disposal" U.S. Patent
2,059,286 November 3 (1936).
133. Adams, A. D., "Activated Carbon: Old Solution to an Old Prob-
lem" Water and Sewage Works, vol. 122, No. 8, p. 46 (1975).
134. Adams, A. D., "Improved Anaerobic Digestion with Powered Acti-
vated Carbon" Presented at Central State Water Pollution Con-
trol Association annual meeting, May 22 (1975).
135. Ventetuolo, T., and Adams, A. D., "Improving Anaerobic Digester
Operation with Powdered Activated Carbon" Deeds and Data-
Water Pollution Control Federation, July (1976).
136. Hunsicker, M., and Almeida, T., "Powdered Activated Carbon
Improves Anaerobic Digestion" Water and Sewage Works, p. 62,
July (1976).
137. Mignone, N. A., "Elimination of Anaerobic Digester Supernatant"
Water and Sewage Works, February (1977).
138. Schroepfer, G. J., and Ziemke, N. R., "Development of The
Anaerobic Contact Process" Sewage and Industrial Wastes, vol.
13, p. 164 (1959)
139. Steffen, A. S., and Bedker, M., "Operations of a Full Scale
Anaerobic Contact Treatment Plant For Meat Packing Waste"
Proc. 16th Purdue Ind. Waste Conf., p 423 (1962).
140. Dietz, J. C., et al., "Design Considerations For Anaerobic Con-
tact Systems" Journal WPCF, vol. 38, p. 517 (1966).
141. Gates, W. E., et al., "A Rational Model For The Anaerobic
Contact Process" Journal WPCF, vol. 39, p. 1951 (1967).
142. McCarty, P. L., "Kinetics of Waste Assimilation in Anaerobic
Treatment" American Institute Biological Sciences, vol. 7 (1966).
143. Babbit, H. E., and Baumann, E. R., Sewage and Sewage Treat-
ment, John Wiley & Sons Inc., NY (1958).
144. Hammer, M. J., and Borchardt, J. A., "Dialysis Separation of
Sewage Sludge Digestion" Journal SED, ASCE, vol. 95, SA5, p.
907 (1969).
55
-------
145. Ghosh, S., et al., "Anaerobic Acidogenesis of Sewage Sludge" 147. DiLallo, Ft., and Albertson, O. E., "Volatile Acids by Direct Titra-
presented 46 WPCF Convention, Cleveland, Ohio (1973). tion" Journal WPCF, vol. 33, April (1961).
146. Ghosh, S., and Klass, D. L, "Two Phase Anaerobic Digestion" 148. Kormanik, R. A., "Estimating Solids Production For Sludge Han-
U. S. Patent 4,022,665, May 10 (1977). dling" Water and Sewage Works, December (1972).
56
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Chapter 3
Aerobic Digestion and Design
of Municipal Wastewater Sludges
Aerobic digestion of municipal wastewater sludges is
based on the principle that with inadequate external
food sources, biological cells will consume their own
cellular material.
The claimed advantages of aerobic digestion are:1
• Volatile solids reduction approximately equal to that
obtained anaerobically.
• Low BOD5 concentrations in the supernatant liquor.
• Production of an odorless, humuslike, biologically
stable end product that can be disposed of easily.
• Production of a sludge with excellent dewatering
characteristics.
• Recovery of more of the basic fertilizer value in the
sludge.
• Few operational problems.
• Low capital cost.
Table 3-1 indicates the types of sludges that have
been studied on a full-scale basis. Operating results from
operating installations indicate that the third and fourth
items above are not correct. Some aerobically digested
sludges, even at very long detention times are not bio-
logically stable and aerobically digested sludge does not
dewater very readily with mechanical equipment.
Today most aerobic digesters are designed using rules
of thumb developed from past experience (table 3-2)
and as the literature has noted19"22 do not always per-
Table 3-2.—Typical present day aerobic digestion de-
sign criteria23
Parameter
Value
Hydraulic detention time, days at 20° Ca
Activated sludge only
Activated sludge from plant operated without primary
settling
Primary plus activated or trickling filter sludge
Solids loading, Ibs volatile solids/ft3/day
12-16
16-18
1&-22
0.1-
0.20
Oxygen requirements, Ib/lb cell destroyed b2.0
Energy requirements for mixing
Mechanical aerators, hp/1,000 ft3 0.5-1.0
Air mixing, scfm/1,000 ft3 20-30
Dissolved oxygen level in liquid, mg/l 1-2
'Detention times should be increased for temperatures below 20° C.
If sludge cannot be withdrawn during certain periods (e.g., weekends,
rainy weather) additional storage capacity should be provided.
"Ammonia produced during carbonaceous oxidation oxidized to ni-
trate.
form as intended. This chapter presents the most up-to-
date design criteria available. Whenever possible full-
scale operating data are presented.
Table 3-1.—Type and reference of full-scale studies on
aerobic digestion of municipal wastewater sludge
Reference on
mesophilic
Reference on
thermophilic
Primary sludge only
Waste activated only
Mixed primary and waste activated
sludge
Waste activated sludge from contact
stabilization
Primary and lime
Trickling filter only
Mixed primary and trickling filter
Sludges containing iron or alum
2,3,4,5,18
7,8
7,8,11
14,15
16
2
2
17,18
6
9,10
10,12,13
CRYOPHILIC—MESOPHILIC—
THERMOPHILIC DIGESTION
For purposes of classification the following three tem-
perature zones of bacterial action will be used through-
out this chapter:
• Cryophilic zone—liquid temperature below 10°C
(50° F)
• Mesophilic zone—liquid temperature between 10°C
to 42°C (50° F to 108°F)
• Thermophilic zone—liquid temperature above 42° C
(108°F)
The effect of temperature on the effectiveness of aer-
obic digestion is still an area of considerable controver-
sy,24 especially in the areas of solids reduction, dewater-
ability and settleability. The data shown in subsequent
sections should help clarify some of the controversy.
57
-------
At the present time considerable research is being
undertaken in the design and operation of thermophilic
aerobic systems,13'24'31 especially auto-thermophilic aerobic
systems.13'27'29'31 Claimed advantages of the thermophilic
aerobic system are:13'30'31
• Higher rates of organic stabilization that allow
smaller volume requirements.
• Higher maintenance energy requirements and higher
microbial decay rates that give smaller amounts of
sludge for disposal.
• Digestion in this temperature range should make
liquid essentially pathogen free.
• All weed seeds should be destroyed.
• Total oxygen demand should be 30 to 40 percent
less than mesophilic since few, if any, nitrifying
bacteria exist in this temperature range.
• Improved solids-liquid separation due to decreasing
liquid viscosity.
• Possible improved oxygen transfer rates because of
the significantly higher coefficient of diffusivity of
oxygen.
VOLATILE SOLIDS REDUCTION
One of the main objectives of aerobic digestion is to
reduce the amount of solids that need to be disposed.
This reduction is normally assumed to take place only
with the volatile content of the sludge, though some
studies24'32 have shown that there can be destruction of
the nonorganics as well. In this discussion solids reduc-
tion will pertain only to the volatile content.
The change in volatile content is normally represented
by a first order biochemical reaction,
dx/dt = -KdX
0)
where
dx/dt = rate of change of volatile suspended solids
per unit of time
Kd = reaction rate constant - day'1
X = concentration of volatile suspended solids at
time t in aerobic digester
The time t in equation (1) is actually the sludge age
in the aerobic digester and, depending on how the aero-
bic digester is being operated (continuous flow without
recycle or with recycle, batch with supernatant decant),
can be considerably greater than the theoretical hydrau-
lic residence time (HRT).
A distinction must be made between biodegradable
volatile suspended solids and nonbiodegradable volatile
suspended solids. Research in this area is quite limited
but the following generalities can be used.
• Approximately 20 to 30 percent of the influent sus-
pended solids of a typical domestic wastewater is
inert.33 Of the remaining suspended solids that are
volatile, approximately 40 percent are inert organics
consisting chiefly of lignins, tannins, and other large
complex molecules.
.40
-8 -35
.30
< .20
cc
A-Pilot Plant Ref (28)
• - Pilot Plant Ref (36)
X- Full Scale Ref (10)
O- Pilol
D- Pilot
• - Piloi
a- Piloi
+- Piloi
t Plant Ref (10)
.1 Plant Ref (11)
.1 Plant Ref (27)
.t Plant Ref (37)
-------
50
§ «
k-
CJ
o 30
> 20
10
0
O ODD
x- Pilot Plant Ref (16)
• - Full Scale Ref (15)
o- Pilot Scale Ref (7)
A- Full Scale Ref (10)
+ - Pilot Plant Ref (36)
A- Pi lot Plant Ref (38)
• - Pilot Plant Ref (39)
o- Full Scale Ref (37)
200 400 600 800 1000 1200 1400 1600
TEMPERATURE (°C) x SLUDGE AGE (days)
1800 2000
Figure 3-3.—Volatile suspended solids reduction as a
function of digester liquid temperature and digester
sludge age.
this time there is not enough data to allow segregation
of Kd by sludge type; therefore, the line drawn through
the data points represents an overall average (Rvalue.
Figure 3-2 indicates the results from one study15 on
the effects of aerobic digester solids concentration on
the reaction rate, Kd. Figure 3-3 shows the effect of
temperature and sludge age on total volatile suspended
solids reduction.
OXYGEN REQUIREMENTS
Activated sludge biomass is most often represented by
the empirical equation C5H7NO2. Under prolonged periods
of aeration, typical of the aerobic digestion process, the
biochemical equation for oxidation is represented by
equation (2).
C5H7NO2 + 7O2 -+ 5CO2 + 3H2O + H + + NO3' (2)
LU Q-
gi
Q.
D >
Z 5
UJ o
O O
>- o
X ^
2°
O UJ
ai I-
o. <
to cc
8.0
6.0
4.0
2.0
TEMPERATURE RANGE >10C
^ \° <10C
o
20 60 100 140 180
SLUDGE AGE (DAYS)
220
Figure 3-4.—Effects of sludge age and liquid termpera-
ture on oxygen uptake rates in aerobic digesters.19
Theoretically, this reaction states that 1.98 pounds of
oxygen are required per pound of cell mass oxidized. In
pilot36 and full-scale10'15 studies where this value has been
evaluated, the range was from 1.74 to 2.07 pounds of
oxygen required per pound of volatile solids destroyed.
For mesophilic systems, a design value of 2.0 is recom-
mended. For thermophilic systems where nitrification
would not exist,13'30'31 a value of 1.4 is recommended.
The actual specific oxygen utilization rate, pounds of
oxygen per 1,000 pounds volatile solids per hour, is a
function of total sludge age and liquid temperature.19'24'38
In one study, Ahlberg and Boyko19 visited several operat-
ing installations and developed the relationship shown in
figure 3-4. Field studies19 have indicated that a minimum
value of 1.0 mg of oxygen should be maintained in the
digester at all times.
MIXING
Mixing in an aerobic digester, treating municipal
wastewater sludge of domestic origin, is considered to
have the following benefits. (Note: It is assumed that a
favorable environment exists to allow development of an
aerobic digestion system.)
• It continues to bring deoxygenated liquid to the
aeration device.
• It keeps the food supply uniformly dispersed and in
constant contact with the growing cells to promote
maximum utilization of the system.
• It keeps the concentration of biological end pro-
ducts at their lowest value by dispersing them uni-
formly throughout the digester.
• It provides environmental uniformity (oxygen, tem-
perature, nutrients, etc.) throughout the digester to
allow the best possible cell development.
• It allows fairly fast dispersion of any toxic material
entering the system, thus, possibly minimizing its
effect on the aerobic process.
There is general agreement that mixing is an important
criterion in the aerobic digestion process. The problem
arises when one tries to evaluate, define or specify a
mixing system.
In recent years it has become popular to use the term
"complete mix" when discussing biological process reac-
tors. The term "complete mix" means that the time for
dispersion of the feed stream is short in relation to the
total hydraulic residence time in the reactor. It is also
defined as sufficient mixing so that concentration gra-
dients of chemical and biological ingredients are uniform
for the particular reaction rates that exist in the basin.
Mixing within the aerobic digestion tank occurs on two
levels: macromixing and micromixing.43 Macromixing deals
with the bulk mass flow within the digester, while micro-
mixing deals with the degree of intermingling of the
system molecules. In biological theory, "complete mix"
assumes micromixing.44
The actual mixing can be performed by a gas system,
mechanical system or a combination of the two.
59
-------
No matter what type device is utilized, the intent is to
achieve mixing through a pumping action. Because of
this relationship, engineers have come to use the term
horsepower/unit volume as some type of parameter to
define mixing in an aerobic digester. Unfortunately, this
term by itself has no meaning. For mechanical type mix-
ers the wide variation in impeller diameters and speeds
can result in similar horsepower but widely different
pumping capacities. For gas mixing systems gas flow,
depth, and bubble size can also result in similar horse-
power but widely different pumping capacities. In addi-
tion, tank geometry and solids concentrations can signifi-
cantly affect power requirements.
Probably the best way to define mixing is from the
TOP VIEW
PROFILE VIEW
LIQUID HEIGHT
D, = EFFECTIVE ZONE DIAMETER FOR MICROMIXING.
D2 = EFFECTIVE ZONE DIAMETER FOR MACROMIXING.
Figure 3-5.—Shear-stress relationship for a thixotropic
pseudo plastic material.
standpoint of zone of influence of an energy source
(figure 3-5). Essentially the zone of influence states that
energy is dissipated as one moves horizontally away
from the energy source. This loss is due to friction
between the fluid molecules which is a function of liquid
density, temperature, and solids concentration. Within a
certain area of the point source there is sufficient en-
ergy to achieve micromixing. There is also a larger area
where bulk flow (macromixing) still takes place even
though there is insufficient energy for micromixing.
Studies45'46 done with point energy sources, in clean
water and with no side boundaries (only surface and
floor boundaries) have indicated that the width of the
micromixing zone is no more than twice the liquid depth,
with the liquid depth being a function of the type of
mixing device utilized and not necessarily the tank liquid
depth.
The effect of tank geometry47 on mixing (as measured
by oxygen transfer rates in clean water) for various
aeration devices (high and low speed mechanical aera-
tors, submerged turbines, oxidation ditch aerator and
diffused aeration) in tanks from several thousand to 1
million gallons (~10 to 3,800 m3), was shown to fall into
three general categories (figure 3-6).
Category 1 is represented by basin geometry A in
figure 3-6. This is the idealized case in which geometry
has no effect on the liquid flow pattern. Each increment
of power into this specific volume has a corresponding
increase in the oxygen supplied.
Category 2 is represented by basin geometry B in
figure 3-6 and has been termed the "flywheel effect."
BASIN GEOMETRY A
BASIN GEOMETRY B
ENERGY INPUT-
ENERGY INPUT-
BASIN GEOMETRY C
ENERGY INPUT-*-
Figure 3-6.—Schematic of zone of mixing influence for
energy source in fluid with only fixed upper and lower
boundaries.
60
-------
140
~S. 120
E
trt
Q
=i 100
8
o
B 80
5 60
DC
O
1 40
8
S 20
Theoretical form
Table 3-3.—General characteristics of raw primary and
waste activated sludge40
I
I
10 20 30
POWER LEVEL, hp/1 mg
Figure 3-7.—Effects of tank geometry on mixing in
clean water as measured by oxygen transfer rates.
Here tank constraints, represented, for example, by a
channel aeration tank, cause a rapid increase in oxygen
supply for small inputs of energy. As the energy per unit
volume increases, the geometry of tanks causes a level-
ing off in transfer.
Category 3 is represented by basin geometry C in
figure 3-6 and has been termed the "choke flow ef-
fect." Here tank geometry interferes with the mixing pat-
tern until a certain energy level is reached. At this point
there is sufficient energy to override the constraint and
allow for complete mixing in the tank contents.
No published studies on field evaluation of the effect
of suspended solids on mixing in aerobic digesters are
available. There have been several such studies48-50 con-
ducted in lagoons with suspended solids in the range of
100 to 400 mg/l and figure 3-7 shows the results. In
general, increased solids concentrations required in-
creased power levels, though the tank geometry50 and
interaction effects of other aerators49 also influenced mix-
ing patterns.
CHARACTERISTICS OF AEROBIC
DIGESTERS
The existing trend in wastewater treatment is to re-
move more and more material from the main liquid pro-
cessing stream. This is frequently done through the use
of secondary biological treatment schemes, chemical
treatment and filtration. The sludge produced can vary
widely and change rapidly even on an hour-to-hour ba-
sis.
Table 3-3 gives specific gravity and particle size dis-
Primary
sludge
Waste activated
sludge
Specific gravity
Particle size
Physical appearance.
1.33-1.4
20% <1 ju
35% 1-100
45% <100
Fibrous
1.01-1.05
40% 1-50 fim
60% 50-180 jam
Slimy, gelatinous
C/3
111
X
en
<
LU
0 RATE OF SHEAR
Figure 3-8.—Power level versus suspended solids.50
tribution on two common type sludges: plain primary and
plain waste activated.
There is little data on the rheology of municipal waste-
water sludge,40 and none could be found on strictly aero-
bically digested sludge. One of the main problems with
collecting data is that such studies are extremely difficult
to perform correctly.41
Even though the majority of raw wastewater sludges
behave as a thixotropic (time dependent), pseudo plastic
material (figure 3-8), it may not be correct to assume
that the sludge within the aerobic digester has the same
general properties. The liquid will have a variable solids
concentration and there is a general reduction in particle
size and shape,38'43 both of which affect fluid viscosity.
Another characteristic of present-day designs is that
the tanks tend to have large surface area to liquid
depth ratios.
SUPERNATANT
It is common practice in most aerobic digestion facili-
ties not to prethicken the sludge but to concentrate it
61
-------
Table 3-4.—Characteristics of mesophilic aerobic
digester supernatant
Reference 9a Reference 19 Reference 52C
Turbidity
NO3-N
TKN
COD
PCvP
Soluble
BOD5
Filtered BOD5
Suspended solids
AIK
S04
Silica
pH
120
40
115
700
70
50
300
6.8
_
2.9-1,350
24-25,500
2.1-930
.4-120
5-6,350
3-280
9-41 ,800
5.7-8.0
30
35
2-5
6.8
150
70
26
6.8
" Average of 7 months of data.
bRange taken from 7 operating facilities.
°Average values.
The drop in pH is caused by an increased concentra-
tion of nitrate ions and a corresponding loss of alkalinity
due to the conversion of NH3-N to NO3-N commonly
called nitrification. Though at one time, the low pH was
considered inhibitory to the process, it has been shown
that over time the system will acclimatize and perform
just as well at the lower pH values.7'38'51
It should be noted that if nitrification does not take
place, there will be very little, if any, pH drop. This
could happen at low liquid temperatures and short
sludge ages or in thermophilic operation.31 Nitrifying bac-
teria are sensitive to heat and do not exist in tempera-
tures over 45°C.52
BACTERICIDAL EFFECTS
Pathogenic organisms in wastewaters consist of bacte-
ria, virus, protozoa and parasitic worms; a good current
review on the subject can be found in Kenner et al.56
Many of these organisms, especially enteric viruses,54
have a strong tendency to bind themselves to sludge
solids.
after digestion. This is done by sending the flow to a
clarifier-thickener or by turning off the aeration device
within the digester for 12 to 18 hours. When this is
done, a digester supernatant is taken off which is nor-
mally returned to the head end of the treatment plant.
Table 3-4 gives supernatant characteristics from several
full-scale facilities operating in the mesophilic tempera-
ture range.
pH Reduction
Figure 3-9 shows the effect of sludge age on digester
pH for mesophilic operation.
8.0 r-
7.0
6.0
5.0
4.0
3.0
Liquid temp at 5° C
Liquid temp at 20° C
I
I
10 30 50
SLUDGE AGE IN AEROBIC DIGESTERS
70
Table 3-5.—Human enteric pathogens occurring in
wastewater and the diseases associated with the patho-
gen"
,56
Pathogens
Diseases
Vibrio cholera Cholera
Salmonella typhi Typhoid and other enteric fevers
Shigella species Bacterial dysentery
Coliform species Diarrhea
Pseudomonas species Local infection
Infectious hepatitus virus Hepatitis
Poliovirus Poliomyelitis
Entamoeba histolytica Amoebic dysentery
Pinworms (eggs) Aseariasis
Tapeworms Tapeworm infestation
Table 3-6.—Pathogenic organisms in sludge1
,57
Type
Raw primary
Trickling filter
Raw waste activated
sludge
Salmonella
(No./ 100 ml)
460
62
93
74
2,300
6
Pseudomonas
aeruginosa
(No./ 100 ml)
46X103
195
110X103
1.1 X103
24X103
5.5 X103
Fecal
coliform
(No. x
106/100 ml
11.4
11.5
2.8
2.0
26.5
Thickened raw waste
Figure 3-9.—Effects of sludge age on pH for mesophilic activated sludge...
aerobic digestion.
9,300
2X103
20
62
-------
Table 3-7.—Thermophilic aerobic digestion time required
for reduction of pathogenic organisms below minimum
detectable level61
Temper-
Type ature
°C
Time required
for lowest
detectable limit
of salmonella
hours
Time required
for lowest
detectable limit
of pseudomonas
aeruginosa hours
Mixture of primary and
waste activated
45
50
55
60
24
5
1
0.5
24
2
2
0.5
UNSTABILIZED
SOLIDS
>•»
*
AEROBIC DIGESTER #1
AEROBIC DIGESTER #2
1
T
^
STABILIZED SOLIDS
Figure 3-10.—Tank configuration for a batch operated
aerobic digester.
Table 3-5 gives a listing of human enteric pathogens
occurring in wastewater sludges along with the diseases
associated with them. Table 3-6 gives some data on
bacterial concentrations of various types of raw sludges.
Researchers have studied pathogenic organism reduc-
tion in both mesophilic56'58'59 and thermophilic digestion.60
Under mesophilic operation, the bactericidal effects ap-
pear to be related to natural die-off with time. For ther-
mophilic operation, the time required for reduction of
pathogenic organisms below minimal detection level is a
function of basin liquid temperature (table 3-7).
DEWATERING
One of the supposed benefits of aerobic digestion is
the production of a sludge with excellent dewatering
characteristics.1 Much of the published literature on full-
scale operations has indicated this is not true,3'4'17'26'61
though there are some published reports of excellent
operating systems.15
Although most recent investigators agree that there is
a deterioration in dewaterability with increasing sludge
age,2'16'17'27'62 there is still debate as to the cause; lack of
sufficient oxygen26'27 reduction in particle size16'17 or con-
centration of biological anionic polymers.63
At this time it can only be recommended that conser-
vative design be used for designing mechanical sludge
dewatering facilities unless pilot plant data indicate oth-
erwise.
TANK LAYOUTS AND OPERATION
Originally aerobic digesters were operated as strictly a
batch operation and this concept is still used at many
facilities (figure 3-10).
Solids are pumped directly from the clarifiers into the
aerobic digester. Eventually, the digester fills up, and the
time required depends not only on the waste sludge flow
but on the amount of precipitation or evaporation. When
the tank is full, the aeration device is turned off for
several hours to allow solids-liquid separation, then a
decant operation takes place. After decanting, thickened
stabilized solids of about 2 to 4 percent in concentra-
tion, can be removed and more waste sludge can be
added.
Many engineers tried to make the process more con-
tinuous by installing stilling wells in part of the digester.
This has proved not to be effective20'64'65 and should not
be incorporated into the design.
The next step was then to provide the aerobic di-
gester with its own clarifier-thickener (figure 3-11).
Solids are still pumped directly from the clarifiers into
the aerobic digester. In this case the aerobic digester
operates at a fixed level with the overflow going to a
solids-liquid separator. Thickened solids are normally re-
cycled back to the digestion tank but when required can
also be removed from the system.
Though initially more costly than a batch operated
system, much of the manual work involved with aerobic
digestion is eliminated.
A third type of system would involve prethickening
before aerobic digestion. This is employed in the cur-
rently being researched auto thermophilic aerobic diges-
tion system (figure 3-12).
In this system, sludge from the clarifiers would go to
some type of thickening device to produce a concentra-
tion greater than 4 percent solids then into the digester.
When operating in this mode, one should not expect any
UNSTABILIZED
SOLIDS
STABILIZED SOLIDS
Figure 3-11.—Tank configuration for a continuous oper-
ated aerobic digester.
63
-------
CENTRATE
AEROBIC DIGESTER
SOLIDS
STABILIZED
SOLIDS
Figure 3-12.—Tank configuration for an auto thermophil-
ic aerobic digestion system.
Figure 3-13.—A floating low speed aerator in an aero-
bic digester.
Figure 3-14.—A diffused air system in an aerobic diges-
tion tank.
further gravity solids-liquid separation to take place after
digestion (see figures 3-13 and 3-14).
SUMMARY
The basic design criteria for aerobic sludge digestion
systems presented in the previous sections are summa-
64
Table 3-8.—Criteria for design of aerobic digestion sys-
tems
Days Liquid temperature
Sludge age required to achieve
40 percent volatile solids reduction.
55 percent volatile solids reduction.
108 4.4°C(40°F)
31 15.5°C(60°F)
18 26.7° C (80° F)
386 4.4° C (40° F)
109 15.5°C (60°F)
64 26.7° C (80° F)
Oxygen requirements Liquid temperature 45° C or
less; 2.0 IDS. oxygen/Ib
volatile solids destroyed
Liquid temperature greater
than 45° C; 1.4 Ibs.
oxygen/lb volatile solids
destroyed
Oxygen residual 1.0 mg O2/l at worst condi-
tions
Expected maximum solids concentration 2.5 to 3.5 percent solids
achievable with decanting (degritted sludge)
Mixing horsepower Function of tank geometry
and type of aeration
equipment utilized
rized in table 3-8. Obviously, operational criteria will
vary with the quantity and biodegradability of material to
be stabilized, as well as temperature and other critical
parameters.
DESIGN PROBLEM
Two designs, a 4 Mgal/d (.18 m3/s) and 40 Mgal/d
(1.75 m3/s), are evaluated. Influent is typical domestic
wastewater of 200 mg/liter biochemical oxygen demand
(BOD5) and 200 mg/liter suspended solids (SS) with no
heavy industrial contributors. Liquid treatment consists of
grit removal, primary treatment, secondary treatment (ac-
tivated sludge) and chlorination. No chemicals are added
to liquid treatment portion.
Sludge Type and Amount
Every million gallons (3,785 m3) of raw plant influent
will generate approximately 1,000 Ibs. (453.6 kg) of dry
primary sludge and 1,000 Ibs. (453.6 kg) of waste-acti-
vated sludge solids.67 Table 3-9 shows how this can be
further broken down.
Based on table 3-9 the sludge generated for the two
design examples would be
Inert nonvolatile
Inert volatile
Biodegradable volatile.
Total
4 Mgal/d design
(Ibs)
4X550 = 2,200
4X510 = 2,040
4X940 = 3,760
4X940 = 8,000 40x940 = 80,000
40 Mgal/d design
(Ibs)
40X550 = 22,000
40X510 = 20,400
40X940 = 37,600
-------
Table 3-9.—Breakdown of inert and volatile suspended
solids per mg of plant influent (Ibs)
Inert Inert Biodegradable
nonvolatile volatile volatile
Primary sludge
Waste activated sludge
Totals
250
300
550
300
210
510
450
490
940
Temperature Effect
Temperature in the aerobic digestion process:
• Affects oxygen transfer capabilities.
• Affects volatile destruction capabilities.
Temperature in aerobic digester is a function of:
• Feed solids concentration.
• Geographical location of treatment facility.
• Tank location and material of construction.
• Type of aeration device utilized.
For this design example the following assumptions will
be made:
• Thermophilic or auto-thermophilic aerobic digestion
will not be considered. This implies average inlet
feed solids to digester under 3.5 percent solids
concentration.
• Lowest liquid temperature expected during winter is
10°C (50° F). During the summer 25.5° C (78° F) is
expected.
quire a temperature-sludge age combination of 475 days.
At the minimum liquid temperature of 10°C., this would
imply a sludge age of 47.5 days. If the system is de-
signed to maintain a 47.5-day sludge age, then during
the summer this combination would be 47.5x25.5 =
1211°C-days. This would give a 49 percent reduction.
Table 3-10 gives various ratios which could be devel-
oped.
Expected Suspended Solids Concentration
in Aerobic Digester Underflow
This is a function of overall detention time, local evap-
oration rate and type of aerobic digestion system em-
ployed (batch or continuous).
Aerobically digested sludge, typically degritted with no
chemical addition, can be gravity thickened to 2.5 to 3.5
percent. For this design a maximum of 3.0 percent is
assumed.
If there is no prior thickening of the raw sludges so
that the average inlet feed solids concentration is under
3.0 percent, then gravity thickening is possible. For this
example, the inlet feed solids concentration for the com-
bined sludge is assumed to be 1.5 percent solids (based
on 4 percent sludge from the primary clarifier and 1
percent sludge from the secondary clarifier).
Oxygen Requirements
Since it is assumed that these design examples would
not be designed for thermophilic aerobic digestion, nitrifi-
cation oxygen demand must be met. From previous dis-
cussions and for design purposes, 2.0 Ibs of oxygen will
be considered as the amount required to oxidize a
pound of cell mass (table 3-11).
Expected Type of Volatile Solids
Destruction
Figure 3-3 showed a plot of volatile suspended solids
destruction as a function of liquid temperature and
sludge age. A minimum of 40 percent VSS reduction has
been chosen for the design example which would re-
Minimum Tank Volume Necessary To
Achieve Desired Results
It was previously noted that a minimum volatile sus-
pended solids reduction of 40 percent was required at
the 10°C liquid level. Based on figure 3-3 this would
imply a minimum sludge age ot 47.5 days.
Sludge age in aerobic digester can be approximated
as follows:
Sludge age =
total Ibs SS in aerobic digester
total Ibs SS lost per day from aerobic digester
total Ibs SS in aerobic digester
(total Ibs SS lost per day in supernatant) + (total Ibs SS wasted per day from system)
(SS cone, in digester)(8.34)(digester tank volume)
[(SS cone, in supernatant)(1 - f) + (SS cone, in underflow)(f)] (8.34)(influent flow)
where:
(influent SS cone.)(percent solids not destroyed)
thickened SS cone.
SS cone, in supernatant—if good solids liquid separa-
tion takes place can expect about 300 mg/l SS in su-
pernatant.
65
-------
Table 3-10.—Various calculated results for volatile suspended solids de-
struction in aerobic digester
4 Mgal/d
design
40 Mgal/d
design
Lbs volatile suspended solids (VSS) destroyed
per day
Winter
Summer
Percent of total solids destroyed
0.4 (2,040+ 3,760) = 2,320 23,200
0.49 (2,040+ 3,760) = 2,842 28,420
Winter
Summer
Percent of biodegradable VS destroyed
Winter
Summer
Original inlet feed VSS/TS
Final VSS/TS
Winter
Summer
2'320XlOO-29%
8,000
X 1 00 35 5%
8,000 J?-^
o oon
X 100 — 61 2%
3,760 01. -A.
X 100 -75 5%
3,760 's.jTo
5'8°°X100 725%
'" 8,000
5,800-2,320
8 000
5,800-2,842
8,000 Jo.u«
29%
355%
61 2%
755%
725%
435%
369%
Table 3-11.—Average pounds of oxygen required per
day for aerobic digestion system
4 Mgal/d design 40 Mgal/d design
Winter 2.0X2,320 = 4,640 46,400
Summer 2.0X2,842 = 5,684 56,840
SS cone, in digester—can range from a minimum
equal to the influent SS concentration to a maximum
equal to the thickened concentration (assume no evapo-
ration). Assume that on the average SS cone, equal to
70 percent of the thickened concentration.
Digester tank volume—million gallons.
For 4 Mgal/d design
Sludge age =47.5 days
SS cone, in digester =(0.7)(30,000 mg/l)
SS cone, in supernatant = 300 mg/l
SS cone, in underflow =30,000 mg/l
47.5 -
(0.7)(30,000)(tank vol)
f
Influent flow =
3.0%
8,000
= 0.35
(0.15)(8.34)
= 63,950 GPD
= 0.06395 Mgal/d
(300X1 - .35) + (30,000)(.35)(0.06395)
_ (21.000 tank vol.)
697
Digester tank volume = 21 000 1-576 m9
Tank geometry function of site location and type of
aeration equipment to be utilized.
For 40 Mgal/d design
Everything the same except for influent flow which =
0.6395.
Tank volume = 15.76 mg.
In addition to the tank volume calculated, additional
volume may be required depending on local weather
conditions and type of downstream sludge-handling facili-
ties.
Tank Layout
For the mesophilic aerobic digestion system being con-
sidered, there are two types of systems to choose from:
the batch operated system (figure 3-10) or the continu-
ous flow through system (figure 3-11).
The original aerobic digestion systems were batch op-
erated; this is still the most prevalent design (figure 3-
10).
Solids are pumped directly from the clarifiers into the
66
-------
aerobic digester. The time required for the tank to fill up
depends not only on the waste sludge flow but the
amount of precipitation or evaporation. When the tank is
full, the aeration device is turned off for several hours to
allow solids-liquid separation, then a decant operation
takes place. After decanting, thickened stabilized solids,
about 3 percent, can then be removed or more waste
sludge would be added.
In the past, many engineers have tried to make this
design more continuous by installing stilling wells in part
of the tank. This has proved not to be effective20'64'65 and
should not be incorporated into the design.
For the continuously operated system, solids are
pumped directly from the clarifiers into the aerobic di-
gester. In this case, the aerobic digester operates at a
fixed liquid level with the overflow going to a solids-liq-
uid separator. Thickened solids are normally recycled
back to the digestion tank but when required can also
be removed from the system.
Though initially more costly than a batch operated
system, much of the manual work involved with aerobic
digestion is eliminated.
Another consideration when sizing the aerobic diges-
tion tank is the relationship between the tank geometry
desired, the type of aeration equipment being utilized,
and the mixing pattern that will develop. An example of
an aerobic digester with a mechanical aerator is shown
in figure 3-13 and one with diffused aeration equipment
is shown in figure 3-14. Figure 3-6 shows the effect of
tank geometry on mixing as measured by oxygen trans-
fer rates.
Assume power cost at $0.03/kwh ($0.83/mJ), no pac-
ing device on the aeration equipment and that oxygen
demand is uniform over 24 hours per day.
Design to handle peak conditions (summer conditions).
For 4 Mgal/d (.18 m3/s) was 5,684 Ibs oxygen/day
(236.8 Ibs O2/hr) (107.4 kg/hr). For optimum tank geom-
etry power bill would amount to $23,225/year. For non-
optimum design power bill could get as high as $38,-
700/year.
Note that winter conditions use less oxygen, 4,640
Ibs/day (193.4 Ibs/hr) (87.7 kg/hr). Using a pacing de-
vice, savings of $3,500 to $5,900/year in power cost
could be realized.
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ties" Journal Environmental Engineering Division ASCE, vol. 103,
No. 5, October p. 815 (1977).
Paredes, M., "Supernatant Decanting of Aerobically Digested
Waste Activated Sludge" Journal WPCF Deeds and Data, October
(1976).
