iLLDGt
4AN/4GCMENT
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MLNICIIAL
SLUDGE
MANAGEMENT
PROCEEDINGS OF THE
NATIONAL CONFERENCE
ON MUNICIPAL
SLUDGE MANAGEMENT
JUNE 11-13, 1974
Pittsburgh, Pennsylvania
Sponsored by:
Allegheny County, Pennsylvania
In Association with:
The Pittsburgh Section of the
American Society of Civil Engineers and
the U.S. Environmental Protection Agency
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Printed in the United States of America
Library of Congress Catalog No. 74-19534
Copyright © 1974
by
Information Transfer, Inc.
1625 Eye Street, N.W.
Washington, D.C. 20006
All Rights Reserved
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CONTENTS
Keynote Address 1
Donald Berman, Department of Public Works, Allegheny County
Overview of Sludge Handling and Disposal 5
].B. Farrell, National Environmental Research Center,
U.S. Environmental Protection Agency
Alternative Methods for Sludge Management 11
Harold Bernard, Environmental Quality Systems, Inc.
Thickening of Sludges 21
Richard I. Dick, University of Delaware
Anaerobic Digester Operation at the Metropolitan Sanitary
District of Greater Chicago 29
Stephen P. Graef, The Metropolitan Sanitary District of Greater Chicago
Metro Denver's Experience with Large Scale Aerobic Digestion of
Waste Activated Sludge 37
David B. Cohen, Metropolitan Denver Sewage Disposal District No. I
High Purity Oxygen Aerobic Digestion Experiences at
Speedway, Indiana , , 55
Daniel W. Gay, Raymond F. Drnevich, Edmund }. Breider and
Kai W. Young, Union Carbide Corporation—Linde Division
Sludge Dewatering 67
Charles W. Carry, Robert P. Miele and ]ames F. Stahl,
Los Angeles County Sanitation Districts
Pressure Filtration—Municipal Wastewater Solids,
Cedar Rapids, Iowa 77
James W. Gerlich, Howard R. Green Company
Heat Treatment and Incineration , 87
Dale T. Mayrose, Dorr-Oliver Incorporated
Drying of Sludge for Marketing as Fertilizer 93
M. Truett Garrett, jr., Houston Sewage Treatment
and. Sludge Disposal Plant
Growth of Barley Irrigated with Wastewater Sludge
Containing Phosphate Precipitants 97
M.B. Kirkham and G.K. Doison, National Environmental Research
Center, U.S. Environmental Protection Agency
in
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IV
Utilization of Digested Chemical Sewage Sludges on
Agricultural Lands in Ontario 107
Steven A. Black, Ontario Ministry of the Environment
The Economics of Sludge Irrigation 115
A. Paul Troemper, Springfield Sanitary District
Composting Sewage Sludge 123
E. Epstein and G.B. Willson, Biological Waste Management
Laboratory, ARS, U.S. Department of Agriculture
Recent Sanitary District History in Land Reclamation
and Sludge Utilization 129
James L. Halderson, Bart T. Lynam and Raymond R. Rimkus,
The Metropolitan Sanitary District of Greater Chicago
Sludge Management in Allegheny County 135
Richard M. Cosentino, Allegheny County Department of Public Works
Trench Incorporation of Sewage Sludge 139
John M. Walker, Biological Waste Management Laboratory,
ARS, U.S. Department of Agriculture
Ocean Disposal Experiences in Philadelphia 151
Carmen F. Guarino and Steven Townsend, Philadelphia
Water Department
Sludge Disposal by Incineration at ALCOSAN 157
George A. Brinsko, Allegheny County Sanitary Authority
Pasteurization of Liquid Digested Sludge 163
Gerald Stern, National Environmental Research Center,
U.S. Environmental Protection Agency
Sludge Handling and Disposal at Blue Plains 171
A Inn F. Cassel and Robert T. Mohr, District of
Columbia Department of Environmental Services
Agricultural Utilization of Digested Sludges from
the City of Pensacola 177
Joe A. Edmisten, Baseline, Inc.
Institutional Problems Associated with Sludge Disposal 183
Kerry /. Brough, Washington Suburban Sanitary Commission
Energy Conservation and Recycling Program of the
Metropolitan Sewer Board of the Twin Cities Area 187
Dale C. Bergstedl, Director of Solids Processing—St. Paul
Sludge Disposal at a Profit? 195
W. Martin Fassell, Barber-Colman Company
Sludge Management System for St. Louis 205
Peter F. Mallei, Metropolitan St. Louis Sewer District
The Environmental Protection Agency's Research Program 211
\\'illiam A. Rosenkranz, U.S. Environmental Protection Agency
Summary of "Pretreatment and Ultimate Disposal of Wastewater
Solids Conference"—held May 21-22, Rutgers University 215
Robert W. Mason, Region 11, U.S. Environmental Protection Agency
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VilSUIIAI
SLUDGE
MANAGEMENT
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KEYNOTE ADDRESS
DONALD BERMAN
Department of Public Works
Allegheny County, Pennsylvania
Wearing another hat, I want to take this oppor-
tunity to extend words of welcome from the Board
.of Commissioners of Allegheny County. Our in-
volvement in a conference of this type, I believe,
indicates our concern about environmental prob-
lems and our willingness to try to do something
about correcting them.
As background information, you should know
that Allegheny County put into operation the
first automatic telemetered air pollution monitor-
ing system in the United States. The data collected
has been used in enforcement proceedings insti-
tuted by the County and is also a vital part of a
warning system which enables us, with the co-
operation of industry, to control emissions from
certain plants during times of weather inversions.
Our Works Department is actively engaged in
implementing a solid waste management program,
and we are assisting local municipalities in im-
plementing regional sewage systems. Our Con-
servation District is involved in surveillance of
erosion and siltation problems. Our Health De-
partment, in addition to its work in the air pollu-
tion field, is the regulatory agency in matters relat-
ing to solid waste and sewage problems. One part
of this latter program—the proper handling of mu-
nicipal sludges—is the subject for discussion at
this conference.
I believe it was Barry Commoner, in his book, The
Closing Circle, who pointed out four basic rules
which we must adhere to in finding solutions to en-
vironmental problems. Rule #1 states that we never
throw anything away. Rule #2 states that every-
thing we do is connected to everything else. Rule #3
states a well-known fact that nature knows best.
And Rule #4 states a fact that we are constantly
faced with—"There is no such thing as a free
,lunch."
With these items as focal points, this august body
will, I trust, help to find ways and means for sludge
management which are equitably, economically
and environmentally acceptable.
We have come to realize that the first rule,
"Nothing is ever thrown away," has a great deal to
do with how we proceed. In our attempt to clean up
the air and water, we sometimes forget that what
we have taken out of these two elements must still
be dealt with. In the field of sewage treatment, the
problem of sludge management is one which has
plagued many engineers and government officials
during the past several decades. It has almost be-
come a critical situation, since we are building more
treatment facilities and expanding existing treat-
ment facilities in order to take more of the pollut-
ants from the water before it is discharged to the
receiving stream. No longer can we afford to hap-
hazardly dump sludge on barren land or at sea. No
longer should we burn sludge just for the sake of
disposing it. No longer will we be able to live with
the premise that ;/ you can't see it, it isn't there.
Many experiments have been tried nationwide.
Some have been successful and some have shown
that additional questions have yet to be answered. I
trust that the discussions at this meeting will
enable pertinent information to be passed back and
forth among those who are seeking solutions. Here
again, in Allegheny County, we are attempting to
solve this sludge problem with a variety programs.
We cannot, however, in any future efforts,
forget Barry Commoner's second rule, "Every-
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2 MUNICIPAL SLUDGE MANAGEMENT
thing is connected to everything else."
For the past several decades, this country has
lived with the belief that bigger is better. We have
grown, we have developed and we have built with-
out any true regard for the after effects of major
disruptions of natural areas. We are now finding
that although we live better, and have more con-
veniences than our fathers thought possible, we
have also created problems which at times seem to
be unsolvable. We then have two areas of concern.
The first is to try to catch up with ourselves and
repair the damage which we have done; and the
second is to see that, in any of our planning for the
future, we do not get into the same bind in which
we now find ourselves.
Paraphrasing Dr. Commoner's second rule,
every action has an equal and opposite reaction. We
must remember that the environment is everything
around us and when we change something, as we
must because of the wants and needs of our in-
creased population, there will be an after effect or
reaction which must be considered in the original
instance.
Although the brunt of your discussions will be
aimed at municipal sludge or that which comes
from treating sewage emanating from homes,
businesses and commercial establishments, we can-
not forget, especially in urban areas, the industrial
sludges which are developed as a part of industry's
program of cleaning up their discharges.
These materials are complex, not only in indi-
vidual plants, but when considering the types of
operations carried on in all of the industrial activi-
ties in this area, the variation in composition some-
times boggles the mind.
In addition to municipal sludges and industrial
sludges, we also face the problem of effective
utilization of the soMwaste or refuse which is gen-
erated in the homes, businesses and industries in
this country. It seems to me, that the brains in this
room should be able to develop some means where-
by these three types of wastes—sewage sludges, in-
dustrial sludges, and solid materials—could be ef-
fectively reused in some manner, either singly or in
combination with one another.
In coming to grips with this expanded scope, we
cannot forget Commoner's Rule #3, "Nature
knows best." While I have a great deal of respect for
engineering talent and wholeheartedly believe that
man's ingenuity is boundless, I also am firmly con-
vinced that there is really nothing new under the
sun. The basic facts of chemistry, physics, elec-
tricity and strength of materials have all been well
established through the ages. For many hundreds
of years man has known about the problems as-
sociated with the disposal of his wastes. For cen-
turies, shepherds have known that the droppings
from sheep could be used as a heat source and
swamps have always given methane gas as a result
of the decomposition of organic materials. It seems
to me that what is required in today's civilized
society is an attempt to use the basic laws of nature
to our advantage rather than to attempt to
circumvent them.
While the basic technology is known, our prob-
lem—yours and mine—is to scale up our labora-
tory or demonstration models to a working size
large enough to handle the problem in a given area.
Remember, however, that a regional plan does not
necessarily mean a regional plant—or we would
have one huge sewage treatment facility in New
Orleans serving the Mississippi and Ohio River
drainage basins.
Let me now go on to Rule #4 which in the minds
of many citizens is the most important—"There is
no such thing as a free lunch." Development and
building and expansion cost money. During the
course of that development, building and expan-
sion, and as a result of that development, building
and expansion, wastes are generated. These ma-
terials, however, are wastes only in the eyes of
those actually involved in the initial growth and in
the eyes of those who are benefiting from the fruits
of that growth. It's readily easy for a consumer to
accept the fact that he has to pay for something re-
quired to build his house or brought into his home,
office or business. It is not so easy to see that the
material that he no longer wants or needs must be
taken away and that there is also a cost associated
with that operation. My concern, and your prob-
lem, is to see that Dr. Commoner's first three rules
are adhered to at as low a cost as is possible, given
the fact that protection of the environment must be
one of our ultimate goals.
I'd like to speak to you for a few more minutes
about two additional rules which cannot be ascribed
to Dr. Commoner.
The first one goes back about a century when
Abraham Lincoln said, "You can't please all of the
people all of the time." In the context of municipal
sludge management, I am sure that whatever solu-
tions may eventually evolve, we will have theo-
rists, purists, environmentalists, governmental of-
ficials and just plain citizens, who will decry our
efforts. I am just as certain, however, that Berman's
Rule must be implemented. This dictum states that
while we can do as much planning and discussing of
numerous alternatives as we care to, while we can
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KEYNOTE ADDRESS
develop all types of "systems" approaches, and eventually lead to the gradual elimination of the
while we can develop "models" to our heart s con- seemingly gigantic problem which faces us today.
tent, no one solution will ever solve all of our Being an eternal optimist, I am confident that our
municipal sludge problems. Rather, a series of ap- sludge management problems will be solved delib-
proaches are required which, I am confident, will erately, rationally, and slowly—but inexorably.
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OVERVIEW OF SLUDGE HANDLING
AND DISPOSAL
J. B. FARRELL
National Environmental Research Center
U.S. Environmental Protection Agency
Cincinnati, Ohio
For about eight years, the Advanced Waste
Treatment Research Laboratory, which is now part
of U. S. Environmental Protection Agency's
National Environmental Research Center, Cincin-
nati, has had an Ultimate Disposal Section. A
primary objective of this group has been to advance
the technology for processing and disposing of the
concentrates from municipal and advanced
wastewater treatment processes. It has thus been
our privilege to observe and to take part in the ef-
fort to improve the methods for handling and dis-
posal of wastewater sludges.
As the, quality of wastewater treatment has im-
proved, sludge handling and disposal have become a
greater problem: The trend is not expected to
change. As more and more municipalities upgrade
facilities to improve effluent quality, the quantity
of sludge will continue to increase. Table 1 com-
pares sludge production in 1972 with the estimated
production in 1985: the amount of secondary treat-
ment sludge will be almost doubled,, and chemical
sludges will be produced in quantity; dewatering
costs will increase more than proportionately
because the biological secondary sludges and
several important types of chemical sludges are un-
usually difficult to dewater.
The operations carried out on wastewater sludge
are conveniently divided into treatment operations,
such as pumping, thickening, stabilization, and
dewatering; and disposal operations, which include
TABLE 1
Trends in Production and Disposal
of Municipal Wastewater Sludge
1972
SLUDGE TYPE
Primary (0.12 lb/ cap-day)*
Secondary (0.08 Ib/cap-da)
Chemical (0.05 lb/ cap-da)
Popul.
(mill.)
145
101
10
Dry tons**
per year
3,170,000
1,480,000
91,000
1985
Popul.
(mill.)
170
170
50
Dry tons
per year
3,720,000
2,480,000
455,000
DISPOSAL METHODS
Landfill
Utilized on land
Incineration
Ocean (dumping and outfalls)
Percent
40
20
25
15
Percent
40
25
35
0
*lb x 0.454 = kg.
**ton x 0.908 = metric ton.
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MUNICIPAL SLUDGE MANAGEMENT
incineration, landfill, and landspreading. The more
important recent developments in treatment are
considered here first.
Treatment Operations
Thickening
If sludges removed from primary and secondary
clarifiers are to be dewatered, their solids content
should be as high as possible, that is, just short of
the point where the "thick" sludge interferes with
the operation of the device used for dewatering.
Thickening is often required. The most commonly
used devices are either gravity or air flotation
thickeners. Air flotation thickeners are generally
more expensive to operate, but have advantages in
certain cases. There is less opportunity for sludge
to become anaerobic, because residence time is less
than in a gravity thickener and air bubbles that
have floated the sludge provide a reservoir of
available oxygen. In secondary plants where the
effluent has nitrified, gravity thickening of waste
activated sludge becomes difficult because the
respiring activated sludge organisms use nitrate as
their oxygen source after dissolved oxygen is con-
sumed. Nitrogen gas is released and this causes a
portion of the sludge to float. If air flotation is
utilized instead of gravity thickening, any tendency
of sludge to float will, of course, be advantageous.
Stabilization
The conventional stabilization processes are
anaerobic and aerobic digestion. The effect of these
processes is to reduce odor, reduce the putrefaction
potential of the sludge, and reduce the concentra-
tion of hazardous microbiological organisms. It is
interesting to separate these positive results and
consider other means by which they can be carried
out. Table 2 measures a number of processes
against these characteristics. It is evident that none
of the processes do all things well. Further study of
Table 2 indicates that certain combinations of
processes would be effective, for example,
anaerobic digestion followed by pasteurization.
These two processes have been carried out in se-
quence in Germany1 to treat sludge used on pasture
lands during the summer months. Radiation has
recently been used following anaerobic digestion2,
again in Germany and for the same purpose.
Costs of combined treatment are additive.
Recently, however, there has been interest in the
use of anaerobic or aerobic thermophilic digestion.
These processes can be operated at 60°C and
destroy pathogenic microorganisms. They effec-
tively combine pasteurization and stabilization into
a single process, and economies can result.
TABLE 2
Attenuation Effect of Well-Conducted
Treatment Processes on Stabilizing
Wastewater Treatment Sludges
PROCESS
DEGREE OF A TTENUA TION
Pathogens Putrefaction Odor
Potential
Anaerobic digestion
Aerobic digestion
Heavy chlorination
Lime treatment
Pasteurization (70°C)
Radiation
Heat treatment (I95°C)
fair
fair
good
good
excellent
excellent
excellent
good
good
fair
fair
poor
poor
poor*
good
good
good
good
poor
fair
poor*
*good for filter cake
Most stabilization processes adversely affect
supernatants and filtrates from subsequent
dewatering operations: anaerobic digestion and
heat treatment produce supernatants rich in
nutrients and BOD; supernatant from heavy
chlorination is high in chloramines and possibly
chloro-organic compounds; and supernatant from
lime treatment is high in pH. Cost of processing
supernatants must be considered in the total cost of
processing.
Processes such as digestion and heat treatment
reduce the mass of sludge to be treated in subse-
quent operations. These changes in mass are a
cause of great confusion in the literature when
costs of complete processing sequences are com-
pared. Costs are most often presented as dollars per
ton of dry solids without specifying whether the
basis is the sludge mass before or after processing.
An unequivocal way of comparing costs of alter-
native processing sequences for a given waste-
water is to base costs on equal wastewater flow
(e.g., 1000 gal. of wastewater).
Dewatering
FILTRATION. The continuous rotary vacuum
filter is the most commonly used device for de-
watering wastewater sludges. Three different
types are used: the drum (cloth on drum, cake re-
moved by doctor blade), cloth belt (cloth belt winds
off drum, sharp bend causes cake to drop off, both
sides of belt are washed, and cloth returns to drum),
and coil (filter surface comprises two layers of
tightly coiled springs, otherwise similar to belt
type).
A new type of drum filter, utilizing top-feed, has
been developed by the Rexnord Corporation for the
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OVERVIEW 7
City of Milwaukee under an EPA grant (see Figure
1). Gravity assists in depositing coagulated solids,
which are denser than water, onto the drum sur-
face. Gravity also assists in removing cake from the
filter surface, so air blowstack to loosen cake is not
needed, and the doctor blade is nearly superfluous.
CENTRIFUGES. Solid-bowl continuous con-
veyor-type centrifuges are the most popular cen-
trifugal device used for sludge dewatering. Cake
solids contents are similar to those obtained with
vacuum filters. These devices are also excellent
classifiers. They are being used with tertiary3 and
primary line sludges4 to classify organic and most
mineral solids except calcium carbonate, which can
be calcined and recovered as CaO.
Basket-type centrifuges are being used to thicken
aerobically digested sludge. This type of centrifuge
will be used to remove solids from the centrate
from Los Angeles County's solid-bowl conveyor-
type centrifuges5.
OTHER METHODS. Filter presses are receiv-
ing corfsiderable interest in the United States, par-
ticularly now that high cake solids is becoming im-
portant. At Cedar Rapids, Iowa6, a filter press has
been installed that uses a precoat of sludge in-
cinerator ash and a body feed of incinerator ash,
lime, and ferric chloride. Vertical presses, used at
several plants in Japan, offer very speedy cake dis-
charge. Precoat is not needed because a section of
the filter cloth is washed during each cycle. The
cloth advances one frame at each discharge cycle.
Thus, a section of cloth is washed after having per-
formed a number of filtrations equal to the num-
ber of frames. Ca(OH)2 and FeCb are the usual
conditioning agents with this filter at Japanese
plants.
Belt filters have been used extensively in Europe
and are being marketed in the United States. They
combine gravity drainage with mechanical pressure
after a cake has been formed. The Carter* belt
filter, presented schematically in Figure 2, il-
lustrates the principle. The capillary suction filter
developed by Westinghouse with the financial sup-
port of EPA combines capillary suction dewatering
with mechanical pressure. A simplified sketch of
the original research unit is presented in Figure 3.
Sludge is placed in a dry belt of a foamed porous
material, which removes moisture from the sludge
by capillary action. A contact roll rides directly on
the partially dewatered sludge layer and applies
mechanical pressure. Water is forced from the
TOP FEED BOTTOM FEED
Figure 1: Drum-Type Vacuum Filters.
LIQUID SLUDGE
T5 o o cr
Figure 2: Schematic of Carter Belt-Filter Press.
Sludge
Compression
Roller
Sludge Feed
Sludge
Cake
'Mention of manufacturer's- name does not constitute EPA
endorsement.
Figure 3: Schematic of Research Capillary Dewatering Unit.
sludge into the porous belt. The sludge cake
transfers to the smooth-surfaced contact roll
where it is collected and removed. The porous belt,
now free of sludge, is washed and squeezed dry. Ac-
tivated sludges, which can be dewatered to over 15
percent solids, can be dewatered to over 18 percent
solids at high rates with this device.
Conditioning
Very few sludges can be filtered without ad-
ditives of some kind. Lime and ferric chloride con-
tinue to be used, particularly when odors are a
problem or when improperly digested sludge is to
be landfilled. The use of organic polymeric con-
ditioning agents continues to grow. The more rapid
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8 MUNICIPAL SLUDGE MANAGEMENT
gains, however, have been made by heat con-
ditioning. Sludge heated to 200° C (392° F) for un-
der an hour, in the presence of air or without air,
loses its gelatinous nature and becomes easily
filterable, generally without chemicals. A substan-
tial portion of the sludge, however, is solubilized
and the filtrate must be recycled to biological treat-
ment.
Heat treatment is a satisfactory means for con-
ditioning sludge. Whether or not it is the most cost-
effective depends on the situation. The process re-
quires a higher degree of operating skill than is or-
dinarily available at wastewater treatment plants.
It causes odors that are sometimes extraordinarily
difficult to eliminate. The additional cost of
biologically treating the filtrate (and handling the
increased biological sludge generated) should be
charged to the process. Even for plants with tem-
porary excess capacity, the loss in capacity caused
by the need to process recycle streams must even-
tually be borne by the wastewater treatment plant.
In Great Britain, where effluent quality re-
quirements are more stringent than those in the
United States, separate treatment of the superna-
tant is needed. At one facility, supernatant is being
biologically treated by separate activated sludge
treatment, which is followed by activated carbon to
remove residual COD. At another plant, the entire
supernatant will be concentrated by flash evapora-
tion and the concentrate then incinerated.
Disposal Operations
Hazardous Substances
Disposal of sludge is turning out to be the pivotal
question in wastewater processing. Sludges con-
tain the concentrated wastes of a community. It is
reasonable to expect that objectionable materials
may be present in sufficient concentration to be
hazardous. The degree of hazard will, of course, de-
pend on the intended means of disposal.
Some typical concentrations of hazardous sub-
stances found in sludge are presented in Table 3.
The hazardous substances are toxic metals, toxic
organic chemicals, and pathogenic microorganisms.
The concentration and quantities of these con-
taminants clearly limit disposal options. For exam-
ple, the high concentrations of pathogens in raw
sludge virtually prohibit disposal to landfill or to
agricultural land. Sludge with a high PCB concen-
tration should not be applied to land if there is a
possibility of leaching. Sludge with high mercury
content may be suitable for disposal to a landfill but
not for incineration. There appears to be no
foolproof disposal method suitable for all sludges.
TABLE 3
Approximate Concentration Levels of
Substances in Municipal
Wastewater Sludges
Sludge parameter
Raw primary Digested primary
% Volatile solids 72.0
% Ash 28.0
Metal concentration
(mg/kg dry solid)
Cd
Zn
Cu
Ni
Hg
Bacterial content
(per 100 ml liquid sludge)
Fecal coliform 1 1 x I06
Salmonella 460
Pseudomonas aeruginosa 46,000
Organic compounds
(mg/kg dry solids)
Polychlorinated biphenyls
Chlordane
DDT
Dieldrin
52.6
47.4
30
1,950
1,000
350
5
0.4 x 106
29
34
n.d.* to 105
3 to 30
n.d. to 1
0.1 to 2.0
*n.d. = not detectable
The type of disposal chosen by a community is a
commitment not easily changed. Increasing
knowledge of hazards, or a change in the nature of
the community's wastes, may indicate that the
wastewater sludges contain too high a concentra-
tion of certain materials for the method of disposal
practiced. Two things are necessary—the com-
munity should have a current knowledge of the
concentrations of hazardous materials in its sludge,
and it should have ordinances which allow it to
prohibit disposal into its collection systems not only
wastes which affect the quality of wastewater
processing but also hazardous wastes which are not
"neutralized" by the disposal method and are a
threat to the environment.
The conventional disposal methods are ocean dis-
posal, landspreading, landfill, and incineration. A
discussion of ocean disposal is beyond the scope of
this presentation. It appears at this time, however,
that disposal of sludge solids by outfalls or by
dumping will diminish in coming years, and that the
level of hazardous materials in their sludges will
have to be reduced by communities continuing to
use these procedures.
Land Application
The use of stabilized sludge to fertilize
agricultural land or reclaim marginal land is a con-
serving use which has been practiced for many
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OVERVIEW
years. Chicago plans to dispose of a major portion
of their sludge by this means7. Coastal cities are
considering it as an alternative to ocean disposal.
Attention has been called to possible hazard to
crops and to human health from a gradual buildup
of toxic metals in soil to which sludge has been
applied over a number of years8. The U. S. EPA is
currently studying suggested guidelines for land
application of sludge.
It is likely that some restrictions will be placed on
the practice of land spreading. Suggestions have
been made to limit both the concentrations of
metals such as Cd, Cu, Zn, and Ni in the sludge and
the maximum loading of sludges on the land8, or
the maximum loading of the metals on the land9.
Knowledge of effects of metals in sludge on crops is
sparse, which makes preparation of reasonable
guidelines difficult. It is expected, however, that for
communities with an unusually high industrial
component in their wastewater, recommended
application rates may be too low for this method to
be competitive with alternative disposal means.
Landfill
Disposition to landfill is the most common way to
get rid of sludge. Sometimes the landfill is a proper-
ly operated sanitary landfill for disposal of solid
wastes and sludge. Often the landfill is an un-
covered dumping site inside the plant grounds.
Small plants often dispose of their sludge to un-
protected sites outside their plant limits—
sometimes in flood plains and of ten without cover.
Little attention has been paid to the disposal
techniques practiced by small plants. Disposal to
sanitary landfills may be impractical because of dis-
tance or because sludges are banned from the land-
fill, and operators often resort to what is essentially
uncontrolled dumping. The EPA is sponsoring
work by the U.S. Department of Agriculture to
develop a method for operating a private sludge
landfill in an ecologically satisfactory way with the
use of simple farm machinery.
A properly operated sanitary landfill is an ex-
cellent place to dispose a sludge high in metals or
persistent organic compounds. The site should be
located where groundwater contamination is not
possible, and any leachate should be collected and
treated.
Incineration
When properly carried out, incineration is a
satisfactory means of disposing of the great majori-
ty of hazardous sludges. Particulates must be con-
tained by modern scrubbing equipment, and
temperature-time characteristics must be adequate
to decompose thermally stable organic compounds.
Sludges containing mercury are an exception
because mercury vaporizes upon incineration and is
not captured satisfactorily by conventional
scrubbers. Even mercury can be captured,
however, if the flue gases are brought to room
temperature and filtered10. A major problem with
incineration is poor operation; this can be corrected
by good operating procedures and modern control
devices.
Trends in Disposal
There has probably not been a more difficult time
for forecasting disposal trends. The picture is very
negative for ocean disposal, although changes are
possible. Land application will be subjected to
guidelines—guidelines that will recommend re-
duced application rates and require more land; land-
fill is satisfactory, but suitable sites are diminish-
ing rapidly; incineration would appear to face a
promising future except that the high cost of fuel,
added to the cost of air pollution control equip-
ment, has escalated the total cost.
Promising areas for the future, which are now
being seriously investigated, are co-incineration
and co-pyrolysis of sludge with solid waste. These
methods will not require supplemental fuel. Co-
pyrolysis may produce usuable fuel gas and char.
When estimating future disposal trends (Table 1),
incineration and pyrolysis increase from 25 percent
in 1972 to 35 percent in 1985. Much of this gain will
be from new methods of co-incineration and co-
pyrolysis.
Certainly disposal costs, as a proportion of
wastewater treatment costs, will rise. The benefit
will be a more secure environment for now and for
the future.
REFERENCES
1. Triebel, W. "Experiences with Sludge Pas-
teurization at Niersverband: Techniques and
Economy," Intern. Res. Group on Refuse Disposal
(IGRD) Inform. Bull. No. 21-31, Aug. 1964-Dec.
1967, pp. 330-390.
2. Suss, A., Matsch, H., Bosshard, E., Schur-
mann, G., and Luscher, O. "An Experimental Ir-
radiation Facility for the Sterilization of Sewage
Sludge," Kerntechnik 16, Jahrgang (1974), No. 2,
pp. 65-70.
3. South Tahoe P.U.D. "Advanced Wastewater
Treatment as Practiced at South Tahoe," pub. U.S.
EPA, 17010 ELQ 08/71 (Aug. 1971), Avail. NTIS,
No. PB 204 525.
-------
10 MUNICIPAL SLUDGE MANAGEMENT
4. Parker, D. S., Zadick, F. ]., and Train, K. E.
"Sludge Processing for Combined Physical-Chem-
ical-Biological Sludges," pub. U. S. EPA, EPA-R2-
73-250 (July 1973).
5. Parkhurst, J. D., Rodrigue, R. F., Miele, R. P.,
Hayashi, S. T., "Summary Report: Pilot Plant Stud-
ies on Dewatering Primary Digested Sludge," pub.
U. S. EPA, EPA-670/2-73-043 (Aug. 1973).
6. Gerlich, James W. "Pressure Filtration of
Waste Water Sludge with Ash Filter Aid," pub. U.S.
EPA, EPA-R2-73-231 (June 1973).
7. Dalton, F. E., and Murphy, R. R. "Land Dis-
posal IV: Reclamation and Recycle," J. W.P.C.F., 45,
No. 7, 1489-1507 (1973).
8. Chaney, R. L. "Crop and Food Chain Effects of
Toxic Elements in Sludges and Effluents," in Proc.
of Joint Conf. on Recycling Municipal Sludges and
Effluents on Land, July 9-13,1973, Champaign, 111.,
pub. Nat. Assn. of State Univ. and Land Grant Col-
leges (Wash., D.C.).
9. Page, A. L. "Fate and Effects of Trace Elements
in Sewage Sludge When Applied to Agricultural
Lands—A Literature Review Study," pub. U. S.
EPA, EPA-670/2-74-005 (Jan. 1974).
10. Perry, R. A. "Mercury Recovery from Pro-
cess Sludges," Chem. Eng. Progress, 70, No. 3, 73
(1974).
-------
ALTERNATIVE METHODS
FOR SLUDGE MANAGEMENT
HAROLD BERNARD
Environmental Quality Systems, Inc.
Rockville, Maryland
The treatment of wastewater is usually
separated into: l) treatment of a liquid phase, and 2)
treatment of a solid phase. There is a natural inter-
change between these two phases, but broadly
speaking, they are separate. The treatment pro-
cesses are usually designed very carefully to
optimize effluent quality; the sludge handling
systems are usually constrained "to do the best we
can with what we have". The situation should be
reversed.
For example, the separated solids from sewage
treatment is generally about 0.5 percent of the total
liquid phase volume. However, sludge disposal
costs usually represent between 30 and 50 percent
of the total costs associated with complete waste-
water treatment. With advanced waste treatment
doubling the quantity of sludges, this cost ratio will
undoubtedly increase. Sludge disposal should re-
ceive at least an equal effort in planning, as does the
wastewater treatment, if not more. It should not be
relegated to being considered only in the final
stages of plant design efforts, after everything else
is "set in concrete".
There are many alternative methods of handling
and disposing of sludge materials from waste-
water treatment plants. No single system is cap-
able of solving all disposal problems. Each disposal
method has advantages and disadvantages; each
can be made to work well under a given set of local
conditions.
For this next iteration, more accurate data would
be necessary. In addition, one may wish to consider
the economics of replacing existing sludge unit
processes. With a computer program this can be
relatively easily accomplished. In addition to being
able to compare ecortomics between processes it is
also possible to determine economics of future
changes in costs to unit processes by conducting
sensitivity analyses. In other words, what would
happen if the cost of power increases, if the cost of
chemicals increases, or if emissions standards
increase, or if close-in land becomes available and
transportation costs dramatically increase? In the
last mentioned situation, future unit cost for trans-
portation becomes important and the flexibility of
the plant to obtain additional dewatering of the
transported sludge becomes critical.
Trade-offs are possible with the use of an
integrated computer oriented approach. How much
extra will it cost, or save, if one approach is used in
anticipation of modifications, sometime in the not
too distant future, can be determined by conduct-
ing some additional analyses.
The end product is that system composed of
various unit processes that will "metamorphosize"
the sludge for ultimate disposal at the "optimum
cost" [not necessarily the least cost].
However, if the approach is limited to a strict
technological and economics exercise, it will not
suffice.
The problem of sludge disposal is further com-
plicated not only by increasing volumes, but also by
such factors as changing character of sludge from
advanced waste treatment processes, reduced land
availability in metropolitan areas, increased fuel
costs coupled with decreasing fuel reserves, and in-
creased emphasis upon the environmental impact
of sludge handling and disposal. These considera-
tions must be superimposed onto the engineering
aspects.
11
-------
12 MUNICIPAL SLUDGE MANAGEMENT
The following presentation of alternative ap-
proaches to sludge handling and disposal will
attempt to indicate the compexities, and the inter-
relationships of sludge disposal processes and
the total environment, as shown in Figure 1 and
Table 1.
final methods utilized. In reality, only several sys-
tems are usually considered without a rigorous
analysis of the impact of other alternatives. This
archaic approach is completely inadequate.
With available data or easily obtainable data, it is
reasonably simple to develop a matric of analytical
Figure 1: Intermedia Interplay.
Sludge Processing Systems
The general characteristics of the various types
of sludges anticipated in BPT and BAT systems
were described in Dr. Farrell's paper, "Overview of
Sludge Handling and Disposal." Figure 2 represents
the alternative processes included in six major
sludge handling and disposal unit operations:
thickening, conditioning dewatering, incineration,
product recovery and ultimate disposal. Table 2
presents the pollutants associated with each unit
operation.
The various combinations that can be
synthesized to form a system can theoretically be
some 44,000. Obviously, consulting engineering
companies do not iterate this many times. The
effect of current processes weigh heavily on the
expressions of costs for each unit operations that
should be considered for potential inclusion in a
sludge management system. The accuracy of the
cost data obtained should be equilibrated to the
degree of the iteration and the relative cost of each
of the systems synthesized. For example, for the
first iteration that may include many variations of
sludge handling systems, the cost data that is used
may constitute that obtained from the particular
treatment plant in question, from company files of
past projects and some budget-type data from
equipment suppliers. The output from this itera-
tion may reduce the number of candidates from the
initial number of let's say 20, to three to six.
The type of information that may be used for this
phase may be in the forms shown in Tables 2 and 3.
-------
ALTERNATIVE METHODS 13
TABLE I1
Wastewater Treatment Solution Problem
/
Transfer to Other Media
(Intermedia Transfer)
Unit
Operations
Screening
Flotation
Coagulation and
Sedimentation
Chemical Addition
Trickling Filter
ActlVtltCQ olllu^C
Lagoons and Sta-
bilization Ponds
Ion Exchange
Activated Carbon
Reverse Osmosis
Chlorination
Spray Irrigation
Ammonia Stripping
Cooling Towers
WATER POLLUTANTS
I -s
^J tr. D £*
^ ~** -2 Cl, > i.
-S' "§ 3 ^ § r° §• •§
^•^^Sg'oS^ "5
oc°g.cg'o,oa .^
*> '~ "> 'S Ci o c ? ? — • ~i.
.i;C ^ C^-1^ fc^ S^QiT
C S. 5 S fe CL 6e — . o S -~'
a£cg°c.^g-o>^ C-g
^i^c's'-^^-S; QF=!S
Q "^ Q *5 t/5 ^, < -^, Q^ h^ N
Air
. .
• [^
• • • • • o
Q
• • • • .
sols
• • • • •
-J
• • • • • s
s
O
• • • • • '*•
•
• • • • • • Aero-
sols
• NU3
• Heat
INTERMEDIA
Water
Compounds
formed
Residue
sludge
Residue
Leachate
BOD,SS
NO3iheavy
metals
Residues
Salt
Runoff
Leachate
CaCO3
TRANSFERS
Land
Residues3
Residues
Residues
Compounds
formed
Residue
Residue
& sludge
Residue
Residue
Residue
CaCO3
It will not be a simple "economics" approach
which will determine the most "effective" sludge
management system to be recommended. The most
"effective" approach will most likely also include
environmental and socio-economic needs that will
permit the ultimate disposal of the sludge in a man-
ner that has the least total impact on the quality of
the total human-environment complex that is
determined for each problem area by the various
participants involved. Each sludge management
concept incurs assessment and cost analysis.
Therefore, before the delineation of the tech-
nology unit costs, and system costs for the various
alternative sludge management concepts, envi-
ronmental and socio-economic factors should be
enumerated and superimposed. These factors will
have positive or negative impacts in the various
technological methods considered and may require
modification of the sludge handling concepts to
more environmentally acceptable systems.
The following socio-economic-environment con-
siderations will affect the total costs of sludge1
management:
Energy
Air Pollution
Health
Land Use
Resource Recovery
Public Attitudes
Secondary Impacts
Surface Water Quality
Nitrogen Impact
Soil Pollution
Cropping Practices
Economic Impact
Social Requirements
Ecological Impacts
The relationship between sludge management
and these environmental facets is shown in Figure 3
-------
14 MUNICIPAL SLUDGE MANAGEMENT
DEWATERING
THERMAL
DESTRUCTION
PRODUCT
RECOVERY
ULTIMATE
DISPOSAL
WASTEWATER
TREATMENT
THICKENING
CONDITIONING
STABILIZATION
PRIMARY
SECONDARY
CHEMICAL
PHYSICAL
i
GRAVITY
FLOTATION
CENTRIFUGATION
ACTIVATED
SLUDGE MOD.
ANAEROBIC
AEROBIC
DIGESTION
THERMAL
CHEMICAL
FREEZING
TRA
ISPORTATION
DRYING BEDS
DRYING LAGOONS
VACUUM FILTERS
FILTER PRESSES
CENTRIFUGES
VIBRATING
SCREENS
THERMAL*
INCINERATION
WITH AND
WITHOUT HEAT
RECOVERY
INCINERATION
WITH REFUSE,
WITH AND
WITHOUT HEAT
RECOVERY
PYROLYSIS
HEAT DRYING
WET OXIDATION
(BY TRUCK, RR, BARGE, PIPE)
SLUDGE
COMPOSITING
SLUDGE-REFUSE
COMPOSITING
FERTILIZER
THERMAL*
CONSTRUCTION
MATERIALS
% ^
LANDFILL
LAGOON
LIQUID SURFACE
SPREADING
LOW M/C SLUDGE
SPREADING
SUBSURFACE
Figure 2: Alternative Solids Handling Processes and Systems.
TABLE 2>
Wastewater Treatment Costs
Treatment
Oil Separation
Equalisation
Coagulation-
Sedimentation
Neutralization
Flotation
Sedimentation
Aeration
Biological
Oxidation
Chlorination
Evaporation
Incineration
Type of
Cost
cca
OM b
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
CC
OM
Model Regression Coefficients
ABC
4.74702
0.64345
4.62325
-0.30103
5.52401
0.86923
4.69897
0.24304
4.59106
0.64345
5.45089
0.64345
4.54407
-0.30103
5.07555
0.09934
4.17609
0.24304
6.11227
-0.7112
5.83373
1.57978
0.92844
-0.17671
0.74646
-0.51016
0.61843
-0.11755
0.98569
-0.10083
0.44964
-0.17671
0.55368
-0.17671
0.23408
-0.51016
0.64300
-0.36057
0.66317
-0.10083
1.00000
-0.24314
0.64339
-0.37205
0.22190
0.0
-0.22358
0.06646
0.00842
0.00586
-0.52716
0.0
-0.02748
0.0
0.0
0.0
0.0
0.06646
0.0
0.07879
0.0
0.0
0.0
0.0
0.0
0.0
Cost ($)
at I million gal /day
Annual
Initial Capital Operating
55,849
42,000
334,202
50,000
38,999
282,416
35,000
119,000
14,999
1,295,000
681,914
15,399
1,750
25,899
6,125
15,399
15,399
1,750
4,399
6,125
2,971
132,998
a CC = Capital Cost
"OM = Operating and Maintenance cost
-------
ALTERNATIVE METHODS 15
TABLE 3'
Residue Disposal Cost Ranges
Disposal
Method
Outfall
Wet Oxidation
Barge (to sea)
Pipeline to Land
Truck to Land
Rail to Land
Drying
Compost
Incineration
Cost Range
($1 Ton-Dry Sludge Solids)
3-5
30-50
10-20
5-20
20-50
30-100
30-50
5-10
40-50
Residue Disposal Costs as a Function
of Distance to Disposal Site
(Dollars/Dry Ton Sludge Solids)
Transportation
Method
Pipeline
Tank Truck
Rail Cars
Distance to Disposal Site (miles)
25
28
40
101
100
100
130
170
200
180
220
180
350
280
390
200
and are discussed in greater detail in the following
text.
For example, sludge includes significant concen-
trations of heavy metals as shown in Table 4. Two
of the alternatives for ultimate disposal are by
incineration and disposal in the land.
Incineration
Figure 4 is a self-explanatory description of the
impact of particulate emissions on human respira-
tory functions.
A recent controversey over emissions from in-
cinerators originally proposed for the Blue Plains
Plant serving the Washington, D.C. metropolitan
area exemplifies the public's concern. The con-
frontation is between "scientific" public citizens
who work and live in the neighborhood of the pro-
posed plant and who feel they will be detrimentally
affected by incinerator emissions. Some of the
pertinent allegations and counter comments from
both EPA and the Utility are reproduced from the
Washington Post newspaper feature article on the
subject in Figure 5.
Land Disposal
Disposal to the land is not without its problems.
Because of the mineral exchange capacity of soil
heavy metals tend to accumulate in the upper soil
layers rather than leaching through the soil mantle.
For instance, zinc, copper and chromium are ex-
tremely toxic to corn.
In one study, zinc concentrations of 306 ppm4 in
the sludge, and a duration which simulated a five
year application, the soil concentration of 4969 ppm
in sand and 1000 ppm in loam, respectively, was
noted. Both concentrations stunted corn yield and
growth dramatically.
At a concentration of 81 ppm of copper in sandy
soils, effect was minimal but at concentrations of
162 ppm, extensive growth depression occurred. A
side effect of the lower concentration was to pro-
duce symptoms of iron deficiency. Chromium
similarly stunted growth, but exhibited greater ef-
fects in silt loam than on sandy soils. It decreased
iron content of the soil and also reduced the avail-
able phosphorous content of the soil and uptake by
the plant.
Lead appears to have very little effect in deep
rooted plants but does cause damage to shallow
rooted plants. However, the form of the lead plays
an important role. Nickel, on the other hand,
appears to cause symptoms which are similar to
those related to calcium deficiencies.
Other heavy metals will have similar effects,
more or less. However, besides the metal, the soil
type, pH, organic matter and the variety of plant
grown will affect the degree of toxicity. It becomes
extremely important, therefore, to conduct appro-
priate tests with a particular sludge, soil type and
crop to determine loading rates, management tech-
nique, treatment and countermeasures.
Pathogens are also of concern when one con-
siders land disposal of sludges. Among the com-
mon pathogens found in these waste materials are
the bacterial pathogens Salmonella, Shizella, Myco-
bacerium, and Vibro comma; the hepatitis viruses,
enteroviruses and adenoviruses; and the pro-
tozoan, Endomoeba histolytica. Pathogens may
survive in sludge treated soils for several months
under favorable climatic conditions. Though
pathogen die-back is relatively rapid, fecal coli-
form, an indicator organism, may persist for some
five months. Pathogenic organisms are largely
screened out near the soil surface and do not leach
through the soil profile.
Moreover, the association of pathogen with
aerosols produced by spraying of sludges into land
-------
16 MUNICIPAL SLUDGE MANAGEMENT
r
Enforcement
Actions
X
xX
etc.
—etc.
Figure 3: Holistic Residue Intermedia Transfer Flow Chart.
is also of concern. Though pathogens may not be a
problem for land disposal, the fact that the litera-
ture notes its survival indicates that the land man-
agement practices should take this aspect into
consideration.
Countermeasures may be as simple as limited
public access, use of additional land for buffer
zones, limitation of spraying to low wind veloci-
ties, and monitoring to assuage public concern.
Resource Recovery
An obvious means for minimizing the cost for
sludge management is to produce a product that
crea tes a monetary value. As long as the cost for the
additional handling is less than the monetary value
received for the product, the cost for sludge man-
agement will be decreased. Milwaukee is doing just
that with their sale of Milorganite. Lately, another
such enterprise has been initiated for the Wash-
ington, D.C. market area. Blue Plains sludge will be
sold to a private entrepreneur on the basis of a
nitrogen assay5. The plant now pays $8.25 per wet
ton (20 percent solids) to haul the sludge. The plant
sludge has a nitrogen assay of 3.5 percent. At this
assay, sludge disposal will cost only $8.50 per ton. If
the assay increases to 6 percent, sludge disposal will
cost only four dollars per wet ton. If methane is pro-
duced and sold to the entrepreneur, additional re-
venues will be received. This would reduce the cost
of sludge disposal by $4.25 per wet ton. The com-
pany foresees selling the dried product as a 6-4-10
fertilizer for a price of about five dollars for a 50-
pound bag. This has obvious marketing limitations
as commercial fertilizer provides a much higher
nutrient content for that price. The impact of this
-------
ALTERNATIVE METHODS 17
TABLE 42
Metals In Sludge
1971-1973
Literature Atomic Absorption
Element
Cd
Cu
Hg
.Ni
Pb
Zn
Geometric
Mean
(ppm)
61
906
14.5
223
404
2420
Spread*
5.89
2.66
5.24
4.54
4.13
2.78
Geometric
Mean
(ppm)
93
1840
3.2
733
2400
6380
*Spread is antilog of standard deviation of log-normal distribution.
commercial venture being successful on the sludge
disposal management aspects is likewise obvious.
However, if the goals are met and concept is pro-
fitable for the entrepreneur, sludge disposal costs
may become as low as one dollar per wet ton.
The recycle of sludges has been practiced for
many years6. The problems lie with cost for trans-
portation, institutional problems, health oriented
problems and public attitudes. Transportation
costs for truck, train, and pipe modes of transport
and for various distances are shown in Table 3. In-
stitutional problems are associated with nuisance
statutes that require certain types of transport con-
tainers and routings or preclude certain areas from
being considered. Health problems are associated
with the viability of pathogens. Public attitudes are
such that they can thwart, delay, or also immea-
sureably increase costs for disposal.
Each of these serve to increase the cost for dis-
posal. Yet each and all of these problem areas must
be seriously considered as part of the overall ana-
lysis, lest we be led down a road of pseudo-
economy.
From the economics point of view, transporta-
tion costs vie as the most important, and of these,
extraneous moisture as the most significant.
A decrease in water content can save millions of
dollars annually. Using the figure noted previously
of $8.25 per ton to transport (20 percent solids)
sludge some 20 miles from the Blue Plains plant in
Washington, D.C. to a U.S. Department of Agricul-
ture site, the annual savings can be approximated as
follows:
Daily load = 300 dry tons
Solids content = 20%
Total daily load = 1500 wet tons
Daily cost - $8.25 x 1500 = $12,370
Annual cost = $4,540,000
If, for example, solids content can be increased to
40 percent with heat treatment,
Total daily load - 750 wet tons
Daily cost = $8.25 x 750 = $6,200
Annual cost = $2,260,000
Annual gross savings in transportation costs =
$2,180,000
Added to this is the compatability of the product
with the environmental health aspects, public atti-
tudes and market value. Detracted from it are the
costs and operational problems associated with this
particular process.
Other means for resource recovery are:
1. Pyrolysis to produce an activated charcoal
which can be recycled to the plant for
advanced waste treatment.
2. Heat recovery wherein the sludge is first de-
watered to about 40 to 50 percent solids and
then burned for its heat value. This can be ac-
complished directly or mixed with other waste
products, coal, or Bunker C fuel oil.
RETENTION (.Z)
80 i
1.0 2.0 3.0 4.0
PARTICLE SIZE (MICRONS)
5.0
Figure 4: Retention of Particulate Matter in Lung in Relation to
Particle Size3.
Composting
Unfortunately composting requires relatively
large areas of land for the composting operations
and buffer zone. Most metropolitan sewage treat-
ment plants do not enjoy this luxury. The opposite
is usually true. Accordingly, most composting
operations require shipment of wet sludge to a re-
-------
18 MUNICIPAL SLUDGE MANAGEMENT
They ar^ asking the question be-
cause hy ?i!.irch, I!i75, I'-luc Plains will
be Incinerating all the sludge it pro-
duce*. Sludge is the solid residue of
effluer.t. Present output is 400 tons a
day. When the plnnt is upgraded to ad-
rfinced treatment, that amount will
rise to 2,400 tons daily, or six pounds
for every fn/milv served by the system.
• The plant will remove pollution
from the water and put It into the air:
an estimated 1'i million pounds of
toxic pollutants will he reli-.-"-e(1 Into
the atmosphere (annually)." (The Kl'A,
using different calculations, puts the
amount at Q-lti.UOO pounds.)
• The incinerator . . . will release
the deadly fumes of oxides of nitrogen
and sulfur dioxide which can ha con-
verted to highly corrosive nitric and
sulfuric acids in the stocks and In the
atmosphere, find which can kill in
the several tens of pails pc-r million
concentration and cause debilitating
disease in a few parts per million con-
centration."
• "The incinerator will put hundreds
of thousands of pounds of pr.rtirukite
matter in the air every year: panicu-
late matter containing strongly irri-
tant sulfales and nitrnies, silica and
highly toxic compounds of mercury.
lead, cadmium, arsenic, vanadium,
chromium, copper, beryllium and
many others."
• The Incinerator will . . .
contribute several per cent
of the average annual air
pollution burden of the
neighboring counties, rnd
will increase the local air
pollution burden near the
.plant by as much as 100 per
cent during the summer
months. It will be a grave
health lia/ard."
Based on maximum capac-
ity, Incinerator pollutant lev-
els would Increase about one-
third above EPA estimates,
District sanitation officials
said the EPA figures, based
on the given average usaye,
are sound. They said reven
Incinerators will sometimes
reach maximum operating ca-
pacity, but at other times they
will be operating at much
lower capacities. rt
Figure 5: From The Washington Post. March 29, 1973. Feature
•\rtnle by Thomas Grubisich.
latively distant site, incurring the above men-
tioned transportation costs. However, a vertical
composting operation at the plant site, which
would require little precious land space, could be
even more attractive than the aforementioned
heat treatment.
Export to Foreign Countries
With both the high cost of fertilizer and its
scarcity, the value of this by-product of waste-
water treatment should increase. The concept is
viable for those treatment plants that are available
to large vessels such as ore carriers or tankers. Each
of these dead head to their points of origin. A load
of valuable (albeit) low grade fertilizer would be
welcomed, especially to underdeveloped countries
that have a poor dollar exchange economy. Be-
cause of the size of these shipments, costs per ton
should be nominal.
More
Not previously mentioned are the possibilities of
modifying the basic wastewater treatment pro-
cesses to make the sludge more compatible for
sludge handling, disposal or sale.
Have you given consideration to even going back
into the collection system? We are all aware of the
relationship of plant and operating costs to plant
size. Regional planning reflects this philosophy.
The following conditions may provide enough in-
centive to reconsider this "the-larger-the-better"
approach:
1. Sludge costs are now 30-50 percent of the
total plant operating costs.
2. Future sludge costs will probably constitute a
greater percentage of plant operating costs.
3. Environmental, ecological, institutional and
public concerns also grow with the size of the
daily accumulation of sludge that must be
disposed of.
4. Large tracts of land, such as the 40,000 acres
used by the Metropolitan Sanitary District of
Greater Chicago, may not be as available or
will be even more remote incurring high land
and transportation costs.
5. Single large plants incur substantial costs fbr
interceptor sewers which become larger and
more expensive to supply outlying
communities.
6. Disposing of sludge in small quantities from
plants located in suburbs have traditionally
not incurred either great cost or public
obstacles.
-------
ALTERNATIVE METHODS 19
Still More
Are these solutions long-term or short-term?
Will incinerators, dryers, or heat treatment con-
cepts be able to get fuel? What impact will pro-
jected increases in fuel cost have? Are there
alternatives or contingency programs that could
handle the sludge, if these are the sole handling and
disposal methods?
If marketing is an inherent part of the economic
flow sheet, what will happen to the market if a
signif iciant number of plants provide similar pro-
ducts in your market area? What will happen if re-
fuse composting also adds to the supply? What is
the local, regional, or national market for various
similar products?
Will these techniques of sludge management
require subsequent laws or regulations to limit the
supply or increase the market? For example, should
import of peat moss, mulch and other soil amend-
ments into a region be prohibited until all of their
fair share of sludge from the sewage treatment
plant serving them is consumed within the region
supplying the sewage? Will this, then, establish a
resource, recovery, and marketing utility?
What will be the impact of 1983 water treatment
requirements? Should the 1985 goals of "no dis-
charge" be considered?
How long into the future should we gaze? What
long-range guidelines should be established for
sewerage authorities to plan their operations and
capital investments?
The time is ripe for consulting engineers to con-
sider the total environmental viewpoint, one that marries
technology to the total environment at a mini-
mum cost.
REFERENCES
1. "Intermedia Aspects of Air and Water Pollu-
tion Control," Socio-Economic Environmental Studies
Series, EPA 6QO/S-73-003, August, 1973.
2. Proceedings of the joint Conference on Recycling
Municipal Sludges and Effluents on Land, U.S. Environ-
mental Protection Agency, U.S. Department of
Agriculture and National Association of State Uni-
versities and Land-Grant Colleges, Champaign, Ill-
inois, July 9-13, 1973.
3. Dautrebande, L., Beckman, H., and Walken-
horst, W. "Lung Deposition of Fine Dust Particles/'
AMA Arch. Ind. Hyg., 16, 179, 1957.
4. Cropper, J. B., Welch, L. F. and Hinesly, T. D.
The Effect of Pb, Cu, Cr, An and Hi on Nutrient Uptake and
Growth of Corn.
5. Feaver, Douglas B. "Conversion of Blue Plains
Sludge to Fertilizer," Washington Post, March, 1974.
6. Agricultural Utilization of Sewage Effluent and Sludge,
An Annotated Bibliography, Federal Water Pollution
Control Administration, U.S. Department of the
Interior, 1968.
-------
THICKENING OF SLUDGES
RICHARD I. DICK
University of Delaware
Newark, Delaware
ABSTRACT
The extent to which sludges are thickened has a
significant influence on the overall cost of sludge
treatment and disposal. Yet, rational approaches to
the design and operation of thickeners to ac-
complish an optimal degree of thickening have not
traditionally been implemented. The purposes of
this paper are to review basic thickening concepts
and to illustrate that appreciable cost savings may
be realized by avoiding the use of conventional, ar-
bitrary, design loadings for thickeners. Instead,
thickeners should be designed to achieve a degree
of sludge concentration which, in concert with
other sludge treatment processes, minimizes
overall sludge treatment and disposal costs.
INTRODUCTION
Thickening inevitably is involved in all schemes
for treatment and disposal of sludges. Often,
separate thickeners are used to reduce the volume
of sludge contributed by wastewater treatment
processes prior to subsequent sludge treatment and
disposal. However, even if a separate thickener is
not provided, thickening is still involved in sludge
treat'ment and disposal schemes. This is because
facilities which separate solids from the
wastewater treatment process and divert them to
sludge handling and disposal facilities normally in-
volve, use of sedimentation basins. Such basins
serve to clarify wastewater prior to discharge and,
indeed, frequently bear the name "clarifier." In ad-
dition to accomplishing clarification, ^these
sedimentation basins also are expected to concen-
trate or "thicken" the solids separated from the
wastewater. The concepts of thickening discussed
in this paper relate as much to the thickening func-
tion of sedimentation basins as to thickening oc-
curring in separate sludge thickeners. In either
case, clarification also is going on and must be con-
sidered in the design.
In spite of the frequent use of separate thickeners
in sludge treatment and disposal schemes, as well as
the more common occurrence of thickening within
sedimentation basins, the design and operation of
such facilities has not usually been accomplished on
a rational basis. Thickeners ordinarily have been
designed using arbitrary design standards with a
little consideration being given to the performance
which should be anticipated or to the possible
benefits of constructing a thickener of different
size. Also, in design, the interaction of thickeners
with other treatment and disposal processes has
not been rationally evaluated. Yet, because the per-
formance of thickeners influences the performance
of other processes, some optimal degree of thicken-
ing must be appropriate for each particular sludge
and sludge treatment and disposal scheme. Similar-
ly, those charged with the operation of thickeners
usually have not explored, on a rational basis, the
manner in which their facilities should be operated
to make optimal use of the installed thickener
capacity.
The technology for making rational assessments
in the design and operation of thickeners would
seem to be available. The purpose of this paper is to
review those concepts and to show their utility in
design and operation of wastewater treatment
facilities. To do this, thickening theory will be brief-
ly reviewed, the interactions of thickening with
21
-------
22 MUNICIPAL SLUDGE MANAGEMENT
other sludge treatment and disposal processes will
be discussed, and the economic implications of
these interactions will be illustrated.
The Rational Analysis of
Thickener Performance
Rational bases for design of thickeners and for
analyzing the performance of existing thickeners
have been presented3 and reviewed2,4 elsewhere
and the concepts will only be capsulized here. The
following discussion is oriented to gravity
thickeners, but is applicable to flotation thickeners
by substituting the rate for the settling velocity and
reversing the direction of the movement of tank
content due to sludge removal.
The basic concept in thickener design is to
provide sufficient area so that the solids loading per
unit area per unit time (the applied flux, ordinarily
expressed as Ib/sq ft/day) does not exceed the rate
at which solids can reach the bottom of the gravity
thickener (or top of the flotation thickener). The
rate at which solids can reach the bottom of a
thickener depends on the rate at which they settle
under the influence of gravity and the rate at which
they are transported through the thickener due to
removal of thickened sludge. That is
Gi = OP; + cm (1)
where Gi is the possible flux of solids through a
layer of concentration a; vi is the gravity settling
velocity of the sludge solids at concentration a; and
u is the bulk downward velocity in the thickener
produced by the removal of sludge from the bottom
of the tank. Equation 1 is an expression of the possi-
ble rate of solids transport per unit area for any
concentration in a continuous thickener (one from
which thickened sludge is continuously
withdrawn). Batch thickeners are a special case in
which the cm term in Equation 1 is zero.
It should be noted that the avi term in Equation 1
depends only on the physical properties on the
sludge and is not susceptible to control by the
designer or operator of the thickener unless
physical, biological, or chemical alteration of sludge
solids (as by use of a polyelectrolyte) is practiced. In
contrast, the magnitude of the an term in the equa-
tion depends on the rate at which thickened sludge
is removed from the bottom of the tank, and is
therefore susceptible to control by the thickener
designer and operator.
For optimal performance of a thickener, sludge
removal equipment must be designed to uniformly
collect thickened sludge from the bottom of the
tank so that
u = Qu/A
(2)
where Qw is the volumetric rate of removal of
thickened sludge from a continuous thickener of
area A. Thus, it is seen that the capacity of a
thickener for receiving sludge solids can be in-
creased by increasing the rate of removal of
thickened sludge. While this may be a desirable
course of action for an overloaded thickener, it con-
flicts with the basic goal of thickening—the produc-
tion of a concentrated thickening underflow. This
is because
Q« =
(3)
and it is desired to maximize the underflow con-
centration, Cu. Equation 3 was obtained from a mass
balance on a thickener receiving feed sludge at a
volumetric flow rate, Qf, with a suspended solids
concentration, cf, assuming that the clarified
effluent from the thickener was essentially free of
suspended solids.
If the relationship between settling velocity, P;,
and concentration, a, is known (see Reference 2 for
procedures and difficulties in determining the
settleability of sludges), and if a value of u is
selected, then the value of the batch flux and un-
derflow flux in Equation 1 can be determined for
each possible concentration of sludge which might
exist in a thickener.
Figure 1 illustrates the variation of these two
terms in Equation 1 with suspended solids concen-
tration and shows the resulting total flux, Gi, possi-
ble for each concentration of sludge which might
exist in a thickener. It is seen that, in the higher
range of concentrations which typically exist in
thickeners the value of G; passes through a
minimum. It is this limiting capacity for transmit-
ting solids to the bottom of a thickener, GL, which
limits the capacity of thickeners. Thus, one must
ascertain that solids are not applied at a rate greater
than GL, or
A = cfQf/Gi
(4)
It should be noted that, because u, the underflow
velocity, is controlled by the designer or operator of
a thickener, the value of Gi is controllable. Thus,
for a thickener receiving a given solids load (cfQf),
the value of GL in Equation 4 can be varied to give
any desired thickener area. However, from Equa-
tion 1, it can be seen that if a high value.of GL is
selected, a high value of u, the underflow velocity,
must also be used. From Equations 2 and 3, it is seen
that the use of a high underflow velocity would
result in the removal of dilute sludge from the
thickener. When a new thickener is being designed,
-------
THICKENING 23
TOTAL
FLUX
TRANSPORT
DUE TO
SEDIMENTATION
TRANSPORT
DUE TO
SLUDGE REMOVAL
G:
"I
Figure 1: Determination of Allowable Loading on a Thickener.
area, and thus underflow velocity, are unknown.
Thus, the solution outlined above becomes a
laborious trial and error situation. This difficulty
can be circumvented by use of a graphical solution2.
This simplified procedure is highly recommended
for design and routine analysis of the performance
of existing thickeners.
Interaction of Thickening with
Other Sludge Treatment and
Disposal Processes
To illustrate the influence of gravity thickening
of the economics of sludge treatment and disposal,
the cost of thickening a typical municipal sludge to
various concentrations was compared with the
savings resulting from the improved thickening in
the cost of various sludge treatment techniques. To
illustrate the effect of the size of the waste treat-
ment facility on the economics of thickening,
calculations were conducted for cities of 10,000,
100,000, and 1,000,000 people.
Sludge Quantities
The following equation was developed to es-
timate the quantities of sludge to be treated by the
various sized cities:
Production suspended solids
of = removed in'
Sludge primary clarifier
nonbiodegradable volatile
solids in raw waste which
become incorporated in
activated sludge
nonvolatile suspended
+ solids carried into
activated sludge process
synthesis of
+ activated sludge
solids
any organic precipitates
+ formed during biolog-
ical treatment
-------
24 MUNICIPAL SLUDGE MANAGEMENT
autooxidation of
biological
solids
suspended solids
lost in
effluent
This equation may be written as
5 -
+ ad-
d-W(i-Pss>css
]Q
(5)
The meaning of symbols in Equation 5 is indicated
below along with dimensions.
a = amount of biological synthesis per unit of
BOD removed, M suspended solids/M
BOD (0.5).
b = fraction of mixed liquor volatile suspended
solids which are autooxidized daily,
dimensionless, (0.12).
CBOD concentration of BOD in raw waste, MIL3,
(178 mg/1).
Cp = concentration of inorganic precipitants
formed during biological treatment, MIL3,
(0 mg/1).
cge = concentration of suspended solids in
effluent from treatment plant, MIL3, (15
mg/1).
egg = concentration of suspended solids in raw
waste, M/L3, (205 mg/1).
f = fraction of volatile suspended solids
entering aeration tank which are not
biologically oxidized, dimensionless,
(0.35).
h = fraction of suspended solids entering
aeration tank which are volatile,
dimensionless, (0.75).
L organic loading intensity in activated
sludge process, M BOD removed/M
volatile suspended solids in aeration tank,
(0.4).
mBOD ~ fraction of BOD entering the secondary
process which is removed (based on
filtered effluent sample), dimensionless,
(0.90).