Ritter, L., "Design and Operating Experiences Using Diffused Aer-
ation for Sludge Digestion" Journal WPCF, vol. 42, No. 10, p.
1782 (1970).
Kormanik, R. A., "Estimating Solids Production for Sludge Han-
dling" Water and Sewage Works, December (1972).
68
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Chapter 4
Thermal Treatment for Sludge Conditioning
INTRODUCTION
The purpose of this chapter is to consider thermal treat-
ment of sludge as a conditioning process to improve
sludge dewaterability by subsequent processes such as
vacuum filter, centrifuge or filter press. Thermal condi-
tioning (also often called heat treatment) involves heating
sludge, with or without the addition of air or oxygen, to
temperatures of 300 to 500° F (150 to 260° C) in a
reactor under pressures of 150 to 400 psig (10.5 to
28.1 kgf/cm2) for periods of 15 to 40 minutes. Thermal
conditioning causes the release of water and organic
material from sludge in the form of a dark brown fluid
or "cooking liquor."
Other thermal treatment processes not discussed
herein include: (1) pasteurization, which operates at low-
er temperatures, in the range of 160°F, and (2) wet air
oxidation, which operates at higher temperatures and
pressures for more complete oxidation of sludge solids.
The EPA Technology Transfer manual on sludge treat-
ment1 describes thermal conditioning, or heat treatment,
as follows:
In heat treatment, temperatures of from 300 to 500° F and pressures
of 150 to 400 psig are attained for protracted periods. Significant
changes in the nature and composition of wastewater sludges result.
The effect of heat treatment has been ideally likened to syneresis, or
the breakdown of a gel into water and residual solids. Wastewater
sludges are essentially cellular material. These cells contain intracellular
gel and extracellular zoogleal slime with equal amounts of carbohy-
drate 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.
Dewatering is improved by the solubility 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 or-
ganic acids, sugars, polysaccharides, amlno 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/l,
COD = 10,000 to 30,000 mg/l, Ammonia = 500 to 700 mg/l, and Phos-
phorus as P = 150 to 200 mg/l. About 20 to 30 percent of the COD Is
not biodegradable in a 30-day period. The volume of cooking liquor
from an activated sludge plant with heat treatment amounts to 0.75 to
1.0 percent of the wastewater flow. Based on BOD5 and solids load-
ings, 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.
Figure 4-1 is a flow diagram for a typical heat treat-
ment system. Major components in the system are a
heat exchanger and a reaction vessel. Heat treatment
STEAM
DECANT
LIQUOR
OFR GAS
_. .
DEWATER-
ING
LIUU
»•
i
CAKE
Figure 4-1.—Typical heat treatment system.
may be used to condition raw or digested sludges and
thus location of the system in the overall treatment train
may vary. If a treatment plant employs anaerobic diges-
tion, heat treatment is more commonly used to condition
the digested sludge. Heat treatment before anaerobic
digestion to improve degradability and energy production
was pilot tested by LA/OMA in Los Angeles.2'3 Heat
treatment may be used in conjunction with incineration ir
a system that recycles waste heat to minimize energy
requirements. These variations in the use of heat treat-
ment in sludge management systems are illustrated in
figure 4-2.
The effect of heat treatment on the chemical composi-
tion of sludge was investigated by Sommers and Curtis.4
Heat treated sludges from plants in Speedway and Terrc
Haute, Indiana were tested to obtain information on the
forms of nitrogen, phosphorus, copper, zinc, nickel, cad-
mium and lead. In general, heat treatment produced
greater than 50 percent reductions in total nitrogen with
essentially no change or a slight increase in phosphorus
and metals concentrations.
69
-------
CONVENTIONAL
SYSTEM
PRIMARY a/OR
WASTE BIOLOGICAL SLUDGE
CAKE
DECANT LIQUOR
DECANT LIQUOR
LA / OMA
SYSTEM
' PRIMARY 8/OR _
WASTE BIOLOGICAL SLUDGE
HEAT
THICKEN
1
1 THERMAL
\ TREATMENT
ANAEROBIC
DIGESTION
DEWATER
I
CAKE
DECANT
LIQUOR
DECANT LIQUOR
ENERGY
RECOVERY
SYSTEM
Figure 4-2.—Heat treatment in sludge management systems.
RAW a/OR
WASTE BIOLOGICAL SLUDGE
THICKEN
DECANT
LIQUOR
WASTE HEAT
DECANT
LIQUOR
ASH
PROCESS DESCRIPTION
Equipment for thermal conditioning of sludge is manu-
factured and supplied in the United States by Envirotech
BSP (Porteous System), Zimpro (wet oxidation), Zurn,
and Nichols. Almost all of the equipment for thermal
conditioning of sludge in the United States has been
supplied by Zimpro or Envirotech. Mayer and Knopp4
reported in January 1977, that 70 thermal conditioning
plants were operating in the United States and Canada
and 43 others were under construction.
Table 4-1.—Size and status of largest thermal condition-
ing installations
Type of plant
With air addition
Operating
Under construction
Without air addition
Operating
Under construction
Number of installations
45
35
25
8
A partial list of thermal conditioning installations is
shown in table 4-1.
Zimpro Process
The Zimpro system is similar to the process illustrated
in figure 4-1 except that air is also added to the reac-
tor. Basic features of the Zimpro process are (1) air
addition to the reactor for oxidation, improvement of
heat exchange characteristics and reduction of fuel re-
quirements, and (2) use of sludge-to-sludge heat ex-
changer. Some of the equipment used in this process is
shown in figures 4-3 and 4-4.
In the continuous process, the sludge is passed
Location
Toronto, Ontario (Ashbridges Bay)...
Cleveland Ohio (Southerly) .
Louisville Ky
Cincinnati, Ohio (Mill Creek)
Flint Mich
Green Bay, Wis
Columbus Ohio (Southerly)
Suffolk Co NY
Toronto, Ontario (Lakeview)
Springfield, Mass
Kalamazoo, Mich
Columbus Ohio
Toronto, Ontario (Highland Creek) . . .
Chesapeake-Elizabeth, Va
Hopewell, Va
York Pa
Billings Mont
Escondido Calif
I
Status
uca
UC
Operating
(1976)
Operating
Operating
Operating
(1975)
Operating
(1976)
UC
Operating
(1975)
UC
Operating
(1971)
Operating
(1972)
UC
UC
UC
UC
UC
UC
Number
of
units
7
5
4
4
3
4
3
2
3
2
3
1
3
1
3
2
2
1
Unit
capacity
(gal/min)
250
280
250
280
250
150
200
205
125
200
125
200
125
150
150
125
100
100
Under construction.
70
-------
Figure 4-3.—Reactor (left), heat exchangers (center),
waste heat recovery boiler (right).
Figure 4-4.—Pump (left), grinder (right).
through a grinder which reduces the size of sludge par-
ticles to about one-fourth inch. Sludge and air are then
pumped into the system and the mixture is passed
through heat exchangers and brought to the initial reac-
tion temperature. As oxidation takes place in the reac-
tor, the temperature increases. The oxidized products
leaving the reactor are cooled in the heat exchanger by
the entering cold sludge and air. The gases are sepa-
rated from the liquid carrying the residual oxidized sol-
ids, usually in a decant tank, and released through an
odor control unit. The oxidized liquid and remaining sus-
pended solids are separated in a decant tank. The de-
cant tank underflow may be further dewatered by sev-
eral methods; the overflow cooking liquor is recycled to
the main plant or treated by a separate system such as
activated sludge, rotating biological disk or anaerobic
filter.
Envirotech BSP Process
This system was formerly called the Porteus process.
The Porteous process was purchased by Envirotech and
various changes have been made in the system. The
basic system components and operation of the BSP sys-
tem are similar to the Zimpro process as illustrated in
figure 4-1. One basic difference is that air is not inject-
ed into the reactor in the BSP system. The BSP systems
also normally employ a water-to-sludge heat exchanger.
Other Processes
The Nichols heat treatment system was previously
marketed as the Dorr-Oliver Farrer system. The Nichols
process is used at a plant serving York, Pa., and there
are five installations of the Farrer system in the United
States: San Bernardino, Calif.; Elkhart, Ind.; Port Huron,
Mich.; Glouster, N. J.; Norwalk, Conn. There is a Zurn
heat treatment system in Mentor, Ohio which serves an
area of Lake County, Ohio.
Thermal Treatment Process Sidestreams
There are both liquid and gas byproducts from any
thermal conditioning system. These sidestreams must be
considered in planning for an accurate comparison with
other processes and in design for a properly operating
system.
Gas Sidestreams
There are four principal sources of odor resulting fron
thermal sludge treatment: (1) vapors from treated sludge
storage (decant tank or thickener), (2) mechanical dewa
tering system exhaust, (3) exhausted air from working
atmosphere in filter and loading hopper areas, and (4)
vapors from strong liquor pretreatment devices. The
odorous gases produced are simple, low molecular
weight, volatile organic substances, consisting of aldeh-
ydes, ketones, various sulphurous compounds, and or-
ganic acids. The odor level source associated with ther-
mal sludge conditioning is dependent to a high degree
71
-------
on the total hydrocarbon content. The odor level and
hydrocarbon content are highest in off-gases from the
heat treated sludge solids separation units, i.e., decant
tank or thickener and mechanical dewatering systems.
Off-gases are best controlled by use of incineration,
adsorption, or scrubbing (or some combination of these
processes).
Water scrubbing plus incineration.—For high hydrocar-
bon airstreams, the highest degree of odor control can
be obtained by water scrubbing followed by incineration.
The scrubbing portion of this system consists of a
packed bed unit which uses plant effluent water at rates
of 20 to 30 gpm (1-3 to 1.9 l/s) per 1,000 ftVm (472.0
l/s). The incineration portion of this system can be ei-
ther direct flame incineration at 1,500°F (815°C) or cat-
alytic incineration at 800° F (427° C). The oxidation cata-
lysts that are commonly used in catalytic incineration are
supported platinum or palladium materials.
Water scrubbing plus adsorption.—In scrubbing meth-
ods, the odorous substances are removed by solubiliza-
tion, condensation, or chemical reaction with the scrub-
bing medium. Scrubbing media that are commonly used
for odor control are potassium permanganate, sodium
hydroxide, or sodium hypochlorite. Two to four pounds
of potassium permanganate are required per pound of
hydrocarbon removed. In the adsorption method, sub-
stances are removed from the odorous gas stream by
adsorption on activated carbon or silica gel. The activat-
ed carbon or silica gel must be capable of regeneration,
usually by steaming. High hydrocarbon sources can be
treated in an odor control system composed of a water
scrubber followed by an activated carbon adsorption
unit. The water scrubber is the same as that described
above. The carbon adsorption 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 ffVmin (472.0 l/s)
gas stream would require a dual bed carbon system
containing 1,800 pounds (816 kg) of carbon per bed.
This sizing would permit an adsorption cycle of 24
hours. After a 24-hour adsorption time, the second car-
bon bed would be placed in the adsorption cycle and
the spent bed would be steam regenerated. The regen-
eration cycle requires low pressure steam at a maximum
of 50 psig (3.5 kgf/cm2) 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.
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 system should be water scrub-
bing using plant effluent at a rate of about 27 gal/min
(1.7 l/s) per 1,000 ftVmin (472.0 l/s). The second and
third stages should be chemical scrubbing using a com-
bination of scrubbing media selected from 5 percent
sodium hydroxide, 3 percent sodium hypochlorite, and 3
percent potassium permanganate. The potassium perman
ganate solution effects the highest degree of hydrocar-
bon reduction and, hence, the highest odor reduction.
One of the most effective multiple scrubber systems con
sists of three stages utilizing plant effluent, 5 percent
sodium hydroxide and 3 percent potassium permanga-
nate.
Liquid Sidestreams
The liquid (cooking liquor) containing materials solubi-
lized during heat treatment of sludge may be separated
from the solids (1) during storage in decant tank, thick-
ener, or lagoon, and (2) in the dewatering step using a
vacuum filter, centrifuge, filter press, sand drying bed or
other method.
The following tabulation shows some of the substances
present in thermal treatment liquor and the general
ranges of concentration.
Constituent
Suspended solids
Chemical oxygen demand
Biochemical oxygen demand.
Ammonia nitrogen
Phosphorus
Color, units
Concentration range
mg/l (except color)
100-20,000
10,000-30,000
5,000-15,000
500- 700
150- 200
1,000- 6,000
The composition of thermal treatment liquor varies
widely depending upon sludge composition and reactor
operating conditions. For a given reactor temperature,
increasing the reactor detention time will generally in-
crease the concentration of soluble organic material in
the cooking liquor. Heat treatment can normally be ex-
pected to solubilize from 40 to 70 percent of the sludge
biomass. As much as 60 to 70 percent of the suspend-
ed solids in waste activated sludge were solubilized in
heat treatment pilot tests in Los Angeles.6
The character of the cooking liquor is somewhat un-
certain and the subject of some debate. The EPA
Sludge Manual1 states: "About 20 to 30 percent of the
COD is not biodegradable in a 30-day period." Based
on pilot scale heat treatment studies of mixed primary
and waste activated sludge, Erickson and Knopp7 esti-
mated that the soluble nonbiodegradable COD was 7
percent of the total cooking liquor COD. Laboratory
tests by Stack, et al.,8 indicated that about 40 percent of
organics in the cooking liquor from heat treatment of
waste activated sludge were resistant to biological oxida-
tion.
The EPA Sludge Manual further states: "Based on
BOD5and solids loadings, the liquor can represent 30 to
50 percent of the loading to the aeration system." Boyle
and Gruenwald9 reported that the heat treatment recycle
liquor BOD contributed approximately 21 percent of the
BOD entering the Colorado Springs, Colorado plant. Stu-
dies by Haug, et al.,6 indicated that recycle of cooking
liquor in the Hyperion plant at Los Angeles would in-
crease the oxygen demand on the aeration system by
about 30 percent.
72
-------
Thermal treatment liquor can be treated by recycle to
the main treatment plant or by separate treatment sys-
tems such as activated sludge, rotating biological disks
or anaerobic filters.
Recycle to main plant.—Thermal treatment liquor often
is recycled through the main treatment plant, being add-
ed 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 and COD of the final effluent will
increase. The effects of recycle can be mitigated to
some extent by storing thermal treatment liquor and re-
turning it to the treatment plant at a uniform rate or
during off-peak hours.
Separate treatment and disposal.—Another method for
handling liquor is to treat the sidestreams separately
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 method is
the installation at Indio, Calif, where aerated lagoons
with long retention provide satisfactory results. Lagoon
effluent is blended with plant effluent for discharge.
Separate treatment prior to recycle.—In order to re-
duce the load on the main treatment plant and maintain
final effluent quality, cooking liquor is often treated sepa-
rately prior to recycle to the main plant. Plain aeration,
extended aeration, and activated sludge treatment have
been used for pretreatment of cooking liquors. BOD
reductions by conventional activated sludge pretreatment
of liquors have been reported as high as 90 percent. It
may be necessary to collect and deodorize aeration
basin off-gases.
THERMAL CONDITIONING COSTS
Thermal conditioning of sludge affects the cost of
other treatment plant processes, decreasing some and
increasing others. Total cost includes direct capital, op-
erating, and maintenance costs for sludge handling plus
or minus the indirect net cost effect of sludge handling
on other treatment plant processes. Added costs result-
ing from heat treatment include: (1) cooking liquor treat-
ment, and (2) treatment of odorous off-gases. Potential
cost savings include reduction in subsequent sludge
treatment and disposal costs because of improved
sludge dewatering.
An EPA10 report presents detailed cost estimates for
thermal conditioning and sidestream treatment. Costs
were based on data from several sources including op-
erating plants, published literature, manufacturers data
and engineering estimates. The following cost information
was developed for thermal conditioning systems (does
not include costs for sidestream treatment):
1. Capital costs for thermal systems vary from about
$50,000 per gal/min ($790,000 per l/s) of thermal
treatment system capacity for a 10 gal/min (.6 l/s)
system to $10,000 per gal/min ($159,000 per l/s)
for a 200 gal/min (12.6 l/s) system.
2. Typical fuel requirements are 900 to 1,000 Btu per
gallon (249 to 277 kJ/l) for systems that do not
employ air addition and 300 to 600 Btu per gallon
(83 to 166 kJ/l) with air addition.
3. Average electrical energy consumption averaged 22
kWh per 1,000 gallons (209 J/l) for plants with air
addition and 10 kWh per 1,000 gallons (95 J/l)
without air addition.
4. Operation and maintenance labor constitutes a sig-
nificant fraction of overall costs, ranging from 6,000
hours per year for a 10 gal/min (.6 l/s) system to
20,000 hours per year for a 200 gal/min (12.6 l/s)
system.
5. Costs for materials and supplies range from $5,000
per year for a 10 gal/min (.6 l/s) system to
$20,000 per year for a 200 gal/min (12.6 l/s) sys-
tem.
The following cost information is related to indirect
costs for treating sidestreams:
1. Increased capital costs primarily result from the
need to increase aeration tank volume and air sup-
ply capabilities.
2. Increased energy is required for aeration capacity
required to treat the recycled liquor.
3. Increased labor is required for maintaining and op-
erating the additional aeration capacity and related
settling and pumping systems.
Costs for treating the off-gas from the thermal treat-
ment system typically constitutes 5 to 10 percent of the
total cost for thermal treatment. Carbon adsorption is
the most costly technique for odor control. Incineration
is most economical in smaller plants and chemical scrub-
bing in larger plants.
Based on unit costs of $7 per hour for labor, $0.03
per kWh for electricity, and $2.80 per million Btu and
amortization of capital costs over 20 years at 7 percent
interest, the following typical costs for thermal condition-
ing were determined (all costs are dollars per ton of dry
solids processed):
Construction costs
O. & M. cost
Sludge
ton/day Direct Indirect Total Direct Indirect Total Total
1....
5....
10....
50....
100....
97.53
30.79
21.45
12.20
10.96
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
256.7"
84.11
60.4(
36.1 1
32.3!
The March 1975 national average construction costs
for thermal conditioning are shown on figure 4-5. These
costs include feed pumps; grinders; heat exchangers;
reactors; boilers; gas separators; air compressors where
applicable; decanting tanks; standard odor control sys-
tems; and piping, controls, wiring and installation serv-
ices usually furnished by the equipment or system manu-
facturer. Not included in the basic thermal treatment
costs are buildings: footings; piping; electrical work and
utilities not supplied by the equipment manufacturer;
73
-------
sludge storage and thickening prior to thermal treatment;
sludge dewatering, incineration or diposal; land; and en-
gineering fees. In escalating costs for 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, equipment-domi-
nated processes such as thermal treatment.
A second curve (curve B) is shown on figure 4-5 and
includes the costs for typical building, foundation and
utility needs for thermal treatment systems. The building
costs represent single-story, concrete or masonry con-
struction with built-up roofing, insulation and heat and
vent systems, and assume that reactors and decant tank
will be located outside of the building. The costs also
include piping and wiring within the building, foundations
for internal and external equipment, and limited amount
of site work. Building sizes provide for easy access to
equipment and control room. For larger installations,
where multiple units are anticipated, space for some
standby equipment is included. Typical building sizes
range from 1,500 square feet (139 m2) for a 10 gal/min
(.6 l/s) plant to 5,250 square feet (488 m2) for a 200
gal/min (12.6 l/s) plant. The construction cost of the
building was estimated to be $36/ft2 ($387/m2).
The curves show a rapid rise in unit construction
costs for plants smaller than about 20 gal/min (1.3 l/s).
The minimum direct cost of a thermal treatment plant is
estimated to be approximately $350,000 regardless of
plant size. For plants above about 150 gal/min (9.5 l/s)
the increased use of multiple treatment units and of
standby units results in a lower limit for unit cost per
gal/min of capacity. This lower limit appears to be in
the range of $9,000 to $12,000 per gal/min ($143,000
to $190,000 per l/s). Data for these larger plants are
sparse, however, and some plants reported lower unit
costs.
The annual fuel requirements based on 8,000 hours ol
operation are shown in figure 4-6. Fuel is used chiefly
as a source of heat to produce steam. The amount of
fuel used is influenced by the reaction temperature, effi-
ciencies of the boiler and heat exchange systems, insu-
lation or heat losses from the system and the degree of
heat-producing oxidation which takes place in the reac-
tor. Some reduction in the unit heat requirement for
increase in plant size is reported. This is believed 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 heat exchangers and reactors
experiencing some oxidation have lower fuel require-
ments.
Typical fuel requirements averaged 900 to 1,000 Btu
per gallon (249 to 277 kJ/l) for plants not practicing air
addition and 300 to 600 Btu per gallon (83 to 166 kJ/l)
depending on the degree of oxidation obtained, for
plants practicing air addition. Curves in this paper are
based on fuel requirements of 900 Btu per gallon (249
kJ/l) for thermal conditioning without air and 500 Btu
e
7
6
5
4
3
2
,000
9
8
7
G
5
4
3
2
100
I
7
6
3
4
3
2
10
--- --
-
—
_-_
—
«---
"
4i
C~
^^^.
-; r- — ^
:URV
^,
*s
CURVE A-
CURV'E B-
1
DATA
POIh
—
-f-
E B
4
_•
THI
INC
AN[
i 1
TS
^ i
4
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i
RMAL T
LUDES B
FOUND
iRE FOR
—
2 3456789 2 34567
f^1
&
ii * — c
f
-URV
<>
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i A
SEATMENT SYSTEM ONL
JI'L'OINGS, UTILITIES'
CURVE A -
—
—
i-
i
4
Y
-4-
-
-
-
9 2 3456783
i,ooo,ooo
10
100
1,000
THERMAL TREATMENT CAPACITY, GPM
Figure 4-5.—Direct construction costs for thermal condi-
tioning.
,100,000
CURVE A-THERMAL CONDITION! G
CURVE B- AIR ADDITION
THERMAL TREATMENT CAPACITY, GPM
Figure 4-6.—Annual direct fuel requirements for thermal
conditioning.
74
-------
per gallon (139 kj/l), corresponding to about five per-
cent oxidation, plants with air addition. These fuel re-
quirements do not include allowances for treatment of
off-gas.
Annual electrical energy usages for the two types of
plants (with and without air addition) are shown in figure
4-5. A separate curve is included on figure 4-7 for
estimating the energy requirements for building needs.
Electrical energy requirements are determined by sizes
and efficiencies of machinery such as sludge and boiler
water pumps, grinders, thickeners and, in plants where
air addition is practiced, air compressors. Electrical ener-
gy is also required for lighting and other building uses.
Average unit energy requirements are 22 kWh per 1,000
gallons (209 J/l) for plants practicing air addition and
10 kWh per 1,000 gallons (95 J/l) for plants without air
addition.
Operation and maintenance labor requirements are
shown in figure 4-8. In this paper operation comprises
time spent collecting and logging data on the process,
controlling and adjusting the various systems and com-
ponents, and laboratory work. The functions covered by
maintenance include cleaning and repairing process com-
ponents, general upkeep of the process area, checking
and repairing of controls and instrumentation, and per-
forming preventative maintenance. Maintenance in figure
4-8 does not include major overhauls which will be re-
quired periodically. In some plants these operation and
maintenance functions may vary or may overlap.
100,000
9
a
7
6
5
4
10,000
c
£
7
s I
100
OPERATION -—,
MAINTENANCE
4 56789
5 6 7 B9
100
4 56789
1,000
THERMAL TREATMENT CAPACITY, GPM
Figure 4-8.—Operating and maintenance labor require-
ments for thermal conditioning.
CURVE A-AIR ADDITION
CURVE B-THERMAL CONDITIONING
CURVE C- BUILDING AND SITE NEEDS
1,000
THERMAL TREATMENT CAPACITY, GPM
Figure 4-7.—Annual direct electrical energy requirements
for thermal conditioning.
In general, maintenance labor is approximately one-
fourth of operating labor, ranging from the equivalent of
one maintenance man for one shift at a 50 gal/min (3.2
l/s) plant to one and one-half men for one shift at a
200 gal/min (12.6 l/s) plant. The amount of maintenance
required depends greatly on the design and operation of
the plant, particularly on equipment and materials used
for construction. It is also dependent on the skill and
knowledge of the maintenance personnel and the design
of, and adherence to, a preventative maintenance pro-
gram.
Annual costs for materials and supplies are shown in
figure 4-9. Curve A shows the normal annual costs for
materials and supplies required to operate and maintain
the thermal conditioning system. These costs are plotted
against thermal treatment plant capacity and include ma-
terials and parts such as seals, packing, coatings, lamps,
bearings, grinder blades, and other items used in sched-
uled and normal maintenance. They also include operat-
ing supplies such as lubricants, cleaning chemicals, boil-
er feed water, and water treating chemicals. These costs
vary from about $5,000 per year for a 10 gal/min (.6
l/s) plant to approximately $20,000 per year for a 200
gal/min (12.6 l/s) plant.
Besides normal, periodic maintenance required for a
plant shown by curve A, additional costs for major over-
haul work are incurred. This work includes such items
as motor rewinding; major overhauls of pumps and com-
75
-------
1,000,000
9
8
2
\ 00 OQQ
e
5
4
3
2
10,000
I
7
6
2
1,000
,
- - -
- - i-
" t
--
—
h-
—
-
-
—
-
— H
—
--I
—
—
- _ _
— —
\
i — "
1
CURVE A
NORMAL ANNUAL COST
,.~ ...
CURVE B ANNUAL COST WITH ALLOWANCE FOR
... 1
• - -- -
- -- --
~ -
B —
^•^^^
^ _^^pZ
t1"
:
PERIODIC OVERHAUL
.—
>-
f
-'
-
-
"
_
^
-
-..
--
--
---
^
--
_...
- j
-i
•'^/\
'<^A
:_rzrr
-
-
X
--
—
X
X
—
—
-
-
-
-
_ _ ._
- --
----
' :::
10,000
TREATMENT PLANT FLOW, MGD
10 20 50 100
10 100
THERMAL TREATMENT CAPACITY, GPM
1,000
Figure 4-9.—Materials and supplies for thermal condi-
tioning.
3 456789
1,000
THERMAL TREATMENT LOADING, TONS/DAY
Figure 4-10.—Direct and indirect costs for thermal con-
ditioning.
pressors; major non-routine rehabilitation or replacement
of heat exchanger tubing piping and controls; and refit-
ting of boilers. This type of work is required at an
average interval of about 6 to 7 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 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 the materials and supplies.
The inclusion of major overhaul work increases the an-
nual materials cost by about 45 percent over that re-
quired for routine and preventative maintenance mate-
rials.
There was considerable variation among the costs for
materials in seemingly similar plants and it appeared that
three factors tended to govern the costs.
1. Preventative maintenance program. In plants where
a good program was practiced, overall costs for
parts supplies generally were lower. Where mainte-
nance was neglected, more major failures were
found to occur with a need for greater expenditure
for parts.
2. 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 mainte-
nance in mind, less maintenance and better mainte-
nance were found and hence less need for replace-
ment was noted.
3. Quality of the water supply. In areas with high
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 re-
quired, resulted in both an increase in replacement
parts for boilers and heat exchangers and an in-
creased amount of chemicals for boiler water treat-
ment and heat exchanger cleaning.
Total costs for thermal conditioning systems, with air
addition, including costs for treatment of cooking liquor
and odorous gas sidestreams are shown in figure 4-10.
Costs in figure 4-10 are based on the following:
1. Cooking liquor treated in the main plant by increas-
ing the size of activated sludge system.
2. Capital costs include an allowance for engineering,
legal and administrative and interest during con-
struction and amortized over 20 years at 7 percent
interest.
3. Electrical energy cost = $0.03/kWh ($0.83/mJ).
4. Fuel cost = $2.80/million Btu ($2.65/GJ).
5. Labor cost = $7.00/hour.
Using the above criteria, total costs for thermal condi-
tioning range from $257/ton ($283/Mg) in a 1 ton/day
(0.9 Mg/day) capacity plant to $32/ton ($35/Mg) in a
100 ton/day (91 Mg/day) plant.
76
-------
DESIGN EXAMPLE
The design example considered herein is a 4 Mgal/d
standard activated sludge plant with the following sludge
characteristics:
Flow
Sludge
type
Primary
Secondary
Total
Total solids
(Ib)
Volatile solids
(Ib)
(gal/min) (Mgal/d)
5,200
4,000
3,120
3,200
5.4
8.3
0.008
0.012
9,200
6,320
13.7
0.020
These sludge quantities were determined with the fol-
lowing assumptions:
1. Raw wastewater suspended solids = 240 mg/l;
BOD = 200 mg/l.
2. Suspended solids removal = 65 percent in primary
treatment and 90 percent overall; BOD removal = 30
percent in primary treatment and 90 percent overall.
3. One-half pound activated sludge produced per
pound BOD removed.
4. Primary sludge is 4 percent solids and is gravity
thickened to 8 percent solids.
5. Waste activated sludge is 1 percent solids and is
thickened to 4 percent solids.
A process and materials flow diagram is shown in
figure 4-11 for a thermal conditioning system of primary
and secondary sludge. The example system utilizes air
addition and assumes that the recycle liquor will be
treated in the main activated sludge plant. Other fea-
tures of this system include the following:
1. One thermal conditioning reactor required.
Flow = 20 gpm (1.3 l/s)
Operating pressure = 350 psig (24.6 kgf/cm2)
Operating temperature = 370° F (225° C)
Operating schedule: 24 hours/day, 7 days/week
Installed horsepower = 85 (63.5 kW)
BOILER
TO ATMOSPHERE
ODOR
CONTROL
TO MAIN PLANT
PRo'rSr® VACUUM ^
PROCESSING ys FILTER
^ AND OR /
DISPOSAL
LOCATION
1 . Primary Sludge
2. Secondary Sludge
3. Recycled Sludge
4. Total Sludge
5. Conditioned Sludge
6. Decant Underflow
7. Vacuum Filter Cake
8. Decant Supernatant
9. Vacuum Filter Filtrate
10 Total Liquid Recycle
11 Decant Tank Exhaust— 81 scfm
12 Vacuum Filter Exhaust-2400 scfm
13 Air to Reactor— 32 scfm
14 Steam to Reactor— 8,000 Ib/day
15. Boiler Feed Water-0 001 Mgal/d (0.7 gpml
16 Vacuum Filter Wash Water— 0.007 mgd {5 gpm)
Mgal/d
0.008
.012
.002
.022
.023
.009
_
.015
013
028
gmp
5.5
8.4
1.4
15.3
16.0
6.3
_
10.4
90
19.4
Ton/day
32
50
11
93
98
36
10
61
56
117
-«
Total
Solids
Ib/day
5,230
4,040
830
10,100
9,760
8,015
7,200
1,730
840
2,570
{°J
Percent
Solids
8.0
4.0
3.6
5.4
5.1
11.1
36.0
_
-
1 1
BOD5
Ib/day mg/l
_
_ _
_ _
_ _
_ _
_ _
_ _
875 7,000
370 3,400
1245 5,300
Figure 4-11.—Thermal conditioning example 4 Mgal/d activated sludge plant.
77
-------
Building area required = 1,115 square feet
(103.6 m2)
2. One decant tank required.
Design loading = 50 Ib/sq ft/day (244 kg/rrrVday)
Diameter = 15 feet (4.57 m)
Side water depth = 10 feet (305 m)
3. Scrubber-afterburner system to treat 81 scfm
(38.2 l/s) odorous gas from decant tank.
Installed horsepower = 3 (2.2 kW)
Building area required = 32 square feet (3.0 m2)
4. Multi-stage scrubber to treat 2,400 scfm (1130 l/s)
odorous gas from vacuum filter.
Installed horsepower = 13 (9.7 kW)
Building area required = 144 square feet (13.4 m2)
In this example, the assumed BOD loading without
thermal conditioning is 6,670 pounds (3025 kg) per day
in the raw wastewater and 4,670 pounds (2118 kg) per
day to the aeration basins. The BOD in the decant tank
supernatant and the vacuum filter filtrate are estimated
to increase the main treatment plant loading as follows:
Decant Vacuum Total
tank filter recycle
supernatant filtrate flow
BOD5, Ib/day 875 370 1,245
Percent BOD5 in raw wastewater .... 13.1 5.6 18.7
Percent BOD5 to aeration basins 18.7 7.9 26.7
REFERENCES
1. "Process Design Manual for Sludge Treatment and Disposal," EPA
Technology Transfer, EPA 625/1-74-006, pp. 6-14—6-16, October
1974.
2. Haug, R. T., "Sludge Processing to Optimize Digestibility and
Energy Production," Journal WPCF, pp. 1713-1721, July 1977.
3. Haug, R. T., et al., "Effort of Thermal Pretreatment on Digestibility
and Dewaterability of Organic Sludges," Journal WPCF, pp. 73-
85, January 1978.
4. Sommers, L. E. and Curtis, E. H., "Wet Air Oxidation: Effect on
Sludge Composition," Journal WPCF, pp. 2219-2225, November
1977.
5. Mayer, M. R. and Knopp, P. V., "The Cost Effectiveness of Ther-
mal Sludge Conditioning," paper presented at the Annual Confer-
ence, New York Water Pollution Control Association, January
1977.
6. Haug, R. T., et al., "Anaerobic Filter Treats Waste Activated
Sludge," Water and Sewage Works, pp. 40-43, February 1977.
7. Erickson, A. H. and Knopp, P. V., "Biological Treatment of Ther-
mally Conditioned Sludge Liquors," Advances in Water Pollution
Research, Pergamon Press, pp. 11-3311-11-3315, 1972.
8. Stack, V. T., Jr., et al., "Pressure Cooking of Excess Activated
Sludge," paper presented at the National Industrial Solid Wastes
Management Conference, University of Houston, March 1970.
9. Boyle, J. D. and Gruenwald, D. D., "Recycle of Liquor from Heat
Treatment of Sludge," Journal WPCF, pp. 2482-2489, October
1975.
10. Ewing, L. J., Jr., et al., "Effects of Thermal Treatment of Sludge
on Municipal Wastewater Treatment Costs," U.S. EPA, Cincinnati,
Ohio EPA-600/2-78-073.
78
-------
Chapter 5
Thickening of Sludge
INTRODUCTION
Sludge thickening is defined as increasing the total sol-
ids concentration of a dilute sludge from its initial value
to some higher value, up to a limit of about 10-12
percent total solids. Thickening is contrasted with "de-
watering" which increases the total solids concentration
to the range of 15-30 percent. Thickening operations
are intended to reduce the volume of sludge to be
further processed and normally constitute an intermediate
step preceding dewatering or stabilization.
The unit processes most commonly associated with
wastewater sludge thickening are gravity thickening, dis-
solved air flotation, and centrifugation. Some of the
heavier sludges, such as raw primary and combinations
of raw primary and some biological sludges, may be
readily thickened with gravity thickeners. Other, more
flocculent sludges, such as those from activated sludge
processes, may require more elaborate methods. The
most frequent applications of the common processes are
summarized in table 5-1.
The selection and design of a sludge thickening sys-
tem is dependent upon many factors including the
sludge characteristics, sludge processing following thick-
ening, and the type and size of wastewater treatment
facility. Each thickening situation will be somewhat differ-
ent. Applications other than those shown in table 5-1
are possible and, in some cases, may provide the de-
sired results.
This paper will discuss in detail the processes of grav-
ity thickening, dissolved air flotation, and centrifugation.
Other newer methods will also be mentioned. First,
sludge characteristics and sludge handling methods will
be discussed. This will be followed by a discussion of
the thickening processes, performance data, and recom-
mended design standards. This material will then be
Table 5-1.—Frequent applications of thickening proc-
esses
Process description
Sludge applications
Gravity thickening
Dissolved air flotation..
Centrifugation
Primary sludge
Combined primary and secondary sludges
Secondary sludges
Secondary sludges
used in a design example which will illustrate the gener-
al approach necessary in thickening alternative evalu-
ation and selection. Bench scale or pilot studies are
frequently required for determining applicability of,
and/or design parameters for, the various thickening
processes. Examples of these will be presented with the
design example. Additionally, equipment capital, opera-
tion, and maintenance cost data will necessarily be pre-
sented to aid in screening the alternatives. As the exam-
ple is developed, the methodology for determining the
most reliable and cost effective process for a given
sludge will be shown.
SLUDGE CHARACTERISTICS AND
HANDLING
Separation of solid matter from wastewater in a set-
tling tank results in a clarified tank effluent and a watery
mass of solids known as "sludge." Many different sludge
types and variations in sludge concentration are encoun-
tered in wastewater treatment. The characteristics of a
sludge prior to thickening will generally depend upon the
type of wastewater treated, the sludge origin (which par-
ticular wastewater treatment process), the degree of
chemical addition for improved settling or phosphorus
removal, and the sludge age. Additionally, the sludge
produced by a specific settling tank will also depend
somewhat upon the design and operation of the unit.
Typical "as removed" sludge concentrations are pre-
sented in table 5-2.
Table 5-2.—Typical sludge characteristics "as removed"
from treatment processes
Sludge type
Range Typical
percent percent
solids solids
Primary (PRI) 2-7 4
Waste activated (WAS) 0.5-1.5 1
Extended aeration (EA) 1-3 2
Trickling filter (TF) 1-4 2
Rotating biological disc (RBD) 1-3.5 2
Combinations:
PRI+WAS 2.5-4 3
PRI + TF 2-6 3.5
PRI + RBD 2-6 3.5
WAS + TF 0.5-2.5 1.5
79
-------
THICKENING
STABILIZATION
DEWATERING
STABILIZATION
REDUCTION
HEAT DRYING
ULTIMATE DISPOSA
Figure 5-1.—Alternative primary sludge disposal process trains.
The lower figures in the range of expected results
may be indicative of settling units processing lighter,
more flocculent sludges or of units operating above their
design capacity. The higher values may be indicative of
the results from units processing easily settled solids or
of units operating below their design capacity. Chemical
additions may result in higher or lower concentrations
depending upon the chemical and dosage utilized. The
"typical" percent solids are indicative of the results ob-
tained from settling tanks operating at design capacity
and treating normal "domestic wastewater."
Treatment and disposal of sludges represent two of
the major problems associated with wastewater treat-
ment. Thickening of the sludge represents but one step
of a total disposal scheme which may include thickening,
stabilization, dewatering, stabilization reduction, or heat
drying prior to ultimate disposal. Figures 5-1 and 5-2
show various primary and secondary sludge disposal al-
ternatives and how sludge thickening may fit into the
total treatment and disposal scheme.
In general, the required degree of thickening is directly
related to the sludge processing method(s) downstream
of the thickener (see figures 5-1 and 5-2). The stabili-
zation stage, in particular, will normally be more suc-
cessful if the solids concentration is kept within the
range that optimizes the rates of biological and chemical
stabilization. Likewise, ultimate disposal of liquid sludge
by land application will generally be less costly when the
solids concentration is maximized but kept within the
range dictated by pumping equipment. Suggested opti-
mum percent dry solids operating ranges for various
sludge handling processes following thickening are
shown in table 5-3.
THICKENING PROCESSES
Gravity Thickening
Gravity thickening of sludges, probably the most com-
mon unit process in use, is relatively simple in principle
so
-------
THICKENING
STABILIZATION
DEWATERING
STABILIZATION
REDUCTION
HEAT DRYING
ULTIMATE DISPOSAL
r> o
O I
0 S
I?
2
O
VACUUM
FILTER
SECONDARY
SLUDGE
O 0
O I
2 rn
2 I
5?
z
o
Figure 5-2.—Alternative secondary sludge disposal process trains.
Table 5-3.—Post thickening process operating ranges
Process type
Operating ranges
optimum sludge
solids, percent
Stabilization
Aerobic digestion
Anaerobic digestion
High pressure wet oxidation..
Low pressure heat treatment.
Lime treatment
Other
Land application
2-4
4-6
4-6
4-6
6-8
6-8
and operation and low in cost. Gravity thickening is
basically a sedimentary process carried out in a unit
which resembles a wastewater settling basin. A typical
unit is shown in figure 5-3. Solids settle to the thickener
SCRAPER BLADES
SLOPE I 4 MINIMUM
UNDERFLOW
ELEVATION
Figure 5-3.—Gravity thickener.
bottom, are then raked to a sludge hopper, and are
periodically removed and discharged to the next proc-
ess. Water separated from the sludge (supernatant) rises
as the sludge settles. This supernatant or overflow con-
taining some solids and probably a high biochemical
81
-------
Table 5-4.—Existing gravity thickener performance data
Location
Rumford Mexico, Me
Kokomo Ind
York, Nebr
Salem Ohio
Middletown, Ohio ...
Feed
WAS
Heat treat3
Combined b
PRI
WAS
Sludge solids
Unthickened
1.2
4-6
0.9
, percent
Thickened
2.7
14-18
6-7
6
3.8
Solids
loading
(Ibs/ft2/day)
5
18
6
1.5
"Contains heat treated primary and waste activated (equal portions).
bContains primary, intermediate (trickling filter), and final (biodisc), proportions unknown.
oxygen demand should be returned to the plant for fur-
ther treatment. Several existing gravity thickener installa-
tions were recently contacted. Data, indicative of equip-
ment performance at that time, are presented in table
5-4.
Gravity thickeners are normally circular in shape and
have a side water depth of about 10 feet (3.0 m). The
tank diameter is a function of the required surface area.
The required surface area is determined by applying
either pilot tested or average recommended solids load-
ing rates to the total solids that the unit will receive
each day. Tank side water depth is influenced by the
desired retention time and equipment availability. Sludge
solids concentrations obtainable by gravity thickening de-
pend upon the sludge type, thickener overflow rate, and
solids retention time. Average recommended solids load-
ing rates and the possible performance for some sludges
are presented in table 5-5.
The values are average ranges only and may or may
not be indicative of the possible results for the particular
sludge in question. A case in point is a community
Table 5-5.—Gravity thickener loading rates and per-
formance
Sludge solids, percent
Sludge type
Primary (PRI)
Waste activated (WAS)
Extended aeration (EA)
Trickling filter (TF)
Biodisc (RBD)
Combinations.
PRI+WAS
PRI+TF
PRI + RBD
WAS + TF .
Unthickened
2-7
0.5-1.5
1-3
1-4
1-35
25-4
2-6
2-6
05-25
Thickened
5-10
2-3
1.5-4
3-6
2-5
4-7
5-9
5-8
2-4
Solids loading
(Ibs/ft2/day)
20-30
4-8
4-8
8-10
7-10
8-16
12-20
10-18
4-8
which gravity thickens a 0.9 percent dry solids waste
activated sludge to 3.8 percent with solids capture of
over 90 percent. The solids loading is 2 to 4 Ibs/ft2 (.91
to 1.81 kg) per day and the hydraulic loading ranges
from 50 to 100 gal/ft2/day (2.0 to 4.1 m3/m2/d). This
plant treats a high percentage of paper mill waste whicl
results in significant concentrations of inorganic solids
escaping the primary tanks. These solids, when com-
bined with the biological sludge, form a floe that has
much better settling characteristics than most waste acti
vated sludges. This results in a better than average
thickened product.
Although the solids loading usually governs gravity
thickener design, the hydraulic loading should also be
checked. Hydraulic loadings in the range of 600 to 800
gal/ft2/day (24.4 to 32.6 m3/m2/d) have been reported
as optimum.1 Also, loadings below 400 gal/ft2/day (16.3
m3/m2/d) have been reported as possibly resulting in
odor problems; recycling of secondary effluent to main-
tain the higher rates has been recommended.1 Much low
er rates, as low as 100 to 200 gal/ft2/day (4.1 to 8.1
rrrVmVd), will often be more applicable. Recycling of
secondary effluent to control odor will dilute the influent
solids. The overall solids thickening performance of the
unit may not deteriorate, however, since dilution will elu-
triate fine solids and reduce the interference between
the settling particles. Polyelectrolyte addition may have
the effect of improving solids capture and thus reducing
solids overflow in the supernatant, but may have little
effect on improving the solids concentration in the un-
derflow. To achieve maximum sludge concentration, a
sludge retention time of one-half to 2 days is normally
required.
Dissolved Air Flotation
Dissolved air flotation is presently the most widely
used method of thickening waste activated sludge. The
system uses air buoyancy to literally float solids to the
surface of a tank to be collected. The main advantage
of this method over gravity thickening is that very light
particles, such as waste-activated sludge solids, can be
removed more completely in less time. A typical dis-
82
-------
solved air flotation system is shown in figures 5-4 and
5-5. The units physically range from small steel package
units to custom designed large units with concrete tanks.
Recycle flow may consist of either underflow from the
unit or recycled plant effluent. It is returned at rates of
up to five times the feed sludge rate, combined with air,
and then pressurized to approximately 60-70 Ibs/in.2
(4.2-4.9 kg/cm2). Since the solubility of air in water
increases with increasing pressure, large quantities of air
ADJUSTABLE FLOAT SKIMMER
CHAIN TENSIONER
\
\ INFLUENT
BACK PRESSURE VALVE
REDWOOD SCRAPER
Figure 5-4.—Dissolved air flotation unit.
UNIT EFFLUEN1
AUX RECYCLE CONNECTION
(PRIMARY TANK OR 1
PLANT EFFLUENT) T
(AIR FEED Cl-*-
AITERNATE' ^ ^Xl
FLOTATION UNIT
TH
-*•
RECIRCULATION PUMP
11 TO 5 1
1 A^
THICKENED SLUDGE
DISCHARGE (FLOAT)
UNIT FEED
"SLUDGE (WAS I
RECYCLE
FLOW
REAERATION PUMP
RETENTION TANK
(AIR SOLUBIIIZATION/
003 TO 005 LB'S DISSOLVED
AIR PER LB OF SOLIDS
Figure 5-5.—Dissolved air flotation system.
go into solution. Later, this recycle flow is allowed to
depressurize as it is mixed with the influent sludge. De-
pressurization releases the excess air out of the recycle
liquid in the form of tiny air bubbles (80 microns). These
air bubbles attach themselves to the sludge solids and
float them to the surface. Thickened sludge is scraped
off the liquid surface by a skimmer mechanism consisting
of a series of paddles. Liquid that is not contained in
the thickened sludge or recycled is discharged from the
system as subnatant. Subnatant may contain high solids
and biochemical oxygen demand, and thus should be
returned to the plant for further treatment.
Data from existing operating full-scale dissolved air
flotation units have been presented in other
publications.2'3 Some of the same installations were re-
cently contacted. Updated performance data for these
and other dissolved air flotation units are presented in
table 5-6.
The effluent sludge (float) percent solids will depend
on many variables including the type and quality of the
feed sludge, recycle ratio, detention time, air to solids
ratio, system pressure, the solids and hydraulic loading
rates, and the amount of chemical aids used. Some
general statements that have been made regarding dis-
solved air flotation thickening of the "average" waste-
activated sludge are as follows:2
1. Increased air pressure or flow will yield higher float
solids and lower effluent suspended solids concen-
tration.
2. Polymer usage will yield higher float solids concen-
tration and improve the subnatant quality.
3. Detention time in the flotation zone is not critical.
Since there are so many variables and each sludge
will react somewhat differently to the dissolved air flota-
tion thickening process, these "general rules of thumb"
Table 5-6.—Recent data for some plant scale DAF units
Location
Indianapolis, Ind
Warren, Mich
Frankenmuth Mich
Columbus Ohio
Nashville, Tenn . .
Xenia, Ohio
Feed
.... WAS3
.... WAS"
WAS
.... WASC
WASd
.... PS.WAS"
.... WAS
Influent
SS (mg/l)
10,000
1 1 ,000
8,000
6,000
8,000
35,000;5,000
4,000
Subnatant
SS (mg/l)
100-1,000
200
90
800
150
100
Float
percent
solids
3.5-4.2
5
3.5-5.5
3.2
3
6
2.5-3.0
Polymer used
Ibs/ton
dry solids
30
40
bO-26
0
0
30
"Contains some primary sludge—proportions unknown.
bMajor flow to plant is brewery waste. Polymer sometimes used to keep sludge from
adhering to skimmers. Sometimes thicken anaerobically digested sludge—similar results with
no polymers required (influent SS 10,000 mg/l).
cJackson Pike facility.
"Southerly facility—units are being used as gravity settlers since they get better results
this way.
'Primary and waste activated are handled by separate units—combined product is 6
percent solids.
83
-------
Table 5-7.—Dissolved air flotation design parameters and expected results
Sludge type
Feed
solids,
percent
Solids
loading
(Ib/ft2/hr)
Air to
solids
ratio
Float solids,
„ , percent
Recycle K
ratio,
percent with™ ,t
Solids capture,
percent
With Withni it
polymer polymer polymer polymer
Waste activated 0.5-1.5 2-3
Primary and waste activated... 3—4 2-4
0.03-0.05 100-500
5-6
4-5
5-8
95-100
85-95
85-95
a Limited experience prohibits listing typical numbers.
may not apply in all cases. Additionally, when the guide-
lines are valid, it is generally only within certain ranges
of the variable parameters. The ranges are typically 40-
70 Ibs/in.2 (2.8-4.9 kg/cm2) for air pressure and 0-40
Ibs (0-18.1 kg) for polymer dosage. Likewise, the deten-
tion time may not be critical once a minimum value of
1.5-3 hours has been attained.
System design is based primarily on a solids loading
rate and the desired air to solids ratio. Additionally,
maximum hydraulic loading rates are usually checked to
avoid exceeding manufacturers' recommendations. If any
flow other than the dissolved air flotation thickener un-
derflow is used for recycle, it must be included in the
unit's total hydraulic loading calculation.
Pilot studies are recommended to determine the appli-
cability of the dissolved air flotation process to the
sludge and to optimize some of the variables. When pilot
studies are undertaken, the full-scale design is based on
the study findings. Since data and sludge samples are
lacking at new wastewater treatment plants, thickener
design must be based on sound engineering judgment
backed up with past experience. Commonly used design
parameters and expected unit performance are present-
ed in table 5-7. It must be emphasized that these are
general guidelines only.
Centrifugation
Centrifugal thickening of sludge is a process which
uses the force developed by fast rotation of a cylindrical
drum or bowl to separate the sludge solids and liquid. In
the basic process, when a sludge slurry is introduced to
the centrifuge, it is forced against the bowl's interior
walls, forming a thin slurry layer or "pool." Density dif-
ferences cause the sludge solids and the liquid to sepa-
rate into two distinct layers. The sludge solids "cake"
and the liquid "centrate" are then drawn from the unit
separately and discharged. The three types of centri-
fuges—basket, disc-nozzle, and solid bowl—all operate
on the basic principles described above. They are differ-
entiated by the method of sludge feed, applied centrifu-
gal force, method of solids and liquid discharge, and to
some extent performance.
The basket centrifuge, as shown in figure 5-6, is a
FEED
POLYMER -i
SKIMMINGS
CAKE ' CAKE
Figure 5-6.—Schematic diagram of a basket centrifuge.
batch type thickening unit. As slurry is fed to the unit,
the sludge solids form a cake on the bowl walls, while
the centrate is discharged over a weir or baffle. Slurry
feed is continued until the centrate solids reach the
maximum tolerable limit. At this point, the unit stops and
a knife wipes the sludge cake off the walls. The sludge
is then discharged from the system through the unit's
open bottom. Of the three centrifuge types, the basket
unit has the capability of producing the driest sludge
84
-------
cake since there is a minimum of disturbance to the
depositing solids. Its use, however, is generally restricted
to smaller plants because of its intermittent operation
and resultant lower capacity.
The disc-nozzle centrifuge, as shown on figure 5-7, is
a continuously operating unit. It is composed of a series
of conical plates which are stacked together to form a
series of narrow channels. Sludge slurry enters the unit
and is dispersed to these channels. The centrate tends
to rise and is discharged from the top of the cones,
while the sludge cake is discharged downward and
through small nozzles in the bowl wall at the cone bot-
toms. High sludge throughput and good solids capture
are possible with these units. Their solids concentrating
capability is limited, however, by the small diameter
(0.05-0.10 in.) (0.13-0.25 cm) orifices through which the
sludge cake must discharge. Additionally, depending
upon the sludge type and previous treatment, degritting
and screening prior to the disc centrifugation may be
mandatory to avoid plugging these sludge discharge ori-
fices and to reduce wear on the machine.
Like the disc centrifuge, the continuous solid bowl
centrifuge is a continuously operating unit. It consists of
a horizontal cylindrical bowl containing a screw type
conveyor. At one end, the bowl necks down to a coni-
cal section that acts as a beach plate for the screw
conveyor. In operation, sludge solids are forced to the
FEED
EFFLUENT
DISCHARGE
SLUDGE
DISCHARGE
RECYCLE
Figure 5-7.—Disc-nozzle centrifuge.
bowl surface and are moved toward the beach plate by
the conveyor where they are discharged from the unit.
The sludge pool level is controlled by adjustable skim-
mers or weir plates. These also function as discharge
points for the centrate. A typical countercurrent solid
bowl centrifuge is shown in figure 5-8. Sludge slurry
enters the unit just before the conical section and
distributes itself along the bowl surface. Sludge solids
are discharged at the cone end while centrate is dis-
charged at the opposite end. Sludge solids do not travel
the full length of the bowl. A second variation of the
solid bowl centrifuge is the concurrent model. In this
unit, sludge slurry is introduced at the far end of the
bowl. Turbulence and interference present at the slurry
inlet point in the countercurrent machine are reduced
with this variation. Also, the slurry must travel the full
length of the bowl before discharge. This may result in
a drier sludge cake.
Centrifuge performance is measured by the percent
solids of the sludge cake and the centrate quality or
total solids captured. Several existing centrifuge installa-
tions were recently contacted. Data, indicative of equip-
ment performance at that time, are presented in table
5-8. The performance of a particular centrifuge unit will
vary with the inlet sludge type and solids characteristics,
the sludge feed rate, and the degree of chemical addi-
tion. Centrifuge performance on a particular sludge will
also vary with bowl design, bowl speed, pool volume,
and conveyor (if present) design. In practice, bowl and
conveyor design are set by the manufacturers. Pool
depth is variable on solid bowl units. Increasing the pool
depth will normally result in a wetter sludge cake but
better solids recovery. Bowl speed is normally variable
on most centrifuge models. Difficulty involved in chang-
ing speeds varies with the manufacturers. An increase in
bowl speed normally results in a drier sludge cake and
better solids recovery. Conveyor differential speed is nor-
mally variable on continuous solid bowl centrifuges. In-
creasing the differential normally results in a wetter
sludge cake and poorer solids recovery. Varying these
parameters will probably result in significant solids
DIFFERENTIAL
SPEED
GEAR
BOX
ROTATING
COVER
MAIN
DRIVE
SHEAVE
FEED
PIPES
CENTRATE
DISCHARGE
"ROTATING
CONVEYOR
-"I V-T>- (SLUDGE
^UJIHJ AND
BEARING CHEMICAL)
BASE NOT SHOWN
SLUDGE CAKE
DISCHARGE
T
Figure 5-8.—Continuous countercurrent solid bowl con-
veyor discharge centrifuge.
85
-------
Table 5-8.—Existing solid bowl centrifuge performance data
Location
Feed
Sludge solids, percent
Unthickened Thickened
Solids
recovery,
percent
Great Northern Paper, Millinocket, Maine . . . .
Kendall Co, Griswoldville, Mass
Miller Brewing Co., St. Louis, Mo
Dubuque, Iowa
. WAS3
WAS3
WASb
WASb
4
3
075-1
115
10-12
7
5-7
6
90
80-85
'Polymers used—quantity unknown.
bPolymers not used.
Table 5-9.—Centrifuge mechanical characteristics and
performance data
Centrifuge type
Parameter
Basket Disc-nozzle Solid bowl
Operation method
Bowl diameter (inches)
Max centrifugal force (G)
WAS feed solids, percent . . . .
WAS cake solids, percent
Solids recovery, percent
Batch
12-60
2,000
0.5-1.5
8-10
80-90
Continuous
8-30
12,000
0.5-1.5
4-6
80-90
6-60
3,200
0.5-1.5
5-8
70-90
changes only within limited ranges. Each performance
improvement must be compared with the additional costs
required to produce it.
Centrifuges have seen more service in dewatering ap-
plications than in thickening applications. When utilized
for thickening, their use is normally limited to the thinner
biological or industrial sludges that cannot be thickened
by less expensive methods. Data on the three centrifuge
types and their possible performance on waste activated
sludge are presented in table 5-9.
Polymers may be required to meet the stated perfor-
mance. The required dosage depends upon the manufac-
turer and may range from 0-8 Ib/ton (0-4.0 kg/Mg) of
dry solids.
Polymer addition generally improves both the percent
solids and the solids recovery. It must be emphasized
that the tabular values are representative of possible
results from an "average" waste activated sludge. Num-
erous sludge and machine variables make consultation
with manufacturers mandatory and pilot tests highly rec-
ommended for each installation.
Other Methods
Thickening of sludge is often a secondary benefit of a
sludge treatment unit having an entirely different pur-
pose. Decanting facilities are provided in aerobic and
anaerobic digesters to remove excess liquids which ha\
risen above the solids layer. In such facilities, sludge
solids concentrations may increase as much as one pe
cent over inlet feed solids concentrations.
New sludge thickening methods are being marketed
each year. One such method is the sludge filter bag
system. In this process, sludge is mixed with polymer
and then held in suspended porous bags. The weight c
the sludge forces water out the bag sides and bottom.
Sludge is held from four to eight hours depending upor
the desired dryness and is then released through a bot
torn opening. Bag life should be about 2 years. This
method has not been in existence long enough to have
been proven reliable.
DESIGN EXAMPLE
Statement of Problem
The problem is to provide sludge thickening facilities
for two communities, both of which have existing con-
ventional activated sludge wastewater treatment plants.
The smaller community has existing wastewater treat-
ment facilities capable of treating 4.0 million gallons pe
day (.18 m3/s). The facilities consist of screening, grit
removal, primary settling, conventional activated sludge
aeration, final settling, chlorination, and sludge lagoonin
Present flow to the plant is 3.5 million gallons per day
(.15 nf/s); the 20 year projected flow is 4.0 million
gallons per day (.18 rrvVs). The plant meets its propose
discharge permit requirements, but the city has been
ordered to abandon the sludge lagoons (which are peri
odically flooded by the receiving stream) and in their
place construct digestion facilities and devise a plan fo
disposal of the digested sludge. The digested sludge w
be dewatered on sand drying beds or hauled as a liqu
to nearby farms. Thickening facilities are required to
reduce the size of the required anaerobic digester, to
insure efficient digester operation, and reduce hauling
costs.
The larger community has existing wastewater treat-
ment facilities capable of treating 30 million gallons per
-------
day (1.31 m3/s). Present flow to the plant is 35 million
gallons per day (1.53 m3/s); the 20-year projected flow
is 40 Mgal/d (1.75 m3/s). The existing treatment system
consists of screening, grit removal, primary settling, con-
ventional activated sludge aeration, final settling, chlorin-
ation, aerobic sludge digestion, sludge dewatering, and
landfilling of dried sludge solids. The existing treatment
scheme will meet proposed permit requirements. There-
fore, all treatment units will be expanded to handle the
20-year flow projections. Anaerobic digestion has been
determined to be more cost-effective than the aerobic
sludge digestion. The aerobic digesters will be aban-
doned as such (will become part of expanded aeration
tank facilities). Thickening facilities are required to re-
duce the size of the required anaerobic digesters, to
insure efficient digester operation, and to improve the
dewatering operation.
Wastewater Characteristics
The wastewater characteristics and removal efficiencies
of the various treatment units are required to determine
the possible solids loading on the thickeners. This infor-
mation may be acquired from plant records or sampling
programs at existing facilities. When these data are not
available (such as in the case of new wastewater treat-
ment plants for new service areas), assumptions based
on sound engineering judgment and previous experience
are necessary. For the sake of simplicity, the wastewater
characteristics and treatment unit removal efficiencies for
the example plants are assumed equal. Raw wastewater
characteristics for the example plants are given in table
5-10.
Treatment Unit Efficiencies
Both plants in this example will meet their proposed
permit requirements by utilizing the existing treatment
processes. Nitrification and phosphorus removal are not
required. Removal efficiencies based on percentages of
the raw "domestic" wastewater characteristics are pre-
sented in table 5-11.
Sludge Characteristics
The characteristics of sludge discharged to the thick-
ening facilities may vary considerably depending upon
Table 5-10.—Raw wastewater characteristics
Table 5-11.—Treatment unit efficiencies
Parameter
Concentration
(mg/l)
BOD5
Suspended solids.
Organic nitrogen..
Ammonia nitrogen
Phosphorus
Grease
200
240
15
25
10
100
Unit
Removal
Parameter efficiency,
percent
Primary settling BOD5 30
SS 65
Aeration and final settling BOD5 60
SS 25
the type and amount of industrial wastes treated, the
sludge origin (which particular treatment unit), the de-
gree of chemical addition for improved settling or phos-
phorus removal, and the sludge age. Ideally, samples of
the sludge will be available for analysis. In lieu of this,
the ranges and typical concentrations shown in
table 5-2 may be utilized.
Existing plant operating data at the example plants
have shown that the unthickened primary sludge con-
tains four percent dry solids; the waste activated sludge,
one percent dry solids. Field experiments at both plants
were conducted by returning the waste activated sludge
to the primaries. This did not seriously alter their opera-
tional characteristics and an unthickened primary sludge
containing 3 percent dry solids resulted. Additionally,
data at these plants have shown that for every pound of
5-day biochemical oxygen demand removed in aeration,
0.5 pound of volatile suspended solids is produced.
Sludge Handling Following Thickening
The required degree of thickening is directly related to
the sludge processing method(s) following thickening.
Suggested optimum percent dry solids operating ranges
for some sludge handling processes following thickening
were presented in table 5-3. In the examples, anaerobic
digestion is to follow the thickening step. Hence, sludge
delivered to the digester should have a solids concentra-
tion between 4 and 6 percent.
For any sludge thickening problem, there will be sever-
al alternative solutions which will result in a sludge prod-
uct in the desired solids range. However, since each
solution will probably not result in the same "guaranteed
average" percent dry solids, the design of the sludge
processing facilities following thickening will also be af-
fected. Consequently, these facilities will also have to be
included in the cost analysis.
Process Alternatives
Gravity Thickening
In the example, a primary (4 percent) and waste acti-
vated sludge (1 percent), or combined sludge (3 per-
cent) is obtained, and a sludge concentration for the
anaerobic digester of 4 to 6 percent is needed. Table
5-5 and past experience indicate that gravity thickening
87
-------
of "normal" waste activated sludge alone will not yield
the required 4 percent solids. Gravity thickening may
yield reasonable results for the combined sludges. Addi-
SETTLING CHARACTERISTICS-8' COLUMN
WASTE ACTIVATED SLUDGE
W.A.S. SUSPENDED SOLIDS = 10,000 Mgal/d
MAXIMUM SOLIDS CONCENTRATION = 2.8%
1/3 VOLUME
120
180 240 300 360
SETTLING TIME (min)
420 480
Figure 5-9.—Settling characteristics—8 foot column
waste-activated sludge.
tionally, gravity thickening primary sludge alone and
waste activated alone, and later mixing the two, is a
possibility. At this point in an actual problem at an exisl
ing treatment plant, bench or pilot studies would be
performed to determine the applicability of gravity thick-
ening to the sludge and to determine design parameters
Examples of results of typical 8-foot column bench
scale tests are shown on figures 5-9 and 5-10. Both
the undiluted and elutriated activated sludges reached
their maximum solids concentrations of 2.8 percent and
2.3 percent, respectively, in less than 3 hours. A similar
test would be made on primary only and combinations
of primary and waste activated sludge.
For the example plants, assume the results of the
tests showed that gravity thickening the sludges will re-
sult in the following: primary sludge, nine percent; waste
activated sludge, 2.8 percent; combined primary and
waste activated sludge, 5 percent.
Dissolved Air Flotation
Reviewing the example problem, there is primary (4
percent) and waste activated sludge (1 percent) or com
bined sludge (3 percent), and a sludge concentration fo
10 •
SETTLING CHARACTERISTICS-8' COLUM
WASTE ACTIVATED SLUDGE
ELUTRIATED (1:1 DILUTION)
INITIAL S.S. OF W.A.S. = 11,600 Mgal/d
S.S. AFTER DILUTION = 5,800 Mgal/d
MAX. SOLIDS CONCENTRATION = 2.3%
30
60 90 120 150 180 210 240 270 300 330 360 390 420 450 480
SETTLING TIME (MINUTES)
Figure 5-10.—Settling characteristics—8 foot column elutriated waste-
activated sludge.
88
-------
the anaerobic digesters of 4 to 6 percent is needed. If
primary sludge is to be thickened alone, gravity thicken-
ing is generally utilized since the costs would be much
less than for dissolved air flotation. Likewise, in the case
of the primary-waste activated combined sludge, gravity
thickening will yield similar results at less cost. This
leaves thickening the waste activated sludge alone by
dissolved air flotation thickening as a possible option.
Dissolved air flotation thickening of the waste activated
sludge, coupled with either unthickened or gravity thick-
ened primary sludge, represents a viable alternative and
will be considered. At existing plants, pilot tests should
be performed to aid in process selection and equipment
design.
Assume a pilot study was completed using dissolved
air flotation thickening on the waste activated sludge.
The variables studied included recycle ratio, air to solids
ratio, solids loading rate, and amount of polymer used.
The system pressure was kept constant. The results,
shown graphically in figures 5-11, 5-12, 5-13, and
5-14, were as follows:
1. Increasing the recycle rate generally yielded higher
percent float solids but also higher effluent sus-
pended solids. A compromise rate was selected for
use in later tests.
2. A concentrated sludge of 4 percent solids could be
consistently achieved with a unit loading of 2
Ib/ft2/hr (9.8 kg/m2/hr) and an air to solids ratio of
0.04. Increasing the solids loading reduced the float
concentration and increased the effluent suspended
solids concentration with and without polymer us-
age.
3. At the recommended loading, an effluent suspended
solids concentration of 50 milligrams per liter with-
out the use of polymers and 20 milligrams per liter
with polymer addition was consistently achieved.
Polymer usage, however, resulted in no clearly
identifiable improvement in the float solids concen-
tration.
4. Very rapid deterioration in the effluent quality oc-
curred when the air to solids ratio fell below 0.020.
Increasing the air to solids rates from 0.040 to
0.250 resulted in only slight reduction in effluent
suspended solids.
As seen from the results, the waste activated sludge
differed somewhat from the experience of others2 and an
CO
Q
Ul
O c
DC 5
LU
D-
cc
I-
•z.
LU 1
O 3
z
o
o
FLOAT
CONCENTRATION
EFFLUENT
SUSPENDED
SOLIDS
800
700
600 ^
CO
Q
500
400
O
CO
Q
LU
O
z
LU
Q.
CO
co
300
200
100
3 4
SOLIDS LOADING (LB/SQ FT/HR)
Figure 5-11.—Float concentration and effluent suspended solids versus
solids loading—without polymers.
89
-------
co
Q
UJ
CJ
CC
UJ
2
O
= 4
UJ _
0 3
O
O
< 9
O 2
FLOAT
CONCENTRATION
/"EFFLUENT
SUSPENDED
SOLIDS
8°.
800
700
600 ~
D
_j
O
CO
500 Q
UJ
400
300
200
100
UJ
ZJ
3 4
SOLIDS LOADING (LB/SQ FT/HR)
Figure 5-12.—Float concentration and effluent suspended solids versus
solids loading—with polymers.
"average" waste activated sludge. A 4 percent float was
obtained with or without polymers.
For the example plants, it will be assumed that dis-
solved air flotation thickening is applicable to the waste
activated sludge and that a thickened sludge of 4 per-
cent solids will be produced at solids loadings of 2
Ib/ft2/hr (9.8 kg/m2/hr) and an air to solids ratio of
0.04.
Centrifugation
The problem at the example wastewater plants is to
produce a 4 to 6 percent dry solids sludge for anaero-
bic digestion from primary sludge (4 percent) and waste
activated sludge (1 percent), or combined sludge (3 per-
cent). Past experience indicates that thickening the pri-
mary or the combined sludge by centrifugation would be
a more costly alternative than gravity thickening. These
alternatives are eliminated from further consideration.
Centrifugal thickening of the waste activated sludge,
however, combined with either unthickened or gravity
thickened primary sludge does represent a viable alter-
native and will be considered. As in the case of gravity
and dissolved air flotation thickening, sludge treatability
and variable optimization make pilot studies highly desir-
able when possible.
For the example, assume a pilot study using a solid
bowl centrifuge was performed as part of the sludge
thickening study on the waste activated sludge. Some
typical data from this pilot test are shown in table 5-12
In the pilot study, the feed rate of the sludge, bowl
speed, and pond setting were varied to determine the
optimum combination to yield a 5 percent sludge. Minor
pond setting changes had little effect on the unit's per-
formance. Operation at 3,200 G produced a sludge
much thicker (12 percent) than needed, while operation
at 1,150 G produced a wet sludge and poor solids
removal efficiency. A force of 2,100 G was selected as
an optimum. At the selected bowl speed, solids recover
and percent solids of the cake were analyzed for differ-
ent sludge feed rates. The data indicated that although
the centrifuge could thicken the sludge to the required
percent, the percent solids could drop from 5 percent
down to 2 percent or increase up to 15 percent, with
only minor feed rate changes. Consistently obtaining the
required 5 percent solids concentration was difficult.
Based on the pilot test data, solid bowl centrifuge thick
ening of the waste activated sludge was not consistent.
90
-------
9 r
55 7
Q
O
co
H 6
LJLJ
CJ
cc
til
S; 5
O
O
z
O 3
O
= 2
FLOAT CONCENTRATION
EFFLUENT SUSPENDED SOLIDS
900
800
700
600
500
CO
Q
_l
O
CO
Q
111
Q
400 I
co
K-
LU
300 3
200
100
0 .02 .04 .06 .08 .10 .12 .14 .16 .18
AIR SOLIDS RATIO
Figure 5-13.—Float concentration and effluent suspended solids versus
air-solids ratio—without polymers.