PBOD ~ fraction of BOD removed in primary
settling tank, dimensionless, (0.33).
pss = fraction of suspended solids removed in
primary settling tank, dimensionless,
(0.6).
Q = wastewater flow rate, L3/T, (at!35gpcd).
S daily production of waste sludge solids,
M/T.
Equation 5 is a modification of Eckenfelder's
Equation 11.36 with the addition of terms to ac-
count for primary sludge, any organic solids
precipitated in the biological reactor9, incorpora-
tion of non-volatile solids contained in the raw
waste into activated sludge, and the loss of solids
over the final sedimentation tank weir. Values of
the various constants as assumed for purposes of
this illustration are indicated in parentheses in the
preceding list. All of these values are subject to
variation from waste to waste and none are
necessarily applicable to any particular plant. In the
absence of information on the amount of inorganic
precipitants formed during biological treatment,
this contribution toward sludge production was ig-
nored. A waste flow rate of 135 gpcd, a per capita
suspended solids loading of 0.23 Ib/day, and a per
capita BOD contribution of 0.2 Ib/day were as-
sumed based on data presented by Loehr7. No al-
lowance was made for the probable variation in
quality and quantity of waste as a function of the
size of the municipality.
Based on the assumed values, sludge production
per million gallons of wastewater flow would be
1,425 Ib/day of which 1,020 Ib/day would be
primary sludge, and 405 Ib/day would be waste
secondary solids. The magnitude of this sludge
production is perhaps on the low-side of reported
experience.
Cost of Gravity Thickening of Sludges
To obtain an indication of current probable costs
of thickening and to achieve a basis for illustrating
the interaction of thickeners with other processes
of sludge handling and disposal, estimates were
developed for the cost of thickening sludge to
various degrees in municipalities of various sizes.
This was done by assuming sludge settling proper-
ties (settling velocity as a function of concentra-
tion), determining the allowable loading on a
thickener to concentrate the sludge to varying
degrees, sizing the thickener, and estimating the
cost of construction and operation of the thickener
of the necessary size.
REQUIRED THICKENER SIZE. As described in
an earlier section, the required size of a thickener is
a function of the extent to which it is desired to con-
centrate sludge and of the settling characteristic of
the sludge being thickened. In this illustration, the
settling properties of a combined primary-
secondary sludge were assumed and expressed in
the form of an equation used by Dick and Young5
= a a~
(6)
-------
THICKENING 25
where vi is the settling velocity of sludge at concen-
tration a and a and n are constants characterizing
the properties of the particular sludge being con-
sidered. For purposes of this illustration, a was
taken as 0.045 ft/min, and n as 2.57, when w is ex-
pressed in ft/min and a in percent.
The allowable solids loading (the limiting flux)
for achieving various degrees of concentration of
the sludge were calculated and are shown in Figure
2 along with the resulting required total thickener
area for a city of 100,000. Because no differences in
sludge production between cities of various sizes
was considered, the required thickener areas for
achieving various degrees of sludge concentration
for cities of 1,000,000 and 10,000 are on an order of
magnitude more or less than the values shown in
Figure 2.
THICKENER COSTS. In addition to requiring an
understanding of factors affecting process perfor-
mance, optimal integration of sludge treatment
processes requires information on the cost of treat-
ment by various techniques as a function of the
level of process performance. Unfortunately,
rational selection, design, and operation of sludge
treatment processes is hampered by a dearth of
4000
2468
SLUDGE CONCENTRATION, PERCENT
such cost information. In the case of gravity
thickening, such data are in particularly short
supply. This is, perhaps, because thickening
normally is the cheapest step in sludge treatment
and disposal and, thus, thickening costs often tend
to be lumped into the cost of other sludge
processing techniques. Additionally, sludges vary
widely in their thickening characteristics, and unit
costs for thickening would be expected to vary
accordingly. As with all current cost estimations,
inflation also imposes complications. In Burd's
review1 of the state of the art in sludge handling
and disposal, it was generalized that separate
sludge thickening costs from two to five dollars per
ton of dry solids. Smith13 presented equations for
the cost of construction of gravity thickeners as a
function of area. In neither case was the thicken-
ing cost related to the degree of sludge
concentration achieved. That was accomplished
here by estimating the cost of the thickeners sized
(Figure 2) to give various degrees of sludge concen-
tration.
Capital costs for thickeners of various sizes were
obtained by adjusting cost data presented by
Smith13 to April, 1974 on the basis of the Engineer-
ing News Record Construction Cost Index (the
April, 1974 value being 1940) and then increasing
the cost by 25 percent to account for contractor's
profit, contingencies, and engineering. The
resulting capital cost equation was
C$ -= [54.3 + 26.3 e-A'1340°] A
(7)
Extensive data on the operation and maintenance
of gravity thickeners as a function of theii^ area
were not available. In the absence of such informa-
tion, costs reported by Smith13 on operation and
maintenance costs for primary clarifiers as a func-
tion of their area were used. It was reasoned that
the equipment and operational requirements were
similar to separate thickeners. Arbitrarily, Smith's
operational and maintenance costs were adjusted
by use of the Engineering News Record Construc-
tion Cost Index to make some allowance for
changes in costs of labor and materials since his
work was published. The resulting equation for an
annual operating and maintenance costs as a func-
tion of thickener area was
OM$ = 2.39A + 189A0-5
(8)
Figure 2: Required Size of Thickeners for Concentrating
Hypothetical Sludge to Varying Degrees in City of 100,000
People.
To obtain an overall cost of thickening to various
degrees, annual costs (operation and maintenance
plus amortization of capital costs) were calculated.
Then, as shown on Figure 3, costs of thickening to
various degrees for various sizes of municipalities
-------
26 MUNICIPAL SLUDGE MANAGEMENT
20
15
CO
o
o
co
cc
o
z
o
10
CO
CC
O
Q
CO
O
O
I
I
I
I
I
I
I
I
024 6 8 10
THICKENED SLUDGE CONCENTRATION, PERCENT
Figure 3: Costs of Thickening Hypothetical Sludge to Varying
Degrees.
could be expressed on the basis of total cost per unit
of sludge production. For this purpose, the ap-
proximate current interest rate on Grade A
municipal bonds (6Vi percent) was used with a 20-yr
amortization period.
Thickening and Dewatering of Raw Sludge
The yield of sludge dewatering devices is in-
creased when water is removed from sludge (as by
gravity thickening) prior to being fed to the
dewatering device. This is because less water must
then be passed through the somewhat im-
permeable sludge cake in the course of dewatering
than would be necessary if the excess water was not
removed previously by thickening. Additionally,
the degree to which sludge can be mechanically
dewatered increases when concentrated sludge is
fed to the dewatering equipment8.
To illustrate the optimal integration of thicken-
ing and dewatering processes, the cost of sludge
dewatering by vacuum filtration was considered.
Then, the total cost of the combination of the
thickening and dewatering processes could be
evaluated to determine the proper design for each
of the two processes.
The effect of feed sludge concentration of filter
yield was taken from data presented by Schepman
and Cornell12 which indicated that
Y = 0.88c« 1.0
(9)
where Y is the filter yield in Ib dry solids/hr/sq ft,
and cu is the concentration of sludge in the
thickener underflow. Extrapolation of the Schep-
man and Cornell data was necessary to include the
range of interests here, but the extrapolated data
agreed closely with information on relationship
between feed solids concentration and filter yield
presented in Quirk10.
Capital costs for vacuum filters were taken from
information presented by Smith13. As with the
capital costs for thickeners, Smith's estimates were
adjusted to the April, 1974 Engineering News
Record Construction Cost Index of 1940 and then
25 percent was added for contractors profit, con-
tingencies, and engineering. Capital costs were
amortized at 6.5 percent for 20 yr. Costs for labor,
power, and maintenance were taken from es-
timates prepared by Quirk10 and, arbitrarily, were
adjusted to current costs by use of the Engineering
News Record Construction Cost Index. Chemical
costs for sludge conditioning were taken as $12/ton
of dry solids and were not considered to vary with
the size of the city or the extent to which the sludge
was thickened.
Resulting total costs for thickening and dewater-
ing are shown in Figure 4. The contribution of
thickening and vacuum filtration (including con-
ditioning) to the total cost is illustrated for the city
of 1,000,000. Total costs curves are shown for all
three cities. The relative contribution for thicken-
ing and dewatering to the total cost for cities of 10,-
000 and 100,000 people can be obtained by com-
paring Figures 3 and 4.
It is seen from Figure 4 that the optimal degree to
which the sludge considered here should be
thickened for this city of 10,000 people of about 8
percent. For the two larger cities, a total cost
became relatively insensitive to the degree of
thickening at a concentration of around 8 percent,
but a true optimum was not reached within the
range of concentrations considered. While the
thickening costs involved in reaching these high
concentrations are in excess of the costs normally
considered for thickening, results would suggest
that, with this sludge and these estimates of capital
and operating costs, more money should be spent
for thickening than is normal practice. However,
-------
THICKENING 27
120
iao -
CO
o
o
co
IT
Q
O
I-
O.
<
O
O
CO
O
VACUUM FILTRATION
CITY OF 1,000,000
THICKENING-CITY
OF 1,000,000
0 2 4 6 8 10
THICKENED SLUDGE CONCENTRATION, PERCENT
Figure 4: Optimal Integration of Thickening and Dewatering.
because sludge properties vary from plant to plant,
the more important point is that great savings in
the combined cost of thickening and dewatering is
possible by use of a rational approach to design of
sludge treatment systems.
Overall Costs of Thickening and
Transporting Sludge by Truck
To illustrate the effect of thickening on another
phase of sludge handling, overall costs of thicken-
ing and subsequent trucking were evaluated for
thickeners designed to achieve varying degrees of
sludge concentration. For this purpose, trucking
costs were taken from estimates prepared by
Riddell and Cormack11 for trucking sludge a dis-
tance of 25 miles. Riddell and Cormack's data
(which were developed for sludge at 3.5 percent
concentration) were adjusted to evaluate the cost of
transporting different volumes of sludge con-
taining the same total amount of dry solids. Figures
then were adjusted for inflated labor and materials
costs by use of the Engineering News Record Con-
struction Cost Index.
Total overall costs for thickening and transport-
ing sludge 25 miles by truck for various sized cities
are illustrated in Figure 5. Again, the breakdown of
costs is shown only for the city of 1,000,000 people,
but the relative contributions of trucking and
thickening for the cities of 100,000 and 10,000 peo-
ple can be obtained by comparing Figures 3 and 5.
As before, a true optimum was not achieved within
the range of sludge concentrations considered.
That is, even though sludge thickening became far
more expensive than usual, the incremental cost
was justified by the reduction in the cost of
transporting the sludge.
SUMMARY AND CONCLUSIONS
Thickening is involved in all schemes of sludge
treatment and disposal. If a separate gravity or
flotation thickener is not used, then thickening still
is involved because it occurs in the sedimentation
tanks which produce the sludge. Thickening has a
great influence on the cost of sludge treatment and
disposal because the cost effectiveness of sludge
treatment and disposal techniques depends on the
concentration of solids in the sludge.
Traditionally, thickeners have been sized in an
arbitrary fashion without regard to the thickening
properties of the sludge being treated or to the
degree of thickening desired. Yet the size of a
thickener does effect the amount of thickening
achieved and this effect can be estimated if the
settling characteristics of the sludge are known.
This allows thickeners to be designed and operated
to achieve any desired degree of sludge concentra-
tion. The degree to which sludge should be concen-
trated in a thickener depends on factors such as the
nature of the sludge, the size of the community,
120
TRUCKING -
CITY OF
1,000,000
THICKENING-CITY
OF 1,000,000
Figure
0 2 4 6 8 10
THICKENED SLUDGE CONCENTRATION, PERCENT
5: Optimal Integration of Thickening and Trucking.
-------
28 MUNICIPAL SLUDGE MANAGEMENT
and the types of other sludge treatment and dis-
posal processes involved in the system.
The effect of designing thickeners to accomplish
varying degrees of solids concentration on sludge
treatment and disposal costs was illustrated herein.
Integration of the design of thickeners with the
design of other processes offers significant poten-
tial for reducing costs. While this approach to the
design of sludge treatment and disposal facilities re-
quires appreciably more information about sludge
treatability than normally is available, the results
suggest that the potential cost savings warrant the
cost of conducting the special studies required.
ACKNOWLEDGMENTS
This work was supported in part by funds
provided by the United States Department of the
Interior as authorized under the Water Resources
Research Act of 1964, Public Law 88-379.
REFERENCES
1. Burd, R. S. "A Study of Sludge Handling and
Disposal," Water Pollution Control Research Series,
Federal Water Pollution Control Administration,
Publication WP-20-4, Washington, D.C.,(1968).
2. Dick, R. I. "Thickening," Advances in Water
Quality Improvement-Physical and Chemical Processes, E. F.
Gloyna and W. W. Eckenfelder, Jr., Editors, Univer-
sity of Texas Press, Austin, Texas, 358-369(1970).
3. Dick, R. I. "Role of Activated Sludge Final
Settling Tanks," ]ournal Sanitary Engineering Division
American Society of Civil Engineers, 96, SA 2, 423-436
(1970).
4. Dick, R. I. "Gravity Thickening of Waste
Sludges," Proceedings of the filtration Society, Filtration and
Separation, 9, 2, 177-183 (1972).
5. Dick, R. I. and Young, K. W. "Analysis of
Thickening Performance of Final Settling Tanks,"
Proceedings of the 2 7th Industrial Waste Conference, Purdue
University, Extension Series, (1972).
6. Eckenfelder, W. W., Jr. "Industrial Water
Pollution Control," McGraw-Hill Book Company,
New York, 275 pp, (1966).
7. Loehr, R. C. "Variation of Wastewater Quali-
ty Parameters," Public Works, 99, 5, 81-83, (1968).
8. McCarty, P. L. "Sludge Concentration-
Needs, Accomplishments, and Future Goals," Jour-
nal Water Pollution Control Federation, 38, 4, 493-507,
(1966).
9. Menar, A. B. and Jenkins, D. "Fate of
Phosphorus in Waste Treatment Processes:
Enhanced Removal of Phosphate by Activated
Sludge," Environmental Science and Technology, 4, 1115
(1970).
10. Quirk, T. P "Application of Computerized
Analysis to Comparative Costs of Sludge Dewater-
ing by Vacuum Filter and Centrifuge," Proceedings
23rd Industrial Waste Conference, Purdue University
Engineering Extension Series No. 132, Part 2, 691,
(1969).
11. Riddell, M. D. R. and Cormack, J.W. "Selec-
tion of Disposal Methods for Wastewater Treat-
ment Plants," Proceedings of Conference on Waste Disposal
from Water and Wastewater Treatment Processes, Universi-
ty of Illinois, Urbana, Illinois, 125-130, (1968).
12. Shepman, B. A. and Cornell, C. F. "Fun-
damental Operating Variable in Sewage Sludge
Filtration," Sewage and Industrial Waste, 28, 12, 1443,
(1956).
13. Smith, R. "Preliminary Design and Simula-
tion of Conventional Wastewater Renovation
Systems Using the Digital Computer, Federal Water
Pollution Control Administration, Water Pollution Con-
trol Research Series, WP-20-9, (1968).
-------
ANAEROBIC DIGESTER OPERATION
AT THE METROPOLITAN SANITARY DISTRICTS
OF GREATER CHICAGO
STEPHEN P. GRAEF
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
ABSTRACT
The prlctice of anaerobic sludge digestion by the
Metropolitan Sanitary District of Greater Chicago
is summarized. Topics discussed include capacities
of five digestion facilities, digester construction
costs, operation and maintenance costs, sludge con-
ditioning and digester flow regimes, heat transfer,
loading intensity, gas production, digested sludge
characteristics, sludge disposal, digester problems
and future improvements in digester operation.
INTRODUCTION
The Metropolitan Sanitary District of Greater
Chicago provides wastewater collection and treat-
ment service for Cook County, Illinois. It operates
three major plants, three small plants and is com-
pleting the construction of a mid-sized advanced
wastewater treatment facility. The District's
1973 average daily flow was 1,420 MGD which
yielded over 800 tons of solids per day to be pro-
cessed and disposed.
Each of the three major treatment works, de-
signed to treat 250, 350 and 1,200 MGD respec-
tively, provide conventional activated sludge treat-
ment. While the 250 and 1,200 MGD plants utilize
high rate anaerobic digestion, sludge from the 350
MGD plant is pipelined to the 1,200 MGD plant for
processing. Anaerobic digestion, however, is not
the only type of solids processing facility at the
1,200 MGD plant. Digestion is complemented by
two additional solids processing units namely, the
heat drying and the Imhoff digestion-drying
processes.
Two of the three small plants (6 MGD, 3 MGD
and 1 MGD) include high rate digester. However,
substantial amounts of undeveloped acreage in the
service areas of these plants result in reduced
digester loading rates. High rate digestion is also
the mode of operation designated for the 30 MGD
advanced wastewater treatment plant now under
construction.
Digestion Capacity in the District
A spectrum of capacities are included among the
digestion facilities operated by the District. These
statistics are summarized in Table 1. Although the
District's digestion facilities are rated according to
tons of dry solids processed per day, TPD, the ac-
companying plant size is listed in column one for
perspective. As noted above the 1,200 MGD plant
utilizes other means for processing solids besides
digestion. Column 3, which lists the mass ratio of
waste activated to primary sludge, was included to
define the feed sludge blend.
Three plants have identical digesters. Each 2.5
million gallon unit has a 110 feet diameter, 33 feet
wall water depth plus a 1:6 sloped conical floor.
Digester volumes are proportionally smaller at the
6 MGD and 1 MGD plants. All existing digesters
are equipped with floating covers, however, fixed
cover units have been considered in future expan-
sion planning. Moreover, the District's digesters
are operated as single stage processes except for the
0.3 ton/day facility. The detention time listed is the
average for 1973, however, the design average de-
tention time at the end of the design period is 14
days for each facility.
29
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30 MUNICIPAL SLUDGE MANAGEMENT
TABLE 1
Digester and Plant Sizes
Plant
(MOD)
1,200
250
**30
6
1
TPD
(Ton 1 Day)
*425
120
70
4
0.3
Acf.Pri
Ratio
75:25
65:35
85:15
65:35
55:45
Volume
(MG)
2.5
2.5
2.5
0.18
0.09
No.
Units
12
8
4
4
2
Detention
Time
(Days)
14
20
18
30
60
*Additional solids to other processes.
**Under construction.
Capital Costs
Capital costs for six District digester construc-
tion projects are tabulated in Table 2. The actual
contract bid price was updated to December 1973
levels using the ENR Construction Cost Index of
1939.0. Cost comparisons in terms of present dollar
value can be made. Note that the cost for Con-
struction Stage III at the 425 TPD facility includes
the cost of four digesters plus four thickeners.
These flotation thickeners are physically identical
to those built six years earlier in Construction
Stage II. As a result the cost comparison between
Stage III, and Stages I plus II is within three and
one-half percent. Furthermore digesters built in
Stages I and II at the 120 TPD facility and those
built in Stages I and II at the 425 TPD facility are
also physically identical. The adjusted costs con-
firm the similarities. Finally, the comparative cost
increase in Stage IV can be explained by the addi-
tional digester appurtenances included in the con-
tract. Additional sludge recirculation pumps,
sludge transfer pumps, piping interconnections,
remote controlled valve operators and instru-
mentation were provided in the Construction
Stage IV contract. Moreover an estimated five per-
cent of the cost was directed toward updating the 8
digesters built in 1962 or 1968. The time lag in con-
struction can be placed in perspective by noting that
the Stage IV contract was bid in November 1971
but the digesters were not placed on line until Jan-
uary 1974. Construction costs for the 70, 4 and 0.3
TPD facilities were not available. These digesters
were built when the plants were initially con-
structed. Since the costs for the various processes
in the plant were not individually itemized in the
bid price, accurate estimates of digester costs could
not be made.
Operating and Maintenance Manpower
Several personnel classifications are involved
in operating the District's digesters. Table 3 lists
the types of manpower and the number of man-
hours per week directly chargeable to digester
operation. Technical supervision, indirect operat-
ing manpower involved in sludge conditioning and
disposal processes and digester maintenance man-
power are not included. However, technical and
plant supervisory costs allocated to digestion have
been estimated at 21 percent of the direct operat-
ing salary costs. The maintenance manhours
average approximately 25 percent of the operating
manhours. The data in Table 3 demonstrate the
TABLE 2
Digester Construction Costs
TPD
(Ton/ Day)
425
120
Construction
Stage
I
11
11!
IV
1
11
Description
4 Digesters
4 Thickeners
4 Digesters & Thickeners
4 Digesters
4 Digesters
4 Digesters
Year
Bid
1962
1962
1968
1971
1962
1967
Cost*
Mi/lions of $
3.10
2.36
5.27
4.14
2.49
2.93
"Based on December 1973 ENR Construction Cost Index = 1939.0.
-------
TABLE 3
Operating and Maintenance Manpower
TPD
(Ton 1 Day)
425
120
*70
4
0.3
Man Hours Per Week
Operating
Engineers
200
200
0
0
TPO
0
40
66
7
Laborer
480
200
4
0
Fireman-
Oiler
40
80
—
0
0
Supervisory Approx. 21 percent of Direct Operating Dollars.
Maintenance Approx. 25 percent of Operating Man Hours.
Base Labor Wage $5.95.
* U nder Construction.
"economies of scale" that accrue with increasing
capacities of the District's digestion facilities.
Conditioning and Flow Regime
As a general rule it is inefficient to discharge
waste activated sludge and primary sludges direct-
ly into a digester. These sludges, especially the
waste activated, are too dilute and should be
concentrated. Table 4 lists the types of sludge
conditioning practiced by the District. Current
practice ranges from gravity thickening at the 0.3
TPD facility to a combination of screening, gravity
thickening, vacuum filtration, cake dilution and
sludge grinding at the 425 TPD digestion complex.
Flotation thickening of activated sludge, aided by
additional chemical flocculants is also practiced.
ANAEROBIC DIGESTER OPERATION 31
At the present time none of the District's
digesters are operated as continuous flow reac-
tors. Although it is recognized that continuous
flow is desirable to minimize shock loading and
smooth the rate of gas production, hardware
limitations to date have prevented the District from
implementing this regime. Continuous feed may be
implemented at the large digestion facilities but
smaller plants, because of limited availability of
operator time, will continue to be fed and drawn on
an intermittent schedule. Reliable meters, level
sensors, valve status indicators, pump speed con-
trollers, timers and remote control equipment can
be used to automate continuous flow operation,
however, quality equipment and a planned main-
tenance program are essential.
Mixing
Mixing is practiced at all District digestion facili-
ties. Designed into each digestion project is the
capability for continuous pumped recirculation of
the digester volume at least once in 24 hours. De-
sign practice also calls for pump and valving
flexibility to permit either recirculating the digester
contents through external heat exchangers or
around them. Besides pumped sludge recirculation
the District has also provided gas recirculation on
most digesters. This form of mixing has not
achieved its full potential since the particulate and
aerosol laden digester gas fouls the compressors
employed to recirulate the gas. Hydrogen sulfide
TABLE 4
Conditioning and Flow Regime
TPD Conditioning
(Ton 1 Day)
425 Screening, Gravity
Thickening, Blending
Feed
Schedule
Once Per Shift
Mixing
Pumped
Recirculation
With Vacuum Filter Cake,
Grinding
120 Gravity Thickening And
Screening
*70 Flotation Thickening
4 Flotation Thickening of
Activated, Gravity
Thickening of Primary
0.3 Gravity Thickening
Semi-Continuously
Computer Controlled
On Day Shift
Feed On Timed Cycle -
Draw 3-5 Times/Week
Pumped
Recirculation
Gas & Pumped
Recirculation
Gas & Pumped
Recirculation
Feed 3 Times/Week Gas & Pumped
Draw 2 3 Times/Week Recirculation
* Under Construction.
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32 MUNICIPAL SLUDGE MANAGEMENT
TABLE 5
Heat Transfer
TPD
Tons 1 Day)
425
120
**70
4
0.3
Heal
Exchanger
External With
Hot Water
External With
Hot Water
Jacketed Draft
Tubes
External With
Hot Water
External With
Hot Water
Area
(Ft*)
9,200
4,000
112
50
Temp. Range %
Raw Sludge Solids
(°F)
60 - 70 4.8
52 72 2.9
4.0
55 72 3.9
53 73 3.5
*BTU/Ton
Dry Solids
(in Millions)
1.04
1.72
1.20
1.23
1.37
*Based On Raising Raw Sludge Temperature From 70 to 95° F.
'Under Construction.
corrosion on the other hand has not been a problem
with District digesters.
Heat Transfer
Four of the five digestion facilities listed in Table
5 utilize external, hot water heat exchangers. This
equipment is readily accessible for service and has
performed satisfactorily for about ten years at
several facilities. The exchanger tubes become
coated with residues from sludge and must be re-
juvenated on a two to five year basis. Table 5 lists
the total transfer surface area at each facility rather
than the manufactures BTU transfer rate since the
transfer rate substantially declines within a few
months after startup. Ninety-five degrees
Fahrenheit is the nominal set point for digester
sludge with a weekly difference between minimum
and maximum temperatures being less than five
degrees. The range of monthly average raw sludge
temperatures for 1973 are also presented.
Column 6 lists the theoretical BTU require-
ments for heating the sludge, having the percent
solids concentration defined in column 2, from 70
to 95° F. In order to illustrate the value of thicken-
ing raw sludge prior to digestion, the BTU require-
ments are expressed on a per dry ton of solids basis.
Note that the 2.9 percent solids sludge at the 120
TPD facility theoretically requires approximately
70 percent more BTU's than the 4.8 percent solids
sludge at the 425 TPD facility. A different type of
heat transfer equipment is being provided at the 70
TPD facility now under construction. Digester
sludge will be heated by six jacketed draft tubes in-
serted through the digester cover into the sludge
mass. A single external boiler serving the four
digester complex will circulate hot water through
the jacket of each draft tube. Performance data on
this type of equipment will not be available until the
plant is placed in service.
Loading Intensity
Besides a minimum design residence time of ap-
proximately ten days another limiting design
parameter for digesters is organic loading rate.
Organic loading rate (OLR) is defined as the pounds
of volatile solids fed for each cubic foot of digester
volume per day. A well operated high rate digester
is normally fed at a rate between 0.1 to 0.3 pounds
of volatile solids per cubic foot per day. Table 6 lists
the range of 1973 monthly average organic loading
rates (OLR) for the various digestion facilities. Al-
though the 425 TPD facility had an OLR typical of
high rates, the District does not intentionally
operate its digesters at the lower loading rates,
OLR. In fact several of its high rate digesters have
been operated at OLR approaching 0.3 with effec-
tive hydraulic residence times on the order of ten
days. Although these experimental operating con-
ditions were maintained for less than five times
the hydraulic time constant or residence time, the
digester performed well and produced an effluent
sludge quality similar to that yielded at an OLR of
0.15 and hydraulic residence time of 14 days.
-------
ANAEROBIC DIGESTER OPERATION
33
TABLE 6
Digester Loading Intensity
TABLE 7
Digester Gas Production
TPD %
(Tons 1 Day) Solids
425
120
*70
4
0.3
4.6 -
2.3
4.0
2.4
2.5
5.2
4.2
4.5
4.0
%
Volatile
61
51
63
55
- 70
- 63
- 73
65
OLR
Lb. Vol. Solid
Ffl
0.13
0.045
0.052
0.036
Day
- 0.17
0.057
- 0.061
- 0.043
Detention
Time
(Day)
14
20
18
30
60
* Under Construction.
It appears feasible to operate the digesters at the
higher loading rates, however, there are several
reasons why the District has not adopted this prac-
tice. Organic loading rate is a function of solids con-
centration, volatile solids content and hydraulic
residence time. Since sludges, which are pre-
dominantly composed of waste activated sludge,
are difficult to concentrate above four percent
solids without employing mechanical dewatering
equipment, concentration is therefore a con-
straint. Furthermore, District sludges are charac-
teristically lower in volatile content than other
municipal digester installations. Finally, hydraulic
residence time, the easiest variable to manipulate in
order to increase or decrease the OLR, has been
fixed by District policy at a minimum of 14 days.
The District's exploratory studies as well as
digester operating records reported in the litera-
ture indicate that satisfactory performance can be
attained at lower hydraulic residence times. The
District however maintains the 14 day minimum as
additional assurance to the regulatory agencies as
well as the community organizations near the Dis-
trict's Land Reclamation site that the sludge is
completely digested.
Gas Production
Rate of digester gas production and gas com-
position are important process variables in digester
operation. These data monitored at the District's
facilities are summarized in Table 7. Column two,
which lists the cubic feet of digester gas produced
per pound of volatile solids destroyed, indicates
that the unit production is typical of other munici-
pal digester installations. Columns three and four
list the total gas production in thousands of stan-
dard cubic per day (TSCFD) at each facility and the
percentage utilized for heating the raw sludge feed,
maintaining digester temperature and heating the
digestion complex buildings. Note that when the
total gas production is hundreds of thousands up to
Gas Production
TPD Ft> % CH4l
(TonsI Day) Lb. Destroyed (TSCFD) Utilized CO2
425
120
*70
4
0.3
14
13
14
N,
17
19
17
.A.
2500
500
12
3800
700
15
N.A.
70
80
85
N
85
- 95
- 95
.A.
1.50 1
N.A.
N.A.
N.A.
.75
*Under Construction.
N.A. Not Analyzed.
several million standard cubic feet per day, a 5 to 30
percent excess is an enormous source of energy.
The District is currently investigating various
means for capturing this energy for other uses.
One potential scheme is steam production for elec-
trical power generation.
Besides maintaining an inventory of the gas pro-
duced, the gas composition is monitored at the 425
TPD facility as an indicator of digester stability and
performance. The volumetric ratio between
methane and carbon dioxide, measured by a gas
partitioner, is determined daily on a sample of gas
from each digester. Detailed gas analyses have indi-
cated that rfcS, Hz, N2 and other gases are negligi-
ble compared to methane and carbon dioxide.
Therefore, a CH4/CO2 ratio of 1.50 to 1.75 indi-
cates a methane composition ranging between 60
and 64 percent by volume. As a result typical fuel
values of the digester of f-gas range from 600 to 650
BTU/ft3 digester gas.
Digested Sludge Characteristics
The quality of digester performance can be gaged
by analyzing the final product. Digested sludge
characteristics reported at the various District
facilities are summarized in Table 8. Columns three
and four indicate the reduction in solids and volatile
matter respectively. The District, however, does
not consider the percentage reduction in volatile
solids to be a significant performance variable. This
variable is dependent primarily on the volatile
content of the feed sludge rather than digester ef-
ficiency. The total volatile acids concentration,
TV A, listed in column 5 is more indicative of the
degree of "complete" digestion since it is the
principal intermediate product in the series
anaerobic digestion reaction.
An important principle is supported by the TVA
data in Table 8. Continuous culture theory indi-
cates that a steady state, the effluent substrate con-
centration from a completely mixed reactor is
-------
34 MUNICIPAL SLUDGE MANAGEMENT
TABLE 8
Digested Sludge Characteristics
TPD
(Ton 1 Day)
425
120
*70
4
0.3
OLR
Lb. - Vol. Sols.
Ffl
.13
.045
.052
.037
Day
.17
.057
.061
.043
Solids
3.5
2.1
2.3
N
- 4.2
3.6
3.4
.A.
Volatile
55 59
45 55
51 59
N.A.
TVA
(mg/L)
50
74
120
N.
150
228
260
.A.
ALK
(mg/L)
2900
1700
2100
N,
4100
2800
3000
.A.
pH
7.0
6.8
7.1
N
- 7.3
7.1
7.6
.A.
*Under Construction.
nearly independent of the influent substrate con-
centration. The data indicate that the District's
digestion facilities support this theory. In spite of
the range of influent concentrations represented
by the OLR, the final effluent concentrations
represented by the TVA values are nearly iden-
tical. This indicates that digested sludge quality is
not adversely affected by operating at the higher
organic loading rates.
Another principal of anaerobic digestion is
illustrated in Table 8. Digested sludge alkalinity is
more or less proportional to the feed sludge solids
concentration. Alkalinity increases due to protein
degradation with its subsequent release of am-
monia. Ammonia reacts with carbonic acid to
produce ammonium bicarbonate. Since the protein
concentration is relatively proportional to the
solids concentration the resulting alkalinity follows
the same proportionality. Note that the alkalinities
in Table 8 demonstrate this relationship with the
feed solids concentrations listed in column 2 of
Table 6.
Methods of Disposal
Sludge disposal is the final step in a sludge man-
agement program. The District, in keeping with the
concept of recycle and reuse, has adopted a pro-
gram of land application as its principal means for
sludge disposal. Table 9 summarizes the disposal
route for each digestion facility.
The District has been involved in refining the
technology of land application of sludge solids for
several decades. Extensive experimental and pilot
studies led to the development of the Prairie Plan
which was recognized as the Outstanding Civil
Engineering Achievement of 1974 by the American
Society of Civil Engineers. The Prairie Plan is the
District's program of reclaiming strip mined land
through application of anaerobically digested
sludge. It is the 425 TPD digestion facility which
serves as the primary source of digested sludge for
TABLE 9
Methods of Disposal
TPD
(Ton/ Day)
Description
425 Barge To Land Reclamation Site, Application, Cropping.
120 Concentration In Basins, Land Application, Cropping
*70 Mechanical Dewatering, Land Application
4 Concentration In Basins, Land Irrigation, Cropping
0.3 Discharge To Interceptor To 120 TPD Plant
* Under Construction.
the Prairie Plan. Solids from the other facilities are
recycled on other land application sites in or near
the Chicago area.
Problems
Anaerobic sludge digestion, although well
suited as a solids processing technique for the Dis-
trict, has encountered problems. Several of these
difficulties, accompanied by the steps which the
District has taken to overcome them, are sum-
marized below.
Thickening
Because of the large proportion of waste acti-
vated sludge in the digester feed, the maximum
solids concentrations attainable have been 2.5 to
3.0 percent by gravity thickening and 4.0 to 4.5 per-
cent by flotation thickening using flocculating
agents. These appear to be firm upper limits. At the
425 TPD facility high feed sludge solids concentra-
tions are attained by vacuum filtering the waste ac-
tivated sludge to a 15 percent solids concentration
and then diluting the cake to the desired concen-
tration. Theoretically any solids concentration
from 3 to 15 percent can be obtained. However,
above six percent solids the sludge viscosity exceeds
100,000 centipoise and its rheological properties
-------
ANAEROBIC DIGESTER OPERATION 35
tend to mimic those of a paste. As a result the target
operating range is 5 to 5.5 percent solids.
Clogging
From the outset of digestion experience at the
District, clogging of recirculation pumps and reduc-
tion of the effective digester volume by rags and
tenacious mats of string, hair and fibers was a
problem. Current design practice calls for the
screening of all digester feed sludges to remove
much of this troublesome material. Moreover,
sludge grinders are employed at the 425 TPD
facility to further insure the homogeneity of the
sludge feed. Screening and grinding appear to
reduce digester maintenance costs in addition to
providing a more uniform feed.
Foaming
Accumulation of foam in the gas space under the
floating cover as well as around and over the edge
of the floating cover has been a problem. Part of the
difficulty is inherent due to the presence of surface
active agents among the products of biological de-
composition. The problem can be accentuated by
undesirable digester feeding practices. If the
digester is fed only once or twice a day, the micro-
bial population metabolizes vast amounts of sub-
strate over a short period of time thus producing
vigorous gasification. Vigorous gasification in turn
results in rapid foam formation. The overall foam-
ing problem can be reduced if the gasification rate is
maintained relatively constant by uniform con-
tinuous feeding several times per shift. Pumped re-
circulation of sludge to the gas space under the
floating cover can also be employed to mechani-
cally destabilize the foam.
Gas Recirculation
Most District digesters employ gas recirculation
for digester mixing. In most instances the moist
foam and grease laden "dirty" digester gases tend to
foul the recirculation compressors. Fortunately
hydrogen sulfide has not been a problem. In order
to increase the mean time between compressor fail-
ure the District has been considering redesigning
and relocating the condensate traps as well as the
possibility of gas scrubbing upstream from the gas
compressors. Both of these areas appear promising.
Supernatant Separation
One other problem, which is appropriate to men-
tion, has been the slow rate at which supernatant
separates from digested sludge. All of the digesters
are operated as completely mixed reactors. As a
result the well mixed digested sludge, which is
supersaturated with carbon dioxide, separates
slowly. The buoyant action of the carbon dioxide
tends to keep the solids in suspension. Gravity con-
centration of digested sludge from approximately
four to six percent solids requires about one month-
This is one of several reasons why the District has
not designed "two stage" digestion facilities. The
total volume of the second stage reactor would be
more than twice the volume of the primary
digesters. Although the problem has not been re-
solved, vacuum degasification of the digested
sludge drawoff has shown some potential.
Other minor difficulties have been encountered
from time to time. These for the most part have
been resolved and the net advantages of anaerobic
digestion have made digestion the primary means
for sludge processing at the District.
Potential Improvements for the Future
Bench and pilot scale research on sludge diges-
tion has advanced significantly beyond the status of
current operating practice. This has created a
potential for improvement in field operations.
Several improvements which may soon be imple-
mented, not only in Chicago but throughout the
country, are the following:
I. Shorter residence times as a result of more
homogeneous feed sludges, improved flow
regimes and more frequent sludge analysis.
2. Uniform digester feeding and withdrawal ac-
complished by automatic mass balance
control.
3. On-line measurement of process stability
indicators such as the rate of methane
production.
4. Digester gas scrubbing in order to (a) produce
a cleaner fuel of higher BTU value; (b) reduce
gas recirculation compressor fouling; (c) pro-
vide pH control by removing the weak acid,
carbonic acid, rather than adding a base.
This list is by no means complete and plant opera-
tors and process engineers will, without a doubt,
implement other improvements. As a result,
anaerobic digestion of municipal sludges, with its
current advantages, coupled with the potential for
significant refinements, should continue to be one
of the principal means for sludge processing.
-------
METRO DENVER'S EXPERIENCE WITH LARGE
SCALE AEROBIC DIGESTION OF WASTE
ACTIVATED SLUDGE
DAVID B. COHEN
Metropolitan Denver Sewage Disposal District No. 1
Denver, Colorado
ABSTRACT
Metro Denver in 1970 converted excess second-
ary aerators to aerobic digesters. The plant scale
system was compared with a pilot open tank oxy-
gen system using both slot and rotating diffusers.
V.S.S. reductions ranged between 11.2 and 47.2
percent for the air system. A significant correla-
tion between V.S.S. reduction and S.R.T. x tem-
perature was observed. Cold shock eliminated ni-
trification for a five month period. When inverte-
brates (particularly rotifers) comprised significant
fraction of the biomass, digestion was maximal.
Supernatant concentration averaged ten percent
of anaerobic supernatants. Biodegradable V.S.S.
auto-oxidation coefficient k = 0.27 for oxygen batch
test. No correlation was observed between D.O.
concentration and V.S.S. digestion rates. Temper-
ature differential increased with increasing load-
ings between ambient and biomass. At loadings >
0.14 pounds V.S.S./ft3/day, oxygen performance is
superior. To continue the economic benefits of re-
duced sludge disposal costs, Metro is considering
conversion of a one million gallon tank to an oxy-
gen aerobic digester with rotary diffusers.
INTRODUCTION
Aerobic digestion may be regarded as
modification of the activated sludge process. Figure
1 depicts the relationship of the aerobic digestion
process to other activated sludge modifications
from "high rate" to "extended aeration" on a time-
concentration continuum (Sa-t values in units of
hours x mg/1). As the biomass concentration in-
creases, settling velocities decrease, food to micro-
organism ratios become infinitesimal and net
sludge synthesis becomes negative because of auto-
oxidation and predation.
The major problem associated with the activated
sludge process involves the disposal of excess waste
activated sludge. There is, therefore, an obvious
economic incentive to aerobically digest as concen-
trated a sludge as possible. The limiting factor in ac-
complishing a high rate of solids reduction in a
thickened waste activated sludge is oxygen transfer
capability. New developments in the field of pure
oxygen fine bubble diffusion technology have
successfully overcome this limitation.
In 1970 the Metropolitan Denver Sewage Dis-
posal District No. 1 (Metro) embarked upon a two-
pronged aerobic digestion program. The first part
of the program involved plant scale aerobic diges-
tion of dilute W.A.S. in four converted secondary
aeration basins (eight million gallongs). The second
part consisted of an extensive research and de-
velopment program to compare diffused air per-
formance with pure oxygen pilot plant aerobic
digestion on a batch feed and continuous feed basis.
Preliminary research data indicated that high
oxygen transfer efficiencies ( > 90 percent) could be
achieved by applying a unique fine bubble oxygen
diffuser in an open tank system (Marox Systems,
F.M.C. Corporation, Englewood, Colorado).
On the basis of the large scale plant experience
with aerobic digestion, as well as the open tank
oxygenation research efforts, a contract was
awarded to Metro in June 1972 by the Environ-
mental Protection Agency for the investigation of
diffused air and pure oxygen aerobic digestion of
waste activated sludge. The diffused air plant scale
phase began on August 1,1972 and was completed
37
-------
38 MUNICIPAL SLUDGE MANAGEMENT
Figure 1: Activated Sludge — Time-Concentration Continuum.
-------
LARGE SCALE AEROBIC DIGESTION 39
on August 31, 1973. The pure oxygen digestion
phase began on November 1, 1972 and continued
until April 1974.
Oxygen Pilot Plant Phasing
The first stage of the oxygen investigation in-
cluded a series of five batch tests for evaluation of
diffuser hardware and system performance. The
batch tests were run using both fresh as well as dif-
fused air digested concentrated waste activated
sludge (three to five percent solids).
The second stage consisted of a continuous feed
system in two 1,800 gallon tanks using a slot-type
diffuser for determining the effect of varying load-
ing rates on system performance. During the five
phases of this stage, the volatile suspended solids
loadings ranged from 0.083 to 0.433 pounds
V.S.S./ft-'/day.
The third stage replicated the second stage but
substituted a rotating diffuser system for the slot
diffusers. The loading rates were 0.43 and 0.60
pounds V.S.S./ft3/day.
Metro Diffused Air System
The plant scale diffused air system was based
entirely on existing aeration equipment consisting
of fine bubble precision tube diffusers. In this sys-
tem "fine bubble" is defined as an average bubble
diameter of approximately 2 to 5 mm. The only in-
novation in the plant scale system consisted in the
shutting off of diffused air in the third and last pass
of the aeration basin once a day for several hours to
allow for solids/liquid separation.
Pure Oxygen Diffusion Systems
The pure oxygen slot diffuser requires recircu-
lation of liquid sludge past gas diffusion bars, to
provide minute oxygen bubbles (average bubble
diameter is 50 to 100 microns). A bubble of 100 mi-
crons diameter would require a water depth of four
feet to obtain 100 percent dissolution before reach-
ing the air/water interface.
Upon completion of the EPA contract, a new type
of gas transfer device developed by F.M.C.-Marox
Systems became available. This device called the
rotating diffuser employs the same shear principle
for small bubble development that is used in the slot
type diffuser. Whereas with the slot type diffuser
the shear is obtained by recirculation of fluid
through a narrow orifice past the gas bars, shear is
obtained with the rotating diffuser by revolving a
diffuser through the liquid at a speed equivalent to
the flow velocity required with the slot diffuser
( ~ 20 ft/second). The major advantage of the rotat-
ing diffuser is that this device does not require pre-
screening (Figure 2).
Figure 2: Slot Diffuser.
Scope of the Study
Variables investigated in relation to aerobic sta-
bilization performance included: time of stabiliza-
tion, volatile solids loading, temperature, oxygen
supplied per unit volatile solids reduced, dissolved
oxygen concentration, oxygen uptake rates, sludge
settleability, supernatant quality, odor levels, de-
waterability and physical/chemical and microbio-
logical characteristics.
-------
40 MUNICIPAL SLUDGE MANAGEMENT
Experimental Data —
Diffused Air System
Table 1 compares the percent change between in-
fluent and effluent for various physical/chemical
characteristics with variations in sludge retention
time.
Considerable confusion exists in the literature
with regard to the solids form that is aerobically di-
gested or reduced. Some authors base their calcu-
lations on total solids, others use total volatile sol-
ids and still others use volatile suspended solids.
This study is based on volatile suspended solids
(V.S.S.) reduction as the criteria for determining
degree of aerobic stabilization. Collodial materials
have not been dealt with separately in this analysis
and are assumed to be part of the dissolved solids
passing through the Gooch filter. It should be noted
that although V.S.S. is the criteria used for deter-
mining the diffused air system performance, bio-
degradable V.S.S. is also considered when discuss-
ing the pure oxygen batch tests.
coefficients of correlation were calculated for sev-
eral environmental-operational functions within
the asymptotic limits previously observed, the most
significant correlation (r = + 0.93) was observed be-
tween V.S.S. reduction and S.R.T. x temperature.
A definition of solids reduction under aerobic
conditions must take into account both solubiliza-
tion of particulates as well as carbon loss in a gase-
ous form. Changes in kinetic equilibrium between
particulate biomass undergoing enzymatic solubil-
ization, and soluble substrate being resynthesized
back to particulate biomass may also account for
differences in biomass reduction calculations.
As T.S.S. conversion increases, the rate of solu-
bilization also increases. During October 1972,
when performance was at a maximum, increase in
the effluent T.D.S. accounted for approximately 30
percent of the T.S.S. converted. In March 1973,
however, when performance was minimal, T.D.S.
increase accounted for only five percent of the
T.S.S. converted.
TABLE 1
Diffused Air System
Laboratory Data
S.R.T. (Days) Vs. Percent Change (Influent-HLffluent)
s. /?. r.
(Days)
3.0 (a)
4.1 (b)
6.3 (c)
8.6 (d)
12.2 (e)
18.3(0
29.8 (g)
Total Suspended Solids
Solids
-11.0
-15.9
-26.0
-24.5
-32.5
-31.0
-28.7
TSS
-14.0
-19.1
34.9
31.5
-46.8
-43.2
-36.0
VSS
-15.8
-22.3
^0.5
-40.6
-50.9
-48.7
-39.3
Total Dissolved
Solids
+ 15.4
+ 17.3
+47.9
+40.2
+98.5
+97.9
+38.9
C.O.D.
-19.0
-27.0
-42.0
^2.2
-50.8
-45.0
-41.2
Nitrogen - N
NO3xlO
+0.11
+0.19
+22.8
+230
+200
+445
+ 135
13 NH4
+20.5
+61.7
+97.1
+44.7
+ 141
+ 121.4
+77.3
T.K.N.
-12.2
-10.4
-31.4
-33.3
-32.9
-44.8
-35.0
Conductivity
fimho
+ 13.4
+ 19.0
+31.9
+44.3
+73.5
+72.3
+46.0
Units
0
+0.07
-0.1
0
-0.45
-0.40
-0.20
A Ik.
CaC03
+0.7
-8.5
-27.8
-13.5
-63.4
-60.0
-36.5
Data Averaged From:
(a) January, February, August 1973
(b) December 1972, March, July 1973
(c) August, September 1972
(d) June 1973
(e) October, November 1972
(0 May 1973
(g) April 1973
Volatile Solids Reduction
Figure 3 indicates the volatile solids reduction
achieved within the spectrum of temperature and
loading conditions encountered during the full
scale plant study. V.S.S. reductions ranged from
11.2 to 47.2 percent. Attempts to relate this per-
formance to a single variable were unsuccessful.
Beyond a certain limiting factor for sludge deten-
tion time, V.S.S. reduction was asymptotic, ap-
proaching but rarely exceeding 50 percent. When
The volatile fraction or V.S.S./T.S.S. ratio of the
aerobically stabilized sludge must be reduced to 60
percent or less in order to avoid potentially obnox-
ious odors, particularly if the stabilized sludge is to
be spread on land. This reduction has been im-
possible to achieve for sludge retention times of up
to 30 days in this study. The volatile fraction of the
aerobically stabilized sludge can be further re-
duced to the requiste 60 percent by either chemical
oxidation (ozonation) or anaerobic digestion of the
aerobic digester effluent.
-------
LARGE SCALE AEROBIC DIGESTION 41
% v.s.s. reduced vs. 9 operational modes(TEMP a. vssO-OADINGS)
J
">i
a
•o
\
i* .
H-
^
%
.&s LOADING f#v
0.18
0.16
0.14
0.12
0.10
O.OB
0.06
0.04
0.02
1
[Low 'TEMP. \ '(MEDIUM 'TEMP. \
I HIGH LOADING]] I HIGH LOADING)
/I6.8% V.S.SA
* reduced '
= 2.8 tons/day
LOW TEMP ]
[MEDIUM LOADING!
/ 17.4 % v.s.s.\
\ reduced '
- 2.8 tons /day
f LOW TEMP ]
. [LOW LOADING]
/ 41.5 % v.s.s. \
* reduced '
= 1.0 tons /day
5 16 17 18 1
.• ^
f MEDIUM TEMP }
IMEIMUM LOADING]
/ 42.7 % «s.s.\
^ reduced '
- 3.6 tons /day
[MEDIUM TEMP.]
lLOW LOADING]
/ 46.2 % v.5S.\
^ reduced '
- 2.3 tons /day
9 20 21 22 23 24 2
— •_
r HIGH' TEMP. ]
[HIGH LOADING J
/20% v.s.s.\
\. reduced /
- 3.1 tons/day
f HIGH TEMP ]
[MEDIUM LOADING J
/ 43.4 % vss. \
\ reduced /
= 4.0 tons/day
f HIGH TEMP. ]
Low LOADING] .
/47.2 % v.ss.\ ~
1 reduced '
= 2.5 ton/day
5 26 27 28 2<
•* — >•
TEMP. (°c)
Figure 3: Metro Diffused Air System.
Influence of S.R.T. Before Aerodigestion
on Volatile Solids Reduction
R. Loehr in his paper Aerobic Digestion — Factors Af-
fecting Design has written, "Different rates of sludge
oxidation and oxygen utilization are due to differ-
ent starting points . . . (but) few authors report the
sludge ages of solids entering the aerobic digester.
The percent volatile solids reduction of a sludge
with a high sludge age will be less than that of a
sludge with a low sludge age. For waste sludges
with a high sludgexage, much of the sludge oxida-
tion has .taken place in the activated sludge (sec-
ondary) system."
The results obtained in the diffused air system at
Metro Denver do not support Professor Loehr's
contention that sludge age prior to aerobic diges-
tion is a major influence on aerobic digestion per-
formance.
-------
42 MUNICIPAL SLUDGE MANAGEMENT
For equivalent S.R.T. conditions (e.g. Septem-
ber — 12.8 days and December 1972 — 12.6 days)
the degree of volatile solids reduction should be al-
most identical. However a 10°C drop in tempera-
ture between September (28.7°C) and December
(18.4°C) resulted in a three fold drop in digestion
rates from 47 percent V.S.S. reduced in September
to only 16.6 percent in December. Solids reduction
is apparently much more sensitive to environ-
mental conditions during digestion (e.g. tempera-
ture) than to sludge prehistory.
Nitrogen Forms
Denitrification occurred when periods of maxi-
mum nitrification coincided with periods of insuf-
ficient oxygen (due to the shutting off of air in "C"
pass for dewatering purposes). Because of the de-
nitrification "split-float" the decanting operation
was discontinued in September 1972.
The impact of sudden cold temperature onset on
nitrification rates is illustrated in Table 2 which
compares various parameters two weeks "before"
and two weeks "after" the first winter snows (No-
vember 15, 1972 S.R.T. constant).
The cumulative decline in liquid temperature of
~1°C per day over a four day period (11/18 thru
11/21/72) resulted in a "cold shock" to the sensitive
nitrifying bacteria. Adaptation of the nitrifying bio-
mass to the colder temperatures is illustrated by
the fact that nitrification was re-established in
April 1973 even though temperatures reached the
minimum monthly average of 15.9°C. During April
the S.R.T. of 29.8 days was sufficient to maintain
high nitrification rates despite the cold tempera-
ture. In July 1973 when biomass temperatures rose
to 28°C and the S.R.T. dropped to 4.3 days nitrifi-
cation inhibition was again observed.
No significant correlation was observed be-
tween temperature standardized oxygen uptake
rate (kzo) and nitrification rates. The highest
TABLE 2
Effect of "Thermal Cold Shock"
on Nitrification Parameters
Effluent
Parameter
Nitrate - N
Ammonia N
•Mkalinitv
PH
Unils
mg; 1
mg I
mg 1
Influent
0.05
35
490
7.0
"Before"
174
145
110
6.0
"After"
0.08
29
364
7.0
degree of nitrification occurred when kzo was at a
minimum.
The most significant correlation (r = +0.96) be-
tween nitrification rates and environmental condi-
tions was found for S.R.T. (days) x temperature
(°C). Nitrate levels in excess of 100 mg/1 N were
observed when the temperatures weighted sludge
retention time factor exceeded 200.
Conductivity
The correlation between total dissolved solids
and electrical conductivity is as expected very high.
Thus, a simple method for estimating degree of sta-
bilization achieved is measuring the change in elec-
trical conductivity between the influent and
effluent.
Microfaunal (Invertebrate) Analysis
Numerical counts were converted to a volu-
metric standard unit using previously determined
dimensions for each particular organism observed.
On a numerical basis, the smaller motile flagellates
and ciliates comprised the great majority of organ-
isms. When the numerical counts were converted
to volumetric standard units, rotifers were found
to comprise the great bulk of the invertebrate
biomass.
Changes in invertebrate populations appear to be
related to environmental stresses, particularly rate
of temperature decline and organic loadings.
August November 1972, when temperatures
were above 22°C and volumetric loadings were less
than 0.1 pounds V.S.S./ft3/day, invertebrate diver-
sity was at a maximum with the rotifer population
assuming nearly half of the total dry weight bio-
mass. During December 1972 when loadings in-
creased to 0.187 pounds V.S.S./ft3/day and tem-
perature declined to 16°C, rotifers disappeared and
ecological diversity declined.
The best performance (as expressed by volatile
suspended solids reductions) was observed to oc-
cur during those months when invertebrate or-
ganisms comprised a significant fraction of the vol-
atile suspended solids under aeration. The rotifer
population appeared to have the most significant
correlation with V.S.S. reduction. The coefficient
of correlation between the rotifer population and
the percent volatile suspended solids reduced was
very high (r = +0.87).
Comparison of Aerobic and
Anaerobic Stabilization
When activated sludge is anaerobically digested
for 30 days, the V.S.S./T.S.S. ratio is usually re-
duced - 20 percent (e.g. from 80 to 60 percent). The
-------
LARGE SCALE AEROBIC DIGESTION 43
amount of carbon lost from the anaerobic digester
as methane and carbon dioxide is greater than the
carbon dioxide lost from a continuous feed aerobic
system where V.S.S./T.S.S. ratios decline by less
than ten percent in 30 days. The concept "stabiliza-
tion" must therefore be operationally defined in re-
lation to odor potential for digested sludges dis-
posed of on land.
When the "partially stabilized" aerobic digester
effluent having a volatile solids residue of about 76
percent is loaded to an anaerobic digester (S.R.T. =
18 days) the V.S.S./T.S.S. ratio is further reduced
to 62 percent. This "double digested" material can
be spread on land without fear of subsequent odor
problems.
Odor Panel Results
The most important indicator of sludge stability
from an aesthetic viewpoint is odor. In order to de-
fine this problem quantitatively, a seven person
panel was formed to periodically monitor the odor
potential of three 50 gallon samples'spread over 10
square foot plots.
Odor results for three sludge mixtures (anae-
robically digested primary sludge, aerobically di-
gested sludge and a mixture (1:1 ratio) of both
sludges) were evaluated during a 28 day period. The
aerobically digested sludge (volatile solids fraction =
75 percent) had the least offensive odor. When this
sludge was mixed in a 1:1 ratio with anaerobically
digested sludge (volatile fraction = 60 percent) a
definitely objectionable odor occurred. The so
called "nonbiodegradable" fraction of the aerobi-
cally digested volatile suspended solids can be
further biodegraded by an adapted anaerobic bac-
terial culture. Thus offensive odors are a definite
possibility if aerobic sludge residues having
V.S.S./T.S.S. ratios greater than 60 percent are al-
lowed to go septic.
Supernatant Quality
Aerobically digested supernatants are much less
concentrated than anaerobic digestion superna-
tants. The C.O.D., T.K.N. and NH4 concentra-
tions in the aerobic digester supernatant averaged
only 13 percent of the concentrations in superna-
tant from an anaerobic digester pilot plant digest-
ing W.A.S. The aerobic supernatant B.O.D. and
T.S.S. concentrations were only ten percent (100
mg/1) of the average anaerobic concentrations.
Dewaterability
Vacuum filter leaf tests indicated that for an
equivalent chemical cost, better vacuum filter per-
formance is obtained with the aerobically digested
sludge, as compared with fresh waste activated
sludge.
The sand filtration rates of the aerobically di-
gested and undigested sludges are approximately
equal. Aerobically digested sludge drained three
times as fast as anaerobically digested sludge on a
volumetric basis. On a solids weighted basis, how-
ever, the anaerobically digested sludge drained 2.5
times faster than the aerobically digested sludge,
and the fresh W.A.S. drained 20 percent faster than
the digested sludge.
Sludge concentration by dissolved air flotation is
affected by the particle surface area available for ca-
tionic polymer conditioning. An inverse relation-
ship between polymer demand and sludge loading
rates was observed. If an optimal loading (0.10
pounds V.S.S./ft3/day) to the digester is main-
tained, significant savings in polymer costs for
sludge thickening can be realized. If very long
S.R.T.'s are maintained, the polymer costs may rise
to double that required for the undigested waste ac-
tivated sludge.
Pure Oxygen Pilot Plant —
Batch Tests
The five batch tests using pure oxygen indicated
that the rate of biodegradable volatile solids reduc-
tion levels off after approximately 15 days. Readily
biodegradable V.S.S. was arbitrarily defined as that
fraction of the total V.S.S. reduced by the end of
each test run. Because of alternate periods of auto
digestion and re-synthesis, the final sample did
not always have the lowest V.S.S. concentration.
Table 3 summarizes the biomass reduction data
for all five batch tests. Of the five tests run, Batch
Test No. 3 was unique in that a previously diffused
air digested material was used as the starting
sludge. Test runs, 1, 2, 4 and 5 were loaded with
fresh waste activated sludge.
The V.S.S./T.S.S. ratio was significantly higher
for the fresh sludge (84.5 percent) as compared
with the air digested waste activated sludge (79.9
percent). The final sample V.S.S./T.S.S. ratio for
test runs 1, 2, 4 and 5 averaged 79.6 percent while
the V.S.S./T.S.S. ratio for run 3 actually increased
slightly to 82.6 percent. All runs experienced alter-
native periods of accelerated endogenous decay fol-
lowed by periods of re-synthesis. The average of
the four runs using fresh W.A.S. showed a net re-
duction of five percent in the V.S.S./T.S.S. ratio.
The volatile suspended solids reduced (based on
the difference between the initial and final sample)
averaged 45.7 percent for test runs 1, 2, 4 and 5,
-------
44 MUNICIPAL SLUDGE MANAGEMENT
TABLE 3
Pure O2 - Batch Tests
Mass Reduction Data Summary
Sample Test
W.A.S.
Undigested
W.A.S.
Undigested
W.A.S.
Undigested
W.A.S.
Undigested
Average
W.A.S.
Air Digested
No.
1
2
4
5
3
Detention
Time
Days
21
15
21
14
21
VSSI TSS Ratio
Initial
0.865
0.826
0.859
0.829
0.845
0.799
Final
0.844
0.750
0.844
0.744
0.796
0.826
A %
-2.1
-7.6
-1.5
-8.5
-5.0
±2.7
% V.S.S.
Reduced*
53.7
54.7
30.5
43.7
45.7
35.3
Biodegradable
Aerobic Digestion Rate Coefficient
"vss
0.143
0.175
0.204
0.273
0.200
0.182
^cod
0.174
0.190
0.182
*Based on initial versus final V.S.S. concentration.
compared with 35.3 percent for test run 3. The
most efficient utilization of pure oxygen is ob-
tained by using a non-digested waste activated
sludge with a high initial V.S.S./T.S.S. ratio.
The only study published to date on aerobic di-
gestion where pure oxygen was used on a large
scale is EPA Report No. 17050DNW02/72. The
aerobic digestion experiments in this study at
Batavia, New York were performed entirely on an
already oxygenated waste activated sludge having
low V.S.S./T.S.S. ratios (67.6 to 74.1 percent). Fig-
ure 4 compares results for runs 6 and 8 at Batavia
with Batch Test No. 5 of this study. The aerobic di-
gestion rate coefficient for readily biodegradable
V.S.S. in the Batavia study was k = 0.12 compared
to k = 0.27 in this study. Figure 4 shows that 20 per-
cent of the readily biodegradable V.S.S. remains in
the Metro batch system at sludge retention time of
six days, compared with 48 percent biodegradables
remaining in the Batavia system.
The instantaneous oxygen uptake rate was
found to be proportional to readily biodegradable
volatile suspended solids above a concentration of
2,000 mg/1. Endogenous respiration must be pro-
portional to the active mass rather than the total
volatile suspended solids. Below 2,000 mg/1,
changing metabolic states make this level of activ-
ity non-linear. In the Metro study, the equation for
oxygen uptake rate for solids concentrated above
2,000 mg/1 was found to be:
O.U.R. = 0.0127 V.S.S.
(Biodegradable) + 39.7
(with a correlation coefficient r = +0.90).
The poor linear relationship that was found to
exist in the Batavia study between O.U.R. and
V.S.S. became a very significant relationship if the
readily biodegradable V.S.S. above 2,000 mg/1 is
substituted for total V.S.S.
The results of the pure oxygen batch test por-
tion of this study yield the following conclusions:
1. A stabilized sludge (40 50 percent volatile
suspended solids reduction) was obtained af-
ter one to three weeks of detention time.
These values are significantly higher than the
20 to 30 percent reduction values reported in
the Batavia report.
2. No correlation was observed during any of the
tests between dissolved oxygen concentra-
tion and V.S.S. digestion rates. The highest
dissolved oxygen rates occurred when the
oxygen uptake rate and the V.S.S. digestion
rate were at a minimum. It appears that above
the minimal concentration required to sus-
tain aerobic metabolism (l.O mg/l) D.O. con-
centrations are a result rather than a cause of
aerobic digestion reaction rates.
3. A high degree of oxygen utilization (~ 92 per-
cent) was demonstrated in the pure oxygen
open tank system (Batch Test No. 5).
4. Erratic variations in V.S.S./T.S.S. ratios dur-
ing the various batch tests may be explained
by the cyclical periodicity of alternate auto di-
gestion followed by re-synthesis of biomass
using previously solubilized nutrients (cryptic
growth).
5. High concentrations of suspended solids re-
sults in a stressful "crowding" situation that is
-------
LARGE SCALE AEROBIC DIGESTION 45
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LEGEND
UNOX RUN No. 8
JNOX RUN No. 6
MAROX RUN No
(1970)*
(1970)*
5
REF: ERA. REPORT NO. ITOSO DNW
02/72
UNOX VSS/TSS RATIO AVERAGED:
RUN No. 6 = 74. 1 %
RUN No. 8 67. 6 %
MAROX V.S.S/TS.S. RATIO AVERAGED
82.9 %
12
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\
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A
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24 6 8 IU 12 14 16 16 20 22 24 26 2
DETENTION TIME (DAYS)
Figure 4: Pure 02 Batch Test Comparison of UNOX and MAROX Data—Reduction of Readily Biodegradable V.S.S. vs.
Detention time.