.20
.22
.24
.26 .28
For the example plants, however, it will be assumed
that centrifugation is applicable to the waste activated
sludge. Also, based on available equipment reliability,
plant operator preference, desired performance, minimum
supportive equipment requirements, and past experience,
the solid bowl continuous centrifuge is selected over the
basket and disc centrifuge for the examples. Available
data from equipment manufacturers and data in table 5-
9 indicate that a product sludge of 6 percent solids may
be reasonably expected.
Other Methods
Decanting may result in some thickening in the diges-
ters. It is not, however, a reliable, consistent method
and does not normally result in appreciable thickening.
Thus, it will not be considered as one of the process
alternatives for the example plants.
New methods, such as the sludge filter bag system,
have not been in existence long enough to have been
proven reliable. Thus, they will not be considered as
thickening process alternatives for the example plants.
Alternative Evaluation
Preliminary Screening
The preliminary screening of sludge thickening alterna-
tives for the example plants was performed in the previ-
ous section. The remaining alternatives at this point are
presented in table 5-13.
The general approach to use, at this point, is to first
determine if any of the remaining alternatives can be
eliminated without performing a detailed cost-effective-
ness analysis. A detailed cost-effectiveness analysis ex-
amining capital and operation and maintenance costs
would then be performed on the remaining alternatives.
Capital costs to be considered may normally include
thickener and supportive equipment costs, land costs,
building or protective structure costs, and, in certain
cases, post thickening treatment unit costs. Other costs
to be considered include power costs, chemical costs,
manpower costs, and maintenance costs. The cost-effec-
tiveness analysis will show which alternative has the low-
est annual equivalent cost.
91
-------
8
7
55
Q
^ 6
CO
1-
LU
(C 5
LLI
~
O
F 4
cc
z
ID o
0 3
2
O
o
^.
< 2
O
u_
1
0
—
«
9
1
I
—
_
"
—
o
^* *^
* ^•*'* __
o ^-^"
^ —
* ^ "**
^^^ ~
0 ° ^^^^
o x^'o o
>^
o o >* —
*"A *
/c?
FLOAT CONCENTRATION
O
o
-
y
\" x- EFFLUENT SUSPENDED SOLIDS
»W... M / * -
800
700
600
500
400
300
200
100
0
.02 .04 .06 .08 .10 .12 .14 .16
AIR SOLIDS RATIO
.18
.20
.22
.24
.26 .28
Figure 5-14.—Float concentration and effluent suspended solids versus
air-solids ratio—with polymers.
Secondary Screening Analysis
Since alternative numbers 1 and 2 both utilize gravity
thickening only, elimination of one of them should be
relatively simple. Wastewater characteristics and settling
tank performance data presented previously will be used
in determining loadings on the required thickeners. For
the examples, differences in density of the sludges are
assumed insignificant and the density is taken as equal
to water. Thickener designs will be based on loading
rates proposed in tables 5-5, 5-7, and 5-9. Designs will
be conservative to assure the desired performance. A
total of two thickeners will be provided with each alter-
native to assure that some thickening will be obtained if
one unit fails. Calculations required for the 4.0 million
gallons per day wastewater plant gravity thickener de-
signs follow:
Alternative No. 1
Definition—Gravity thicken primary sludge; gravity thicken waste acti-
vated sludge.
Primary sludge
Quantity: 4x240x8.34x0.65 = 5,204 Ibs/day (2360 kg/day)
Volume. 5,2047(0.04X8.34) = 15,600 gals/day (59,050 I/day)
Required thickener: 5,204/20 Ib/ft2/day = 260 ft2 (24.2 m2) or an 18.2
ft (5.55 m) dia. unit
Recommended thickener: one 20 ft (6.10 m) dia., 10 ft (3.05 m) deep
unit
Thickened product: 5,2047(0.09x8.34) = 6,933 gals/day (26,240 I/day
Thickener cost: $64,000
Waste activated sludge
Nonbiological- 4X240X8.34X0.25 = 2,002 Ibs/day (908 kg/day)
Biological: 4x200x0.60x8.34x0.5 = 2,002 Ibs/day (908 kg/day)
Total quantity: 4,004 Ibs/day (1816 kg/day)
Volume: 4,004/(0.01 x 8.34) = 48,010 gals/day (181,740 I/day)
Required thickener: 4,004/4 Ibs/ft2/day = 1,001 ft2 (93.0 m2) or a 35.'
(10.88 m) dia. unit
Recommended thickener: one 35 ft (10.67 m) dia., 10 ft (3.05 m) de«
unit
Thickened product: 4,004/(0.028 X 8.34) = 17,146 gals/day (64,905
I/day)
Thickener cost. $98,000
Combined product
[(6,933X9)+ 17,146 (2.8)]/(6,933 + 17,146) = 4.59
24,079 gals/day (91,950 I/day) of 4.59 percent sludge
Alternative No. 2
Definition—Gravity thicken combined sludge.
Combined sludge
Nonbiological: 4x240x8.34x0.9 = 7,206 Ibs/day (3269 kg/day)
Biological: 2,002 Ibs/day (908 kg/day)
Total quantity: 9,208 Ibs/day (4177 kg/day)
Volume: 9,208/(0.03x8.34) = 36,803 gals/day (139,315 I/day)
92
-------
Table 5-12.—Pilot centrifuge results
Run
No.
1
2
3
4
5
6 ...
7 .. .
8 ....
9 ...
10 ....
11 ....
12 ....
13 ....
14 ....
15 ....
16
17
18
19
20 . . . .
21 ....
22 . . . .
23 ....
24 . . . .
25 . . . .
26 ....
27 ....
28 . . . .
Feed
Rate
gpm
136
108
168
177
183
. 25.2
. 25.7
. 21.6
. 23.0
. 35.4
. 23.6
. 12.2
. 10.7
. 11.1
. 22.2
27.3
27.3
28.3
44.6
. 59.0
—
. 23.0
. 25.4
. 44.0
. 44.5
. 40.4
—
. . 63.2
sludge
Concen-
tration
percent
SS
0.799
.859
.817
.925
.918
.833
.845
.809
.813
.809
.782
.790
.699
.757
.757
.779
.737
.793
.777
.760
.786
.760
.750
.751
.745
.701
.487
.725
Centrate
Rate
gpm
12.5
6.8
15.8
16.2
10.0
24.0
240
13.0
17.2
22.2
22.2
10.0
10.0
9.7
20.0
26.1
26.1
26.1
42.8
42.8
23.1
17.6
24.0
42.8
42.8
27.3
42.8
42.8
Concen-
tration
percent
SS
0.027
.018
.077
.034
.020
.230
.072
.024
.027
.039
.136
.018
.015
.014
.026
.191
.152
.039
.236
.034
.032
.023
.078
.349
.165
.030
.040
.061
Cake
Concen-
tration
percent
SS
9.7
2.3
11.5
10.7
2.0
12.7
10.9
2.0
3.1
2.1
11.5
4.3
10.5
5.8
7.5
13.6
13.3
10.2
15.1
2.8
—
3.2
12.8
14.7
12.1
2.1
1.2
2.1
Mechanical conditions
Percent Bowl
solids speed,
recovered rpm
97
99
91
96
99
73
92
98
96
97
86
98
98
98
97
63
79
95
70
97
—
98
90
55
78
97
98
94
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
3.250
3,250
3,250
3,250
3,250
3,250
3,250
3,250
4,000
4,000
4,000
4,000
4,000
4,000
4,000
4,000
Bowl-
Pond conveyor
setting differential
(rpm)
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-3/4
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
8-5/8
4.0
9.2
3.1
4.2
73
3.3
4.8
6.3
2.8
2.8
4.1
59
4.1
5.4
5.4
2.7
4.0
5.4
4.4
6.0
5.6
8.0
4.0
4.0
5.6
7.1
7.1
6.1
Table 5-13.—Example sludge thickening alternatives
Alternative
Sludge thickening method
Primary Waste activated Combined
Number 1 . . . .
Number 2
Number 3
Number 4
Number 5
Number 6
Gravity
None
Gravity
None
Gravity
Gravity
DAF
DAF
Centrifuge
Centrifuge
Gravity
Required thickener: 9,208/8 Ib/ft2/day = 1,151 ft2 (106.9 m2) or two
27.1 ft (8.26 m) dia. units
Recommended thickener: two 30 ft (9.14 m) dia., 10 ft (3.05 m) deep
units
Thickened product: 9,208/(0.05x 8.34) = 22,082 gals/day (83,590 I/day)
Thickener cost: $160,000
The analysis has shown that capital costs for alterna-
tive No. 2 are slightly less than those for alternative No
1 ($160,000 versus $162,000). Additionally, a thicker
sludge would be obtained with alternative No. 2 (5 per-
cent versus 4.6 percent). This would result in additional
cost savings in the digestion facilities. A similar analysis
for the 40 million gallons (1.75 m3/s) per day plant
resulted in a $167,000 unit (60-foot (18.29 m) diameter)
for the primary sludge, and a $489,000 unit (110-foot
(33.53 m) diameter) for the waste activated sludge (tota
cost $656,000), or two $305,000 units (85-foot (25.91 m
diameter) for the combined sludge (total cost $610,000)
Thus, on the basis of capital costs, alternative No. 1 is
deleted from further consideration.
Alternative No. 6 appears to be a viable solution for
our example plant. However, an initial check of the
thickened sludge product should be made since a
sludge that is too concentrated can actually cause mor<
problems in the anaerobic digestion facilities than a
sludge which is too thin.
93
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Alternative No. 6
Definition—gravity thicken primary sludge; thicken waste activated
sludge by centrifugation. Three shifts (24 hours) 7 day per week oper-
ation of gravity thickeners at both plants and of centrifuges at 40
Mgal/d (1.75 m3/s) plant; two shifts (15 hours) 5 day per week opera-
tion of centrifuges at 4 Mgal/d (.18 m3/s) plant.
Primary sludge
4 Mgal/d (.18 m3/s) 40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 5,204 (2360 kg/day) 52,040 (23,605 kg/day)
Volume (gals/day) 15,600 (59,050 I/day) 156,000 (590,500 I/day)
Recommended thickener
4 Mgal/d—one 20 ft (6.10 m) dia., 10 ft (3.05 m) deep unit
40 Mgal/d—one 60 ft (18.29 m) dia., 12 ft (3.66 m) deep unit
Thickened product
4 Mgal/d—6,933 gals/day (26,240 I/day) of 9 percent sludge
40 Mgal/d—69,330 gals/day (262,400 I/day) of 9 percent sludge
Thickener cost
4 Mgal/d—$64,000
40 Mgal/d—$167,000
Waste activated sludge
4 Mgal/d (.18 m3/s)
40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 4,004 (1816 kg/day) 40,040 (18,161 kg/day)
Volume (gals/day) 48,010 (181,740 I/day) 480,100 (1,817,400 I/day)
Recommended thickener
4 Mgal/d—one 75 gpm unit (4.73 l/s)
40 Mgal/d—one 667 gpm unit (42.08 l/s)
Thickened product (daily average based on 7 day week)
4 Mgal/d—8,002 gals/day (30,290 I/day) of 6 percent sludge
40 Mgal/d—80,020 gals/day (302,900 I/day) of 6 percent sludge
Thickener cost (based on one unit)
4 Mgal/d—$89,000
40 Mgal/d—$280,000
Combined product
4 Mgal/d—[(6,933 X9) + (8,002 X 6)]/(6,933 + 8,002) = 7.39
14,935 gals/day (56,535 I/day) of 7.39 percent sludge
40 Mgal/d—149,350 gals/day (565,360 I/day) of 7.39 percent sludge
The calculations show that a 7.4 percent solids sludge
would result. This exceeds the 4 to 6 percent solids
recommended for efficient digester operation. Thus, alter-
native No. 6 is eliminated from further consideration.
Detailed cost analyses are required for screening the
remaining alternatives.
Cost-Effectiveness Analysis
Design of the thickener units (based on data previous-
ly presented in this paper) and capital costs for those
units will be presented first for the remaining alterna-
tives. Other costs will then be analyzed.
Alternative No. 3
Definition—thicken waste activated sludge with dissolved air flota-
tion; no thickening of primary sludge; two shifts (15 hours) 5 days per
week operation of DAF units at 4 Mgal/d (.18 rrvVs) plant; three shifts
(24 hours) 7 days per week operation of units at 40 Mgal/d (1.75
m3/s) plant.
Waste activated sludge
4 Mgal/d (.18 m3/s)
Required DAF equipment
4 Mgal/d—<4,004X7)/(15X5X2.0 Ib/ft2/hr) = 187 ft2 (17.4 m2)
40 Mgal/d—40,040/(24x 2.0 Ib/ft2/hr) = 834 ft2 (77.5 m2)
Recommended DAF equipment
4 Mgal/d: two 100 ft2 units (9.3 m2)
40 Mgal/d: two 400 ft2 units (37.2 m2)
Thickened product (daily average based on 7-day week)
4 Mgal/d—(4,004/0.04x8.34) = 12,002 gals/day (45,430 I/day)
40 Mgal/d—120,020 gals/day (454,330 I/day)
Thickener cost
4 Mgal/d—$82,000
40 Mgal/d—$205,000
Combined product (unthickened primary-t-thickened
WAS)
4 Mgal/d—15,600 + 12,002 = 27,602 gals/day (104,490 I/day) of 4 per-
cent sludge
40 Mgal/d—276,020 gals/day (1,044,900 I/day) of 4 percent sludge
Alternative No. 4
Definition—gravity thicken primary sludge; thicken waste activated
sludge with dissolved air flotation. Three shifts (24 hours) 7 days per
week operation of gravity thickener at both plants and of DAF unit at
40 Mgal/d (1.75 rrrvs) plant; two shifts (15 hours) 5 days per week
operation of DAF unit at 4 Mgal/d (.18 m3/s) plant.
Primary sludge
40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 4,004 (1816 kg/day) 40,040 (18,161 kg/day)
Volume (gals/day) 48,010 (181,740 I/day) 480,100 (1,817,400 I/day)
4 Mgal/d (.18 m3/s) 40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 5,204 (2360 kg/day) 52,040 (23,605 kg/day)
Volume (gals/day) 15,600 (59,050 I/day) 156,000 (590,500 I/day)
Recommended thickener
4 Mgal/d—one 20 ft (6.10 m) dia., 10 ft (3.05 m) deep unit
40 Mgal/d—one 60 ft (18.29 m) dia., 12 ft (3.66 m) deep unit
Thickened product
4 Mgal/d—6,933 gals/day (26,240 I/day) of 9 percent sludge
40 Mgal/d—69,330 gals/day (262,400 I/day) of 9 percent sludge
Thickener cost
4 Mgal/d—$64,000
40 Mgal/d—$167,000
Final sludge
4 Mgal/d (.18 m3/s) 40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 4,004 (1816 kg/day) 40,040 (18,161 kg/day)
Volume (gals/day) 48,010 (181,740 I/day) 480,100 (1,817,400 I/day)
Recommended thickener
4 Mgal/d—one 200 ft2 unit (18.6 m2)
40 Mgal/d—one 800 ft2 unit (74.3 m2)
Thickened product (daily average based on 7-day week)
4 Mgal/d—12,002 gals/day (45,430 I/day) of 4 percent sludge
40 Mgal/d—120,020 gals/day (454,300 I/day) of 4 percent
sludge
Thickener cost
4 Mgal/d—$55,000
40 Mgal/d—$91,000 (built-in-place unit, equipment only)
Combined product
4 Mgal/d—[(6,933 x 9) + (12,002 x 4)]/(6,933 +12,002) = 5.83
18,935 gals/day (71,680 I/day) of 5.83 percent sludge
40 Mgal/d—189,350 gals/day (716,800 I/day) of 5.83 percent sludge
Alternative No. 5
Definition—thicken waste activated sludge by centrifugation; no
thickening of primary sludge. Two shifts (15 hours) 5 days per week
operation of centrifuge units at 4 Mgal/d (.18 m3/s) plant; three shifts
(24 hours) 7 days per week operation of units at 40 Mgal/d (1.75
m3/s) plant.
94
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Table 5-14.—Thickener product and anaerobic digester requirements
Digester influent sludge
Digester volume (ft3) Digester cost
Alternative
Volume (gals/day)
Percent
solids
4 Mgal/d 40 Mgal/d 4 Mgal/d 40 Mgal/d 4 Mgal/d 40 Mgal/d
Number 2
Number 3
Number 4
Number 5
50
40
5 83
4 68
22082
27602
18,935
23602
220 820
276 020
189,350
236 020
58034
71 938
49,683
61 661
580 340
71 9 380
496,830
616610
$789 000
877 000
742,000
806,000
$4 074 000
5310000
3,425,000
4,361 ,000
Notes: If thickeners were not used, digester influent sludges would be as follows:
Alternative No. 2— 4 Mgal/d, 36,803 gals/day of 3.0%
40 Mgal/d, 368,030 gals/day of 3.0%
All other alternatives— 4 Mgal/d, 63,610 gals/day of 1.74%
40 Mgal/d, 636,100 gals/day of 1.74%
Digester design is based on the thickened sludge, 85° F. temperature, 20 days detention, 75 percent
sludge volatile content, 2,302 pounds of dry solids per million gallons of wastewater and the volatile sludge
loading factor method. Digester costs are based on two high rate units for each plant.
Final sludge
4 Mgal/d (.18 m3/s) 40 Mgal/d (1.75 m3/s)
Quantity (Ibs/day) 4,004 (1816 kg/day) 40,040 (18,161 kg/day)
Volume (gals/day) 48,010 (181,740 I/day) 480,100 (1,817,400 I/day)
Recommended thickener
4 Mgal/d—two 38 gpm units (2.40 l/s)
40 Mgal/d—two 334 gpm units (21.07 l/s)
Thickened product (daily average based on 7 day week)
4 Mgal/d—4,004/(0.06x 8.34) = 8,002 gals/day (30,291 I/day) of
6 percent sludge
40 Mgal/d—80,020 gals/day (302,910 I/day) of 6 percent sludge
Thickener cost
4 Mgal/d—$116,000
40 Mgal/d—$324,000
Combined product
4 Mgal/d—[(15,600 X 4) + (8,002 X 6)]/(15,600 + 8,002) = 4.68
23,602 gals/day (89.340 I/day) of 4.68 percent sludge
40 Mgal/d—236,020 gals/day (893,440 I/day) of 4.68 percent sludge
The design calculations for the various alternatives
indicate that they will result in different sludge moisture
contents and sludge volumes. These data and the resul-
tant required anaerobic digester volumes and costs are
summarized in table 5-14. As shown by the data, con-
siderable digester cost savings are possible with the
thicker sludges.
The example plants are located in the Midwest. There-
fore, the problem of possible freezing temperatures
needs to be addressed. Except for icing of weirs and
possibly a thinner product sludge, exposed gravity thick-
ener operation should not be seriously affected in freez-
ing weather. Flotation and centrifuge equipment, how-
ever, should be located in heated enclosures to prevent
freezing of the exposed piping and to protect corrodible
components from the elements. Besides housing the
thickening equipment, the structure should also provide
space for polymer feed equipment, and for polymer stor-
age if polymers are to be used. At the example plants,
assume that existing building space is fully utilized and,
thus, any thickener building would be new construction.
The required building space and associated costs for
alternatives utilizing flotation or centrifugal thickening are
presented in table 5-15. Polymers are required with al-
ternatives Nos. 4 and 5. Storage space for a 30-day
supply has been included in the required building area.
All capital costs for the alternatives have been summa-
rized in table 5-16.
Power requirements and associated costs vary with
the type and size of thickeners utilized. Gravity thicken-
ing systems require power for the operation of raw and
Table 5-15.—Required thickener building space
Thickener description
Building description
Alternative
#3-4 Mgal/d.. .
#3-40 Mgal/d..
#4-4 Mgal/d....
#4-40 Mgal/d ..
#5-4 Mgal/d ..
#5-40 Mgal/d ..
Type
DAF
DAF
DAF
DAF
Centrifugal
Centrifugal
Unit
size
2-100 ft2
2-400 ft2
1-200 ft2
1-800 ft2
2-38 gpm
2-167 gpm
Area
(ft2)
1,520
2,750
1,150
2,050
770
1,000
Height
(ft)
12
14
14
10
10
10
Building
cost
$84,000
136,000
75,000
"181,000
49,000
58,000
'includes concrete tankage.
95
-------
Table 5-16.—Capital costs
Alternative
description
(Mgal/d)
Thickeners
Supportive
equipment
Building
Anaerobic
Total
Number 2.
Number 3.
Number 4.
Number 5.
4 Mgal/d plant
$160,000
82,000
119,000
116,000
$18,000
28,000
46,000
28,000
$84,000
75,000
49,000
$789,000
877,000
742,000
806,000
40 Mgal/d plant
$967,000
1,071,000
982,000
999,000
Number 2
Number 3
Number 4
Number 5
610,000
205 000
258 000
324 000
24,000
44,000
68000
44000
_ „ _
136,000
181 000
58000
4,074,000
5,310000
3 425 000
4 361 000
4,708,000
5,695,000
3,932 000
4 787 000
thickened sludge pumps and the sludge collector drive.
Dissolved air flotation systems also require power for
raw and thickened sludge pumps, but additionally for a
recirculation pump, reaeration pump (if present), bottom
collector drive, skimmer drive, air compressor, polymer
feed system (if present), and heating and lighting of the
thickener building. Centrifugal thickening systems require
power for the raw thickened sludge pumps, bowl drive,
conveyor drive (if present), polymer feed system (if pre-
sent), and heating and lighting of the thickener building.
Since the required anaerobic digester volume differs with
the four alternatives, the yearly sludge heating require-
ments will also vary. These sludge heating costs need to
be included in the thickener cost-effectiveness analysis
since they are directly related to thickening process.
Total operating horsepower, thickener building heating
requirements, and associated power costs for the vari-
ous alternatives, excluding digester heating costs, are
presented in table 5-17. Building lighting costs were
determined insignificant and are not presented. Operating
horsepower figures include influent and effluent sludge
pump motors which total as follows: Alternate No. 2: 4
million gallons per day (.18 rrrVs)—1 horsepower (.75
kW), 40 million gallons per day (1.75 m3/s}—5 horse-
power (3.73 kW); Alternate No. 3: 4 million gallons per
day (.18 m3/s)—1-1/2 horsepower (1.12 kW), 40 million
gallons per day (1.75 m3/s)—4-1/2 horsepower (3.36
kW); Alternate No. 4: 4 million gallons per day (.18
Table 5-17.—Thickening power requirements and costs
Alternative
(Mgal/d)
Power requirements
Equipment Heating
(operating hp) (Btu/year)
Yearly power costs
Equipment Heating
Total
Number 2, gravity thickener
Number 3, DAF thickener
Number 4, gravity thickener
Number 4, DAF thickener
Number 5, centrifugal thickener.
Number 2, gravity thickener
Number 3, DAF thickener
Number 4, gravity thickener
Number 4, DAF thickener
Number 5, centrifugal thickener.
5
50
2.5
40
42.5
11
140
4
110
106
4 Mgal/d plant
42.5
1.85X108
1.63X108
8.60 X107
40 Mgal/d plant
114
$1,306
5,817
653
4,653
4,944
— 2,874
3.91 X108 36,581
— 1,045
2.08 X108 28,743
1.12 X108 27,697
5,306
29,788
$765
675
355
1,620
855
$1,306
6,582
653
5,328
5,299
2,874
38,201
1,045
29,598
5,981
30,643
465 28,162
96
-------
m3/s)—2-1/2 horsepower (1.87 kW), 40 million gallons
per day (1.75 m3/s)—6-112 horsepower (4.85 kW); Alter-
nate No. 5: 4 million gallons per day (.18 m3/s)—1-1/2
horsepower (1.12 kW), 40 million gallons per day (1.75
m3/s)—4-1/2 horsepower (3.36 kW). Power costs for
equipment operation are based on a rate of $0.04 per
kilowatt-hour ($1.11/mJ). Power costs for heating the
building are based on using fuel oil at a cost of $0.45
per gallon ($.12/1). The cost associated with heating the
sludge in the anaerobic digesters and the total power
costs for each alternative are presented in table 5-18.
In developing heating costs for the digester, it was as-
sumed that auxiliary fuel (fuel oil at a cost of $0.45 per
gallon ($.12/1)) would be required 50 percent of the
time.
Polymers are required for dissolved air flotation thick-
ening and may be required for centrifugal thickening of
the waste activated sludge. Polymer requirements quoted
by the various equipment manufacturers vary consider-
ably for the same type process. Average polymer re-
quirements based on several submittals and data from
existing installations and the associated costs are pre-
sented in table 5-19.
Labor associated with operating and maintaining the
thickening equipment varies with the complexity of the
process. The continuously operating gravity thickener re-
quires a visual inspection only once a shift, whereas the
more complex dissolved air flotation and centrifuge sys-
tems should be checked every 2 or 3 hours. The in-
spections should be visual checks on the product quality
and also on the operating conditions of all system com-
ponents. Additional time for startup and shutdown of
either the dissolved air flotation or centrifuge systems
must be included if they are not operated on a continu-
ous 24-hour basis (Alternatives Nos. 3, 4, and 5 for the
4 Mgal/d (.18 m3/s) plant). Startup and shutdown time
Table 5-18.—Digester heating costs and alternative total
power costs
Table 5-19.—Polymer requirements and costs
Alternative
(Mgal/d)
Number 2
Number 3
Number 4
Number 5
Number 2
Number 3
Number 4
Number 5
Digester heating
(Btu/year) (Cost/year)
4 Mgal/d plant
1 6831 X109 $6749
2 0820 X 1 09 8 61 5
1.4563X109 6026
1 7875 X109 7397
40 Mgal/d plant
1 5628 X1010 64 668
1 9415X1010 80338
1 3426 X1010 55556
1 6851 X1010 69729
Table 17
power
costs
(cost /year)
$1 306
6582
5,981
5,299
2874
38201
30643
28,162
Total
yearly
power
costs
$8055
15197
12,007
12,696
67542
118539
86199
97,891
Polymer Polymer cost
renuireri
Alternative
Number 3 and 4- 4 Mgal/d DAF) ..
Number 3 and 4—40 Mgal/d DAF)
Number 5- 4 Mgal/d centrifugal)
Number 5—40 Mgal/d centrifugal) . .
(Ib/ton
of dry Unit
solids) ($/lb)
35 008
35 008
6 1 80
6 1 80
Yearly
total
$2,046
20,460
7,892
78.92C
probably amounts to a total of about 1 hour per day.
Routine sampling and testing of the thickener influent
sludge, effluent sludge, and supernatant is required for
any type thickener. The tests involved are essentially the
same regardless of thickener type or size. Testing must
be done more frequently, however, on DAF and centri-
fuge systems than on gravity systems. Routine mainte-
nance includes such things as lubricating equipment and
daily washdown or cleanup operations. At least once a
year, all thickeners should be dewatered, throroughly
inspected, and repaired, as necessary. Painting of cor-
rodible components will probably be necessary at 5-year
intervals. Solid bowl centrifuge conveyors may have to
be resurfaced or replaced after 5,000-10,000 hours use,
depending upon the amount of grit in the sludge and
conveyor construction. A summary of the yearly opera-
tion and maintenance time and the associated costs for
each alternative are presented in table 5-20.
Maintenance materials costs were developed from in-
formation provided by equipment manufacturers and date
from existing installations. The materials costs shown in
table 5-20 are estimates and, hence, may not be indica-
tive of the costs associated with any one particular man
ufacturer's equipment. These costs may be described as
percentages of the thickener system capital costs as
follows: gravity thickening, 0.3 percent for small installa-
tions and 0.2 percent for larger installations; dissolved
air flotation, 1 percent for small installations and 0.9
percent for larger installations; centrifugation, 5.2 percen
for small installations and 3.1 percent for larger installa-
tions.
Power, chemicals, and operation and maintenance
yearly costs have been summarized in table 5-21. Since
the power requirements for the gravity thickening alter-
native (alternative 2) are low and chemicals are not
required, it has the lowest yearly operating cost of all
the alternatives. Although the centrifugation alternative
(alternative 5) has power costs similar to those of the
dissolved air flotation alternative (alternative 4), the year
ly operating cost is considerably higher due to much
higher chemical and operation and maintenance costs.
The alternatives' total capital costs and total yearly
costs previously derived in tables 5-16 and 5-21, re-
spectively, are repeated in table 5-22. The data show
97
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Table 5-20.—Operation and maintenance time and costs
Alternative
description
Operator's time
(hrs/year)a
($/year)6
Maintainer's time
(hrs/year)a
($/year)b
Material
cost
($/year)
Total cost
($/year)
Number 2, gravity ....
Number 3, DAF
Number 4, DAF
Number 4, gravity
Number 5, centrifugal.
4 Mgal/d plant
483
1,416
868
373
1,659
$2,415
8,496
5,208
1,865
9,954
252
586
293
126
264
$1,260
3,516
1,758
630
1,584
,$535
1,100
830
245
C6,000
$4,210
13,112
7,796
2,740
17,538
10,536
40 Mgal/d plant
Number 2
Number 3
Number 4,
Number 4
Number 5,
qravitv
DAF
DAF
Qravitv
centrifugal . . .
483
2496
1,408
373
2,739
2415
14 976
8448
1 865
16,434
440
804
402
220
445
2200
4 324
2412
1 100
2,670
1 260
2240
1 215
380
C1 0,000
5875
22 040
12075
3 345
29,104
15420
Time variances are due to equipment and operating time differences noted in the alternative defini-
tions.
"Costs are based on $5/hr wage for gravity thickener operators/maintainers; $6/hr wage for DAF or
centrifuge operators/maintainers.
cCosts are based on replacing conveyor after 7,500 operating hours.
Table 5-21.—Yearly operating cost summary
Table 5-22.—Cost summary and rank
Alternative Operation
description Power Chemicals and Total
(Mgal/d) maintenance
Number 2.
Number 3 .
Number 4..
Number 5.
4 Mgal/d plant
$8,055 —
15,197
12,007
12,696
$2,046
2,046
7,892
40 Mgal/d plant
$4,210
13,122
10,536
17,538
$12,265
30,355
24,589
38,126
Number 2
Number 3
Number 4
Number 5
67 542
1 1 8 539
86,199
97,891
20460
20,460
78,920
5875
22040
15,420
29,104
73,417
161 039
122,079
205,915
Alternative
description
Capital
costs
Yearly
Ranking operating Ranking
costs
4 Mgal/d plant
Number 2 $967,000 1 $12,265 1
Numbers 1,071,000 4 30,355 3
Number 4 982,000 2 24,589 2
Numbers 999,000 3 38,126 4
40 Mgal/d plant
Number 2
Number 3
Number 4
Number 5
4 708 000
5 695,000
3,932,000
4 787,000
2
4
1
3
73,417
161,039
122,079
205,915
1
3
2
4
that for the 4 million gallons per day (.18 m3/s) plant,
the least expensive option in terms of both capital and
operating costs is gravity thickening of the combined
sludge followed by anaerobic digestion (alternative 2).
Note that there is only 3 percent difference between the
capital cost of alternative 2 and the third most expen-
sive alternative (in terms of capital costs—alternative 5).
The results for the 40 Mgal/d (1.75 mVs) plant are
somewhat different than those for the 4 Mgal/d (.18
m3/s) plant. In this case, the least costly alternative in
terms of capital costs does not correspond with the
least costly one in terms of yearly operating costs. Addi-
tionally, for the 40 Mgal/d (1.75 m3/s) plant, the least
costly alternative (capital costs) is not alternative No. 2
(as was the case for the 4 Mgal/d (.18 m3/s) plant) but
alternative No. 4. Also, in this case, there is a 22 per-
cent difference between the capital cost of the least
expensive and the third most expensive alternative. Since
the lowest capital cost and lowest operating cost alter-
natives do not correspond, a present worth analysis
98
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Table 5-23.—Present worth analysis 40 Mgal/d alterna-
tives No. 2 and No. 4
Construction cost
Project cost8 . .
Project contingency
Total cost for facilities
Constant 1-20 O & M. costs
Salvage value . . .
Present worth
Initial project
PW. constant O. & M
Subtotal
P W salvage value
Total present worth
Average annual equivalent cost
Alternative
No. 2
$4,708,000
5,273,000
527,300
5,800,300
73,400
1,527,200
5,800,300
800,800
6,601,100
423,300
6,177,800
566500
Alternative
No. 4
$3,932,000
4,482,500
448,300
4,930,800
122,100
1 ,247,000
4,930,800
1,332,100
6,262,900
345,700
5,917,200
542,600
a Includes costs associated with engineering, legal and administrative,
inspection, surveying, soil borings, start-up and generation and mainte-
nance manual, and interest during construction.
would be required to make the final selection. Although
the alternative capital cost rankings varied with plant
capacity, the yearly operating cost rankings did not.
Gravity thickening of the combined sludge followed by
anaerobic digestion had the lowest operating costs; cen-
trifugation of the waste activated sludge or thickening of
primary sludge, followed by anaerobic digestion, had the
highest. A present worth analysis is presented in table
5-23. This analysis shows that alternative No. 4 (gravity
thickened primary sludge and DAF thickened waste acti-
vated sludge) has the lowest average annual equivalent
cost for the 40 Mgal/d (1.75 m3/s) facility.
SUMMARY
The purpose of this paper has been to describe, in
detail, those thickening methods which are currently uti-
lized, and to present the general approach necessary in
evaluation of thickening alternatives by means of a de-
sign example. The methods presented can be used to
analyze a thickening problem at any wastewater treat-
ment plant, regardless of its size or complexity. The
results of the design example are valid for the assump-
tions made. Any change in problem definition could
mean a different solution.
Recommendation of a particular process should be
geared to available operation and maintenance person-
nel. Considerably more skill is required to operate and
maintain dissolved air flotation and centrifuge equipment
than gravity thickeners. The final recommended alterna-
tive process will be one that is agreed upon by the
owner, the engineer, and the regulatory agency.
REFERENCES
1. USEPA, "Process Design Manual for Upgrading Existing Wastewa-
ter Treatment Plants," USEPA Technology Transfer EPA-625/1-74-
004a, October 1974.
2. USEPA, "Process Design Manual for Sludge Treatment and Dispos-
al," USEPA Technology Transfer, EPA-625/1-74-008, October 1974.
3. Water Pollution Control Federation, "Operation of Wastewater
Treatment Plants,"—Manual of Practice No. 11, WPCF, Washington
D.C., 1976.
4. Metcalf and Eddy, Inc., "Wastewater Engineering," McGraw-Hill,
New York, 1972.
99
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Chapter 6
Review of Developments in Dewatering
Wastewater Sludges
INTRODUCTION
This chapter reviews the sludge dewatering operating
experiences which have occurred over the past 4 to 6
years and assesses the impact of these results on future
designs. Particular emphasis is placed on innovative con-
cepts and equipment.
It should be noted that practically all of the innovative
development of new dewatering equipment in the last 4
to 6 years occurred first in Europe (particularly in the
Federal Republic of Germany) and Japan, and has only
begun to be transferred and utilized in the United States
very recently. It will also become apparent that older,
previously dominant equipment and concepts have, in
many instances, been replaced as a direct consequence
of operating results.
In attempting to incorporate the latest and best equip-
ment and concepts into current design, the U.S. design
engineer must be aware of plant operational results with
various alternate systems.
Alternative dewatering equipment and procedures can-
not be evaluated in isolation, but only as part of an
overall system conceptual design. The interrelationship
between sludge processing and liquid stream processing
should always be considered. Previous works1'2 graphical-
ly illustrate the adverse effects of equipment or systems
that provide less than 90 percent capture of influent
solids, and thereby illustrating the profound effect of the
choice of a dewatering system on the operability and
cost-effectiveness of the liquid processing system. Events
of the past 4 to 6 years have further verified this princi-
ple.
There have been indications that the selection of the
type of activated sludge system could have a strong
effect on the relative severity of associated sludge proc-
essing problems.
Table 6-1 lists the effect of various activated sludge
process modifications on yields of excess biomass and
on sludge processability.
While this table is a summary estimate, the trends and
principles involved are accurate. Given the above infor-
mation, it is understandable why some states have
banned the "High Rate" version of the activated sludge
process. Regardless of statutory positions, results at
plants incorporating "High Rate Activated Sludge" are
sufficient to deter its use if the resultant sludge is to be
disposed of in other than liquid form.
In selecting a dewatering system, an item of real con-
cern is the choice of the final or ultimate disposal meth-
od for the sludge or its residue. Indeed, the available
options for final disposal should be known prior to se-
lection of the dewatering system. Fortunately, some of
the new dewatering equipment, by virtue of producing
higher solids content dewatered cake and by offering
the capability to eliminate inorganic conditioning solids in
dewatered cakes, provides considerably more flexibility
than was previously available in matching up a dewater-
ing process and an ultimate disposal process.
Dewatering is essentially always preceded by thicken-
ing and conditioning, and frequently by stabilization. The
essential role of dewatering is to transform a dilute wa-
ter slurry into a damp, moist cake form for either direct
final disposal or for drying as a final product, or for
reduction via an incineration or other combustion proc-
ess prior to final disposal.
In evaluating dewatering processes it is essential to
Table 6-1.—Excess biomass production and sludge processability from
various activated sludge processes
Process
variation
High rate
Conventional
Extended aeration
Pounds biochemical
oxygen demand
per 1,000 ft3
100-1 000
20-40
10-25
Food to
microorganism
ratio
0.4-1 .5
0.2-0.4
0.05-0.15
Pounds W.A.S.
pound biochemical
oxygen demand
removed (typical)
1.07
0.4
0.15
Sludge
processability
Poor
Good
Variable
101
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Table 6-2.—Autothermic combustion4
Parameter
Case A Case B
Gross calorific value
Percent combustible matter in solids .
Percent solids for autothermicity
17,400
60
41.8
29,100
75
18.5
consider more than the direct operating costs, the pro-
duction rate, and the dry solids content of the dewater-
ed cake. The evaluation should include complete materi-
al balances (Quantified Flow Diagram 1, 2) around the
dewatering process with a concurrent evaluation of the
effect of all recirculation streams on other preceding unit
processes, and the effect of all dewatered cake proper-
ties on the processes subsequent to dewatering, includ-
ing final disposal.
To illustrate this point, note in table 6-2 that the
percent dry solids level at which autogenous incineration
occurs is a function of the calorific value of dry solids
in dewatered cake, which in turn varies with the chemi-
cal composition of the solids. The requisite dry solids
level for self sustaining combustion varies from 18.5 to
41.8 percent depending on these factors which are in
turn materially affected by the unit processes to which
the sludge has been subjected prior to dewatering.
ANALYSIS OF RECENT PLANT OPERAT-
ING RESULTS AND IMPLICATIONS FOR
DESIGN
What lessons should the past 5 years of plant operat-
ing results bring to bear on current and future designs?
The following five points which bear consideration:
1. The effect of choice of type of biological process
on sludge processing, and vice versa.
2. The effect of the inclusion of biomass on the
sludge processing system.
3. The effects of processing discontinuity on biomass
or mixed sludge processability.
4. The importance of painstaking analysis of plant re-
sults.
Relative operability and maintainability of various
5.
sludge processing systems or units.
Type of Biological Process Chosen
As previously noted in table 6-1, the selection of the
High Rate activated sludge process variation can result
in a plant having to process a mixed sludge with 65
percent or greater biomass content. Further, that particu-
lar type of biomass is normally much more difficult to
process than other types. While imposition of other de-
sign constraints may have resulted in utilization of the
High Rate process in certain cases, it is apparent that a
current overall system evaluation of alternate conceptual
designs, particularly in the light of operating experiences,
would usually not support the use of the High Rate
system.
Results have also shown that the extended aeration
process, unless kept within certain food to microorga-
nism (F/M) and solids retention time (SRT) ranges can
cause sludge processing problems. These factors further
strengthen the need for adequate testing of sludges from
alternate biological processes prior to selection of same.
Effects of Inclusion of Biomass
The results of the past 5 years are reflected in the
following list:
1. Gravity thickening of mixtures of primary and ex-
cess biomass sludges is usually ineffective (unless
flocculants are used).
2. Recycling of biomass to primary clarifiers is nearly
always a self-defeating process which causes more
problems than it cures.
3. Inclusion of biomass in a mixture with primary
sludge causes settling problems in conventional
two-stage anaerobic digestion systems. This, plus
the need to maximize gas production frequently
makes single stage complete mix anaerobic diges-
tion the process of choice for stabilization prior to
dewatering in plants where sludge stabilization is
required prior to dewatering.
4. Biomass causes poor settleability in elutriation
tanks. These tanks can be modified to serve as
post digestion thickening tanks (with use of floccu-
lants). This is essential for economic dewatering.
5. Inclusion of biomass makes the careful selection of
dewatering systems, including pretreatment process-
es such as conditioning and thickening, essential to
successful design.
Processing Discontinuity and Sludge
Storage Effects
The following list delineates the pitfalls inherent in ex-
cessive accumulation of sludge within a plant brought on
by discontinuity in removal by the dewatering process,
either by excessive unplanned down time, or by design.
1. Development of septicity.
2. Destruction of some of the bioflocculation of the
biomass.
3. Partial solubilization through prolonged aqueous
contact.
4. Increased hydration and more sensitivity to shear
(pumping, etc.).
5. Deterioration of processability occasioned by all
four of the preceding.
102
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Methods of Analyzing Plant Operating
Results
In considering the significance of plant results and
relevance to design decisions, the following four con-
cepts bear consideration:
1. The use of single static numbers as bench marks
for a dynamic, interrelated system can be seriously
misleading.
2. Appreciation of the "Inertia" inherent in moderate
and large plant processing systems is necessary.
3. There is a paramount need to maintain "Steady
State" conditions as much as possible.
4. Recycle or sidestreams should be minimized within
reasonable ranges.
In developing design criteria, it is frequently assumed
that dewatering equipment can be sized using steady
state flow conditions for the overall system with some
allowance for peaking. These assumptions are reason-
able as long as reliable conditioning, thickening, and
dewatering equipment are installed. However, if sludge
removal operations are interrupted for lengthy periods or
fundamental changes are made, then the standard fac-
tors may not be applicable in terms of order of magni-
tude. While some properly aerated sludge storage ca-
pacity is beneficial, storage usage should be minimized
and septicity avoided whenever dewatering is used.
The length of time required to reestablish equilibrium
or steady state conditions in moderate or large size
plants with significant inventories of sludge is much
longer than would normally be anticipated. This "Inertia
Factor" is calculable through the use of mathematical
models. From experience, in large plants, it can take
several months to fully evaluate the effect of changes.
The need to maintain a "Steady State" or equilibrium
removal rate of sludge sufficient to prevent overaccumu-
lation within a plant is paramount. Once an accumulation
problem develops, rapid resolution via accelerated re-
moval rate procedures will prevent further difficulty.
Particular attention must be paid to processes which
inherently cause heavy recycle loads. Processes or
equipment which cause heavy recycle loads can have a
negative effect on sludge removal rate. If large quantities
of sludge have accumulated in a plant either because of
heavy recycle loads or from a shut down period, normal
operating schedules will require alteration. In order to
clean out such an accumulation the "Sludge Removal
Rate" during the transition "Clean Out" period prior to
reestablishment of a normal equilibrium must be much
greater than the normal rate. Unfortunately, if the over-
accumulation is due to processes or equipment which
cause a significant recirculation load of biomass, the
aeration system of the plant will, during the "Clean Out"
period of higher than normal sludge removal rate, be
extremely overloaded and will also produce more excess
activated sludge than normal. Another effect is that
sludge storage renders biomass more difficult to process
and results in a much greater amount of recirculation
than normally would be predicted by "standard condi-
tion" testing figures and criteria.
Relative Operability and Maintainability of
Various Dewatering Systems and Units
The reliability and maintenance characteristics associ-
ated with various types of conditioning-dewatering proc-
esses, equipment, and brands is very important to the
municipality and its personnel, and ultimately to the pub-
lic who pays the bill. In addition to the need to keep
units operating to prevent sludge accumulation and its
attendant bad effects, maintenance costs are a very
important factor in overall system costs.
The only truly accurate source of reliability and main-
tenance cost data is actual plant operational results. To
justify professional process and equipment selection, the
design engineer should acquaint himself thoroughly with
reliability and maintenance parameters by visiting existing
installations and obtaining accurate information from op-
erating personnel. It is also necessary to sort out when
problems are due to poor plant maintenance practices
and when they are due to inherent process or equip-
ment characteristics. If performance data are not avail-
able then they should be specified and a guarantee
provided by the supplier.
The current methodology of bidding and selection of
suppliers to equipment municipal plants has been, in
some cases, a cause of some of the reliability and main-
tenance problems now being experienced. The bidding
documents or plans and specifications should include
cost factors for maintenance and life cycle, and should
be sufficiently complete to ensure that truly equal equip-
ment specified is provided. If this is not done, and the
job is awarded on a strictly lowest price basis, inferior
processes and equipment can be selected.
CONDITIONING FOR DEWATERING
The following list delineates the normal functions of
conditioning for dewatering:
1. Flocculation of suspended solids (particularly fines).
2. Washing out the alkalinity of anaerobically digested
sludge (the original purpose of elutriation).
3. Promotion of rapid formation of a stable drainable
cake.
4. Promotion of cake release from filtration support
media.
5. Enhancement of cake fuel value.
6. Prevention of scale formation and corrosion inhibi-
tion.
The methods used to accomplish the above functions
are as follows:
1. Chemical addition (inorganic).
2. Chemical addition (organic flocculants).
3. Elutriation (new function).
4. Heat treatment (conversion).
103
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5. Ash addition (cake release).
6. Coal addition (fuel value).
7. Polyphosphonate addition (scale inhibition).
Chemical Conditioning
In organic chemical conditioning the most notable oc-
currence has been the increase in total availability of
metal salts, such as ferric chloride, due to the entry into
the market of firms recovering the products from waste
acids.
The dosage and costs for chemical conditioning vary
substantially for activated sludge plants depending on
the type of biological system employed and the overall
sludge processing system. For this reason, unless other-
wise noted, all quotations of typical dosage figures as-
sume that plants are well designed, not involving proce-
dures or systems which are known to materially increase
conditioning demand and costs. An example of the latter
would be to pipeline sludge for several miles prior to
dewatering.
In the organic polyelectrolyte flocculant area, there
have been several developments of consequence to de-
watering processes:
1. New high charge density, high molecular weight
materials in dry powder form, which are more effi-
cient in conditioning the difficult sludges, have be-
come available and are used widely.
2. A new class of compound, the "Mannich" cationic
products, which have different performance charac-
teristics, have been introduced, almost entirely as
liquid products. These materials produce a floe and
drainage characteristic more akin to that produced
by ferric chloride.
3. Emulsion form cationic products of high charge
density and molecular weight have been developed
and are used.
Elutriation
This process had been applied successfully to digest-
ed primary sludge, was misapplied to mixtures of primary
and biomass sludges, and then adapted very successful-
ly as a flocculant aided postdigestion thickening process
to facilitate cost-effective dewatering.1
Heat Treatment
This type process, sometimes called "Thermal Condi-
tioning," is covered in detail in Chapter 4.
City utilities which have had dewatering experiences of
note, some written up in the literature and some not,
are:
Kalamazoo, Mich.
Colorado Springs, Colo.
Chattanooga, Tenn.
Chicago, III.
Columbus, Ohio
Perth, Scotland
• Ft. Lauderdale, Fla.
• Port Huron, Mich.
• Flint, Mich.
• Lakeview, Ontario
• Green Bay, Wis.
In Great Britain, where the most and earliest installa-
tions of the Porteous and Farrer heat treatment process-
es were made, the heat treatment process has been
largely abandoned. In one case, a new plant, never
used, has been offered for sale.
British water authorities detected significant quantities
of refractory organic material in the effluent from plants
dewatering heat treated sludges. The authorities conse-
quently banned recycle of cooking liquors into biological
treatment systems which discharge into rivers subse-
quently used as sources of drinking water, since the
biological systems are incapable of removing the refrac-
tory organic material.
An additional development in dewatering heat treated
sludges has been the need to chemically condition
sludges in a number of cases, either on a spasmodic or
regular basis. In the case of Port Huron, Mich. (Farrer
System), which employs centrifuges for dewatering, rou-
tine use of flocculants at the rate of $8/ton ($8.82/Mg)
of sludge dewatered has been found necessary. Other
heat treatment plants have found flocculants necessary
to promote cake formation to obtain reasonable solids
capture.
To help alleviate scaling problems, Grand Rapids has
found it necessary to condition heat treated sludge with
$3/ton ($3.31/Mg) of polyphosphonat.es.
Various other chemicals have been found necessary to
raise the pH of sludges, to condition boiler feed water,
and to solvent wash scale from heat exchangers.
DEWATERING EQUIPMENT TRENDS
The following is a list of the types of dewatering
equipment or processes normally used in municipal
wastewater sludge processing:
1. Drying beds.
2. Rotary vacuum filters.
3. Horizontal solid bowl centrifuges.
4. Pressure filters.
5. Continuous belt filter presses.
6. Rotating cylindrical devices.
7. Imperforate basket (batch)centrifuges.
8. Lagoons.
Drying beds are widely used at a large number of
plants, particularly moderately sized plants in sunny
climes, but not restricted to same. As will be seen, they
have been the subject of recent developmental improve-
ment activity, both with regard to improved capacities
and mechanical removal facilities.
Whereas rotary vacuum filters were once the common
of mechanical dewatering systems, their incidence of
selection has rapidly decreased due to energy costs, the
104
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problem of cake pick up with certain sludges, and lack
of ability to provide as dry a dewatered cake as various
other devices.
Horizontal solid bowl centrifuges, particularly of the
new low speed type, are still popular where a very high
solids cake is not essential. Their popularity has dwin-
dled to some extent due to energy considerations.
Pressure filters of the ordinary recessed chamber type
have been installed in a few U.S. plants. Results have
been mixed insofar as overall performance is concerned,
despite the attainment of somewhat higher total cake
solids levels (without necessarily improving the ratio of
sewage solids to water) compared to Rotary Vacuum
Filters or Solid Bowl Centrifuges. Major problems are
cost, maintenance, and the frequent need to use high
percentages of inorganic conditioners.
The new continuous belt filter presses have become
the most widely selected dewatering devices for munici-
pal sludge dewatering. Their rapid growth in popularity is
due to ease of operation, low energy consumption, and
the ability (in some models) to produce dewatered cakes
with solids contents much greater than obtainable with
Rotary Vacuum Filters, Centrifuges, or conventional Pres-
sure Filters.
Rotating cylindrical devices, such as the Pernutit DCG,
have been installed in some plants. Their use has been
primarily at small plants and as the first stage of a dual
system which includes an inclined multiroll press (MRP)
for further cake dewatering.
Imperforate basket batch centrifuges have been in-
stalled at a few small plants where a low solids, relative-
ly fluid cake is tolerable.
Lagoon drying is now frequently applied.
DEWATERING METHODOLOGY
Wastewater sludges all form cakes during the dewater-
ing process which are compressible to some degree and
by virtue of this fact and their inherent water binding
nature tend to require application of conditioning proc-
esses to facilitate a reasonable dewatering rate.
The various sludges may be indexed or characterized
by determination of the "Specific Resistance to Filtra-
tion." They may also be characterized by being subject-
ed to standardized bench scale dewatering test proce-
dures (Filter leaf or Buchner funnel tests).
An important facet for design consideration is that
dewatering of wastewater sludges is a "Cake Filtration"
process. The cake which forms during dewatering is the
primary filtration media and relative cake structure and
form throughout the dewatering process will largely de-
termine the efficacy of the system.
In assessing the cost-effectiveness of the pretreatment
methods aimed at improving dewatering it is essential
that the effect of these processes on the type of cake
formed be considered. In most municipal wastewater
treatment plants, if the following steps are effected, a
mixed primary and biological sludge will result which is
amenable to a cost-effective dewatering process yielding
a dewatered cake suitable for either reduction or direct
ultimate disposal in an economic fashion:
1. Maximization of solids capture in well-designed pri-
mary basins so as to provide as much typically
easy to process "Primary" sludge as possible. This
precludes high recycle loads of W.A.S. or thickener
overflows or heat treat cooking liquors to the pri-
mary basins.
2. Selection of biological process variation with rea-
sonable assessment of the amount and type of ex-
cess biomass which will be produced and will have
to be processed. This usually precludes use of
"High Rate Activated Sludge" processes and some
Extended Aeration designs.
3. Use of gravity sludge thickeners only for straight
primary sludge, or if this is not possible, provision
of flocculant dosage capability to ensure reasonable
solids capture and underflow thickened sludge sol-
ids contents when mixed primary-biological sludge
is being thickened.
4. Use of dissolved air flotation or centrifugal thicken-
ing for excess activated sludge prior to mixed
sludge anaerobic digestion, or prior to dewatering if
stabilization is not to be included.
5. If anaerobic digestion of mixed sludge is employed,
use of a single stage complete mix process and a
post digestion thickening process, either gravity set-
tling or dissolved air flotation (DAF).
6. Use of a conditioning process which does not re-
sult in creation of a heavy recycle load, either in
the form of suspended or dissolved solids or in the
form of BOD5 or chemical oxygen demand (COD) or
refractory organics. Likewise the conditioning pro-
cess should not destroy any significant amount of
the matrix forming material in the sludge solids
which will form the cake in the dewatering process,
and should not alter other cake properties requisite
to the succeeding processes.
7. Selection and use of a dewatering device which is
of rugged design, readily maintainable and will pro-
vide a minimum solids capture of 90 percent and a
cake solids content amenable to succeeding proc-
esses. It is, for all practical purposes, always nec-
essary to condition municipal sludges prior to de-
watering.
DRYING BEDS
Sludge drying beds are frequently referred to as
"Sand Beds." In most cases except instances wherein
"paved drying beds" or wedge water screens are used,
sand is the primary drainage and cake support medium.
The recent and continuing development of various types
of Drying Beds prompts the use of that term, rather
than Sand Bed.
Drying Beds are still the most common method of
municipal wastewater sludge dewatering. The only reasor
they are not widespread in use is that they have not
105
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been the subject of any significant degree of develop-
ment and improvement. This situation is changing as
municipalities become more cognizant of their viability
and relatively low cost of construction, operation, and
maintenance when properly designed. An additional pre-
vious deterrent to their use has been the frequent lack
of inclusion of mechanical sludge removal capability and
an understandable dislike by operating personnel, occa-
sioned by a need for manual removal. This deterrent can
be and has been removed in many cases by relatively
minor design modifications to facilitate mechanical re-
moval.
An additional previous deterrent to selection of the
drying bed alternative is that the "Ten State Standards"
do not reflect the application of conditioning to sludges
prior to dewatering. The use of "Ten State Standards"
criteria, which assume no sludge conditioning, can result
in excessive land requirements and the resultant acquisi-
tion costs artificially inflate cost estimates for the drying
bed alternative.
A well-designed and properly operated drying bed can
produce a drier sludge than any mechanical device.
They are also less sensitive to the influent solids con-
centration.
On the negative side, drying beds are generally appli-
cable only to digested or stabilized solids. Though they
are particularly suitable for small installations and the
"Sun Belt," drying beds are used successfully in treat-
ment plants of all sizes and in widely varying climates
(i.e., Chicago southwest treatment plant, the
largest plant in the world).
Drying beds may be roughly categorized as follows:
1. Conventional rectangular beds with side walls, lay-
ers of sand and then gravel with under drainage
piping to carry away the liquid. They are built ei-
ther with or without provision for mechanical remov-
al and with or without either a roof or a green-
house type covering.
2. Paved rectangular drying beds with a center sand
drainage strip with or without heating pipes buried
in the paved section and with or without covering
to prevent incursion of rain.
3. "Wedge-Water" drying beds which include a wedge
wire septum incorporating provision for an initial
flood with a thin layer of water, followed by intro-
duction of liquid sludge on top of the water layer,
controlled formation of cake, and provision for me-
chanical cleaning.
4. Rectangular vacuum assisted sand beds with provi-
sion for application of vacuum as a motive force to
assist gravity drainage.
Mechanism
On drying beds, the dewatering initially proceeds by
drainage and then continues by evaporation. The propor-
tion and absolute amount achieved by drainage will vary
depending on whether or not the cake has been condi-
tioned, and its overall drainage characteristics. An impor-
tant consideration is the relative time period required for
the cake to develop cracks which expose additional
sludge to evaporation effects. Since one of the main
functions of conditioning is to flocculate and immobilize
the smaller "fines" particles in the sludge cake it is
immediately apparent why a conditioned sludge slurry
dewaters in a fraction of the time required for an uncon-
ditioned sludge. The completion of the drainage period
is substantially delayed in an unconditioned sludge by
migration of the fines to the sludge cake sand interface
resulting in some plugging of the uppermost layer of
sand. Maintenance of porous, relatively open structure
within the cake is also essential to evaporation rate.
Conventional Rectangular Beds
Drying bed drainage media normally consists approxi-
mately as follows:
1. The top layer is 6 to 9 inches (15.2 to 22.9 cm) of
sand, usually with an effective size of 0.3 to 1.2
mm and a uniformity coefficient less than 5.
2. About 8 to 18 inches (20.3 to 45.7 cm) of gravel
with size gradation of 1 /8 to 1.0 inch (0.3 to 2.3
cm). The top three inches (7.6 cm) of the gravel
layer are preferably 1/8 to 1/4 in. (0.3 to .6 cm)
size.
3. Underdrain piping with a minimum diameter of 4
inches (10.2 cm) is often vitrified clay with open
joints spaced 8 to 20 feet (2.44 to 6.10 cm) apart.
Recently, plastic pipe is being used to prevent pos-
sible cracking when front end loaders are run
across the bed for sludge removal. If a gridwork of
concrete runways is provided for the front end
loader, the selection of pipe is not critical.
Drying beds are frequently enclosed by glass. The
glass enclosures can materially improve the performance
of the beds, particularly in cold or wet climates. Experi-
ence has shown that in some cases only 67 percent of
the area required for an open bed is required with en-
closed beds. The degree to which, at specific locations,
the space requirement could be reduced and the sludge
loading increased by use of translucent roofing or total
glass enclosure is a function of site rainfall, temperature,
and sunlight prevalence.
Unfortunately, mechanical removal methods have not
normally, in the past, been used with glass enclosed
beds. Obviously the adaptation would not be either diffi-
cult or expensive.
Table 6-3 describes the typical design criteria for
open drying beds.
The combination of the use of chemical conditioning
plus design to permit mechanical sludge removal coupled
with the use of either a translucent roof or complete
glass enclosure with ventilation louvers dramatically low-
ers the space requirement for conventional drying bed
use and should be the first alternative considered for
dewatering in most plants.
The sidestream from drying bed operation consists of
106
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Table 6-3.—Criteria for design of open conventional dry-
ing beds
Type digested
sludge
Primary and humus
Primary and activated...
Primary and activated...
Pre-
treatment
None
None
Chemically
conditioned
Area
(sq.ft./cap.)
1.6
3.0
0.64
Sludge loading
dry solids
(Ib/sq. tt./yr.)
22
15
55
the drainage liquor which may be augmented by rainfall
in the case of open beds. The additional drainage water
is not normally a problem. The drainage water is usually
relatively innocuous and can be recyled into the plant
with impunity.
Drying times in open beds also vary due to climate,
type of sludge, and whether or not it has been condi-
tioned. In good weather, an average of 45 days is rea-
sonable for unconditioned sludge. This period can be
reduced to 5-15 days or less via conditioning.
A typical case study of the use of conventional drying
beds follows.
TAMPA, FLA.—CURRENT PLANT
The current Tampa plant is a primary treatment facility
featuring anaerobic digestion and sand drying beds for
sludge dewatering. The plant is designed for a flow of
36 Mgal/d (1.58 m3/s) and is normally treating 40
Mgal/d (1.75 nrvVs). On occasion, alum and polyelectro-
lyte are used in the liquid treatment phase to meet the
current interim effluent standards.
Drying Bed Details and Operations—
Existing Tampa Primary Plant
Thirty-three beds, each 125 by 60 feet (38.10 by
18.29 m) are employed. The rectangular beds employ a
drainage medium of two sizes of graded sand above
two layers of differently sized stone or gravel. The beds
are usually refurbished every 2 to 3 years, at most.
Current anaerobically digested primary sludge production
is estimated to be 56,000 gallons (211,980 I) of 3.0
percent dry solids content per day. This is equivalent to
14,000 pounds/day (6350 kg/day) of dry solids. With 33
beds of 7,500 square feet (696.8 m2) area each, the
total available drying area is 247,500 square feet (22,993
m2).
The 33 older drying beds at Tampa are not covered
so the drying cycle varies somewhat due to rainfall vari-
ation. Nonetheless, the operation has been so successful
that the new expanded advanced waste treatment (AWT)
plant which will be in operation shortly is also equipped
with drying beds for sludge dewatering. Tampa has for
about 3 years regularly used polyelectrolytes for condi-
tioning the sludge on its way into the drying beds. Dry-
ing time to liftable cake conditions without conditioning
used to run 30 days minimum. With chemical condition-
ing, the drying time varies from 8 to 15 days depending
on rainfall pattern.
Tampa features front end loader mechanical removal
of dried sludge cake from the beds. One man can easily
empty one bed in 6-8 hours. Previous removal methods
involved use of 5 men for 1-1/2-2 days to remove
sludge from one bed.
Figure 6-1 is a photograph of the mechanized sludge
removal equipment used at Tampa on the drying beds.
Current operating procedure involves pumping about
55,000 to 60,000 gallons (208,200 to 227,130 I) of di-
gested sludge onto a bed with in-line dosing of cationic
liquid polyelectrolyte at a dosage rate of about 50
pounds per ton (25 kg/Mg). The price of the liquid
cationic polymer is $0.13 per pound (0.29 per kg) on an
as is, liquid basis making the conditioner cost $6.50 per
ton ($7.17 per Mg) of dry solids.
Taking the estimated bed loading volume of 56,000
gallons (211,980 I) of 3.0 percent sludge and an aver-
age drying time of 11.5 days, the solids loading rate on
the current Tampa beds is 60 pounds/square foot/year
(292.8 kg/m2/year). It should be noted that current
practice is to produce a very dry cake as shown in
figure 6-2.
TAMPA, FLA.—NEW ADVANCED WASTE
TREATMENT PLANT
Tampa has installed and is now starting up a new
plant which features biological nitrification and denitrifica-
tion with chemical addition for phosphorous removal. The
Figure 6-1.—Mechanized sludge removal at Tampa.
107
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Table 6-4.—Tampa AWT plant, estimated annual aver-
age unstabilized byproduct solids production
Year
Item
Figure 6-2.—Dried cake appearance on bed at Tampa
before removal.
new plant is designed for a treatment capacity of 60
million gallons per day (2.63 m3/s).
Aerobic digestion for advanced waste treatment plant
(AWT) sludges and anaerobic digestion for primary
sludge plus 32 new sand drying beds (each 100 by 140
feet 30.48 by 42.67 m) were included in the new facility.
While the original concept of the new facility was to
aerobically digest the excess biological solids and to
dewater them separately on the new drying beds, along
with the AWT chemical solids, considerable flexibility was
designed into the plant and the eventual process config-
uration to be utilized will be selected on an empirical
basis. There is some apprehension regarding the energy
costs for aerobic digestion which was designed into the
plant as an option prior to the surge in energy prices. If
aerobic digestion proves too costly, anaerobic digestion
of mixed sludges will be evaluated.
DESIGN EXAMPLE—DRYING BEDS—
6O MGAL/D (2.63 m3/s) PLANT
The design of the new Tampa AWT plants' drying
beds serves as an example of the design of this type
system for a large plant in a subtropical climate.
Estimates of quantities of unstabilized sludge solids to
be encountered in the new plant are summarized in
table 6-4.
Faced with processing the daily volumes of sludges
shown and considering the acceptable results previously
achieved at Tampa with anaerobic digestion, further cal-
culations of the amounts of sludges which would result
from anaerobic digestion of primary solids and aerobic
digestion of AWT solids were carried out and results are
listed in table 6-5.
1976
1985
Primary solids slurry
Ibs/day (dry) 37,000
percent solids 5.0
gals/day 89,000
AWT solids slurry
Biological solids
Ibs/day (dry)
Chemical solids
Ibs/day (dry) 31,000
Total 75,000
percent solids 3.0
gals/day 300,000
Combined solids slurry
Ibs/day (dry) 112,000
percent solids 3.5
gals/day 389,000
37,000
5.0
89,000
44,000 71,000
48,000
119,000
3.0
476,000
156,000
3.3
565,000
Table 6-5.—Tampa AWT plant, estimated annual aver-
age stabilized byproduct solids production
Year
Item
1976
1985
Primary solids slurry
Ibs/day (dry) 14,000
percent solids 3.0
gals/day 56,000
AWT solids slurry
Biological solids
Ibs/day (dry)
Chemical solids
Ibs/day (dry) 31,000
Total 69,500
percent solids 5.0
gals/day 169,000
Combined solids slurry
Ibs/day (dry) 83,500
percent solids 4.4
gals/day 225,000
14,000
3.0
56,000
38,500 57,500
48,000
105,500
5.0
253,000
119,500
4.6
309,000
A series of sludge solids stabilization, dewatering, and
disposal options were then reviewed for reliability, envi-
ronmental impact, and capital plus operating and mainte-
nance costs. Table 6-6 summarizes these cost results.
Based on the comparative costs shown and on other
108
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Table 6-6.—Tampa AWT plant, alternative byproduct
solids systems total cost comparison
Estimated com-
parative costs —
$1 ,000,000
Rank Description
Capital Avera9,e
annual
Average
annual
cost per
ton raw
solids3
1 Air dry—with chemicals—cake
2
3
4
5
6
7
8
9
10
to user
Air dry — cake to user
Air dry — with chemicals — cake
to landfill
Air dry — cake to landfill
Kiln dry — without anaerobic
digestion
Kiln dry — with anaerobic
digestion
Mechanical dewatering
Liquid spray
Liquid slurry to user
Incineration
$11.67
14.14
11.67
14.14
15.18
16.07
15.87
23.79
23.65
21.47
$2.75
2.84
3.31
3.40
b3.44
"3.50
3.84
4.32
4.38
4.71
$96.52
99.81
116.16
119.46
"120.68
b1 22.76
134.85
151.78
153.79
165.49
aBased on 78 tons per day (dry) raw byproduct solids.
bNet after revenue deduction from sale of product.
Table
beds
6-7.—Tampa AWT plant, design criteria—drying
Design year—1985
Air drying beds
Annual
average
Maximum
month
Volume each drying bed (gals at 12" fill
depth) 65,000
Area each drying bed (ft2) 8,690
Number of drying beds 140
Total area (ft2) 1,216,600
Drying time (days)
Solids loading (Ibs/ft2/yr).
Dried solids
Ibs/day (dry)
percent solids
Ibs/day (wet)
tons/day (wet)
cu ft/day (wet)
29.5
35.85
119,500
40.0
298,800
149
3,900
65,000
8,690
140
1,216,600
19.6
53.79
179,300
40.0
448,300
224
5,800
evaluation factors, the alternate of air drying (drying
beds) with use of flocculants was chosen as the most
cost effective.
The total estimated capital cost for the air drying sys-
tem being installed at Tampa currently, including all pip-
ing, auxiliaries such as equalizing storage, site work,
engineering, underdrainage system, etc., was $4,671,000
including $941,000 contingency.
The drying bed operational design criteria are as
shown in table 6-7.
PAVED RECTANGULAR DRYING BEDS
WITH CENTER DRAINAGE
A good example of this type of system is that at
Dunedin, Fla. Figure 6-3 is a photograph of the Dunedin
beds.
As can be seen, the two beds in the left portion of
the photo contain previously loaded sludge which is dry-
ing. The two empty beds on the right are ready to be
loaded.
The Dunedin plant is of interest due to use of a
unique heated drying bed system.
Plant process features:
1. An average flow of 2.5 Mgal/d (.11 m3/s) of pri-
marily domestic wastes.
2. Liquid treatment via primary sedimentation followed
by conventional activated sludge. The plant original-
ly used a contact stabilization system but was con-
verted to conventional activated sludge with positive
results.
3. Primary sludge is subjected to two stage anaerobic
digestion with a Pearth gas recirculation system.
4. The excess activated sludge is thickened in a DAF
unit and most of the thickened WAS then goes into
the anaerobic digester system. Some of the WAS is
subjected to aerobic digestion, but no more than
necessary due to the energy consumption of same.
Figure 6-3.—Paved rectangular heated drying beds,
Dunedin, Fla.
109
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(The operation of the DAF unit is well managed, as
is the entire plant, and the plant is a good refer-
ence point for the proper application of DAF thick-
ening in a smaller plant.)
5. The digested sludges are processed in three differ-
ent ways. A portion is dried on the heated drying
beds prior to use as a soil additive. Some of the
sludges are dewatered on an existing rotary vacu-
um filter when this is required. An additional portion
is disposed of in liquid form via tanker.
6. The digester gas is burned in a hot water heating
system. The heated water is circulated through pip-
ing in the paved portion of the drying beds.
The Dunedin plant has four drying beds (75x25 feet
each) (22.86 x 7.62 m) or 7500 ft2 (696.8 m2) of evapora-
tive drying area. The drainage drying area, due to the
type of construction is only a fraction of the evaporative
area. The beds are heated, as noted, but are not cov-
ered and the Tampa Bay area has a high average annu-
al rainfall. Polyelectrolytes are used to condition the
sludge.
Sludge drying time (averages) to liftable condition is 5
days normally and 12 days in rainy periods. The beds
are charged with 5,000 gallons (13,930 I) of a 2.6 per-
cent dry solids content sludge at a time. Thus the load-
ing rate varies from 18 to 43 pounds (6.35 to 19.50 kg)
of dry solids sludge per square foot (.09 m2) per year.
With a 5-day drying period the 4 beds are capable of
dewatering about 13 dry tons (11.8 Mg) per month.
Certainly the capacity of 43 pounds per square foot per
annum (209.8 kg/m2/year) achieved at Dunedin is sever-
al times greater than the Ten States Standards for con-
ventional open beds.
WEDGEWATER DRYING BEDS
Wedgewater "Filter Beds" or drying beds were de-
signed to introduce sludge slurry onto a horizontal rela-
tively open drainage media in a fashion which would
yield a clean filtrate and also give a reasonable drainage
rate.