-------
46 MUNICIPAL SLUDGE MANAGEMENT
in-imitable to successful growth and repro-
duction of invertebrate organisms in the batch
test digester. The addition of cationic high
molecular weight polymers to the sludge dur-
ing dissolved air flotation may also adversely
affect the ecological diversity of this system.
At the end of Batch Test No. 1, only one of the
common invertebrate group (micro flagel-
lates) could be observed in a viable condition,
while no moving invertebrate organisms
could be observed at the end of Batch Test No.
2. In the pure oxygen batch tests, mass decay
and endogenous respiration are accomplished
almost entirely by bacteria rather than higher
invertebrate organisms.
6. The relative differences in performance be-
tween the aerobic digestion oxygen studies at
Batavia and this study may be a result of dif-
ferences in mixing energy, initial volatile sol-
ids concentration, different methods of trans-
ferring oxygen from the gaseous to the liquid
phase and carbon dioxide/pH differences
between the initial and final samples. A
master's thesis by Thomas J. Weston of
Michigan Technological University con-
cluded that the oxygen unit achieved lower
solids reduction and exhibited lower oxygen
uptake rates than the air unit. The author
mentioned that in his closed tank system oxy-
gen digested sludges had significantly lower
pH value (4.9) and concluded that pH toxicity
may have affected the biological activity of the
system. The average volatile solids reduction
in Weston's pure oxygen digester was only
23.7 percent compared with 41.3 percent in
the air digester for an equivalent detention
time.
The unique nature of the Marox fine bubble dif-
fuser allows for purging of carbon dioxide and vol-
atile organic acids from the open tank system. Thus
the final pH is more alkaline than in the Batavia
study. The continuous recycling of the liquid sludge
through the gas/liquid dif fuser to create minute gas
bubbles aides in purging COz from the system, and
also involves a high degree of mixing energy which
can be related to the high rate of volatile solids re-
duction achieved.
Pure Oxygen Continuous Feed Pilot Plant
— Slot Diffuser
The major objective of this part of the test pro-
gram was to determine the breakdown loading rate
to the system. The first three phases were de-
signed to duplicate the loading range that had been
applied to the diffused air system (0.083 to 0.196
pounds V.S.S./ft3day). The last two phases of this
test investigated the performance at much higher
loading rates (0.326 to 0.433 pounds
V.S.S./ft.'/day).
At the lowest loading rate (0.083 pounds
V.S.S./f t3/day) there was virtually no difference be-
tween air and biomass temperatures. At the high-
est loading rates (0.433) the temperature differen-
tial increased to 20°C (i.e., ambient temperature
averaged 8.4°C while the biomass in Tank B aver-
aged 28.6°C).
Table 4 summarizes the aerobic digestion per-
formance for the five phases of the flow through
pilot plant using the slot diffuser. Aerobic diges-
tion performance (measured as percent V.S.S. re-
duced) declined only slightly from 47.1 percent at
the lowest loading rate to 38.8 percent at the
highest loading rate, and averaging 42.7 percent for
the entire period. The maximum loading rate be-
yond which performance breaks down was there-
fore not determined.
Pure Oxygen Continuous Feed Pilot Plant
— Rotating Diffuser
The major objectives of these tests were to dem-
onstrate the capability of the rotary diffuser to
aerobically digest thickened W.A.S. without pre-
screening with a high degree of 02 transfer effi-
ciency ( > 80 percent), and a high rate of V.S.S. re-
duction ( > 30 percent).
The first objective was demonstrated satisfac-
torily during a sixty day test period. No indication
of diffuser plugging appeared, despite the removal
of prescreening. All of the other objectives were
also realized and the overall performance exceeded
expectation. Table 5 summarizes the aerobic di-
gestion performance for the two test runs using the
rotating diffuser.
Test Run No. 1 was loaded at 0.60 pounds
V.S.S./ft3/day while the loading for Test Run No. 2
was reduced to 0.43 pounds V.S.S./ft3/day.
The pounds of oxygen supplied per pounds
V.S.S. reduced, averaged 1.54 for the two runs with
the second test averaging 1.36. This performance
can be compared with phase 5 of the flow through
slot diffuser test which required 2.68 pounds Oz
supplied per pound V.S.S. reduced. Thus, the re-
sults of the second rotating diffuser test repre-
sents a reduction of 50 percent as compared with
the slot diffuser test at identical loadings. The
S.R.T. and hydraulic detention time averaged 5.3
days and 4.1 days respectively, indicating the feasi-
bility of high volumetric loadings and low space re-
quirements for large scale operations.
-------
LARGE SCALE AEROBIC DIGESTION 47
TABLE 4
Pure O2 - MAROX Flow Through Pilot Plant
Solids Data Summary (V.S.S.)
Loading
Phase ft VSSjFpjDay
I Mean
Min.
Max,
II Mean
Min.
Max.
Ill Mean
Min.
Max.
IV Mean
Min.
Max.
V Mean
Min.
Max.
Total Mean
159 Days
Phase I -6/11-
Phase 11 7/14
Phase III 10/20
0.083
0
0.135
0.139
0
0.188
0.196
0.092
0.293
0.326
0.230
0.421
0.433
0.301
0.579
0.235
7/13/73
-8/22/73
- 11/19/73
Feed
tt/Day
38.0
0
81.9
64.3
0
92.4
90.3
42.3
134.9
150.1
127.0
193.8
199.2
152.3
266.2
108.4
Phase
Phase
Waste
tnventory Retention
tt/Day kLbs/Day (Lbs.) S.R.T.
19.0
0
171.5
35.8
0
76.8
52.9
31.2
98.0
88.3'
59.4
138.4
120.9
90.9
186.2
63.4
IV 11/20-
V 12/14-
+ 1.1 855 63.3
+ 1.1 884 25.3
-3.8 806 16.6
+2.4 866 10.2
+ 1.0 921 7.9
+0.36 866 13.7-
12/13/73
1/13/74
Time (Days) # 02/tt VSS Digested Aerob. Digest.
Hydraulic Respired
"22.3 N.A.
18.8
28.2
15.7 N.A.
11.0
23.3
10.7 1.94
6.4
21.9
6.7 1.70
5.8
12.8
5.4 1.49
5.0
6.4
9.1 1.70
Supplied tt/Day
6.93 17.9
3.76 27.4'
—
2.96 41.2
2.37 59.4
2.68 77.3
3.10 44.6
*including spills
%
47.1
42.6
45.6
39.6
38.8
42.7
TABLE 5
Pure 02 MAROX Flow Thourgh Pilot Plant - Rotating
Diffuser Solids Data Summary (V.S.S.)
1.
2.
Test Run
Mean
Min.
Max.
Mean
Min.
Max.
Loading
Lbs V.S.S. 1 Ft* 1 Day
0.60
0.47
0.93
0.43
0.34
0.51
Feed
Lbs 1 Day
137
108
213
99
79
118
Waste
Lbs 1 Day
92
64
130
66
59
82
Inventory
Lbs 1 Day
-3.0
+0.7
Lbs
424
386
Retention
S. R. T.
4.7
3.1
6.2
5.8
5.2
6.6
Time (Days)
Hydraulic
3.7
2.3
4.7
4.5
4.1
5.4
Aerob.
Lbs 1 Day
48.4
31.5
.Dig.
%
35.3
32.0
Total Mean
57
Days
0.515
118
79
-1.6
405
5.3
4.1
40.0
33.7
Test Run No. 1 -3/10-4/10/74
Test Run No. 2-4/11 -5/5/74
The data obtained with the rotary diffuser rep-
resents a significant breakthrough in the technol-
ogy of pure oxygen aerobic digestion, by obtaining
a high degree of mass reduction at economic oxy-
gen utilization levels in thickened waste activated
sludge ( ~ 5 percent T.S.) without screening prob-
lems. This technology represents an attractive al-
ternative to other processes for W.A.S. handling
and disposal.
Comparison of Diffused Air and Oxygen
Performance
A major difference between the air and oxygen
systems relates to the temperature ranges experi-
enced with each system. Whereas air system tem-
peratures were subject to sudden changes, the pure
oxygen pilot plant system was conducted indoors,
and was therefore not subject to cold temperature
shock. Significant increases in biomass tempera-
-------
48 MUNICIPAL SLUDGE MANAGEMENT
ture occurred with the oxygen systems particular-
ly during the initial batch tests.
The sudden decline in oxygen uptake rates with
increase of temperatures above 40°C indicates that
the mesophilic biomass may have been replaced by
thermophilic bacterial culture. The intermediate
temperature range of 40 to 50°C is an inefficient
range for accomplishing rapid volatile suspended
solids reduction. During the flow through pure
oxygen pilot plant testing using the slot diffuser,
the biomass temperatures increased in direct rela-
tion to the loading rates, with the maximum in-
crease of 20°C being experienced at the highest
loading rate (0.043 pounds V.S.S./ft3/day). The
ability to maintain high loading rates in thickened
waste activated sludge suggests the possibility of
thermophilic aerobic digestion with accelerated
rates of V.S.S. reduction, if the oxygen system
were to be enclosed and insulated to conserve the
heat generated. It would, however, be necessary to
ensure that the thermophilic cultures were con-
sistently maintained above the minimum tempera-
tures required for optimal growth and develop-
ment, in order to avoid cycling between mesophilic
and thermophilic conditions with subsequent un-
predictability of biological performance.
A further difference between the air and oxygen
systems relates to the influence of sludge reten-
tion time prior to aerobic digestion on the ultimate
rate of V.S.S. reduction. While the sludge reten-
tion time of the activated sludge system prior to
loading the aerobic digester appeared to have little
influence on the rate of volatile suspended solids re-
duced, the opposite was the case during the batch
tests with pure oxygen.
It appears that the major factor determining di-
gestion rates is not S.R.T. per se, but rather the ini-
tial V.S.S./T.S.S. ratio. Very small differences in
this ratio were observed in the activated sludge
loaded to the diffused air aerobic digester. A signif-
icant reduction in the V.S.S./T.S.S. ratio was noted
however in the air digested sludge loaded to the
pure oxygen pilot plant.
Endogenous respiration ensures that the efflu-
ent from an aerobic digester will have a lower vola-
tile solids ratio than the initial sample. There does
not appear to be any advantage in two stages of
aerobic digestion, that is diffused air digestion fol-
lowed by pure oxygen digestion. If a pure oxygen
system is available, the best use of plant resources
would indicate that activated sludge be loaded di-
rectly to the oxygen system, without an inter-
mediary diffused air step.
A quantative measure of sludge stability is the
specific oxygen uptake rates (kr). Whereas, in the
oxygen batch tests kr of less than 1.0 was achieved
in ten days, the flow through air and oxygen sys-
tems kr ranged from four to six. The high oxygen
uptake rates are attributable to metabolic resyn-
thesis oxygen requirements using lysed metabo-
lites. No significant correlation between kr and ni-
trification rates was observed with either the air or
the oxygen system. Similarly, no relation was ob-
served between the dissolved oxygen concentra-
tion and the rate of aerobic digestion up to 16.6
mg/1. Contrary to some statements in the litera-
ture regarding the influence of dissolved oxygen
concentrations on digestion levels, it appears that
dissolved oxygen concentration is an effect rather
than a cause of changing oxygen uptake rates.
When oxygen uptake rates decline, dissolved oxy-
gen concentrations tend to increase, if the oxygen
supply is constant.
The solids specific oxygen uptake rate (kr) aver-
aged 7.1 for the air system compared with 5.0 in "A"
tank and 4.3 in "B" tank of the pure oxygen sys-
tem. On the basis of the differential between ini-
tial and final V.S.S./T.S.S. ratios achieved in both
the air and the oxygen systems, an empirical stand-
ard for a stabilized sludge of kr < 5 appears
reasonable.
The percent reduction in solids forms achieved
for both the air and oxygen systems versus loading
rates and detention time is depicted in Figure 5. At
loadings greater than 0.15 pounds V.S.S./ft3/day,
the percent V.S.S. reduction is greater with the
oxygen system than the air system.
Conversely, at loading rates below 0.10 pounds
V.S.S./ft3/day the reduction of all solids forms is
greater with the air system than the oxygen
system.
The degree of solubilization achieved is greater at
all loading rates for the oxygen system than for the
air system. The oxygen system is better able than
the air system to maintain a high degree of volatile
suspended solids reductions at very high loading
rates that cannot be maintained in the air system
because of oxygen transfer capability limitations.
Conversion rates of total kjeldahl nitrogen to ni-
trates, ammonia and nitrogen gas were signifi-
cantly different in the oxygen and air systems. For
the air system (during those periods when tem-
perature conditions were favorable for nitrifica-
tion) most of the T.K.N. conversion was exhibited
as an increase in NOa. Approximately half of the
T.K.N. was converted to nitrate, while one-quarter
of the kjeldahl nitrogen conversion was expressed
as an increase in ammonia concentration. The re-
mainder of the T.K.N. conversion was due to deni-
trification during periods of low dissolved oxygen
-------
LARGE SCALE AEROBIC DIGESTION 49
100-
90-
80-
70-
60-
50-
40 J
30 i
ki
to
5
+ 10-
0 •
-10-
20-
30-
40
50
60
70
80
90
100
100-
80
70
60
50
40
30
+10-
K.
^
30-
40
50
60
70
80-
90-
100 •
Ar
.S.(^IR)-H
4«.
M II
T
\
&
q p
.TD.S. (A
T.S.S.
V.S.S.Oil
R)
y-I.U.b. lUz't
UR)
R)
T.D.S. (OzUxlO)
T.S. (Os)
^
V.S.S. (02)
, ( Ibs. V.S.S.,
FIG. 57 A
S, (/SIR)
o — 10 o>
-------
50 MUNICIPAL SLUDGE MANAGEMENT
concentration. Whereas a very good correlation
was observed between nitrification and S.R.T. x
temperature for the diffused air system, no such
correlation was observed for the oxygen system.
High nitrification rates were observed in several of
the oxygen batch tests, but minimal nitrification
occurred in the flow through tests. It is assumed
that the conditions of crowding at high loading
rates as well as polymer conditioning and high mix-
ing energy do not provide a suitable environment
for rapid growth and reproduction of nitrifying
bacteria.
Nitrate concentrations in the air system in-
creased by a factor of 10s between influent and ef-
fluent at a S.R.T. of 8.6 days. The highest nitrate
levels observed in any of the oxygen phases was less
than 5 mg/1 NOa-N.
Ammonium ion concentrations were signifi-
cantly higher in the oxygen system than the air sys-
tem at all loading rates. At organic loadings of 0.187
pounds V.S.S./ft3/day, ammonia increased in the air
supply by only twelve percent versus 150 percent in
the oxygen system. It is apparent that the oxygen
system converts more kjeldahl nitrogen to
ammonia, whereas the air system (at equivalent
loadings) oxygenates more of the ammonia to
nitrates.
Figure 6 compares V.S.S. reduction rates with
loading rates for both the oxygen and air systems.
At loading rates below 0.08 pounds V.S.S./ft3/day
the air system results are equal or better than the
oxygen system. Up to loading rates of 0.14 pounds
V.S.S./ft3/day the performance of the oxygen air
systems was roughly equal. Above 0.14 pounds
V.S.S./ft3/day the performance of the air system
declines rapidly, whereas the oxygen system con-
tinues to perform well up to loading rates as high as
0.6 pounds V.S.S./ft3/day.
Figure 7 shows the oxygen utilization efficiency
(expressed as pounds 02 supplied per pound V.S.S.
reduced) versus loading rates. Three different
ranges of oxygen efficiency related to three differ-
ent oxygen transfer methods are illustrated. The
best diffused air performance requires 15 pounds of
02 supplied per pound V.S.S. reduced. The best per-
formance for the pure oxygen slot diffuser system
requires 2.3, and the best pure oxygen rotating dif-
fuser system performance requires 1.36. Whereas
with the diffused air system, the optimal loading
rate for high oxygen transfer efficiency was 0.08,
the optimal loading range for the pure oxygen sys-
tem was the highest loading possible. With the dif-
fused air system, the change in oxygen transfer ef-
ficiency varied with temperature and liquid depth,
ranging between 5.2 to 19.3 percent. The major fac-
tor influencing oxygen transfer efficiency with the
oxygen system was the degree of plugging experi-
enced. With the slot diffuser oxygen efficiencies as
high as 93 percent were experienced in one of the
batch tests. This level could not be consistently
maintained for several months at a time during
flow through testing. With the substitution of the
rotating diffuser for the slot diffuser, oxygen
transfer efficiencies in excess of 90 percent were
consistently achieved for several months of con-
tinuous operation. In order to ensure high percent
transfer efficiencies with an open tank pure oxy-
gen system, it is necessary that dissolved oxygen
never exceeds the saturation concentration at the
air/liquid interface.
Comparison of dewaterability of aerobically di-
gested sludges from the diffused air and oxygen
systems indicate no significant differences in spe-
dific resistance (r = 109 sec2/gr) between influent
and effluent samples for sludge retention times be-
tween three and thirteen days. Differences in fil-
ter leaf performance tests were however observed.
For a dilute sludge without polymer condition-
ing, aerobic digestion improves vacuum filter per-
formance by reducing the volatile fraction. After
digestion of polymer thickened sludge, however,
there is an adverse effect on vacuum filter perfor-
mance related to the decrease in solids concentra-
tion. The effect of detention time on flotation
polymer demand for the diffused air system is di-
rectly related to particle size and T.S.S./V.S.S. ra-
tio. The higher the S.R.T. of the sludge in the air
system, the higher the ultimate polymer demand.
The invertebrate population as a part of the total
volatile suspended solids biomass was reduced by
80 percent between the diffused air aerobic di-
gester effluent and "A" tank of the oxygen system,
and was further reduced by 77 percent between "A"
and "B" tank. Temperature changes and loading
stresses influenced population dynamics in the dif-
fused air system, while it is assumed that the con-
ditions of crowding and the high rate recycling mix-
ing energy in the pure oxygen system created an
environment which was inimicable to growth and
reproduction of higher invertebrate forms.
FUTURE PLANS
Since the initial conversion of 8 M.G. secondary
aeration capacity to aerobic digesters in 1970, in-
creased loadings to the secondary process have
necessitated a cutback in the aerobic digestion facil-
ity to 4 M.G. By 1975, all of the presently operat-
-------
LARGE SCALE AEROBIC DIGESTION 51
60-
*iO
4n-
2n-
10
V,
\
o
1
"--,
•- — ^
^^~—»
o
^>
\
\
1
1
1
j
1
o
~-^-^
1*— —
\
\
\
\
\
\
•^-^
M
<
ETRO
NORM
Mi
CO
^-^
DENVE
AL W.
^ROX
gCENTR
^^
R AIF
vs. ( =
OXYGE
MED W
.^^ o
DIFF
;0.8%
N Dl
A.S. (>
-^
USION
T.S.S.)
FFUSK
4.5 %
•— _
)N
T.S.S.)
^-«^^c
ik
"0 .05 .10 .15 .20 .25 .30 .35 .40 .45 .50 55 .60 £5
1
LOADING RATE, Ibs. V.S.S./ft3/day
Figure 6: V.S.S. Reduction vs. Loading Rates.
ing aerobic digesters will have to be converted back
to secondary aeration basins. In order to continue
to obtain the benefits of reduced sludge disposal
costs resulting from aerobic digestion, Metro Den-
ver is at present considering the conversion of a 1
M.G. sludge holding tank to a pure oxygen aerobic
digestion system using the rotating diffuser. The
oxygen digested sludge would be further digested
anaerobically for about two weeks to ensure a final
product having less than 60 percent volatile frac-
tion suitable for land application.
ACKNOWLEDGEMENT
This project has been funded by the Environ-
mental Protection Agency (Contract No. 68-03-
0152) under the direction of Dr. J. E. Smith, Jr., Ad-
vanced Waste Treatment Research Laboratory,
Cincinnati, Ohio.
BIBLIOGRAPHY
1. Cohen, D. B. A Low Cost Open Tank Pure Oxygen
System for High Rate Total Oxidation: Proc. 6th Int.
Conf., Water Pollut. Res., Jerusalem Pergamon
Press, Oxford & N.Y., 1973.
2. Cohen, D. B. and Puntenney, J. L. "Metro
Denver's Experience with Large Scale Aerobic Di-
gestion of Waste Activated Sludge," 46th Annual
Conference, Water Pollution Control Federation,
Cleveland, Ohio, 1973.
3. McDowell, M. A., et. al. Continued Evaluation of
Oxygen Use in Conventional Activated Sludge Processing,
E.P.A. Report No. 17050 DNW-02/72, 1972.
-------
52 MUNICIPAL SLUDGE MANAGEMENT
50
45
40
35
1 "
\ 25
MET^O OIFOISED
AIR SY
TEM
20-
15-
10-
\
PURE
02- SLC T DIP USER
SYSTEM
_ Jf
o
PURE 0
ROTATIf 0
:d±
DIFF
SYJ
SER
TEM
0 .05 .10 .15 .20 .25 .30 .35 40 .45 .50 .55 SO $5 ro
LOADING RATE-lbs. V.S.S./ft.3/doy
Figure 7: Air vs. Oxygen Aerobic Digestion—O'Supplied/V.S.S. Reduced vs. Loading Rates.
-------
LARGE SCALE AEROBIC DIGESTION 53
4. Benedek, P., et. al. "Kinetics of Aerobic Sludge
Stabilization," Water Research, 6, 91-97, 1972.
5. Baldwin, C. W. "Effect of Mixing on Aerobic
Sludge Digestion," Michigan Technological Univ.
(Masters Thesis), 1970.
6. Weston, T. J. "The Application of High Purity
Oxygen to Aerobic Sludge Digestion," Michigan
Technological University (Masters Thesis), 1972.
7. Reynolds, T. D. "Aerobic Digestion of Thick-
ened Waste Activated Sludge," Water & Sewage
Works, Ref. No. R 118-123, 1973.
8. Randall, C.W. and Koch, C. T. "Dewatering
Characteristics of Aerobically Digested Sludge,"
journ. Water Pollut. Cont. Fed., 41:5, R215-238,1969.
9. Lindstedt, K. D., et. al. "Aerobic Digestion for
Waste Activated Sludge Solids Reduction," Water &
Sewage Works, 118:166-169, 1971.
10. Loehr, R. C. "Aerobic Digestion: Factors Af-
fecting Design," Water & Sewage Works, Ref. No.
R169-180, 1965.
11. Andrews, J. F. and Kambhu, K. Final
Progress Report. Thermophilic Aerobic Digestion of Or-
ganic Solids Wastes, Clemson Univ., So. Carolina,
1971.
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION
EXPERIENCES AT SPEEDWAY, INDIANA
DANIEL W. GAY, RAYMOND F. DRNEVICH, EDMUND J. BREIDER
AND KAI W. YOUNG
Union Carbide Corporation—Linde Division
Tonawanda, New York
ABSTRACT
A full scale aerobic digestion study utilizing high
purity oxygen was performed at the Speedway,
Indiana Water Pollution Control Plant. The major
purpose of this study was to investigate the
possibility of attaining and operating at elevated
temperatures without an external heat source. The
existing UNOX System" was modified so that the
entire wastewater flow from the primary clarifier
could be handled by one of the two available reactor
trains while the other train served as an aerobic
digestion unit. The oxygen aerobic digestion
process showed the capability of sustaining high
temperatures (>31°C) during winter operation
through the conservation of energy produced by
endogenous decay. Volatile suspended solids
reductions in excess of 43 percent were obtained
with retention times as short as 11.6 days.
Utilizing the data obtained from these tests a full
scale aerobic digestion system using high purity
oxygen was designed for Speedway, Indiana. The
economic evaluation of the design indicates that the
annual cost for aerobically digesting Speedway
waste sludge is $53,400 which represents a cost of
$34.00 per ton of dry solids treated.
INTRODUCTION
Aerobic digestion is a process designed to
stabilize waste primary and/or waste activated
sludge through the catabolism of aerobic
microorganisms. Catabolism, in the form of
*UNOX is a registered trade mark of UNION CARBIDE
CORPORATION. The UNOX System is covered by patents in
the U.S.A. and foreign countries.
endogenous metabolism, is the self destructive
activity that results in the breakdown of complex
materials within the microorganism. This cellular
destruction involves the release of energy and a
reduction in the population of viable organisms
within the system. Generally, the rate at which the
biodegradable volatile suspended solids (BVSS)
concentration decreases within a digester is used as
a measure of the rate of this endogenous
metabolism occurring within that system.
If the cell mass of a microorganism can be
approximated by the formula CsHyNOz then the
catabolic activity in an aerobic digestion system can
be represented by either one of the following two
equations:
CsH7NO2 + 502 — 5 COz + 2HzO + NH3 + (1)
CsHyNOz + 702^5 COz + 3H20 + NO3 + H (2)
Equation (l) represents a process operated at
conditions that inhibit the growth of organisms
known to cause nitrification. From this expression
it is possible to calculate a theoretical oxygen
requirement (1.4 Ibs. Ch/lb. BVSS reduced as well as
a theoretical respiration coefficient (l.O moles COz
produced/mole Oz used). In designing an aerobic
digestion unit utilizing either pure oxygen or air it
is imperative that the oxygen requirements and
respiration coefficient be known so that the oxygen
supply and mass transfer equipment can be
properly sized. In a closed system the COz
concentration in the gas has an adverse effect on
the driving force for Oz dissolution.
Equation (2) is indicative of a system where the
conditions are acceptable for the propagation of
nitrifying organisms. In this case the theoretical
oxygen demand and respiration coefficient for the
55
-------
56 MUNICIPAL SLUDGE MANAGEMENT
process are 1.98 Ibs. Oz/lb. BVSS reduced and 0.71
moles COz produced/mole Ch used, respectively.
As was presented earlier, the catabolic activity of
microorganisms involves the release of energy.
Andrews and Kambhu1 indicated that the energy
released by the exothermic reactions represented in
equations (l) and (2) can approach the heat of
combustion of waste sludge, 9,000 BTU/lb. VSS
destroyed. However, some energy is required for
cell maintenance. According to McCarty2, energy is
required for mobility, the prevention of
undesirable flow of solutes across membranes, and
the resynthesis of proteins that are constantly
degraded due to kinetic instability. Therefore the
difference between the heat of combustion and the
maintenance energy is the free energy liberated
through the digestion process.
The kinetics of aerobic digestion are generally
accepted to be a first order rate phenomenon. The
rate of BVSS oxidation is first order with respect to
the BVSS concentration (Equation 3).
d(BVSS) = -KD (BVSS)
dt
(3)
Further, the variation in the endogenous decay
(Ko) coefficient is believed to follow an Arrhenius
relationship (Equation 4). Therefore, as the
temperature of the system increases the rate of
stabilization increases.
KD KDZO 0
(T-20)
(4)
Because of the exothermic nature of digestion
reactions and the increase in rate of digestion with
temperature it was anticipated that the totally
enclosed pure oxygen system would provide a
number of significant advantages over an open air
system for aerobic digestion. Because of the high
utilization of oxygen expected for the process, less
heat should be lost in the form of (l) sensible heat of
the vent gas and (2) heat of vaporization of water
within the vent gas and thus a higher temperature
should be attained. Also the reduction in heat loss
due to the covered oxygen system tankage should
permit better retention of the energy liberated by
digestion and, consequently, higher operating
temperatures.
Literature data3-4-5 indicate that the performance
of aerobic digesters is very sensitive to the D.O.
level. Consequently, the EPA recommends8 a
minimum D.O. concentration of 2 mg/1 be
maintained to insure proper performance. As
already indicated, at the higher temperatures the
rate of endogenous decay increases but,
unfortunately, the driving force for O2 dissolution
decreases. This decreased driving force at elevated
operating temperatures manifests itself as an
increase in the power necessary to dissolve the
oxygen required for the aerobic digestion process.
In air digesters the inordinately high dissolution
power required to maintain a D.O. concentration of
2 mg/1 when operating at elevated temperatures is
oftentimes an uneconomic operating mode.
Because of the high gas phase oxygen purities (and
the concomitant increased driving force), it was
anticipated that an oxygen aerobic digester could
economically operate at elevated temperatures and
high D.O. concentration. Therefore, the
Speedway, Indiana study was initiated to
demonstrate the inherent operating advantages of
using high purity oxygen for aerobic digestion.
Procedure
The Speedway, Indiana Wastewater Treatment
Plant Facility consists of primary sedimentation
units, a UNOX System, a Zimpro LPO System for
treating the combined waste sludges (oxygen
activated sludge and primary), and a vacuum filter
for dewatering the heat treated sludge before land
disposal.
The plant was designed for 7.5 mgd with a
maximum storm capacity of 18 mgd. The actual
flow to the system during the operation of the
digestion process in late 1972 was about 4 mgd.
The UNOX System at Speedway is presented in
Figure 1. The plant consists of two biological
reactor trains with each train containing a series of
four concurrent gas-liquid stages. During the
aerobic digestion study the two trains were
operated as completely independent systems. The
south side of the reactor train was chosen to handle
the 4 mgd of wastewater because it is better
instrumented and has access to two clarifiers. The
aerobic digestion reactor, north side, required only
one clarifier which was used as a holding tank for
the aerobically digested sludge.
Each stage of the aerobic digestion reactor had
the dimensions 22 ft.x22 ft. and operated with a 16
ft. liquid depth. The total volume of the digester
was 232,000 gallons. The clarifier was 65 ft. in
diameter with a 10 ft. liquid depth.
Aeration and mixing in the digestion unit was
performed by the existing equipment. Each stage
contained a 45° P.B.T. surface aerator, with a
supplementary bottom mixer. The horsepower
ratings for the mixers were 10, 7.5, 7.5 and 7.5 for
stages one through four, respectively.
The aerobic digester was operated on a semi-
continuous basis. The waste UNOX System
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION 57
From
Oxygen Supply
L
Wastewater
Feed
From
Primary
Sludge
Well
Valve Closed r
during study
*
North Reactor
(Aerobic Digester)
South Reactor
(UNOX System)
Waste
Sludge
Line
Figure 1: UNOX System at Speedway, Indiana (Schematic).
activated sludge was fed to the four stage digester
system daily over a nine hour period. During
operations with both primary and secondary
sludges, the primary sludge was pumped to the
digestion unit twice each day—Monday through
Friday. The digested sludge was stored in clarifier
number 3 which was periodically pumped to
Speedway's sludge disposal system.
Daily samples of feed sludge, Stage 1 contents,
and Stage 4 contents were taken and analyzed by
Speedway Treatment Plant personnel for total
suspended solids (TSS), volatile suspended solids
(VSS), and chemical oxygen demand (COD). Daily
measurements of temperature, dissolved oxygen
(D.O.), gas flows, and gas purities were also
obtained on Stages 1 and 4. The average ambient
temperature was also recorded daily. Oxygen
uptake rates, settling tests and pH measurements
were performed three times each week. Weekly
composite samples from Stages 1 and 4 were also
analyzed for total Kjeldahl nitrogen (TK.N),
ammonia (NHs), and total phosphorus. The data
were collected for each phase of operation only
after the digester had been run for at least one
retention time (SRT) to insure that the digester
was properly acclimated.
RESULTS AND DISCUSSION
The Speedway aerobic digestion studies were
divided into two phases. During Phase I (9/20/72 -
11/5/72) the digester was fed waste activated
sludge only. Phase II (11/7/72 - 12/20/72) was
operated with a mixture of primary and secondary
sludge.
Table 1 contains the characteristics of the sludges
fed to the aerobic digesters in this study. The TSS
of the secondary sludge ranged from 1.8 to 2.8
percent, values fairly typical of sludges from a
UNOX System treating primary effluent. The TSS
of the primary sludge ranged from four to six
percent so that the combined feed to the digester
during Phase II averaged approximately 3.0 percent
-------
58 MUNICIPAL SLUDGE MANAGEMENT
TABLE 1
Characteristics of Sludges Fed to Digesters
PHASEI
Temperature, °F
PH
COD, 10,000 mg/1
TSS. 10,000 mg/1
VSS, 10,000 mg/1
Alkalinity, mg/ 1
as CaCOs*
UNOX
Overall
A verage
67.0
6.6
2.98
2.14
1.64
—
System Sludge
Range
62-74.0
6.2-6.8
2.50-3.70
1.82-2.60
1.40-1.90
_
UNOX
Overall
Average
60.0
6.7
3.16
2.41
1.69
2250
PH A
System Sludge
Range
55-65
6.2-6.9
2.9-3.6
2.0-2.74
1.48-1.90
2220-2940
S E II
Primary
Overall
A verage
60.0
6.8
6.27
4.74
2.91
3690
Sludge
Range
57-65
6.4-7.2
4.2-8.2
3.8-6.0
2.4-3.9
2070-4830
*For the week ending I2/17/72.
total suspended solids with a volatile fraction of
0.66.
General operating experience gained from this
study included the ease at which the process could
be controlled. It required only one retention time
for the solids in the digester to build to steady state
concentrations. The operation of the digester did
prove to be very sensitive to the D.O. level
maintained in the reactor. A rapid increase in the
rate of digestion was qualitatively observed when
the D.O. of the reactor was increased from
< l.Omg/1 to values greater than 2 mg/1. However,
maintaining D.O. concentrations of 2 mg/1
required drastic decreases in the system oxygen
utilization to increase the mass transfer capabilities
of the existing dissolution equipment which was
not designed for aerobic digestion. Since there was
insufficient oxygen production capability to sustain
prolonged operation at reduced digester oxygen
utilization, the dependency of KD on D.O.
concentration was not developed in detail.
Table 2 presents the average operating
conditions observed during each of the phases of
operation. Because the oxygenation facility used as
the aerobic digester was not specifically designed
for this purpose, problems were encountered with
respect to the staging characteristics of the unit.
The fact that the TSS values for Stages 1 and 4 are
similar during both phases of operation is a result of
back mixing. The interstage openings were
designed for 3.75 mgd of continuous plant influent
flow plus recycle. The 13,000 to 21,000 gpd feed on
a semi-batch basis was not sufficient to permit the
system to operate as a four stage unit. The actual
systems's performance could have been that of a
completely mixed tank for Phase I and a two or
three stage unit (because of the increased feed flow)
for Phase II.
Some of the interesting data contained in Table 2
are the temperatures of the reactor. In Phase I the
digester maintained an average temperature of
91°F even though the feed temperature was 67°F
and ambient air was less than 45°F. During Phase II
temperatures as high as 91°F were obtained in the
digester even though the feed and ambient
temperatures averaged less than 61°F and 28°F
TABLE 2
Average Operating Conditions for the
Oxygenated Aerobic Digestion System
at Speedway, Indiana
Phase 11
(57% UNOX
Phase I System Sludge
by Wl.)
Flow Rate, gpd 14,250 20,700
Feed Temperature, ° F 67 60.8
Retention Time, days 16.3 11.6
Stage 1
TSS, mg/ 1
VSS, mg/ 1
% VSS
Temp., °F
D.O., mg/1
Stage 4
TSS, mg/ 1
VSS, mg/1
'( VSS
Temp., °F
D.O., mg/ 1
Average Ambient Temperature, °F
Solids Loading Rate
P VSS, Day Ft-1
12,800
9,080
70.9
90.8
1.6
13,100
9,220
70.4
91.1
1.2
45.5
0.063
20,700
12,900
62.3
88.2
0.55
18,300
11,500
62.8
88.8
0.55
28
0.106
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION 59
respectively. This ability to maintain high
temperatures during winter conditions is a result of
the heat liberated by the microorganisms through
endogenous respiration. No external source of heat
was utilized in these experiments. The increased
temperature rise in Phase II was probably a result of
increased heat generation due to the higher loading
and the higher VSS concentration of the feed.
Table 3* reports the volatile solids reduction data
obtained during this study. Overall VSS reductions
of 43.9 and 43.0 percent were achieved during
Phases I and II respectively. Insufficient analytical
data were generated to enable a determination of
the average biodegradable fraction of the influent
volatile solids. As a result, an accurate appraisal of
the digester performance in terms of the approach
to complete digestion was not possible.
Qualitatively however, the digested sludge
appeared very stable. On many occasions the
digested sludge was stored in Clarifier No. 3 for
extended periods of time (approximately 15 days)
with no significant odor problems. Notwithstand-
ing this lack of analytical quantification however, a
theoretical determination of the percent approach
to complete digestion is presented below.
The kinetic model presented earlier can be
applied to the data collected at Speedway, by
solving equation (3) for BVSS and thus obtaining:
BVSS t = t
BVSS t = O
1 + KD tQ
(5)
Applying equation (5) to each stage of a multistaged
system and combining into a single expression
results in equation (6)
BVSSt = t = BVSSt = o ( 1 + KDtQ,n)
(6)
where n equals the number of stages to be
considered.
To properly utilize equation (6) the effect of
temperature on the KD constant must be
established. Essentially the-0 value of equation (4)
must be obtained for the sludge in question.
Unfortunately this was not done during the
Speedway test. However, Figure 2 does contain a
generalized correlation of data obtained from the
literature1,7 as well as tests performed on similar
sludges at Union Carbide's laboratories6. Applying
the corresponding KD value from Figure 2 to the
data obtained for VSS reduction it was anticipated
that the actual number of digestion stages could be
calculated for various BVSS fractions in the feed.
Figures 3 and 4 present a graphical display of this
attempt for Phases I and II, respectively.
For example, equation (6) would predict a BVSS
reduction of 78 percent in a four stage digester
operating under the conditions observed at
Speedway during Phase II. Thus, if 48 percent of
the influent VSS were biodegradable the
corresponding VSS fraction degraded would be
approximately 37 percent. Point "A", shown on
Figure 4, represents this reduction of VSS and
BVSS for a theoretical four stage digester. In the
Speedway digester, as mentioned earlier,
backmixing was a problem due to the equipment
used in this study. This high degree of backmixing
encountered leads one to assume that the number
of theoretical stages was significantly less than the
actual number (four). If the number of theoretical
TABLE 3
Summary of Volatile Solids Reduction
Speedway High Purity Oxygen Aerobic
Digestion Experiments
Volatile Solids
Feed Retention Digester Loading Rate % VSS
Phase Sludge Type Time, Days Temp. °C ttVSS/Day/Ft3 Reduction
I
II
UNOX System
WAS*
UNOX System
WAS* & Primary
16.3
11.6
32.8
31.6
0.0629
0.106
43.9
43.0
*WAS = Waste Activated Sludge.
'Tables 4 & 5 contain weekly summaries of the data obtained
during each phase of this study.
-------
60 MUNICIPAL SLUDGE MANAGEMENT
TABLE 4
Summary of Aerobic Digestion Data
(Speedway, Ind.) for Phase I
(UNOX System Sludge Only)
Starling Dale: 9/20/72
Week Ending
Measured Data
Feed
Flow Rate, GPD
TSS, mg/l
VSS, mg/ 1
% VSS
Temp., °F
Stage 1
TSS, mg/ 1
VSS, mg/ 1
% VSS
Temp., °F
D.O.a
02 Purity, %
CO2 Purity, %
Stage 4
TSS, mg/ 1
VSS, mg/ 1
% VSS
Temp., °F
D.0.a
O2 Purity, %
CO2 Purity, %
ISR, ft, hr.
Miscellaneous
Average Ambient Temp., °F.
O2 Feed, cfd
O2 Purity in feed, %
Vent Gas Flow, cfd
Calculated Data
Solids Reduction
Retention time, days
% VSS Digested
' '•'(• TSS Reduction
Loading Rate, ffVSS/day/ft-1
Oxygen Consumption
O2 Consumption; Ibs/lb VSS
O2 Utilization, %
CC>2 Generation, Ibs/lb 02
Used
10/15/72
15,000
22,100
16,700
75.6
72
12,900
8,900
69.0
92.5
1.9
51.9
>20
13,800
9,930
72.0
93.0
1.4
40.0
>20
1.22
47
15.5
40.8
37.3
.0674
10/22/72
13,400
21,100
16,400
77.7
66
11,400
7,890
69.2
93.4
2.0
48.6
>20
1 1 ,300
7,740
68.5
93.9
1.5
40.7
>20
1.89
43
19,000
= 90
13,200
17.3
52.7
46.6
0,0590
1.06
72
10/29/72
14,300
20,200
15,900
78.7
66
13,000
9,240
71.0
90.3
1.2
41.7
>20
13,600
9,400
69.0
90.4
0.67
36
>20
0.9
45
19,700
S90
15,400
16.2
40.7
32.5
.0609
1.34
70
11/5/72
14,300
22,000
16,700
75.9
65
13,800
10,300
75.0
87.0
1.2
38.0
42.7
13,700
9,820
72.0
87.0
1.3
31.0
42.7
0.75
47
22,000
S90
17,400
16.2
41.2
37.8
.0642
1.48
74
(0.70)
A verage
14,250
21,400
16,400
76.6
67
12,800
9,080
70.9
90.8
1.6
45.0
42.7
13,100
9,220
70.4
91.1
1.2
36.9
42.7
1.19
45.5
20,200
= 90
15,300
16.3
43.9
38.6
.0629
1.29
72
(.70)
"4 Ft. off bottom.
stages at Speedway during Phase II was two, as
suggested earlier, Figure 4 would indicate that
more than half of the influent VSS was
biodegradable.
The COD reduction capabilities of the process
were also measured at Speedway. Average COD
reductions of 51.4 and 56.9 percent were obtained
for the respective phases of operation. Little
information is presented in the literature with
respect to the amount of COD reduction expected
through aerobic digestion. This is believed due to
the prime interest of investigators in VSS
reduction and the difficulties associated with
measuring the parameter, e.g., the total COD
ranged from 20,000 80,000 mg/l for the
Speedway studies.
The Oz consumption data obtained during this
test were established by making a gas phase O2
balance around the digester. The average O2
consumption ratio of 1.29 and 1.85 Ibs. Ozllb. VSS
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION 61
reduced were obtained for the waste UNOX sludge
and the UNOX primary mixture, respectively.
These values are very close to those reported in the
literature for similar sludges. The increased Oz
requirement for the primary sludge is believed to be
due to a large quantity of organic substrate that rs
metabolized in the digester, producing cellular
material which is then digested.
The O2 utilization values for both phases
averaged approximately 70 percent. This low value
was due to the inherently high CO2 production of
the digestion process, the inability of the existing
system to maintain a high D.O. (D.O. > l.Omg/l)
at lower oxygen purities, and the poor gas staging
characteristics encountered at the low gas flow
rates required by the process.
TABLE 5
Summary of Aerobic Digestion Data
(Speedway, Ind.) for Phase II
(Mixture of UNOX System & Primary Sludges)
Starting Date: 11/7/72
Week Ending
Measured Data
Feed (% UNOX System Sludge
by wt.)b
Flow Rate, GPD
TSS, mg/ 1
VSS, mg/1
% VSS
Temp., °F
Stage 1
TSS, mg/ 1
VSS, mg/ 1
% VSS
Temp., °F
D.O.
62 Purity, %
CO2 Purity, %
Stage 4
TSS, mg/ 1
VSS, mg/ 1
% VSS
Temp., °F
D.O. a
O2 Purity, %
CO2 Purity, %
ISR, ft/hr.
Miscellaneous
Average Ambient Temp., °F
O2 Feed, cfd
O2 Purity in Feed, %
Vent Gas Flow, cfd
Calculated Data
Solids Reduction
Retention time, days
% VSS Digested
% TSS Reduction
Solids Loading Rate,
#VSS/day/ft3 '
Oxygen Consumption
O2 Consumption, Ibs/lb VSS
02 Utilization, %
CO2 Generation, Ibs/lb O2
Used
a 4 Ft. off bottom
b Based on TSS
11/26/72
1
54.6
18,600
30,800
20,800
67.2
63.0
20,900
13,600
65
88
.7
59
26
18,900
12,300
65.0
88.0
0.6
50
37
0.37
35
42,900
S90
12.5
40.7
38.7
0.0967
1.92
65
12/3/72
55.4
20,700
29,200
18,900
64.9
62.0
20,800
12,900
62
88
.6
62
\
18,200
11,400
62.0
,90.0
0.6
53
0.5
35
50,500
S90
11.2
39.8
36.1
0.103
«?
1.89
65
12/10/72
57.9
20,100
31,700
20,700
65.4
60.0
21,600
13,200
61 ,
91
0.3
66
19
18,400
11,300
61.0
91.0
0.4
58.8
30
0.46
25
59,700
= 93
11.5
45.4
41.9
0.112
1.90
65
12/17/72
60.0
20,700
30,900
20,300
65.7
58.0
19,500
12,000
61
86
0.6
57
20
17,800
11,000
61.8
86.0
0.6
48.0
29
0.86
18
54,400
=93
36,900
11.2
45.9
42.5
0.113
1.78
67.8
(0.5)
A verage
57.0
20,700
30,600
20,200
66.0
60.8
20,700
12,900
62.3
88.2
.55
61.0
21.7
18,300
11,500
62.8
88.8
,0.55
52.4
32.0
.55
28
51,900
=91
36,900
11.6
43.0
39.8
0.106
1.87
66
(0.5)
-------
62 MUNICIPAL SLUDGE MANAGEMENT
O
O
V — Young6, Mzo"0-12- 1/d»V
Q —Andrews,1 (Kd)20 = 0.10, I/day
^ --Jaworskl7, (Kd)20 = 0.10, I/day
[•] — Jaworskl7, (Kd)2o " °-19' 1/dav
11-20), °C
40 50
-10 0 10 20 30
Figure 2: Effect of Temperature on the Decay Constant, Kd.
Figure 3: Speedway Aerobic Digestion, Phase I, Percent VSS
Digested Vs. Number of Stages.
1 2
Figure 4: Speedway Aerobic Digestion, Phase II, Percent VSS
Digested Vs. Number of Stages.
Because of the poor method of COz
measurement available to the operators at
Speedway, the respiration coefficient obtained for
the process was much lower than the 1.0 or 0.7
moles CCh/mole C>2 predicted through equations
(1) and (2), respectively. A value of 0.5 moles
COi/mole Oz was obtained using Draga tubes for
CO2 measurements on grab samples of gas.
A limited amount of settling and thickening data
was obtained during this program. All the data
were obtained using a one liter graduate cylinder
stirred at a rate of one RPM. This data is plotted in
Figure 5 and demonstrates that the settling
velocities of the aerobically digested sludge at
Speedway, Indiana fall within the range of those
observed for oxygen activated sludge.
Finally, the weekly composite samples obtained
from Stage 4 of the digester for nutrient analyses
indicated that:
(a) there was hardly any nitrification occurring
at the temperatures observed
(b) the NH3-N concentration was increased due
to nitrogen solubilization from 50 mg/1 in the
feed to 100-120 mg/1 in the digester super-
natant, and
(c) the soluble phosphorus concentration in-
creased from 60 mg/1 to 100 mg/1.
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION 63
OXYGEN SLUDGC "BAND"
s
£\ UWOX System Sludge Only
O UNOX System + Primary
Initial Solids Cone, mg/1
Figure 5: Aerobic Digested Sludge Settling Data.
Economics
The test work done at Speedway, as previously
mentioned, was accomplished using tankage that
was specifically designed to operate as a secondary
oxygen activated sludge system. The relatively
small liquid and gas flows through the system
during the aerobic digestion study resulted in a
number of inefficient operating characteristics.
These inefficiencies, of course, resulted in very
distorted system economics. Therefore, based on
the process information collected during this study,
an aerobic digestion system was designed (but has
not been installed) to treat the primary and waste
activated sludge generated by the Speedway facility
at design year operating conditions. The economics
presented herein therefore are considered typical
oxygen aerobic digestion costs for small scale low
strength waste municipal facilities with primary
treatment.
The major unit operations at the Speedway,
Indiana facility were designed for influent
hydraulic conditions of 7.5 MGD. The existing
oxygen generator is a four tons per day pressure.
swing adsorption type unit and the dissolution
system is as described earlier. Since the plant
influent flow observed during the aerobic digestion
studies was only slightly more than half this design
flow, extrapolation of some of the plant influent
conditions was necessary to insure a properly
designed and integrated aerobic digester.
Therefore, for this analysis it was assumed that the
primary and secondary operating characteristics
are as shown on Table 6. These characteristics are
consistent with the anticipated design year
hydraulic conditions and the wastewater character
presently observed at the Speedway, Indiana
facility.
The data shown on Table 6 were used as the basis
for the aerobic digestion system design. The
aerobic digester therefore has been designed for an
average waste sludge volume of 31,840 gallons per
day with a total suspended solids concentration of
approximately 3.3 percent. The design temperature
of the feed to the aerobic digester was assumed as
20°C with the feed stream dry solids content being
composed of 55 percent waste primary sludge and
45 percent waste secondary sludge. This
composition is slightly different than the feed
characteristics observed during the digestion
studies because not all of the primary waste sludge
generated during the study was fed to the aerobic
digester. Therefore, based on the plant influent
conditions and the secondary system sludge
production characteristics shown in Table 6, the
55:45 primary to secondary waste sludge ratio was
determined.
A schematic layout of the Speedway plant with
an incorporated aerobic digester is presented in
Figure 6. The aerobic digester will be a single train
TABLE 6
Design Year Operating Characteristics
of the Primary and UNOX Systems
at Speedway, Indiana
Influent Conditions:
Flow
BOD5
COD
SS
PH
Temp.
7.5 mgd
175 mg/1
350 mg/1
175 mg/1
7
20° C
Primary System Operating Characteristics:
Underflow TSS
Underflow VSS
Dry Solids Wasted
Total Waste Volume
= 4.75%
= 2.9%
= 4690 Ibs/day
= 11,840 gallons/day
UNOX System Operating Characteristics:
Retention Time
Biomass Loading
Underflow TSS
Underflow VSS
Dry Solids Wasted
Total Waste Volume
1.48hrs.
0.38 Ibs. BODj/lb. MLVSS/day
2.4%
1.69%
3915 Ibs/day
= 20,000 gpd
-------
64 MUNICIPAL SLUDGE MANAGEMENT
Primary
Clariflers
Oxygen Generation
Facility
Type = PSA
Capacity = 7.5TPI
Digester Dimensions (Internal)
Each Stage 44' x 44' x 15'
Liquid Depth 11'
Stage Liquid Volume 159300 gal.
UNOX System
r-0
Secondary
Clarifiers
To
' Disinfection
Oxygen Aerobic Digester
To
Disposal
Figure 6: Speedway, Indiana Plant Layout with Oxygen Aerobic Digestion (Schematic).
two stage system with a liquid volume of
approximately 318,600 gallons. Each stage has
internal (excluding wall, bottom, or cover thickness
allowances) dimensions of 44 ft.x44 ft.xisft. with a
liquid depth of 11 feet. In ground tankage was
specified to utilize the thermal insulating capacity
of the ground. The design was also based on the
worst case average ambient air temperatures
expected during the winter season. Therefore,
summertime operating temperatures may be
somewhat higher. The two stage design was
selected rather than a three or four stage system
because of both operating and economic
advantages. The relatively large surface
dimensions of each stage facilitate adequate mixing
and mass transfer at the specified liquid depth.
Increasing the number of stages would necessarily
result in decreasing the liquid depth in each stage
(due to the decreased surface dimensions of each
stage) to insure the maintenance of adequate
mixing and mass transfer. Furthermore the heat
losses of the two stage system are less than the
multistage systems because of the reduced exposed
surface area to volume ratio. The decreased heat
loss, of course, results in the potential for higher
system operating temperatures.
The two stage system also has economic
advantages since it requires fewer interstage
baffles and fewer mechanical aerators than the
multistage systems. Capital investment savings can
therefore be realized because less concrete is
required for the tankage, and advantage can be
taken of the "economy of scale" costs of the larger
(but fewer) mechanical aerators required. The
combination of the operating and economic
advantages inherent in the two stage design
indicated that this was the optimum configuration
for this particular location.
The retention time of this system at design
conditions is ten days. Based on the aerobic
digestion studies conducted at Speedway, this
system will achieve approximately a 44 percent
reduction of VSS at a temperature of 36°C. The
system is designed to maintain a D.O.
concentration of 2 mg/1 at an oxygen utilization of
between 65 and 70 percent. The oxygen
requirement, based on the consumption
-------
HIGH PURITY OXYGEN AEROBIC DIGESTION 65
characteristics observed during the digestion
studies, is 3.5 tons per day.
Each digester stage is equipped with a mechanical
aerator having a submerged, supplemental mixing
device. Table 7 shows the installed and operating
energy requirements for mixing and mass transfer
at design conditions. Also included on Table 7 is the
power necessary to generate the required oxygen.
To reflect accurate system economics the oxygen
generator was sized to satisfy the oxygen
requirements of both the UNOX System and the
aerobic digestion system (7.5 tons per day). The
power, capital investment, and operating costs of
oxygen production for the aerobic digestion system
were then taken as a fraction (0.46) of the power
and costs associated with oxygen production for
the entire Speedway treatment facility.
The installed cost of the Speedway aerobic
digestion system is estimated to be $510,000. This
cost includes the dissolution equipment, tankage,
oxygen generation equipment, and all
instrumentation required for a completely
integrated, fail safe unit operation, in addition to
TABLE 7
Aerobic Digester Design
Criteria Summary
Design Criteria:
Flow, gpd
Total Suspended Solids, mg/1
,%
Volatile Suspended Solids, mg/1
.%
Feed Stream Temperature, °C
Ambient Air Temperature, °C
pH
Operating Characteristics:
Retention Time, days
Operating Temperature, °C
VSS Reduction, %
Average D.O., mg/1
Oxygen Utilization, %
Oxygen Required, tons/day
Energy Requirements:
31,840
33,000
3.3
21,000
2.1
20
-2
6.8
10
36
44
2
65-70
3.5
Oxygen Dissolution
Oxygen Generation
Miscellaneous .
Total 159
Operating
Power, BHp
67
88
4
Installed
Power, Hp
80
monies for the installation and startup of all the
above equipment. The costs of unit operations (e.g.,
thickeners, vacuum filtration, centrifugation, etc.)
following the aerobic digester have not been
included in the cost of the aerobic digestion system
because their use is a function of the ultimate
sludge disposal objectives.
Amortizing the capital investment at a six
percent interest rate over the 25 year economic life
of the plant and assuming a power cost of 1.3 cents
per KWH, the annual cost of aerobically digesting
the Speedway waste sludge is $53,400. This yearly
figure represents an average cost of $34.00 per ton
of dry solids treated.
It should be noted that these costs are specific to
the Speedway facility. Normally an aerobic digester
is integrated with a UNOX System design in the
most cost effective manner by considering, among
other things, (l) the overall effective transfer
efficiencies of each unit operation, (2) the relative
quantities of primary and secondary sludges to be
treated, and (3) the degree of treatment required
from each unit operation. Since the original
Speedway facility was designed without aerobic
digestion, this economic optimization procedure
was somewhat restricted.
Furthermore, these costs do not reflect many of
the intrinsic advantages of an oxygen aerobic
digester. Since an oxygen digester is covered, the
aerating gas is vented through a single vent stack;
thus effective odor control is achieved and the
biological aerosol problem typical of air aerobic
digesters is eliminated. The covered tankage
operating at elevated temperatures also eliminates
the freezing problem often encountered in
northern climates. Also, the covered digester acts
essentially as a respirometer, providing real time
response to the sludge loads placed on it. It is
therefore possible to automate the oxygen
production unit to respond to these changes in
sludge load, thereby causing the system to use only
the appropriate power for oxygen generation
commensurate with the sludge loading being
processed. Therefore, although the actual costs of
oxygen aerobic digestion are dependent upon
individual circumstances, the costs presented
herein can be considered representative for small
scale municipal facilities with primary treatment.
ACKNOWLEDGEMENT
We wish to acknowledge the City of Speedway,
Indiana and the Speedway Water Pollution Control
Plant personnel for their contributions to this
study. Without their cooperation, this program
could not have been performed.
-------
66 MUNICIPAL SLUDGE MANAGEMENT
REFERENCES
1. Andrews, J.F., and Kambhu, K.
"Thermophilic Aerobic Digestion of Organic Solid
Wastes," Final Progress Report, Clemson University,
Clemson, South Carolina, May, 1970.
2. McCarty, P. L. "Thermodynamics of
Biological Synthesis and Growth," 2nd Int. Con/. On
Water Pollution Research, Tokyo, Japan, August, 1964.
3. Ahlberg, N.R., and Boyko, B.I. "Evaluation
and Design of Aerobic Digesters," Journal WPCF,
Vol. 44, No. 4, 634, April, 1972.
4. Stein, R.M. "A Study of Aerobic Sludge
Digestion Comparing Pure Oxygen and Air,"
Master Thesis, Vanderbilt University, Nashville,
TN, 1971.
5. Hamilton, Ohio's Experience with Aerobic
Digestion, Preliminary Report prepared for review
by the E.P.A., 1970.
6. Young, K. W. "Aerobic Digestion of Sludge at
Thermophilic Temperatures Using High Purity
Oxygen," Report No. 1. Isothermal Batch Studies
of Air Activated Sludge Using 14-liter Fermentors.
Process & Product Development Department,
Engineering Memorandum No. 5279, May 15,
1972.
7. Jaworski, N., Lawton, G.W. and Rohlich, G.A.
"Aerobic Sludge Digestion," Advances in Biological
Wtrste Treatment, The MacMillan Company, New
York, 1963.
8. Burd, R.S. "A Study of Sludge Handling and
Disposal," Water Pollution Control Research Series
Publication No. WP-20-4, May, 1968.
APPENDIX A Nomenclature
A Hm Energy Utilized for cell maintenance
(BTU/lb. of VSS)
A He Energy Released through combustion
(BTU/lb. of VSS digested)
tQ = t Retention Time of digestion, (days)
Xo VSS of sludge leaving digester, (mg/l)
KD Endogenous rate, (I/day)
Ki (Fractional) efficiency of energy transfer by
biological reaction through heterogene-
ous microorganisms
Xi VSS of sludge fed to the digester, (mg/l)
fs Fraction of total VSS digested
BVSS Biodegradable Volatile Suspended Solids,
(mg/l)
KDZO Endogenous rate at 20°C, (l/day)
T Temperature in degrees centrigrade
0 Temperature coefficient
n Number of stages in the digester
D.O. Dissolved Oxygen Concentration
P.B.T. Pitched Blade Turbine
-------
SLUDGE DEWATERING
CHARLES W. CARRY, ROBERT P. MIELE AND JAMES F. STAHL
Los Angeles County Sanitation Districts
Whittier, California
ABSTRACT
Commencing in April, 1970, an extensive sludge
dewatering investigation was undertaken at the
Los Angeles County Sanitation Districts' 380 mgd
Joint Water Pollution Control Plant. Discharge
requirements set by the Los Angeles Regional
Water Quality Control Board necessitated that a
minimum of 95 percent of the suspended solids in
the digested sludge be removed. Various
combinations of sludge conditioning (polymer,
chemical, thermal) and sludge dewatering
(centrifugation, pressure filtration, vacuum
filtration) were examined, the results of which
indicated that five conditioning-dewatering
systems were capable of meeting the required
effluent quality. An economic evaluation was made
of each system, from which a two stage
centrifugation system was found to be the
alternative of lowest cost. The system consisted of
the existing horizontal scroll centrifuges and
imperforate bowl basket centrifuges with polymer
conditioning for the second stage basket machines.
The composite sludge cake from the system will be
hauled to a sanitary landfill for ultimate disposal.
INTRODUCTION
One of the most difficult problems in
wastewater treatment is the processing and
disposal of sludge, and recently the complexity of
the problem has been magnified by increasingly
stringent quality standards for treated waste-
waters. Additionally, in many instances expanding
urban development has limited the land area avail-
able for sludge disposal and has necessitated the
dewatering of sludge prior to its ultimate disposal.
In the mid 1950's the Los Angeles County Sanita-
tion Districts recognized this problem and installed
horizontal scroll centrifuges for sludge dewater-
ing at its Joint Water Pollution Control Plant
(JWPCP) a 380 mgd primary treatment facility
located in the city of Carson, California. Figure I
shows a schematic of the JWPCP treatment and dis-
posal system. In addition to municipal and indus-
trial wastes entering the plant through several
trunk sewers, five water renovation plants located
upstream from the JWPCP discharge raw and
waste activated sludge into the tributary trunk
lines. Basically, the treatment plant consists of pri-
mary sedimentation, anaerobic digestion of the
settled solids and horizontal scroll centrifugation
(30 percent S. S. recovery) of the digested sludge.
The primary effluent from the sedimentation
tanks, along with centrate from sludge dewatering,
is discharged to the Pacific Ocean at White's Point
through a series of .submarinejoutfalls using multi-
port diffusers at a distance of about two miles
offshore and a depth of 150 to 200 feet. The
effluent is chlorinated to comply with ocean
bacteriological standards.
The dewatered cake from the centrifuges is
spread on land adjacent to the dewatering site for
open air drying, aided by mechanical turning of the
sludge on the drying beds. The dried sludge is
collected from the beds -and-soU-to a-local fertilizer
manufacturer for use as a soil conditioner.
Over an extensive time period, the monitoring of
ocean waters surrounding the JWPCP outfall
system identified settleable and floating material
67
-------
68 MUNICIPAL SLUDGE MANAGEMENT
which were attributable to centrifuge centrate
discharge along with primary effluent. In early
1970 a major research effort was initiated by the
Districts, the purpose of which was to investigate
methods for improving solids capture during
sludge dewatering. In September of 1970 the Los
Angeles Regional Water Quality Control Board
established new effluent standards for the JWPCP.
Compliance with the new standards required a
major supplementation to the existing treatment
facilities. Additional primary sedimentation
capacity would be needed to bring about greater
suspended solids removal from the raw sewage.
Moreover, the new standards mandated a criterion
for a sludge dewatering system which would be
capable of recovering at least 95 percent of the
suspended material in the digested sludge.
The sludge dewatering research program
involved a comprehensive review of existing
technology and a pilot plant evaluation of those
systems having documented process performance.
These systems included centrifuges, vacuum filters
and pressure filters. Because of the potential
economics offered by a dewatering scheme that
would use the existing horizontal scroll
centrifuges, each pilot dewatering system was
evaluated1 with respect to its ability to dewater
centrate from the existing centrifuges as well as
digested sludge. The major criterion used in
evaluating a dewatering system was an effluent
suspended solids of less than 1,500 mg/1.
Processes for Sludge
Conditioning
Initial testing of the pilot systems indicated that
dewatering without some form of prior condition-
ing of the sludge failed to produce the desired solids
recovery. Four types of sludge conditioning were
evaluated: thermal conditioning, conditioning with
cationic polymers, chemical conditionng with ferric
chloride and/or lime, and fly ash conditioning.
In the thermal conditioning process, sludge is
heated under pressure to a temperature normally
greater than 310°F. At these temperatures and
pressures bound water associated with the solid
matter in the sludge is released, the sludge is more
easily dewatered and dryer cakes should be
obtained. To evaluate the process on the JWPCP
sludge a 200 gph pilot unit was procured.
PRECHLORINATION
GRIT TO LANDFILL
GRINDERS
QAS TO
SLUDGE
DENSITY
MEASUREMENT
AUTOMATIC
SLUDGE FEED
CONTROLS
CENTRATE
FLOTATION
TANK
CENTRIFUGES
Figure 1: Schematic Diagram of the Joint Water Pollution Control Plant.
-------
DEWATERING 69
The initial investigations with the heat
treatment system were directed towards
determining the optimum temperature—detention
time relationship for the digested sludge. In this
determination it was important to note that the
thermal conditioning process results in the
solubilization of a significant amount of the
suspended material in the sludge. To minimize the
solubilization of such organic material, and thus
minimize the soluble oxygen demand, it is desirable
to operate a thermal conditioning system at the
lowest possible temperature. With cognizance of
this phenomenon, optimum conditions in the
operation of the unit were defined as those that
resulted in the lowest suspended solids
concentration after dewatering. Time-temperature
tests were conducted at two sludge detention
times—30 and 40 minutes and at temperatures
ranging from 165° to 201°C (330° to 395°F). Results
of these tests indicated that at the 30 minute
detention period, the supernatant suspended solids
from a one hour laboratory settling test were
reduced from approximately 6,000 mg/1 to 3,500
mg/1 by increasing the temperature from 165° to
182°C (330° to 360°F). The supernatant suspended
solids concentration was not further reduced by
temperature increased above 182°C (360°F). A
similar trend occurred at a sludge detention time of
40 minutes, the differences being that the
minimum supernatant suspended solids level was
reached at a temperature of 177°C (350°F), and the
supernatant suspended solids concentration at that
temperature was approximately 3,000 mg/1. It was
concluded from the testing that the optimum
thermal conditioning temperature for the digested
sludge was 177°C (350°F) at a detention time of 40
minutes. Under these operating conditions the
suspended solids concentration of the digested
sludge decreased from approximately 40,000 mg/1
to 30,000 mg/1 following thermal conditioning.