The Wedgewater Filter Bed (figure 6—4) consists of a
shallow rectangular watertight basin fitted with a false
CONTROLLED DIFFERENTIAL HEAD IN VENT
. BY RESTRICTING RATE OF DRAINAGE
-VENT
WEDGEWATER SEPTUM-
OUTLET VALVE TO CONTROL
RATE OF DRAINAGE
Figure 6-4.—Cross section of a wedgewater drying bed.
floor of wedgewater panels. These panels have slotted
openings of 1/4 MM and produce a total open area of
8 percent. The boundary of this false floor is made
watertight with caulking where the panels abut the walls,
An outlet valve is fitted in one wall of the bed to com-
municate with the underside of the wedgewater decking.
The controlled drainage rate is obtained by first intro-
ducing a layer of water into the wedgewater unit to a
level above the septum. The sludge is then slowly intro-
duced and in effect, under the proper conditions, floats
on the water layer. After the proper amount of sludge
has been introduced, the initial separate water layer and
drainage water is allowed to percolate away at a con-
trolled rate. The exact procedure varies somewhat with
different types of sludges. It is apparent that for this
concept to perform as intended the sludge and the ini-
tial water layer must be relatively immiscible.
The wedgewater technique is designed to permit con-
trolled formation of a cake at the crucial sludge/support
media interface before any significant quantity of fines
migrates to the interface or into the openings of the
septum or escapes in the filtrate. Since polyelectrolyte
flocculants promote rapid cake formation and bind up
fines they are now used in conjunction with Wedgewater
Filter Bed installations processing municipal sludges.
Each square foot (0.9 m) of wedgewater can normally
dewater between 1/2 Ib (.23 kg) and 1 Ib (.45 kg) of
dry matter per charge. The loading rate depends on the
initial solids concentration of the waste sludge applied.
Most sludges can be dewatered to a handleable condi-
tion of 8 to 12 percent solids within 24 hours. This
process is most practical for the smaller treatment plant
which has an average daily flow of 500,000 gal/day (.02
m3/s) or less. Sludge loading rates of 182-365 Ib/ft2 per
year (882.2-1781.2 kg/m2/year) are normal.
Results with Wedgewater units at 2 U.S. plants are
described in the following paragraphs.
ROLLINSFORD, N.H.
This plant produces an excess biological sludge at the
rate of 150 gallons per day (567 I) at 2 percent dry
solids content. A wedgewater unit, as shown in figure
6-5, is used to dewater the sludge to a solids content
of 8 percent, which is liftable.
A polyelectrolyte conditioner is used in the process.
Calculations from the data in the reference cited show
that conservatively assuming 2 drying cycles per day for
the 15' by 6' (4.57x1.83 m) unit, the production rate
could be 1.1 Ib/hr/sq ft (5.4 kg/m2/hr), or 570 Ib/sq ft
year (2780 kg/m2/year) which is, of course, an order of
magnitude greater than the dewatering rates normally
associated with conventional drying beds. These results
are tempered by the fact that 8 percent, while a liftable
condition for this sludge, is not a particularly high solids
content. It is apparent, however, that higher than 8 per-
cent solids would be readily obtainable with increased
drying times while still maintaining a very high annual
solids loading, if such a higher solids content were re-
quired.
110
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WEDGEWATER PANELS
15'0" X MULTIPLES
OF 3' 0"
x 3" X 3" WOODEN STRIPS
(4) SIDES
PLAN VIEW OF UNIT
Q
SIDE ELEVATION
Figure 6-5.—Wedgewater drying bed, Rollinsford, N.H.
END ELEVATION
3" VENT PIPE
DUNEDIN, FLA.
Additional results on the wedgewater system are re-
ported from work at Dunedin, Fla. At that location, the
biological sludge was dewatered to a solids content of
10.4 percent in 22 hours through the mechanism of the
wedgewater element, use of support water, and the re-
stricted drainage procedure, without the use of polymer
flocculants.
There are 18 U.S. installations of the wedgewater sys-
tem. Several are industrial applications but most are
installed at small plants of the contact stabilization type.
A tillable unit, more or less similar to the lift and
dump mechanism of a dump truck, is available to facili-
tate removal of sludge when slightly fluid cake can be
tolerated or when removal by rake is feasible. The sup-
plier, Hendricks Manufacturing Co. of Carbondale, Pa.,
also supplies design recommendations for mechanical
removal via small front end loader when indicated. A
1 square foot (0.09 m2) bench scale test model is avail-
able for test purposes.
The stainless steel wedgewire septum in the 15- by
6-foot (4.57 x 1.83 m) Rollinsford unit would cost $4,500
at today's prices.
VACUUM ASSISTED DRYING BEDS
At the 4.5 Mgal/d (.20 nrrVs) Sunrise City, Fla., con-
tact stabilization plant, a vacuum assisted
drying bed system has been used for the past 18
months to dewater the 2 percent dry solids sludge.
Principal components of the system are:
1. A rigid multimedia filter top surface.
2. An intermediate void filled with stabilized aggregate.
3. A low impermeable barrier, consisting of reinforced
concrete. (It would alternatively be pre-fabricated
fiberglass.)
Figure 6-6 is a photograph of one of the two drying
bed units showing the sludge being fed onto the surface
of the upper multimedia in one of the beds.
m
-------
Figure 6-6.—Rapid sludge dewatering beds, Sunrise
City, Fla.
The following sequence of operations is used:
1. Sludge is fed onto the filter surface by gravity flow
at a rate of 150 gallons (567 I) per minute to a
depth of 12 to 18 inches (.30 to .46 m).
2. Filtrate is drained through the interconnected voids
of the stabilized aggregate to a sump, from which
it is pumped back to the plant by a self-actuated
submersible pump.
3. As soon as the entire surface of the rigid filter is
covered with sludge, the vacuum system is turned
on to maintain a vacuum of one to 10 inches (2.54
to 25.4 cm) of mercury on the intermediate void
area.
Figure 6-7.—Vacuum assisted drying beds, Sunrise City,
Fla.
Under favorable weather conditions, this system dewa-
ters the 2 percent solids aerobically digested contact
stabilization sludge (a difficult high bound water content
sludge) to a 12 percent solids level in 24 hours without
polymer use, and to the same level in 8 hours if floccu-
lant is used. The 12 percent condition is liftable. The
sludge will further dewater to about 20 percent solids in
48 hours.
The sludge cake is removed from the filter surface
either manually, mechanically by a small hydrostatic drive
front-end loader such as a Melroe Bobcat 520, or by a
vacuum truck.
Controlled tests of this type system have shown that a
sludge loading rate of 306 pounds per square foot year
(1490 kg/m2/year) is attainable.
At Sunrise City plant (figure 6-7), the two 20 feet by
40 feet (6.10x12.19 m) vacuum drying beds are proc-
essing a substantial portion of the total plant load. The
photograph below shows the appearance of a bed at
the end of the drying period and also shows the proxim-
ity to a local athletic field.
The vacuum assisted drying bed system at Sunrise
City is a proprietary system now designed and supplied
by International Sludge Reduction Co.
DESIGN EXAMPLE—DRYING BED FOR
4 MGAL/D (.18 m*/s) PLANT
Basic Assumptions
These assumptions are as follows:
1. The sludge to be processed is an anaerobically
digested mixture of primary and WAS at 4 percent
dry solids content. It is a mixture of 60 percent
primary sludge and 40 percent WAS with the WAS
originating from a conventional activated sludge
system.
2. Ultimate disposal is to be by hauling to a sanitary
landfill, or to farmland or other horticultural use.
3. Equilibrium sludge removal rate of 2.5 tons (2.3
Mg) of dry solids per day to be maintained.
4. The plant is located in the Middle-Atlantic section
of the United States.
Alternate Units for Consideration or Eval-
uation
For a plant of this size, depending on site limitations,
either conventional enclosed drying beds or vacuum as-
sisted enclosed drying beds should be considered. The
economics and other constraints of final disposal, such
as length of truck haul and final solids content require-
ments would bear consideration. Land area availability
would materially affect the choice between gravity or
vacuum assisted drying beds. If excess methane was
available from anaerobic digestion, consideration could
be given to use for heating the enclosed bed air space
during the winter.
112
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For the purposes of this example it is assumed that
sufficient land area is available for either gravity or vac-
uum assisted drying beds.
Evaluation Procedure
The general sequential procedure recommended to be
followed would be similar to that fully described on page
37 in the RVF design example. The only variation would
be that bench scale and/or pilot plant tests on the
drying bed dewatering characteristics of the sludge
would probably have to be planned and carried out
entirely by the consulting engineering firm and the client
for the conventional enclosed bed option. On the vacu-
um assisted bed option the suppliers have developed
small scale testing procedures and could be involved in
the work.
Pilot Scale Tests
Since temperature conditions could affect the sizing of
enclosed beds it is suggested that, in the absence of
available data from existing plants in the same general
area with equivalent sludges, a small greenhouse type
test installation would be in order. Ready-made unitized
small greenhouse enclosures intended for the homeowner
are now available at modest prices and could be adapt-
ed to enclose a small drying bed for test work on both
options.
Design Calculations
It is assumed that the test work has shown that by
enclosing the beds and using in line flocculant condition-
ing the average bed loading for the conventional gravity
system is 55 Ib/ft2 per year (268 kg/m2/year) and for
the vacuum assisted option is 110 Ib/ft2 per year (537
kg/m2/year).
1. Since drying bed operation is a batchwise proce-
dure a sludge storage or surge vessel should be
provided to contain the thickened digested sludge
and serve as a feed tank for the drying beds.
2. Sludge volume rate would be 14,000 gallons/day
(53,000 l/d) or 98,000 gallons (371,000 I) per week,
so a 100,000 gallon (378,500 I) surge vessel would
be required as a feed tank.
3. Assuming tests showed a 12 inch (0.30 m) bed fill
level to be practical, for the conventional gravity
beds loaded at a conservative loading of 47 Ib/ft2
per year (229 kg/m2/year), five beds, each 65 feet
by 120 feet (19.81 x 36.58 m) would be adequate.
4. The use of five beds would permit the bed filling
procedure to average less than two per week on
an annual basis.
5. For the vacuum assisted bed option using a con-
servative design loading of 91 Ib/ft2 per year (444
kg/m2/year) would result in selection of four 50
feet by 100 feet (15.24 by 30.48 m) drying beds.
Additional Considerations
The system should include for either of the two op-
tions, mechanical sludge removal via a front end loader.
An important point in evaluating the two options would
be a determination of the energy requirements involved
in operating the vacuum system in that option.
FUTURE OF DRYING BEDS
An objective review of past results and consideration
of the developments of the past 5 to 7 years in modify-
ing and increasing the dewatering capacity and improv-
ing the mechanical removal capabilities of drying beds
must lead to the conclusion that they should be much
more widely used than at present.
It seems clear that a judicious combination of the
following aspects would in many locations make drying
beds the dewatering system of choice:
1. Provision in the bed design for mechanical removal
via front end loaders a la Tampa, etc.
2. Provision for conditioning of the sludge on its way
into the bed with polyelectrolytes or equivalent as
needed.
3. Inclusion in the design of a translucent roof, or a
total greenhouse type enclosure with adequate ven-
tilation and odor control systems.
4. Where required for capacity purposes some form of
vacuum assistance (a la Sunrise City, Fla.) for in-
creasing the drainage rate and enhancing evapora-
tion where indicated.
If these aspects were included In conceptual designs,
the design criteria in terms of square footage of bed
area required would be many times less than the figures
listed in the Ten State Standards. As a result of this an
overall system evaluation of cost-effectiveness would
surely result in more widespread use of drying beds than
is currently the case.
ROTARY VACUUM FILTERS
There are three normal types of rotary vacuum filters
and they are described in table 6-8.
The first (drum) type was largely displaced by the
latter two due to cloth plugging problems associated
Table 6-8.—Types of rotary vacuum filters
Type
Support media
Discharge mechanism
Drum.
Coil...
Belt...
Cloth
Stainless steel coils
Cloth
Blowback section/doctor blade
Coil layer separation/tines
Small diameter roll, flappers, doctor
blades
113
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with the use of lime and ferric chloride/lime conditioning
systems. The drum type filter does not exhibit cloth
plugging problems with polyelectrolyte flocculants.
The coil filter has been widely used and does have a
positive release mechanism. Care must be exercised with
coil filters to ensure a sufficiently rapid rate of cake
formation to prevent loss of fines through the more open
media involved during the initial phase of cake forma-
tion. This is a relatively infrequent problem and if the
fines problem does occur it is usually symptomatic of
predewatering processes which have destroyed a sub-
stantial portion of the matrix forming material in the
sludge(s) or of inadequate conditioning. Such pretreat-
ment processes will be detrimental in some manner to
any dewatering device.
Belt type filters were introduced to permit continuous
washing of the cloth and ostensibly overcome effects of
plugging by lime or fines. This concept was erroneous in
most cases since the belt washes were not particularly
effective in removing lime. In several plants which had
early installations of the Drum type filter and later instal-
lations of Belt filters side by side, the purported advan-
tages of the Belt filters proved to be illusory. Belt type
filters are particularly prone to cake discharge problems.
Rotary vacuum filters produce typical results when in-
organic chemicals are used for conditioning. The results
appear in table 6-9.
While the data in this table above and the following
one are representative, they should not be used for
design purposes if the actual sludges to be dewatered
are available for lab and/or pilot test work. It should
also be noted that the cake solids figures shown in this
table include the significant amounts of ferric chloride
and lime used so the actual sewage solids content is
lower than what is shown. For instance, the correction
would typically bring the net sewage solids of a 22
percent cake down to a correct figure of 18 percent.
There are instances where a combination of ferric
chloride and polyelectrolyte is employed to maximize ro-
Table 6-9.—Typical rotary vacuum filter results for
sludge conditioned with inorganic chemicals
Table 6-10.—Typical rotary vacuum filter results for
polyelectrolyte conditioned sludges
Chemical dose
(percent)
Type sludge
Ferric
chloride
Yield Cake
(Ib/hr/ solids
ft2) percent
Lime
Raw primary
Anaerobically digested primary .
Primary and humus
Primary and air activated
Primary and oxygen activated ..
Digested primary and air
activated
1-2
1-3
1-2
2-4
2-3
4-6
6-8 6-8
6-10 5-8
6-8 4-6
7-10 4-5
6-8 5-6
6-19
4-5
25-38
25-32
20-30
16-25
20-28
14-22
Type sludge
Chemical
cost
($/ton)
Yield Cal
(Ib/hr/ft2) soli
Raw primary
Anaerobically digested primary
Primary and humus
Primary and air activated
Primary and oxygen activated
Anaerobically digested primary and
air activated
1.5-3
3-6
4-8
5-18
5-15
6-22
8-10
7-8
4-6
4-5
4-6
3.5-6
25-
25-
20-
16-
20-
14-
tary vacuum filter production rate. This is frequently the
case where the sludge has a high grease content and
tends to stick to the filter cloth on belt type filters.
Aluminum chloride or aluminum chlorohydrate are alsi
effective inorganic conditioning agents and where plant
have existing rotary vacuum filters, the availability of
such materials as waste byproducts of industrial plants
worth exploration.
Typical results for polyelectrolyte conditioned sludges
are described in table 6-10.
In point of fact, more of the sludge processed in
plants equipped with rotary vacuum filters is conditione
with polymer flocculants than with inorganic conditioner
The chemical cost is normally about the same for the
use of polyelectrolytes or inorganic conditioners. The u
of polyelectrolytes largely prevails because of more coi
venient handling, less extensive preparation facilities, ai
freedom from corrosion problems, plus the elimination c
significant quantities of inorganic solids in the dewatere
cake.
On the other hand, some plants must use inorganic
conditioners to obtain cake release, provide matrix forr
ing material in the cake, or to facilitate lime addition fc
ultimate disposal.
With a digested mixture of primary and excess active
ed sludge, in most plants, rotary vacuum filters will prc
duce dewatered cakes with cake solids contents within
the 18-22 percent range, which is almost always too
wet for autogenous incineration or some composting
processes. These facts, plus energy costs have causec
the selection rate for rotary vacuum filters to wane coi
siderably.
The sludge feed to rotary vacuum filters should neve
be below 3 percent dry solids content and preferably
should be greater than 4 percent if reasonable produo
tion rates are to be attained.
AUXILIARY DEVICES FOR ROTARY
VACUUM FILTERS
To obtain higher solids cakes from rotary vacuum fil-
ters (RVF), three companies have developed devices
114
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which can further dewater the filter cake. These devices
are, in some cases, specifically designed as add-ons to
existing filters or in others, supplied as integral parts of
the rotary vacuum filter.
The items of reference are:
1. The Eimco Hi-Solids filter.
2. The Parkson Magnum Press high pressure section.
3. The Komline Sanderson Unimat high pressure sec-
tion.
Eimco Hi-Solids Filter
This device combines normal rotary vacuum filtration
with a batch type adjunct pressure filter. The cake while
still on the rotary vacuum filter belt feeds into a small
co-joined stage where it is subjected on one side to
pressure from a rubber diaphragm (50-150 Ib/in.2 g or
3.5-10.5 Kg/cm2) while on the other side (below the
belt) a vacuum is applied to facilitate drainage. Since
this is a batch procedure with the rotation of the rotary
vacuum filter being momentarily interrupted while the
pressure and vacuum are applied in the pressure cham-
ber section, some lowering of production occurs.
Eimco supplies this unit as an integral system and
also supplies the press portion as an add-on device for
existing conventional rotary vacuum filters. This device
was tested on pilot scale at Washington, D.C., and in-
creased the cake solids content from a normal 17 per-
cent up to a level of 25 percent. The sludge tested was
a rather difficult to process mixture of primary and sec-
ondary sludges.
Parkson Magnum Press
This unit (more fully described in the section on Con-
tinuous Horizontal Belt Filters) was evaluated on pilot
scale at Washington, D.C., for dewatering filter cake
from the existing rotary vacuum filters. Filter cake of 18
percent dry solids content was further dewatered to
35—40 percent dry solids with no further conditioning
employed.
Commercial availability of this unit hinges on success-
ful conclusion of development work required to enable
design of a mechanical method of transmitting filter cake
from the rotary vacuum filter to the auxiliary press sec-
tion without degrading the processability of the cake.
Komline Sanderson Unimat
A pilot model of the medium and high pressure sec-
tions of the Unimat was evaluated at Washington, D.C.,
on the cake from the rotary vacuum filters and produced
a cake of 38 percent dry solids. Once again mechanical
development work would be required to facilitate an
installation.
In summation, the three devices briefly described
above offer real promise for providing a means to fur-
ther dewater sludge cake from existing rotary vacuum
filter installations where such a procedure is in order.
DESIGN EXAMPLE—ROTARY VACUUM
FILTRATION 4 MGAL/D (0.18 mVs)
PLANT
Basic System Assumptions:
The sludge is an anaerobically digested mixture of
primary and excess activated sludge which has been
thickened to 4 percent solids via a flocculant aided
post-digestion thickening process. System design has
been such that the sludge mixture is about 60 percent
primary and 40 percent secondary sludge. The sludge is
available for testing.
The ultimate disposal method for the sludge is to be
by hauling dewatered cake to either a sanitary landfill,
or for disposal on farmland, or for composting and horti-
cultural use.
The sludge removal rate required is to average 2.5 dry
tons (2.3 Mg) per day and the cake must possess suffi-
cient dimensional stability to preclude flow out of a
truck.
Alternate Units for Consideration and/or
Evaluation
1. A Coil filter.
2. A Belt type filter.
3. A Drum type filter.
Evaluation Procedure
The sequence to be followed in the evaluation and
design is planned as follows:
1. Verification of the amounts and relative degree of
uniformity of the flow of sludge to be dewatered.
This is to be obtained by review of plant operating
data.
2. Diagnostic bench scale dewatering tests of the
sludge, repeated several times during different oper-
ational periods to assure uniformity. It is absolutely
essential that these tests and any pilot tests be
done on site with fresh sludge.
3. Review of the above results with interested candi-
date suppliers and then repetition of the bench
scale tests in conjunction with suppliers personnel.
4. A pilot dewatering test series should then ensue,
particularly if there is any doubt about any facet of
the dewatering operation. This should be carried
out with at least two of the potential suppliers.
5. Summation of design data should be prepared by
the consulting engineer. Each potential supplier
should be asked to prepare and transmit a report
of the bench and pilot test work including their
design recommendations, including equipment re-
quired, sizing, delivery time, etc., together with
"budget price quotes" and estimates of annual op-
eration maintenance costs, and life cycles of the
various items of equipment.
6. A detailed design should then be prepared and
plans, specifications, conditions of contract, etc.,
115
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forwarded to those suppliers whose equipment and
performance have qualified them to enter a firm
price quotation.
7. From the design and overall system cost data avail-
able, and with full consideration of relative equip-
ment reliabilities, a selection of the supplier can
then be made.
Bench Scale Tests
The "Buechner Funnel" test procedure is well docu-
mented and all suppliers of rotary vacuum filters are
very familiar with it. The "Filter Leaf" test procedure is
likewise readily available.
Normally the Buechner Funnel test, employing a cake
support media identical to that to be employed will sup-
ply all the required information needed. However, if the
dewatered cake shows real signs of sticking to the filter
media, then a leaf test to check this property may be in
order.
In the Buechner Funnel test it is important to:
1. Determine dewatering rate, time to vacuum break
and resultant cake solids after a simulated cycle.
2. Analyze the filtrate for suspended solids, BOD5,
COD, and total dissolved solids.
3. The data from (2), along with analogous sludge
feed data should be used to determine exactly
what total solids capture is being obtained.
4. The cake release characteristics should be carefully
assessed. If a problem is indicated, a leaf test can
be run to observe whether or not the cake falls
freely from a vertically held leaf. If it doesn't, then
a Belt filter will cause release problems.
Pilot Tests
Most suppliers have packaged pilot units which can
be wheeled in for testing. This is advisable, in most
cases.
It is important that the sludge quality during the com-
parative pilot plant tests be reasonably comparable. This
can be verified by concurrent "Buechner Funnel" test-
ing.
Design Calculations
1. Operating cycle to be 35 hours per week (7
hours/day). This permits start-up and wash-up times
within an 8 hour shift
2. One filter, with adequate supply of key spare parts
to be maintained.
3. Size of vacuum filter.—Production rate has been
determined via pilot testing to be 5 Ib/hr/ft2 (24
kg/m2/hr), but to provide a margin of safety, 4
Ib/hr/ft2 (20 kg/m2/hr), will be used. Steady state
sludge removal rate requirement is 35,000 pounds
(15,870 kg) per week. With a 35 hour-per-week
schedule, weekly filter capacity at 4 pounds per
hour per square foot (20 kg/m2/hr) is 140 pounds
(63.5 kg) perft? 35,000 pounds/week (15,870
kg/wk) -s-140 pounds (Ibs/ft2/wk) per square foot
per week (685 kg/m2/wk) = 279 square feet (26 m
of filter area required. The nearest standard size
filter is 300 square feet (28 m2), so a single unit c
this size is chosen.
4. Sizing of auxiliary equipment.—In each case the
details of sizes of vacuum equipment, conveyors o
other system required to get the dewatered cake
into the truck for hauling, and the chemical dosing
equipment for sludge conditioning must be devel-
oped, and priced.
5. Sludge storage capability.—The one shift per day-
five day per week mode of operation plus the use
of a single filter will require provision of several
days storage capacity for the digested sludge. Thii
could potentially be provided by a combination of
the inherent surge capacities of the digestion tank;
and post digestion thickening tanks, or by provisio
of a separate storage tank equipped to ensure ho-
mogeneity of feed to the RVF.
For a sludge of the type described, a cationic poly-
electrolyte flocculant would probably be used for condi-
tioning. The testing and selection of suitable conditionin
agents would necessarily be carried out in conjunction
with the series of bench scale and pilot test programs
used to select and size the rotary vacuum filters. As
part of the selection process for suitable conditioners,
data should be obtained and reviewed on:
1. Price, dosage rate, and availability of both polyele<
trolytes and inorganic conditioners in the particular
locale.
2. The system required for solution preparation and
application, and its cost.
3. The storage stability (shelf life) of the conditioner i
its form as supplied and in stock solution for use.
4. Handling characteristics, safety aspects and corro-
sion properties of the material in dry and liquid
forms.
5. Previous experience with the same materials at oil-
er plants with similar sludges.
DEWATERING SYSTEM CONSIDERATIONS
Auxiliary equipment such as sludge conveyor or re-
moval facilities, chemical mixing and feed equipment, ar
sludge feed pumps are usually available from the rotary
vacuum filter supplier.
Polymer solution preparation and dosing equipment is
also frequently available from the polymer supplier or
from an equipment supplier other than the rotary vacuu
filter supplier.
An Energy Audit should be a part of every system
evaluation. The Energy Audit should include not only ar
estimate of the power consumption of the dewatering
equipment and its immediate auxiliaries, but also the
impact of the particular dewatering system on the overs
treatment process system. In this regard, the assessmer
should specifically include the impact of the
conditioning/dewatering system on both the post dewa-
116
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tering portion of the system and the pre-dewatering por-
tion of the system. The latter facet makes preparation
and consideration of "Quantified Flow Diagrams" for
both the conditioning/dewatering system and the overall
treatment system mandatory to cost effective design.
For purposes of comparison, the rotary vacuum filter
in this design example would require a vacuum pump of
30 horsepower (22 kW), and filtrate pump of 3 horse-
power (2.2 kW). To make a complete energy audit, all
the auxiliary equipment data, and the other points men-
tioned in the previous paragraph would have to be as-
sessed.
DESIGN EXAMPLE—ROTARY VACUUM
FILTRATION—4O MGAL/D (1.75 mVs)
PLANT
Basic System Assumptions
These would be the same as in the preceding design
example for a 4 Mgal/d (0.18 m3/s) plant except that
the required removal rate would be 25 tons of dry solids
per day (22.7 Mg/day).
Other Considerations
The following parts of the Design Example would be
the same as for the 4 Mgal/d plant (0.18 m3/s) in the
preceding example:
1. Alternate units for consideration and evaluation.
2. Evaluation procedures.
3. Bench scale testing.
4. Pilot tests.
Design Calculations
1. Operating cycle.—To be either a seven day per
week, 24 hour per day operation or five day per
week, 24 hour per day operation depending on
reduction and final disposal processes chosen.
2. Size and number of Rotary Vacuum Filters re-
quired.—Production rate to be conservatively taken
at 4 pounds/hour/sq ft (20 kg/hr/m2). At 350,000
pounds (159 Mg) per week the weekly capacity of
a square foot of filter area for a seven day opera-
tion (allowing 2 hours/day downtime average for
clean up and maintenance) is 4 pounds/hour/sq ft
(20 kg/hr/m2) x 154 hours per week or 616
pounds/week/sq ft Dividing 350,000 pounds per
week by 616 (3) gives a filtration area requirement
of 568 square feet (53 m2). A similar calculation for
a five day operation gives a filtration area require-
ment of 793 square feet (74 m2). In either the
seven day/week or five day/week options, two 500
square foot (46 m2) rotary vacuum filters would
normally be specified to provide sufficient capacity
and redundancy.
3. All of the other facets of the design procedure
would be the same as in the 4 Mgal/d (0.18 m3/s)
example.
General Comment—Rotary Vacuum Fil-
ters
The RVF was, for many years, the common device
for dewatering municipal sludges. Their frequency of use
had persisted longer in the United States than in the
rest of the world.
Operating problems such as the cake pick-up difficul-
ties, poor cake release from belt filters with sticky sludg-
es, and the maintenance requirements associated with
vacuum producing equipment have existed in numerous
cases. Solids capture problems associated with either
the effect of less than adequate cake formation rate in
some relatively open media filter installations or with
cake recycle due to sticking problems have also oc-
curred. While these problems could be moderated in
many cases by revision of conditioning methodology or
mechanical changes, they are deterrents to widespread
continued usage.
More universal deterrents to the continued selection of
RVF's are:
1. The energy and maintenance costs associated with
operating vacuum systems.
2. The inability to produce nearly as dry a cake as
other newer devices.
These comments are made to encourage the design
engineer to review current operating and cost experi-
ences at existing plants prior to making a design deci-
sion.
CONTINUOUS BELT FILTER PRESSES
This general type of device, which employs single
and/or double moving belts to continuously dewater
sludges through one or more phases of dewatering was
originally developed, and in subsquent years modified
and improved, in West Germany. The earliest concurrent
U.S. development was under the aegis of the late Brian
Goodman, at Smith and Loveless Division of Ecodyne.
The scope and depth of development of this newer
type device has been much more pronounced in Europe
than in the United States until the past 3 to 4 years.
Within those past 3 to 4 years, many different models of
the same type device, differing in configuration and ca-
pability, have been introduced into the U.S. market.
While there is general agreement that the Continuous
Belt Filter Press (CBFP) materially extends capabilities
for improved dewatering of sludges, the U.S. design en-
gineer is faced with a real task in selecting the optimum
device from the many which are now available. But that
task must be dealt with if advantage is to be taken of
this technological breakthrough.
U.S. installations of the latest and best models are just
now coming onstream. To review actual operating per-
formance on particular sludges, usage of available mo-
bile pilot test units, coupled with site visits is in order.
There is considerable operating experience available at
existing European sites. The old conundrum that Europe-
an sludges are different and results are not applicable
117
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should be treated with the contempt it deserves, since it
is inaccurate.
Original Concept and Evolutionary Devel-
opments—Continuous Belt Filter Presses
Figure 6-8 illustrates the single level device originally
marketed by Klein of Germany and their U.S. licensee,
R. B. Carter.
Practically concurrent with this development was Brian
Goodman's work with the Smith & Loveless Concentrator
which is described later.
This type unit was successful with many normal mixed
sludges. Typical dewatering results for digested mixed
sludges with initial feed solids of 5 percent are to give a
dewatered cake of 19 percent solids at a rate of 6.7
Ib/hr/sq ft (32.8 kg/hr/m2) with a chemical conditioning
cost of $4.10/ton ($4.52/Mg). In general, most of the
results with these units closely parallel those achieved
with rotary vacuum filters. They do have advantages in
that there is no sludge pickup problem which sometimes
occurs with rotary vacuum filters, and they have a lower
energy consumption.
These results are satisfactory for many installations
and the Continuous Belt Filter Press of this first type or
its immediate successor, a two-level unit of the same
basic design and concept (see figure 6-9) has in the
past 5 years become the most frequently selected dewa-
tering device around the world.
There have been additional developments of the basic
principles of the Continuous Belt Filter Press and several
third generation units from various companies are now
available. In a broad sense these latest improvements
may be described as:
1. The addition of some form of continuous mechani-
cal thickening device as the initial stage of a Con-
tinuous Belt Filter Press.
2. The addition of additional medium and/or high
pressure press sections to the Continuous Belt Fil-
ter Press, and variations in the cake shearing
mechanisms to obtain additional dewatering.
PRESS BELT
SLUDGE
FEED
SLUDGE FEED
CHEMICAL
FLOCCULANT
ADDITION
(• \-
FILTERj
BELT
f f
PROCESSED
DIRTY WASH WATER,
FILTRATE, AND
RECYCLE POLYMER
DISCHARGE
PATENTS APPLIED FOR
Figure 6-8.—Original concept: continuous belt filter
press.
Figure 6-10.—Conceptual schematic: R.B. Carter series
31732—CBFP.
118
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DISCHARGE
END
10" APPROX.1
Figure 6-11.—R.B. Carter series 31/32—CBFP.
3. In the second or high pressure zone (4 atmo-
spheres) the sludge is sandwiched between two
sieve belts. Large mesh openings are possible be-
cause the sludge has developed structural integrity
at this point.
4. A serpentine configuration makes up the Shear
Zone at the end of the second pressure zone
wherein by stretching the belts and sludge cake
over smaller rollers, a squeezing action expels more
water from the cake.
As will be noted subsequently in more detailed de-
scriptions of each unit, the advanced third generation
CBFP's give cake dry solids contents equivalent to those
achieved with pressure filters.
In addition to the Carter Series 31/32 device, other
suppliers of similar third generation type devices are:
Company
Unit
Komline Sanderson Unimat
Parkson Co Magnum Press
Ashbrook Simon Hartley Winklepress
Carborundum Sludge Belt Filter Press
Tait Andritz SDM
There are also other Continuous Belt Filter Presses
which are more advanced than the original first genera-
tion type units. These are also described later.
Categorization of Continuous Belt Filter
Presses
Only units which have at least two phases built into
their operation, and which yield cakes which are truly
dewatered and dimensionally stable (nonflowable) can
logically be classified as Continuous Belt Filter Presses.
The Dual Cell Gravity (DCG) Concentrator as supplied
by Permutit when used in series with the Permutit multi-
ple roll press (MRP) is a system which performs as a
continuous dewatering device in a fashion analogous to
the first generation CBFP.
All of the variations start with a gravity drainage zone
followed by various combinations of shear and different
levels of pressure (or vacuum) applied to the gravity
drained cake. Rather than attempting to lump presses of
different configuration into rigid categories, each will be
described and results listed.
SMITH AND LOVELESS (S & L) SLUDGE
CONCENTRATOR
This device, as described in reference 10, was devel-
oped and is marketed by the Smith and Loveless Divi-
sion of Edodyne. It is essentially a "Gravity-Pressure"
filtration unit which uses an endless, variable speed,
relatively open mesh filter screen to retain flocculated
solids while the bulk of liquid passes through the screen.
Solids from the gravity drainage stage pass into the
second or pressure stage where three sets of compres-
sion rollers fjrther dewater the cake. The pressure in-
creases with each set of rollers. The dewatered sludge
falls off the belt into a discharge chute for removal.
The S & L Concentrator is offered in two models of
varying size. Typical dewatering capacities claimed are
described in table 6-11.
As will be noted this device does not give as dry a
cake as some of the other more complicated machines.
It has found usage at certain plants which can utilize
cake solids levels as shown. The unit uses only 5 horse-
power versus a normal 40 horsepower for a rotary vac-
uum filter.
PERMUTIT DCG—MRP
This system consists of a dual cell gravity unit fol-
lowed in series by a multiple roll press. In reference to
the schematic cross section of the DCG, this first drain-
age section forms a plug of fluid sludge in the first fine
mesh nylon cell and then the plug is further dewatered
in cake form in the second cell (see figures 6-12 and
6-13).
The relatively moist cake from the DCG is conveyed
to the MRP, an inclined dual continuous spring loaded
belt which further dewaters the sludge cake.
Table 6-11. — S & L sludge concentrator
estimate
Type of sludge
Anaerobic digester primary
Aerobic digester W.A.S. . . .
W.A.S
Estimated
dewatering rate
(Ib/hr)
Model Model
40 80
250 500
250 500
225 450
performance
Polymer
dosage
Ib/ton
15
10
10
Cake
solids,
percent
12
10
10
119
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DRIVE ROLL
AND SPROCKET
ASSEMBLY
NYLON FILTER
CLOTH
DEWATERING
CELL
SLUDGE
INLET
CON
VEYOR
FILTRATE DISCHARGE
Figure 6-12.—Cross section of a Permutit DCG.