This decrease was thought to be the result of
dilution water added by the steam required to raise
the temperature of the sludge and in part by the
solubilization of suspended material.
Once the optimum temperature-detention time
relationship was developed, a pilot gravity
thickener was evaluated on the heat treated sludge.
Detention times ranging up to two hours were
investigated, along with overflow rates varying
from 200 to 650 gpd/sq.ft. From an evaluation of
the thickener, it was concluded that supernatant
suspended solids of approximately 3,700 mg/1
could be obtained by gravity thickening of
thermally-conditioned sludge for one hour at an
overflow rate of 225 gpd/sq.ft. The corresponding
underflow solids concentration was between nine
and ten percent. Further dewatering investigations
indicated that the combined process of thermal
conditioning and gravity thickening resulted in an
effective pretreatment system.
Polymer conditioning was used in conjunction
with centrifugation, vacuum filtration and
pressure filtration, while chemical conditioning
was investigated for the vacuum filtration and
pressure filtration processes. Fly ash conditioning
was evaluated solely in conjunction with pressure
filtration. The effectiveness of all of the
conditioning agents is discussed in the subsequent
section regarding dewatering processes.
Sludge Dewatering
Centrifugation: Horizontal Scroll
Horizontal scroll centrifugation studies were
directed towards the processing of the JWPCP
digested sludge through the existing centrifuges.
However, studies on the centrifugal dewatering of
heat-conditioned digested sludge were also
conducted with a pilot scale (six inch diameter)
horizontal scroll centrifuge.
The evaluation of the base performance of the
existing centrifuges was carried out in a manner
which enabled the effect of variations in sludge feed
rate and bowl pool depth to be independently
assessed. Considered in this respect were primary
digested sludge feed rates between 200 and 400
gpm, and pool depths between 1.0 and 3.5 inches.
The rotational speed of the bowl was held constant
at 1300 rpm. The differential speed, i.e., the speed
difference between the bowl and scroll, remained
fixed at 15.3 rpm. The bowl speed was held con-
stant at 1300 rpm because previous experience had
shown that the maintenance associated with higher
speeds was excessive. Figure 2 shows the effect of
pool depth on solids recovery for various feed rates.
It can be seen that increasing the pool depth
resulted in higher solids recovery. The exact
opposite relationship was found for the cake solids
concentration. It can also be seen that for a given
pool depth increasing the feed rate resulted in a
decreased suspended solids recovery and, while not
shown, it was observed that this also resulted in an
increase in cake solids concentration. Considering
that one criteria for an acceptable dewatering
process was a suspended solids recovery in excess
of 95 percent, it was significant to note from Figure
2 that the maximum recovery obtained was 55
percent. The cake solids concentration associated
with this recovery was approximately 35 percent.
To increase the suspended solids recovery, a
number of cationic polymers were investigated for
-------
70 MUNICIPAL SLUDGE MANAGEMENT
£55
UJ
O
IU 45
in
o 30
UJ
0 25
z
UJ
0. 20
J I
FEED RATE
200 gpm
250 gpm
300 gpm
350 gpm
400 gpm
: PRIMARY DIGESTED
SLUDGE fl> 3.8-4.0% S.S.
MACHINE SIZE: 36" X 96"
BOWL SPEED • 1300 rpm
I I I I I I I I I I
2.0 3.0 4.0 5.0
POOL DEPTH, inches
6.0
7.0
Figure 2: Relationship Between Suspended Solids Recovery and
Pool Depth in a Horizontal Scroll Centrifuge.
use as conditioning agents. For the polymers
investigated the centrifuge feed rate was
maintained at 250 gpm, with the bowl speed held
constant at 1300 rpm. For each polymer dosage the
pool depth of the centrifuge was adjusted to obtain
the maximum solids recovery. Shown in Figure 3
are the results of the evaluation for two typical
polymers. While the performance of each polymer
tested was slightly different from that of the
others, it was generally concluded that to obtain a
centrate containing a suspended solids centration
of 1,500 mg/1 or less (96 percent recovery), a
polymer dosage of approximately ten Ibs/ton was
required. Corresponding cake solids ranged from 19
to 23 percent by weight. However, the
performance of the centrifuge with polymer
conditioning was erratic, At times it was not
possible to duplicate the performance shown in
Figure 3. This inconsistency in performance was
thought to be the result of day-to-day variations in
the characteristics of the digested sludge which
interfered with and partially negated the activity of
the polymer.
With regard to gravity thickened thermally-
conditioned digested sludge, a six inch diameter
pilot horizontal scroll centrifuge was used to
evaluate its dewatering properties. It was
determined that when the solids in the underflow
from the thickener were dosed with approximately
three Ibs/ton of polymer and centrifuged, a solids
recovery approximating 98 percent was achieved
with corresponding cake solids of 25 percent.
Blending this centrate with the supernatant from
the thickener resulted in a combined suspended
solids concentration of 3600 mg/1. Without
polymer addition the recovery was 80 percent,
while cake solids remained at 25 percent by weight.
Centrifugation: hnperforate Basket
An imperforate bowl basket centrifuge operates
in a batch manner, using the same principles as a
scroll centrifuge with the exception that cake
removal is intermittent, not continuous. A basket
centrifuge rotates around a vertical axis while scroll
centrifuges generally operate in a horizontal
position. Flow enters the machine at the bottom
and is directed toward the outer wall of the basket.
Cake continually builds up within the basket until
the quality of the centrate, which overflows a weir
at the top of the unit, begins to deteriorate. At that
point, feed to the unit is stopped, the machine
FEEDRATE
PRIMARY DIGESTED
SLUDGE ® 3.8-4.0% S.S.
250 gpm
MACHINE SIZE i 36" X 96" ffl MAXIMUM
POOL DEPTH
BOWL SPEED : 1300 rpm
CAKE
POLYMER SOLIDS
16-19
18-23
I I
(O 0 2 4 6 8 10 12 14 16 18 20 22 24
POLYMER DOSAGE, Ibs/ton
Figure 3: Horizontal Scroll Centrifuge Performance with
Polymer Conditioning.
decelerates, and a nozzle-skimmer apparatus
removes the liquid layer remaining in the unit. The
skimmed contents are discharged through a hose.
This is accomplished while the centrifuge is
running at full speed. The skimmer is then
retracted and the bowl is decelerated to a very slow
speed whereupon the remaining dryer cake is
peeled from the wall with a large bladed knife. The
knifed contents fall through open quadrants at the
bottom of the basket for conveyance to a discharge
point. Upon retraction of the knife, the solids
discharge cycle is completed. The bowl is
reaccelerated to full speed and the feed cycle
reinitiated. The buildup of solids in the bowl
during the the feed cycle is such that those solids
closest to the bowl wall contain the least amount of
moisture, with the moisture content increasing
towards the center of the basket.
The machine was evaluated as a second stage
system to remove the solids remaining in the
centrate from the existing horizontal scroll
centrifuges. Initially a machine having a 30 inch
diameter was examined. The unit was operated at a
bowl speed to produce a G-force of 1300 at the
outer wall of the bowl. With the feed rate varied
from 15 to 50 gpm the maximum suspended solids
-------
DEWATERING 71
recovery without sludge conditioning was
approximately 80 percent. Using several of the
cationic polymers that had produced satisfactory
results in the horizontal scroll centrifuge
evaluation, it was shown that the necessary
suspended solids recovery of 95 percent could be
achieved with a basket unit. However, to obtain
more accurate data for full scale projections it was
decided to continue the evaluation with a 40 inch
diameter unit because this unit possessed most of
the features of a full scale machine. Like the 30 inch
unit the 40 inch machine had a G-force of 1300 at
the bowl wall. In the operation of the unit the feed
rate was varied from 20 to 60 gpm. This resulted in
feed cycles ranging from 10 to 30 minutes. For all of
the cycles approximately three minutes were
needed to skim the liquid layer and knife the
remaining cake solids in the bowl. With regard to
the feed rate to the basket and its effect upon
suspended solids recovery, the results can be seen
in Figure 4. For the data shown the suspended
solids feed concentration ranged from 2.5 to 3.0
percent, and to normalize this variation and that of
the hydraulic feed rate, a mass feed rate in Ib/hr was
utilized. Figure 4 indicates that without sludge
conditioning the suspended solids recovery was
below 80 percent and decreased noticeably with an
increasing feed rate. However, as can also be seen,
with a cationic polymer dosage in the range of two
to three Ibs/ton it was possible to achieve a
suspended solids recovery of approximately 96
percent, which resulted in a centrate suspended
solids concentration of approximately 1500 mg/1. It
is also of note that contrasted with the response
achieved with no polymer addition, the suspended
solids recovery of the polymer conditioned sludge
was not noticeably affected by the range of feed
rates. The effect of the mass feed rate on the
composite cake solids concentration was shown in
Figure 5. It can be seen that increasing the mass
feed rate resulted in a decreased cake solids.
However, for a given feed rate increasing the
polymer dosage from one Ib/ton to four Ibs/ton
resulted in an increased cake solids concentration.
Polymer dosages in excess of four Ibs/ton did little
to increase the cake solids concentration. At a
dosage of four Ibs/ton cake solids of 20 to 22 percent
were obtained, with a corresponding suspended
solids recovery in excess of 95 percent, resulting in
a centrate suspended solids concentration of
approximately 1500 mg/1.
In summary, it was determined that a series
system combining the existing horizontal scroll
centrifuges as a first stage, and basket machines as
the second stage, would produce a composite cake
o
o
UJ
70
" so
en
9 50
_l
o
40
30
WITH CATIONIC POLYMER ADDITION 2-3lb/ton
-NO SLUDGE CONDITIONING
FEED : HORIZ. SCROLL CENTRATE
<3> 2.5 - 3.0 % S.S.
MACHINE DIAMETER: 40"
BOWL SPEED : 1540 rpm (1300 G's)
CATIONIC
J I | I
POLYMER
I
I
100
200 300 400 500 600
MASS FEED RATE, Ib/hr •
Figure 4: Effect of Polymer Conditioning on Solids Recovery in
an Imperforate Bowl Basket Centrifuge.
of 25 percent solids by weight and a centrate
suspended solids concentration of 1500 mg/1. This
could be accomplished with a cationic polymer
dosage of four Ibs/ton to the basket centrifuges,
and no sludge conditioning for the horizontal scroll
units.
Pressure Filtration
The majority of the dewatering research
conducted on the pressure filter used digested
sludge as feed, because very early in the evaluation
it was discovered that pressure filter dewatering of
the centrate from the existing horizontal scroll
centrifuges was not practicable. This was so
because of the extremely wet cakes that were
produced. It was felt that these wet cakes were
mainly caused by the fine nature and low
concentration of the suspended material in the
centrate. Pressure filtration of digested sludge
could not be accomplished without some form of
conditioning. Therefore, the performance of the
pressure filter was assessed on sludges conditioned
by either chemicals (lime and ferric chloride),
polymers, flyash or heat. All attempts to dewater
polymer conditioned sludge proved to be totally
unsuccessful due to rapid blinding of the filter
media. Consequently, further evaluation of this
type of conditioning was discontinued. An attempt
was also made to thicken the digested sludge with
polymers as a prelude to chemical conditioning in
the hope that lower chemical requirements would
/esult. However, such was not found to be the case.
The pilot unit was basically comprised of a sludge
conditioning tank, two pressure tanks, an air
compressor, a sludge transfer pump, and the
pressure filter. The filter had a total filter area of 18
sq.ft., with the filter cloth being constructed of a
monofilament polypropylene material. Wire mesh
screens were used as a backing for the filter media.
-------
72 MUNICIPAL SLUDGE MANAGEMENT
W
O
CL
FEED iHORIZ SCROLL CENTRATE
ffl 2.5 - 3.0 % S.S.
MACHINE DIAMETER: 40°
BOWL SPEED : 1540 rpm (1300 G's)
SOLIDS RECOVERY: 95%
POLYMER :CATIONIC
I I I I I I
0 100 aOO 300 400 500 600 TOO BOO
MASS FEED RATE,lb/hr
Figure 5: Effect of Polymer Conditioning on Cake Solids in an
Imperforate Bowl Basket Centrifuge.
The operation of the unit was in a batch or cyclic
mode and consisted of: (1) precoating of the filter
surfaces, (2) sludge conditioning, (3) feed cycle
(pressurization of the system), and (4) depressur-
ization and cake discharge. The independent
variables which control the operation of a pressure
filter are the type of sludge conditioning, type of
precoat, feed cycle time and feed pressure. All of
these variables exert some influence on cake
dryness and filtrate suspended solids. Precoat of
the filter is necessary to prevent blinding and
insure that the cake can discharge cleanly.
Diatomaceous earth and flyash are two materials
which are suitable for this purpose. In this work,
the type and amount of precoat was kept constant
for each form of sludge conditioning studied. When
the sludge was conditioned by chemicals or heat,
diatomaceous earth was used for the precoat; with
ash conditioning, ash was used for the precoat.
With sufficient conditioning the filtrate usually
contained less than 100 mg/1 of suspended solids.
Consequently, filtrate quality was not a major
concern in evaluating the pressure filter or in
determining the operational criteria for its
operation.
The majority of the pilot plant testing was done
using lime and ferric chloride as conditioning
agents. Various dosage combinations were in-
vestigated along with different feed cycles.
Optimum results obtained by analysis of all the
data indicated that a 40 percent cake could be
produced with a two-hour feed cycle and chemical
dosages of 500 Ibs. of lime/ton, and 120 Ibs. of ferric
chloride/ton. However, because of the high
chemical requirements, almost one-third of the
solids in the cake was attributable to the condition-
ing agents. For these conditions the overall solids
loading rate to the filter was 0.7 lbs/hr/ft2.
Flyash conditioning was investigated as an
alternative to chemical conditioning. The use of
flyash is dependent upon incineration of the
produced cake to obtain the ash conditioning
material. Initially, studies were carried out using
2000 Ibs/ton of flyash as a body feed material.
Without the use of lime, a 37 percent cake was
generated in a two-hour feed cycle; the solids
loading rate, however, was low. When 450 Ibs/ton
of lime as Ca(OH)z was also added, generated cake
dryness was increased to 47 percent solids by
weight. This indicated the importance of lime
addition for raising pH of the flyash conditioned
sludge. Tests were run to determine the effects of
increasing the ash dosage to 3000 to 4000 Ibs/ton.
For runs under similar conditions, conditioning
with 4000 Ibs/ton of flyash produced a dryer cake
than with the lower ash dosage. At the higher ash
dosage, an increase in the feed cycle time effected
an increase in cake dryness. As noted, a small
amount of lime was used to raise the pH of the
conditioned sludge and induce coagulation.
Following a one-hour feed cycle, a discharged cake
of 43 percent solids by weight was generated.
Increasing the feed cycle to three hours served only
to increase cake dryness slightly. A corresponding
reduction in solids loading was also effected. While
the resulting cake was about 50 percent solids by
weight, consideration was also given to the fact
that two-thirds of the solids were recycled ash.
Further analysis revealed that the ratio of water to
sludge solids in that cake was the same as that
optimally obtained with chemical conditioning.
With .regards to heat conditioning, the
dewatering characteristics of the pressure filter on
both thickened and unthickened thermally-
conditioned digested sludge were examined. The
results indicated that thickening was required prior
to filtration; however, no additional sludge
conditioning was required, excepting the
diatomaceous earth precoat for the filter. The
optimum filter operation resulted in a filtrate
suspended solids of less than 100 mg/1, a cake solids
concentration of 38 percent, with a two-hour feed
cycle. When the filtrate was combined with the
supernatant from the gravity thickener the final
effluent had a suspended solids concentration of
less than 3,000 mg/1.
Vacuum Filtration
The pilot vacuum filtration studies encompassed
an evaluation of a rotary drum coil filter and a
rotary drum cloth belt filter. With either unit
attempts to dewater the centrate from the existing
horizontal scroll machines were completely
unsuccessful. The failure was due to the large
percentage of fine material in the centrate, which
-------
DEWATERING 73
resulted in the lack of significant cake buildup on
either the coil or cloth units. To alleviate the
situation, polymers and chemicals were used to
condition the centrate. However, the improvement
in cake formation was slight and certainly not
enough to merit further investigations. It was also
not possible to dewater the primary digested sludge
in any of the units without incorporating some
form of conditioning. For both types of units the
conditioning agents used were polymer, chemical
(ferric chloride and/or lime), and thermal
conditioning with and without intermediate
thickening.
Coil Filter
The pilot plant evaluated at the JWPCP was
equipped with a three-foot diameter drum having a
one foot wide face. This provided a total filter area
of approximately nine sq.ft. The filter employed
stainless steel coil springs arranged in a corduroy
fashion in two layers. Loading rate on the filter and
the type and amount of conditioning agent were
the major variables investigated.
For cationic polymer conditioning of digested
sludge the results obtained with the coil filter can
best be described as an all or nothing process. At
polymer dosages below five Ibs/ton solids recovery
was sparsely achieved, and when achieved the
generated cakes were thin and discharged poorly.
Between five and nine Ibs/ton the solids recovery
generally increased with increasing dosage but was
quite erratic. At polymer dosages of ten Ibs/ton
solids recovery stabilized between 90 and 98
percent; however, polymer dosages beyond this did
nothing to enhance the situation. At polymer
dosages of ten Ibs/ton the solids recovery remained
relatively unaffected by increased loading rates
up to 18 lbs/hr/ft2. Cake solids averaged
approximately 18 percent by weight and remained
within a constant range of 16 to 20 percent. With
regard to chemical conditioning, combinations of
ferric chloride and lime were used. The results
indicated that a ferric chloride dosage of 80 Ibs/ton
and a lime dosage between 500 and 600 Ibs/ton
produced the optimum suspended solids recovery.
At these conditions a solids recovery of 90 percent
was achieved, with cake solids of approximately 26
percent by weight. Dewatering of thermally
conditioned sludge on the coil filter was attempted.
Cake formation, however, was negligible. When an
intermediate thickening step was used, a dry (30
percent) filter cake was produced; the filtrate,
however, contained a suspended solids
concentration of 20,000 mg/1.
Of the three types of conditioning investigated,
polymer conditioning was the only one that allowed
effluent quality criteria to be met. Although this
type of conditioning produced the wettest cake (18
percent),it also resulted in the highest solids
recovery (95 percent).
Belt Filter
The pilot-scale belt filter was equipped with a
three-foot diameter drum having'a one-foot wide
face. Filter leaf tests conducted with six different
synthetic cloth materials enabled three to be
selected for pilot testing. In the actual pilot plant
work, best results were achieved with one belt
material regardless of the type of conditioning.
Only those results achieved with the one belt will
be discussed. In addition to the types of filter
material the other variables investigated were
loading rate and type of conditioning. Laboratory
tests revealed that cloth belt filtration of heat-
conditioned digested sludge would not be possible
without intermediate thickening. Similar tests also
revealed that digested sludge would not filter
directly unless preconditioned with at least ten
Ibs/ton of a cationic polymer or 400 Ibs/ton of lime
as Ca(OH)2.
With regard to chemical conditioning, tests using
lime as a conditioning agent were run at dosages
from 400 to 800 Ibs/ton. Acceptable filtrate quality
occurred at loading rates up to 3.0 lbs/hr/ft2;
however, the cake produced at this loading was thin
and did not readily discharge from the belt. Results
of the pilot tests indicated that optimum conditions
of filtrate quality and cake discharge were obtained
at a lime dosage of 600 Ibs/ton and a loading rate of
1.5 lbs/hr/ft2, yielding filtrate suspended solids of
200 mg/1 and cake solids of 35 percent by weight.
For polymer conditioning, a dosage of ten Ibs/ton
resulted in a filtrate suspended solids of
approximately 500 mg/1 regardless of the solids
loading rate. However, the resultant cake solids
were in all cases wet and thin and lacked adequate
discharge characteristics. Thermally conditioned,
gravity thickened digested sludge was successfully
dewatered with the belt filter. Using the underflow
from the thickener a maximum loading rate of 3.3
lbs/hr/ft2 was achieved, yielding a filtrate
containing 1,300 mg/1 of suspended solids and a
cake solids concentration of approximately 37
percent. No additional polymer or chemical
conditioning was required to achieve this
performance. Blending of the filtrate with the
thickener overflow produced an effluent that
contained approximately 3,200 mg/1 of suspended
solids.
In general, it can be said that the belt filter was
able to capture a majority of the suspended material
from digested sludge conditioned thermally,
-------
74 MUNICIPAL SLUDGE MANAGEMENT
chemically, or with polymers. Cake formation,
however, differed markedly with each form of
conditioning and had a significant effect on the
loading rates required for producing a freely
discharging cake.
Composite System Evaluation
Shown in Table 1 are the dewatering systems
that met the effluent suspended solids criteria of
1500 mg/1 or less. It should be noted that for the
system comprised of thermal conditioning and
gravity thickening, followed by belt vacuum
filtration of the thickener underflow, it was felt
that a separate biological treatment plant would be
needed to reduce the high soluble BOD created
during thermal conditioning. In addition, the
effluent from the biological treatment plant would
easily meet the suspended solids criteria. It should
also be noted that for the other four systems the
final effluent BOD was less than or equal to 1000
mg/1.
In selecting the systems shown in Table 1,
consideration had to be given to the economic
feasibility and to reliability of performance. Several
combinations—in particular, pressure filtration of
thickened thermally-conditioned sludge or
horizontal scroll centrifugation of polymer
conditioned, thickened thermally treated sludge—
would likely meet the criteria if the resulting
effluent were given additional treatment.
However, such schemes were obviously
uneconomical and hence were not considered
further. With regard to performance reliability,
polymer addition to the horizontal scroll
centrifuges produced centrate that met the effluent
criteria on occasion, but because of the
nonreproducible results, this system was not
considered a viable alternative. Performance of the
basket centrifuge, the pressure filter, and the belt
and coil vacuum filters was very reliable
throughout the research project.
Economic Evaluation
Cost estimates were prepared for the systems
listed in Table 1 to provide a rationale for selecting a
full scale process. The estimates include capital and
operating costs for the five selected dewatering
systems and costs for ultimate disposal. In
preparing the estimates the quantity of digested
sludge solids used was 350 tons/day, with an
additional 40 tons/day of solids being contributed
from a future digester cleanings system. Thus a
total solids quantity of 390 tons/day, having a
suspended solids concentration of 3.8 percent was
assumed as the influent for all systems. With
regard to ultimate disposal incineration was not
considered as a viable alternative because of the
geographic limitations of the Los Angeles Basin,
and thus disposal to a sanitary landfill had to be
utilized. The paramount expenses involved in
landfill disposal are vehicles to transport the sludge,
loading facilities at the dewatering site, sludge
storage and landfill disposal fees. The moisture
TABLE 1
Summary of Performance for Dewatering Systems
System
Horizontal Scroll
and
Basket Centrifuge
Pressure Filter
Coil Vacuum Filter
Belt Vacuum Filter
Belt Vacuum Filter
Mode
of
Conditioning
Cationic Polymer
to
Basket
Ferric Chloride
Lime
Cationic Polymer
Lime
Thermal and
Gravity Thickening
Conditioning
Dosage
(Ib/ton)
4
120
600
10
600
--
Effluent
S.S.
(mg/1)
1,500
100
1,500
800
3,000b
Cake
Solids
(%)
25
40
20
35
35
Effluent"
BOD
(mg/1)
1,000
200
1,000
500
5,000b
a Estimated from a limited number of tests.
h Biological treatment of the effluent will reduce the BOD to 1.000 mg/1 and the suspended solids to less than
500 me 1.
-------
DEWATERING 75
TABLE 2
Summary of Cost Estimates for Sludge Processing Systems
Itema'b
Horizontal
Scroll +
Basket
Centrifuges
Pressure
Filter
Vacuum
Filter -
Polymer
Vacuum
Filter -
Chemical
Vacuum Filter
Thermal
<-
Thickening
Dewatering System
Capital1-$ 7,800,000 15,000,000 5,200,000 11,800,000 18,450,000
O&M-S 890,000 2,350,000 1,350,000 1,500,000 1,050,000
Annual
Cost-$/ton 13.80 31.00 14.50 22.00 24.90
Hauling System
Wet Tonnage - tons/day 1,560 1,300 1,950 1,500 860
Capital-$ 5,000,000 4,400,000 6,400,000 4,900,000 3,150,000
O&M-S 1,800,000 1,550,000 2,300,000 1,750,000 1,100,000
Annual
Cost-$/ton 22.60 20.30 28.90 22.00 14.00
Total System
Capital - S
Annual
Cost - $/yr.
Annual
Cost - $/ton
12,800,000
5,180,000
36.40
19,400,000
7,300,000
51.30
11,600,000
6,190,000
43.40
16,700,000
6,270,000
44.00
21,600,000
5,550,000
38.90
aQuantity of sludge to be dewatered 390 tons/day. Disposal by truck hauling to a sanitary landfill located 30 miles
from JWPCP.
''Capital Costs amortized at 6% for 10 years, excepting trucks and other vehicular equipment 6% for 3.4 years.
content of the dewatered sludge was considered a
direct function of all these costs.
A summary of the capital, operation and
maintenance, and yearly cost for all of the systems
is shown in Table 2. To provide for an effective
economic comparison between each system,
common cost factors for hourly labor rates, power
and fuel were used. All capital costs, excepting
trucks and other necessary vehicular equipment,
were amortized over a ten year period. This was
deemed necessary because of the state of flux
regarding standards on effluents discharged to the
ocean, and thus it was felt that the dewatering
system selected might only have a useful life of ten
years. As indicated in Table 2, the unit costs of the
dewatering systems ranged from less than $14/ton
to $31/ton, with the lowest cost system being two
stage centrifugation. One of the economic
advantages of this system was the ability to utilize
the existing horizontal scroll centrifuges. In this
regard it should also be noted that the capital cost
shown for the thermal conditioning—vacuum
filtration system included a 2 mgd biological
treatment plant. This was needed to provide
acceptable limits for the suspended solids and BOD
concentration of the system filtrate. With regard to
the ultimate disposal systems, it can be seen that
the annual costs ranged from $14/ton to ap-
proximately $29/ton, and that the costs varied
directly with the total quantity of wet sludge to be
hauled. The system producing the lowest amount
of wet sludge was thermal conditioning and
thickening followed by vacuum filtration. Thermal
conditioning solubilizes a portion of the digested
sludge solids and this fact, coupled with the
relatively dry cake obtained when filtering the
thickened conditioned sludge (35 percent solids),
results in an appreciably lower quantity of sludge
for disposal than the remaining systems.
When considering the combined costs for
dewatering and ultimate disposal, the results as
shown in Table 2 indicate that the two stage
centrifugation system has the lowest overall cost.
Based on the total system cost, this system was
chosen as that to be implemented at the JWPCP.
The economic advantage of this system was
certainly influenced by the previously mentioned
short life expectancy of the system and the present
existence of the first stage horizontal scroll units.
From the viewpoint of intangible benefits, the
JWPCP treatment plant staff has over the years
gained valuable knowledge regarding the operation
and maintenance of centrifuges and the choice of
the two stage system allows for the continued use
-------
76 MUNICIPAL SLUDGE MANAGEMENT
of this knowledge. In addition, the utilization of the
existing horizontal scroll centrifuges assured a
continuation of the present use of the sludge as a
soil conditioner. The other alternative dewatering
systems would have produced sludges much finer
in particle size distribution, and in the case of lime
conditioning more alkaline, and these properties
could certainly have presented problems in their
use as effective soil conditioner.
SUMMARY
Pilot plant studies conducted at the JWPCP
ascertained that five conditioning-dewatering
systems were capable of meeting the established
criteria of 95 percent solids recovery from an
anaerobically digested sludge. Based on a solids
quantity of 390 tons/day, an economic evaluation of
each alternative was made the result of which was
the selection of a two stage centrifugation system.
The system utilized existing horizontal scroll
centrifuges followed by imperforate bowl basket
centrifuges. Cationic polymers at a dosage of 4
Ib/ton were added to the influent of the second
stage unit. The composite cake from both units will
be hauled by truck to a sanitary landfill for ultimate
disposal. The total system cost was estimated to be
$36.40/ton.
-------
PRESSURE FILTRATION—MUNICIPAL
WASTEWATER SOLIDS,
CEDAR RAPIDS, IOWA
JAMES W. GERLICH
Howard R. Green Company
Cedar Rapids, Iowa
ABSTRACT
From pilot plant studies of numerous
dewatering processes, Cedar Rapids selected the
pressure filter and constructed the first major
installation in the United States. Pilot dewatering
studies indicated that fly ash was an effective filter
aid reducing chemical conditioning costs from 20 to
4 dollars per ton dry solids. The full scale plant
utilizes sludge cake incinerator ash as a condition-
ing agent.
A detailed nine month plant evaluation indicated
the full scale plant exceeded performance of the
pilot plant. Economic evaluations were made of
operation and equipment. Some conclusions:
1. Pressure filtration of wastewater sludges is
an effective and economical process.
2. Ash filter aid increases dewatering production
and decrease chemical costs. Incinerated
sewage sludge ash can be recycled as a filter
aid. Power plant (coal) fly ash is most effec-
tive sludge conditioner.
3. A detailed pilot plant program is of great value
in design of a full scale plant.
4. Some chemicals in combination with ash filter
aid further improves dewatering efficiencies.
5. Technology and experience is extremely
limited in the field of pressure filtration of
wastewater solids.
INTRODUCTION
Cedar Rapids has used pilot plant studies to
obtain design data for sludge dewatering facilities.
Detailed pilot plant studies were conducted with
several dewatering processes and the pressure
filter system was selected as the most economical
process. Test data compiled during the pilot scale
pressure filter program was interpreted as a basis
of design for a full scale pressure filter plant. This
test data from the pilot plant at that time
represented nearly all of the analytical data
available on the operation of the pressure filter in
the United States.
During the course of the dewatering studies it
was observed that fly ash was an effective filter aid
which cut chemical conditioning costs from about
20 to 4 dollars per ton dry solids. Economic
evaluations of handling power plant fly ash were
less favorable than those of on-site incinerated
sludge ash and therefore multiple hearth sludge
cake incineration with recycled sludge ash was
constructed. Early information from Europe
indicated that sludge ash due to its irregular and
fine particle shape was a better sludge conditioner
than fly ash. Field operation on both sludge ash and
fly ash proved this to be incorrect.
For the application at Cedar Rapids, the pressure
filter and the multiple hearth incinerator was the
most economical combination of dewatering
incineration processes. Evaluating capital
investment and fuel operating costs, this
combination was estimated to be 10.3 percent less
cost per ton of dry sewage solids dewatered and
incinerated over the vacuum filter-incinerator
'combination. Other dewatering systems
considered were even less favorable in cost
comparison. Some other factors favoring the
pressure filter selection are: less building space, less
operator attendance, closed system with fewer
odor problems, drier cake, greater solids capture,
77
-------
78 MUNICIPAL SLUDGE MANAGEMENT
clear filtrate, lower power requirements,
expandable capacity of filter unit.
The full scale pressure filter sludge dewatering
facility was designed on the basis of experiences
with the pilot scale pressure filter and with the
expectations that the performance of the pressure
filter could be improved. It was planned to have the
pressure filter and accessories as completely
automated as was practical and to monitor and
control most functions from a central console.
Summary of Design Data
Total Solids
Volatile Solids
Percent Solids in Sludge
Solids Source
Digester Solids
56,000 Ibs/day
26,400 Ibs/day
5.5
Primary and 2-Stage
Trickling Filters
Pressure Filler
Two units each 83 plates, 64 inch, 207 cu. ft.
3400 sq. ft., expandable to 100 plates.
Pressure Filter Loadings
Ash/Sludge Ratio 1.5:1
Filtration Time per Cycle Hr. 1.5
Total Cycle Time-Hours 2.0
Total Filtration Time Hr/Day 16
Cake Moisture-Percent 52
Yield Filter Cake-Lbs/
Sq.Ft./Hour 0.75
Filtrate Suspended Solids Nil
Chemical Required None
Other Plant Data
Population:
Present
20 Year Future
BOD Equivalent
Plant Flow:
Average, MOD
Wet Weather, MGD
Construction Sludge
Dewatering Completed:
1:1
1.0
1.25
16.7
60
0.65
Nil
None
113,000
172,000
810,000
28.6
64.4
1972
General Description of the Pressure Filter
Process
Solids are periodically pumped from the
secondary digesters to the solids holding tanks.
Bottom solids from the sludge holding tanks are
removed by pump on a demand basis and sent
through preconditioning facilities to the pressure
filters where the sludge is dewatered.
The sludge preconditioning facilities consist of
sludge grinders, mix tanks where ash and/or
chemicals are added as a filter aid, contact tanks
where slow mix of filter aids and flocculation occur,
and variable rate sludge pumps to feed the filter.
The pressure filters consist of a series of plates
covered with nylon, or similar, filter cloth. Sludge is
pumped through the filter leaving a solids
deposition on the filter cloth. This solids formation,
or filter cake, is periodically discharged at the end of
the filter cycle. The filter cycle may be one to two
hours depending upon numerous variables.
Cake discharged from pressure filter is broken
and then fed to the multiple hearth furnace for
incineration. Filtrate removed from the solids in the
pressure filter is discharged back through the plant
for further treatment.
Details of a typical pressure filter plate are shown
in a cross-section view in Figure 1. Conditioned
sludge is pumped to the filter by variable speed,
variable capacity, variable pressure pumps,
automatically controlled to decrease the output
capacity as the pumping head increases until it
reaches a maximum stall pressure of approximately
225 psi. Prior to beginning of the filter cycle, the
pressure filter is precoated to protect the filter cloth
from blinding or clogging due to possible grease
content or fine particles, and to form a shear plane
between the cloth and cake to assure free and clean
discharge of the cake. Filter precoat is a mixture of
ash carried to the filter cloth with recycled filtrate
water. Immediately after precoat, the sludge feed to
the pressure filter is automatically applied through
motorized control valves to begin the filtration
cycle. The duration of the filtration cycle may be
determined by time, pressure, or rate of filtrate
flow. Usual operation will be controlled by rate of
filtrate wherein the filtration continues until a
predetermined rate of filtrate flow is observed
across the V-notch weir. At the end of the filtration
cycle the filter core feeding the individual filter
plates remains filled with wet, soft sludge slurry.
This soft core is blown out using air prior to
opening the filter to discharge the cake. The filter
cake is discharged to the bunker below the pressure
filter and is sheared into smaller particles as it
passes shear cables stretched across the opening
below the filter. From the cake bunker it is
conveyed to the multiple hearth incinerator and the
incinerated ash is recycled to process. The process
schematic and photographs are shown in Figure 2.
Study Results
Digested sludge at Cedar Rapids is unusually
difficult to dewater. It became apparent early in the
operation of the full scale plant that to assure
reasonable operation on a continuous basis, a
-------
PRESSURE FILTRATION 79
Figure 1: Cross Section View and Flow Diagram of Plate Section.
sludge conditioning monitor was necessary.
During the pilot plant operation sludge feed was
conditioned on a batch basis with admixtures
carefully predetermined. Filterability of a sludge
can be defined as the ease at which the sludge gives
up water. It is desirable to be capable of objectively
'describing a numerical value to a sludge to give us a
meaningful value for operation. Specific resistance
has proved valuable at Cedar Rapids as a sludge
conditioning monitor in a day-to-day operation. By
using the specific resistance, an operator can:
1. Determine, whether a sludge is conditioned
properly.
2. Evaluate and optimize new methods, of
conditioning such as organic polymers.
3. Evaluate the filterability of the unconditioned
sludge.
The resistance meter is essentially a pressurized
Buchner funnel and is simple to operate by the
attending personnel. Specific resistance becomes
meaningful when it becomes known what value a
sludge must have to filter well. Unconditioned
sludge may have a value greater than 366 (366 x
1012cm-2). Generally speaking, at Cedar Rapids a
conditioned sludge must have a specific resistance
of less than ten to filter well. Knowing this limit, we
can optimize sludge conditioning (ash and
chemicals) using the resistance meter or bench
filter much more easily and rapidly than by a trial
and error method on the full scale filter.
Filter Performance
The design data for the pressure filter was
originally developed using fly ash for sludge.
conditioning. The full scale plant used sludge ash
recycled from the incinerator. Soon after the plant
was on line it became apparent there was a lack of
correlation between the pilot plant study and the
performan.ee of the full scale filter, and that the full
scale plant could not successfully operate at any
reasonable sludge/ash ratio without using ferric
chloride and lime. Consequently a program and
study was set up comparing the two different
ashes. Figure 3 indicates the comparative
filterability between sludge/ash and fly ash
conditioning. This data indicates that fly ash is 2.5
to 3.0 times more effective for conditioning
digested sludge per unit weight of fly ash than
sludge ash. Additional comparisons have been
made using fly ash and sludge ash for conditioning
other sludges including raw primary sludge,.
straight domestic sludges, other complex industrial
wastes-domestic sludges. In all cases it has been
seen that fly ash was consistently better than
sludge ash. Studies by others have found a
difference between sludge ash and fly ash, but in
reverse order, therefore it appears that the fly ash
and possibly the sludge ash in Cedar Rapids are
unique, and results cannot be directly applied to
other cities. Fly ash and sludge ash can differ in
three fundamental ways: chemically, by size and by
shape. Chemically, fly ash and sludge ash are quite
dissimilar. Fly ash is approximately 50 percent silica
and sludge ash is approximately 50 percent calcium
oxide. Other components, such as ferric oxide and
alumina are present in different quantities. The
effect of these higher levels of iron and aluminum
salts in fly ash is not known, and further research is
required. Fly ash has a smaller particle size than
does sludge ash. Ash fractions of smaller size<45M,
are significantly far more effective for sludge
conditioning than ash of larger size, <45M-
Concern has been expressed that in sludge
filtration processes using recycled incinerated
sludge ash as a filter aid, the recycling of ash
through the incinerator may result in a shift of ash
particles to the smaller particle sizes. This concern
is based on the premise that the fine fraction of ash
is not effective in sludge conditioning and may in
effect be deleterious. Ash samples were collected
and stored and later extensive tests using the same
-------
80 MUNICIPAL SLUDGE MANAGEMENT
SCHEMATIC SLUDGE OEWATERING a i NCI NE RAT ION
Figure 2: Proposed Layout.
-------
PRESSURE FILTRATION 81
zoo
rILTER PERFORMANCE
SLUDGE ASH
5 -
~
DIGESTED
\
^ i
X
SLUDGE
MAIN PLANT
"r" WITH NO ASH "308
% S = 5.5
1 l l l
^-FLYASH
\
\
i r
1.0 1.5 2.0 2.5 3.0
ASH/SLUDGE RATIO
3.5
4.0
Figure 3.
digested sludge, indicated that a decreasing ash
particle size enhances the ability of the ash to
condition the sludge. Samples conditioned with
fractions of ash below 32ju.had less filter resistance
than the original mixture and the most effective
fraction of ash was that fraction below 15 it. It
appears that the recycle of ash, with the subsequent
increases in the fraction of fines when recycled
through an incinerator, is not a problem.
Early in the operation of the pressure filter, it
also became apparent that the digested sludge could
not be conditioned satisfactorily with sludge ash
alone. After extensive testing, ferric chloride and
lime dosages were determined which are still
basically in use today. Filter performance curves
were established following the filtration rate (yield)
in terms of pounds per hour per square foot, vs.
time in hours at various sludge solids
concentrations. A typical curve for 5.5 percent
solids is shown in Figure 4. Performance indicates
that filter yield increases in almost direct
proportion to increased raw sludge solids density at
a ratio of about 1.8/1, that is, if the sludge density is
increased times 2, the resulting filter yield is
increased times 1.8.
It is apparent that properly conditioned sludge
filters satisfactorily at both low and high sludge
densities, and that yield is principally influenced by
time to completely fill the cavity forming the
predetermined cake volume. In practice it became
apparent that the limiting factor to improving yield
was the ability to pump conditioned solids to the
pressure filter in sufficient capacity to develop the
maximum rate of filtration. Sludge cake is
discharged from the pressure filter and is about 58
^
A/S RATIO
O 1:1 Log
A 1.5:1 Log
^
l""*ll"'>«» A *•
/ t
^1.511
YIELD = -0.1571
|0 YIELD =-01803
SOLIDS= 5.5%
— s
k« ^**~»»r
^
U***V> f~*****~^
A "V
(TIME1+0.22520
[TIMEH 0.18972
'***»«-> ^"C
~**-M
^ -^^^^
>.» C
1.5 a.o
TIME (hours)
Figure 4: Filter Performance.
inches in diameter, TVi inches thick, weighs
approximately 200 pounds. Each pressure filter
produces 83 cakes per cycle, and at normal
operation cake moisture content is in the range of
36-38 percent and the appearance is dense, dry and
textured. The specific weight of the discharge filter
cake varies from 107 to 114 pounds per cubic foot
dependent upon the moisture content and the ash
ratio. Cake discharged from the filter to the cake
storage bunker is sheared by a series of cables
which tends to bulk the broken cake. Bulked filter
cake was determined to be approximately 47
pounds per cubic foot, but this was for cake not
exposed to the impact and compressive forces due
to dropping. Cake discharged from the bottom of
the hopper under compressed conditions of
superimposed loads of ten to twelve feet depth had
a specific weight of 83 pounds per cubic foot.
Conditioned sludge applied to the pressure filter
forms a cake structure which almost immediately
serves as the principal filter media. The filter cloth
has served only as the base structure for this
development, and after the cake has formed to 1/16
or 1/8 inch thickness, the influence of the filter
cloth is negligible for continued filter performance.
As the cake continues to form, the filter void is
filled, the total resistance increases rapidly
developing full pressure differential in possibly 30-
40 minutes. Continued full pressure differential of
about 225 psi is desirable to assure most of the free
water has the time to travel through the cake and
underdrainage system to be disposed of as filtrate.
The noteworthy point after the development of the
full cake formation is that prolonged high pressure
does very little to further dewater the cake.
For a seven month period the filtrate averaged 74
mg/1 suspended solids while the sludge solids.
-------
82 MUNICIPAL SLUDGE MANAGEMENT
averaged 4.60 percent solids giving a removal of
suspended solids of 99.99984 percent. The BOD
and COD of the filtrate are due primarily to
dissolved organics such as volatile acids and are not
necessarily affected by the filtering process.
Dewatering Raw Primary Sludge
In-plant arrangements were made to dewater
raw primary sludge on a test basis.. The filterability
of raw sludge is generally much better than
digested sludge at Cedar Rapids. Better
performance was maintained at lower ash/sludge
ratios with a 1:1 ash/sludge ratio valid for all solids
concentrations. Pressure filter cake averaged 54-
56-58 percent solids for filter feed solids of 5-6-8
percent respectively. Volatile solids content was
much higher providing good incinerator feed and
no difficulty was experienced in conveying the raw
sludge filter cake. Filtrate suspended solids showed
no basic difference from digested sludge, averaging
79 mg/1; however, a significant increase occurred
in BOD and COD values. The mean value of filter
runs are:
Digested: COD mg/1 510; BODs mg/1 300;
BODs/COD (%) 56
Raw: COD mg/1 7080; BODs mg/15700;
BODs/COD (%) 81
Process Evaluation
Labor and chemical costs were based on actual
costs in 1972, including labor benefits. Hourly rates
were about $4.50 ± for different classifications of
operators. Management and laboratory costs are
not included. The dewatering facilities were bid in
December 1968, and all construction costs have
been interpreted to 1972 increased costs. All costs
were evaluated to filtration capacity regardless of
the operating capacities experienced at any given
time during the study.
The total cost for pressure filtration is composed
of four separate costs. These costs are labor for
maintenance and operation, power, chemicals for
conditioning, and capital investment, see Table 1.
Labor costs were determined, as described
before, to be $3.82 per hour filtration. Power costs
were determined in the same manner to be $0.80
per hour filtration. Likewise capital costs were
determined to be $9.55 per hour filtration. In the
compilation of the data, each cost was deter-
mined as:
Cost/Ton cycle time (hours) x cost factor
tons dry solids/filter
Cycle time represents the time to complete a
filter run. Added to filtration time, approximately
0.4 hour is required to discharge the cake, refill the
filter with filtrate, and precoat, and is referred to as
turn-around time.
Cycle time (hours) = filtration time (hours) 4
'0.4 hours turn-around
The cost factor represents the dollar per hour
value placed upon each cost.
Chemical costs were determined for each cycle by
measurement of actual quantities of lime and ferric
chloride used to condition the sludge.
Figure 5 shows cost data for 5.5 percent feed
solids. Costs for other solids concentration varied
as follows:
% Total Solids Feed 4.5% 5.5% 6.5%
Operating $/Ton 5.83 4.69 3.83
Capital 12.05 9.71 7.91
Total(including chemical) 26.83 21.69 18.20
Having operated the pressure filter pilot plant at
an early date when no analytical data was available
for evaluation and guidance, and having developed
the design criteria for a full scale plant from pilot
studies, Cedar Rapids has observed the process
performance without bias to other installation
performances. The full scale plant process is highly
automated and therefore reflected some of the
malfunctions associated with equipment failure
from numerous interrelated units. Some
unsatisfactory process performance was
experienced during the early days of start-up and
check-out. Much of this was related to inadequate
training of the operating personnel, particularly
those of the equipment manufacturer. It is
understandable that the first major installation
would offer some degree of challenge on the start-
up and that experience would make the next one
easier. A general evaluation of the pressure filter
process confirms the performance predicted by the
pilot plant study, and that the process offers
distinct advantages over other forms of sludge
dewatering based upon Cedar Rapids' sludge.
Performance can be maintained over a wide range
of feed sludge concentrations, with particular note
in the low solids level ranging to 2.5 percent.
Proper precoat is essential to satisfactory filter
performance and precoat pressure is an important
control for assuring good cake formation. Normal
precoat pressure for clean filter media starts at
about 25 psig and ranges upward to 40 psig. Above
this operating range it appears that gradual
deterioration of filter performance often occurs To
-------
PRESSURE FILTRATION 83
TABLE 1
Costs for Pressure Filtration
Man-Hours/Hour Filtration
$ I Hour Filtration
Operation Foreman
Assistant
Maintenance Foreman
Assistant
0.90
0.33
0.17
0.33
1.23
$4.13
1.37
0.76
1.37
$5.50
2.13
$7.63
$3.82/hour filtration
0.80/hour filtration
0.065/pound
0.0114/pound
Labor costs per filter (* 2 units)
Power costs 40 KWH/hour filtration x $0.02
Chemical Costs: Ferric Chloride $130/ton
Lime 22/ton
Capital Costs: 20 years at 4!4%, 5'/4days/week - 286 working days/yr.
Building $ 417,000
Equipment 1,255,000
$ 1,672,000 = $ 19.10/ hour * 2 units
$ 9.55/hour filtration
PERCENT OF TOTAL SOLIDS
Q
_J
UJ
.D
,4
,2
.0
,8
.6
.4
,2
0
(
\ <
\
\
\
\
OPER
rn^T
5
I
\
\
\
* vc
\
ATING *£
c
\
\
CAPIT
COST:
D
\
AL ^[
7
\x
V*»
:_/
^^r
L
x— ».<
^~- »-^
TOTA 1
"1
^•••^ — ^i
k.
COSTS 1
^--t
MCL. CHE
WICAL CO
: :
?'^H!s:^
PT^
a
D 5 1015 20 25 30 35 40 45 5
COSTS (tt/Ton.)
Figure 5.
operate within the precoat pressure range it is
necessary to have properly conditioned raw sludge
applied to the filter and to have had the proper
quantity of ash precoat. Improperly conditioned
raw feed sludge will cause some buildup on the
media and thus gradually raise the successive
precoat pressures. Often this problem can be
corrected by dropping the quantity of precoat from
150 to 50 pounds, thus forming a new cleavage
plane encouraging the discharge cake to remove
some accumulated precoat. Improper quantities of
precoat may cause a gradual buildup whereby the
excess ash creates a shear plane distant from the
filter medias so that when the cake breaks away
excess precoat remains on the media. Operating
experience which had reduced the precoat:
pressure, or at least prolonged the need for a
complete filter wash is that of extending filter fill
cycles. During the fill cycle, filtrate water is
pumped into the filter and by continuing
recirculation of the overflow it is sometimes
possible to perform some degree of washing. This
practice is not totally successful but it may reduce
the precoat pressure by 10 psig which may extend
the need for a complete filter wash for a few days.
Estimated filter runs between media washes is
about 100 runs, or 150 to 200 filter hours of
operation. The method of washing a pressure filter
could stand much improvement. The wash rod
originally furnished with a series of high pressure
-------
84 MUNICIPAL SLUDGE MANAGEMENT
nozzles is no longer used. A more successful system
has been a single high pressure nozzle with a broad
discharge operating at about 750 pounds pressure.
Commercial grade detergent is used in the wash-
ing solution.
Cake and incinerated ash grinders were
originally installed because the German process
recommendations stressed this requirement.
Experience at Cedar Rapids confirmed that cake
grinding was undesirable and that incinerated ash
grinding was unnecessary. Cake passing through
the grinder tended to knead to a plastic-like ball and
under high moisture content of about 55 percent
into the grinder, came out more as an extrudable
paste.
Ash feeders used at Cedar Rapids are of the
gravimetric type. These were installed for accurate
proportioning of ash. Feeder problems .are
associated with both the unpredictable fluidizing
and compaction characteristics of ash in storage. It
is difficult to cover both extreme properties of ash,
and experience would suggest the most simple
feeder, such as a volumetric feeder to be more
trouble-free.
Cake handling facilities were the cause of
many problems, primarily because no experience
had existed in discharging filter cake to bunker
storage. The bulk density of broken filter cake in
the bunker was greatly underestimated at about 48
pounds per cubic foot which is reasonable under
normal conditions. However, dropping 200 pound
cakes into bunker storage recompacted the mass to
about 83 pounds per cubic foot at the drag conveyor
discharge.
Filter feed pumps are hydraulic driven ram
pumps having a variable capacity, variable head
characteristic. Pumps having these characteristics
are desirable as the filter cycle pressure develops
and the input diminishes, however, a more suitable
primary pump should be provided to meet the early
demands of the filter, particularly on a filter
installation as large as Cedar Rapids. Considerable
experimentation was carried out by the plant
operating personnel wherein filter performance
was greatly improved by placing all four filter feed
pumps on one filter rather than the normal two
units. Prolonged slow feed rates to the pressure
filter to form satisfactory cake development is
uncertain from our observations with sewage
sludges and ash precoat. Where a precoat system is
not used, it may be desirable and even necessary to
slowly develop a cake on the cloth to provide a
protective zone so that the solids are not driven into
the cloth upon increasing pressure. With a precoat
system the protective zone is already established
and through-put should be as rapid as possible.
Pressure filter plate warpage has been the major
.problem from Day One. Some of this has been
attributed to the early day of operation when the
plates were inadequately shimmed, however,
adequate shimming of the stay bosses has not
totally eliminated this problem. Warpage occurring
in the plate diaphragm transfers bending to the
plate frame which in turn accelerates plate gasket
deterioration due to warped' gasket seating plane.
To date, over 100 plates have been, or will be,
replaced due to material fatigue and failure. Most
failures have occurred as ruptures of the
diaphragm either around the stay bosses, or at the
top or bottom at the rim. It is apparent that once
structural change has occurred, no practical means
of compensating for that change has been
developed for this design and the only solution is to
replace the bent or distorted plates without further
experimentation. There are two basic causes for
plate warpage, either improper shimming or the
application of dissimilar cloth to a given plate.
Inadequate precoat may eventually lead to a
pressure differential across a plate, however, this
should be obvious before damage may occur and
corrective remedies taken.
Many of the numerous problems and failures
have been attributed to poor shop workmanship
and field service. Many modifications have been
made and after three years, they continue to be
made. Equipment and process improve with
experience of people in specific applications,
therefore, it is reasonable to assume that the
pressure filter for sewage sludge dewatering will be
improved in both design and operation.
SUMMARY
Looking back to the expectations of the pilot
plant study, and to the process performance
observed in the full scale plant, it is obvious that the
pressure filter process is dependable, economical,
and offers distinct advantages over other forms of
sludge dewatering based upon Cedar Rapids'
sludge. The process has achieved a higher quality
product with greater capacity in the full scale plant
than was predicted by the pilot plant study. Cake
quality is maintained over a broad range of sludge
solids concentrations, particularly at the low solids
level (2.5 - 4.5 percent) which were not previously
experienced in the pilot studies. It is also obvious
that with the limited pressure filtration experience
available, it would be ill-advised to design an
application without extensive pilot study, which
-------
PRESSURE FILTRATION 85
study should include a prototype pressure filter in
addition to the minimal bench scale studies. The
product of pressure filtration is an increase in solids
concentration in the cake with a decrease in
suspended solids content in the filtrate. Compared
to other systems for sludge dewatering, the
advantages for pressure filtration may be
summarized as follows:
1. The quantities of sludge conditioners are
usually reduced.
2. Filtration efficiency is maintained over a broad
range of sludge characteristics.
3. Increased solids content of the filter cake.
4. Extremely low filtrate suspended solids and
BOD.
5. Minimal operation and manpower
requirements.
6. Minimal maintenance of equipment.
7. More convenient cake disposal due to lower
moisture and smaller volumes.
8. Total costs, capital plus operation, are
competitive.
-------
HEAT TREATMENT AND INCINERATION
DALE T. MAYROSE
Dorr-Oliver Incorporated
Stamford, Connecticut
With increased regulatory agencies' re-
quirements for higher quality treatment of
municipal wastes, biological treatment, particularly
some form of the activated sludge process, has
become the most common method of treatment in
most areas of the country. As a result, municipal
wastewater treatment plants are faced with dis-
posal of a greatly increased volume of primary and
waste biological solids.
Sludge Disposal Alternates
As illustrated in Figure 1, the alternates are
digestion and dewatering versus heat treatment
and dewatering versus dewatering and combus-
tion. With digestion, there is approximately 80
cubic feet per million gallons of dewatered sludge
for final disposal—without digestion, 130 cubic feet
per million gallons. With heat treatment, there is
approximately 65 cubic feet per million gallons of
sludge for final disposal. The lesser amount of
sludge after heat treatment is due to a drier cake
than when dewatering the digested sludge. The
dewatering and combustion requires disposal of
only four cubic feet per million gallons and this is an
inert ash.
Heat treatment has been practiced in this coun-
try for the past several years. Basically, heat treat-
ment is nothing more than heating waste sludges to
approximately 400°F, and maintaining it at this
temperature for approximately 30 minutes. The
result of heat treatment will be a sludge with great-
ly improved dewaterability characteristics. The im-
proved dewaterability of the sludge will result in
drier cakes discharged from the dewatering device.
-DEGRIT ^-THICKEN »DIGEST-
-HEAT TREATMENT
• DEWATER
80 CU.FT./MG
DEWATER
65 CU.FT./MG
COMBUSTION
4 CU.FT./MG
Figure 1: Sludge Disposal Alternates.
Figure 2a, b and c will illustrate the improved
settling rate of a 50 percent mixture of raw primary
and waste activated sludge which has been heat
treated.
Results of Heat Treatment
Table 1 illustrates the comparison between
dewatering heat treated sludge and chemically con-
ditioned sludge. The sludges for these tests were
approximately 50 percent primary and 50 percent
activated. Test runs with heat treatment marked
"C" produced cakes of 39 percent plus total solids
while the capture was 95 percent plus without any
chemicals.
Dewatering the same sludge but without heat
treatment, indicated as "NC", produced cakes of
only 18.9 percent total solids. Capture in the cen-
trifuge was only about 52 percent without
polymers on the raw sludge. In order to achieve the
captures of 95 percent plus when dewatering the
raw sludges, chemical costs of approximately $13
per ton of dry solids were required. Solids capacities
of the centrifuge with heat treated sludges were
double those when dewatering raw sludges.
Table 2 summarizes the effects of heat treatment
at a full-scale plant (6000 gph). The sludge at the
87
-------
88 MUNICIPAL SLUDGE MANAGEMENT
Figure 2a: 0 Minutes.
TABLE 1
Swindon Pilot Plant Tests
Centrifuge Tests
RP + AS
Figure 2b: 2 Minutes.
Figure 2c: 5 Minutes.
Ih/hr. 1)S '", TS
Chemical Cost
SI Ton
72C
75C
54C
63NC
65NC
'
i l
160
34
41.6
41.3
39.6
18.9
18.')
98.4
95.2
97.9
51.9
96.6
0.00
0.00
0.00
0.00
12.95
Swinilon. l-nglaiul
Heat I reatment Conditions: Temperature = 400° F
Reaction Time = 30 Minutes
time of the test was a mixture of approximately 60
percent primary and 40 percent waste activated
sludge. Note the effect of decreasing heat treat-
ment temperature on the decreasing centrifuged
cake concentration.
Recovery in the centrate was lower than ex-
pected and simultaneously cake concentration was
higher. To compensate for this situation the pool
depth in the centrifuge will be increased by chang-
ing the regulating rings to a smaller inner diameter.
With this change, cake concentrations of 40 percent
will be obtainable with centrate recoveries in
the range of 90-95 percent when heat treatment
temperatures are approximately 385°F. Product
rates of 800 Ibs/hr. D.S. could be obtained con-
sistently from each centrifuge when the dewatered
cake concentration was at least 40 percent.
Advantages of Heat Treatment
This experience using mechanical dewatering
devices on heat treated mixtures of primary plus
waste activated sludge has indicated the following
advantages over dewatering the corresponding raw
chemically conditioned sludge:
A. Double the cake solids concentration in the
dewatering unit is possible.
B. Double the cake solids capacity of the dewater
device is possible.
C. Significant reduction or elimination of
chemical conditioning agents, depending on
the ratio of primary to secondary solids being
heat treated, and on the type of dewatering
unti being used, can be achieved.
D. When combined with combustion, a smaller
incinerator can be utilized due to lower water
content in the feed cake.
E. No fuel will normally be required for in-
cineration. When the ratio of volatile matter
to water in the dewatered heat treated sludge
cake is approximately 0.4, the dewatered cake
is autogenous.
Heat treatment of sewage sludges does produce
liquors high in soluble BOD. This is due to the so-
called hydrolyzation that takes place and also to the
leaching of the cell water from the structure during
heat treatment. In those plants in which primary
plus waste activated sludges are heat treated, the
recycled heat treatment liquor contains a dissolved
BOD load approximately equal to 25 percent of the
primary settled sewage.
Like the supernatant from a digester, handling
the sludge heat treatment liquors is part of the
-------
HEAT TREATMENT AND INCINERATION 89
TABLE 2
San Bernardino Farrer Plant
Operating Results
Centrifuge Operation (w/o Chemical Cond.)
Temperature
°F
385
385
385
365
360
Reaction Time
(Minutes)
30
30
30
30
30
Feed
Cone.
(%TS)
10.3
12.4
14.9
13.0
9.7
Cake
Cone.
(%TS)
55.5
53.2
51.9
35.5
33.3
Prod.
Rate.
(tt/Hr.D.S.)
485
547
833
804
690
Recovery
%
90
89
83
82
83
solids handling system which is true of all return
streams from the solids handling system, such as
thickener overflow, centrate or filtrate.
•Of course, if the cost of handling the liquors from
heat treatment is such that it offsets the overall
economics of heat treatment in the first place, then
there would be little reason to even consider the
process. However, our data indicates that the BOD
in the liquors is biodegradable and can be handled
without offsetting the overall economics of the
heat treatment plant.
Dorr-Oliver's practice is that in those sewage
treatment plants where the wet end aeration tanks
are sized for six hours detention time, the heat
treatment liquors can be returned directly to the
head of the sewage treatment plant. In those cases
where the aeration detention time would be less
than the nominal six hours, the Dorr-Oliver Sludge
Heat Treatment System would incorporate a
pretreatment of the heat treatment liquor before
returning this liquor to the head end of the sewage
treatment plant.
Incineration
Incineration of primary and biological solids has
been practiced in the United States for many years.
The fluid-bed incinerator and the multiple hearth
incinerator are the most common units utilized in
the municipal market.
Dorr-Oliver has supplied a total of 94 fluid-bed
reactor systems for handling biological sludges. Of
the 94, a total of 74 are for incineration of municipal
sludges. The remaining 20 installations are in-
cinerating biological sludges produced at industrial
waste treatment plants.
Of the 74 municipal installations, five in-
stallations include heat treatment, two of which are
in operation.
Table 3 shows performance data from eleven
FluoSolids* installations. The plants shown do not
incorporate heat treatment prior to incineration.
Two of the installations are at primary treatment
plants, four at plants burning sludges produced at
trickling filter plants, and five are burning sludges
produced at activated sludge plants.
The average design capacity of the eleven
installations is 764 Ibs/hr. dry solids. The actual
average operating capacity is 851, which is 11.4 per-
cent higher than the design capacity. Average
design auxiliary fuel requirement is 57.6 gal/ton
dry solids. The actual average fuel oil consumption
is 48.2 gal/ton dry solids.
The average design power requirement is 280
kwh/ton dry solids. The actual power consumption
is 248 kwh/ton dry solids.
The volatile content in the ash was 0.49 percent,
as compared to a design content of 3.1 percent.
The stack emission was 0.0318 grains/scf, as
compared to a design of 0.12 grains/scf.
The average cost of incineration is $30.44 per ton
of dry solids for power, fuel oil, and polymers re-
quired to chemically condition the sludge.
This figure is based on a fuel oil cost of 30
-------
•-O
o
TABLE 3
FS Performance Data
Plan/
Liberty, N.Y.
Ocean City,
Md.
Barstow, Cal.
Northwest
Bergen, N.J.
Upper Merion
I'wp.. Penna.
Port Wash.,
N.Y.
Arlington.
N.Y.
New Windsor,
N.Y.
Bath, N.Y.
Lorain, Ohio
Somerset-
Raritan
Average
Type
oj
Sludge
Prim +
T.F.
Prim
Prim
Prim +
WAS
Prim +
T.F.
Prim +
T.F.
Prim +
WAS
Prim '
T.F.
Prim +
WAS
Prim +
WAS
Prim -•-
WAS
FS Heal
Reactor Re-
Dia. Covery
6'
6'
T
12'
9'
9'-6"
9'
7'
9'-6"
14'
12'
No
No
No
Yes
Yes
No
No
No
No
No
Yes
Capacity
tt/Hr. D.S.
Design
282
350
500
1100
865
860
700
570
605
1400
1170
764
Actual
338
445
552
1169
918
865
742
666
657
1635
1376
851
Au.\. Fuel Po\\'er
Gal/ Ton D.S. KW HI Ton D.S.
Design
102.8
48.0
36.0
41.5
18.4
64.5
-
56.6
113.9
40.0
55.0
57.6
Actual Design Actual
53.3
22.9
31.9 239 210
57.0 267 243
14.4
85.5 252 261
.
75.5
85.5 400 344
32.2 274 181
23.8 247 247
48.2 280 248
< r 1 <
//; .
Design
3.0
3.0
-
4.0
-
3.0
3.0
3.0
3.0
3.0
3.0
3.1
i/a tile
•ls/7
Actual
0.31
0.85
-
0.59
-
0.4
0.3
0.4
0.4
0.7
0.5
0.49
Stack
Emissions
Grains/ SCF
Design Actual
.
-
O.I 0.025
O.I 0.018
-
0.1 0.025
-
_
0.1 0.044
_
0.2 0.047
0.12 0.0318
c
Z
n
C
a
n
m
m
m
2
H
-------
HEAT TREATMENT AND INCINERATION 91
a fluid bed incinerator utilizing polymers to
chemically condition the sludges is shown.
The heat treatment-fluid bed system utilizes
heat recovery from the reactor to produce steam
required in the heat treatment process.
The fluid bed system with chemical conditioning
utilizes heat recovery to preheat the fluidizing air
to 1000°F.
The combination of heat treatment and incinera-
tion offers the opportunity to improve the
economics of sludge disposal by greatly improving
the dewaterability of the sludge and by significantly
reducing or eliminating chemical conditioning
agents utilized in sludge dewatering and auxiliary
fuel required to sustain combustion in the in-
cinerator.
APPENDIX A
Dorr-Oliver Solids Handling System
Cost Evaluation
Design Basis
Plant Flow at 20 MGD
Solids at 34,285 Lbs/D;
Hrs./Wk.)
2,000 Lbs/Hr. (120
Heat Treatment, Centrifuge
FS Combustion with Heat
Alternate No. 1
Dewatering,
Recovery.
Alternate No. 2 Chemical Conditioning,
Centrifuge Dewatering, FS Combustion with
Heat Recovery.
Basic Equipment
Heat Conditioning
Centrifuges
FS Reactor
Alternate
No. 1
6000 GPH
4 - 16-L's
1 13'0
Alternate
No. 2
16-L's
17'0
6
1
Operating Requirements & Costs
Alternate
No. 1
Boiler Feed Water
Lbs./Hr. NazSOa 0.19
Lbs./Hr. NaCl 3.73
Cost/Ton D.S. $ 0.71
Power
KWH/Hr.
Cost/Ton D.S.
250
3.00
Alternate
No. 2
0
0
0
240
2.88
Fuel
Gallons/Hr.
Cost/Ton D.S. $
Polymers
Lbs./Hr.
Cost/Ton D.S. $
Potable Water
Gallons/Hr.
Cost/Ton D.S. $
Start-Up Fuel
Gallons/Hr.
Cost/Ton D.S. $
Labor
Men/Hr.
Cost/Ton D.S. $
Maintenance
Cost/Ton D.S. $
Cost of Heat Conditioning Liquors
Cost/Ton D.S. - $ (140 HP)
Total Operating Cost
Cost/Ton D.S. $
Capital Costs
(Erected System)
Capital Costs
(Civil Works to
House & Support
System)
Total Capital Costs
Amortized Costs/
Ton D.S.
Total Operating
Cost/Ton D.S.
Total Cost/Ton D.S.
Operating Cost Basis
Power = $0.
Fuel = $0.
Polymers = $1.
Na2SO?, = $0.
NaCl = $0.
Potable Water = $0.
Labor = $4.
Alternate
No. 1
0
0
0
0
960
0.14
3.83
1.15
2
9.00
7.66
1.25
22.27
1,771,000
708,400
2,479,400
20.66
22.27
42.93
012 KWN
30/Gallon
35/Lb.
,07/Lb.
,0155/Lb.
,015/1000
.50/Hr.
Alternate
No. 2
43.2
12.96
15
20.25
0
0
0.42
0.13
2
9.00
7.80
0
53.02
1,234,000
493,600
1,727,000
14.40
53.02
67.42
Gallons
-------
DRYING OF SLUDGE FOR MARKETING
AS FERTILIZER
M. TRUETT GARRETT, JR.
Houston Sewage Treatment and Sludge Disposal Plant
Houston, Texas
Evaluation of the drying of sludge for marketing
as fertilizer usually is made after less complicated
processes such as sand bed drying, lagooning, or the
spreading of liquid sludge on land are found not to
be feasible; and that mechanical processing will be
required. The comparison then is between drying
with the sale of the dried material as fertilizer and
incineration with land disposal of the ash. These are
not just choices in the final sludge process, but must
be evaluated for various water reclamation
processes on a system basis.
Incineration may be applied to any type of sludge
following mechanical dewatering by centrifugal,
floatation, or filtration processes. However, in
order to produce a marketable fertilizer it is
necessary to keep the nitrogen content as high as
possible, and minimize the moisture, ash, grease,
and cellulose. This normally means drying
activated sludge, as anaerobic digestion increases
the ash content, and primary sedimentation tank
sludge is high in grease, fiber, and even
carbohydrates. There are a number of local dryer
installations that dry digested sludge for sale as an
excellent soil conditioner. Heat dried activated
sludge has been produced for nearly fifty years in
Chicago, Milwaukee, and Houston and sold in all
parts of the United States, Canada, and Mexico. In
the market, it must compete with other sources of
organic nitrogen such as dried blood, fish scraps,
tankage, and cottonseed meal.
Although the material may be priced on the basis
of its nitrogen and phosphorus content, the cost to
the buyer includes the cost of freight on the total
weight of the material. The cost of the freight on
the inert material ultimately affects the price that
the producer receives for the sale of the material.
Thus the effect of low nitrogen on the price can be
more than linear, and can cause the material to be
unmarketable when transportation is involved.
Therefore, it is well to consider the things that
affect the nitrogen content of sludge.
The principal components of activated sludge are
microbial cells, grease, cellulose fiber, and inorganic
insoluble materials. The analytical procedures
usually used—nitrogen, ash, ether soluble material,
and crude fiber—do not accurately follow this
classification. Microbial cells contain both inorganic
and ether soluble material. Disregarding this minor
discrepancy, the cells may be estimated to be equal
to the volatile matter minus the crude fiber and
ether soluble material. It has been found at
Houston that when the cells are evaluated on this
basis they contain about ten percent nitrogen. The
nitrogen content of the dried sludge depends upon
the degree of dilution of the cell content by ash,
grease, fiber, and moisture. This is illustrated in
Figure 1 which shows how ash affects the nitrogen
content. The phosphorus content is not predictable
in a similar manner because it may be part of the
cells or incorporated as an inorganic precipitate.
Ash content increases during wet weather and
decreases during dry periods. Food processing
wastes with low ash content tend to increase the
cells without increasing the ash, and thus, increase
the nitrogen content.
The processes used at Houston are shown in
Figure 2. The water reclamation processes are
designed to be compatible with the sludge disposal
scheme of drying and marketing as fertilizer.
93
-------
94 MUNICIPAL SLUDGE MANAGEMENT
ACTI-'ATTD 3LU
PEP CEIT 0~
25
ASK
10
E+7
30
ASH
L'OF COKPOIIEOTS
65
CELLS
10
t4""
35
ASH
60
CELLS
10
E+r
HO
A~!!
55
CELLS
10
E+F
1*5
ASH
E+F = tthe
r Soluble
50
CELLS
10
E+F
15
CELLS
b
H20
6
F20
6
HgO
6
H20
6
H-,0
lilTROGETI
PER CeiT OF
PRODUCT
6.13
5.66
5.19
4.72
4.24
•Material plus Crude Fiber
Figure 1: Variation of Nitrogen in Product as Cells Are Diluted
with Other Components.
MUNICIPAL
WASTEWATER
BAR SCREEN
ACTIVATED SLUDGE
AERATION TANKS
RFTUMI SLUDGE EXCESS SLUDGE
SETTLI1IG TANKS SETTLING TANKS
CHLORIHATIOH AND
DETENTION TANKS
RECLAIMED WATER
TO RIVER
SLUDGE HOLDING
TANK
VACUUM FILTERS
RECLAIMED SOLIDS
TO FERTILIZER INDUSTRY
Figure 2: Water and Solids Reclamation Processes at Houston,
Texas.
Bar screens were installed to minimize problems
in the activated sludge process. In the early 50's,
screenings were ground and returned to the flow.
The shredded rags were found to reagglomerate
into large wads of waste. The grinder was removed
and screenings hauled to a sanitary landfill. A
philosophy evolved, "If you take it out keep it out."
This same philosophy was then applied to the
organic return pumps on the grit washers. The
removal of stringy material is beneficial to filter
operation, since strings accumulate on the agitator
and on the scraper blade. This has been particularly
noticeable when bar screens have been taken out of
service for repair and sewage bypassed around
them.
The grit removal facilities remove large particles
and thus offer some protection to the pumps, but
they have little affect on the ash content of the
sludge. Where air distribution is maintained in the
aeration tanks there is no deposition of sand.
The original activated sludge plants at North Side
and South Side did not have primary settling tanks.
As drying was added to the operations, it became
part of the system to omit them. This minimizes the
exposure of raw sewage and eliminates one unit
operation.
The activated sludge tanks have been sized to
provide eight hours detention based on the raw
sewage flow in dry weather. The final settling
tanks provide for an overflow rate of 1800 gpd/ft.2
at peak flow.
Excess sludge is taken from a thickener fed mixed
liquor. The overflow, which is normally of good
quality, goes to the plant effluent. In the
enlargement under construction at the North Side
plant, new thickeners are included with recycle of
the overflow to the aeration tanks (consideration
has been given to the need to be able to sample
influent sewage prior to the addition of return
flows from the sludge processing operation.) The
thickeners are designed for a solids loading of 10
Ib/day/f t2. The operation of the thickeners is critical
to the sludge filter operation and requires regular
manual attention. If there is no appreciable return
of solids from the sludge drying plant, then the
mixed liquor flow to the thickeners gives a measure
of the growth rate of the sludge in the activated
sludge plant.
When all flows from the drying plant are
recycled, stand-by drying capacity becomes
important. Control of the activated sludge process
is lost when solids cannot be dried as fast as they are
produced. As solids accumulate in the aeration
tanks, the sludge age increases and the filterability
often decreases. The ability to catch-up from high
loads or plant shut-downs is necessary. When
operation is 24 hours/day, 365 days per year, a 25
percent spare capacity would permit catch-up in
eight days following a two day shut down. When
operation is based on less than 21 shifts per week,
then the unused shifts are available for catch-up.
The holding tank is sized for one hour detention
time to provide for brief interruption of sludge
pumping without interruption of the dryers, and to
provide sufficient sludge for orderly shut-down in
event of total pump failure.
Sludge conditioning is a key step in the filter
operation. Ferric chloride has been the coagulant at
Houston. The dose required is about ten percent of
the dry solids. This produces a pH of about 2.9 in
the sludge and pH 3.8 at the cake discharge. The
iron content of the sludge is only slightly increased
and ferrous iron and other divalent cations are
released into the filtrate. Lime has not been helpful
except in large doses to give a pH of 10. This is not
-------
MARKETING AS FERTILIZER 95
desirable as fertilizer manufacturers often blend
ammoniacal compounds with the sludge, and the
high pH would cause the release of ammonia.
Moreover, the nitrogen would be diluted by not
only the calcium carbonate, but also, the ferrous
iron which would be retained as ferrous hydroxide.
The vacuum filters are 12 feet diameter by 16
feet face, having 600 square feet of filter surface.
The filterability of the sludge has varied about
four-fold in the past decade—in 1964, the yield was
3 Ib/hr/sf and it has been as low as 0.75 Ib/hr/sf in
the last two years. Coincidental with this change in
filter yield has been an indicated decrease in fiber
from 7.5 percent to 0. There was also a
corresponding increase in nitrogen. Laboratory
studies at Houston have indicated that bacteria
metabolizing cellulose fiber grow at 23 percent per
day at laboratory -temperature of 72°F. This
corresponds to a sludge age of 4.3 days. The source
of fiber is paper in the sewage, but the likelihood of
restoring the fiber content by stopping its
degradation is very low. Effluent BOD
requirements of 12 or 10 mg/1 which are indicated
for the near future will require a sludge age of
five to eight days which is more than adequate for
biological degradation of the fiber. Therefore, new
drying plants are being designed on the basis of
current experience.
The sludge dryers are C-E Raymond Flash
Dryers (Figure 3) with 14 feet diameter cyclones
and heat exchangers for high temperature deodor-
ization. They have a rated evaporative capacity of
12,000 Ib. water per hour while producing a
product with five percent moisture. They are
normally operated at a loading of 8,000 to 10,000 Ib.
of water per hour. Deodorization temperature is
controlled at around 1100°F and the stack
temperature is about 500°F. The fuel used is
natural gas and the heat input is about 22 million
BTU per hour.
Dried sludge is conveyed in screw conveyors to
minimize the dust emission. The vapors from
sludge conditioned with ferric chloride are acidic
and corrosive, so stainless steel troughs are used.
Hard iron bearings have been found to be
successful. The material is weighed and then
screened on six by six mesh screens. The oversize
material is mostly trash and is hauled to a sanitary
landfill. The screened material is stored until
shipped. For shipment the material is fed to an
elevator, weighed in automatic batch scales, and
allowed to flow by gravity into a boxcar. A "car
trimmer", a high speed conveyor about 15 feet long,
is used in the car to throw the material to the end of
the car.
•M REFRACTORY
!••••• HOT GAS TO DRYING SYSTEI
••••• DRYING SYSTEM
UMIA COMBUSTION AIR
QDBQ DEODORIZED GAS
Figure 3: The C-E Raymond Flash Drying System for Sludge
Drying Only . . with Deodorization.
The Houston dried activated sludge is sold under
the copyrighted trade mark of Hou-Actinite. The
marketing has been through a contract with a
brokerage firm. The contract which is for a five
year period is awarded on the basis of competitive
bidding. The material is sold to fertilizer
manufacturers who usually blend it with other
materials to produce a balanced fertilizer. The
revenue to the City of Houston is approximately
$21 per ton. The material is priced on the basis of
analyses by the City laboratory using official
methods of the Association of Official Agricultural
Chemists (AOAC). The market is greatest in the
winter and spring, and least in summer. It has been
necessary at times to store as much as 20 percent of
the annual production before the market picks up in
the fall.
Through the use of the activated sludge process
and the drying and marketing of the solids
removed, Houston and other cities are treating
sewage to a sparkling effluent and recycling the
solids to beneficial use in an aesthetically acceptable
form.
BIBLIOGRAPHY
1. Bryan, A.C. and Garrett, M.T. Jr. "What Do
You Do with Sludge? Houston Has an Answer."
Public Works, p.. 44, Dec. 1972.
2. Billings, C.H. and Smallhorst, D.F. Manual of
Wastewater Operations, Chapter 20, Texas Water
Utilities Assoc., Austin 1971.
-------
GROWTH OF BARLEY IRRIGATED WITH
WASTEWATER SLUDGE CONTAINING
PHOSPHATE PRECIPITANTS
M. B. KIRKHAM AND G. K. DOTSON
National Environmental Research Center
United States Environmental Protection Agency
Cincinnati, Ohio
ABSTRACT
Barley (Hordeum vulgare L., var. Bearded), grown
for 16 weeks in pots of loam soil, was irrigated for
seven weeks with wet sludges containing
precipitated phosphates to see if chemically-treated
sludges could be used agriculturally. Raw primary
sludge and primary sludges from alum and ferric
chloride addition to raw wastewater, each either
limed or unlimed, were added at two different
rates, corresponding to a total application of 23 or
46 m tons/ha/yr. The elemental composition of the
plants and the total and extractable phosphorus in
the soil were determined. The results showed that
barley plants irrigated with the three types of
sludges grew as well as those supplied with in-
organic fertilizer. Fertilized plants, however, yield-
ed more grain than sludge-treated plants. Slight
variations in nutrient content of the sludge-treated
plants appear to be due to differences in the concen-
tration and availability of the elements in the soil.
Differences in total soil phosphorus among the
treatments were not significant. The limed and un-
limed alum-treated soils and the unlimed iron-
treated soil had significantly more extractable
phosphorus. Liming significantly decreased extrac-
table phosphorus only in the iron-treated soil.
Ninety-two percent of the phosphorus added by
the sludges was still in the sludge crust at the end of
the experiment. The results demonstrated that the
presence of phosphate precipitated by Al or Fe did
not limit growth, and that sludges containing
phosphate precipitants can be used, at least on a
short-term basis, to grow barley.
Phosphorus in the form of phosphate is one of
the major uncontrolled pollutants in wastewater3.
It is a primary nutrient for algal growth in surface
waters. Removal of phosphate in sewage is now
required by law in several states, particularly those
with plants discharging effluents into waters that
drain into the Great Lakes. Phosphorus is
precipitated by the polyvalent cations in aluminum
salts, iron salts, or lime, which can be added at
various points during wastewater treatment.
Removal of phosphorus by aluminum or iron salts
is especially desirable because extensive modifica-
tion of the sewage treatment process is not needed.
In the most frequently used procedure, the salt is
mixed with the wastewater just before the pri-
mary clarifier. Many smaller plants in the United
States and Canada are using this method8.
Aluminum or iron salts are added to wastewater
in an atomic ratio of 1.5 to 2.5 metal atoms per P
atom. The mechanism of P removal is obscure, but
it is pictured as being effected by the formation of
the metal phosphate and the metal hydroxide. Ad-
sorption may take place, and/or the phosphate may
be included in a complex metal hydroxide ligand6.
If plants are grown on land spread with the
sludge, the phosphate precipitated into sludge dur-
ing wastewater treatment may be in a form un-
available for plant growth. In fact, the excess metal
hydroxide may react with the natural soil
phosphate and make it less available. Jansson (Upp-
sala, Sweden) reported on P availability in soils
spread with chemically treated sludges. His results,
in what appears to be the only published study on
the subject10, indicate that iron and alum sludges
with excess hydroxides did not bind exchangeable
97
-------
98 MUNICIPAL SLUDGE MANAGEMENT
phosphate in the soil, and P availability was not
decreased. Under field conditions and with
adequately-limed soils, application of alum and iron
sludges resulted in a slow but positive phosphate
reaction. On acid, lime-poor soils, the effect was
unfavorable. But such soils, Jansson pointed out,
are not consistent with rational land use and they
should be limed. In greenhouse studies, he found
that corn grew just as well on a loam soil fertilized
with superphosphate as on the same soil treated
with alum sludges.
Before wastewater sludge is applied to land, it is
most commonly stabilized by digestion or chemical
treatment5'7/, which can reduce odors and the
pathogen level1'4'12. Digestion decreases the
biodegradability potential of the sludge. This is
desirable because localized high temperatures or
anaerobic conditions caused by rapid breakdown of
the sludge are less likely to occur and cause adverse
growing conditions. Chemical treatment of sludge
with lime is effective in minimizing odors and
probably is better than digestion in reducing the
number of pathogens. The ultimate biodegradabili-
ty potential of sludge is not reduced by lime, but it
does slow the degradation rate.
The object of this investigation was to determine
the availability of P in Al-primary and Fe-primary
sludges stabilized with lime before their disposal on
land. To accomplish this aim, the growth of barley
in pots of soil with and without inorganic fertilizer
was compared with the growth of barley on soil
amended with the following four types of sludges:
1) lime-treated primary sludge with no Al or Fe
salts; 2) lime-treated primary sludge containing Al
or Fe salts; 3) primary sludge with Al or Fe salts, but
with no lime; 4) primary sludge with no Al or Fe
salts and no lime.
Materials and Methods
Three sludges were obtained from the Lebanon,
Ohio (30 miles from Cincinnati, Ohio) sewage
treatment plant: a raw sludge from conventional
primary treatment (referred to as primary sludge);
a raw sludge from primary treatment of
wastewater with 93 mg/1 of alum (Ah(SO4)3 •
14HzO) added as a phosphorus precipitant
(referred to as Al-primary sludge); and a raw sludge
from primary treatment with 48 mg/1 FeCb as a
precipitant (referred to as Fe-primary sludge). In
both chemical treatment cases, the atomic ratio of
added metal to total P in the raw wastewater was
2/1.
After being transported to Cincinnati from
Lebanon, the sludge sat overnight in a refrigerator
(5°C) to allow the solids and supernatant to
separate. The precipitate was adjusted to two per-
cent solids on a dry-weight basis with the use of the
supernatant. This two percent sludge was analyzed
for pH, total solids (TS), volatile solids (VS), total
Kjeldahl nitrogen (TKN), ammonia nitrogen (NHs),
chemical oxygen demand (COD), and for the
elements Fe, Mn, Zn, Na, B, Al, P, N, K, Ca, and Mg.
Procedures described in Standard Methods1* were
used to analyze for pH, TS, VS, TKN, NHa, and
COD.
Ca, Mg, Na, and K analyses were performed in
the following manner. A 1.00-g sample of dry
sludge was heated with 20 to 30 ml of 1:4 HC1 (20
percent HC1 solution) on a hot plate. After boiling
for a few minutes, the sample was cooled and
filtered into a one-liter volumetric flask. The
residue was washed, and the filtrate was diluted to
one liter. The sample was then analyzed following
standard procedures for wastewater analysis14.-
Al, Cd, Cu, Fe, Mn, Ni, Pb, Zn, and P analyses
were conducted as follows. A 1.00-g sample of the
dried sludge was heated with 10 ml of 1:1 HzSO4 +
10 ml concentrated HNOa until the first ap-
pearance of dense white fumes. After cooling, 2
ml of a 3:1 (vol/vol) HNOs + HC1O4 acid mixture
was added. Digestion was continued until the first
appearance of dense white fumes. The sample was
cooled and 30 to 40 ml of water was added. After
filtering through a Gooch glass filter crucible into a
one-liter flask and diluting to volume, it was
analyzed on a Perkin-Elmer 303 Atomic Absorption
Spectrophotometer for the metals and on a
Technicon Autoanalyzer for P. The sludge crust
was analyzed for P by the same procedure.
The experiment, which lasted 110 days (9 July to
26 October, 1973) consisted of eight kinds of
treatments, each at two levels, replicated three
times for a total of 48 pots. The treatments were as
follows:
1. soil (unfertilized)—water
2. soil (fertilized)—water
3. soil—unlimed sludge (no Al or Fe)
4. soil—limed sludge (no Al or Fe)
5. soil—unlimed Al sludge
6. soil—limed Al sludge
7. soil—unlimed Fe sludge
8. soil—limed Fe sludge
Water and sludge (two percent solids) were added
at two levels: 235 ml/wk and 470 ml/wk. Each
application was equivalent to a depth of 1.25 cm and
2.5 cm of liquid sludges; or 2.5 m tons/ha (1.11
tons/A) and 5.0 m tons/ha (2.23 tons/A) of dry
sludge solids. For the limed sludges, a ten percent
-------
GROWTH OF BARLEY 99
CaO slurry was added until a pH of 11.8 was
reached.
Fifteen barley seeds (Hordeum vulgare L., var.
Bearded) were planted in each of the 48 plastic pots
(23 cm tall; 18 cm diameter) on 9 July 1973 (Day 1).
The pH of the loam soil ranged from 7.4 to 8.1, and
the cation exchange capacity was 20 meq per 100 g.
The appropriate sludge (with or without lime, Al,
or Fe) was added two times, on 25 May and on 29
May 1973, to the soil surface before the seeds were
planted. Chemical fertilizer was mixed into the soil
of the six fertilized pots before the seeds were
planted, to supply 227-99-187 kg per ha of N-P-K,
respectively. The plants were thinned to ten plants
per pot on 16 July.
Distilled water was added ten times to the pots (a
total of 3,045 ml/pot) between the time that the
sludge was added to the soil (29 May) and the time
the first sludge applications were made after plan-
ting (3 August). Leaching did not occur until the
last day of distilled water additions (27 July) before
sludge applications, and then only a small amount
of leachate collected in the pans under the pots.
Between 3 August and 14 September, 235 ml or 470
ml of sludge was poured weekly on the soil surface
(seven additions). The control pots (fertilized and
unfertilized) received 235 ml or 470 ml of distilled
water. Four days after each sludge or distilled water
addition, 235 ml of distilled water was added to
every pot. From 14 September to 26 October, 235
ml of distilled water was added twice a week to each
pot (11 additions). Table 1 shows the days on which
sludge and water were added to the pots. There
were nine applications altogether of sludge,
amounting to 22.5 m ton/hectare (10 ton/acre) of
dry sludge solids for the low level of addition, and
45 m ton/hectare (20 ton/acre) for the high level.
The plants were grown in a greenhouse in Cin-
cinnati, Ohio, under natural light conditions. After
the plants were thinned (16 July), there were ap-
proximately 24 cloudy days and 77 sunny days until
the end of the experiment. A cooler in the
greenhouse kept the temperature below about
32°C. Temperature varied from 11°C to 32°C and
the humidity from 40 percent to 98 percent. Plant
height, pot weight, and par evaporation rate were
recorded weekly. >
On 26 October, the plants were harvested by cut-
ting them 1 cm above the sludge or soil surface,
separating them into stems, leaves, and grain, and
measuring fresh weight. The plant samples were
oven dried at 80°C, weighed, and ground in a Wiley
mill. Samples were digested in concentrated HzSO4
with a Cu catalyst16 and analyzed for total N
(micro-Kjeldahl steam distillation technique), P
TABLE 1
Timing and Amount of Sludge or Distilled
Water Additions to Pots Containing a
Loam Soil in which Barley Plants Grew
Date
25 May
29 May
5 June
25 June
2 July
9 July
13 July
16 July
20 July
24 July
25 July
27 July
3 Aug
8 Aug
10 Aug
14 Aug
17 Aug
21 Aug
24 Aug
28 Aug
31 Aug
4 Sept
7 Sept
11 Sept
14 Sept
18 Sept
21 Sept
25 Sept
28 Sept
2 Oct
5 Oct
9 Oct
12 Oct
16 Oct
19 Oct
23 Oct
26 Oct
Sludge
Day added
45 235 or 470
41 235 or 470
34
14
7
1 (planting)
5
8
12
16
17
19
26 235 or 470
31
33 235 or 470
37
40 235 or 470
44
47 235 or 470
51
54 235 or 470
58
61 235 or 470
65
68 235 or 470
72
75
79
82
86
89
93
96
100
103
107
110 (harvest)
Distilled water added
to control pots (no Distilled water
sludge, but with or added to
without fertilizer) all pots
ml
460
235
235
235
235
235
470
235
235
470
235 or 470
235
235 or 470
235
235 or 470
235
235 or 470
235
235 or 470
235
235 or 470
235
235 or 470
235
235
235
235
235
235
235
235
235
235
235
(Autoanalyzer), K, Mg, Ca (flame photometer) and
trace elements (Atomic Absorption Spec-
trophotometer).
At the end of the experiment, a glass tube (1 cm
diameter) was used to take two soil samples—one
from the top half and one from the bottom half of
the soil in each pot. The soil was analyzed for total P
with the use of NazCOs fusion technique9 and for
extractable P with the use of the Bray No. 1 test13
which employs a 0.03 N NH4 F in 0.025 N HC1 ex-
tracting solution at soil to solution ratio of 1:10.
-------
100 MUNICIPAL SLUDGE MANAGEMENT
TABLE 2
Characteristics of Sludge from
the Lebanon, Ohio Sewage Treatment
Plant (Average of Four Samples)
Characteristic
Primary
sludge
A l-primary
sludge
Fe-primary
sludge
Sludge before adjustment to 2% solids
Total Solids, % 7.6 2.9 2.8
Volatile solids, % 64.2 66.5 68.3
Sludge alter adjustment to 2% solids
Total solids, %
pH
COD
Total nitrogen
Ammonia nitrogen
Ca
Mg
K
Na
P
Al
B
Cd
Cu
Fe
Mn
Ni
Pb
Zn
2.1
5.7
29,500
695
92
26.1
3.2
0.8
1.0
9.1
5.9
<0.1
-------
GROWTH OF BARLEY 101
TABLE 3
Height of Barley Plants Grown in a Loam Soil without
Fertilizer, with Fertilizer, or with Primary,
AL-Primary, and FE-Primary Sludges*
No
Fertilizer
Unlimed
Time
after
planting
davs
9
10
11
12
16
19
24
32
39
46
53
60
67
74
81
88
95
102
109
t
235
ml
wk
9
10
11
12
19
24
32
40
41
41
41
43
43
44
45
46
45
45
45
Fertilizer
Unlimed
t t
470 ' 235
ml
wk
8
9
10
11
17
22
30
35
37
40
45
50
51
51
51
51
51
50
50
ml
wk
11
11
12
11
17
21
28
37
45
55
55
54
56
57
57
58
57
57
57
t
470
ml
wk
8
9
11
13
21
26
30
34
37
42
47
53
56
55
55
56
55
55
56
Primary sludge
Unlimed Limed
235
ml
wk
11
11
12
12
21
26
31
39
41
42
46
48
48
51
49
50
51
50
51
470
ml
wk
10
10
11
14
19
24
33
37
40
44
47
50
52
53
51
53
51
51
51
235
ml
wk
cni'i i-
11
12
12
13
21
25
34
39
41
43
47
49
50
50
49
50
51
50
49
470
ml
wk.
12
12
13
13
20
26
35
38
41
45
48
52
51
52
53
52
53
51
50
Al-primary
sludge
Unlimed Limed
235
ml
wk
10
10
11
11
18
23
31
38
40
40
46
46
46
45
48
48
48
47
48
470
ml
wk
10
10
11
11
18
24
33
37
38
40
42
45
47
49
49
49
48
47
47
235
ml
wk
8
8
10
11
17
24
33
37
39
40
44
44
42
45
45
46
47
45
45
470
ml
wk
8
9
9
10
18
23
34
37
39
40
45
43
45
47
46
43
43
42
42
Fe-primary
sludge
Unlimed Limed
235
ml
wk
8
9
10
10
17
23
32
38
38
39
42
45
48
46
46
47
48
47
48
470
ml
wk
6
7
8
8
15
21
31
37
38
41
45
49
51
52
52
52
51
50
49
235
ml
wk
1
1
8
8
14
21
31
36
36
38
40
41
44
45
45
46
47
45
45
470
ml
wk
8
9
10
10
17
22
32
39
40
41
47
46
50
50
50
50
50
48
48
* In addition to the 235 or 470 ml/wk sludge or distilled water irrigations, each pot received 235 ml/ wk distilled water. Day 68 was
the last day sludge was added. From then until the end of the experiment, each pot received 470 ml/wk distilled water.
+ Only distilled water added to these pots.
++ Average coefficient of variation: 0.15
sludge produced significant effects on the concen-
tration of extractable P. Soils treated with unlimed
Al-primary, limed Al-primary, and unlimed Fe-
primary sludges, had significantly more extractable
P than the other soils. Liming significantly reduced
extractable P in the soil treated with Fe-primary
sludges. Liming may have reduced the extractable P
in soil treated with primary s,ludge, but significance
level was less than 95 percent.
The elemental composition of the stems, leaves,
and grain of the harvested plants are presented in
part in Table 6. Concentration of P, N, and K in the
three parts of the plants are shown for the various
soil treatments and their levels. The data can be
readily scrutinized for the effect of sludge loading,
limed versus unlimed sludge, and the different
sludge type, or fertilizer level on concentrations of
P, N, and K. There is no indication of a significant
effect of any of these factors. Concentrations of
other elements were examined in the same fashion
and similarly showed no significant effects. Median
values of the concentrations for the 16 soil
treatments are presented in Table 7 for all of the
elements analyzed. Cd, Ni, and Pb were at low con-
centrations in all of the plant parts.
Changes in pH were monitored once after the
primary sludge was added to the surface of the pots
(Table 8). Within 24 hours after adding the unlimed
sludge, soil pH had risen to the pH value of the soil
with no sludge. Also, within 24 hours, the soil
treated with 235 ml of the limed sludge had a pH
similar to that of the control soil. After 48 hours,
the soil receiving 470 ml of the limed sludge had a
pH that was not higher than that of the control soil.
DISCUSSION
The results showed that barley plants irrigated
with primary, Al-primary, and Fe-primary sludges
grew as well as those supplied with inorganic fer-
tilizer. Fertilized plants, however, produced more
grain than plants treated with 235 ml of sludge per
-------
102 MUNICIPAL SLUDGE MANAGEMENT
week. The grain yields from the plants that receiv-
ed the higher sludge applications approached the
yield for fertilized pots. Sludge fertilized plants
yielded more grain than unfertilized plants, but
statistical significance at the 95 percent level could
not be demonstrated.
The higher grain yield obtained with plants
supplied with fertilizer as compared with plants
fertilized with sludge may be the result of the
different method of addition: fertilizer was mixed
uniformly into the soil at the beginning of the ex-
periment whereas sludge was added periodically
over the course of the experiment and not mixed
into the soil.
The growth response and grain yield for the
three different types of sludge were quite similar.
There is no indication that availability of nutrients
(particularly P) is less from sludges that contain Al
and Fe.
No discernible differences were observed
between the elemental content of plants grown on
sludge treated soils and the control soils with and
without fertilizer. Differences in concentration and
availability of the elements in the soil itself appear
to have been the major factor in affecting the
elemental composition of the plants. This conclu-
sion is supported by work at Battelle-Northwest,
now in progress. The Battelle results showed no
significant difference in macro- and micro-nutrient
concentrations between soils treated with fer-
tilizer, unlimed sludge, or limed sludge. Non-
uniformity within the soil system evidently con-
tributes enough scatter to dominate effects of the
different treatments.
The total amount of P added by the sludge and
the amount of P measured in the sludge crust at the
end of the 16-week experiment are given in Table 9.
An average of 92 percent of the P remained in the
crust. King and Morris11, at the end of a two-year
study where they added 40 cm of digested sludge to
fields with coastal Bermuda grass or rye, found that
the sludge crust contained 49.5 percent of the P
added by the sludge. The soil contained 46.7 per-
cent of the P, and the plants removed 3.8 percent of
the P. Therefore, over a two-year period, a con-
siderable amount of the P in the sludge crust was
incorporated into the soil. It was, however, in un-
available form. Even though the digested sludge of
TABLE 4
Dry Weight of 110-Day-Old Barley Plants Grown in a
Loam Soil without Fertilizer, with Fertilizer, or
with Chemically-Treated Sludges
Application No fertilizer Fertilizer primary sludge
rale* Unlimed Unlimed Unlimed Limed
A l-primary
sludge
Unlimed Limed
Fe-primary
sludge
Unlimed Limed
ml/wk
_gramst_
Stems
235
470
3.8
3.1
5.4
2.4
4.3
6.0
4.1
5.3
5.1
6.7
4.0
5.5
4.3
4.4
4.3
5.0
235
470
3.0
2.4 ++
3.7
3.2 ++
3.3
5.2
Leaves
2.6
3.9
3.0
5.1
3.4
4.5
3.2
4.9
2.9
5.4
Grain
235
470
235
470
4.3
3.8++
11.1
9.3
8.3
3.6++
17.4
9.2++
6.1
7.3
13.7
18.5
5.3
7.7
6.4
8.0
Total (excluding roots)
12.0
16.9
14.5
19.8
5.7
7.4
13,1
17.4
6.5
8.2
14.0
17.5
6.1
6.5
13.3
16.9
*See first two footnotes Table 3.
t Average coefficient of variation: 0.10
++ Poor germination in these pots caused low yields.
-------
GROWTH OF BARLEY 103
TABLE 5
Total and Extractable Phosphorus in a Loam Soil with No
Fertilizer, with Fertilizer, and with Primary, Al-Primary, and Fe-Primary
Sludges after the Harvest of 110-Day-Old Barley Plants
Application
rare*
ml/wk
A 1-pprimary
No fertilizer Fertilizer Primary sludge sludge
Unlimed Unlimed Unlimed Limed Unlimed Limed
^
Total
Fe-primary
sludge
Unlimed Limed
235
470
235
470
1100
1240
28
32
980
1410
39
25
1130
1440
60
46
890
1120
Extractable
32
32
1510
1710
144
147
1680
2560
137
144
1640
2070
9.1
95
1430
1890
60
46
* See first two footnotes under Table 3.
f Average coefficient of variation: 0.20.
TABLE 6
Concentration of P, N, and K in 110-Day-Old Barley
Plants, Grown in a Loam Soil with Various Types and
Levels of Sludge and Fertilizer
Factors and Levels
Sludge Sludge
Concentration of Nutrients (%)
Type
None, no
fertilizer
None,
fertilizer
Primary
Al-primary
Fe-primary
Limed Loading
no low*
high*
no low *
high*
no low
high
yes low
high
no low
high
yes low
high
no low
high
yes low
high
P
0.15
0.11
0.11
0.10
0.13
0.16
0.16
0.14
0.12
0.13
0.10
0.10
0.16
0.11
0.11
0.10
Stem
N
1.19
0.72
1.60
0.49
1.49
1.94
1.81
1.69
1.67
2.04
1.50
1.92
1.71
1.64
1.68
1.76
K
1.69
1.65
2.98
1.74
1.68
1.48
2.19
1.92
2.03
1.77
2.49
2.36
1.98
2.46
2.20
2.14
P
0.10
0.18
0.15
0.18
0.16
0.11
0.17
0.16
0.13
0.13
0.16
0.14
0.12
0.19
0.15
0.12
Leaves
N
1.62
1.37
2.29
1.56
2.50
2.19
2.68
2.40
2.06
2.08
2.44
2.13
2.41
2.79
2.19
2.40
K
1.78
1.87
2.10
1.76
1.42
0.98
1.64
1.54
1.70
1.63
1.91
1.82
2.01
1.74
1.87
1.81
P
0.29
0.31
0.35
0.28
0.38
0.44
0.39
0.45
0.41
0.32
0.30
0.36
0.31
0.33
0.35
0.34
Grain
N
2.25 C
1.53
2.33
1.82
2.48
2.68
2.28
2.55
2.38
2.48
2.42
2.32
2.18
2.36
2.33
2.50
K
).95
.16
.41
.34
.25
.17
.35
.13
.32
.03
.17
.30
.32
.05
.07
.18
* low and high loadings of water
f + mean values of 3 replicates
-------
104 MUNICIPAL SLUDGE MANAGEMENT
TABLE 7
Median Concentration of Elements in
110-Day-Old Barley Plants Grown
in a Loam Soil with and without
Fertilizer, and with Various Sludges
Element
P
N
K
Ca
Mg
He
Mn
Zn
B
Al
Cd
Ni
Pb
Stems
0.12
1.68
2.08
0.38
0.10
80
<0.05
22
-------
GROWTH OF BARLEY 105
TABLE 8
pH of a Loam Soil after Application of
Limed (pH 11.8) or Unlimed (pH 5.7) Primary Sludge
Depth of soil
sample in pot
cm
pHof
sludge
Sludge
application
Time after sludge application
ml
-hr-
24
48
0-7
7-14
14-2!
0-7
7-14
14-21
0-7
7-14
14-21
0-7
7-14
14-21
0-7
7-14
14-21
5.7
5.7
5.7
5.7
5.7
5.7
11.8
11.8
11.8
11.8
11.8
11.8
No
/
235
235
235
470
470
470
235
235
235
470
470
470
sludge added
, „
' » "
6.6
6.5
6.7
8.7
8.7
8.7
8.8
8.8
8.8
7.8
7.8
7.8
7.8
7.9
7.9
7.9
7.9
7.7
7.6
7.9
7.9
8.0
8.1
8.0
7.8
7.9
7.8
7.6
7.7
7.7
7.8
7.8
7.8
7.2
7.3
7.2
7.3
7.4
7.4
7.4
7.5
7.6
TABLE 9
Total P Added by Sludge or Fertilizer to a
Loam Soil in which Barley Was Growing and
the P Content of the Sludge Crust at the
End of the 110-Day Growth Period
Treatment
P added by
sludge or
fertilizer
P in sludge Change in
crust at P content
harvest of crust
Primary, unlimed sludge,
" limed "
235-ml rate
470-ml »
235-ml *
470-ml »
Alum-Primary, unlimed sludge,
» » limed »
Iron-primary, unlimed "
" " limed "
235-ml rate
470-ml »
235-ml »
470-ml »
235-ml "
470-ml »
235-ml "
470-ml »
0.39
0.77
0.39
0.77
1.13
2.26
1.13
2.26
1.17
2.34
1.17
2.34
-grams-
No fertilizer,
„
Fertilizer
„
235-ml rate (water)
470-ml »
235-ml »
470-ml -
1.96
1.96
1.14
2.54
1.10
2.24
0.89
1.77
1.21
1.70
101
112
97.4
99.1
75.6
75.6
103
72.6
-------
106 MUNICIPAL SLUDGE MANAGEMENT
the Environment, Toronto, Ontario, Canada, 212
pp, 1972.
3. Dean, R.B. "Ultimate Disposal of Wastewater
Concentrates to the Environment," Environmental
Science and Technology, 2: 1079-1086, 1968.
4. Dean, R.B., and Smith, J.E., Jr. "The Properties
of Sludges," Proc. of the Joint Conference on Recycling
Municipal Sludges and Effluents on Land, Champaign, Il-
linois, July 9-13, pp. 39-47, 1973.
5. Dean, R.B., and Smith, J.E., Jr. "Disposal and
Recycling of Wastewater Sludge Containing Lime,"
presented at the Quadrennial Intl. Lime Con-
ference, Berlin, Germany, May 1974. Published in
Vortrage Lectures Conferences, 3rd Intl. Symposium
on Lime, B undes verband der Deutschen
Kalkindustrie e.V. Koln, 9 May 1974.
6. Farrell, J.B., Salotto, B.V., Dean, R.B., and
Tolliver, W.E. "Removal of Phosphate from
Wastewater by Aluminum Salts with Subsequent
Aluminum Recover," Chem. Eng. Progress
Symposium Ser., 64 (90): 232-239, 1968.
7. Farrell, J.B., Smith, J.E., Jr., Hathaway, S.W.,
and Dean, R.B. "Lime Stabilization of Primary
Sludges," /. Water Pollution Control Fed., 46: 113-122,
1974.
8. Hathaway, S.W., and Farrell, J.B. "Thickening
Characteristics of Aluminum and Iron Primary
Municipal Wastewater Sludges," presented at
Research Symposium on Pretreatment and Ul-
timate Disposal of Wastewater Solids, Rutgers
Univ., May 21-22, 1974.
9. Jackson, M.L. Soil Chemical Analysis, Prentice-
Hall, Inc., Englewood Cliffs, N.J., 498 pp, 1958.
10. Jansson, S.L. "Klarschlammverwertung in
der Landwirtschaft Schwedens," Stand und
Leistung agrikulturchemischer und agrar-
biologischer Forshung, 22: 61-66, 1972.
11. King, L.D., and Morris, H.D. "Land Disposal
of Liquid Sewage Sludge: IV. Effect of Soil
Phosphorus, Potassium, Calcium, Magnesium, and
Sodium," ]. Environ. Quality 2: 411-414, 1973.
12. Lund, E., and Ronne, V. "On the Isolation of
Virus from Sewage Treatment Plant Sludges,"
Water Research, 7: 863-871, 1973.
13. Olsen, S.R., and Dean, L.A. "Phosphorus,"
1035-1049. In C.A. Black, D.D. Evans, J.L. White,
L.E. Ensminger, and F.E. Clark (ed), Methods of Soil
Analysis, Part 2. Chemical and Microbiological Properties,
Amer. Soc. Agron., Inc., Madison, Wisconsin, 1965.
14. Taras, M.J., Greenberg, A.E., Hoak, R.D.,
and Rand, M.C. (ed), Standard Methods for the Examina-
tion of Water and Wastewater, 13th ed., Amer. Public
Health Assn., Washington, D.C. 874 pp, 1971.
15. Truog, E. "Liming in Relation to Availability
of Native and Applied Phosphates," 281-297. In
W.H. Pierre and A.G. Norman (ed), Soil and Fertilizer
Phosphorus in Crop Nutrition, Academic Press, Inc.,
New York, 1953.
16. Voss, R.E., Hanway J.J., and Dumenil, L.C.
"Relationship Between Grain Yield and Leaf N,P,
and K Concentrations for Corn (Zea mays L.) and the
Factors That Influence This Relationship," Agron.
}., 62: 726-728, 1970.
-------
UTILIZATION OF DIGESTED CHEMICAL SEWAGE
SLUDGES ON AGRICULTURAL LANDS IN ONTARIO
STEVEN A. BLACK
Ontario Ministry of the Environment
Province of Ontario, Canada
INTRODUCTION
Sewage sludge is a by-product of the sewage treat-
ment plant process and its subsequent treatment
and disposal is considered to be the most costly
phase of sewage treatment. Although a number of
alternatives exist for the handling of sewage
sludge, the predominant method used in Ontario
involves anaerobic digestion followed by dumping
into sanitary landfills or disposal onto farmers'
fields.
Keeping in step with the overall concept of
recycle, reuse and reclamation, Ontario is now
recommending, wherever possible, the utilization
of digested sewage sludge on articultural lands.
Interim guidelines and regulations governing
sewage sludge application to agricultural lands
have been established by the Ontario Ministry of
the Environment in conjunction with agricul-
tural and health concerns. Through provision of
adequate controls and continual upgrading as new
technology is developed, these guidelines and
regulations will ensure that this practice may be
pursued with maximum benefit while minimizing
hazards of soil, crop and ground and surface water
contamination.
Sludge Production
With a population of some 7.7 million people
(1971 Census), Ontario has a total of some 360
sewage treatment plants of which 230 are
mechanical, primary or secondary plants (71 per-
cent are secondary) with the remainder being
lagoons. The largest plant has a design capacity of
180 MIGD, only six percent of the plants have a
capacity greater than 10 MIGD and 71 percent have
a design capacity of 2.0 MIGD or less.
If one assumes that sludge production from
sewage treatment is equivalent to 0.5 percent of the
flow, there are approximately 4.3 million gallons of
sludge at from three to six percent solids produced
per day in Ontario which must be suitably disposed
of. The cities of Toronto, Hamilton, London and
Ottawa account for about 50 percent of the sludge
production.
Ontario has two basically different kinds of
sludge disposal problems. The highly populated
metropolitan areas produce massive volumes of
sewage sludge and cost of transportation and dif-
ficulty of finding suitable disposal sites are major
factors in the choice of sludge disposal systems at
their relatively large sewage treatment plants. At
the small sewage treatment facilities dispersed
throughout the remainder of the Province, disposaj
sites can usually be found within realistic travelling
distances and any form of sludge thickening or
dewatering generally only adds to the disposal cost.
Present Sludge Disposal Practices
Present sludge disposal practices used in Ontario
include incineration, lagooning, dumping into
sanitary landfills, disposal onto farmers' fields, and
drying and stockpiling for home garden usage.
Metro Toronto, Hamilton and London each have
sludge incinerators and between them incinerate
approximately 41 percent of the sludge produced in
Ontario. About 70 percent of the remainder of the
sludge is disposed of onto farmers' fields.
For the major percentage of the sewage treat-
ment plants in Ontario, land application of the
107
-------
108 MUNICIPAL SLUDGE MANAGEMENT
sludge is perhaps the most satisfactory solution to
the sludge disposal problem. It is not necessarily
cheap, averaging out at about $32.00 per ton dry
solids when compared to reported incineration
costs ($10 to $50 per ton dry solids total U.S.) but
for a small municipality incineration is out of the
question.
Although land application is the most common
method of sludge disposal in Ontario, in most in-
stances it is looked upon as a disposal rather than a
utilization practice. Sludge application rates used
are generally higher than they should be and the
farmers, although realizing the benefit from the
sludge, seldom take into account the nitrogen and
phosphorus so added when applying commercial
fertilizers.
Agricultural Value of Sludge
The value of sewage sludge as a fertilizer and soil
conditioner has been well documented. Sewage
sludges have a favourable effect on soil properties
by increasing field moisture capacity, non-capillary
porosity, cation exchange capacity, organic matter
content and soil aggregation. All of these factors
tend to render the soil more suitable to plant
growth through improved aeration, greater ease of
root penetration, increased rate of water infiltra-
tion and improved availability of nutrients.
As a fertilizer, sewage sludge contains on the
average, about four percent nitrogen and two to
three percent phosphorus by weight. An applica-
tion of three to four tons of sludge per acre will
provide adequate nitrogen and phosphorus for a
healthy crop of corn. One thousand gallons of
sludge will have a fertilizing value of about $7.50.
Sewage sludges also contain varying amounts of
potassium and other macronutrients essential for
plant growth.
One must not, however, overlook the possible
dangers involved in its usage and its misuse.
Besides containing the major nutrient materials,
sewage sludge also contains varying concentrations
of heavy metals and complex organic compounds
which could lead to crop contamination and soil and
water pollution.
Probably the greatest concern in Ontario at pre-
sent in the agricultural utilization of sewage
sludges relates to their heavy metal content and the
possible long term effect these metals may have on
the soils, crop and subsequent food chain.
Although several of the heavy metals contained
in sludge are considered to be essential for plant
growth, and some Ontario soils are naturally
deficient in one or more of these metals, there is
often a narrow margin of concentration between
when an element is a nutrient and when it becomes
a toxicant. In addition, many of the metals con-
tained in sludges are non-essential to plant life and
their presence in the soil must be viewed with
suspicion.
Nitrogen, if applied in excess to the soil, may
readily leach out into the groundwater.
Phosphorus on the other hand is tied up in mineral
soils in the uppermost layers of the soil. If the soil
phosphorus level is permitted to build up too high,
soil erosion and runoff may contribute high levels
of phosphorus to the receiving stream. Sludge
application rates must therefore be geared towards
optimum usage of these nutrients by the crop.
Ontario's Phosphorus Removal Program
As a result of the 1969 International Joint Com-
mission report1 recommending that phosphorus
discharges from all sources in the Lower Great
Lakes be reduced to the lowest practical level, the
Province of Ontario announced a policy requiring
the installation of phosphorus removal facilities at
municipal and institutional wastewater plants in
both the Lower Great Lakes areas and in inland
recreational waters.
Initially, the policy required a minimum removal
of 80 percent of the phosphorus from wastewater
plant influents with the need for higher levels of
removal to be determined by further studies of the
receiving waters. This criterion was subsequently
superseded in the Lower Great Lakes by the sign-
ing, in April, 1972, of the Canada-United States In-
ternational Agreement on Great Lakes Water
Quality2 which called for an effluent objective of 1
mg/1 total phosphorus.
Permanent phosphorus removal facilities must
be operational by December 31, 1973 in the most
critically affected areas of the Province, by
December 31, 1975 for those facilities discharging
to waters deemed to be in a less critical condition,
and three years after notification in all other areas
of the Province where problems are found to exist.
Figure 1 outlines the scheduled phosphorus
removal compliance dates for the southern section
of the Province of Ontario.
Under this program, approximately 100
mechanical plants are affected by the 1973 date, and
a further 50 by the 1975 date, leaving 72 for future
notification.
Land disposal of sewage sludges has been prac-
ticed in Ontario for many years without the iden-
tification of any specific problems. The ultimate
disposal of sludge was generally of no concern to
the sewage treatment plant operator, however, and
-------
UTILIZATION ON AGRICULTURAL LANDS 109
LEGEND
DEC. 31 , 1973
o::c. 31, 1973
DEC. 31, 1975
D
ALL PLANTS
' PLANTS LARGER
THAN 1 mgd OR
WHERE LOCAL
PROBLEMS
DEMONSTRATED
PLANTS LARGER
THAN 1 mgd
STUDIES REQUIRED
TO DETERMINE
NEED
Figure 1: Province of Ontario—Southern Section. Phosphorus Removal Program Scheduled Compliance Dates.
no particular effect was placed on evaluating this
method of disposal.
With the onset of the Province-wide phosphorus
removal program, however, whereby about 85 per-
cent of all the sewage treatment facilities in On-
tario will be practicing phosphorus removal by the
end of 1975, a much closer look is now being taken
at this method of disposal with a strong emphasis
on utilization. More consideration is being given to
the benefits and hazards involved with the sludge
contained nutrients, heavy metals, long chain
organics and the micro-organisms on soils, crops
and ground and surface waters.
Chemical Sludges for Land Utilization
As all phosphorus removal facilities in Ontario
will employ chemical precipitation processes, con-
siderable concern has been expressed as to the
acceptability of the chemical sludges so produced
for land utilization.
The use of chemicals in sewage treatment in-
creases the weight of solids produced, generally in-
creases the volume of sludge to be disposed of and
alters its composition. The chemicals will
precipitate out more of the heavy metals and
phosphorus and will affect the nitrogen fractions
and the sludges become more chemical in nature.
The metals used for phosphorus precipitation,.
aluminum, iron and calcium are themselves
however, abundantly found in natural soils and the
extra sludge solids will tend to dilute the extra
metals and phosphorus precipitated. Thus, it is felt
that when considering their effects on agricultural
soils, normal and chemical sludges may be treated
alike, except for possible effects on nitrogen
availability and sludge decomposition rates.
Because of the desire to ensure the acceptability
of the practice of digested sewage sludge utilization
on agricultural land, the governments of Canada
and Ontario are supporting a comprehensive
program of coordinated studies on sewage sludges
under the Canada/Ontario Agreement on Great
Lakes Quality3.
-------
110 MUNICIPAL SLUDGE MANAGEMENT
Canada/Ontario Agreement Research
Under the Canada/Ontario Agreement3, joint
funding has been provided by the governments of
Canada and Ontario for the upgrading of sewage
treatment facilities in the Lower Great Lakes Basin.
A total of six million dollars has been provided over
a five year period to conduct related research
studies.
One of the areas of high priority for research
work involves sludge treatment and disposal, and a
total of seven projects dealing with sewage sludge
utilization on agricultural lands are being funded.
These projects may be divided into the five main
areas as listed in Table 1.
TABLE 1
Research Projects - C/OA
/ - Characterization of Sewage Sludges
Nutrients
Heavy Metals
Organic Compounds
Virus
2 - Laboratory & Greenhouse Studies
Heavy Metal Availability
Sludge Decomposition
j - Field Trial Studies
Plant Uptake
Runoff
4 - Lysiineier Studies
5 - Application Equipment
Projects are well underway in attempting to
characterize sewage sludges as to variability both
within a single source and between different
sources in relation to heavy metals, organic com-
pounds and viruses as well as nutrients.
Laboratory and greenhouse studies are being
carried out in an attempt to determine the rate of
decomposition of sewage sludges in soils as affected
by the availability of nitrogen and the soluble-
insoluble relationships of heavy metals. Extrac-
tants are being evaluated for one which may be
used to determine the crop available portion of
heavy metals under specific soil environments.
Long term field studies have been initiated to
determine the maximum rate of sludge application
on an annual and perennial crop, as influenced by
soil texture, which will maintain crop quality and
yield and avoid contamination of groundwater.
Long term runoff studies are also being carried out
in an attempt to relate sludge application rate to the
quality of runoff from two different slopes.
Lysimeter studies are being funded to determine
the environmental effects of chemical sludge dis-
posal on land including effects on soil properties,
plant growth and the transport of organic and in-
organic constituents of chemical sludges within
soils and to groundwater.
A further study evaluating various types of
application equipment in relation to the physical
damage of wet soils has now just been completed.
Out of these studies it is anticipated that
regulations may be drawn up to effectively control
the utilization of sewage sludge as an organic fer-
tilizer on agricultural land without adversely affec-
ting soil, crop, ground or surface waters with
nutrients, heavy metals, organic materials or
pathogenic organisms.
Guidelines on Land Utilization of Sewage
Sludges
Until the current studies of the land utilization
of sewage sludges are completed, it would be inop-
portune to establish firm regulations concerning
this method of sludge disposal. Therefore, the
Ministry of the Environment, in consultation with
the Ministry of Agriculture and Food, University of
Guelph and Ministry of Health has adopted a set of
interim guidelines to be used in the meantime. Ul-
timately, some of these guidelines will be added to
regulations which already exist concerning the
location and management of the site.
The interim guidelines cover the items as listed in
Table 2 and are outlined in Appendix A. Guidelines
are also being prepared in relation to the heavy
metal content of the sludge. Under these new
guidelines, there will be acceptable and non-
acceptable sludges for land application. Non-
acceptable sludges will be those which contain one
or more heavy metals in excess of maximum
allowable concentrations. A maximum allowable
concentration in the sludge for each metal will be
set from a knowledge of the heavy metal content of
TABLE 2
MOE Interim Guidelines
1 - Site Location
2 - Land Characteristics
3 - Site Management
4 - Application Rates
5 - Application Equipment
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UTILIZATION ON AGRICULTURAL LANDS 111
domestic sewage sludge and the degree of treat-
ment available for the various industrial wastes
contributing heavy metals to municipal
wastewaters. Such a guideline should provide a
positive incentive to the municipalities to enforce
industrial bylaws covering industrial discharges to
sewers as well as protecting the agricultural lands
where sludges are applied.
REFERENCES
1. "Pollution of Lake Erie, Lake Ontario and the
International Section of the St. Lawrence River,"
Volume 1, Summary, International Joint Commis-
sion, 1969.
2. Canada/United States International Agree-
ment on Great Lakes Quality, April, 1972.
3. Canada/Ontario Agreement on the Lower
Great Lakes, August, 1971.
APPENDIX A
Guidelines for Utilization
of Processed Organic Waste
by Land Application
NOTE.-
a) The following pertains to the utilization of
processed organic waste which has undergone
proper anaerobic or aerobic digestion or other
suitable processing at a municipal water pollu-
tion control plant.
b) It is intended that the method of land applica-
tion entail the utilization of processed organic
waste in the agricultural industry, as opposed
to merely disposing of the material.
Emphasis should be placed on the aspect of
utilization as stated, embodying the proposed
principles relating to rates of applications,
since these will markedly reduce the potential
for run-off loss. It is suggested that the lower
limit of a range be used on sloping land. For
minimum distances to water course, use same
figures for winter as for summer.
I. SITE LOCATION
1.1 The site should be remote from surface
water courses. The minimum distance
between the site and the surface water
course should be determined by the land
slope as follows:
Maximum
Sustained
Slope
0 to 3%
3 to 6%
6 to 9%
greater
than 9%
Minimum Distance to Watercourse
For processed organic waste
application during May to Nov.
inclusive
200 feet
400 feet
600 feet
No processed organic waste
to be applied unless special
conditions exist
For processed organic
waste application during
Dec. to Apr. inclusive
600 feet
600 feet
No processed organic
waste to be applied
No processed organic
waste to be applied
1.2
1.3
1.4
1.5
1.6
The site shall be at least 300 feet from in-
dividual human habitations.
The site shall be at least 300 feet from
water wells.
The site shall be at least 1,500 feet from
areas of residential development.
No spreading shall be done when there is
more than 3 inch frost or a solid ice layer on
the surface of the soils, particularly when
there is no snow cover. (It is considered that
waste applied to a snow cover when there is
little or no frost in the soil below the snow
will be relatively safe from run-off.)
Above values are for rapid to moderately
rapid permeability. For moderate to slow
permeability the distances should be
doubled and spreading suspended during
March and April when run-off is expected.
Use footnote from item 2 to determine
permeability classification.
In Northern Ontario suspend spreading on
moderate to slowly permeable soil during
April and May.
The groundwater table during processed
organic waste application should be not less
than 3.0 feet from the surface for soils with
moderate to slow permeability. For soils
with rapid to moderately rapid permeabili-
ty, the groundwater table should be not less
than 5.0 feet from the surface.
2. LAND CHARACTERISTICS
2.1 The land slope and soil permeability will
determine the time of year that processed
organic waste may be applied as follows:
3. SITE MANAGEMENT
3.1 When sludge is applied to agricultural land,
the land is to be for those crops specified in
these Guidelines. Dairy cattle should be ex-
cluded from pasture land. These restric-
tions on land use shall apply from the date
of application until the end of the calendar
year during which the processed organic
waste has been applied.
1.7
1.8
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112 MUNICIPAL SLUDGE MANAGEMENT
3.2 The boundaries of the site shall be marked
(e.g. with stakes at corners) so as to avoid
confusion regarding the location of the site
during processed organic waste application,
or during the taking of soil or crop samples.
The markers should be maintained until
the end of the current or subsequent grow-
ing season, whichever is applicable.
3.3 Soil tillage and processed organic waste
application should where possible, follow
the contours of the land (to maintain a con-
tour furrow system). Passage of processed
organic waste spreading vehicles over the
land should be minimized to reduce com-
paction of the soil (e.g. the allowable
processed organic waste application rate in
cu. yds./A/yr., could be achieved after one
or two passes).
3.4 Special precautions may be required where
the possibility of localized surface water
runoff problems exist.
Max. Sustained
Slope
0 to 3%
3 to 6%
b to 9%
Soil Permeability"
any
rapid to moderately
rapid
moderate to slow
rapid to moderately
rapid
Allowable Duration
of Application
Southern Northern
Ontario Ontario
12 mo/yr 12 mo/yr
12 mo/yr 12 mo/yr
10 mo/yr 9 mo/yr
(May to (June to
Feb.) Feb.)
7 mo/yr 6 mo/yr
(May to (June to
Oct.) Oct.)
'Soil permeability classification shall be in accordance with Tables 1 and 2 of the
Ministry of Agriculture and Food's publication entitled "Drainage Guide forOntario."
The type of soil should be determined with the use of County Soil Maps available
through the Ministry of Agriculture and Food
Organic wastes are best applied to un-
plowed soil with the residues of the pre-
vious crop present to control runoff. This is
particularly useful for winter spreading.
3.5 Where processed organic waste application
is carried out by tank truck, untiled land
should be given preference to tiled land.
Where tiled land is used the processed
organic waste hauling contractor should
request instructions from the landowner,
with regards to minimizing the possibility
of damage to the tile system.
4. PROCESSED ORGANIC WASTE APPLICATION
RATES
4.1 In determining the allowable rate of
processed organic waste application for a
particular parcel of land, the objective shall
be to match as closely as possible the quan-
tity of nutrients removed from the soil by
the harvesting of the crop. The allowable
rate will thus be determined by the nutrient
content of the particular processed organic
waste and the nutrient uptake capabilities
of the particular crop under consideration.
The processed organic waste hauling con-
tractor shall adhere to the application rate
(in cu. yd./A/yr.) specified in the Certificate
of Approval issued by the Pollution Control
Planning Branch of the Ministry of the En-
vironment. The suitability of processed
organic waste application rates may, if re-
quired, be monitored by soil analyses
and/or crop analyses. The collection of soil
or crop samples shall be the responsibility
of the Pollution Control Planning Branch.
4.2 The processed organic waste shall be
spread uniformly over the surface of the
land.
4.3 The water pollution control plant operating
agency is to keep records of the location of
all the sites used for the disposal of its
processed organic waste and the processed
organic waste quantities disposed of at each
site, each week (e.g. Volume of processed
organic waste in cu. yds., and weight of
processed organic waste solids in tons). The
operating agency shall ensure that at least
every 2 months, samples of the processed
organic waste are submitted for thorough
analysis (e.g. total solids, volatile solids, pH,
nitrogen, phosphorus, potassium, ether ex-
tractables, heavy metals, etc.).
5. PROCESSED ORGANIC WASTE HAULING AND
SPREADING EQUIPMENT
5.1 Equipment should be maintained in good
working order at all times and should be
cleaned on a regular basis.
5.2 Before the tank can be used for any other
purpose, permission must be obtained from
the appropriate authority such as, the
Pollution Control Planning Branch, the
Ministry of Health, the local Medical Of-
ficer of Health, etc.
5.3 The processed organic waste should be
spread at least as wide as the widest part of
the spreading equipment to minimize the
number of passes required to spread the
processed organic waste on the site.
5.4 Some method should be provided to control
the spreading valve by the driver of the
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UTILIZATION ON AGRICULTURAL LANDS 113
spreading equipment while the vehicle is in
motion.
5.5 The spreading valve should not be opened
until the spreading equipment is in motion.
5.6 The spreading valve should be of the "fail
safe" type (i.e. self-closing) or an additional
manual stand-by valve should be employed
to prevent the uncontrollable spreading or
spillage of the processed organic waste.
5.7 Care should be taken under windy con-
ditions to avoid spreading out of the ap-
proved area.
5.8 The hauling equipment should be so
designed to prevent the possibility of
spillage, the dissemination of odours, and
other public nuisances during transport.
5.9 If the processed organic waste is
transferred from the hauling equipment to
separate spreading equipment, the transfer
should be carried out under controlled con-
ditions to preclude spillage, perhaps using a
closed-type transfer system.