CAKE
DISCHARGE
SLUDGE INLET
J
Figure 6-13.—Cross section of a Permutit MRP.
Typical performance on the DCG-MRP (Caldwell, N.J.)
indicates dewatering of an anaerobically digested mixture
of primary and humus sludge from a feed solids of 4-5
percent yielding a dewatered cake of 15 percent dry
solids with polymer costs of $8 to $10 per ton ($9 to
$11 per Mg).
The DCG-MRP has worked reasonably well at small
plants with noncontinuous dewatering schedules. Some
problems have been noted with maintainability of the
early units and some modifications are in process.
INFILCO DEGREMONT FLOC-PRESS
This is a two stage unit of French origin featuring a
horizontal belt gravity drainage area on a woven synthet-
ic fiber belt followed by a press section. The partially
dewatered cake is sandwiched between the lower belt
and a rubber pressure belt (adjustable hydraulic loading)
to provide cake solids levels similar to that which is
obtained in rotary vacuum filters or centrifuges (see fig-
ure 6-14).
There are 46 world-wide Floe-Press installations and
there were five in the United States as of January 1976.
A notable U.S. installation is at Medford, N.J.11 At Med-
ford, a 0.9 Mgal/d (0.04 m3/s) contact stabilization
plant, a two meter wide Floe-Press replaced an existing
rotary vacuum filter which has been shut down. The
results are shown in table 6-12.
The horsepower consumption is 6.25 (4.7 kW) for the
Floe-Press versus 22 (16.4 kW) for the previously used
rotary vacuum filter. The RVF had provided similar cake
solids but poorer solids capture. Polyelectrolyte costs
are in the $11-15/ton ($12-16/Mg) range. The filter bell
is still in excellent condition after almost a year of oper-
ation. The wash water rate is 22 gpm (1.4 l/s) at 50
Ib/in.2 (3.5 kg/cm2) and plant effluent water is used.
The Floe-Press system includes a mounted sludge con-
ditioning chamber and other auxiliaries such as chemical
conditioner and sludge feed systems, conveyors for
sludge removal and automated control panels.
Output in pounds per foot of belt width per hour is
quoted at 134-268 (200-400 kg/m) for an anaerobically
digester mixture of primary and W.A.S. at a feed solids
of 3.5 to 9 percent, the Medford, N.J., Floe-Press is 16
WOVEN SYNTHETIC
FIBER BELT
PRESSURE BELT
HYDRAULIC JACK
ROTARY DRUM
FLOCCULATOR
DISTRIBUTION
BOX
AIR
ACTUATED
PINCH
ROLLERS
BOTTOM
DRAIN PAN
PRESSURE BELT
FLEXIBLE
SCRAPER
OR DOCTOR
BLADE
SAFETY
SHUT
.DOWN
MONITOR
BELT WASH '
"**"" SPRAY
SUPPORT NOZZLES RUBBER COVERED
ROLLERS DRUM
Figure 6-14.—Infilco Degremont Floe-Press.
Table 6-12.—Floe press results—Medford, N.J.
Feed solids, percent
Cake
Filtrate suspended solids (PPM).
Percent solids capture
Averages
3-4
17-19
100
98
120
-------
feet 1-1/4 inches long, 10 feet 4-3/8" wide, and 10 feet
6 inches (4.9 mx3.1 mX3.2 m) tall.
The Floe-Press is available in belt widths varying from
a nominal 3 feet (0.9 m) to a nominal 10 feet (3 m) with
effective belt areas of 32.28 square feet to 96.84 square
feet (3-9 m2). For the larger units, only additional width
must be provided for.
PASSAVANT VAC-U-PRESS
This is a German development which features the fol-
lowing:
1. A continuous press utilizing gravity and vacuum
drainage followed by a pressure zone.
2. Conditioned sludge is evenly distributed on a mov-
ing belt which initially drains by gravity and then by
virtue of vacuum boxes beneath the belt.
3. The compression belt is applied on top of sludge
on the lower belt to form a sandwich.
4. The two belts are subjected to pressure by going
under tension around large dewatering cylinders.
Pressure is then applied to alternating sides of the
belt by smaller pressure rolls.
5. Dewatered sludge is discharged and belts are con-
tinuously back-washed.
6. The Vac-U-Press is enclosed in a fiberglass rein-
forced polyester housing to control noise and odor.
Typical sizing data are as shown in table 6-13.
There are five U.S. installations of the Vac-U-Press, all
of the BFP-200 model. Indications are that it gives a
dewatered cake slightly drier than a rotary vacuum filter.
A mobile test unit is available for rental.
TAIT ANDRITZ SDM and SDM-SM
Andritz, an Austrian equipment firm, first developed a
continuous double belt filter dewatering device for use
on various industrial sludges. In the past two years Tait
Andritz of Lubbock, Tex., has sold and installed 43 of
these devices at 28 total U.S. locations for dewatering
of various industrial and municipal sludges. The 1977
world-wide installation list shows 68 locations where
these devices are in use. Twenty of these locations are
on municipal sludges. The industrial installations are in
some cases on straight 100 percent biomass sludges.
The dewatering in the Tait Andritz unit(s) is achieved
Figure 6-15.—Tait Andritz—SDM-SM model.
by passage of the sludge through a gravity dewatering
zone, into a wedge zone for pressure dewatering, fol-
lowed by higher pressure dewatering in a module zone.
The module zone can be either an S configuration (off-
set rolls), or a press configuration (pressure loaded rol-
lers).
Main design features are: variable speed drive for
belts and conditioning drum; pneumatic belt tensioning
and pressure adjustment during operation; pneumatic belt
tracking; and in the industrial SDM model, use the end-
less belts.
Figure 6-15 shows the SDM-SM model (seamed belts)
designed for municipal operation where unattended
round-the-clock operation is not necessary.
Table 6-14 summarizes reported operating results.
The results shown in table 6-14 tend to indicate that
the Tait Andritz CBFP's will normally produce a cake
solids content somewhat higher than that obtainable in a
rotary vacuum filter. Further, more definitive results on
the two versions (either the "Press Module" or the "S"
Module equipped) of the basic device will be forthcom-
ing during 1978. In this vein, it is understood that Bur-
lington, Wis. (an installation discussed later) has recently
ordered several units.
The Tait Andritz SMD device (Industrial) has an excel-
lent performance record (ease of maintenance, etc.) in
dewatering biological and mixed sludges in the paper
industry.12'13
Data on the size of the three SDM-SM models avail-
able are shown in table 6-15.
Table 6-13.—Passavant Vac-U-Press—sizing data
Model
Number
BFP075
BFP125
BFP200
Belt
width
26-1/2
43-1/2
... 72-1/2
Length
(ft/in.)
14-9
14-9
14-9
Width
(ft/in.)
4-1
5-8
8-2
Height
(ft/in.)
5-3
5-3
5-3
Drive
motor
(hp)
1.5
3
3
Active
belt area
(Ib/ft2)
90
150
250
Nominal
capacity
(gal/hr)
1,500
2,500
4,200
121
-------
Table 6-14.—Tait Andritz—SDM-SM results
Percent
dry solids
rype sludge
Feed Cake
Throughput3
gal/min
Dry solids
(Ib/hr)
Polymer
cost
($/ton
dry
solids)
Raw primary
Primary and W.A.S.
Unox ext. aer
5-7 22-26 10-14
3-5 20-25 15-20
1-2 18-23 20-25
300-500 4-7
200-350 4-8
200-250 8-10
aPer 20 inches of working belt width.
Table 6-15.—Tait Andritz SDM-SM—machine sizing data
Size and type
SDM 40
SDM 60
SDM 80
Working
belt
width
(in.)
40
60
80
Overall dimensions
Length
(in.)
152-1/2
186
186
Width
(in.)
75
114
134
Height3
(in.)
75
83
83
Weight
(Ib)
5,513
14,333
17,640
Conn
H.P.
load
3-1/2
5-3/4
5-3/4
Belt spray
consumption
(gal/min)
18-24
30-37
35-45
"Height will vary according to drive system used.
ASHBROOK SIMON-HARTLEY WINK-
LEPRESS
The Winklepress was developed by Gebr. Bellmer KG.
of Germany. Simon-Hartley of the United Kingdom mar-
kets U.S. units through a subsidiary, Ashbrook Simon-
Hartley of Houston, Tex.
Figure 6-16 is a schematic conceptual drawing which
shows that the device employs two endless synthetic
fiber mesh sieve belts to convey and dewater condi-
tioned sludge. After an initial gravity sandwich drainage
stage, the primary belt meets the second belt and forms
a vertical sandwich drainage section. The two belts,
which are under tension, then carry the sludge along an
arrangement of staggered rollers where multiple shear
force action areas squeeze out remaining free water.
The sieve belts are continuously washed.
While there are a number of operational installations in
Europe, as of November 1, 1977 none of the U.S. instal-
lations under construction had started operation. (See
tables 6-16 and 6-17.)
KOMLINE SANDERSON UNIMAT GM,H-7
CONTINUOUS BFP
Komline Sanderson manufactures its version of the
German Unimat under license from Mull-Abwasser-Trans-
portanlagen-GMBH, Elversberg, West Germany.
HORIZONTAL
DRAINAGE
SECTION
ROTARY DRUM
CONDITIONER
REAGENT
FEED
VERTICAL
DRAINAGE
SECTION
FINAL
DEWATERING
SECTION
X . ' , . . « ,
y,..,..
BAND
WASH
Figure 6-16.—Schematic of an Ashbrook Simon-Hartley
Winklepress.
122
-------
Table 6-16.—Winklepress test results (from supplier)
Dry solids
Capacity feed
Digested primary and humus
Digested primary and WAS
Feed
38
5.7
35
4.8
Cake
362
36.3
363
38.5
Filtrate
(mg/l)
85
95
90
75
Polymers
(kg/m3)
0.182
0.165
0.165
0.182
m3/h
meter
7.5
6.5
7.5
7.5
gal/min
33.0
28.6
33.0
33.0
Table 6-17.—Winklepress size and capacity data
Input width
Winklepress
size
Nominal
capacity of
digested sludge
mm
inches m3/h gal/min
0
1
2
3
4
200-300
500-800
1 000-1 300
1,500-1,800
2,000-2,300
8-12
20-32
39-51
59-71
79-91
2-3
5-8
10-13
15-18
20-23
8.8-13
,22-35
44-57
,61-79
,88-101
The most advanced model of the modularized Unimat
(figure 6-17) which is designed for maximum cake dry-
ness and throughput is the GM2H-7. This press consists
of four stages:
1. Gravity drainage (actually a thickening stage)
2. A mild pressure stage
3. A medium pressure stage
4. A high pressure stage
The initial gravity drainage stage is a continuous belt
of pockets which are formed by folding a rectangular
piece of cloth. This is a separate belt. After thickening
in this first stage the sludge dumps into a different belt
which moves over a gravity drainage tray prior to dump-
ing onto another belt on a succeeding tray (and a dif-
FLOCCULATED
SLUDGE
•GRAVITY DRAINAGE
STAGE
SLUDGE DUMPS TO NEW BELT:
INTERNAL WATER INCREASED
SLUDGE DUMPS
TO NEW BELT-
INTERNAL WATER
RELEASED
MILD
PRESSURE
STAGE
MEDIUM PRESSURE STAGE
Figure 6-17.—Komline Sanderson Unimat GM2H-7.
—- GRAVITY DRAINAGE BELT
—} PRESSURE BELTS
@ ADJUSTABLE PRESSURE ROLLS
123
-------
ferent belt) where a small amount of pressure is added
by small diameter rollers. Then it is moved to the third
tray of the mild pressure section (and back on the origi-
nal pressure belt) and subjected to slightly more pres-
sure before going into the medium pressure stage. All
the rollers in the medium pressure stage are adjustable
for pressure optimization. While passing over the medium
pressure rolls, the cake sandwich between the belts is
flexed from one side to the other. Each of the large
diameter drums has smaller diameter rolls which apply
pressure as the sandwich passes over the drums. Every
other roll is perforated for water removal. Pressure is
applied to the cake by tension on the belts as the belts
go around the drums and by the small diameter rollers.
The belt tension is, however, relatively low and all syn-
thetic media is used instead of stainless steel in the
long axis.
The cake now goes to the high pressure stage which
can be thought of as two caterpillar tractors standing
upright with the tracks butting together. As in the medi-
um pressure section the pressure is adjustable through
springs.
In applications where a very high dry solids in the
cake is not imperative, the unit is available without the
high pressure section.
In addition to the previously mentioned nomenclature
and model system the Unimat series is available in three
models:
Model S Gravity stage
Model SM Gravity and medium pressure stages
Model SMH Gravity, medium and high pressure stages
Table 6-18.—Active filtration surface areas and retention
times
Machine
Machine model width
(meter)
S
M
1
2
3
1
2
3
Active
filtration Retention time
surface area (min)
(ft3)
S
68
136
204
5 roll
101
203
305
L S
104 1.2 to 6
208
312
7 roll 5 roll
190 5 to 19
380
570
L
2 to 9
7 roll
10 to 36
ALL
32.9
65.6
98.4
ALL
2 to 6
Note: When using 2 or more sections, the retention time and active
surface areas are cumulative
Table 6-18 lists the design features of this series.
There were 69 European locations employing the Uni-
mat as of November 1976, with practically all of them
processing municipal sludges of some type, including
straight 100 percent biomass.
Table 6-19 lists reported results.
While at the time of writing this, no Unimat systems
are yet operating in the United States, 16 units have
been sold and some will be operative by early 1978.
A mobile test unit is available and considerable U.S.
test work was carried out on site during 1977.
Performance of Unimat on Washington,
D.C., Mixed Sludge
At Blue Plains the Unimat GM2H-7 dewatered a sludge
mixture of 1 part primary plus 2 parts W.A.S. to a dry
solids content of 27 to 33 percent at rates of 196 to
206 Ib/hr/ft (292 to 307 kg/hr/m) width. Polymer costs
were mostly between $8.76 to $9.20 per dry ton ($9.66-
$10.14/Mg) with a solids capture of 95-98 percent. On
the existing rotary vacuum filters a total dry cake solids
of 22-24 percent (including solids resulting from use of
5-7 percent ferric chloride and 15-20 percent lime) is
normally obtained. Because of the large variation of the
sludge quality, the lime dosage for the rotary vacuum
filters reaches 30-40 percent on occasion.
At Blue Plains, the dewatered vacuum filter cake was
fed to the M2H sections of the Unimat and the cake
solids were increased to 37-40 percent at a feed rate
of 366 Ib/hr/ft (545 kg/hr/m) width with no auxiliary
conditioner dosage.
Performance of Unimat on Columbus,
Ohio, Southerly Plant Sludge
At Columbus Southerly plant, the anaerobically digest-
ed mixture of primary and W.A.S. was dewatered to a
cake solids content of 36-39 percent at a rate of 228-
305 Ib/hr/ft (341-455 kg/hr/m) width. Solids capture
was 90-95 percent and polymer costs $8-$14/ton
($9-$15/Mg). Feed solids were 3-4 percent dry solids.
Thus an autogenous cake is feasible with this difficult
sludge.
It is quite apparent that the K.S. Unimat press is one
of the CBFP's newly introduced into the U.S. from Ger-
many which has the capability to effectively dewater
mixtures of primary and W.A.S. sludges to a dry solids
content high enough to be in the autogenous incinera-
tion range.
PARKSON MAGNUM PRESS
This device, of Swedish origin, is manufactured and
sold in the United States by the Parkson Corporation of
Ft. Lauderdale, Fla.
The Magnum Press is an advanced or third generation
type CBFP designed to maximize dry solids content of
dewatered cake. The Magnum Press has three stages
and can best be described by reference to the cross
sectional side-view of figure 6-18.
124
-------
Table 6-19.—Dry solids of cake and polymer dosage
Unimat
Model S
Model SM
Model SMH
Type of sludge feed cone.
(percent D.S.)
Fresh-primary — raw (4-6 percent)
Fresh primary and trickling filter (3—5 percent)
Fresh primary and activated (3-5 percent)
Anaerobically digested primary and activated (4-9 percent)
Activated — 100 percent W.A.S. (0.5-1.0 percent)
After gravity
stage
(percent D.S.)
12-18
10-15
10-15
14-24
8-12
After gravity
and medium
pressure
(percent D.S.)
25-35
22-32
17-27
25-35
17-20
After gravity
and medium
and high
pressure
(percent D.S.)
30-45
28-40
25-35
30-45
17-23
Typical polymer
dosage
(Ibs/ton D.S.)
60-85
60-100
60-100
5.0-8.5
70-100
UPPER SCREEN
HYDRAULIC CONTROLLED
SQUEEZE BELL TENSION
HIGH PRESSURE
SQUEEZE BELTS
PNEUMATIC CONTROLLED
SCREEN GUIDANCE
PNEUMATIC CONTROLLED
SCREEN TENSION
DOCTOR
BLADE
DOCTO
BLADE
PNEUMATIC CONTROLLED
SCREEN TENSION
VARIABLE SPEED
DRIVE MOTOR
SCREEN WASH
SPRAY NOZZLES
SCREEN WASH
COLLECTING TRAY
PLOUGH AND ROLLER
SECTION (OPTIONAL)
SCREEN
WASH
SPRAY
NOZZLES
DRAINAGE
ROLLER
COLLECTING TRAY
Figure 6-18.—Cross section of a Parkson Magnum Press.
LOWER SCREEN
PNEUMATIC CONTROLLED
SCREEN GUIDANCE
The initial stage is a unique gravity drainage section.
In addition to normal dewatering occurring by gravity
from a properly conditioned sludge, the sludge can be
subjected to a light pressure involved by rollers and be
turned by plows (both optional). The partially formed
cake then proceeds to the low pressure stage where the
second polyester screen belt comes into play on the top
forming a sandwich that is fed into the second or low
pressure stage. In the low pressure stage perforated
press rolls of decreasing diameter subject the cake to
continuously increasing pressures. In the last or high
pressure stage the cake is subjected to very high pres-
sure that is adjustable, depending on the application.
The high pressure is generated by a series of 1 inch
wide flat belts that press the screens against a perforat-
ed roll uniformly from side to side. This feature allows
125
-------
Table 6-20.—Magnum press size data
Model
MP-20
MP-40 ..
MP-60
MP-80 . .
Screen
width
(nominal)
20"
40"
60"
... . 80"
Weight
(tons)
3.8
4.4
4.8
6.0
Overall dimensions
A-width
4'
5'-8"
7'-4"
9'
B-height C-length
7-9" 14'-10"
7'-9" 14-10"
7'_9" 14'-10"
7<_9" 14'-10"
Screen wash
water flow
rate @ 100
gal
12 gal/min
24 gal/min
36 gal/min
48 gal/min
the sludge to be subjected to high pressure for a long
period of time without producing an excessive load on
the screens. The pressure is adjustable through the use
of two hydraulic cylinders.
This final high pressure stage of the Magnum Press
can also be employed in a modular fashion to further
dewater filter cake from existing Rotary Vacuum Filter
installation.
The Parkson Magnum Press is available in four sizes
as shown in table 6-20.
As of December 1977, nineteen Magnum Presses had
been sold worldwide. There are seven Japanese installa-
tions, nine in Europe, and three in the United States.
The first U.S. unit (at Mobil Oil Co.) processing straight
excess biological sludge is just now commencing opera-
tion.
Parkson has a mobile Magnum Press and a smaller
pilot unit, both of which have been used to carry out
on-site tests at various U.S. locations.
Performance of Magnum Press at
Washington, D.C.
A 0.25 meter pilot unit was evaluated on the various
sludges at Blue Plains plant. The following two figures
show the results obtained with various mixtures of pri-
mary and excess activated sludges (including phosphorus
removal sludges resulting from iron salt use).
In assessing results of dewatering work at Blue Plains
it is important to note the following:
1. The normal mix is 32 percent raw primary/68 per-
cent raw secondary sludges (on a weight percent
dry solids basis). The primary is gravity thickened
to 9.5 percent and the secondary is DAF thickened
to 5.5 percent. The resulting 6.8 percent solids mix
is filtered on RVF's to about 18 percent (without
lime).
2. The Blue Plains plant has an abnormally large
amount of a difficult to process excess activated
sludge due primarily to the use of a high rate acti-
vated sludge biological treatment system. This sys-
tem was apparently chosen because of certain site
and capacity constraints.
As can be seen in figure 6-19, the Magnum Press
produced a dewatered cake of 30 percent dry solids
126
PRESSED
VACUUM
FILTER
CAKE
0 10 20 30 40 50 60 70 80 90 100
PERCENT PRIMARY (WT. PERCENT DRY SOLIDS)
100 90 80 70 60 50 40 30 20 10 0
PERCENT SECONDARY (WT. PERCENT DRY SOLIDS)
Figure 6-19.—Magnum Press results, Blue Plains.
content at a rate of 244 Ib/hr/ft (364 kg/hr/m) belt
width.
It should also be noted that a straight interpolation of
the data in figure 6-20 indicates that at a more normal
sludge ratio of 60 percent primary and 40 percent sec-
ondary, even with the high rate W.A.S., the production
rate would be 17 percent greater and the cake solids
would be 34 percent. As shown in figure 6-20, polymer
dosages varied from 5.5 to 1.6 pounds per ton (2.8 to
0.8 kg/Mg) of dry solids and solids recoveries varied
from 95 to 98 percent.
The Magnum Press was also tested for dewatering the
filter cake from the existing RVF's. Cake solids of 35-42
percent were obtained at rates of 244 to 853 Ib/hr/ft
(364 to 1273 kg/hr/m) belt width. There is mechanical
development work required to design equipment to trans-
fer the filter cake to such a press.
-------
0 10 20 30 40 50 60 70 80 90 100
PERCENT PRIMARY (WT. PERCENT DRY SOLIDS)
100 90 80 70 60 50 40 30 20 10 0
PERCENT SECONDARY (WT. PERCENT DRY SOLIDS)
Figure 6-20.—Magnum Press results II, Blue Plains.
Magnum Press Performance at Los
Angeles/Orange County Metropolitan Ar-
ea (LAOMA)
The Magnum Press mobile unit was evaluated on sev-
eral mixtures of the sludges being studied in this major
research and development project.
While the results in table 6-21 are impressive and
may well be acceptable for the system, it is also appar-
ent that the dewatering devices' performance is penal-
ized by attempting to dewater an unthickened sludge. It
is strongly suspected that if the LAOMA sludges were
thickened a much higher capacity and cake solids would
be realized, in addition to being operable at a much
lower polymer dosage.
Magnum Press Performance—Other
Locations
A bench scale Magnum Press has been evaluated at
various other locations in table 6-22.
It is significant to note that the Magnum Press will
function with inorganic conditioning agents to extend the
flexibility of the unit and to reduce polymer costs.
CARBORUNDUM SLUDGE BELT FILTER
PRESS
Carborundums' Pollution Control Division at Knoxville,
Tenn., manufactures and sells a unit called the Sludge
Belt Filter Press (SBFP). This unit is based on the de-
sign of Rittershaus and Blecher of Germany who devel-
oped the "Dreibandpresse."
The Carborundum unit incorporates two unique fea-
tures: stainless steel wire supported belts and oscillating
pressure rollers.
As can be seen in figure 6-21, the gravity drainage
section of the SBFP includes two phases involving a
dumping of the partially drained sludge from the initial
belt onto a second drainage belt prior to the incidence
of the upper sandwiching belt. The two belt cake sand-
wich then proceeds around a large diameter roll into a
further pressurizing section involving smaller diameter off-
set pressure rollers in a two level configuration. Thus, in
effect, the Carborundum SBFP has a two stage gravity
drainage section plus two additional pressureshear
stages to successively expose the cake to increasing
degrees of shear and pressure.
Carborundum is also bringing out a newer model with
a "Pre-Concentrator" stage in the same vein as the
Unimat and R. B. Carter Series 31/32 devices.
The current Carborundum SBFP is available in 2 mod-
els. Table 6-23 shows the dimensions.
This unit was introduced into the United States in
1977 so no U.S. commercial scale operating data are
yet available. A pilot unit is available for testing and the
supplier quotes the results as shown in table 6-24.
Additional field U.S. results are now available from
Table 6-21.—Performance of magnum press—Los Angeles/Orange County
metropolitan area
70
30
Sludge mixture
(digested mix)
Prim-30 WAS
Prim-70 W.A.S
Dry solids,
percent
Feed Cake
1 .8 29
2.1 21
Capacity-
dry solids
(Ib/hr/m)
360
320
Polymer
($/ton
dry solids)
12.60
21.40
Percent
solids
recovery
96
88
127
-------
Table 6-22.—Performance of Magnum Press—various sludges8
Dry
solids,
percent
Location
Blue Lake, St. Paul, Minn. . .
Lake Charles, La.
Richardson, Tex.
Industry
Sludge mixture
45-Prim.b
55-W.A.S.
Prim + W A S
Digested prim +
W.A.S. + alum
W.A.S.
Feed
5.3
2 9
4 1
3.5
Cake
35
29-34
26-27
d 22-23
Capacity-
dry solids
(Ib/hr/m)
1,260
580
615
500
Flocculant
($/ton
dry solids)
14
12
C11
d17
Percent
solids
recovery
98
95
95
95
"All results from 0.25 meter bench scale press.
bConcentrations by volume.
°Costs using 75 Ib/ton FeCI3 plus 5 Ib/ton polymer: straight polymer = $16/ton.
dValues shown are for 100% polymer usage: use of 30-55 Ibs/ton FeCI3 will increase cake solids to
net of 27% at slightly lower capacity.
Table 6-24.—Carborundum SBFP results
Figure 6-21.—Carborundum sludge belt filter press.
Table 6-23.—Carborundum SBFP
Model
Approximate overall
Belt dimensions (inches)
width
(in.)
Length Height Width
135.
215.
39 160
70 160
96 69
96 100
Carborundum and German full-scale installations have
been in operation for several years.
R. B. CARTER SERIES 30 PRESSES
R. B. Carter of Hackensack, N.J., is the U.S. licensee
of Klein of Germany, the developers of three successive
Type sludge
Capacity
(gal/hr)
Feed
solids
(percent)
Cake
solids
(percent)
Polymer
cost
($/ton
dry solids]
Primary + W.A.S
Anaerobically digested
primary + W.A.S
W.A.S
900
1,300
1 100
4-6
4-9
4
34-37
26-40
16-20
9
10
11
generations of continuous belt filter presses, each of
increasing capability in either capacity or cake solids
content realized.
The original single level Klein device which was intro-
duced in Germany in about 1969, the Carter Series 30
(a two level unit), and the lastest multistage unit, the
Carter Series 31/32 CBFP (based on the Klein "S"
press) were described in a preceding section dealing
with the evolution of the CBFP. The early single level
device has been superseded by the two level Series 30
and the multistaged Series 31/32.
R. B. Carter Series 3O Installations,
Dimensions and Results
As of July 1976 there were 21 U.S. installations of the
Carter Series 30 CBFP that were either operating or
were on order. The 21 installations involved 36 units. Of
these installations, 8 were for industrial sludges and 13
municipal.
The series 30 units are available in 3 sizes as shown
in table 6-25.
128
-------
Table 6-25.—Carter series 30—overall dimensions
Table 6-27.—Single level press—R. B. Carter type,
Lenham Works, East Kent, U.K.
5/30
10/30
15/30
Table
Model
6-26. — Performance
width
(inches)
53
73
93
data — Carter series 30
Weight
(ibs)
2,500
3,500
4,500
CBFP
Type sludge
Primary + humus +
W.A.S
Straight humus..
Dry solids,
percent
Feed
4.5
45
Cake
22
18
Capacity8
(Ibs dry
solids/ hr)
72
49
Polymer
($/ton)
5.64
8.00
Percent
solids
capture
96-99
96
Type sludge
_ .. . "0.5 meter belt width x 3.0 meter length—Wm. Jones, Chem. Eng.
Solids LW
content _ .. Polymer
•—Q as? «r
sq.ft) f* Table 6-28.—Lenham Works—cost analysis, first genera-
Feed Cake > tion CBFP
Primary + W.A.S 4-5 20-30 6.5-12 4-8
Anaerobically digested primary +
WAS 6-8 20-30 10-20 4-8
Extended aeration (no primary
treat) 2-4 16-24 6-10 2-6
The Series 30 is typically about 12 feet long and five
feet tall. Quoted typical results for the Carter Series 30
model are shown in table 6-26.
A mobile pilot unit of the Series 30 has been used in
onsite test work.
Performance of a CBFP of the Carter
Series 30 Type in the U.K.
In addition to the quoted typical results above addi-
tional insights into the capabilities of the Carter Series
30 units can be gained by study of references 14 and
15. The latter reference is an exhaustive study by the
U.K. Department of the Environment (D.O.E.) on an in-
stallation of the British version of the first generation
Carter type press. This study was carried out over many
months by the D.O.E., an agency of the government, at
Lenham Works in East Kent.
Different mixtures of sludges were processed to deter-
mine applicability of the single level first generation
CBFP, including operability, maintainability, and all cost
factors as well as dewatering capacity.
Typical results are shown in table 6-27.
As will be noted the normal mixed sludge is not a
difficult one and results were essentially equivalent to
dewatering with an RVF. However, it is doubtful that an
RVF would have achieved results on straight secondary
sludge similar to those shown.
The Lenham plant is a small plant designed to treat a
dry weather flow of 0.11 Mgal/d (0.005 m3/s) and actu-
Item
$/ton dry solids
Polymer
Wash water
Power
Operating labor (inc. super.).
Total operating
Capital costs
Total (ex. maint.)a
4.90
1.94
0.66
12.00
19.50
46.00
65.50
aMaintenance estimate+ 3/4 hour/1,000 hours operation.
ally processing about one half of design flow. The plant
includes primary, trickling filter and activated sludge op-
eration. Though the normal sludge mixture is a relatively
easy to process material, the performance of the first
generation CBFP was viewed as highly successful.
The cost analysis (table 6-28) showed a total operat-
ing and capital cost of $65.50 per ton ($72.20/Mg) of
dry solids dewatered. Maintenance costs were low.
Performance of an R. B. Carter Series 3O
CBFP——Hutchinson, Minn.
At Hutchinson, Minn., a Series 30 Carter CBFP has
been operating for many months on a municipal sludge
from an activated sludge plant. Figure 6-22 is a photo
of the unit.
At Huntchinson, the waste activated sludge is fed to
the CBFP at a solids concentration of 1-1.5 percent
resulting in a cake solids content of 13-15 percent and
dry solids throughput of 340 pounds per hour (155
kg/hr). While this performance is satisfactory it could be
greatly improved by prethickening to a solids content
more logical for maximum dewatering capability.
129
-------
Figure 6-22.—Carter series 30 CBFP.
R.B. CARTER SERIES 31 CBFP
The basic design characteristics of this unit have been
delineated in ealier sections. Essentially it consists of an
initial "Reactor Conditioner" system which performs the
dual function of conditioning and prethickening followed
by two successive pressure zones and a shear zone
under pressure.
The Series 31 device also comes in 3 sizes, 5/31,
10/31, and 15/31 which differ in widths. The largest
unit, the 15/31, is designed for a nominal feed of 85
gal/min (5.4 l/s) of typically a 5 percent mixed sludge.
Complete systems, including the chemical feed system,
pumps, controls and erection costs are usually priced at
slightly less than $2,000/gal/min ($31,160/l/s) or
$170,000 for an 85 gal/min (5.4 l/s) Series 15/31 unit.
Solids capture in the Series 31 normally averages 95
percent plus. Connected electrical power, including
sludge pumps and conditioner system pumps totals not
more than 15 horsepower (11.2 kW).
Sizing of a building or space for a two unit Series 31
system, including polymer preparation system, and con-
veyor sludge removal system indicates a floor space
requirement of about 36 feet by 18 feet (11.0 mx5.5
m). Height requirement is 13 feet 6 inches (4.1 m) mini-
mum.
While there are quite a few operating installations of
the Series 31 type unit (Kleins or Wm. Jones "S" Press)
around the world, U.S. commercial units were just com-
ing on stream during 1977.
Performance of R. B. Carter Series 31
CBFP at Hamilton, Ontario
The Carter Series 31 mobile pilot unit has been tested
at several North American locations including Hamilton,
Ontario, among others.
On a digested mixed primary and W.A.S. sludge at
Hamilton, a 27 percent dry solids cake was obtained
130
which compared very favorably with a 16 percent cake
being obtained at the same time on the existing Rotary
Vacuum Filters. Hamilton was experiencing some problem
with fines recirculation and accumulation within the sys-
tem at the time and no doubt even more favorable
results would be realized in a situation with normal
sludge conditions.
Performance of R. B. Carter Series 31
CBFP at Parkersburg, W. VA.
At the Borg Warner Co., two 15/31 Carter units are
dewatering a pure excess biological sludge. Feed solids
are 0.5 to 2.0 percent with a cake solids content of
25-33 percent. Capacity averages 1500 pounds (682 kg)
of dry solids per hour per machine.
Performance of R. B. Carter Series 31
CBFP at Scituate, Mass.
A Carter Series 31 unit equipped with a Reactor-
Thickener was evaluated on the difficult aerobically di-
gested extended aeration sludge at the Scituate, Mass.
plant. Results are shown in table 6-29.
In a cost comparison, the engineers involved estimated
that a production level of 3 dry tons (2.7 Mg) per day
for a 5-day week either 2 Carter Series 31 CBFP's (60
inches wide) with Reactor-Thickener first stages: or two
250 ft2 (23 m2) DAF units plus two 200 ft2 (19 m2) RVF's
would be required. Equipment costs for the CBFP option
were estimated at $222,000 and for the second option
at $425,000. Horsepower requirements were estimated at
26 hp (19 kW) and 200 hp (149 kW) respectively for the
two systems.
DESIGN EXAMPLE—CONTINUOUS BELT
FILTER PRESS
Basic Assumptions
These assumptions are identical to those used in the
example for design of a Rotary Vacuum Filter System:
1. Anaerobically digested mixture of primary and
W.A.S. at 4 percent solids content, 60 percent pri-
mary and 40 percent W.A.S.
Table 6-29.—Carter CBFP—Model 5/31, aerobically dig-
ested extended aeration sludge scituate, Mass.
Test
1
2
Percent
dry solids
Feed Cake
2 18
3 16
Sludge
feed
(Ibs/dry
solids/hr)
88
255
Solids
capture,
percent
91
98
Polymer
cost
($/ton
dry solids)
a26
"11
aCationic polymer A used.
bCationic polymer B used.
-------
2. Ultimate disposal by hauling to either a sanitary
landfill, or to farmland, composting or other horticul-
tural use.
3. Equilibrium sludge removal rate of 2.5 tons (2.3
Mg) of dry solids per day required.
Alternate Units for Consideration or
Evaluation
Any of the twenty or so varieties of continuous BFP's
available from 11 different companies. Depending on the
length of the truck haul and the cake dryness require-
ments for final disposal the design engineer would pre-
screen the many alternates and select perhaps three
companies to work with in proving specific devices and
carrying out bench and pilot scale qualification
trials.