5.10 Backup equipment should be readily
available in the event of breakdown of
equipment on the highway or site.
5.11 A pumped or pressurized spreading
method must be considered the preferred
technique to obviate an even and consistent
spreading of the processed organic waste
on the site.
-------
THE ECONOMICS OF SLUDGE IRRIGATION
A. PAUL TROEMPER
Springfield Sanitary District
Springfield, Illinois
The existing plant of the Springfield Sanitary
District was completed and placed into operation in
1928. Since that time, the District has utilized
several of the commonly used methods of anaerobic
sludge disposal, including sludge drying beds and
shallow sludge lagoons. Because of the decreased
demand for municipal sewage sludge in the early
1960's and a considerable change in farming prac-,
tices in the area, the District decided to experiment
with disposal of the sludge on our own land. It was
reasoned that it would be more economical to apply
it to our land in the liquid state and thus save the
cost of bulldozing the sludge from lagoons, loading
it, hauling it and spreading it in the dry state on our
own land.
A plot of approximately eight acres of very poor
agricultural land was set up for test purposes. A
road was built, the land was cleared, ravines were
filled and a pipe line was built from the digester area
to the sludge disposal area. Discharge of sludge to
the experimental plot was begun in 1965.
Without resorting to a recital of the various
problems encountered in the course of this ex-
perimental work, it nevertheless becomes obvious
that the development of this process of sludge
irrigation was arrived at by rationalization of a
logical sequence of sludge disposal methods. Since
funds were not available for a highly scientific
study, it was necessary that we carry on our studies
in a most rudimentary manner and within the very
limited funds available to the District for this pur-
pose. We made our mistakes as we proceeded and
corrected them as best we were able. Through all of
this trial and error operation, it still became obvious
that liquid disposal of anerobically digested sludge
to the land was a feasible and economic method of
sludge disposal.
While a project such as this is designed primarily
as a means of disposal of sludge, one of the side
benefits from it is the fertilization and buildup of
the soil to the point where a better crop yield is ob-
tained. Records were kept on the yield from the
area on which sludge was applied as compared to an
area immediately adjacent on which sludge was not
applied. These are summarized in Table 1. During
the crop year 1965, a yield of 107 bushels of corn
per acre was obtained from the area on which liquid
sludge was applied as compared with a yield of 88
bushels per acre on the adjacent area with no
sludge. This worked out to a net dollar yield per
acre of $65.19 for that area receiving the sludge.
The increased yield in dollars per acre as compared
to the adjacent land was $18.05 per acre. During
the year 1966, the crop was not as good throughout
the entire area. A yield of 76 bushels per acre was
obtained from the area on which sludge was applied
as compared to the yield of 39 bushels per acre on
the area receiving no sludge. This resulted in a net
dollar yield per acre of $49.02. The increased yield
in dollars per acre was $44.03. During the year
1967, it was disasterous climatically for this type of
operation. A wet spring delayed planting to the
point where corn could not be planted on the test
plot because of the lateness of the season. As a
result, soybeans were planted on four of the five
terraces, the fifth being too wet to plant. A wet
summer produced an excellent crop; however, an
extremely wet fall prevented harvesting of the crop
and the soybean crop was ultimately abandoned
and left in the field. As a result, there is no com-
115
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116 MUNICIPAL SLUDGE MANAGEMENT
TABLE 1
Tabulation of Crop Yields and Sludge Application
Crop Yield- Bui Ac.
Crop
Year
1965
1966
1967*
1968
1969
1970
1971
Sludge
Irr.
Area
107
76
62
82.6
No
Liquid
Sludge
Area
88
39
32.5
70.5
Increase
Crop
Yield
Bui Acre
19
37
29.5
12.1
Net S
Yield
Per
Acre
$65.19
49.02
-20.00
**7.07
**9.86
Corn
Price
S/Bu.
$0.95
1.19
0.897
0.954
Increased 5
Yield Per Ac.
of Liquid
'Sludge Area
Over Other
$18.05
44.03
26.46
11.54
Actual
Acres
Planted
1
1
0
4
2.9
0
Sludge
Applied
Thousand Tons D.S.
Gallons
3,415
2,360
290
1,105
2,260
Per Acre
106
67
11**
36**
64
Cost Per
Ton of
Dry Sol.
$0.97
1.77
17.05
4.46
2.62
Avg.
81.9 57.5
24.4
$22.23 $0.998
*Terraces tiled at cost of $8,005.73
**Based on 7 acres
***Excludes 1967 data in average
parison during the year 1967 and the net dollar
yield per acre was a minus $20.00 because of the ex-
pense occurred in planting the crop and the fact
that it was not harvested. During the year 1968,
four out of the seven acres were planted in corn
with the other three being utilized for sludge dis-
posal. Also, dried sludge removed from the sludge
lagoons had been applied to the adjacent land to
build up its fertility, so the direct comparison with
sludged and unsludged land did not apply as well.
However, there was a yield during the 1968 season
of 62 bushels per acre on the area on which liquid
sludge had been applied as compared to 32.5
bushels per acre on the adjacent land. This resulted
in a dollar yield per acre of $7.07 as spread over the
entire seven acre test plot. It also would have
amounted to an increased yield of $26.46 per acre
based on the actual acres planted in corn in that
year. During the year 1969, 2.9 acres out of the
seven acre plot were planted in corn; the other four
acres being utilized for sludge disposal. Dried
sludge had continued to be applied to the adjacent
land to build up its fertility. The land with liquid
sludge application had a yield of 82.6 bushels per
acre of corn during that year as compared to 70.5
bushels on the adjacent land without the liquid
sludge. This resulted in a dollar yield per acre of
$9.86 when spread over the entire seven acre test
plot. The increased yield amounted to $11.54 per
acre based on the actual acres planted. During the
year 1970, no comparison between the two areas is
available. Crops were not planted in the entire area
for the year 1971 because of the imminence of con-
struction in this particular area. Average produc-
tion on the sludged areas over the four years of
record was 81.9 bushels of corn per acre while on
the unsludged area it was 57.5. This is an increase in
yield of 42 percent. It may be concluded that while
the dollar value of crops harvested is a factor to be
reckoned with, it is not a controlling factor in the
operation of such an area.
The basic reason for operation of a liquid sludge
disposal area is, of course, disposal of sludge. Dur-
ing the period March 1965 through February 1966,
a total of 3,415,000 gallons of sludge or 1,695,400
pounds of dry solids were applied to the test plot.
This amounted to 211,925 pounds of dry solids per
acre of 106 tons per acre. This was applied during
the period July 1 to September 1. Corn had been
planted in the plot and sludge was not applied until
the corn was approximately 18 inches high and had
been cultivated once. It was then applied by run-
ning it down the corn rows as often as the ground
was dry enough to take it. This was discontinued on
September 1 to allow the field to dry for harvesting.
After harvesting in November, the land was plow-
ed and disced and sludge applied through the winter
until March 1 when it was discontinued to allow the
land to dry for planting.
The period March 1966 to February 1967 was not
as favorable climatically for this type of operation
as was previously explained. In spite of the poor
year during this period, a total of 2,360,000 gallons
of sludge or 1,075,925 pounds of dried solids were
applied to the test plot. This amounted to 134,491
pounds of dried solids per acre or 67 tons per acre.
It was pointed out previously that the period
March 1967 to February 1968 was disastrous
climatically for this type of operation. Some sludge
was applied to the test area; however, this
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ECONOMICS OF IRRIGATION 117
amounted to a total of 290,000 gallons or 157,201
pounds of dry solids. This was only applied on the
fifth terrace, the other four terraces being planted
in soybeans which could not be irrigated with
sludge during the growing period. Using these
figures over the entire test plot area, this amounted
to 19,650 pounds of dry solids per acre or 10 tons
per acre disposed of on the test plot.
During the period March 1968 to February 1969,
1,105,000 gallons of sludge were applied on three of
the seven acres of the test plot, the other four acres
being utilized for crop production. A total of
503,805 pounds of dry solids were applied or 71,972
pounds or 36 tons per acre of dry solids when
applied over the entire seven acre tract.
The years March 1969 to February 1970 and
March 1970 to February 1971 were not reported on
as previously explained. During the year March
1971 to February 1972, no crop was planted on the
area because of anticipated construction. However,
sludge was continued in application over the entire
seven acres. A total of 2,260,000 gallons of sludge
were applied during this period or a total of 891,351
pounds of dry solids. This worked out to an aver-
age of 127,336 pounds of dry solids or 64 tons
applied per acre on the entire seven acre tract.
Attempts were made to make an economic
analysis of the sludge disposal operation. The land
had been purchased immediately prior to 1965 at a
cost of $459.96 per acre. Road construction in the
area to allow operations added $46.40 per acre.
Clearing and filling of ravines accounted for $48.12
per acre. Terracing of the land amounted to
$812.51 per acre. The pipeline to supply sludge to
the area amounted to $536.46 per acre. The tiling of
the terraces done in 1967 amounted to $1,143.67
per acre.
It was assumed at that time that the life of the
area for sludge disposal purposes would be ten
years and it was also assumed that the land would
increase in value to $800.00 per acre because of
building up soil fertility. If it is assumed that one-
half of the cost of tiling the land is spread over this
ten year period, we may then arrive at an annual
cost of the land (exclusive of interest) on the invest-
ment of $459.96 plus $46.40 plus $48.12 plus
$812.51 plus $536.46 plus $571.80 minus $800.00
all divided by 10 which equals $167.53 per acre.
As was previously pointed out, the net yield per
acre in 1965 was $65.19. Subtracting this from the
$167.53 annual cost would give a cost of $102.34
per acre as the cost of sludge disposal. Dividing the
$102.34 by 106, the tons of dry sludge applied per
acre, would give a cost of $.97 per ton of dry solids
as the cost of sludge drying and disposal (exclusive
of digestion and pumping costs).
Applying the same figures for the crop year 1966
in which a net dollar yield of $49.02 per acre was
achieved and during which 67 tons of dry solids
were applied per acre, we would arrive at a cost of
$1.77 per ton of dry solids as the cost of sludge dry-
ing and disposal. During the year 1967, which was
previously pointed out as being disastrous
climatically and during which time the net dollar
yield per acre of minus $20.00 was obtained and
only 11 tons of dry solids applied per acre, a cost of
sludge drying and disposal of ,$17.05 per ton of dry
solids was obtained. During the crop year 1968 in
which a net dollar yield per acre of $7.07 over the
entire seven acres was obtained and in which sludge
disposal to the extent of 36 tons of dry solids per
acre over the entire seven acres was obtained,
would give a cost of sludge drying and disposal of
$4.46 per ton of dry solids. Again applying the same
mathematics to the year 1971 in which no crop was
planted or no income was received from crop and
during which time 64 tons of dry solids were
applied per acre, the cost of sludge drying and dis-
posal amounted to $2.62 per ton of dry solids.
If we average the five years of 1965/66, '67, '68
and '69, we obtain an average net yield of $22.23
per acre. Also averaging the sludge application dur-
ing the years 1965, '66, '67, '68 and '71, we would
arrive at an average of 56.8 tons of dry solids
applied per acre. If we drop 1967, the four year
average would amount to $2.46 per ton of dry
solids as the cost of sludge drying and disposal.
While we experienced some very good annual
results, we also experienced some disastrously poor
ones. However, in spite of this, we felt that the
method had application. We, in fact, had sufficient
confidence in the method that it was decided after
the 1968 season to go to this method of sludge dis-
posal for the anaerobically digested sludge produc-
ed at the existing plant of the Springfield Sanitary
District and to also utilize this method of sludge dis-
posal for aerobically digested sludge at a new plant
at the District.
The Springfield Sanitary District has been com-
pleted and has recently begun utilization of perma-
nent liquid sludge application areas in two
locations. An approximate 30 acre site is being
utilized for this purpose at the existing Spring
Creek Plant of the District. Also, a 36 acre site is
being utilized for liquid disposal of aerobically
digested sludge at the new Sugar Creek Treat-
ment Works. A permanent underdrainage system
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118 MUNICIPAL SLUDGE MANAGEMENT
is provided at each installation to carry the under-
flow from the sludge irrigation area back to a pump
pit from which it is pumped back into the aeration
tanks of the treatment works. A permanent system
of force mains to allow spray irrigation of the
sludge is provided. The force main system is buried
at sufficient depth and valved in such manner that
it can be utilized for sludge application in incle-
ment winter weather without danger of freezing.
The force main system is laid to grade so that fol-
lowing each sludge application, the contents of the
force main will drain back to the pump pit and be re-
turned to the aeration tanks. Both of these installa-
tions were utilized during the winter of 1973-74
and proved the feasibility of winter operation.
While we do not recommend this, on one occasion
we did spray irrigate sludge when the temperature
was 10 degrees below zero.
Costs of developing the sludge disposal areas at
these plants are possible to break down as based
upon the low bid received on each contract. The
itemization of the various costs involved in the
sludge disposal system at the Sugar Creek Plant are
tabulated in Table 2. The itemization of the costs
based on the low bid at the Spring Creek Plant are
tabulated in Table 3. Operation costs are nominal at
the Sugar Creek Plant consisting of a small amount
of operator attention (approximately one hour per
week) and electric power costs. Operation costs at
the Spring Creek Plant are higher because of
TABLE 2
Development Costs
Sugar Creek Sludge Disposal Area
Bid Prices of Low Bidders on Project
(Bids taken September 14, 1971)
Pumping Station
Structure (Est. 1/3 of $94,932)
Equipment (Est. 1/3 of $36,072)
Sludge Comminutor
Total Pumping Station
Sludge Distribution
1,820' of 8" C.I. Force Main at $9.90
5,735' of 6" C.I. Force Main at $7.05
8" Valves - 8 at $250
6" Valves -44 at $175
Fittings -9,415 tt at $0.60
Spray Nozzles 6 at $440
Total Sludge Distribution System
Disposal Area Underdrainage
23,610' of 4" perforated PVC pipe
at $2.00
1,300' of 6" perforated PVC pipe
at $2.64
640' of 8" perforated PVC pipe
at $4.00
Underdrainage Pump Pit & Controls
Total Underdrainage System
Miscellaneous
4,000' of woven wire stock fence
at $2.00
Electrical (1/20 of $253,700 -
total plant elect, bid)
Total Miscellaneous
Total Development Cost (Exclusive of Land)
$206.950.75 $5,792.07 per acre development
35.73 cost of disposal area
IMIU! Cost
2,075' x 750' 35.73 acres
35.75 at $750 per acre land cost $26,797.50
$31,644.00
12,026.00
6,352.00
$18,018.00
40,431.75
2,000.00
7,700.00
5,649.00
2,640.00
$47,220.00
3,432.00
2,560.00
6.593.00
$ 8,000.00
12,685.00
$50,022.00
$76,438.75
$59,805.00
$20,685.00
$206,950.75
$233,748.25
Total Disposal Area Cost
S233.748.25 $6,542.07 per acre total cost
35.73 of disposal area
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ECONOMICS OF IRRIGATION 119
TABLE 3
Development Costs
Spring Creek Sludge Disposal Area
Bid Prices of Low Bidders on Project
(Bids taken March 28, 1972)
Screening & Pumping of Sludge
Structure $ 6,275.00
Equipment 15,768.00
Total Screening & Pumping $22,043.00
Sludge Distribution
6,456' - 6" C.I. force main at $3.25 $20,962.50
900' - underdrain return at $8.56 7,704.00
25 - spray irrigation headers &
risers at $321.00 8,025.00
2 - spray nozzles at $496.00 992.00
Total Sludge Distribution System $37,683.50
Disposal A rea Underdrainage
35,500' - 4" perf. PVC pipe at $2.50 $88,750.00
1,300' - 6" perf. PVC pipe at $3.25 4,225.00
2,475' - 12" perf. PVC underdrain
at $8.30 20,542.50
Underdrain lift station 2,657.00
Underdrain lift station equipment 8,294.00
Total Underdrainage System $124,468.50
Miscellaneous
Pipe & Foot Bridge relocation $10,523.00
Prevision & adjustment of bridge 365.00
Pipe supports & Braces on bridge 1,340.00
Electrical (1/20 of $134,939 -
total plant elect, bid) 6.746.95
Total Miscellaneous $18,974.95
Total Development Cost (Exclusive of Land) $203,169.95
$203.169.95 $6,772.33 per acre development
30 cost of disposal area
Land Cost
20 acres at $459.96 per acre $ 9,199.20
10 acres at $500.00 per acre 5,000.00
Total Land Cost $14,199.20
Total Disposal Area Cost
$203,169.95 + $14,199.20 = $7,245.63 per acre total cost
30 of disposal area
problems with the screening and pumping installa-
tion; however, they are expected to be comparable
to the Sugar Creek Plant when pumping and
screening problems are resolved.
Data on sludge irrigation at the Spring Creek
Plant since start up are tabulated in Table 4, and
similar data for the Sugar Creek Plant are tabulated
in Table 5. The data in both instances has been com-
puted on a per acre per year basis to allow for com-
parison with installations elsewhere. It will be
noted that a much heavier hydraulic application
was made in the case of the aerobically digested
sludge, as would be expected, but the pounds of
solids applied per acre were in reasonably the same
range. The anaerobic sludge was applied at a rate
approximately 4-3/4 inches depth per year compared
to the aerobic sludge application of 14'-% inches
depth per year. The anaerobic sludge was applied at
the rate of 27 tons per acre per year of dry solids
compared to 22-Vi tons for the aerobic sludge.
We are well aware that there are many questions
yet to be answered with regard to this method of
sludge disposal. Answers to these questions will
evolve as more experience is obtained by ourselves
and others. We believe that because of the under-
drainage system, we are in a position to collect data
on this method of sludge disposal that no other pre-
sent installation has available to them. We are,
therefore, planning on an extensive data collection
project at both plants which we would hope to
report on at a later time. Hopefully, this will be
helpful in answering some of these questions.
-------
120 MUNICIPAL SLUDGE MANAGEMENT
TABLE 4
Spray Irrigation of Anaerobically
Digested Sludge at
Spring Creek Plant
TABLE 5
Spray Irrigation of Aerobically
Digested Sludge at
Sugar Creek Plant
Date
10/16/73
10/17/73
10/18/73
10/19/73
10/22/73
10/23/73
11/1/73
11/2/73
11/15/73
11/19/73
11/28/73
11/30/73
12/1/73
12,7/73
I2/ 13/73
12/26/73
12/27/73
12/28/73
1/15/74
1/16/74
3/4/74
3, '28/74
Total
Average
Annual Per
Acre Average
Gallons
60,000
60,000
50,000
60,000
80,000
70,000
80,000
80,000
80,000
70,000
60,000
15,000
40,000
80,000
40,000
120,000
60,000
50,000
230,000
180,000
100.000
100,000
1,765,000
129,360
%
Solids
5.38
4.93
4.92
4.80
4.89
4.92
5.24
4.87
4.92
5.68
5.27
4.75
5.16
4.90
4.67
5.25
5.42
5.18
5.24
4.57
4.99
4.92
5.04
%
Volatile
41.6
43.1
43.2
41.9
42.6
48.7
43.8
44.3
42.3
44.1
45.0
38.5
46.2
45.3
40.3
44.6
46.0
45.5
42.4
47.6
45.4
43.7
43.9
Lbs.
Solids
26,922
24,653
20,516
23,994
32,635
28,740
34,995
32,500
32,876
33,185
26,371
5,942
17,196
32,693
15,562
52,492
27,121
21,601
100,580
68,672
41,649
41,033
741,928
54,378
Lbs.
Volatile
11,199
10,609
8,872
10,057
13,822
14,051
15,324
14,382
13,916
14,629
11,868
2,288
7,932
14,811
6,276
23,382
12,478
9,828
42,585
32,722
18,907
17,929
327,867
24,030
SUMMARY
While there are admittedly unanswered ques-
tions, it is yet possible to define a number of a posi-
tive factors with regard to this disposal method.
1. Irrigation of cropland with liquid digested
sludge is a valid method of disposal of sludges from
wastewater treatment works.
2. Spray irrigation of the sludge is the most prac-
tical method of application that we have tried.
3. Irrigation of cropland with liquid digested
sludge compares favorably on an economic basis
with any of the presently accepted methods of
sludge disposal if land is available for use of this
method.
4. Operation of this method is quite simple, re-
quiring a minimum of operation attention.
5. The method need create no odor nuisance if
the sludge is reasonably well digested before
application.
Dale
10/3/73
10/5/73
10/9/73
10/18/73
10/19/73
10/29/73
10/31/73
11/12/73
11/13/74
11/16/73
11/19/73
11/27/73
11/28/73
11/30/73
12/4/73
12/5/73
12/7/73
12/10/73
12/21/73
12/26/73
1,4,74
1/9/74
1/10/74
1/14/74
I/ 15/74
1/18/74
1/25/74
2/1/74
2/8/74
2/15/74
2,26/74
3; 6/74
3/20/74
Total
Average
Annual Per
Acre Average
%
Gallons Solids
43,000
298,000
170,000
85,000
85,000
213,000
43,000
341,000
128,000
107,000
107,000
107,000
213,000
128,000
213,000
298,000
298,000
277,000
171,000
320,000
277,000
319,000
107,000
107,000
149,000
234,000
256,000
288,000
330,000
341,000
341,000
332,000
203,000
.30
.41*
.41*
.41*
.41*
.41*
.41*
.19*
.19*
.19*
.19*
.19*
.19*
.13
.17
.68
.52
.14
.78
.51
.14
.11
.13
.21
.19
.34
.45
.36
.39
.43
.53
.55
.76
6,929,000
1.35
395,437
%
Volatile
51.3
52.7*
52.7*
52.7*
52.7*
52.7*
52.7*
54.9*
54.9*
54.9*
54.9*
54.9*
54.9*
56.1
56.6
56.3
56.8
57.2
59.4
60.5
58.8
59.7
58.2
60.8
61.2
63.7
60.9
57.9
54.8
55.3
55.7
53.6
49.8
56.07
Lbs.
Solids
4,654
35,039
19,980
9,983
9,983
25,042
5,048
33,832
12,697
10,615
10,615
10,615
21,134
12,057
20,779
41,748
37,772
26,334
25,383
40,287
26,334
29,526
10,080
10,793
14,780
26,143
30,958
32,654
38,253
40,655
43,498
42,904
29,797
789,972
45,084
Lbs.
Volatile
2,388
18,465
10,529
5,261
5,261
13,197
2,660
18,574
6,971
5,828
5,828
5,828
1 1 ,603
6,764
11,761
23,504
21,454
15,063
15,077
24,374
15,485
17,627
5,866
6,562
9,045
16,653
18,853
18,906
20,963
22,482
24,228
22,997
14,839
444,896
25,390
* Average concentration of tank contents used since sampler not yet
available.
6. Planning of a disposal area for spray applica-
tion must take into account wind drift if nuisance
from this is to be avoided.
7. Establishment of a permanent disposal area
with a sloped distribution system allows operation
in quite severe winter weather without particular
problems.
8. Provision of a good underdrainage system
minimizes the possibility of ground water con-
tamination.
-------
ECONOMICS OF IRRIGATION 121
9. While the method will work most advan- 10. In this day of concern over advantageous use
tageously in an open soil, use of the method is not of our natural resources, this method is particularly
confined to areas with an open soil; and it is an attractive, since it utilizes as a resource what
applicable method even in areas of a rather tight previously has been an odious end product of the
soil. wastewater treatment process.
-------
COMPOSTING SEWAGE SLUDGE
E. EPSTEIN AND G. B. WILLSON
Biological Waste Management Laboratory
Agricultural Research Service
U.S. Department of Agriculture
Beltsville, Maryland
In the past, sewage sludge has been used
occasionally as an additive in solid refuse (garbage)
composting V,4, but seldom has it been composted
by itself. Recently, increased interest in sludge
disposal on land has pointed up a need for more
study and information on sewage sludge
composting as an option in this process. Proper
composting of sludge would not only dewater it and
destroy objectionable odors but would also destroy
any disease organisms during the compost heating
process. Furthermore, compost is an aesthetic
product which can be handled easily and used in
urban areas.
To investigate sludge composting at a pilot plant
level, studies were initiated in the fall of 1972 by the
United States Department of Agriculture in
cooperation with the Maryland Environmental
Service and the Blue Plains Wastewater Treatment
Plant in Washington, D.C. 3,5. This project is
located on the grounds of the ARS Agricultural
Research Center at Beltsville, Maryland.
Composting systems generally fall into three
categories: (a) pile, (b) windrow, and (c) mech-
anized or enclosed systems J,2,4. The method
selected at Beltsville was the windrow system,
where thermophilic microorganisms generate heat
as a result of biological waste oxidation. Convective
air provides microorganisms with oxygen.
Convective forces cause the air to rise as it is
heated. Porosity and size of windrow will deter-
mine the rate of air exchange. If the windrow is too
large, dense, or wet, it may become anaerobic, thus
producing undersirable odors. A small porous win-
drow, on the other hand, may permit such rapid air
movement that temperatures remain low and
composting is delayed.
The preliminary studies at Beltsville involved the
use of three bulking agents—sawdust, shredded
paper, and wood chips—mixed with various
quantities of sludge. Twenty-two combinations of
materials including both raw and digested sludge
were tested. The sludge and bulking materials were
mixed in approximately five cubic yard batches and
stacked in a cone-shaped pile to simulate the effect
of a windrow. The piles were turned every few days
with a front-end loader. Temperatures and oxygen
levels within the piles were monitored to determine
the effectiveness of convection aeration. Bulk
density and moisture content were measured to
estimate the stage of composting. These tests were
conducted outdoors during January, February, and
March 1973. Pile temperatures were affected more
by frequent heavy rains than by ambient air
temperatures. All trials showed some signs of
composting. The shredded paper-sludge mixtures,
however, soon settled into a compact mass that air
could not penetrate. Both the chips and sawdust
continued to compost satisfactorily. Based on these
studies, wood chips were selected as the most
satisfactory bulking agent when incorporated with
sludge at a volumetric chip ratio of 3:1 sludge.
Site and Operations
Figure 1 indicates the location of the composting
site and the various facilities. The project covers 80
acres, of which 15 acres are used for the composting
operation. The remaining area is for isolation,
screening, and drainage water disposal. An aerial
view of the area is shown in Figure 2. Main sections
of the site are:
123
-------
124 MUNICIPAL SLUDGE MANAGEMENT
IRRIGATIOH AREAS
WASH
AREA ..COMPOST PAD
V
Figure 1: Composting Site, Agricultural Research Center, Beltsville, Maryland.
1. Windrow pad—a 5-acre area paved with 14
inches of crushed stone. This area can accom-
modate heavy equipment.
2. Wood-chip storage area.
3. Compost storage, curing, and screening area.
4. Weighing station and office building.
5. Truck wash area.
6. Runoff storage pond and irrigation disposal
system. The entire site, including the pad, is
graded to collect all runoff in a detention
pond. Several wells were installed around
the site to monitor the ground water quality.
7. Research area.
The major equipment used in the operations
consists of:
1. Cobey-Terex* composter
2. LeafCo Roto-shredder
3. Screen
4. Front-end loaders
5. Dump trucks
Supplementary equipment consists of spring-tooth
harrow, tractor and sprayer, irrigation system, and
truck scales.
The compost operations are depicted in the flow
chart (Figure 3). Details of the major operations are
as follows:
1. A layer of wood chips 12 inches deep and 15
feet wide is placed on the paved area. The sludge is
dumped on top of the chips and spread with a front-
end loader. A ratio of three parts chips to one part
sludge by volume is used. The composting machine
then forms the sludge and chips into a windrow.
(Either of the composting machines can be used for
this operation.) Several turnings (about eight to ten
times) are necessary to adequately blend the two
materials.
2. The windrow is normally turned daily with
the composter; however, during rainy periods,
turning is suspended until the windrow surface
layers dry out. Temperatures in the windrow under
proper composting conditions range from 55 to
65°C. Turning mixes the surface material to the
center of the windrow for exposure to higher
temperatures. The higher temperatures are
equivalent to temperatures needed for
pasteurization and thus effectively kill most
pathogenic agents. Turning also aids in drying and
'Trade names are included to provide specific information, and
do not imply endorsement by the U.S. Department of Agri-
culture.
-------
COMPOSTING 125
Figure 2: Compost Site.
increasing the porosity for greater air movement
and distribution.
3, The windrows are turned for a two week
period or longer depending on the efficiency of
composting; i.e., achieving temperatures above
50°C. The compost row is then flattened for
further drying.
4. Material removed from the windrow is
further cured for 30 days. Curing further stabilizes
the compost and provides additional time for
pathogen destruction. This curing can take place
either before of after screening. Screening
separates the bulk of the wood chips from the fine
material. The wood chips are reused in subsequent
composting and generally last through five
composting cycles. A system of forced aeration is
being tested to accelerate curing.
From April to mid-June 1973, about 200 wet tons
of sludge were processed weekly. From mid-June to
mid-September sludge delivery increased to 400
wet tons per five-day week. Sludge delivery rose
from an average of 400 wet tons per five-day week
in the latter part of September to over 930 wet tons
on the week of October 8. A large portion of the
sludge delivered at that time was raw or
undigested. During September, October, and
November, odor problems occurred and, because of
considerable public protest, operations essentially
ceased during December. From January to the
present, about 50 wet tons of digested sludge per
day have been composted without odor problems.
The odor problems were believed to result from:
(a) Overloading the site—projected capacity of this
site was 100 to 150 wet tons per day; and on
October 24, 283 wet tons of raw sludge were
delivered; (b) receiving raw instead of digested
sludge; (c) possible heavy use of chemicals such as
FeCb and lime added in the wastewater treatment
plant; and (d) climatic conditions including air
inversions and stagnation, and wet periods that
delayed composting.
The experience of the past year suggests that
with digested sludge, present operating proce-
dures could be used satisfactorily in this area.
Winter operations required extra effort and longer
detention in the windrows. Several modifications
seem promising for winter composting and will be
-------
126 MUNICIPAL SLUDGE MANAGEMENT
Compost
SLUDGE ' V A
CHIPS 3 V .
4
l
Mixing
1
>.
Windrov
Composting
1
1
^
Drying
«^.
Curing
_»
Sjrjen-
' Chit) Recvcline
Reload
Figure 3: Compost Process.
evaluated this coming winter when additional
research is planned.
Desirable modifications and improvements in
operations and site design are suggested below.
1. The windrow pad should be paved with
concrete to provide a stable pad with better
drainage. The present pad's instability is due to:
(a) high water table and poor drainage, and
(b) tearing up by heavy equipment. Water and
sludge are held in depressions which could cause
odor problems as well as machinery slippage. Also,
the present pad was constructed from locally
available crushed stone which is from serpentine
rock that contains enough nickel to contaminate
the resulting compost. Loose and decomposed
stones from the pad are often found in the sieved
compost.
2. A partially covered site may be necessary to
compost in the winter and during rainy periods.
Possibly in a covered or enclosed system, odors,
which generally develop during the first six days of
processing, may be controlled. This concept needs
further investigation.
3. The stockpile areas, chip storage, and curing
areas are too small at the present site. Additional
space is needed for drying the wet windrows. ,
4. The wash area should be built so that the
sludge washed from trucks would be collected into
a septic tank or sewage system, which should be
cleaned out periodically.
5. The present access roads are dusty and are
difficult to maintain during snowy winter months.
These should be improved.
6. If odor-masking facilities are necessary, a
better system should be designed. Possibly a pipe-
mast system in the direction of the prevailing winds
would be more adequate.
Although the present equipment can do an
adequate job, the following improvements are
desirable.
Composter
I. The composter should be able to build a
windrow approximately five to six feet high with a
density of 50 pounds per cubic foot.
2. The composter should be able to mix materials
well with as few passes as necessary to obtain a
good mix.
3. The composter drive system and the wheel
drive system should be separate. The wheel drive
system should have variable speed control to
compensate for changing pad and material
conditions. This will reduce wheel slippage and
allow for better mixing.
Screen
1. The motors should be dust-proof or a
hydraulic system installed.
2. The hopper should be redesigned to be capable
of handling full loads from front-end loaders.
3. Transferring of material to the conveyer
needs improvement because the conveyer is too
narrow and results in too much slippage.
Operations
Two major operational problems have been
encountered in the past year: (l) how to compost
digested sludge under adverse weather conditions;
and (2) how to compost raw sludge without
producing odors.
Digested sludge composts slower than raw
sludge, particularly during wet, cold periods,
possibly because of the lack of sufficient digestible
energy material for rapid biological oxidation. Our
-------
COMPOSTING 127
research shows that adding material, such as
chopped hay, straw, or shredded paper, greatly
accelerates composting of digested sludge. Thus,
adding material to the wood chips to enhance
composting in cold, wet periods may be necessary.
In the improved process temperatures are
increased probably because of reduced air
movement and reduced moisture content, result-
ing in better granulation of the sludge.
Research is being conducted on microbiological,
chemical, and mechanical means of preventing
odors during composting of raw sludge.
Management
A compost site should be managed by an
individual familiar with composting technology and
composting equipment. Good coordination is
needed between the sewage treatment plant and
the composting site. If possible, research and
technical assistance should be an integral part of
the composting operation. If the research arm is a
separate unit, good coordination and relations are
absolutely necessary. Facilities, such as space on the
pad, that do not interfere with the normal
operating procedures must be present for research
or development. As technological problems devel-
op (such as odors or reduced composting
temperatures), the site manager should be able to
communicate effectively with the research
personnel. They in turn should be able to respond
to the problems immediately.
An efficient compost removal, marketing, or
public distribution system needs to be developed
that does not interfere with normal operations or
traffic at the site. Visitors should not interfere with
operations. Essentially, a well-defined organiza-
tional structure is necessary which cordinates the
activities of site operations, treatment plant
operations, research and development, marketing,
and public relations.
For greatest efficiency and lowest cost, locating
the composting site as an integral component of the
wastewater treatment plant would be desirable.
Public Relations
Public relations can often spell the success or
failure of a waste treatment project. Requirements
in this area will be considered for both the planning
and operation stages.
Planning Stage
The planning stage is crucial, and should be
initiated well'in advance of the project. It must be
well-organized and staffed by tactful public-
relation individuals. The following are some of the
points to be considered:
1. Contact local organizations and explain
project—What problems can be encountered?
What procedures will be used when problems
occur? Types of organizations include:
League of Women Voters
Isaack Walton League
Community citizen groups
Garden clubs
2. Organize a steering committee from local
organizations.
3. Provide information to local papers, citizen
group papers, etc.
4. Contact political units; i.e., City Council, local
politicians, Congressmen, etc. Try to find
individuals who have an interest in the program
and involve them.
Operation Stage
1. Develop a youth program, such as a
beautif ication and horticultural program in schools
and youth organizations. Involve youth in the
community improvement and recreational
projects.
2. Stimulate adult interest through groups like
garden clubs.
3. Provide compost material for community
projects.
4. Possibly provide limited amount to local
citizens for personal use.
5. Publicize use of compost on public lands
instead of other material (peat, humus, top soil)
that normally must be purchased. Explain what this
represents in savings to the taxpayer.
6. Maintain public relations with community
groups and constantly provide information to local
and citizen papers. If problems develop, inform the
public immediately of your awareness and attempts
to solve the problems.
Economics and Costs
Table 1 provides information on costs of
composting on a site capable of handling 200 wet
tons per day on a seven-day per week basis. The
following cost estimates do not include land
acquisition, hauling sludge to the site, and compost
distribution. We have not attempted to balance
these costs with direct benefits (sales of product) or
indirect benefits (cleaner environment, improved
soil).
In cold, humid climates, part or all of the pad may
have to be covered. The cost of composting sewage
sludge will also be affected by whether the runoff
could be recycled into the sewage system. The
-------
128 MUNICIPAL SLUDGE MANAGEMENT
TABLE 1
Estimated Annual Operating Costs for Processing 200 Wet Tons
Per Day (20 Percent Solids) of Digested Sludgea
Operational Cost
Labor costs, including insurance and taxes
(7 men - 40 hrs./wk.) $90,000
Management and overhead 24,000
Fuel and electricity 14,000
Site maintenance 10,000
Equipment maintenance 10,000
-Supplies 14,000
Wood chips 140.000
$302,000
DIRMI* Annual
Equipment % Capital Cost
1 Composter $120,000 34.5 $41,400
I Composter (backup) 120,000 20.0 24,000
1 Front-end loader large... 80,000 34.5 27,600
I Front-end loader small.. 40,000 34.5 13,800
Dump truck & pickup 34,000 34.5 11,730
Scale 12,400 21.5 2,666
Mobile office trailer 55,000 24,5 1,225
Irrigation, Deodorizer 10,000 24.5 2,450
Tractor, harrow, sprayer... 6,000 24.5 1,470
Screen 50,000 34.5 17,250
Miscellaneous equipment. 10,000 24.5 2,450
$487,400 $146,041
* Depreciation, interest, repairs, maintenance, and insurance.
Construction
Concrete Compost pad
(5 acres) $500,000
Storage Area 50,000
Runoff and pond 30,000
Roadways 30,000
$660,000
13
13
13
13
$65,000
6,500
3,900
3.900
$85,800
Investment — Equipment $487,400
Construction 660,000
Total $1,147,400
Annual Costs — Capital Costs
Equipment $146,041
Construction 85,800
Total Capital Costs $231,841
Operating Cost 302,000
Total Annual Costs $533,841
Cost per ton of wet sludge $7.31
aWe thank Dr. Gar Forsht, Economic Research Service, USDA, and Mr. Ronald Albrecht, Maryland Environmental Service, for their
assistance in cost estimation.
annual cost in Table 1 for the 200-wet-tons-per-
day site is estimated at $7.31 per wet ton or $30.00
per dry ton. Increasing the capacity to handle 600
wet tons per day will reduce the cost to $5.15 per
wet ton or $21.12 per dry ton of sludge. Wood chips
contribute over $2 per wet ton to the costs, most of
which is for chip hauling. A waste product that
could be used for bulking and would otherwise cost
money to get rid of would reduce these costs.
Municipalities should not view composting of
sewage sludge as a potential money maker, but as a
means of reducing sludge disposal cost from
wastewater treatment plants. Major assets of
compost are that it is an aesthetically pleasing,
easily handled material that can be used in urban
environments without causing odor or nuisance
problems.
REFERENCES
1. Golueke, C.G. and H.B. Gotaas, Public Health
Aspects of Waste Disposal by Composting, Am. J. Public
Health, 44:339-354, 1954.
2. Poincelot, R.P. The Biochemistry and Methodology of
Composting, Conn. Agr. Expt. Sta. Bull., 727, p. 38,
1972.
3. Walker, J.M. and G.B. Willson. Composting
Sewage Sludge: Why?, Compost Sci. 14, 1973.
4. Wiley J.S. and J.T. Spillane. Refuse-Sludge
Composting in Windrows and Bins, ]. Sanitary Engr.,
Div. Amer. Soc. Civil Engr., 87:33-52, 1961.
5. Willson, G.B. and J.M. Walker. Composting
Sewage Sludge: How? Compost Sci. 14, 1973.
-------
RECENT SANITARY DISTRICT HISTORY IN
LAND RECLAMATION AND SLUDGE UTILIZATION
JAMES L. HALDERSON, BART T. LYNAM, AND RAYMOND R. RIMKUS
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
INTRODUCTION
Vi
Area Served
The Metropolitan Sanitary District of Greater
Chicago, an organization chartered under the
statutes of the State of Illinois, serves an 860 square
mile area with a population of approximately 5 V^
million persons. The non-domestic waste load, in-
cluding industrial, commercial, infiltration and
storm-water, adds the equivalent of an additional 5
Vi million persons. All of the area served is located
within Cook County Illinois and is composed of the
city of Chicago as well as approximately 120 other
cities and suburbs.
Forms of Sludge
Three major treatment plants handle the daily
flow of 1.4 billion gallons. The major treatment
process of heated anaerobic digestion, Imhoff
digestion followed by sand bed drying, and heat
drying of vacuum filtered waste activated sludge,
produce approximately 625 dry tons of solids per
day.
Heat dried sludge is disposed of thru a contractor
who transports the total output of this process to
the southern states and Canada for agricultural
use. The Imhoff sludge from the sand drying beds is
removed to a storage area for additional dewatering
and decomposition. Final disposal has been by oc-
casional contract and pickup from the general
public.' In recent months all of the output of the
anaerobic digesters of the major plant, West-
Southwest has been sent to Fulton County for
storage prior to land application. On a volume basis
this amounts to approximately 7000 wet tons per
day.
Of the three sludge forms being processed the air
dried sludge has the most desirable properties for
land utilization. Essential plant nutrient analysis
averages 4-6-0.1 for nitrogen (N), phosphorus
(PzOs) and potassium (KzO) while dry matter con-
tent varies from 30 to 70 percent. However, the air
dried sludge is much more valuable, on a dry matter
basis than are the other sludges, because of the
much greater stabilization which it has undergone.
One appears to be justified in considering the
organic content of the air dried sludge to be essen-
tially humic matter. As such, its importance for
rebuilding topsoil would be well appreciated by the
agricultural community.
Heat dried sludge has an N-PzOs—KzO analysis
of approximately 6-5-0.5 with about a five percent
moisture content. However, the valuable com-
ponents of alkalinity, and humic content are essen-
tially missing because of relatively little biological
stabilization prior to the drying operation.
Anaerobically digested sludge, on the other hand,
has considerable alkalinity, 3-4000 mg/1 but the
solids content only averages four percent as it
comes out of the digesters. Analysis shows the di-
gested sludge to average 6-5-0.5 for N-PzOs and
KzO. Lagooning concentrates solids to eight
percent.
Projects to Date in Land Reclamation
Northwestern University Campus
In April of 1968 the Sanitary District, at the re-
quest of Northwestern University officials, began a
program of applying digested sludge to University
owned land. A five acre peninsula had been con-
structed from dredged sand by the University. On
129
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130 MUNICIPAL SLUDGE MANAGEMENT
top of the sand an 18 inch clay layer was placed to
hold the sand in place and to provide sufficient
water holding capabilities for vegetation. A rate of
100 dfy tons per acre of digested sludge was applied
to the soil by the ridge and furrow method of irriga-
tion. Test wells for water monitoring indicated no
detrimental effects due to infiltration. Soil struc-
ture and pH were improved to the extent that
shrubs and an excellent grass cover could be es-
tablished and maintained.
Ottawa, Illinois
At a 37 acre site near Ottawa, Illinois, the Libby-
Owens-Ford Company disposed of waste silica
sand from a glass manufacturing operation.
Because of the nature of the sand, the site was bare
of vegetative cover so that moderate winds caused
severe dust problems. Digested sludge was applied
to the site by gated pipe irrigation methods. The ini-
tial soil pH of approximately eleven was reduced to
near neutral and sufficient organic matter was
added to the soil so that a good vegetative cover of
grass could be established and maintained.
Hanover Park
The village of Hanover Park, Illinois, located in
northwestern Cook County, has a 6 mgd treatment
plant serving the residential area. In 1968 an eight
acre site was developed for investigating the effects
of sludge fertilization on agricultural crops. The
site was prepared so that surface and subsurface
water could be collected for analysis. Six plots were
established and have been planted to field corn
during each of the subsequent years. Heavy metal
analysis of corn plant tissue and of the grain has
been the major research interest. To date, results
indicate that corn grown under such conditions
does not differ from corn grown under
conventional practices except for an increased
protein content of the grain.
Calumet Farm
At the Calumet Sewage Treatment Plant a rub-
bish disposal site of approximately 60 has been
reclaimed for agricultural cropping purposes. Sur-
face debris has been removed and sludge applfed so
that a productive soil has been formed. At the end
of the 1973 growing season an accumulated total of
237 dry tons per acre had been accomplished over
the five years of sludge application. Application has
been done entirely by flood irrigation practices as
the fields are essentially level. Field corn and wheat
have been the crops grown to date at this site.
Paho Project
The Shawnee National Forest located near Car-
bondale, Illinois has considerable acreage of strip
mined land within its confines. Generally, the sur-
face water leaving the mined areas has pH values in
the 3.0 range. This prevents most forms of
biological growth in and along the receiving
streams. In addition to the pH problem, a rock
problem exists such that use of the lands for
cultivated purposes is economically not feasible.
In 1970 The National Forest Service in coopera-
tion with The Sanitary District conducted an
application rate study on four test plots. Dry sludge
solids were applied at rates of up to 100 tons per
acre where the applied material was digested
sludge. Various grasses were planted on the plots
following sludge application. Companion plots
received applications of agricultural limestone and
commercial fertilizer.
Only on the plot with the highest application rate
of sludge did a substantial grass growth occur.
Testing of soil pH indicated that change in the pH
was primarily responsible for vegetative growth.
The plots receiving limestone tended to have acid
leaching through the soil at a later date. This
resulted in a reversion of soil pH's and loss of
vegetative vigor.
As a result of the pilot plot trials The National
Forest Service has prepared a 190 acre site for
sludge application. At the present time a contractor
is removing sludge from a lagoon at the Calumet
Plant site and is transporting it to the application
site and will apply it over a period of several years.
The Sanitary District has also cooperated with the
Forest Service on this larger scale project. Exten-
sive water monitoring is being done on the site to
determine the effects of the sludge application and
subsequent vegetative establishment..
Arcola Project
For the past several years a private firm has
applied lagooned digested sludge to a 900 acre
agricultural site at Arcola, Illinois. On occasion,
loading rates of 150 dry tons per acre per year have
been accomplished under the supervision of the Il-
linois Environmental Protection Agency. The firm
has the responsibility for all phases of the opera-
tion, starting with sludge removal from the lagoon.
A unit train is used for transportation of sludge to
the site with application being done by traveling
sprinklers or by moldboard plow incorporation.
Elwood Agronomy Research Center
In conjunction with the University of Illinois, a
research center for agronomic studies has been
operated at Elwood, Illinois since 1968. A total of 44
plots, each of 10 feet by 50 feet, have been used to
study several soil types under sludge application.
Plot borders are isolated from surrounding
groundwater by plastic sheets with total water
drainage being collected for analysis. The facility
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RECENT SANITARY DISTRICT HISTORY 131
was designed to provide a means of determining the
accumulative concentration changes of plant
nutrients, non-essential heavy metals, and organic
carbon, along with the change in biological status of
soils and water from cropped land irrigated with
various rates of digested sludge.
To date, one of the significant research results
has been the indication that application of freshly
digested sludge can inhibit or prohibit seed ger-
mination. However,' if the sludge is applied ap-
proximately one week prior to planting or if the
sludge has been lagooned for some time prior to
application, germination will proceed normally.
Offensive odors from well digested sludge
applications have not been a problem.
The Fulton County Land Reclamation
and Utilization Site
Land Acquisition
In the fall of 1970 the Sanitary District made an
initial purchase of land in Fulton County, Illinois,
approximately two hundred miles away from the
sludge treatment facilities. The land was a com-
bination of place land and strip mined land. Of the
strip mined land, some areas had been partially
leveled so that grazing operations could be under-
taken.
Fulton County, Illinois is one of three counties in
Illinois which traditionally lead the state in coal
production. Over the past several years, an average
of 1650 acres per year has been stripped in the
county. /Since approximately 40,000 acres of such
strip mined land already exist in the county, it was
obvious to concerned county officials that
something must be done to counteract this erosion
of the economic base of the county. As a result,
Fulton County officials and District officials got
together.
Steering Committee
At an early date a steering Committee was
formed which had the responsibility of a multidis-
ciplined advisory group to the District.
Represented on the committee are University
research personnel, State Water Survey personnel,
University Extension Service, Federal and State
Soil Conservation personnel, elected county
officials, representatives of various local com-
munities, citizen organizations and District
personnel. Their task was to review the various
proposals offered by the District and to suggest
modifications for maximizing benefits of the
proposals to all parties.
Transportation System
A transportation system was developed for mov-
ing digested sludge directly from the digesters and
hauling it by barge down the Illinois River. At the
downstream end a dock was constructed for han-
dling the barges and associated pumps. The sludge
is removed from the barges with portable pumps
which discharge into the suction line of booster
pumps. From this point the material is pumped
through an underground 20 inch pipeline a distance
of 10.8 miles to holding basins.
Holding Basins
The holding basins were constructed near the
center of the planned utilization facility. Four in-
dividual cells comprise the total storage capacity of
approximately 8.1 million cubic yards. Each basin
was lined with a two foot thick compacted clay liner
to prevent seepage and one basin is ringed with a
number of wells for purposes of collecting ground
water to detect seepage from the basins.
The basins receive sludge every day of the year
barring exceptionally heavy ice or flood conditions
on the river, and mechanical breakdowns. Two
functions are served by the basins: to accumulate
sludge without the need of immediate application,
and to separate liquid from solids. Separation per-
mits application of a sludge with a solids concentra-
tion which can be different from the sludge being
input to the basins.
Distribution System
,A conventional dredge is used to remove sludge
from the holding basins. It has a cutter head which
can reach depths in excess of 30 feet and is moved in
an oscillatory manner when removing settled
solids. The dredge discharges into a floating pon-
toon line which conveys the sludge to several large
holding tanks.
From the holding tanks the sludge is fed to two
pumps in series which have a collective capability of
delivering 1200 gpm at 80 psi. The output of the
distribution pumps is conveyed through a surface
layed, ten inch, steel line out to the fields for
application. Each of the presently installed eight
distribution lines services an area of approximately
250 acres.
Within the field, portable, eight inch, aluminum
irrigation piping conveys the sludge to the various
areas. Traveling sprinklers do the major amount of
sludge application and they are connected to the
aluminum line with a five inch diameter 660 foot
long hose. In some instances a tandem disk
equipped with a distribution manifold is connected
to the five inch hose for incorporating sludge as it is
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132 MUNICIPAL SLUDGE MANAGEMENT
applied. Either application method can cover a max-
imum area of approximately ten acres with a single
settling of the aluminum pipe. Sludge is applied
during the growing months of May through Oc-
tober with the distribution pipeline being flushed
with water and then drained for winter periods.
Site Preparation
Prior to sludge application each field is leveled by
construction equipment to maximum slopes of ap-
proximately six percent. Berms are placed around
the field so that all surface water runoff is directed
to adjacent retention basins for temporary storage
and analysis prior to release to the water course.
Retention basin capacity is designed to receive the
100 year frequency storm, which for the Fulton
County area amounts to a bit over five inches of
water. Rocks and other debris are removed from
the field during site preparation. Those areas that
were scarified and which will not become part of
the productive field are seeded to permanent grass
for erosion control.
Environmental Protection System
The system is designed to operate in a fail safe
manner. Complete surface water collection is ac-
complished by directing application field runoff to
retention basins. The water is then analyzed prior
to release to show that it meets State water quality
standards. In addition, several small streams that
run through the property are monitored at points
where they enter and leave District Property. The
State Water Survey, IEPA and the County Health
Department also monitor some of these streams as
well as several other locations within the property.
Numerous shallow wells have been located
throughout the property for purposes of supplying
ground water for monitoring purposes. Shallow
wells for ground water monitoring purposes sur-
round the holding basin that was put into operation
first. Extensive use of grassed waterways reduces
the sediment load that leaves the fields during
heavy rains. The waterways also provide for ad-
ditional utilization of nutrients prior to entry of the
runoff into retention basins.
Cropping Program
The basic aim of the Sanitary District is to be able
to apply as much sludge to a particular location as
the environmental limitations will permit. In this
regard, the agricultural cropping program is a vital
component. Information indicates that somewhat
less than half of the applied nitrogen in this system
ends up in the soil and is thus available for plants.
The remaining portion evolves to the atmosphere
as gaseous nitrogen. To the present date, nitrogen
has been the primary parameter by which loading
rates were determined. Of all conventional
agricultural crops, field corn has been the crop that
used the greatest amount of nitrogen and
presented the fewest management difficulties dur-
ing its production.
The Sanitary District procures the services of
local farming organizations through competitive
bidding on crop production contracts. The contrac-
tor is essentially responsible for all phases of the
crop from "bag to bin". During the growth of the
crop the District supplies the required fertility to
the crop by sludge application. Marketing of the
crop has been done by contract through local com-
mercial grain dealers.
Production records indicate that when sludge is
applied to strip mined land, corn yield has been in-
creased by approximately a factor of four when
compared to those strip mined fields which received
no sludge. Because strip mined soils have no
organic matter to speak of, they have relatively lit-
tle ability to contain adequate amounts of soil
moisture. Therefore, it appears important that
sludge be applied in the liquid form until soil
organic matter is built up to a sufficient level.
Many good agricultural soils range from three to
five percent in organic matter. An application of
100 dry tons per acre of the District's air dried
sludge would change the soil organic matter con-
tent by approximately one percent. At this rate the
entire daily solids output of the District, 625 dry
tons, could only improve six acres per day by an
organic matter change of one percent. On an an-
nual basis this approximately equals the acreage
which is strip mined in one county of one state,
Fulton County, Illinois. Conservation of a valuable
commodity must receive greater attention.
Research Studies
The District's Research and Development
Department is studying quite a number of factors
connected with the long range changes that might
result from sludge application in an agricultural
setting. In addition to the above mentioned
parameters that are being tested, lakes on the site
are periodically sampled for biological specimens
ranging from microorganisms to fish. Grain and
plant tissue analysis is conducted on the crops being
grown.
In cooperation with the University of Illinois
School of Veterinary Medicine a grazing study is
underway which involves approximately 100 head
of beef brood cows. The cattle consume forages
produced entirely from sludge fertilized lands.
During the summer the cows directly graze an
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RECENT SANITARY DISTRICT HISTORY 133
irrigated crop while during the winter they graze
stubble fields or are in dry lot. The cows and their
calves are being examined for parasitic changes,
heavy metal concentration changes and changes
due to disease producing organisms.
A number of small plots have been established on
strip mined soil near the holding basins. Studies on
these plots involve crop response to sludge fer-
tilization, soil response to sludge fertilization, and
the effects, on soil water, of sludge migration down
through the soil profile. Because of variable en-
vironmental conditions it is sometimes unreliable
to extrapolate data collected from plots in a
different locale.
Multiple Use Facilities
Throughout the early development and im-
plementation of the reclamation site, considerable
emphasis has been placed on multiple utilization.
Various integral parts of the site have been
developed for public uses such as boating, camping,
fishing and hiking while other parts have been
devoted to improving the habitat for wildlife.
Several hundred acres of land, within which are
sludge recycle fields, has been leased to the county
government. They in turn are responsible for
managing the area for public utilization. The State
of Illinois Department of Conservation is
cooperating in the wildlife habitat improvement
and stocking of the strip mined lakes for fishing. Ef-
forts continue on the project for reestablishment of
a native population of giant Canada geese.
Future Developments
Application Rates
At present, the Illinois Environmental Protection
Agency has approved application rates on the
Fulton County site of 75 dry tons per acre per year
for strip mined land and 25 dry tons per acre per
year for place land. These rates pertain to liquid ap-
plication wherein the solids content might reach a
maximum of eight percent. Over a period of five
years the application rates are reduced to a steady-
state rate of 20 dry tons.
Infiltration rates for the clay soils of the area
restrict the amount of water that can be applied
over and above a normal annual rainfall of ap-
proximately 35 inches. It appears that an average
year would result in approximately four acre inches
of sludge being applied to the soils. This factor
would limit maximum dry matter application to ap-
proximately 36 dry tons per acre per year if eight
percent solids are in the irrigant. Therefore, it
appears that in the near future, the District will be
strongly considering application of a sludge which
can be handled as a dry material. Several major
benefits of such a move would be that annual
application limits could be achieved in a single
application, organic matter could be built up in the
soils at a much more desirable rate, and that sludge
could be incorporated shortly after application to
result in much less nutrient and particulate loss
from the field due to erosion.
The concentration of heavy metals in the soil is a
factor that can be controlled to any desired degree.
One can monitor the soil for metal concentration
and the crop for toxicity indications. If, and when,
crop toxicity is encountered one can relieve the
metal concentration in the soil by tilling more deep-
ly. The normal plow layer is considered to be eight
inches. It is presently possible to till to a depth of ap-
proximately 36 inches with existing equipment.
More than a four-fold reduction in concentration
would result from such action. Fears that there are
no practical responses to too high of a metal con-
centration in the soil appear to be unfounded.
Reclamation and Strip Mining
Some of the land that the District is now leveling
and reclaiming has been laying in an unproductive
condition for a great number of years. The land has
become overgrown with low quality trees and vast
amounts of soil has been conveyed to nearby
streams over the years. In considering the total cost
to society for such practices, it does not appear
reasonable that such a time span need exist
between strip mining and reclamation.
Recent State of Illinois laws have required
current strip mined spoils to be leveled to slopes of
no greater than 15 percent. However, this practice
can only be viewed as a partial solution to the
problem. Long slopes of only several percent on
bare soil cause serious erosion problems. This con-
dition is coupled with the fact that soils devoid of
organic matter take an exceedingly long time to es-
tablish adequate vegetative cover. Before vegeta-
tion protects the soil from erosion, ditches are
formed which concentrate water flow and cause
still more serious erosion. The process is a never
ending cycle as soil must be moved to correct the
ditch problem and the process is repeated.
The missing key to the reclamation of these soils
is organic matter. The incorporation of sludge into
freshly leveled mine spoil immediately after strip-
ping appears to present the most desirable benefits
for sludge utilization and land reclamation.
Nowhere in agriculture are such quantities of
organic matter available at a cost which would be
comparable to that of sludge.
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SLUDGE MANAGEMENT IN ALLEGHENY COUNTY
RICHARD M. COSENTINO
Department of Public Works
Allegheny County, Pennsylvania
I am delighted to have the opportunity to par-
ticipate in a Conference which brings together the
interest and expertise of individuals so deeply con-
cerned with the subject of sludge and to share with
you, information on existing activities and poten-
tial practices dealing with this timely topic in
Allegheny County.
We are pleased to relate that as a conservation
and resource recovery concept, the recycling of
municipal sludges to the land is a practice of long
standing in Allegheny County. While this method
of disposal is apparently serving the County in an
environmentally acceptable manner, there is a
growing awareness that the advent of advanced
wastewater treatment processes and the need to
attain higher degrees of pollutant removal will
create as much as a 50 to 80 percent increase in
sludge quantities. In addition, these sludges will
contain chemicals introduced as part of the
wastewater treatment process that may alter the
character of the waste material.
It would be appropriate to provide you with some
pertinent background information on our County.
Allegheny County embraces an area of about"745
sq. miles with a population of about 1.6 million,
residing in 129 separate municipalities. Of that
population, approximately 97 percent is served by
sewerage systems with the remaining three per-
cent still utilizing private on-lot systems, such as
septic tanks. There are about 45 sewage treatment
plants with design flows ranging from 0.1 mgd to
the Alcosan facility with a design flow of 150 mgd.
The number of plants with design flows under 0.1
mgd approaches 100. Those facilities serving small
industries, commercial facilities, and institutions
are slowly being abandoned as a result of the con-
struction of new and enlarged municipal plants and
the extension of new and existing interceptors.
In terms of sludge generation, it is estimated that
between 150 and 175 tons of dry solids are pro-
duced daily by all the plants in the County. Of that
amount, the Allegheny County Sanitary Authori-
ty (Alcosan) facility produces on the average about
113 tons. Since another member of this panel will
describe sludge management at Alcosan, I shall not
attempt to do so except to relate that incineration is
the final reduction process.
Although an average of 75 percent of the sludge
generated in the County is processed by combus-
tion, most of the balance is digested anaerobically
with a small amount digested aerobically. The latter
method of processing has found acceptance with
consultants in several of the new plants, especially
those designed under 3 mgd. An example of that
type of facility is one which serves the greater
Pittsburgh Airport and four surrounding com-
munities and utilizes the contact stabilization
process with aerobic digestion of sludge for present
low flows averaging 0.80 mgd. As the volumes ap-
proach the design capacity of 2.5 mgd. then high lime
treatment of sludge with thickening and vacuum filtering may
be considered as an alternative to an aerobic diges-
tion. While stabilizing the sludge by adding lime of a
pH of about 12 is not a new treatment concept the
possibility of its use here is comparatively recent. It
is well understood that most soils and crops are
benefited by lime treatment; therefore, agricultural
applications of lime stabilized sludges may be
thought of as environmentally feasible. R. B. Dean
and J. E. Smith, Jr. of the EPA, in their paper titled
135
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136 MUNICIPAL SLUDGE MANAGEMENT
the "Properties of Sludge" indicate that lime inac-
tivates most heavy metals by precipitation and the
use of high limed sludge as a soil amendment would
pose only minimal risks. In this regard, the only
treatment facility in the County which could con-
tain heavy metals in its sludge to any appreciable
degree would be the Alcosan facility. Practically all
the other plants treat a fairly homogenous
domestic waste.
You may ask about the Commonwealth of Penn-
sylvania's role in the area of sludge management
and how their enforcement posture affects the
County. Under Department of Environmental
Resources Rules and Regulations Article I, Land
Resources, Chapter 75 "Solid Waste Management,
Paragraph 75.116" sewage solids, liquids, and
hazardous waste, Section (B) reads "Sewage sludge
shall be digested properly and dried to ap-
proximately 80 percent moisture contents by
weight. Septic system cleanings shall not be al-
lowed except as approved by the Department." To
our knowledge, sludges being disposed in sanitary
landfills meet that criterion because until only
recently, there were no landfills approved for that
purpose. Within the past six months however, one
of the seven permitted landfills in the County con-
structed a leachate treatment facility which
employs a lime-acid-aeration process. The operator
of another fill has connected his leachate flows to
the sanitary sewer of a treatment facility. Two
other sanitary landfills located in strip mined areas
are approved even though treatment facilities do
not exist. In spite of a lack of treatment, these fills
are being monitored by means of wells to assure
that pollutants are not reaching surface waters.
Now you may feel compelled to question how we
can boast about recycling of sludge when sanitary
landfills are becoming readily available for disposal.
Our reaction is that sanitary landfills would have to
be necessarily considered as disposal sites of last
resort. The continued application of sludges on
agricultural land meeting the landfill criteria is ex-
pected and encouraged. However, it is most ob-
vious that state and County controls for sludge
disposal on farm land or home gardens or wherever
are much more difficult to enforce since there are
many more sites to inspect. If there is a breakdown
in sludge treatment efficiency or a lack of complete
treatment in accordance with State standards then
the potential public health implications become
magnified. We are all too familiar with the gastro-
enteric effects of pathogenic bacteria and viruses
attributed to human wastes.
How is the Pennsylvania Department of En-
vironmental Resources coping with this problem?
In cooperation with the Allegheny County Health
Department, the State now requires all new
applicants and also those treatment facilities being
upgraded or expanded to provide the following type
of information:
1. Type of sludge and physical characteristics.
2. Groundwater module phase 1 or water quali-
ty management module 5 (with required
maps).
3. General Site information.
(a) Location of private water supplies in the
area.
(b) Surface water drainage characteristics.
(c) Assurance that the site is compatible with
the requirements of local ordinances.
4. Method of operations.
(a) Quantity of sludge to be deposited.
(b) Frequency of spreading.
(c) Method of spreading (how and what type
of equipment).
(d) Method of working into soil.
(e) Provisions for adverse weather handling
and storage.
While this regulatory approach appears to be
operating well for the treatment facilities men-
tioned previously and classified as new and upgrad-
ed, an enforcement gap exists with those facilities
at the secondary level of treatment which are not
yet under orders to improve their process.
However, within the short term future, the
Department of Environmental Resources and the
County will be able to account for practically all
large scale disposal of sludge on land.
You generally have a blueprint of existing ex-
periences with sludge in this County. What of the
future? What new concepts or perspectives lie
ahead? Given the impetus provided by the En-
vironmental Protection Agency's R. J. Schneider in
his paper at last summer's Conference on "Recycl-
ing Municipal Sludges and Effluents on Land", the
County is preparing to undertake a few demonstra-
tion projects which will enable either the County
and/or operating authorities in the County to
develop cost effective programs with alternatives
for recycling of potential sewage pollutants
through products of agriculture and silva culture as
stated in Title II of the amended Federal Water
Pollution Control Act of 1972.
The County, through the services of a consul-
tant, has applied to the Environmental Protection
Agency for a grant to initiate a project titled
"Restoration of Strip Mine Lands Using Municipal
Treatment Plant Residues and Fly Ash". Since our
area is blessed with an abundance of plant residues
fly ash, and strip mine areas which constitute our
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ALLEGHENY COUNTY 137
major mode of solid waste disposal, we feel a project
of this kind, while not necessarily unique in itself,
represents a potential technical benchmark for the
County.
The project entails the following:
OBJECTIVE
To conduct a feasibility study for renovation of
strip mine land in one Township of the County by
either mixing municipal sludges with ambient soils
and spoils or a combination of municipal sludges
and fly ash. Unlike existing projects, it is proposed
to combine a need for disposing the maximum quan-
tity of residue with the need to restore strip mine
lands for a community resource.
- Following a demonstration of the feasibility of
the concept, a field demonstration will be initiated
embodying all the precepts outlined in the feasibili-
ty study (Phase II). Following the successful com-
pletion of the demonstration, a full scale operation
will be considered (Phase III).
SPECIFIC OBJECTIVES
(l) Inventory of existing acid mine drainage.
(2) Sampling and characterization of residues.
(3) Feasibility of application of residues.
(4) Socio-economic benefits that will be realized.
(5) Assurances that State and County pollution
abatement programs will aid in restoration of
the waterways.
(6) Method description, development of
demonstration project, and full scale opera-
tion.
(7) Preliminary engineering to determine
capital, operating costs and benefits that will
accrue to the project.
Some of the highlights of the project include:
Surface and sub-surface water quality and quan-
tity data will be obtained to determine existing con-
ditions, for monitoring requirements, for site
modifications, for engineering requirements, and
for operational requirements.
Physical and chemical analyses of the soils and
spoils will be obtained for design of the greenhouse
and site cropping studies.
"Greenhouse" studies will be conducted using
site soil, sludge(s) and fly ash with various proposed
grasses and forest crops to be determined by the ul-
timate use of the site(s).
Replicate plots at the proposed site will be utilized
to verify the ranges of sludge applications and
growth phenomena.
One management alternative for handling
sludge during the winter months is to compost
sludges for subsequent cover of the municipal
sludges being applied to the uncropped lands to
minimize odors.
At the present time, the application is being held
in abeyance until July 1, of this year for resubmis-
sion to the Federal Government. The State has
given the County a green light on the project
providing certain conditions are met such as the
submittal of an erosion and control plan.
Another innovative research project in which the
County intends to participate is the processing of
dewatered undigested sludge produced at the Pine
Creek water pollution control facility now un-
der construction adjacent to the County's first solid
waste transfer station. The consulting engineer for
the McCandless Township Sanitary Authority
plans to design treatment units arranged so that
any of the several modifications of the activated
sludge process may be employed.
Since the facilities will be constructed in four
stages, beginning with 3 mgd flow capacity units,
the consultant will propose that the filter cake
produced from thickened and dewatering un-
digested sewage sludge solids, conditioned with
ferric chloride and sufficient lime to maintain a pH
of ~\ 2, free from odors and pathogenic organisms,
be transported by tank truck to the transfer station
for mixing with solid wastes and subsequent dis-
posal in a landfill, as a temporary expedient. As
treatment plant capacity increases then future
provisions for another method of sludge processing
such as incineration will be evaluated.
The actual logistics for services and details of this
unique arrangement will be developed within the
next year. The state at this stage has given this
proposal an experimental approval pending sub-
mission of all required and supporting engineering
data.
It would be most relevant to add that the current
energy crisis will provide us with the opportunity
to examine the advantages of burning combustible
materials from the transfer station with the sludge.
Of course, this change in operation at the station
would require extensive physical additions and
modifications to the facility for separation and
shredding. Another possibility, depending upon the
success of our strip mine project, would entail the
combining of the organics from the station with the
sludge to increase the viability of that project. It
would be worthwhile to mention that the County
anticipates the construction of two additional
transfer stations as part of our solid waste manage-
ment program.
The last system which the County may consider
at the site would be the use of a high performance
fluid bed reactor. This system would utilize solid
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138 MUNICIPAL SLUDGE MANAGEMENT
waste as a fuel to dispose of all the sludge in a pollu-
tion free manner. The system requires the installa-
tion of blowers, magnetic separators, air classifiers,
shredders, scrubbers, and a thermal energy
recovery system. Naturally, the residue emanating
from the operation would have to be disposed of in
a landfill unless the market value for ferrous
materials, glass, aluminum, and other non-
magnetic metals warrants reclamation.
In addition to the potential systems which have
been described, the County will encourage and
promote the use of the available sludge processing
capacity at various plant locations throughout the
County. If the County is able to designate, with the
cooperation of about a dozen authorities, certain
treatment facilities where operators of small
facilities may transport their liquid sludges for ul-
timate processing, we may then be better assured
that indiscriminate dumping of waste material
is reduced and the environment, as a result,
enhanced.
In conclusion, Allegheny County is on the
threshold of an era of significant experimentation
and demonstration, not only in sludge management
but also in solid waste management areas which are
so closely intertwined. We are hopeful that our ef-
forts and accomplishments will not only benefit the
County but also serve as a model for similar en-
vironmental activities throughout the Country.
BIBLIOGRAPHY
1. Dean, R. B. and Smith, Jr. J.E. "The Properties
of Sludge", Environmental Protection Agency.
2. Pennsylvania Department of Environmental Rules and
Regulation, - Chapter 75 - Solid Waste Management.
3. Schneider, R. J. "A Regional View on the Use
of Land for Disposal of Municipal Sewage and
Sludge", Environmental Protection Agency.
4. Environmental Quality System, Inc. "Renova-
tion of Strip Mine Land Using Municipal Treat-
ment Plant Residues and Fly Ash", Allegheny
County Proposal to Environmental Protection
Agency.
5. Kane, John T. "Pine Creek Sewage Treatment
Plant, Letter Report to McCandless Township
Sanitary Authority," Chester Engineers.
6. Combustion Power Co., Inc. "Liquid and Solid
Waste Disposal Systems for Municipal Wastes."
-------
TRENCH INCORPORATION OF SEWAGE SLUDGE
JOHN M. WALKER
Biological Waste Management Laboratory
Agricultural Research Service
U.S. Department of Agriculture
Beltsville, Maryland
ABSTRACT
Entrenchment seems a feasible method for
simultaneously disposing of sewage sludges and
improving marginal agricultural land, particularly
for dewatered (20 percent solids) raw-limed sludge.
The primary problem will be to avoid pollution of
groundwater with nitrate-nitrogen, as demon-
strated in test with sewage sludge (5 and 20 per-
cent solids) placed in 60-cm (two foot) wide
trenches of different depths and spacings. For de-
watered sludge, application rates were 800 and
1200 Mt/ha (350 and 500 tons/acre) dry solids,
respectively, in trenches 6JD cm wide x 60 cm deep x
60 cm apart and 60 cm wide x 120 cm deep x 120 cm
apart.
Entrenchment prevented contamination of sur-
face water, buried pathogens permitting their
demise during sludge decomposition, promoted
slow nitrogen release, and favored denitrification.
Nineteen months after sludge entrenchment,
fecal coliform and salmonella bacteria had not been
detected in soil more than a few centimeters from
the entrenched sludge. No downward movement of
heavy metals had been detected, and metal uptake
by crops had been moderate. Nitrate movement
had occurred, causing increased levels in under-
drained water. Groundwater in monitoring wells
'Based in part on a study by the Agricultural Research Service of
the U.S. Department of Agriculture in cooperation with the
Maryland Environmental Service of the Maryland Department
of Natural Resources, the District of Columbia Bureau of
Wastewater Treatment, and the Office of Research and
Monitoring of the U.S. Environmental Protection Agency.
had not shown increases in any pollutants except
chloride that might have come from the sludge.
Recommendations are given for running a sludge
trenching operation.
INTRODUCTION
The lack of environmentally acceptable
procedures for sewage sludge disposal seriously
hinders adequate treatment of municipal
wastewater. The better the wastewater treatment,
the greater the amount of sewage sludge generated
and requiring disposal.
Although all wastewater treatment plants have a
serious sludge disposal problem, the 1.14millioncu
meter (300 million gallon) per day Blue Plains Plant,
which serves much of the Metropolitan
Washington, D.C. Area, has a particularly acute
one. Since 1972 Blue Plains has been required to
reduce the BOD of the secondary effluent dis-
charged into the Potomac River. They had planned
to reduce this BOD by treating the effluent with
either FeCl3 or alum, but they have had no en-
vironmentally and politically acceptable alternative
for disposing of the resulting greatly increased
quantities of sludge—much of which would be un-
digested.
Early in 1972, the District of Columbia Govern-
ment (DC), the Environmental Protection Agency
(EPA), the Maryland Environmental Service
(MES), and other State, county, and local agencies
and groups launched a cooperative effort with the
Agricultural Research Service (ARS) of the United
States Department of Agriculture, to find an en-
vironmentally acceptable procedure for sewage
sludge disposal.
139
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140 MUNICIPAL SLUDGE MANAGEMENT
A comprehensive research-demonstration study
was undertaken for evaluating the trenching of
raw and digested sewage sludge to improve
agriculturally marginal soils and simultaneously
provide an economically feasible, environmentally
sound, and politically acceptable alternative for
sewage sludge disposal. These studies included:
(1) Field testing on large-scale to develop feasible
all-weather procedures for hauling and incor-
porating sewage sludge in the soil in trenches;
(2) Characterizing the proposed treatment site by
investigating its hydrologic properties as well as the
biological and chemical properties of the surface
and underground waters and the soils; (3) Testing
a drainage control system for the site;
(4) Establishing a program to monitor the move-
ment, form, persistence, etc., of sludge nitrogen,
heavy metals (zinc, copper, cadmium, nickel, etc.)
and pathogens in soil, underground and surface
waters, and plants growing on the site; and
(5) Supporting laboratory and greenhouse studies.
Field Entrenchment of Sludge
Site Preparation
We selected a 30-hectare (75-acre) experimental
site which seemed to offer excellent possibilities
for improvement with sludge, for monitoring, and
for drainage control. It was readily accessible to
heavy equipment, distant from residential develop-
ment, and had very sandy soils, which made it an
excellent site to test whether pollutants from en-
trenched sludge would move into the groundwater.
Approximately 50 soil borings were made to map
the water table and underlying impervious clay
layers. Many of these borings were converted into
monitoring wells. All of the wells in the immediate
plot area were cased, grouted, and sampled before
sludge was incorporated.
Based on the map of the water table and underly-
ing impervious soil layers, diversion drains (surface
and subsurface) were installed at several locations
(Figure 1). Test ditches showed that the soil was so
(Approx. Scale -- 1 inch 200 feet)
Wooded
Wooded
f-^- ~v^~^, J1-0 nuau
/ ^^^~-^-~-^~-V^^^N^N^~
Wooded
Fipun- I: Pnrti.il bite Map (Sludge Entrenched in the Plot Area).
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TRENCH INCORPORATION 141
sandy and wet that open ditches for conventional
tile laying would collapse. A special trenching
machine with shields behind the trenching wheel
(Table 1) was used to lay a 12.5-cm (five inch) cor-
rugated plastic drain tube which was slotted and
screened. A laser beam guidance system was used
to accurately place the tile line on a 0.1 and 0.2 per-
cent grade. A drainage catchment pond with a 3,785
cu meter (one million gallon) capacity was con-
structed down slope. This pond could hold about
two months' normal drainage from the site.
Sludge Entrenchment
The engineering firm of Whitman, Requardt, and
Associates worked with ARS to establish and test
trenching procedures for incorporating digested
and raw-limed sewage sludge into soil. Test
treatments used are listed in Table 2. Standard
equipment items were rented (Table 1).
TABLE 1
Heavy Equipment Used at Beltsville
in the Trenching Pilot Study8
Site Preparation - Pond Construction and Drainage
1 Speicher VT600 - 6 wheel drive trencher with laser beam automatic
grade control
1 Caterpillar - D-6
1 Caterpillar - D-7
I International 0.7 M3 (7/8 yd3) backhoe
Sludge Entrenchment
Cleveland JS-36 Trencher with tiltab!: and traversible
60 cm (24-inch) wide trencher
wheel
Caterpillar - 977 loader with 3.4 M3 (4-1/2 yd3) bucket
Caterpillar crawler loader with 1.9 M3 (2-1/2 yd3) bucket
Caterpillar - D-8
Caterpillar - D-7 - with winch
Numerous- 10-wheel, 12 M3 (16 yd3) dump trucks
23 M3 (6,000-gallon) pneumatically operated tank truck
Caterpillar - motor grader
50-cm (20-inch) farm disk
a Mention of trade names, contractors, consultants, and commercial products is for the
convenience of our readers and does not imply endorsement or recommendation for use by
the United States Department of Agriculture.
TABLE 2
Sludge Entrenchment Data
Treatment
a &b
I
11
II
IV
Va
b
Trench
Depth
cm. foot
60(2)
120 (4)
60(2)
120 (4)
60(2)
120 (4)
Spacing^
between trenches
planned, actual
ft. ft.
2 3.0
4 4.9
2 2.1
6 8.7
2 3.0
4 4.9
Sludge
Typeb
Digested
Digested
Raw-Limed
Raw-Limed
None
None
Actual
Solids
%
27.6
27.6
18.5
9.3
NA^1
NA
Ratec
dry solids
MT/ha, tons /a
830 (360)
1150 (500)
740 (320)
130 ( 55)
( 0)
( 0)
Plot Sizes
a &
ha
6.3
0.1
0.05
0.05
b each
acre
(0.75)
(0.25)
(0.12)
(0.12)
bDigested and liquid raw-limed from Blue Plains and dewatered raw-limed from Fairfax plants.
f Assuming 0.85 wet tons/cu. yd. and ten percent trench overfill.
dNot applicable.
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142 MUNICIPAL SLUDGE MANAGEMENT
The prime goal was simulation of full-scale
trenching of dewatered sludge at the rates
generated at Blue Plains. Twelve-cubic meter (16-
cubic yard) dump trucks were used for hauling. The
initial plan was to discharge the sludge onto the
ground near the open trenches so that front-end
loaders could gather up the sludge and place it in the
trenches. This plan proved unsatisfactory because
the jelly-like sludge was very difficult to scoop up
with the loader buckets. Both the trucks and front-
end loaders tracked sludge all over the soil surface,
making it very wet and slippery and difficult for the
trencher to operate. Furthermore, the trucks
tracked the sludge onto the site roads and the
highway.
Temporary storage pits were established into
which the sludge was dumped and from which the
loaders obtained, the sludge. This was a con-
siderable improvement. The pits, approximately 2
meters deep x 4 meters wide x 12 meters long (6.5 x
13 x 26 feet), were pushed into the soil with a
bulldozer. Sludge still fouled the pit entrances and
the soil surface near the trenches, so pit entrances
had to be continually scraped with a bulldozer. Oc-
casionally, to keep the trencher from slipping, the
sludge impregnated soils in the trench area were
also scraped. Pits were closed as they became im-
passable and/or too far from trenches.
Trenches, dug along the contour, were filled with
sludge by using front-end loaders (Figure 2). The
loaders buckets held more sludge than a segment of
60- x 60-cm trench the length of the bucket could
contain. To guide the filling considerable care and
skill of operators as well as a field assistant standing
by the trenches were needed. Usually, trenches
were ten percent overfilled. As the trencher dug a
new trench, properly spaced from the first, it
simultaneously covered the previous trench with
the diggings (Figure 3).
Because the trencher lacked grouser bars on the
track pads it slipped in wet weather. The trencher
could only be operated in one direction because the
operator's seat was located on one side, which
caused visibility problems. The trencher operator
had to see the previous trench and needed guid-
ance from a field assistant to keep the trenches
straight, properly spaced, and at the proper depth.
The trencher operated at approximately six meters
(25 feet) per minute in dry weather when digging
60-cm (two-foot) deep trenches. When trenches
were 120 cm (four feet) deep, the walls of the very
sandy soil often collapsed. Lateral force from the
adjacent previously filled and covered trenches
aggravated the situation. An occasional increase in
the spacing between trenches helped prevent
collapse.
On a dry surface the rubber-tired front-end
loader was very mobile, fast, and efficient in filling
the trenches, but it often got stuck in wet weather.
Although it could usually work itself free, in so do-
ing it tore up the soil surface. Under wet conditions,
crawler-loader was better but slower and less
mobile than the rubber-tired loader, and con-
siderably disrupted the soil surface. The wet, dis-
rupted soil surface required leveling and scraping
with a bulldozer and/or several days' drying before
trenching could be continued.
The dump trucks were not ideal for hauling
sludge. They leaked sludge liquid onto the highway,
would not unload completely, and often became
stuck at the site. Sludge adhered to the truck tires,
mud flaps, and chassis and dropped onto the soil
surface and highway.
The on-site gravel roads were barely adequate in
wet weather. They had to be graded continuously
and gravel added to maintain a crown and allow
drying.
Even with all the difficulties involved with learn-
ing a new operation and the less than ideal equip-
ment, sludge was trucked to the site and incor-
porated in trenches at the expected rate of sludge
production at Blue Plains. For example, 450 filter-
cake tons (20 percent solids) of digested sludge
were incorporated into 120 cm (four foot) deep
trenches (plot Ha) in six hours, using only the 977
rubber-tired loader. This was equivalent to a rate of
600 filter-cake tons per eight-hour day (calculated
from Table 2). Approximately 2,000 filter-cake tons
of sludge were incorporated into plots la and Ib(0.6
hectares—1.5 acres) under very wet conditions in
27 hours, not counting time off during rain. Both
front-end loaders were used. This again was
equivalent to a rate of 600 filter-cake tons per eight-
hour day.
With the standard equipment used, trenching
sludge in the rain was not possible. Modifications of
procedures have been recommended to improve
the cleanliness and desirability of the operation and
make it nearly possible to operate during all kinds of
weather. These modifications include installation
of grouser bars on the trencher, use of cement
trucks for hauling sludge and discharging it directly
into trenches in dry weather or into special
bulldozer-pulled trailers in wet weather, and un-
loading the trailers into the trenches with a Moyno
pump. Clean sludge-handling equipment and
prompt filling and covering of raw-limed and
digested sludge, dewatered within the past 24 to 48
hours, is necessary to prevent malodor problems.
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TRENCH INCORPORATION 143
2- Trencher Digging a Tretirh and Loader Pilling It with
Figure 3: Tnmchef Digging a New Trench wi*h the Diggings UwE-ring Simultaneously, the Previutii Trench Thai Had
Been Filled with Sludge.
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144 MUNICIPAL SLUDGE MANAGEMENT
Liquid Sludge Incorporation
The procedure used to incorporate liquid sludge
into trenches was unsatisfactory. Because of
logistical problems and the desire not to backfill im-
mediately, all trenches were dug before sludge in-
corporation. Approximately a 2.5-meter (eight-
foot) spacing between trenches was necessary to
prevent collapse in the sandy soil (Table 2). Liquid
sludge was successfully discharged into the
trenches pneumatically from a large tank truck.
Dewatering occurred very slowly and additional
sludge could not be added. The 120-cm (four-foot)
deep trenches could only be one-third to one-half
filled or sludge would run out during backfilling
with spoil.
Costs
Projected costs for trenching sew/age sludge have
been taken from a report by Resource Management
Associates, Inc. (See Reference List) prepared for
the Maryland Environmental Service. These costs
are summarized briefly in Table 3. Capital costs are
mainly for site development and equipment. Most
site development funds are for on-site roads on two
40-hectare (100-acre) sites. Capital funds are also in-
cluded for drainage and water containment at the
two sites.
The equipment costs are for purchasing 14 pieces
of equipment such as crawler dozers and loaders,
trenchers, sludge pump trailers, a motor grader,
TABLE 3
Estimated Costsa'b for Trenching
Costs for two-year operation at 400 filter-cake tons per day
CAPITAL
Site development (two separate 100 acre-sites) $576,000
Loader hopper at treatment plant 20,000
Equipment 492,000
OPERATING (annual)
Capital amortization and interest (6%) $593,000
Equipment operation 110,000
Labor 233,000
Miscellaneous 5,000
Costs per filter-cake 400 filler-cake tons per day level
CAPITAL
OPERATING
Total $10.17
"l-rom costs estimated by Resource Management Associates, Inc.,
for the Maryland Environmental Services
''Costs do not include land acquisition, transportation, or any resale
value of equipment at the end of 2 years
etc., which would be transported between each site
as necessary. The capital costs do not include
purchase or rental of land or equipment resale
value at the end of two years.
Operating costs are mostly for amortization of
capital costs (six percent) over a two-year period,
equipment operations, and labor. Labor funds were
projected for 13 employees. The operating costs do
not include sludge transportation. A total cost of
$10.17 per ton of filter-cake sludge was estimated
for trenching at a rate of 400 filter-cake tons per
day. This cost is competitive with that of other
sludge disposal methods.
Environmental Effects
Environmental effects of high rate sludge
application to land were studied in groundwater
observation wells, in underground and surface
drainage water, in soil below and around en-
trenched sludge, within entrenched sludge, and on
crops.
Nineteen months after entrenchment, no
salmonella and fecal coliform bacteria, no nitrogen,
and no metals were detected in groundwater wells
that came through soil from the sludge.
Measurements of drainage water and soil under
trenches, however, suggested that nitrate pollution
would probably become a problem as sludge
weathers (dries out) and becomes more aerobic.
Fecal coliform and salmonella bacteria were not
detected more than a few centimeters below the
trenches and the organisms were only detected in
surface drainage water at the time of sludge incor-
poration due to sludge spillage on the soil surface.
Laboratory studies suggest that viruses can move
in soils, but not likely far or fast enough to con-
stitute a hazard. Studies on virus movement
through soil are continuing. Heavy metals had not
been detected moving down into soil out of en-
trenched sludge.
Sludges weather (dry out) in soils after entrench-
ment. Root penetration speeds the process (Figure
4). The sludge is changed from a malodorous,
dense, anaerobic, water-shedding material to an
aerobic porous peatlike consistency in which
nitrification is facilitated and denitrification in-
hibited.
Considerable nitrate-nitrogen apparently is lost
by denitrification, (the bacteriological conversion
of nitrate to nitrogen gas, which occurs under
anaerobic conditions with a carbon energy source
required). Nevertheless, nitrate levels were in-
creased in the soil under the trenches and in
drainage water. Drainage and impoundment of in-
filtrating rain water will likely be necessary. This
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TRENCH INCORPORATION 145
4 .Crtfss-Sectfonjl Excavation of bntrenched Digested Sludge 17 Months (Oi't 17, 1°73J Showing Degree nf Weathering
r A = 'W*^[h"rrfr
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146 MUNICIPAL SLUDGE MANAGEMENT
impounded water, if contaminated with excessive
levels of nitrate, could be applied as irrigation water
on surrounding land, where crops would remove
the nitrate.
Metal availability to plants apparently increased
as sludge weathered. DTPA-TEA-chelate-
extractable metals (an index of plant available
metals) and plant uptake showed this trend. Metals,
however, were apparently less available in the most
weathered sludges as compared to intermediately
weathered sludges in a greenhouse study. Overall
uptake of metals by crops growing on the trench
study plots has been moderate.
Digested sludges (60-cm trenches) were about
one-half weathered to peatlike consistency 19
months after entrenchment, and raw-limed
sludges were approximately one-fifth peatlike.
Studies will continue on the fate of pathogens,
nitrogen, and metals as weathering continues.
Growth of crops like fescue, alfalfa, rye, and trees
in the sandy infertile soils in our studies seems to
have benefited from entrenched sludge. Detailed
data and discussions of environmental effects may
be found in our research reports on trenching (See
Reference List).
RECOMMENDATIONS
General
Trenching seems to be a suitable procedure for
high rate disposal and application of sewage sludge
to land. Trenching would be an appropriate system
to use when low-rate (fertilizer-rate) surface
application of sludge is not feasible, e.g., with raw
sludge. Properly used, the procedure seems en-
vironmentally safe and compatible with use of the
land for some agricultural purposes. Trenching
would not be appropriate in prime agricultural land
because of subsoil being brought to the surface and
the amount of trace elements applied. Because we
have only been able to study the effects of
trenching for a short time under limited conditions,
any system using the trenching procedure now for
land application of sludge should include careful
monitoring.
Desirable Site Characteristics
A good site should have: A substratum suitable
for establishment of a drain system, a good location
for a holding pond, good vehicular access, a rural
location, slopes less than 15 percent where sludge is
to be applied, and soil of marginal agricultural
value—first choice would be a heavy soil underlain
with an impermeable stratum. Sites with sandy
soils, while less desirable, may still be suitable. If
fissured rocks are present, they should be at least
three meters (ten feet) below the surface.
Survey
To determine its suitability, the potential trench
application site must be studied carefully to
characterize: (a) The surface and subsoil; (b) the
topography; (c) the distance to fissured rock;
(d) location of any hard, impervious layers;
(e) location of permanent and perched water
tables; (f) the direction and flow of underground
waters; (g) potential for underground drainage;
(h) areas for suitable waterholding ponds; and
(i) adequate access for heavy trucks and other field
equipment.
Site Drainage
A drainage network should be installed with the
average depth 120 to 150 cm (four-five feet). Pond
storage capacity should be adequate to hold drained
water for approximately two months. If con-
taminated, the water could be applied on surround-
ing land for purification by crop utilization and
percolation through soil. Drainage and surface
water control should be under the guidance of any
agency like the Soil Conservation Service.
Trenches
Trenches should be dug on the contour at the
time the sludge is available. The trenches should
then be covered the same day that they are filled. A
trenching machine should have cleated tracks and a
rear-mounted digging wheel that is movable from
side to side and tiltable. For maximum benefit and
decreased nitrate hazard, limed sludges should be
placed in trenches no more than 75 cm (30 inches)
deep and 60 cm (24 inches) wide and 60 to 75 cm (24
to 30 inches) apart edge to edge. Sludges placed in
narrow trenches (less than 60 cm (24 inches) wide)
would result in greater soil sludge contact and more
aerobic conditions. Narrow trenches would
probably favor more rapid nitrification with less
denitrification, and consequently increase the
danger of nitrate pollution of groundwater.
Sludges
Sludges should be entrenched at high disposal
rates when use of low fertilizer application rates (25
to 55 dry MT per hectare annually or 10 to 25 dry
tons per acre annually) mixed into the soil surface
to a depth of 15 cm (six inches) of soil is not possible.
The low-rate application of sludge to the soil surface
compared with high-rate application in trenches
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TRENCH INCORPORATION 147
would yield full agricultural benefit of sludge
nutrients and avoid the potential movement of
nitrate and excessive metal accumulation in soils.
Sludges to be entrenched should first be limed
and dewatered. The pH of the sludge at time of
dewatering should exceed 11.5 to reduce survival of
pathogens and to lower the potential for metal ac-
cumulation by crops. The metal content of sludges
should be as low as possible to further decrease
potential for excessive uptake of metals by crops.
Raw sludges should not be applied to land except
in trenches because of the potential pathogen
hazard and odor problem associated with surface
incorporation. Metals content is less and apparent-
ly risk of nitrate movement is less from a given
volume of raw than of digested entrenched sludge.
Hauling and Filling
A sealed cement-type truck is recommended for
hauling the sludge from the wastewater treatment
plant«to the trench incorporation site. This truck
could also then be driven directly to the trenches
when the soil is dry and capable of bearing the load.
The sludge could then be unloaded from the cement
truck via its own extended discharge chute. In wet
weather, the cement truck could discharge the
sludge into a trailer outfitted with a Moyno pump.
A bulldozer could then pull the high flotation trailer
near the trench so that the sludge could be unload-
ed via the pump into the trenches.
Preparation for Seeding
To prevent erosion and permit soil stabilization,
the trenched area should be left ridged until
weather is suitable for leveling and seeding. When
leveling freshly filled and covered trenches, a
bulldozer or some other suitable tracked vehicle
should be used at right angles to the trenches. Deep
cross-ripping of the entrenched sludge is un-
necessary in sandy soil. Its possible benefit should
be determined in clay soil. Based on soil tests and
the crop to be grown, fertilizer and lime should be
applied and worked into the soil surface. The lime
and fertilizer requirement could be reduced by sur-
face application of approximately 23 to 45 dry MT
per hectare (10 to 20 dry tons per acre) of digested
sludge.
Crops
Crops should be limited to grass the first year. In-
itially the trenches can be leveled and cultivated
only at right angles. If row crops are subsequently
grown, they should be planted on the contour to
prevent excessive erosion. Because of uncertainty
on availability of metals to crops grown on trenched
soils, crops grown should not be used in the food
chain unless monitored to determine their safety.
Monitoring
Since monitoring is extremely important and so
little is known about the long-term effects of
trenching on the environment, it should be the
responsibility of a qualified trained individual
responsible to a governmental institution, such as
the State Department of Health. Monitoring
should begin before sludge is applied and continue
for at least five years after application. Monitoring
for a large-scale trenching operation might include
determinations as suggested below. A smaller
operation would require less monitoring.
Background samples should be taken from
strategically located groundwater wells a month or
two before sludge is applied. These wells should be
located both inside and on the downflow side of the
underground water coming from the entrench-
ment site. Background analyses should include
some of the following determinations: fecal
coliforms, PCB's, chlorinated hydrocarbon
pesticides, alkalinity, organic nitrogen, nitrate
nitrogen, ammonium nitrogen, chlorides, pH,
solids, BOB, COD, phosphate, calcium, sulfate,
manganese, zinc, cadmium, nickel, copper, lead,
mercury, potassium, magnesium, sodium, and
specific conductivity. If techniques are available the
water might also be analyzed for viruses. A similar
analysis for the background parameters should be
made 6 to 12 months after sludge incorporation and
then yearly thereafter for at least five years. Well
water should be sampled more often (monthly to
tri-monthly) depending upon the well location, and
analyzed for fecal coliform, chloride, ammonium
nitrogen, and nitrate nitrogen.
At least two complete sets of background
analyses, preferably at three-month intervals,
should be made of residential wells located within a
one- to two-mile radius of the area of sludge en-
trenchment.
Composite samples should be collected and
analyzed monthly from streams draining the area,
from major subsurface collector lines draining the
area, and from ponds holding drainage water. This
sampling should begin two months before sludge
application, continue at monthly intervals
thereafter, until one year after all sludge has been
incorporated, and then periodically for two to four
years. Analyses of these samples might include
fecal coliforms, pH, dissolved oxygen, BOD and
COD, chloride, ammonium, nitrate, total nitrogen,
and phosphorus.
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148 MUNICIPAL SLUDGE MANAGEMENT
Representative soil samples should be collected
to the depth of proposed entrenchment in the area
to be treated and analyzed prior to sludge incor-
poration for cation exchange capacity, texture, and
pH. It also may be useful to know the soil levels of
potassium, phosphorus, soluble salts, chlorides and
metals like zinc, copper, cadmium, nickel, lead and
mercury.
Before sludges are entrenched, they should be
continuously monitored during each day for pH at
the treatment plant. At the time of dewatering the
pH should exceed 11.5. A composite sludge sample
should be carefully assembled biweekly from daily
composited samples and analyzed for chloride, fecal
coliforms, salmonellae, and metals like zinc, copper,
nickel, cadmium, lead, and mercury. Less frequent
analyses for viruses, total and ammonium nitrogen,
phosphate, potassium, CaCOs equivalent and per-
cent volatile solids may also be helpful.
Crops grown on the area to receive sludge should
be sampled and analyzed semiannually for at least
five years for uptake of zinc, copper, cadmium,
nickel, lead, and mercury. The same crops grown on
nearly similar soils should be analyzed as a control.
Research
Considerable additional research is needed on
sludge handling and incorporation in soil and in
trenches that will result in minimum hazard to the
environment and maximum benefit from sludge
disposal and land reclamation. A portion of the
funds for the trenching operations should be
allocated for research. Research personnel should
cooperate to determine the environmental effects
of even larger scale trenching operations than
previously studied. Other research would include
studies on: (a) Placement of raw and digested
sludge with and without lime and other chemical
treatment in trenches in different types of soils, on
which the surface and underground water, the en-
trenched sludge, and the soil surrounding the
sludges would be carefully studied for movement
and survival of pathogens; movement of nitrogen,
the metals (zinc, copper, nickel and cadmium),
chlorides, and COD; pH; weathering; and degree of
plant root penetration; (b) movement of viruses
through soils both in the field and in the laboratory;
and (c) availability of heavy metals to crops. Rou-
tine composite samples of different sludges and
their effluents at treatment plants should be taken
and analyzed for metals biweekly. These analyses
can be used to identify more clearly the effects of
chemical treatment and at what point in treatment
different sludge components are separated out.
The metal measurements in the sludges, along with
metal measurements of wastewater at suspected
discharge points, can be useful to identify sources
of metal pollution, i.e., from point-sources, general
industrial and domestic sources, and/or street
runoff. These determinations should also be very
useful for reasonably predicting the potential
usefulness of the tested sludges on land.
Summation
These recommendations on trenching
procedures are based on data from limited ex-
periments over a short time. We believe, however,
that this research shows that dewatered sludge can
be trenched safely by following our present
recommendations. The most likely difficulty is that
excessive nitrogen from the sludge might reach un-
derground water. This nitrogen problem can be
minimized by underdraining the entrenchment site
and retaining the drained water for irrigation of
surrounding land.
Implementation of Land
Use-Disposal Sludge
Special approaches are needed to implement not
only a sludge trenching operation, but any land use
disposal system for sludge. Because of political, en-
vironmental, and energy limitations, the primary
method for sludge disposal is rapidly shifting to
land use. This a radical change from considering that
sludge should be merely incinerated, landfilled, or
dumped in the ocean.
Land use for disposal of sludge requires a new set
of specialized skills. Wastewater treatment
authorities must now learn all about the limits of
sludge application to land, including presence in
sludge of pathogens, heavy metals, salts, nitrogen,
and phosphorus. They also must be aware that soil
type, its acidity, and the crops can limit sludge use
on land. They may discover that sludge cannot be
used on land until its heavy metal content has been
reduced, because once these metals are added to
soils they remain and may later become hazardous.
They soon will discover that sludge application to
soil is limited in poor weather and when crops are
growing. They will discover that regulations for
sludge use on farmland will be restrictive and con-
servative with different rules than for placing
sludge in trenches. They finally will discover,
quickly, that not nearly enough is known about
long-term effects of sludge on soils, water, crops,
and the food chain. Although they will be tempted
to dismiss land use as a means of sludge disposal,
land use of sludge will soon be one of the only
available methods of land disposal.
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TRENCH INCORPORATION 149
Because of weather limitations and all the other
uncertainties about land use of sludge, every land
use system must have one or more back-up sludge
use or disposal alternatives. For example, a
trenching operation should have a landfill, in-
cineration, land spreading, or some other back-up
or combination of these back-up alternatives.
The problem areas listed in Table 4 will likely be
encountered in any sludge use disposal operation.
To avoid these problems we must be aware of the
limits of sludge application to land and seek ade-
quate consultation on establishing the proposed
land use system. With trenching, changing some of
the recommendations can be beneficial, while other
changes e.g., ignoring the weather, using trenches
too narrow or too deep, providing no drainage,
using leaking trucks, failure to cover sludge
promptly—may be disasterous and lead to a court
order to cease operations. Finally, the public must
TABLE 4
Problem Areas in Trying to Implement
a Sludge Land Use - Trenching Operation
(1) Budget
(2) Coordination of treatment plant personnel with land use
personnel to avoid establishment of a treatment process yielding
a sludge that cannot be used on land
(3) Availability of proper equipment, e.g., sealed hauling
(4) Untrained and/or inexperienced supervisors and operators
(5) Monitoring
(6) Uncertainty of procedures and needs to avoid adverse effects of
sludge on the environment
(7) Alternative back-up sludge disposal and/or use procedures
(8) Contracts with firms for sludge use disposal must specify
procedures so that proper careful operation is encouraged
(9) Early and continued public involvement
be involved from the beginning and aware of the
problems with free discussion of possible solutions
in arranging for sludge disposal.
I hope that we can find needed answers for im-
plementing land use systems for sludge disposal as
soon as possible. Some answers, however, will
come slowly. Meanwhile, land use operations for
sludge disposal must be clean, prompt, backed up
with disposal alternatives to use during periods of
rain or poor sludge quality, and have the active sup-
port of the public.
REFERENCES
1. Walker, J. M., W. D. Surge, R. L. Chaney, E.
Epstein, J. D. Menzies, and W. H. Harrington,
"Trench Incorporation of Sewage Sludge," Draft
Report by Agricultural Research Service and Mary-
land Environmental Service for District of Colum-
bia and Environmental Protection Agency, June
1974.
2. Incorporation of Sewage Sludge in Soil to Maximize
Benefits and Minimize Hazards to the Environment,
Agricultural Research Service, USDA, Beltsville,
Md. An interim report by Agricultural Research
Service for Maryland Environmental Service, Dis-
trict of Columbia, and Environmental Protection
Agency, June 1972.
3. Land Containment Sites for Undigested Sewage Sludge.
Report by Whitman, Requardt, and Associates for
Maryland Environmental Service, June 1972.
5. Sludge Utilization Project—Operations Plan and
Procedures. Report for Maryland Environmental Ser-
vice by Whitman, Requardt, and Associates,
Baltimore, Md., August 1972.
-------
OCEAN DISPOSAL EXPERIENCES
IN PHILADELPHIA
CARMEN F. GUARINO AND STEVEN TOWNSEND
Philadelphia Water Department
Philadelphia, Pennsylvania
INTRODUCTION
Ocean disposal of digested sludge in Philadelphia
is a very simple "unit" operation. First, the sludge is
digested, using standard-rate anaerobic digesters,
and pumped to an on-site lagoon for thickening.
This step results in thickening of the sludge from
seven percent to about 12.5 percent. From the
lagoon, the sludge is pumped to the barge via a ten
inch Ellicott Dredge. The barge has a capacity of
two million gallons and is equipped with an 18 inch,
hydraulically operated, bottom dump valve. The
barge is towed downriver and out to a prescribed
disposal area where the sludge is discharged at ap-
proximately 30,000 gallons per minute over a six
mile course. No known harmful effects have oc-
curred as a result of this operation.
Except for one "catch," the story might end at
this point with a few additional comments on the
economic benefits and operational improvements
for the system. That "catch" is called the Marine
Protection, Research, and Sanctuaries Act of 1972
(The Act). This Act is the first comprehensive
Federal waste disposal control regulation and the
discussion that follows will focus on its conse-
quences, both good and bad.
Philadelphia's sludge disposal experiences may be
divided into three parts: 1) our experiences prior to
the passage of the Act, 2) the Act itself and the
effects of its passage on ocean disposal, and 3) the
implications of information being developed as a
result of the Act's requirements. Hopefully, the dis-
cussion will illustrate the need for integrating laws
and regulations with sound sludge management
concepts.
Choosing and Continuing
Ocean Disposal
Three wastewater treatment plants in
Philadelphia serve an area of over 360 square miles.
The Northeast Plant is now an intermediate-type
activated sludge facility while the Southeast and
Southwest Plants use primary treatment. The com-
bined capacity is 450 MGD and all of the sludge
produced is anaerobically digested and barged to
sea. In order to meet more stringent discharge stan-
dards, the City is now in the design stage of a $300
million plant expansion program that will result in
full secondary treatment at all plants.
Ocean disposal of sludge began in 1961 when it
became apparent that, due to growth in the
drainage area, available lagoon space would not be
sufficient. The decision to begin use of the ocean
followed a survey of disposal alternatives which
found ocean disposal to be the most economic and
practical method. Since 1961, Philadelphia has
barged approximately 960 million gallons of sludge
to the ocean.
The site chosen in 1961 and used until May, 1973,
is a rectangular area (one mile by two miles) located
13 miles off Cape Henlopen, Delaware. The site
was selected by considering favorable ocean
currents (dilution), depth of water (60 ft.), and the
marginal value of shellfishing in the area.
In early 1970, the environmental soundness of
our ocean disposal program began to come under
review and criticism. Because of the great concern
expressed by citizens, various agencies, and
government groups, the City contracted with the
Franklin Institute Research Laboratories (FIRL)
151
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152 MUNICIPAL SLUDGE MANAGEMENT
and Jefferson Medical College, both located in
Philadelphia, to take samples of the waters and
sediments at the disposal site and determine if any
harmful effects had occurred.
The FIRL study focused on an area within a six
mile radius of the center of the disposal site.
Monthly cruises were made to examine the water
column, sediments, and biota. The study was com-
pleted in February, 1972.
Dissolved oxygen levels in the water were near
saturation. Coliform bacteria were found in the
trail of a discharging barge, as would be expected.
Positive counts existed, however, only while the
plume could be seen, a period of about two hours.
Examination of the sediments showed clean
sand, gravel, and pebbles. Tests for heavy metals in
sediments showed levels in the disposal area to be
insignificantly different from contiguous areas of
the ocean. Nowhere was there evidence of a sludge
blanket.
The diversity of species was high and the animals
present appeared healthy. Scuba and TV examina-
tion of the bottom revealed normal conditions for
the New Jersey coastal region.
The conclusion reached by FIRL was that over
ten years of discharging sludge had done little or no
perceptible damage to that area of the ocean. While
this study has been criticized as being incomplete or
inconclusive, the fact remains that a year long
study by competent and reputable organizations
was unable to detect any significant effects.
Recent Regulations and Their Effects
In October, 1972, the United States enacted the
Marine Protection, Research, and Sanctuaries Act.
The law established regulations and criteria, to be
implemented by a permit system, for all forms of
waste discharge (except pipeline) to the ocean. The
permit system took effect on an interim basis on
April 23, 1973, pending development of final
criteria. The final criteria were promulgated on*Oc-
tober 15,1973, and the Act went into effect in final
form on February 13, 1974.
A detailed discussion of the implementing
criteria of the Act is beyond the scope of this paper
but a brief treatment of the now final regulations is
pertinent. Basically, the criteria contain categories
of potentially hazardous materials which are
regulated according to concentration definitions.
For example, under title "Other Prohibited
Materials" are organohalogen compounds, mer-
cury compounds, cadmium compounds, and
petroleum products. A dumper must show that his
waste contains these materials in less than trace
concentrations, as defined in the Act, before a
permit can be issued.
Another title by which dumpers are regulated is
"Material Requiring Special Care" which lists a
wide range of materials that must meet the re-
quirements of a Limiting Permissable Concentra-
tion (LPC). The LPC is based upon the results of
bioassay testing, an application factor (.01), and
definitions of a mixing zone and a release zone.
Other requirements of the permit structure in-
clude site monitoring, extensive waste analysis,
research into alternatives to ocean dumping, and
public hearings.
The first direct effect of the permit system on
Philadelphia was the movement of the disposal site
from 13 miles to 50 miles off the coast. Our permit,
issued first on April 23,1973, required that the new
(50 mile) site be in operation by May 8, just two
weeks after issuance. The reasons for the move are
still unknown but the effect has been a barging cost
increase of 175 percent, possible adverse interac-
tion with a nearby acid waste site, and continued
opposition from coastal areas.
Another immediate effect was a greatly ex-
panded analysis program due to a requirement for
analyzing some 40 parameters on each barge leav-
ing our treatment plants. This program is still in
effect at a cost of $40,000 per year for outside
laboratory services plus almost 20 percent of our
own laboratory man-hours. The program has
shown that Philadelphia's sludge meets all re-
quirements, except mercury and cadmium in the
"solid phase." Procedures for Hg and Cd, the two
most critical parameters, are still in draft form, but
indications are that the sludge will exceed the
criteria for these metals. This question will be dis-
cussed further in a later section.
The permit also required extensive monitoring of
the new disposal location. A $250,000 (per year)
contract is now being processed to do the monitor-
ing work. Monitoring should begin in August of
this year.
The Act makes provision for research into alter-
natives when a dumper exceeds the criteria. This
has led to several comprehensive reports and
studies that explore the viability of various alter-
natives.
On February 13,1974, due to passage of the final
criteria, the City received another interim ocean
dumping permit. The requirements of this permit
are similar to those mentioned above with the ex-
ception of a requirement for an extensive industrial
source control program. The program is intended
to determine the sources, relative contributions,
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OCEAN DISPOSAL 153
and controllability of heavy metal sources within
the City's drainage area.
Control of Ocean Use
As indicated by the requirements outlined
above, a tremendous amount of information has
been generated as a result of the Act's passage. In
this light, Philadelphia has been very active in
reviewing its position on sludge disposal and how
regulations such as the Marine Protection Act will
ultimately affect the environment. To date, all of
the data developed and reviewed leads to the con-
clusion that controlled, well-managed, ocean dis-
posal is the best sludge disposal method presently
available to Philadelphia. Further, it appears that
the Act, as it stands now, is overly aggressive in
protection of the ocean while not taking into ac-
count similar potential hazards of the various alter-
natives to ocean use. There is a lesson in en-
virorimental management to be learned from this
situation. Realizing that the discussion of this point
may be academic with regard to the Marine Act, the
comments that follow may help in considerations of
the many disposal options being presented at this
conference.
At present, analysis shows that Philadelphia's
digested sludge exceeds the criteria of the Act for
only two elements, mercury and cadmium in the
"solid phase." Two possibilities for checking appear
immediately in (l) the analysis and (2) the criteria.
Analysis for many of the materials listed in the
Act, including Hg and Cd, had not been routinely
performed on sludge until the permit system took
effect. Problems have arisen as to confirmation of
the resulting data. For example, our laboratory per-
formed the prescribed mercury test for six months
under our first permit. The results varied con-
siderably, from 33.6 mg/1 to 0.2 mg/1, using the
same procedure so that when our new permit was
being considered, the sludge could not be adequate-
ly compared to the criteria. During this period, we
conducted a round-robin confirmation of the data
using three analytical methods and five
laboratories and found that mercury concen-
trations were ~ 1.0 mg/1 (total).
Even with confirmed data, comparison to the
criteria could not be made since the criteria gave
concentrations in liquid and solid phases. Is sludge a
two-phase substance? This depends entirely on the
method specified for preparing the samples, and
none were available. Recently, a draft form of a new
procedure for Hg and Cd was developed by the
Regional EPA. This method, by definition, makes
the sludge exceed the criteria for Hg and Cd in the
solid phase. Should sludge be considered as a liquid,
the sludge would exceed the criteria by 1 ppm for
cadmium.
There are many such draft procedures from the
various EPA regions and yet the law is being im-
plemented on a national basis. It would seem that
the law should not precede valid analytical
procedures by which the law is implemented.
The wording of the Act is unclear as to what is in-
tended by the distinction between solid and liquid
phases when all other substances are limited by the
bioassay test. The criteria state that mercury con-
centrations in the barge must be less than 0.75 mg/kg
in the solid phase and 1.5 mg/kg in the liquid phase.
Philadelphia's discharge rates and bioassay results
are well within the criteria but we may be forced to
abandon the ocean by concentration values that
have already met the toxicity limitations. No con-
sideration seems to be given for dilution which
drastically reduces the effective concentration of
the waste. Since toxicity is directly proportional to
concentration, i.e., availability, limiting discharge
by such low concentration values of the waste
before dumping seems inappropriate.
Interaction of wastes in a disposal area is another
key question. Mentioned earlier, one result of our
move to the 50 mile disposal site was a possible in-
teraction with a nearby acid waste site. Recent EPA
survey cruises in the area have reported some metal
increases in the sediments. While these findings
have not been statistically validated, they do point
to the possibility of adverse interaction. One
hypothesis is that, because the acid waste contains
FeClz, it is causing flocculation and some deposition
of sludge solids in the area.
The interaction of two wastes may produce
effects at the site totally different from the effects
of the materials considered individually making
comparison of individual wastes to the criteria
meaningless. It may be that grouping of compatible
waste discharges is needed to avoid hazardous in-
teractions or, on the other hand, to promote any
beneficial effects.
The questions raised regarding the validity of the
criteria, if nothing else, show very definitely that
much more study is needed towards developing
proper ocean management schemes. The Marine
Protection Act has accomplished a great deal to this
end. We should be investigating maximum loading
rates for specific areas. We should be investigating
sludge dispersion characteristics in the ocean. We
should be carefully monitoring all disposal
operations and establishing complete baselines. We
should be investigating movement of hazardous
materials in the food chain. But we should not
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154 MUNICIPAL SLUDGE MANAGEMENT
eliminate the ocean as a disposal alternative before
these questions have been adequately answered. In
the same way some materials could have an adverse
effect, eliminating ocean disposal could result in the
loss of the best possible choice, as the next section
will point out.
Alternatives to Ocean Disposal
If disposal of digested sludge is potentially harm-
ful to the ocean, then the same potential exists for
other methods like land disposal or incineration.
Digested sludge, by its very nature, contains
materials that are potentially harmful to the en-
vironment if improperly managed. Investigation of
the alternatives is not a problem but how to then
evaluate the relative impacts of the alternatives has
not been clearly established. It is implicit in the
passage of the Marine Act that these
benefit/damage relationships are known for the
ocean but this is insufficient without the same con-
sideration for other disposal means. Further, once
the acceptable means of distributing pollutants are
established for other segments of the environment,
a comprehensive study of the relative value of each
segment must be made. Only after this is ac-
complished can municipalities make design
decisions on the best means of ultimate disposal.
The reports and efforts resulting from our per-
mit requirements for research into alternatives
warrant discussion at this point to complete the
sludge disposal picture for Philadelphia.
There are three basic choices for ultimate dis-
posal of sludge solids. In a report entitled "Report
on the Management of By-Product Solids from
Water Pollution Control Plants" by our con-
sultants, Greeley and Hansen, the alternatives
available to Philadelphia were evaluated on the
basis of economics, practicality and environmental
concerns.
Economically, all alternatives to ocean disposal
were shown to be much more costly to the City.
The present cost to the City for ocean disposal
(including the recent 175 percent increase) is ap-
proximately $17.00 per dry ton of raw sludge. The
average cost of the nine disposal alternatives con-
sidered was $50.00 per dry ton, an increase of 194
percent. The incineration alternate is estimated to
cost on the order of $62.00 per dry ton, a 265 per-
cent increase. Ocean disposal is by-far the most
economically attractive disposal method.
The practicality of each alternate was considered
by first estimating the implementation times. The
study estimates from 7.5 to 12.5 years for im-
plementation of a land spreading operation and
from 5.0 to 7.5 years for incineration. It is also
pointed out that both may include operational
problems. For example, large areas of available land
do not exist in the highly populated Northeastern
United States. This means that long distance
transportation methods are needed which would
increase operating difficulties, adverse public
reaction, energy consumption and costs.
To give an idea of the land requirements, a recent
EPA Policy Statement (draft form) has set up
guidelines that fix sludge application rates on
agricultural soils. The rate is based on the cation ex-
change capacity (CEC) of the receiving soil and the
zinc equivalent concentration (A.D.A.S., England),
in the sludge to be applied. Using analyses of Zn,
Cu, and Ni in Philadelphia's sludge and a CEC of 25
meq/100 g-soil (maximum expected in
Northeastern U.S.), it would require approximately
2000 acres of new land every year for Philadelphia
to dispose of its annual sludge production. Thus, by
the year 2000, we would have applied sludge to
some 52,000 acres of land. It can be shown that the
equivalent loading rate on the land area under our
present ocean disposal site is approximately two dry
tons per acre per year. The EPA Policy Statement would
allow loading on the land, with no dilution, of 54 dry
ions per acre per year. Other sections of the Policy leave
some question as to whether we would be allowed
to apply sludge at all to agricultural land but that
discussion must wait for another time.
Also, land spreading may not be done in winter
months due to runoff problems and would require
large storage facilities. Composting is expected to
have problems due to cold weather reducing pile
temperatures although the extent of these weather
effects is not yet known. Incineration processes re-
quire stack gas scrubbing to meet air pollution stan-
dards and will require extensive maintenance. Of
all the alternatives, ocean disposal was shown to
have the least operating problems.
The environmental evaluation showed similar
potential hazards are associated with all disposal
methods. The potential for heavy metal introduc-
tion into the food chain exists for both ocean and
land disposal. Clams and other filter feeders could
ingest the metals and thus be available for
harvesting and ultimately human consumption. In
a similar manner, crops grown on sludge enriched
soil could be ingested by livestock and also be
available for human consumption.
Microorganisms associated with sludge pose
threats whether introduced directly to ocean
waters or indirectly through percolation to ground
waters. Air pollution problems are unique to in-
cineration processes since operation of air scrubber
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OCEAN DISPOSAL 155
equipment has been insufficient to date. Incinera-
tion is also a high energy consumer, as is land
spreading, when compared to barging to sea.
The report states that both sea disposal and land
disposal have the advantage of supplying nutrients
to nutrient deficient areas. The study also points
out however, that ocean disposal has the advantage
of diluting potentially hazardous materials that
land disposal does not, while land disposal does
provide somewhat better control.
Overall, ocear disposal did not appear more
hazardous to the environment than the other
methods considered and if aesthetics and public
reactions are included, it appears as the best choice
again.
Another result of our ocean permit is the in-
vestigation of source control of hazardous
materials such as metals. Philadelphia's Industrial
Waste Unit is now conducting an extensive survey
of area industries to identify controllable sources.
Data generated to date indicates that few industries
discharge significant quantities of problem heavy
metals and even fewer are controllable through
pretreatment. That is, even if these sources could
reduce their metal concentrations, it appears that
the total heavy metal movement in the drainage
area would not be changed appreciably. Indications
are, therefore, that a high "background" level exists
in urban areas that is controllable only through life-
style changes.
SUMMARY
Philadelphia's recent experience with ocean dis-
posal has led into almost every area of sludge
management and its related problems. Implemen-
tation of the Marine Protection Act has pointed out
that regulation should not result in ocer-control to
the point of creating more or equally severe
problems in other areas. Further, the ways in which
these problems might appear has been discussed to
illustrate that management should be the main ob-
jective of environmental regulation and not
prohibition.
The discussion has pointed out that a major
benefit of regulation is the incentive to fill the gaps
in "state-of-the-art" knowledge in all areas of
sludge disposal. This conference and its proceed-
ings can make a very signficiant contribution to this
effort.
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SLUDGE DISPOSAL BY INCINERATION
AT ALCOSAN
GEORGE A. BRINSKO
Allegheny County Sanitary Authority
Pittsburgh, Pennsylvania
In recent years, research and design engineers
seem to have neglected the difficult problems of
sludge handling and disposal in favor of the more
glamorous problems associated with the liquid por-
tion of wastewater treatment. The liquid portion
has not proved as exasperating to sanitary
engineers as has the disposal of the semi-solid
resjdue of wastewater treatment, the sewage
sludge. What to do with the solids from constantly
escalating treatment procedures is rapidly becom-
ing a major problem to those of us responsible for
operating a modern wastewater treatment plant.
Not only is solid disposal a major problem, it is
without a doubt, the most costly of all the
processes. Pittsburgh is no exception.
In the mid-1940's when "Alcosan," the Allegheny
County Sanitary Authority was being designed,
the Authority's engineers, Metcalf & Eddy of
Boston, recommended that Pittsburgh incinerate
its sludge. To talk incineration in the mid-1940's
was not a problem, but to do it today, it is like dis-
cussing Judge Sirica with the Watergate
defendents. Nobody likes them. No one likes in-
cineration, find some other method of disposal and
locate it someplace else,—especially "someplace
else." This was true in Pittsburgh, it is true in
Chicago, and it is true in most other cities in the
United States. Pittsburgh accepted the challenge,
now complicated by increasing volumes of sludge
from both domestic and industrial sources, re-
duced land availability and now reduced public
tolerance of air and water pollution. Pittsburgh
elected to incinerate and in doing so, eliminated its
sludge disposal problems. Before I go into sludge
disposal, a brief history of the Allegheny County
Sanitary Authority (ALCOSAN) is in order.
In 1945, under the Clean Streams Act passed by
the Pennsylvania Legislature, the City of
Pittsburgh, along with neighboring municipalities
and industries, were ordered to cease the discharge
of their untreated wastes into the waters of the
Commonwealth of Pennsylvania. The Allegheny
County Sanitary Authority (better known as
Alcosan) was incorporated for the purpose of
handling this problem and to handle it on a county-
wide or regional basis.
Figure 1: Service Area.
Alcosan serves the City of Pittsburgh, 75
municipalities and approximately 7000 industries,
an area of 225 square miles with a population of 1,-
250,000 (Figure l). The wastewater collection
system includes 69 miles of interceptors which vary
from eight to twelve inches in the outer-most areas
to IOl/2 feet in diameter at the Main Pump Station at
the treatment plant. It generally follows the banks
157
-------
158 MUNICIPAL SLUDGE MANAGEMENT
of the three rivers and two large creeks, and is
almost entirely gravity flow with the exception of
three small pumping stations and three ejector
stations (Figure 2). Approximately 39 miles of the
interceptor system are in deep rock tunnel. The
treatment plant is located in the City of Pittsburgh,
three miles downstream from the Golden Triangle
on the north bank of the Ohio River.
Alcosan's wastewater treatment facilities are
spread over 48 acres and cost $18 million to con-
struct in 1956. The construction cost of the 69 miles
of interceptor sewer lines, including pumping and
ejector stations was $64,500,000. The complete
construction costs for all facilities amounted to
$100 million and was entirely financed by a bond
issue. The Bonds are being redeemed from revenue
received by Alcosan from sewage charges, based on
water consumption. The current rate is 37Vz cents
per 1000 gallons.
From 1959 to October 1973, the facilities were
rated as providing intermediate treatment but in
reality were only slightly better than primary. With
an average flow of 150 mgd, BOD removal aver-
aged 35 percent and suspended solids 55 percent.
The average total solids removal was 135,000 Ibs.
per day with the balance of over 90,000 Ibs. dis-
charged into the Ohio River. The settled solids, at a
concentration of six percent were pumped to tanks
where it was thickened or concentrated to 16 to 20
percent by an anaerobic flotation process (Laboon
process) This process entailed heating the sludge-to
100° F, and then pumping it into large holding tanks
where it was allowed to stand for three to five days.
Escaping carbon dioxide gases carried the sludge to
the surface. The subnatant liquor was withdrawn
and returned to the head end of the plant for treat-
ment. The concentrated solids were pumped to the
incinerator building, flash-dried, and incinerated.
Final disposal of sludge solids was accomplished
in one or more of the four C.E. Raymond Flash-
Drying incinerators (Figure 3). In the flash-drying
process sludge particles are dried in suspension in a
" .1 > ^
\ CHURCHILL J ^-x. '
\_--rs. ' "«s^/
7
-V^' r
) »»«!• II CIA,. /
Figure 2 Location of Sewage Works.
-------
INCINERATION 159
Figure 3: Flow Diagram for Sludge Incineration.
stream of hot gases, removing instantaneously
almost all of the moisture. The concentrated
primary sludge containing 16 to 20 percent solids
were pumped to the mixer of the flash drying
system where it was blended with previously dried
sludge. The blended semi-moist sludge was fed to
the dual cage mills where it contacted the hot gases
from the incinerator. In the cage mills, drying was
accomplished in a matter of seconds. The fine dried
sludge particles were dispersed throughout the hot
gas stream, and from the cage mills, the gas-borne
particles were carried to cyclone separators to
remove the dried sludge from the moisture laden
and considerably cooled gases. A portion of the
dried sludge was automatically returned to the mix-
er for blending with incoming wet concentrated
sludge and the remainder was fed by closed con-
veyors to the incinerators for burning. Malodorous
gases produced during the evaporation process
were drawn into the deodorizing section of the fur-
nace for destruction.
In 1970, Alcosan completed the first step in a
$48,000,000 modernization plan designed to
further upgrade plant performance. At that time,
the coal fired incinerators were converted to clean-
burning natural gas.
From 1960 to 1973, Alcosan concentrated or
thickened an average of 27,584 dry tons of solids a
year at a cost of $8.68 per dry ton. Incinerator costs
averaged from $12.10/dry ton in 1961 to
$49.09/dry ton of solids in 1972. The concentrated
sludge averaged 16 percent solids and had a thermal
value of 6000 BTU/pound dry solids. With an 84
percent water content, auxiliary fuel was required
to maintain a specified combustion temperature. In
1970, when coal and gas were used as auxiliary
fuels, an average of 0.4 Ibs. of coal/lb. of dry solids
along with 1.1 cii. ft. of gas/lb. of dry solids was
necessary to complete the combustion cycle.
In October 1973, the secondary treatment
facilities, utilizing the step-aeration activated
sludge process were completed and put into opera-
tion (Figure 4). BOD removal now averages 91 per-
cent and suspended solids 90 percent. Total solids
removal was increased to 200,000 Ibs./day. At
Alcosan, activated sludge is not wasted from the
final clarifiers but is returned to Pass 1 of the 4-Pass
aeration system. Solids in Pass 1 are maintained
around 10,000 parts or one percent. Activated
sludge is wasted from Pass 1 to the head-end of the
existing primary sedimentation tanks where it is
mixed with the incoming raw sewage and resettled
with the primary solids. This mixture of raw and
waste activated sludge solids at a concentration of
five percent are pumped from the primary sedi-
mentation tanks through disintegrators and into
mixing tanks where slow speed mixers insure a
homogeneous mixture prior to sludge condition-
ing by polymer application. The sludge is then
pumped to the vacuum filters. A battery of ten 575
square feet stainless steel coil-type filters dewater
the combined sludge. The dewatered sludge at 20 to
30 percent solids is then carried by belt conveyors
to the existing incinerators. The incineration
process is the same as was employed for incinera-
tion of concentrated primary sludge. The incinera-
tor ash together with the residue from the Venturi
wet type scrubbers is settled in the existing ash set-
tling tanks. The supernatant from these settling
tanks is returned to the head end of the plant for
treatment. The ash is hauled to a sanitary landfill
properly approved by the regulatory agencies.
Figure 4: Schematic of Wastewater Flow, Alcosan Wastewater
^Treatment Plant, Pittsburgh, Pennsylvania.
The present incinerators actually are
evaporators of water, and were designed to
evaporate water from sludge with an 84 percent
water content using supplemental heat. The
greater solids concentration of the vacuum filter
-------
160 MUNICIPAL SLUDGE MANAGEMENT
cake have markedly increased the capacity of the in-
cinerators. Figure 5 shows the amount of wet
sludge that can be incinerated at sludge solids con-
tent of 16 percent, 20 percent and 25 percent. It
should be noted that a constant amount of water is
being evaporated. Also significant is the marked in-
crease in the pounds of dry solids that can be dis-
posed of through a given incinerator. Thus, be-
cause of higher solids concentration in the vacuum
filter sludge cake, the efficiency of the burning pro-
cess is greatly increased. Gas consumption in 1973
was reduced by 40 percent over 1972. The savings
of 248 million cu. ft. is enough gas to serve about
1340 homes for one year.
POUNDS OF DRY SOLIDS
INCINERATED PER HOUR
POUNDS OF WATER
EVAPORATED PER
HOUR
PERCENT SOLIDS
IN SLUDGE
Figure 5: Incinerator Capacity.
In 1973, with the introduction of the vacuum
filters, 24,184 dry tons of filter cake were
dewatered at a polymer cost of $2.63 per dry ton
and a total dewatering cost of $19.92 per dry ton.
Incinerator costs averaged $40.35 per dry ton with
a total vacuum filter and incineration cost of $60.27
a dry ton. The filter cake averaged 22 percent solids
and had a BTU value of 7800 BTU/lb. dry solids.
Auxiliary gas consumption amounted to 5.7 cu.
ft./lb. dry solids.
Of course, auxiliary fuel requirements will vary
with an increase or decrease in solids content,
sludge volatile content, heat loss from the incin-
erators, presence of incombustible chemicals, and
excess air usage. Fuel costs can also be reduced by
eliminating or reducing temperature cycling. Since
shutdown or start-ups of incinerators accounts for
as much as 180,000 cu. ft. of gas per incident, the
largest and easiest reduction in fuel consumption
can be obtained by eliminating or reducing many of
the start-ups or shut down incidents.