For the purposes of this example it will be assumed
that a dry solids content cake of at least 28 percent is
required. Accordingly, units such as the R. B. Carter
Series 31, Komline Sanderson Unimat, Parkson Magnum
Press, Ashbrook-Simon Hartley Winklepress, and Carbo-
rundum Sludge Belt Filter Press would certainly be con-
sidered. Certain models of the Tait Andritz, Infilco De-
gremont Floe-Press and Passavant Vac-U-Press would
require at least preliminary consideration with further
study dependent on estimates of capabilities from the
supplier firms.
Evaluation Procedure
The systematic procedure for evaluation would be
identical to that described in the RVF design example.
Bench Scale Tests
Most of the equipment suppliers have laboratory or
bench scale test equipment and procedures which indi-
cate general acceptability of their units. In most cases,
unless the sludge to be dewatered is an unusually easy
one, pilot scale testing will yield much more accurate
design criteria and should be pursued. Most companies
have mobile pilot or full size units.
Design Calculations
1. Operating cycle to be 35 hours per week (7
hours/day), permitting start-up and wash down
times within 8 hour shift.
2. One CBFP with adequate spare parts to be main-
tained.
3. Size of CBFP.—Production rate proves to be 50
GPM (3.2 l/s) of 3-4 percent feed sludge giving
rate of 228-305 Ib/hr/ft (341-455 kg/hr/m) width
(from pilot test runs). Solids capture is an accept-
able 93-98 percent in all tests. Cake solids with
complete press (all sections, including high pressure
stage) in use is 38 percent. Without high pressure
section, cake solids are 30 percent. Polymer dos-
age is consistent. Design Engineer must then as-
sess added capital and O/M costs for high pres-
sure section and effect of 8 percent drier cake on
haulage costs to determine which unit is to be
chosen. A single CBFP of two meter width would
be adequate if several days sludge storage surge
capacity were provided. Alternatively 2 one meter
wide units could be chosen.
4. Sizing of auxiliary equipment.—Same as described
in RVF design example. If, for example, a Komline
Sanderson Unimat were the selected unit, the basic
machine is just under 24 feet (7.3 m) long, width
requirement is 5 feet 2 inches (1.6 m) at base with
the upper drive motor making upper width need just
under 8 feet (2.4 m). Height of the Unimat is 10
feet 2 inches (3.1 m). The same considerations
apply to selection of a suitable flocculant system,
sizing of conditioning system and overall "Dewater-
ing System Considerations" as noted in the RVF
design example.
DESIGN EXAMPLE—CONTINUOUS BELT
FILTER PRESS—40 MGAL/D (1.75 mVs)
PLANT
Basic Assumptions
1. Anaerobically digested mixture of primary and
W.A.S. at 4 percent dry solids content, 60 percent
primary and 40 percent W.A.S.
2. Ultimate disposal by either composting or incinera-
tion, both systems requiring'a minimum cake solids
content of 30 percent.
3. The sludge removal rate to be an average of 25
dry tons (22.7 Mg) of solids per day.
Alternate Units for Consideration
Same comment as in 4 Mgal/d (0.18 m3/s) example
preceding.
Evaluation Procedure
The same procedure as described in the RVF design
example could be used, except:
1. Determination of the calorific value of the dewater-
ed cake produced in pilot tests would be essential
for evaluating efficacy of incineration and to ensure
whether or not autogenous incineration would be
achieved in burning periods (there is no such thing
as totally autogenous incineration since startup and
shutdown procedures require fuel usage regardless
of cake characteristics). Nonetheless, self-sustaining
combustion would at least minimize fuel consump-
tion.
2. Review of the suitability for composting could be
carried out with experts in that field.
Bench Scale and Pilot Tests
Same as in 4 Mgal/d (0.18 m3/s) example.
131
-------
Design Calculations
1. Pilot results show that 50 gal/min (3.2 l/s) of 3-4
percent sludge will yield a cake solids of 38 per-
cent at a production rate of 228-305 Ib/hr/ft (341-
455 kg/hr/m) width, with adequate 93-98 percent
solids capture and usage of polymer at $10 per ton
($11/Mg) of dry solids.
2. Operating cycle.—To be based on 3 shifts/day, 7
days per week and 22 hours/day unit operating
time since incineration requires continuous operation
to minimize fuel consumption.
3. Sizing of CBFP.—50,000 pounds/day (22,730 kg/
day).
Meter Daily production/unit
width (pounds)
FILTER CLOTHS
FIXED END
16,500
33,000
49,500
two
On the above basis 4 one meter units or 2
meter units would be chosen.
4. Summation.—All other facets of the design proce-
dure would be similar to the 4 Mgal/d (0.18 m3/s)
RVF design example.
PRESSURE FILTERS
The original main focal point for the development of
the plate and frame, and recessed chamber types of
pressure filters was Stoke-on-Trent, United Kingdom. The
slurriers incident to the manufacture of pottery and china
are particularly difficult to dewater and as a result pres-
sure filters were employed.
These types of pressure filters, particularly the re-
cessed chamber type have been frequently designed into
the U.K. wastewater treatment plant sludge dewatering
systems.
A few U.S. installations of pressure filters have also
been made in the past few years.
Pressure filters are batch devices and to some extent
because of the level of development of feed and chemi-
cal dosage systems normally use substantial quantities of
metal salt and lime for conditioning. These chemicals
require relatively extensive handling systems requiring
considerable maintenance. This is one of the factors
which has slowed acceptance of pressure filters outside
the United Kingdom.
Essentially, a pressure filter consists of a series of
vertical plates, usually recessed, covered with cloths to
support and contain the cake, mounted in a framework
consisting of head supports connected by two heavy
horizontal and parallel bars or an overhead rail. Figure
6-23 shows a cross section of a pressure filter.
Conditioned sludge is pumped into the pressure filter
at increasing pressure. Presses are normally supplied to
operate at either a nominal 100 lb/in.2g (7 kg/cm2) or
225 lb/in.2g (16 kg/cm2). Cake building time or sludge
feed time is normally 20 to 30 minutes followed by a 1
to 4 hour pressing period. The press is then opened
and the filter cake falls off into the removal system.
132
SLUDGE IN
FILTRATE DRAIN HOLES
Figure 6-23.—Cross section of a partial pressure filter.
While pressure filters will generally produce a cake
solids content 10-20 percent points drier than a rotary
vacuum filter, some portion of these total cake solids
are lime and metal salt rather than sewage solids. Ca-
pacities of pressure filters are usually about 10 to 20
percent of the loadings achieved on rotary vacuum fil-
ters.
Significant developments in Pressure Filter technology
are the diaphragm press and other membrane type
presses which are discussed later.
Since an excellent survey of three operating U.S. in-
stallations was available, a review of those case histories
is the most applicable way to present a perspective on
conventional recessed chamber type presses.
CASE HISTORY—KENOSHA, WISCONSIN
This is a 26 Mgal/d (1.1 m3/s) plant with a primary
and activated sludge system.
1. The sludges are mixed, gravity thickened, anaerobi-
cally digested, and then dewatered in Nichols (Ed-
wards & Jones) pressure filters. The dewatered
cake is given to farmers who land spread from
manure spreaders.
2. Chemical dosage is 3 percent ferric chloride and
25 percent lime (both on a dry solids sludge basis)
3. Digested sludge at 3-7 percent solids is dosed in
line with ferric chloride and lime is added in a
subsequent mix tank with slow speed mixing.
4. Two Moyno pumps feed the two presses simulta-
neously. The Moynos have worked very well. Fil-
trate is returned to head of plant.
-------
5. Cycle includes maintenance of 100 lb/in.2g (7
kg/cm2) for 30 minutes and total cycle time is
2-1 /3-2-112 hours. Operate 16 hours per day, 7
days per week to produce 12 tons (10.9 Mg) per
day of dry solids cake at 35-38 percent solids.
Cake thickness is one inch.
6. Two Nichols-Edwards & Jones pressure filters, with
80—4 feet by 4 feet (1.2 m x 1.2 m) plates (rubber-
coated steel) used.
7. One operator in continuous attendance.
Results
Table 6-30 shows the good handleable press cake
and clear filtrate.
Problems
High chemical dosage and costs have been experi-
enced. Cake is actually about 25 percent added chemi-
cal so analysis is really about 65 percent water, 26
percent sewage sludge and 9 percent inorganic chemi-
cal. Net sludge production must be reduced by 25 per-
cent to get actual figures. Excessive wear in cloths and
stay bosses causes serious maintenance problems. Filter
cloths were replaced 3 times in 2 years ($3,000 per
press per change). Severe ammonia odor problems have
occurred in press room (effect of lime and high pH).
Comment
Despite problems noted above there have no extensive
forced downtime periods in the 2 years of operation.
Much of the chemical consumption might be eliminated if
the alkalinity of the digested sludge were washed out in
a properly designed and operated elutriation system us-
ing flocculants. Why use pressure filters when the wet
cake is disposed of on land by a manure spreader?
BROOKFIELD, WIS.
This plant design includes a primary and activated
sludge system and contact stabilization. Flow is 2
Mgal/d (0.09 m3/s). 80 percent Primary Sludge-I-20 per-
cent Secondary Sludge is mixed, pumped through a
grinder, diluted with recycled incinerator ash (0.5 Ib/lb
sludge), conditioned with lime (15-18 percent) and Ferric
Chloride (5-7 percent), pressed and fed to a 5 hearth
Table 6-30.—Costs—pressure filtration, Kenosha, Wis.
Costs
$/ton
Labor $7.43
Chemicals 20.17
Power '. 1.71
Maintenance 3.25
Total.
32.56
incinerator. 95 percent of incinerator ash is recycled.
The incineration is not autothermic and uses natural gas.
Pressure filters are standard Passavant design with forty-
six 52" (1.3 m) diameter plates of steel and have been
operated for 1-1/2 years.
Results
Plant personnel state that no major operating problems
have been encountered. There have only been two
"Sludge Blowing Incidents" in the 1-1/2 years of opera-
tion. Press cloths have had to be replaced every 6
months at a cost of $3,600 per shot. The press cake,
which contains a large amount of inorganic conditioning
agents and recycled ash averages 45 percent total sol-
ids. The press cake is only 30-40 percent volatile so
the ratio of water/sewage solids is quite high.
Comments
1. The mixed sludge being processed is a relatively
easily dewaterable material which is high (80 per-
cent) in primary content and high in fibrous materi-
al. Indeed the high fiber content has caused prob-
lems in the press cake breaking operation.
2. No records are available on natural gas consump-
tion and no cost data on the system have been
made available.
3. The system appears to be a complex high capital
and high operating and maintenance cost one
which is difficult to rationalize, particularly at a
plant with such an easily processable sludge.
4. The plant has two components of interest to other
potential press filter designs: the wet sludge grinder
and the slow speed cake breaker.
Conclusions on U.S. Results to Date
Reference 16 from which the above results came, is
an excellent review of the current U.S. installations.
The conclusions from reference 9 are as follows:
1. In looking at the two types of presses, we found
some advantages with the lower pressure design.
Essentially, it is a much simpler operation. The re-
cycling of incinerator ash seemed to provide few
benefits, particularly because it only complicated
the operation with additional material handling
equipment.
2. In general, we found that filter presses are an ac-
ceptable method for dewatering sludge. Theoretical-
ly, they should always produce an autocombustible
sludge cake. But, practically, we know of no instal-
lation anywhere that can achieve this. The ash re-
circulation is probably the limiting factor. (The inor-
ganic conditioning agents also contribute to the
problem.)
3. Filter presses seem to be quite capable of handling
different sludge concentrations and different types
of sludge feed. Proper conditioning, especially with
lime, is the key to good operation. Vacuum filters
are not quite so adaptable.
133
-------
4. The necessity of using high lime for conditioning
could be a drawback. Lime handling is always diffi-
cult.
5. Prior to a large scale installation, pilot plant work
should always be performed to evaluate the dewa-
tering characteristics and chemical requirements.
6. Filter presses have a higher capital cost than vacu-
um filters. The presses also usually have a higher
operational cost. Their real advantage is in greatly
reducing the costs of final disposal for the sludge
cakes. A detailed economic analysis of the total
system is needed before deciding for or against
filter presses.
POLYELECTROLYTE CONDITIONING FOR
PRESSURE FILTERS
Due to the more prevalent previous incidence of the
use of filter presses in continental Europe and the Unit-
ed Kingdom, and also due to innovative work there, the
successful use of certain polyelectrolytes in conditioning
sludges for dewatering in pressure filters has been real-
ized at a number of locations.
Farnham Pollution Control Works,
Thames Water Authority, U.K.
This plant is a primary and trickling filter installation.
Humus sludge is recirculated to the primaries, the mixed
sludge gravity thickened, and then dewatered on two
filter presses. Operating pressures are 85-100 lb/in.2g
(6-7 kg/cm2).
Initially the plant used aluminum chlorohydrate for
sludge conditioning. Figure 6-24 is a flow diagram of
the dewatering system.
The Farnham plant experienced severe filter cloth
blinding problems and proceeded to carry out diagnostic
trails with various conditioning agents to rectify the
problem. They found that by converting the system to
use Allied Colliods Zetag 63 polyelectrolyte the cloth
blinding problems were alleviated sufficiently for the two
presses to cope with the sludge load. (See Table 6-31
for dewatering results.)
CASE HISTORY—THORNBURY STP, U.K.
Reference 17 describes exhaustive test work on the
use of polymers for conditioning sludge for dewatering
via recessed chamber pressure filters.
FILTRATE
CAKE
DISCHARGEE
FILTER PRESSES
CHEMICAL
STORAGE
TANK
CHEMICAL
j DILUTION
-h^TANK
i
.j
FILTRATE
CAKE
DISCHARGEE
KEY
» ALUMINIUM CHLOROHYDRATI
BATCH CONDITIONING
-->-ZETAG 63 IN-LINE
CONDITIONING
Figure 6-24.—Farnham Plant dewatering system.
134
-------
Table 6-31.—Farnham dewatering results
Conditioning
agent
Aluminum chlorohydrate (batch) . . .
Aluminum chlorohydrate (in-line)...
Zetag 63 (batch)
Zetag 63 (in-line) . ..
Ferric chloride and lime (batch)...
Ferric chloride and lime (in-line)...
Dose
(% on
dry solids)
2.5
2.5
0.2-0.3
0.2-0.3
3
25
3
25
Cost
($/ton
dry solids)
22.00
22.00
6.70-10.10
6.70-10.10
14.80
14.80
CST range
during cycle
(seconds)
10-65
10-32
8-14
8-45
8-15
Pressing cycle
time range
(hours)
6-18
6-12
6-9
3-6
3-13
3-5
aResults not available.
By virtue of using in-line conditioning and observing
logical procedures the results shown in table 6-32 were
achieved.
The Thornbury works processes a mixture of 45 per-
cent primary sludge and 55 percent of mixed sludges
from adjacent secondary treatment plants. In addition to
illustrating successful use of polyelectrolytes, the article
delineates other significant facts relative to pressure filter
design.
MEMBRANE USE—PRESSURE FILTERS
References 18 and 19 describe the successful upgrad-
ing of the production rate in conventional recessed
chamber pressure filters by equipping same with alter-
nate "membrane" plates. This retrofitting process causes
each of the chambers formed between the standard re-
cessed plate and the membrane plate to be subject to
the squeezing action of a membrane at will during the
press cycle. The membrane plate is a steel reinforced
rubber plate in which the rubber membrane is inflatable
by air pressure. After the initial filling period in a press
cycle, when the filtrate rate falls off, the sludge feed
Table 6-33.—Conventional versus membrane press
Severn Trent Water Authority18
Table 6-32.—Thornbury, U.K.—pressure
filtration
Conditioner
Aluminum chlorohydrate
Polyelectrolyte (Zetag 94)
dry solids
Feed
46
4.6
(Primary + secondary
Cake
38
37
sludge)
Conditioner
cost
($/ton
dry solids)
23.40
4.60
Press
cycle
(hrs.)
4.9
4.9
Type press
used
Conventional....
Membrane
Cycle
(minutes)
390
87
Cake
thickness
(inches)
1.25
0.7
Dry
solids
(percent)
28
27
Weight
(Ibs)
1,227
558
Output
(Ib/hr/
press)
186
385
(Raw feed sludge—3.9% dry solids—2.0% alum chlorohydrate cond.)
pump is stopped and the membrane inflated to give a
pressure up to 150 lb/in.2(10.5 kg/cm2) to squeeze the
partially formed cake and obtain quick dewatering.
As can be seen in table 6-33 following, though a
thinner cake results, the overall filtration cycle is so
much shorter that the total throughput doubles or even
triples in some cases.
The suppliers of the rubber membrane plates claim
that new installations of the membrane type unit are less
expensive overall due to the increased capacity of the
membrane units.
A somewhat analogous but different type of variable
volume pressure filter is described in the following sec-
tion.
DIAPHRAGM TYPE PRESSURE FILTERS
As described in reference 20, a new type of pressure
filter, employing flexible rubber diaphragms between the
chambers of a pressure filter, has recently been intro-
duced into the United States. This type device was de-
veloped in Japan and there are several operating instal-
lations there.
At least two versions of this new type of pressure
filter have been tested and are available in the United
135
-------
States. The earliest one was supplied by NGK Insulators
Ltd., of Nagoya who have now licensed Envirex division
of Rexnord for U.S. sale of their device. Ingersoll Rand
has the U.S. rights to the Lasta automatic diaphragm
pressure filter. There are indications that Dart Industries
and Industrial Filters OMD of Chicago have devices
based on similar principles.
Figure 6-25 is a diagram of the I. R. Lasta press that
illustrates the operating principles.
As will be noted in figure 6-25 the feed slurry enters
the top of the chamber between the filter cloths and
gradually fills the chamber. After a cake is formed the
diaphragm is expanded by water under pressure to 250
lb/in.2g (17.6 kg/cm2) which squeezes and dewaters the
cake. The filter plates are then automatically opened and
the cake discharged. Cloth washing ensues before an-
other pressing cycle.
It is claimed that the length of the cycle is shorter
than for conventional presses because of the improved
control of the relationship between cake formation and
pressure build-up.
Table 6-34 lists dimensional data on the I. R. Lasta
press.
The most detailed report on these devices is Refer-
ence 20 which describes the extensive pilot work at
Blue Plains with the Envirex-NGK Locke diaphragm
press. This Envirex unit is highly automated and in wort
at Blue Plains (mixture of primary and W.A.S. sludges),
it produced a 40 percent total dry solids cake using 20
percent lime and 10 percent ferric chloride dosages. Th
only problem is that when the correction is made for th
inorganic conditioning solids present in the dewatered
cake, the percentage of dry sewage solids in the cake
relative to water content is only about 28 percent.
This new type pressure filter does offer much im-
proved capabilities over conventional pressure filters for
extremely difficult to dewater sludges. Pricing figures
available indicate that the units will be priced about
eight times the price of a conventional pressure filter, si
the need must be clear and obvious.
CENTRIFUGES FOR DEWATERING
Horizontal solid bowl decanter type centrifuges have
been used for wastewater sludge dewatering for a num-
ber of years. They were popular for primary sludges wit
low grit content in coastal resort areas with large swing
in loadings because of ease of operation, quick startup
and shutdown and ease of odor control. Attempts to
adapt these relatively high speed devices (g forces of
1000 + ) to heavy duty operation in large cities or for
use with mixed sludges containing significant quantities
FILTERING I
COMPRESSION II
CAKE DISCHARGE
WASHING OF
FILTER CLOTHS IV
FILERING
CHAMBER
PRESSATE
DIAPHRAGM
HIGH
PRESSURE
WATER
O O
o o
o
SHOWERS
Figure 6-25.—I. R. Lasta diaphragm pressure filter.
136
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Table 6-34.—I. R. Lasta automatic filtering press
600 IT
800 FT
1,000
1,250
1,500
Size of
filtering
plate
im (24")
im (32") ..
mm (40")
mm (50")
mm (60")
Number of
filtering
chambers
8
14
20
14
20
26
20
26
32
26
32
38
32
38
44
"area9 He"ht Length
m2
4
7
10
13
19
24
30
39
48
62
77
91
112
113
154
ft2 mm
43 2,050
75
108
140 2,485
204
258
323 2,845
420
516
667 3,200
829
979
1,205 3,620
1,431
1,657
ft. mm
7 2,660
3,650
4,640
8 3,490
4,930
5,920
9 5,240
6,230
7,220
10 6,555
7,545
8,535
12 8,205
9.225
10,245
ft.
9
12
15
13
16
19
17
20
24
22
25
28
27
30
34
Width
mm ft.
1,610 5
1,800 8
2,100 7
2,600 9
3,050 10
of biomass were previously plagued by two problems:
1. Erosion of the surfaces exposed to high speed im-
pingement of abrasive materials caused mainte-
nance problems.
2. Prior to the development of polyelectrolytes capable
of providing a reasonable clean centrate and avoid-
ing serious fines recirculation problems, solids cap-
ture was inadequate.
In the past 5 years or so six steps were taken which
have helped this type device gain a wider use:
1. Development and use of new high molecular weight
cationic shear resistant polyelectrolytes.
Use of lower rotational speeds to reduce turbu-
2.
3.
lence, power costs, and erosion wear problems.
Use of a concurrent flow pattern for sludge and
centrate to minimize turbulence.
4. Adjustable variation of speed differential between
the bowl and the sludge removal scroll.
5. Use of longer bowls with smaller diameters.
6. Provision of extremely large units at plants with
large sludge removal needs producing an economy
of scale.
Various manufacturers have combined some of the
above features in their newer models. This resulted in a
surge of popularity about 4 years ago. Since the energy
crisis the degree of popularity of centrifuges, even with
the above mentioned improvements, has slackened be-
cause of energy costs.
Once again, the pioneering development work on
these devices was carried out primarily in West Ger-
many. The most practical description of these develop-
ments is contained in references 21 and 22 which are
excerpted in the following section.
CASE HISTORY—CENTRIFUGAL DEWA-
TERING—WUPPERTAL-BUCHENHOFEN,
GERMANY
Reference 21 is a comprehensive article relating re-
sults obtained at Wuppertal-Buchenhofen plant with a
low speed concurrent flow type unit. This is a combined
municipal-industrial treatment plant treating 1,200,000
population equivalent. After primary and biological treat-
ment the mixed sludges are thickened to 3-4 percent
and anaerobically digested, followed by sludge settle-
ment and decantation, thence dewatering.
After initial trial work the authority asked for competi-
tive tenders from various suppliers of centrifuges with
performance requirements as follows:
1. Capacity of each centrifuge: 40-60 rrvVhour of
sludge with feed of 2.5-3 percent dry solids.
2. Minimum cake solids: 20 percent.
3. Centrate maximum suspended solids of 0.2 percent.
4. Maximum polyelectrolyte dosage permissible of 3.6
kg/Mg of dry solids (100 gm/m3).
5. Maximum permissible power consumption of 1 kWh
per cubic meter of sludge feed including ancillary
equipment such as pumps, flocculant metering sta-
tions, etc.
6. Guaranteed life of screw conveyor = 10,000 hours.
7. Provision of a package plant with a minimum ca-
pacity of 40 m3/h for a 4-month trial period under
a leasing agreement.
137
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Table 6-35.—Effect of speed differential on throughput
and dry solids
Speed differential
Table 6-36.—Side by side comparison—process results
Flocculant dosage (g/m3)
Dry solids carried by dis-
charge, percent
Dry solids carried by cen-
trate (undissolved solids)
Ideal throughput (m3/h)....
60
26
0.35
33
80
285
0.25
37
60
24
017
43
80
23
0.07
45
60
205
0.12
40
80
20
0.07
48
KHD Industrieanlagen AG Humboldt-Wedag of Cologne
(U.S. Licensee—Bird Machine) won the contract and
initially installed two S3-2 type low speed concurrent
flow centrifuges with capacities of 20-30 m3/h each.
These units met the agreed performance guarantees but
when the full civil installation was completed they were
replaced, as planned, by two of the larger S4-1 units (of
the same basic type) but with capacities of 40-60 mVti
each.
Power consumption for the complete dewatering plant
was 0.9-0.95 kWh/m3 with S3-2 units and improved to
0.75-0.8 with the larger S4-1 units. Disage of Zetag 92
polymer (Allied Colloids) averaged 60-80 gm/m3.
The article contains much data on the effect of centri-
fuge dewatering variations on overall process perform-
ance and sludge disposal costs.
A significant factor studied was that of the effect of
the differential in speed between the scroll and the bowl
(see table 6-35).
As can be seen in table 6-35, a 28.5 percent dewa-
tered cake at a reasonable throughput of 37 nrrVhour
and centrate suspended solids of 0.25 percent can be
obtained with flocculant dosage of 80 g/m3 by using a
speed differential of 2 instead of 6.
The paper claims and purports to show that very large
capacity centrifuges of the improved low speed-concur-
rent flow type, when operated in a lower differential
speed mode can offer significant capital and O/M cost
savings where large volumes of sludge are to be proc-
essed.
Unit costs are given as follows:
Operating—Deutsche mark 36.40/ton (DM
40.12/Mg) dry solids
Annual Capital—Deutsche mark 47.60/ton (DM
52.477 Mg) dry solids
CASE HISTORY—CENTRIFUGAL DEWA-
TERING—STOCKHOLM, SWEDEN
Stockholm has operated three high speed centrifuges
for a 3 year period and also has operated a new low
speed concurrent flow unit on the same sludge for
1-1/2 years.
Table 6-36 shows the results obtained with the two
different types of centrifuge.
Centrifuge design
sludge identification
Anaerobically digested primary
plus waste activated with
alum sludge
Low speed
High speed
No. of operation units
Flow rate per unit
Percent feed consistency
Percent cake solids
Percent solids recovery
Polymer type . .
Polymer dosage
1
190 gal/min
3
16-18
95-98
Allied Colloids
Cationic
6 Ibs/ton
3
90 gal/min
3
16-18
95-98
Percol # 72f
12 Ibs/ton
While table 6-36 only shows the improvement realized
by reduction in polyelectrolyte costs by about $9/ton
($10/Mg) (which is a considerable savings), table 6-37
illustrates the additional advantages for the low speed
design.
Wear played an important part in displacing the high
speed centrifuges in favor of the low speed centrifuges
at this particular plant. The low speed centrifuge was
inspected after 2,000 hours of operation and found to
have only 1/18 of the wear of the high speed alterna-
tive. The abrasive protection on the low speed machine
conveyor blades is tungsten carbide, while the protectior
on the high speed machine is equivalent to an alloy
called Stellite 1016. The Stellite material is considered
inferior to the tungsten carbide hardness values ap-
proach Rc-69. Experience shows that if both materials
had been similar that the wear rate would still have
favored the low speed design by as much as a five to
one ratio.
Summarized in table 6-38 is the annual cost analysis
of the operation of these two types of centrifuges in-
stalled side by side. The low speed unit clearly has the
Table 6-37.—Side by side comparison machine parame-
ters
Centrifuge design
Low speed
High speed
Bowl diameter 36"
Bowl length 96"
Centrifugal force 511 x G
Unit flow rate 190 gal/min
Unit pool volume 196 gallons
Sigma factor 1.15X107 cm2
Unit motor size rating 100 hp
Absorbed horsepower 0.3 gal/min
Noise level at 3 ft 80-85 dBa
Wear at 2,000-hour inspection 1/2 mm
25"
90"
1,878 x G
90 gal/min
73 gallons
5.3 X107 cm
180 hp
0.6 gal/min
95-100 dBa
9 mm
138
-------
Table 6-38.—Side by side comparison annual cost—
profile
Table 6-40.—Basket centrifuge operation—Burlington,
Wis., WWTP
Centrifuge design
Tons/year per unit
Power expenditure
Polymer expenditure
Maintenance expenditure
Amortized equipment
Total annual cost
Low speed
12,483
$0.06/ton
$9.00/ton
$1 .21 /ton
$1 .50/ton
$12.33/ton
High speed
5.913
$1.19/ton
$16.00/ton
$8.30/ton
$2.44/ton
$27.93/ton
Feed rate (gal/min)
Dewater rate (Ibs D.S./hr)
Hours required/week
23
104
168
17R
147
0
R— ft f\\ T ^
^0
f\9
88
397
44
99
48
30
13-15 (T)
14
47
Table 6-39.—Dimensional data—low speed centrifuge
Model No.
HB 2500
HB 3700
HB 6400
Overall
length
(in.)
138
139
276
Overall
width
(in.)
80
72
150
Overall
height
(in.)
36
41
71
Weight
(Ibs)
6500
9,400
3440
edge in all categories. Power consumptions are one-half
(1/2) that of high speed unit. With respect to polymer
consumption, the low speed centrifuge in this particular
case utilized 44 percent less cationic polymer than the
high speed centrifuge. With respect to conveyor mainte-
nance, we have modified the high speed centrifuge fig-
ure to reflect a ratio of conveyor resurfacings more in
the category of 5 to 1 than the 18 to one margin
indicated by the actual side by side installation. The
category entitled "Amortized Equipment" includes the
cost of centrifuge, the motor, and the starter, and is
expressed on a tonnage basis and reflects an amortiza-
tion rate of 7 percent interest over a 20-year period.
Electrical usage rate was assumed to be 0.02/kWh and
polymer (Allied Colloids Percol 728) was figured at
$1.50/lb ($3.30/kg).
While the larger size of the low speed unit would
account for a minor portion of the above noted superior-
ity, it is abundantly clear that the lower speed concur-
rent flow unit is superior from a cost-effectiveness stand-
point.
Dimensional Data—Centrifuges
Table 6-39 shows dimensional data for one brand of
the newer low speed centrifugals.
CASE HISTORY—CENTRIFUGAL DEWA-
TERING—BURLINGTON, WIS., WWTP
The experiences at Burlington are described (in an
outstanding fashion) in reference 23.
The Burlington plant treats an average flow of com-
bined municipal and industrial wastes at DWF level of
1.5 Mgal/d (0.06 m3/s) and a wet weather flow of 2.0
Mgal/d (0.09 m3/s).
The treatment plant employs contact stabilization (12
hour aeration time, 25 percent return rate, MLSS of
2000 mg/l). The F/M ratio is 0.2 to 0.5. A sludge age
varying from 5 to 12 days is employed, including aera-
tion and aerobic digestion time.
The above described liquid treatment system results in
sludge disposal requirements of 160,000 gallons (606 m3)
of W.A.S. per week or 3400 pounds (1545 kg) per day,
about 27,000 gallons/day (102 mVday).
The plant was designed for ultimate liquid sludge dis-
posal by lagoon. When this disposal option was cur-
tailed, dewatering studies ensued. Needless to say, the
sludge dewatering problems are significant. It is a clas-
sic example of the problems which result when a plant's
liquid treatment system is designed for liquid sludge dis-
posal and then dewatering is required.
A batch, cycling, basket centrifuge was tested, pur-
chased, installed and has been operated for some time.
The essence of the results of the full scale performance
is listed in table 6-40.
As can be seen, despite the high polymer cost, the
overall cost analysis showed the total operation to be
more cost effective with polymer use.
It should be noted that if the city could start again
from square one, it is certain that, now having to dewa-
ter sludge, and knowing the overall energy costs of the
type total system involved, a different liquid treatment
system would be chosen.
Additional valuable insights in the referenced paper
relate to the correlations between activated sludge sys-
tem operating parameters and resulting sludge processa-
bility.
REFERENCES
1. Process Design Manual Sludge Treatment and Disposal, Technolo-
gy Transfer, U.S. EPA, Washington, D.C. (1974).
2. Camp Dresser & McKee, Report No. PB-255-769, NTIS, Spring-
field, Va. June 1976.
3. Jones, Edgar R., P. E., "Sludge Production Rates, District of
Columbia," Ecoletter of Chesapeake WPCF and Water and Waste
Operators of Md., Del., and D.C., vol. 3, No. 2, p. 4., Spring
1977.
4. Gale, R. S., "Recent Research on Sludge Dewatering." Filtr. Se-
par. (September-October, 1971), pp. 531-538.
139
-------
5. Corrie, K. D., "Use of Activated Carbon in the Treatment of Heat- 15.
Treatment Plant Liquor," Water Pollution Control (U.K.) 1972, pp.
629-635.
6. Stack, V. T., Jr., Marks, P. J., and Garvey, B. T., "Pressure 16.
Cooking of Activated Sludge," paper by Roy F. Weston, Inc.
7. Reports by Greeley & Hansen to the city of Tampa. 17.
8. Crockford, J. B., Sr., and Sparham, V., "Developments to Upgrade
Settlement Tank Performance, Screening, and Sludge Dewatering
Associated with Industrial Water Treatment," Purdue Industrial
Waste Conference, May 1975, pp. 1072-1083. 18.
9. Personal Communication, Dr. Dan Swett and Mr. Mike Riise of G.
C. One Ltd., Suite 605, 2700 N.E. 135th St., North Miami, Fla.
33181. 19.
10. Bell, J. A., Higgins, R., and Mason, Donald G., "Dewatering, a
New Method Bows," Water and Wastes Engineering, April 1977,
pp. 33-41.
11. Eichmann, Bruce W., "Dewatering Machine Solves Sludge Drying 20.
Problems," Water and Sewage Works, October 1977, pp. 99-100.
12. Creek, John, "Tait Andritz SDM Sludge Dewatering Machine," 21.
WWEMA Conference paper, April 20, 1977, Altanta, Ga.
13. Keener, Phillip M., and Metzger, Larry R., "Startup and Operating 22.
Experience With a Twin Wire Moving-belt Press for Primary
Sludge," vol. 60, No. 9, September 1977, TAPPI, pp. 120-123. 23.
14. Grove, G. W., Exxon Research & Engineering, "Use Gravity Belt
Filtration for Sludge Disposal," Hydrocarbon Processing, May 1975.
Department of the Environment, U. K., Directorate General Water
Engineering, R & D Division, Project Report No. 4, Sewage
Sludge Dewatering by Filter Belt Press.
Cassel, A. F., "Review of U.S. Filter Press Operations," paper
presented at Chesapeake Section, WPCF, June 1976.
White, M. J. D., and Baskerville, R. C., "Full Scale Trials of
Polyelectrolytes for Conditioning of Sewage Sludges for Filter
Pressing," Journal of Institute of Water Pollution Control, No. 5,
1974.
Heaton, H. M., "The Practical Application of the Membrane Filter
Plate to Increase Filter Press Productivity and Overall Economics
Filtech 77, September 20-22, 1977, Olympia, London.
White, M. J. D., Bruce, A. M., and Baskerville, R. C., "Mechanic:
Dewatering of Municipal Sludge in the U.K.—Laboratory to Full
Scale," presented at conference, Theory, Practice, and Process
Principles for Physical Separations, California, 10/10 to 11/4/77.
Cassel, A. F., "Update on Filter Press Operations," paper presen
ed at Chesapeake Section, WPCF, June 1977.
Reimann, D., Kommunalwirtschaft, No. 9, September 1974, pp.
343-352.
Guidi, E. J., "Why Low Speed Centrifugation," Presented at Ohio
WPCF, Columbus, June 16, 1976.
Pietila, K. A., and Zacharias, D. R., "Full Scale Study of Sludge
Processing and Land Disposal Utilizing Centrifugation for Dewater
ing," Paper presented at Central States WPCF, May 1977.
140
- U. S. GOVERNMENT PRINTING OFFICE. 1978-760-566/8 Region No. 5-11
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