To say the least, sludge disposal by incineration is
the most costly of all methods of disposal. Alcosan
is no exception to this. Thirty-seven percent of our
operating budget is tied in with our incinerator
operation with 50 percent of our overall
maintenance budget going towards the up-keep of
the incinerators. As reflected earlier, disposal costs
are high and the problems are many.
Alcosan has experienced its share of problems
with the incineration of solids. In 1959, shortly
after the start-up of the incinerators, sludge odors
were detected in our stack gases. We were flooded
with complaints. The odoriferous drying vapors
were short circuiting and going out into the stack.
Studies were conducted by our staff and our con-
sultants, Metcalf & Eddy, and Combustion
Engineering Inc. Modifications were made to the
furnaces. Four new ports, 2 feet by 5 feet were cut
into the target wall so that the discharge of four
gases was directed into the combustion chamber,
the hottest part of the furnace. A stainless steel baf-
fle attached to the bottom of the target wall and ex-
tending 32 inches into the combustion chamber in-
creased the contact time of the foul gases with the
high temperatures to three seconds. Studies also
indicated that to destroy odors at Alcosan a com-
bustion chamber temperature of 1300° F was
needed. The increased contact time, the four new
ports directing the foul gases into the hot part of
the furnace, and slightly higher preheater tem-
peratures of 1300° F all combined, eliminated our
odor problem.
The start-up of our new vacuum filter operation
in 1973 brought a return of the problem experi-
enced in 1959: a flood of odor complaints. Odors
were detected coming from the three ventilating
systems from within the vacuum filter building.
One system handles the air from the basement and
the first floor, the second system removes the air
from the vacuum filter floor with the third system
handling the gases from the ten separate vacuum
pumps that are designed to mechanically remove
the water and with it, foul gases from the sludge.
The air from the three systems is ducted through a
one inch thick activated carbon filter and then dis-
charged to the atmosphere. Our investigations
revealed the activated carbon that was supposed to
have a life of one year was completely spent in a
matter of a couple months. An attempt to have the
carbon regenerated here in Pittsburgh failed for, as
yet, no one was set up to refire the carbon. Hoping
to save time and because of the municipal authori-
ty act* a contract was set up for the purchase of an-
"Municipal Authority Law—Any purchase in excess of
$1,500.00 has to be advertised and set up for competitive
bidding.
-------
INCINERATION 161
other set of filters. However, with President
Nixon's wage and price control, Phase IV pro-
hibited the manufacturer of the filters to raise the
cost of the filters. The manufacturer refused to
make the filters. New carbon was purchased and
the filters recharged. The spent carbon was sent
out to a local firm that was finally able to regenerate
activated carbon. This was not the answer for the
life of the carbon was greatly reduced by high
quantities of sulfides in the sludge.
In an effort to alleviate the odor situation, ex-
periments were conducted utilizing a chemical to
oxidize the odor producing sulfides. Chlorine diox-
ide was found to be very effective. An application of
0.23 ml/gallon of sludge controlled the odors to an
acceptable level. However, this was only a tem-
porary solution and an expensive one for the
chlorine dioxide dosage had to be increased 0.8
ml/gallon and resulting costs were $5.90 per dry
ton of solids.
We feel that a permanent solution to our problem
is thermal destruction of the odoriferous gases.
Our consultants were instructed to redesign the
exhaust system and duct the foul gases from the
vacuum pumps into the incinerators for final
destruction. However, such modifications are not
made instantly. Drawings and specifications had to
be drawn up and a contract let. On May 17, 1974
the Contractor started work on the necessary
revisions.
During the period of redesigning our foul air
system, additional experiments were conducted
utilizing hydrogen peroxide to oxidize the odor
producing sulfides in the sludge. Initial tests in-
dicate that 0.18 ml/gallon of sludge reduced the
soluble sulfides to .02 ppm a level supposedly free of
odors. However, the quality and age of the sludge
will affect greatly the quantity of hydrogen perox-
ide necessary to reduce the level of sulfides in the
sludge and thus treatment costs. The cost of
hydrogen peroxide treatment averages $.79 per ton
of dry solids at an application of 0.36 ml/gallon of
sludge. Recently hydrogen peroxide was added at a
rate of 2.6 ml/gallon of sludge, resulting in a c*ost of
$5.61 per ton of dry solids. Without a doubt, it has
proven to be the most effective chemical for com-
bating our odor problems. However, escalating
dewatering and odor control costs could justify the
evaluation of the Zimpro process as a possible sub-
stitute for the present methods employed by
Alcosan.
In addition to the odor problems, incinerating the
mixture of primary and waste activated solids also
has created a problem. The increased heat value of
the sludge has resulted in higher incinerator
temperatures. In the past combustion chamber
temperatures were normally around 1650° F.
However, we now find the temperatures to be
around 1900° F and a considerable amount of slag-
ging occurs. The furnaces must be taken off line to
remove the accumulation of slag, resulting in the
loss of valuable furnace time. To reduce. the
temperature and eliminate the slagging problem a
new duct system has been designed to carry fur-
nace combustion air into the upper part of the com-
bustion chamber.
With all the problems associated with sludge dis-
posal by incineration, why then did Alcosan elect to
go this route in the first place? In 1950, a study of
the various methods of sludge disposal in primary
treatment plants revealed that primary sludge was
low in nitrogen and thus unsuitable for use as com-
mercial fertilizer. In addition, the low auxiliary fuel
cost coupled with reduced availability of land near
Pittsburgh or any other metropolitan area, made it
more advantageous for Alcosan to go to sludge dis-
posal by incineration. Would this be true today? Yes
and No. To incinerate just to reduce the overall
quantity is not the answer. However, in as much as
solid waste (garbage) is also a problem, the answer
may be the incineration of a mixture of sewage
solids and solid waste materials with the generation
of steam as a final product, a product .that today is
in great demand.
REFERENCES
1. Brinsko, George A. "Allegheny County
Sanitary Authority, Past-Present Problems -
Motivation," New England Water Pollution Con-
trol Association, June 10, 1970.
2. Brinsko, George A. "Incinerology in the
Pittsburgh, Pennsylvania Story," Water Pollution
Control Federation, 44th Conference, San Fran-
cisco, California, October 5, 1971.
3. Farrell, J.B., Olexsey, R.A. "Sludge Incinera-
tion and Fuel Conservation," News of Environmen-
tal Research in Cincinnati, May 3, 1974.
-------
PASTEURIZATION OF LIQUID DIGESTED SLUDGE
GERALD STERN
National Environmental Research Center
U.S. Environmental Protection Agency
Cincinnati, Ohio
ABSTRACT
Digestion of sludge removes most, but not all, of
the pathogens and parasites that can be harmful to
man and animals. Although there is no evidence
that land spreading of digested or otherwise
stabilized sludge has caused disease to man or
animals, the concern that the remaining pathogens
may contribute to disease cannot be overlooked.
Pasteurization, an effective method for disinfecting
liquid digested sludge, is discussed. Topics covered
are pasteurization temperature and time
relationship, heat required, fuel sources, transferr-
ing and dispersing the heat with liquid sludge,
equipment, estimated pasteurization costs, and
maximum temperature for applying pasteurized
sludge on lawn grasses.
The conclusions are that pasteurization at 70°C
for 30 to 60 minutes destroys pathogens; sufficient
methane gas, from anaerobic digestion, is available
as the fuel source; direct steam injection and ef-
ficient mixing of the steam and liquid sludge is
recommended; costs approximate ten dollars per
ton of sludge solids for small plants; and cooling the
pasteurized sludge to 60°C is sufficient to permit its
use on lawn grasses.
INTRODUCTION
Four broad groups of pathogens that can be dis-
seminated by sewage sludge are viruses, bacteria,
cysts, and oocysts of protozoa and worm eggs.
Sludge digestion removes most, but not all, of the
pathogens and parasites that can be harmful to man
and animals.
There is no evidence that proper land spreading
of digested or otherwise stabilized sludges, which is
a widespread practice, has caused disease to man or
animals. Nevertheless, the concern of health
authorities that the pathogens may contribute to
human and animal diseases cannot be overlooked.
Disinfection may be needed where people or
animals come into contact with sludge.
Pasteurization is an effective method for dis-
infecting sludge. In West Germany and
Switzerland, for example, pasteurization is re-
quired when the sludge is spread on pastures dur-
ing the summer growth season.
Pasteurization Effectiveness on Sludges
Pasteurization implies heating to a specific
temperature for a time period that will render
harmless, or destroy, undesirable organisms in
sludge. Roediger1 has shown (Table l) that
pasteurization at 70°C (158°F) for 30 minutes
destroys pathogens found in sludge.
Strautch2 followed the destruction of various
types of salmonella found in humans and animals.
He showed that pasteurization at 70°C (158°F) for
30 minutes destroyed pathogenic intestinal bacilli
in digested sludge.
Pasteurization of digested liquid sludge is being
studied at the National Environmental Research
Center, Cincinnati, Ohio. In laboratory tests, direct
steam injection was applied to about 0.014m3 (3.75
gallons) digested sludge in a 0.019m3 (five gallons)
carboy, and in a field test to about 3.4m3 (900
gallons) digested mixed sludge held in a 6.8m3 (l,-
800 gallons) tank truck. Test results are shown in
Table 2. Sludge temperatures of 75°C (167°F) for
one hour destroyed pathogens and reduced in-
dicator organisms to less that 1,000 counts/100 ml
163
-------
164 MUNICIPAL SLUDGE MANAGEMENT
TABLE 1
Temperature and Time for Pathogen
Destruction in Sludges
Microorganisms
Exposure time (minutes) for destruction
at various temperatures (° C)
50 55 60 65 70
Cysts of Entamoeba
histolytica
Eggs of Ascaris
lumbricoides
Brucella abortus
Corynebacterium
diphiheriae
Salmonella typhi
Escherichi co/i
Micrococcus pygogenes
var. aureus
Mycobaclerium tuberculosis
var.
Viruses
5
60
60
45
30
60
4
5
20
20
25
(below detectable limits). At 70°C (158°F) for one
hour, the pathogens were destroyed though
coliform indicator concentrations sometimes
remained above 1,000 counts/100 ml.
These studies show that pathogens and parasites
in liquid digested sludge can be destroyed or inac-
tivated by pasteurizing at 70°C for 0.5 to one hour.
Requirements for Pasteurizing Liquid
Digested Sludge
The major factors in pasteurizing liquid digested
sludge are the heat required, fuel sources, method
for transferring and dispersing the heat
throughout the liquid sludge and type of equipment
employed. Although the solids concentration in li-
quid digested sludge varies, five percent solids will
be assumed.
Heat Required
The assumptions to determine the heat required
for one ton (English) of digested solids at five per-
cent solids concentration in liquid sludge are:
1. The amount of liquid digested sludge is 2,000
lb/0.05 or 40,000 Ib (4,800 gallons).
2. Ambient temperature of the liquid digested
sludge is assumed to be 17°C (63°F).
Pasteurization temperature is 70°C (158°F).
3. Twenty percent additional heat is assumed for
maintaining 70°C for 0.5 to one hour.
4. Specific heat capacity, (Btu/(lb) (°F), is 1.0.
The total heat required is:
(40,000) (1) (158-63) (1.2) = 4.6 x 10* Btu/ton of
sludge solids*
Fuel Sources
Two practical sources of fuel are:
1. Purchase of fuel; that is, oil or natural gas.
Both natural gas and Number 2 fuel oil are
clean burning fuels and do not require pre-
heating. However, these fuels are restricted in
their availability. Number 6 oil is more
available but its use requires preheating and
sophisticated air pollution equipment. Treat-
ment plants using aerobic sludge digestion will
have to purchase fuel for the pasteurization
treatment.
2. Using methane gas produced from anaerobic
digestion as fuel is technically feasible. Raw
sludge contains about 60 percent volatiles.
About 50 percent of the volatile solids are
destroyed by digestion and 70 percent of the
solids remain after digestion. Therefore, the
amount of raw sludge sent to the digester is
1/0.70 or about 1.43 tons. Thus, about 0.43
tons (860 Ib) of volatiles are destroyed by
digestion.
The fuel value of the methane gas for each
pound of volatiles destroyed is about 10,000
J. The heat content available is, therefore,
'To convert Btu on (English) to kilocalories/ton (metric) multip-
ly by 0.278.
-------
PASTEURIZATION 165
TABLE 2
Pasteurization Test Results7
Organisms/ 100 ml
Test
No.(l)
L-l
L-2
L-3
L-4
L-5
L-6
L-7
Using
Temp.
0°C
13.52
S9-643
15
60-69
16
63-66'
15
67-75
30
68-73
18
77-85
14
87-91
Steam
Time
(hours)
1
2
1
1
1
2
1
1
1
Salmonella
sp.
7.3
N.D.4
N.D.
>23
N.D.
9.3
N.D.
23
N.D.
N.D.
29
N.D.
3
N.D.
240
N.D.
Pseudomonas Total
aeruginosa
20
N.D.
N.D.
>9.1
N.D.
21
20
150
N.D.
N.D.
>1100
N.D.
7.3
N.D.
43
N.D.
aerobic counts
2.5 x 10'
2 x 106
1 x 105
3.4 x 108
7 x 105
7 x 107
6.3 x 10'
2.5 x 10s
6.4 x 10'
1.3 x 10'
1.7 x 109
<3 x 10'
1.2 x 10s
6 x 105
1 x 10s
3 x 106
Fecal
coliform
6x 105
9000
B.D.L. *
1.5 x 10'
B.D.L.
7.7 x 10'
6000
2x 10'
B.D.L.
B.D.L.
9.9 x 10'
5000
1.9 x 10'
B.D.L.
1 x 10'
B.D.L.
Fecal % Dilution
streptococci after past.
16 x 104
B.D.L.
B.D.L. 18
30 x 10"
B.D.L. 32
2.3 x 10'
9 x 10" 14
5x I06
B.D.L.
B.D.L. 22
2.7 x 106
5000 12
6.5 x 10"
B.D.L. 52
6.5 x 10"
B.D.L. 59
Gun
P-l
26
38-55
1.5
>240
>240
93
N.D.
7.9 x 107
1.7 x 107
5x ID'
5x 10'
1.7 x 105
4.2 x 10" 10
Through Copper
Tube with
3/16-inch
P-2
12
holes
25
70-83
59 7
1
1.5
>240
N.D.-
N.D.
N.D.
16
N.D.
N.D.
'N.D.
1.8 x 10s
4.4 x 105
4.5 x 105
3.8 x 10s
8.4 x 106
B.D.L.
B.D.L.
B.D.L.
2.1 x 105
B.D.L.
B.D.L.
B.D.L. 58
Notes:
(1) L-numbers — Laboratory Tests
P-numbers — Large-Scale Tests
(2) Original Digested Sludge Temperature (typical)
(3) Pasteurization Temperatures (typical)
(4) N.D. — None Detected («3/100 ml)
(5) Below detectable limits of analysis (< 1000/100 ml)
(6) Presence of Pseudomonas aeruginosa and relatively high fecal streptococci suggests that heat did not penetrate the
sludge.
(7) After cooling with air to 59° C.
860 Ib x 10,000 Btu/lb of volatiles destroyed or
about 8.6 x 106 Btu. Assuming 20 percent loss
of fuel value in producing the heat energy
(e.g., in a steam boiler) leaves about 6.85 x 106
Btu. This quantity of heat energy is more than
sufficient to meet the need for 4.6 x 106
Btu/ton of digested sludge solids at five per-
cent concentration. Methane gas from
anaerobic digestion is successfully being used
for large-scale pasteurization of liquid
digested sludge at Niersverband, Germany4.
Transferring and Mixing the Heal with Liquid Sludge
Direct steam injection is more effective than in-
direct heat exchange. Both organic fouling and in-
organic scaling on heat exchanger surfaces reduce
the heat transfer efficiency4. In one study5, about
30 minutes were required to heat water to 100 psi
in a 200-gallon reactor fitted with a steam jacket
and coils; however, four hours were required to
heat sludge in the same reactor. Because direct
steam injection is used, the steam should not con-
tain chemicals that are incompatible with land
spreading of pasteurized digested sludge. Extra
precaution is required to insure proper deaeration
and softening of the boiler feed water to protect the
steam boiler from excessive scaling and corrosion.
For complete effectiveness, each sludge particle
(and solution) must receive the full pasteurization
treatment. Because the thermal conductivity of
-------
166 MUNICIPAL SLUDGE MANAGEMENT
water" is low**, it is important to avoid thick
pockets of unstirred liquid sludge so that heat can
be transferred in a reasonable time period. Es-
timates were made of the time needed to reach
pasteurization temperatures for unstirred liquid
digested sludge layers (semi-infinite) ranging from
0.625-cm to 5.08-cm (1A to 2 inches) thick using the
unsteady-state heat transfer procedures described
by McAdams6 and the following assumptions:
1. Heat transfer by convection is negligible
because thick sludge particles are a non-New-
tonian liquid with a yield factor that inhibits
' convective heat transfer.
2. Heat can be transferred through the liquid
sludge layer two ways. The first is heat
transfer in one direction only, from the bulk of
the liquid sludge through the liquid sludge
layer to the walls of the pasteurizer. The other
is two-direction heat transfer, that is, heating
the liquid sludge layer simultaneously from
opposite directions perpendicular to the liquid
sludge layer.
The time estimates are shown in Table 3 for two
sets of heat transfer conditions. Note that the
heating time increases by a factor of four when the
thickness of the unstirred sludge layer doubles.
Also, -two-direction heat flow reduces by one
fourth the time needed for heat transfer, and in-
creasing the heat transfer driving force reduced the
time needed for pasteurization of the liquid sludge
layers approximately in proportion to the change in
the ultimate driving force. Nevertheless, even un-
der more favorable conditions, the time estimates
for heat to be transferred through thick unstirred
liquid sludge layers are relatively long, thus
demonstrating why thick pockets of unstirred li-
quid sludge must be avoided. In-line heating by in-
jecting steam into the pipe through which the liquid
sludge is flowing may be more effective than injec-
ting steam directly into a large tank. Also, a multi-
ple lance that ejects steam in a circular motion may
possibly provide enough mixing to prevent thick
unstirred liquid sludge pockets.
Type of Equipment
Two major equipment items needed for
pasteurization of liquid digested sludge are a steam
boiler to produce the steam and a tank to hold the
digested sludge during pasteurization. This equip-
ment need is deliberately oversimplified to show
that only limited information is available on
'Liquid sludge at five percent solids concentration contains 19
parts water to one part solids.
"0.035 calKhr) (cm-) (°C/cm) at 20°C or 0.34 BtuKhr) (ft*)
(°F;ft) at o8°F.
pasteurizing liquid sludges. For instance, this
writer is unaware of any treatment plant pasteuriz-
ing liquid sludges in the U.S. In Germany and
Switzerland, pasteurization is used to disinfect li-
quid sludge spread on pastures during the summer
growth season.
Triebel4 describes sludge pasteurization at the
plant at Niersverband, Germany. Heat recupera-
tion is used to conserve the heat energy. Figure 1
shows the steps in a one-stage heat recuperation
pasteurization process. From the concentrated
digested sludge tank, the sludge is pumped into a
preheating chamber where it is heated from 18°C
(64°F) to 38°C (112°F) by the vapors from the blow-
off tank under 0.1 atm. From the preheater, the
sludge is pumped at 1 atm to the pasteurizer where
direct steam injection heats the sludge to about
70°C (158°F). The sludge is then transferred to a
retention tank where it is held for 30 minutes.
Next, the sludge is transferred to the blow-off tank
where it cools to 45°C (113°F) under 0.1 atm. The
vapors are used to preheat the incoming new
sludge. The sludge is further cooled to 35°C (99°F)
at 0.051 atm in a second blow-off tank. The vapors
from the second retention tank are condensed and
discarded. This one-stage heat recuperation
pasteurization sludge process is considered
economical if the minimum daily sludge flow is 200
to 250 m3/day (53,000 to 66,250 gpd). If the sludge
quantity is 400 to 500 m3/day (106,000 to 132,500
gpd), two-stage heat recuperation is considered
economical. For over 1,000 m3/day (over 265,000
gpd) sludge flow, three-stage heat recuperation
appears economical.
Even with the energy crisis and high fuel costs,
recovery and reuse of the heat from pasteurized li-
quid sludge may not be feasible for smaller treat-
ment plants because of the substantial capital in-
vestment required for heat vapor compressors.
A pilot field test conducted at the Lebanon
Sewage Treatment Plant near Cincinnati, Ohio,
demonstrated that liquid digested sludge can be
pasteurized in a tank truck. This pasteurizing ap-
proach may be useful for small treatment plants.
Approximately 3.4 m3 (900 gallons) of digested
mixed sludge was pumped into a 6.8 m3 (1,800
gallons) tank truck. Steam was directly injected into
the liquid sludge using an 8.5 liter/minute (2.2 gpm)
condensed steam cleaner. The liquid sludge
temperature was increased from 25°C (77°F) to
80°C (176°F) in 1 hour. The wet steam temperature
was approximately 118°C (244°F). The
temperature was maintained between 70°C (157°F)
and 83°C (181°F) for approximately 30 minutes.
The destruction of pathogens is shown in Table 2,
-------
PASTEURIZATION 167
TABLE 3
Hours Needed to Reach Pasteurization
Temperature of Unstirred Liquid
Sludge Slabs with Thickness Range from
0.25 Inches to 2 Inches
Sludge thickness
(In.)
Condition A
Sludge temperature 20° C
Bulk sludge temperature 75° C
Desired sludge
temperature of slab 70° C
Condition B
Sludge temperature 20° C
Bulk sludge temperature 80° C
Desired sludge
temperature of slab 65° C
CASE1: One-direction, unsteady-state heat transfer
0.25 0.14
0.50 0.57
0.75 1.29
1.00 2.3
2.00 9.25
CASE 2: Two-direction, unsteady-state heat transfer
0.25 0.04
0.50 0.14
0.75 0.32
1.00 0.57
2.00 2.3
0.06
0.22
0.5
0.9
3.6
0.02
0.06
0.13
0.22
0.90
Test No. P-2. The 58 percent dilution of the
pasteurized liquid sludge was due to using wet
steam and a poorly insulated truck. The theoretical
dilution is a minimum of eight percent, though in
practice 15 percent is probably more realistic with
dry steam. Closer temperature control could be
achieved by better control devices and experience.
Costs for Pasteurizing Liquid Digested
Sludge
The cost of pasteurization was calculated by
Triebel in 1967* for German conditions (Table 4).
He showed that pasteurization cost per ton of
sludge solids decreases with increased sludge flow.
The heat for pasteurization was derived from the
methane gas produced by anaerobic digestion and
no heating costs were included.
Desk-top estimates were made for today's condi-
tion of the cost for pasteurizing one ton and four
tons digested sludge solids per day. The assump-
tions are 20 percent heat loss in a steam boiler,
operation of the pasteurization equipment two
hours/day for one ton sludge solids and eight
hours/day for four tons sludge solids, sizing the
equipment to pasteurize 1.4 tons sludge solids
(seven days liquid sludge accumulation over a five-
day period), purchase of Number 6 fuel oil at
$2.50/106 Btu or use of methane gas from
anaerobic digestion and one man-hour/pasteur-
ization treatment.
Basis: One ton of digested sludge solids/
pasteurization treatment.
Fuel Costs
4.6 x 1Q6 x $2.50
1 x IQ6 x 0.80
= $14.40
(The fuel cost has increased 3.5 times since a
similar cost estimate was made in 1972)7.
Capital Equipment Cost
Approximately 4.6 x 106 Btu is needed to pas-
teurize one ton of sludge solids at five percent con-
centration over a two-hour period. It is assumed
that 80 percent, or 3.8 x 106 Btu is needed to raise
the liquid temperature from 17°C to 70°C in one
hour. Assuming steam at 1,100 Btu/lb, the size of
the steam boiler is:
3.8 x 1Q6 x 1.4
1,100
= 4,840 Ib of steam/hour
Inquiry made of several small steam boiler sup-
pliers indicates that the capital cost for the boiler
and auxiliary equipment is about $6.75/lb of steam
generated. The capital cost is estimated to be 4,840
(Ib) x $6.75/lb = $32,700 for the steam boiler, plus
$4,500 for pumps, pipes, and the pasteurizer tank
(120 ft3) for a total of $37,200. The average life of
the equipment, when properly maintained, is about
25 years. At six percent interest, the annual cost
factor is $37,200 x 0.0783 = $2,900/year or about
$8/day. This is the capital cost for pasteurizing one
ton of sludge solids/day. When pasteurizing four
times/day, the capital cost is $2/ton of sludge solids.
The capital cost will increase by about 20 percent
if methane gas is used instead of Number 6 fuel oil.
-------
168 MUNICIPAL SLUDGE MANAGEMENT
Preheater, 0.1 atm.
To the vacuum pumps
0.1 atm.
Blow-off
tanks
Concentrated
digested sludge
\18°C
Storage basin
Pumps
Pump
Storage basin Pump
Sludge
Heating steam
Vapors
Vacuum (air)
Water
Figure 1: Diagram of Sludge Pasteurization in the Group-Sewage Plant of the Niersverband Viersen Illustrating One-Stage Heat
Recuperation (After Triebel, Reference 4).
This additional capital cost is needed to clean the
methane gas before use. However, the additional
capital cost is usually less than the cost of fuel oil.
Labor Cost
Labor cost is $6/man-hour, plus ten percent for
cleaning, maintenance, etc., or $6.60 for each 1.4
ton sludge solids pasteurized; orabout$4.75/tonof
sludge solids.
TABLE 4
Dollar Costs for Pasteurizing
Digested Sludge*
Tons of digested sludge Tons per day Cost per ton of
pasteurized annually (365 day-year) sludge solids (1967)
0.78 x I03
1.56 x I01
5 x 10'
10 x 10-'
2.14
4.28
13.7
27.4
S8.60
6.42
1.85
1.30
Electrical Power, Chemicals, and Replacement Part Costs
These costs are minor when compared to other
pasteurization costs. Electrical power needed is
about 14 kw-hr per pasteurization treatment. Total
cost of the electrical power plus chemicals (for cor-
rosion control of the steam boiler) plus repair parts
is estimated at $2/ton of sludge solids.
Summary of Pasteurization Costs
Total costs for pasteurization of one ton of
sludge solids at five percent concentration in liquid
digested sludge are:
1. One liquid sludge pasteurization/day
•After TriebeH; 1967 rale of exchange, 4 German marks = Sl.OO.
Number 6
Oil
Heal energy 14.40
Capital cost 8.00
Labor 4.75
Electrical, chemical repairs 2.00
$29.00
Methane Gas
From Anaerobic Digestion
$17.00
-------
PASTEURIZATION 169
2. Four liquid sludge pasteurization/day
Number 6 Methane Gas
Oil From Anaerobic Digestion
Heat energy 14.40
Capital cost 2.00
Labor 4.75
Electrical, chemical repairs 2.00
$23.00
$11.00
These figures show the cost saving of substituting
methane gas instead of purchasing Number 6 fuel
oil and operating the pasteurization facilities four
times per day. Note that pasteurization costs still
approximate ten dollars per ton of sludge solids for
relatively small treatment plants.
If quick turn-around time is required for loading
and unloading pasteurized liquid digested sludge, it
may be of advantage to install a heated storage
tank,. No-cost estimate was made for a holding
tank. Its cost will probably be a minor fraction of
the total cost of pasteurization.
Maximum Temperature for Spreading
Pasteurized Sludge on Lawn Grasses
Studies conducted at the National Environmen-
tal Research Center in Cincinnati showed that
pasteurized sludge can be applied to growing
grasses if the temperature at the soil surface does
not exceed 60°C (140°F). Because evaporative cool-
ing of sprayed sludge can reduce the temperature
significantly and heat may be lost in transit, direct
cooling may not be necessary in most cases. No
adverse effects are expected if hot sludge is applied
to bare soil before crops are started.
SUMMARY
1. Pasteurizing at 70°C (158°F) for 30 to 60
minutes destroys pathogens in digested liquid
sludge.
2. Sufficient methane gas is produced by
anaerobic digestion for the fuel needed to
pasteurize digested liquid sludge.
3. Direct steam injection is more efficient than
indirect heat transfer for pasteurizing liquid
sludge.
4. Thick, unstirred sludge pockets must be
avoided for effective pasteurization.
5. Small treatment plants can pasteurize liquid
digested sludge at reasonable costs.
6. After pasteurization, liquid sludge needs to be
cooled to only 60°C (140°F) before it is sprayed on
grass.
ACKNOWLEDGEMENTS
The author gratefully acknowledges the
assistance of the following people: Dr. R. Dean, Mr.
G. Dotson, Mr. E. Grossman, Dr. M. Kirkham, Mr.
B. Kenner, Mr. H. Clark, Mrs. G. Valentine, Mrs. J.
Wilson, Miss J. Fley, and Mrs. M. Curry at the
National Environmental Research Center, Cincin-
nati, Ohio; Mr. R. Bryant formerly at the National
Environmental Research Center, Cincinnati, Ohio;
Mr. J. Whittaker, Lebanon Sewage Treatment
Plant, Lebanon, Ohio; Mr. J. Pfeiffer, Miami Steam
Cleaners, Cincinnati, Ohio; and Mr. J. Veale,
Bosmac Septic-Tank Service, Covington, Ken-
tucky.
LITERATURE CITED
1. Roediger, H. "The Technique of Sewage-
Sludge Pasteurization; Actual Results Obtained in
Existing Plants; Economy", International Research
Group on Refuse Disposal (1RGRD) Information Bulletin
Numbers 21-31, August 1964 to December 1967, pp.
325-330.
2. Strautch, D. "Hygenic Considerations in
Sludge Treatment", 2nd International Water Con-
servancy Conference, Jonkoping, Sweden,
September 1972.
3. Burd, R. S. "A Study of Sludge Handling and
Disposal", U.S. Department of the Interior, FWPCA
Publication, WP-20-4, May 1968, p. 248.
4. Triebel, W. "Experiences with Sludge Pas-
teurization at Niersverband; Techniques and
Economy", International Research Group on Refuse Dis-
posal (IRGRD) Information Bulletin Numbers 21-31.
August 1964 to December 1967, pp. 330-390.
5. Weinberg, M. S., Weiss, H. K., Palanker, A. L.,
and Sheffner, A. L. "Sludge Conditioning Using
SOz and Low Pressure for Production of Organic
Feed Concentrate", Contract 14-12-813 for the
Office of Research and Monitoring, U.S. Environ-
mental Protection Agency, Washington, D.C.
20460, p. 55.
6. McAdams, W. H. Heat Transmission, McGraw-
Hill Book Company, Inc., 1942, Chapter 2.
7. Dotson, G. K., Dean, R. B., Stern, G. "The Cost
of Dewatering and Disposing of Sludge on the
Land", Chemical Engineering Progress Symposium Series,
129, AIChE, "Water" 1972, 69, pp. 217-226.
-------
SLUDGE HANDLING AND DISPOSAL
AT BLUE PLAINS
ALAN F. CASSEL AND ROBERT T. MOHR
District of Columbia Department of Environmental Services
Washington, D.C.
ABSTRACT
Blue Plains Wastewater Treatment Plant is a
regional facility serving most of the Metropolitan
Washington Area. The plant is now undergoing ex-
pansion and the addition of advanced wastewater
treatment. In the past, anaerobically digested
sludge was all disposed on the land. At the present
time both raw and digested sludge is undergoing
disposal by a variety of techniques including land
spreading, burial, composting, and thermal
dehydration. For future disposal of sludge, a com-
bination of methods, including land spreading,
composting, thermal dehydration, and incineration
are now under evaluation. The District's priorities
are to work within all regulatory guidelines and
process the sludge into a reusable commodity.
Overview of Blue Plains Plant
The District of Columbia Government, Depart-
ment of Environmental Services operates the Blue
Plains Wastewater Treatment Plant. The plant ser-
vices a 725 square mile area including the entire
District of Columbia and portions of Maryland and
Northern Virginia. The current plant facilities in-
clude primary and secondary treatment, with
sludge processing by anaerobic digestion, elutria-
tion, and vacuum filtration (Figure l). The plant
was designed for 240 MGD, but operated at an
average flow of 294 MGD during fiscal year 1973.
Effluent quality averaged 49 mg/1 BODs during
that period.
A major construction effort at the site is now un-
derway with completion scheduled for January 1,
1978. Flow capacity will be expanded to 309 MGD
and advanced waste treatment will be added at a
total cost of approximately $360,000,000.00. The
effluent requirements will be: BODs = mg/1, total
phosphorus =0.22 mg/1, and total nitrogen = 2.4
mg/1. To meet these requirements, the plant will
construct additional primary and secondary
facilities, biological nitrification and denitrification
reactors for nitrogen removal, and filtration and
disinfection facilities (Figure 2). Ferric or aluminum
salts will be added in a two-stage process for
phosphorus removal. A solids processing unit
which includes air flotation thickeners, vacuum
filters and multiple-hearth incinerators will handle
all sludges produced.
Sludge Disposal - Past
During the period from 1938 to the present, the
plant went through several distinct changes in
operation which resulted in improved treatment of
wastewater concurrent with increased sludge
production. From 1938 through 1960 the facility
was only a primary treatment plant, with sludge
processing by conventional anaerobic digestion,
elutriation, and vacuum filtration. Sludge quan-
tities gradually increased from an average of 19 to
33 dry tons per day. Final disposal was by applica-
tion to farmland, use as a soil conditioner by the
Federal government, or by burial, At that time the
most important aspect of sludge processing was the
production and use of methane gas which was
burned to produce electricity to operate the entire
plant.
Throughout the 1950's and 1960's government
and private demand for the filter cake as a soil con-
ditioner increased steadily. In 1960, secondary
171
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172 MUNICIPAL SLUDGE MANAGEMENT
Aerated
Grit Chamber
Primary
Sedimentation
Secondary
Aeration
Secondary
Clarif I catio n
Sludge
Thickening
Digestion
Elutriation
Vacuum
Filtration
Vacuum
Filtration
Dry! ng
Market
Figure 1: District of Columbia Wastewater Treatment Plant— Present Facilities.
treatment and high-rate digestion came on line and
the quantities of sludge gradually increased. To
make the sludge more acceptable, it was hauled to a
yard on the plant site, mixed with earth (three parts
dirt/one part sludge), limed, windrowed, and al-
lowed to dry. The mixture was shredded and given
to the public. In 1964 odor problems and reported
cases of salmonella caused the Health Department
to require sludge aging for one year prior to dis-
tribution and also to restrict its distribution during
the summer months.
The sludge disposal problem became aggravated
in the late 1960's when polymers for elutriation and
filtration were first used. The polymers succeeded
in increasing the capture of fines and greatly im-
proved the wastewater quality, but sludge quan-
tities doubled. Polymer treated sludge contained
more moisture as discharged from the filter (75 - 80
percent) and also was not easily dewatered in the
sludge yard. The sludge yard gradually became a
large storage area and demand for the product
dropped off considerably. In 1970, the plant started
using contract haulers to dispose of the filtered
sludge, and to clear the stockpile of sludge in the
yard. Most of this sludge was then used in making
top soil for the final cover of various landfill pro-
jects in the area. In 1972, the sludge yard was ex-
cavated for future construction of advanced waste
Row Wastewater
Filtration
4-
Dieinfection
Gravity
Thickeners
Flotat ion
Thick eners
L
to river
Blending
Vacuum
Filtration
Incineration
Ash
fo
"Dispose I
Figure 2: District of Columbia Wastewater Treatment Plant— Future Facilities.
-------
BLUE PLAINS 173
TABLE 1
Sludge Quantities
Year
1939
1950
1961
1970
1973
1974
1978
Wet tons per Day
(% solids)
60 (32.4%)
78 (30%)
106 (30.7%)
310(24%)
418 (20.2%)
expected 550 (20-23%)
expected 2160 (20%)
Disposal Cost
($/wet ton filter cake)
$1.25
$1.28
$2.00
$6.85
$8.25
treatment facilities, allowing at present only a small
area for emergency stockpiling.
Table 1 summarizes some of the historical quan-
tities and costs for sludge disposal.
Sludge Disposal - Present
Currently, the production and disposal of sludge
is tied to an interim agreement (until new incinera-
tion facilities are completed) among the local
political jurisdictions. When the plant operates nor-
mally (primary plus secondary treatment), it
produces an effluent quality of approximately 50
mg/1 BODs (120,000 pounds per day BODs dis-
charged to the river), and also an average of 310 wet
tons per day of digested sludge as filter cake. Under
the interim agreement, the plant has installed
facilities to add metal salts to secondary treatment
to reduce the BOD discharged to the river to less
than 100,000 pounds per day (40 mg/1 BODs). Such
addition of chemicals to the required one-half of
plant flow produces an additional 240 wet tons per
day of undigested (raw) sludge as filter cake. The
two types of sludges, raw and digested, are handled
separately. For digested sludge filter cake the op-
tions for final disposal are either land spreading or
composting. For the raw sludge filter cake the op-
tions are burial by the trenching method, com-
posting or thermal dehydration.
Digested Sludge
LAND SPREADING. For the annual year 1973,
the digested sludge was used for reclaiming
marginal lands by plowing or discing into the soil.
The hauling and final disposal was handled by a
private contractor at a cost of $6.85 per wet ton.
During that year, all available land sites in the
District of Columbia for sludge disposal were
exhausted, and the remainder went into Prince
Georges County, Maryland for reclamation of
marginal lands.
The year 1974 brought a new hauling contract at
a new price ($8.25 per wet ton) and an increased
public awareness as to final disposal sites and
methods. At one point, EPA obtained a Federal
court injunction preventing us from shutting down
the plant because we could not dispose of the
sludge. An agreement was finally reached between
all local jurisdictions involved to each handle their
fair share of the sludge. Sludge is now proportioned
to each of the local counties based on their average
wastewater flows.
Because of this increased local involvement, land
disposal is, of necessity, becoming more of a
science. All parties involved are taking precautions
to protect against the possibility of health hazards,
odor problems, and heavy metal contamination of
the soils to which sludge is applied. Possible sites
are divided into one of three categories; agricultural
land, marginal land, and high-rate disposal lands. In
all cases, health hazards are minimized by the use of
special trucks for hauling, with sealed tailgates,
sometimes tarpaulin covered. The sludge may not
be stockpiled at the site for more than 24 hours and
in general it must be buried under a layer of earth.
Disposal on agricultural land is subject to many
restrictions. Only certain types of crops may be
grown, such as corn and soybeans, and these may
not be for direct human consumption. To protect
against heavy metal uptake by plants, the pH of the
soil must be maintained above 6.5 and a limit of 15
dry tons per acre is tentatively being proposed in
Maryland. The exact application rate on
agricultural land is calculated by the following for-
mula:
Dry tons per acre =
(8.15 x 103) (CEC)
zinc equivalent (mg/kg
dry sludge)
CEC = cation exchange capacity of the soil
Zinc equivalent of sludge =Zinc + 2 (Copper)+8
(Nickel) expressed as mg/kg dry sludge
An additional requirement is that the cadmium
content of the sludge must be 1.0 percent or less
than the zinc concentration. If the cadmium con-
tent exceeds this value, the sludge may not be used
on any agricultural lands. The above formula was
developed by the University of Maryland, Depart-
ment of Agronomy, based on some work per-
formed by the U.S. Department of Agriculture.
Typical metal contents of Blue Plains sludge are
shown in Table 2. The plant performs a composite
sample analysis for metals biweekly. A procedure is
now being established in Maryland whereby a
farmer who wishes to receive sludge on his proper-
ty can have the soil analyzed and a recommendation
-------
174 MUNICIPAL SLUDGE MANAGEMENT
TABLE 2
Heavy Metal Content
of Blue Plains' Sludges
Mi'lal
Zinc
Copper
Nickel
Cadmium
Lead
Raw Sludge
1200 mg/kg(dry)
300
35
9
400
Digested Sludge
2500 mg/kg(dry)
750
45
24
780
for application rate determined by the local health
department.
At this time cadmium is the most restrictive ele-
ment and we are investigating possible sources in
our wastewater collection system. When all the in-
formation is developed as to the cause-effect
relationship of the elements we will intelligently be
able to implement an industrial waste ordinance for
users of the wastewater collection system.
The problem of heavy metal contamination in
sludge must be faced no matter how the sludge is to
be processed. Work is underway at the EPA-DC
Pilot Plant to determine the fate of heavy metals in
various wastewater treatment sequences.
Disposal of sludge on marginal lands is limited to
fifty dry tons per acre, again because of possible
heavy metal contamination. The sludge is generally
plowed or disced into the soil. Because of odor and
possible runoff problems, plowing is the preferred
method. Land in this category includes gravel pits,
beltway interchanges and land scheduled for
recreational use.
High rate disposal of sludge is generally ac-
complished by the trenching method. Trenches ap-
proximately two to four feet deep and two feet wide
are dug, filled with sludge, and covered with earth.
The disadvantage is that the sludge is no longer a
resource, and stabilization and dewatering may
take up to five years. Up to 500 dry tons per acre
may be applied by this method.
COMPOSTING. Sludge composting with wood
chips is another currently operable process now
under examination. Filter cake sludge is hauled
twenty-one miles to a fifteen acre site at Beltsville,
Maryland. The sludge cake is mixed with wood
chips in a 3/1 chip to sludge ratio and piled into
windrows five feet high. The windrows are turned
daily for approximately fourteen days. The mixture
is then screened and the wood chips recycled. The
compost is aged for an additional thirty days in a tall
pile to ensure pathogen destruction before
distribution. To date nearly all the compost
produced has been used by local agencies as a soil
conditioner. The finished product is an excellent
peat moss substitute.
The composting project is a joint effort with
USDA's research laboratory at Beltsville and
Maryland Environmental Services. The project
originally grew out of a need to handle the interim
produced raw sludge at Blue Plains. Raw sludge
composting, although technically a viable process,
produces undesirable odors for a site such as at
Beltsville. Because of these odor problems, the
composting of raw sludge is now limited to special
research testing. The site is now handling routinely
50 wet tons per day of digested sludge without any
problems. We are continuing work on the optimiza-
tion of digested sludge composting and odor abate-
ment with raw sludge composting. At the present
time, D.C. considers that composting digested
sludge is a viable option for its future needs, and we
are in search of a more permanent, and closer site.
Raw Sludge Disposal
As mentioned above, the production of raw (un-
digested) sludge is necessary because of the interim
treatment program. The disposal of this sludge is
quite difficult because it is not biologically stabilized
and therefore produces severe odor problems. In
fact, the interim chemical treatment program has
been sporadic due to numerous problems en-
countered with raw sludge disposal. Trenching has
been tried but it is generally unfavorable except in
emergency situations due to a lack of land disposal
sites. Composting has been tried, as mentioned, but
is restricted because of odors. Research on com-
posting small batches of raw sludge, or blends of
raw and digested sludges, is still underway. The
only immediately available option now open to us is
a thermal dehydration process. A contract
negotiated between Maryland Environmental Ser-
vices (MES), D.C. Department of Environmental
Services, and a private concern will result in the
Companys' processing up to 240 wet tons per day of
raw sludge on a research-demonstration basis.
Their product, dehydrated sludge, will be
marketed as a 6-4-0 organic fertilizer (6 percent
nitrogen, 4 percent phosphoric acid, 0 percent
potash). The contractor has built a single-train dry-
ing plant at the Blue Plains site. The final product
(50-60 dry tons per day) is their property to market.
If the market price of their product increases above
a set value, D.C. and MES will also obtain a net
lowering of the disposal price paid to the contrac-
tor.
The process is as shown in Figure 3. Vacuum
filtered sludge is delivered in trucks and conveyed
-------
BLUE PLAINS 175
Raw Sludge
Air
Hopper
Mixer
1
Dryer
Collectors
I
Storage
Hopper
Odor
Scrubber
Air
to
Exhaust
Compactor
Granulator
Screen
Product
Storage
Bulk
Bagged
Figure 3: Sludge Drying Process.
to the wet sludge hopper. Sludge is pumped to the
mixer, mixed with previously dried product and fed
to the dryer. Air, heated to 1100 °F, is blown into
the dryer, a toroidal shaped unit. Dried sludge is
collected in cyclone collectors and a bag house. The
air is scrubbed with a chemical solution for odor
control before discharge. The dried sludge is air
conveyed to a storage hopper. From this hopper the
product is either backmixed with fresh sludge or
sent on to final processing, which includes
compaction, granulation and screening. The final
product, a pellet sized material is either shipped out
in bulk or bagged and sent directly to market.
The fuel used for drying is now #2 fuel oil. We are
in the process of designing a sludge gas pipeline and
accessory equipment to be used as the primary fuel
source.
Construction of the plant is now completed and
they are now in the start-up phase of the project.
Sludge Disposal - Future
The scheduled expansion at Blue Plains will
greatly increase the quantities of sludge to be
processed. The new solids processing facilities have
been designed for an average daily production of
431 dry tons per day. This includes all wastewater
treatment sludges as well as all water treatment
sludges produced in purifying the city's drinking
water. To handle these quantities, a conservative,
all-weather system has been designed. A new
solids-processing building is nearing completion. It
will house air-flotation thickeners for waste ac-
tivated sludge, sludge blending tanks, thirty
vacuum filters, and eight multiple hearth in-
cinerators. The capital cost for the building, flota-
tion thickeners, and blending units was ap-
proximately $13,200,000.00 The vacuum filter por-
tion was bid at approximately $7,800,000.00. The
incinerators are estimated to cost approximately
$20,000,000.00.
The incinerators incorporate the best state of the
art for controlling stack emissions. Pollution con-
trol equipment consists of a high energy venturi
scrubber (40 inches of water pressure drop), a two-
plate impingement jet scrubber, and a direct flame
afterburner. The major operating expense for the
incinerators will be the fuel oil requirements. At
the average design rate, the usage will be ap-
proximately 15.6 million gallons per year of #2 fuel
oil. Thirty percent of that total is required for the
afterburner. We are currently awaiting EPA's ap-
proval to bid the incinerator portion of the solids
processing facility.
Recognizing the high cost of incineration of raw
sludge and the shortage of fuel, we are currently
evaluating other methods for sludge disposal. The
heavy metal content of our sludges are low enough,
that with a good industrial ordinance, we should
have little problem in meeting any proposed land
disposal standards. Consequently, we have set our
priorities to reuse the sludge on land whenever
-------
176 MUNICIPAL SLUDGE MANAGEMENT
possible. Drawing upon the experience of the
1960's and the present time, we recognize the need
to make the sludge acceptable for public and private
use. But we also recognize the need for a multi-
faceted disposal method, one that is not totally sub-
ject to the weather nor to any other biological or
chemical upsets. The options include:
a) all raw sludge disposal, thermally dehydrated,
marketed as fertilizer.
b) combination-raw sludge, thermally dehy-
drated, marketed as fertilizer; digested sludge,
disposed on land or composted. Methane from
digestion used as fuel for drying.
c) combination-raw sludge, incinerated;
digested sludge, disposed on land or
composted. Methane from digestion used as
fuel for incineration.
d) raw sludge-incinerated; digested sludge, ther-
mally dehydrated, marketed as fertilizer, us-
ing methane as fuel for drying.
e) raw sludge-incinerated.
We have not yet decided on which option to
choose. Full evaluation of the thermal dehydration
process and marketability of the product is
necessary. Another winter's operation on com-
posting should fully describe design and
operational parameters. The distribution of the
compost must be defined as to what quantities the
Washington Metropolitan Area can absorb.
In the fall of 1974, the EPA-DC Pilot Plant will
start work on a unique system for sludge disposal.
The system will incorporate the best aspects of
anaerobic digestion and thermal dehydration and
make the two processes more compatible. Methane
gas produced by high rate digestion is of sufficient
quantity to totally dry the residual sludge after
vacuum filtration. The problem with digestion is
that the process converts a large portion of the in-
soluble organic nitrogen to soluble ammonia, and
the residual sludge contains only 2.5 - 3.0 percent
nitrogen, which limits its marketability. The pilot
scale process will separately treat the digester
supernatant to recover the ammonia as a crystalliz-
ed ammonium salt, which can be used to upgrade
the total nitrogen content of the dried sludge. The
process includes raising the pH of the supernatant
stream, air stripping to remove the ammonia and
then recapturing the ammonia by contact with sul-
furic acid. The resultant ammonium sulfate is recir-
culated to build up the concentration and a
sidestream run to a crystallizer. The solid am-
monium sulfate will be blended with the dried
sludge.
In general, because of the energy situation, we
are looking closer at the digestion process, and the
use of the resultant methane as a resource.
Methane for drying or incineration is especially at-
tractive economically. We are even now running a
project to operate a motor vehicle with purified
sludge gas.
In summary, we are desperately attempting
define a permanent solution to the sludge disposal
problem. With each apparent solution, there
appears a new stumbling block and always more in-
formation is required. We only wish that the
technology of sludge disposal could be as well-
defined as the technology of wastewater treatment
and air pollution control. Some of the efforts at
Blue Plains will hopefully define some solutions.
NOMENCLATURE
BODs five-day biological oxygen demand
MGD million gallons per day
mg/kg milligrams per kilogram on a dry weight
basis
mg/1 milligrams per liter
-------
AGRICULTURAL UTILIZATION OF DIGESTED
SLUDGES FROM THE CITY
OF PENSACOLA
JOE A. EDMISTEN
Baseline, Inc.
Pensacola, Florida
The concept of turning a liability into a profit is
becoming a reality for Pensacola, Florida. Before
1974, the daily load of 80 to 90,000 gallons of
digested liquid sewage sludge from the Pensacola
Main Street Sewage Treatment Plant was not only
a useless noxious material, the city had to truck the
sludge 18 miles and pay to dump it in the county
sanitary landfill. In late 1973, the County Com-
mission of Escambia County Florida and the City
Council of Pensacola through the Intergovern-
mental Program Office agreed to share the costs of
a one year experiment to determine the feasibility
of using liquid digested sludge for agricultural
production on deep sandy coastal plains soils. The
project unites efforts of the University of Florida
and the University of Florida Agricultural Re-
search Center near Jay, Florida, the University of
West Florida, the Florida Department of Pollution
Control, the Florida Department of Health and
Baseline, Inc. (a private consulting firm). The
ultimate goal of the project is to establish safe pro-
cedures for deposition of 80,000 gallons per day of
sludge produced at the Pensacola Main Street
Sewage Treatment Plant.
Now at the end of April, 1974, an average of
30,000 gallons per day of liquid digested sludge is
being applied to crops at the Agricultural Research
Center near Jay, Florida. The composition of the
sludge from the Main Street Plant is presented in
Table 1 in parts per million and in Table 2 as pounds
per acre inch.
Experimental design is based on greenhouse
experiments at the University of West Florida and
the ARC Jay, Florida under the direction of Dr. Joe
A. Edmisten. Edmisten's greenhouse experiments
TABLE 1
Elemental Composition of Pensacola's
Liquid Digested Sludge
Element
mg/liter(ppm)1
mglkg(ppm) (Dm basis)1
Calcium
Magnesium
Potassium
Phosphorus
Aluminum
Iron
Manganese
Copper
Zinc
Sodium
Chromium
Lead
Silicon
Titanium
Nickel
Mercury
Cobalt
Cadmium
Lithium
Boron
Arsenic
Selenium
Molybdenum
Nitrogen
60.0
17.0
30.0
12.0
400.0
166.0
1.8
3.9
120.0
85.0
4.3
0.5
264.0
0.0
3.0
0.024
0.8
0.011
0.014
—
—
-
-
1,787.0
(0.18%)
1,764.7
500.0
882.4
352.9
11,764.7
4,882.4
52.9
114.7
3,529.4
2,500.0
126.5
14.7
7,764.7
0.0
88.2
0.7
23.5
0.3
0.4
—
—
—
-
52,558.8
(0.18%)
(0.05%)
(0.09%)
(0.04%)
(1.18%)
(0.49%)
(0.35%)
(0.25%)
(0.78%)
(5.26%)
'Wet basis—the liquid digested sludge sample contained 96.6 percent
water (3.4 percent solids).
2Dry matter basis.
were designed to establish the rates of sludge
application that could be tolerated by tomatoes,
corn and a variety of horticultural materials. The
liquid sludge was applied to the potted corn and
-------
178 MUNICIPAL SLUDGE MANAGEMENT
TABLE 2
Analysis of Pensacola's Sludge Taken in Early 1973
Sample
Taken
1/29/73
2/5/73
2/21/73
Average
Elements in pounds/ acre
Ca
183
122
204
107
Mg
14
10
16
13
K
6
6
9
7
P
3
3
2
3
X/
98
69
105
91
Fe
35
30
37
34
Mn
0.7
0.6
0.7
0.7
Cu
3
2
3
3
Zn
28
23
29
27
Na
11
11
11
11
Cr
3
2
3
3
A*
3
2
3
3
Si
60
51
65
59
TV
0
0
0
0
Ni
0.2
0.2
0.3
0.2
Previous analyses indicate about 200-300 Ibs. of nitrogen per acre inch of sludge.
A six inch column of sand removes about 95 percent of the BOD.
Corn yields were increased from 96-118 bu/acre using 4.7 acre inches of sludge on a previously well fertilized area.
tomatoes at rates of zero, two, four and eight
inches per week. At the end of the experiment,
composition of the plants was ascertained as to
total nitrogen (Table 3), zinc, copper and man-
ganese (Tables 4 and 5). The application rates of
four and eight inches per week were found to
drown plants and were discontinued. While the
plants could tolerate up to two inches per week, the
optimum rate of application was established at
about one inch per week. Analysis of plant tissues
indicated that accumulations of heavy metals such
as zinc and/or copper could be expected. Edmisten's
and Lutrick's greenhouse work forms the basis for
rates of sludge applications in the field experi-
ments now in progress on the ARC farm.
The field experiments are being conducted under
the direction of Dr. Monroe Lutrick of the ARC,
Jay, Florida. Specific goals of the project include the
following:
1. Three experiments, having four replications
each, with treatments of 0, 3, 6, 9, 12 and 15
acre inches of sludge per year will be estab-
lished. Applications of the liquid digested
sludge will be made prior to planting on an
annual basis. Corn, sorghum and soybeans
will be the three crops used. Soil samples will
be taken to 24-inch depths at 6-inch in-
tervals prior to sludge application. Soil and
plant samples will be taken for analyses when
the plants are in early anthesis. Grain and soil
samples will be taken when the grain is
mature.
2. A similar experiment will be applied on pine
timber land that has been chopped and is in
various stages of regrowth.
3. A large depression will be filled with the liquid
digested sludge to determine if the material
can be stored without leakage for later distri-
bution on agricultural lands.
TABLE 3
Average Nitrogen Content of Tomato and
Corn Leaves in Percent Dry Weight Grown
on Sludge in Greenhouse Experiments
Treatment
1A inch/week
I inch/ week
2 inch/week
2 inch/ week
controls
Tomatoes
% N in leaf
1. 6
1. 9
2.2
died
1. 5
Corn
% N in leaf
1. 7
2.1
-
-
1.6
TABLE 4
Micronutrient Content Averages of
Tomato Leaves Grown on Sludge
in Greenhouse Experiments
Treatment
ppm
1 inch/ week
2 inch/ week
4 inch/ week*
controls
105
120
110
18
31
38
40
8
614
666
717
508
*This heavy application killed plants after two-three weeks.
TABLE 5
Micronutrient Content Averages
of Corn Leaves Grown on Sludge
in Greenhouse Experiments
Treatment V'"PP'"
Cu
ppm
Zn
ppm
1 inch week
1/2 inch week
controls
90
60
20
40
18
12
660
590
420
-------
AGRICULTURAL UTILIZATION 179
4. In order to determine if there are accumula-
tions of the heavy metals or certain microor-
ganisms in tissues (liver, blood, muscle, fat,
etc.) of steers grazing pasture grasses which
have been fertilized with liquid digested
sludge, two groups of steers will be fed in
drylot for approximately six months and
slaughtered. One group of steers will be fed
the dry ration without any dried sludge while
the dry ration of the other group will contain
approximately five percent dried sludge.
Animal tissues will be collected and examined
for pathological lesions and for heavy metal
and microbiological assays.
After many false starts and hesitations due to dif-
ficulty in obtaining key pieces of equipment, the
experimental project is now in full swing. As of
mid-May an average of five tanker loads of sludge
per day were being hauled to the experimental
fields. Each tanker load is about 6,000 gallons
resulting in the 30,000 gallons/day figure. On days
when the soils are too wet to allow the direct ap-
plication of sludge to the soil, the tankers are un-
loaded into the holding lagoon (Figure 1) located in
a depression on the farm.
Figure 1: A Dock Can Be Seen Protruding into the Sludge
Lagoon. Along this Dock Are Located Water Sampling Tubes
Designed to Sample Surface Soil Water from Depths Varying
from One to Twelve Feet Deep.
Normally the 6,000 gallon tanker is unloaded
directly into the 3,000 gallon liquid manure
spreader. The special transfer pump will unload one
half of the 6,000 gallons in about ten minutes
(Figure 2). The spreader, pulled by a large Ford
tractor leased for the one year experiment, is very
effective and accurate in the application of the
sludge to bare ground as well as plots already hav-
ing plants (Figure 3). Average time to get to the
field plots, apply the fertilizer and return for the
second load to empty the tanker is 14 minutes.
Figure 2: Much of the Delay in Getting the Sludge Project Going
Centered Around These Machines. The Liquid Manure
Spreader Cost $5,200 and had to be Escorted to Pensacola. The
Transfer Pump with Accessories Cost $1,135. The Pump Func-
tions at a Rate of 300 Gallons Per Minute and will thus Load the
3000 Gallon Spreader in Ten Minutes. The Large Ford Tractor
Needed to Pull the Spreader Is Valued at $10,000 and Is Leased
with IPO Funds for $1,500 Per Year.
The field plots on which the sludge is being
applied at various rates are 120 by 40 feet in size.
The 40 foot width is ideal for the designed "throw"
of the spreader so it moves at a moderate pace be-
tween the plots (Figure 4). From the field plot
diagram (Figure 5) distribution, crop and treat-
ment can be seen. Three crops will be grown in the
experiment, corn, sorghum and soybeans. Each
crop will have 24 plots in which four will receive no
sludge, four receive three inches, four receive six
inches and so on through 9,12 and 15 inches for the
growing season.
Various tests for groundwater contamination,
accumulation of toxic heavy metals, bacterial con-
tamination of soil, water and animals are being
regularly completed. In Figure 6 one can see the top
of a 70 foot deep well near the test holding lagoon
from which water will be taken at regular intervals
for such tests. In the right background of Figure 1,
one can see a pier built out into the sludge holding
lagoon. The pier gives access to water sampling
devices from which water can be taken from the soil
directly below and in the holding lagoon at depths
of one to twelve feet.
Similar deep wells and shallow soil water samples
are located in and around the 72 field test plots
presented in Figure 5.
Other precautionary tests included in the ex-
periments are regular testing of surface and sub-
surface soils for fecal coliform bacteria. In Figure 7,
Dr. Lutrick is shown collecting these soils for
microbiological tests which are being performed by
-------
180 MUNICIPAL SLUDGE MANAGEMENT
Figure 3: A Large Part of the $80,000 Funding of the Project
went to Buy a New Tanker to Transport the Liquid Sludge to the
Experimental Farm. Here One 6000 Gallon Tanker Is Unload-
ing While Another Waits the 28 Minutes Needed to Spread the
Two 3000 Gallon Loads with the Spreader. It Takes the
Spreader Five Minutes to the Field, Four Minutes to Spread the
3000 Gallons and Another Five Minutes to Return to the
Tanker.
Figure 4: The Liquid Manure Spreader Is Shown in Action. It Is
Designed to Spread at 750 Gallons Per Minute and Throws the
Material Evenly Over a 40 Foot Area to Its Left. Here a Low Area
Is Being Covered Several Times Before Planting. In Front of It
Are Patches of Corn That Were Planted in Areas Previously
Treated Which Will Continue to Receive Sludge Up to a Pre-
scribed Level. The Treatments Vary from Up Through 3, 6, 9
and 12 Inches of Sludge for the Growing Season.
the regional laboratory of the Florida Department
of Health to determine the fate of human fecal
bacteria in the sludge, soils, plants and
groundwater.
Further precautions that bacterial hazards will
not be produced by this agricultural utilization of
sludge are seen in the sub-experiment in which
dried sludge is fed to test animals. Six steers (Figure
8) were bought for this experiment. Three of the
steers are being fed 100 grams of dried sludge in
their daily ration. The other three serve as con-
trols and receive no sludge. The six steers will be
slaughtered after six months and their tissues will
be carefully tested for bacterial and/or heavy metal
contamination. During the feeding experiment the
six test animals will be monitored as to growth,
behavior and general response to the ingestion of
the 100 grams of dried sludge which approximates
the daily consumption of one gallon of the fresh
liquid sludge.
Greenhouse experiments continue and are de-
signed to further test the effect of sludge on the
growth of corn and to ascertain if the water leached
from the experiment will contaminate the soil
water. In Figure 9, one set of plants is shown from
an entire set in which soils of the Troup and Red
Bay series have been placed in large cylinders.
Water from sludge applied to the top of the cylinder
is collected at the bottom for chemical analysis of
(NO3) nitrate, (CD chlorides and (K) potassium.
The differential growth rates associated with rates
of sludge application will be noted in Figure 9 where
the tube getting no sludge is on the right and the
tubes getting 3, 6, 9, and 12 inches are progres-
sively taller to the left.
40
"o
6J
3
15
3
15
3
15
6
9
6
9
6
9
0
12
0
12
0
12
15
3
15
3
15
3
9
0
9
0
9
0
12
6
12
6
12
6
0
12
0
12
0
12
3
15
3
15
3
15
6
6
6
6
6
6
9
9
9
9
9
9
12
3
12
3
12
3
15
0
15
0
15
0
7
a.
o
.
!
z
in
inches of sludge applied: 0.3,6,9,12 and IS.
Figure 5: The Field Plot Diagram for Number, Distribution,
Crop and Treatment with Pensacola's Sewerage Sludge.
-------
AGRICULTURAL UTILIZATION 181
Figure 6: Several Deep Wells Have Been Drilled to Depths of up
to 70 Feet at Key Sites In and Around the Experimental Areas.
Here a Well is Shown Near a Sludge Holding Lagoon. Water will
Be Drawn from this and Other Wells at Regular Intervals and Be
Tested for Bacteria, Nutrients and Toxic Heavy Metals.
Figure 7: Dr. Lutrick Collecting Soils for Microbiological Tests.
The sludge research project has fired the energy
and imagination of the entire staff and faculty of
the experiment station. Almost all of the senior
faculty members of the experiment station are in-
volved in the experiment in some way. The station
nematologist hopes that the accumulation of
organic matter will allow a significant (Vz) cut in the
amounts of nematocides needed to control cyst
nematodes, a serious pest on soybeans. There is a
series of small test plots in which a wide variety of
grasses and other crops are being grown with vary-
ing rates of sludge applications. No results are
available from these test plots at this time but the
crop responses to sludge appear to be in keeping
with earlier greenhouse experiments in which yield
increases are proportional to sludge applications up
to the point of flooding.
Figure 8: Six Young Steers Were Purchased at a Price of $886.49
as a Part of the Experiment. Dr. Bertrand Is Currently Feeding
Three of the Six a Sludge Supplement of 100 Grams Each Day to
Test the Theory that Animals Might Be Harmed by the Direct
Consumption of the Dried Sludge from Grass to which It Has
Been Applied. So Far in Two Months of Feeding the Three Test
Steers Are Doing as Well or Better Than the Three Control
Steers. The Food with 100 Grams of Added Sludge Is Con-
sumed Readily by the Test Animals.
Figure 9: The Greenhouse Experiments Started by Dr.Edmisten
at the University of West Florida, Pensacola, Florida are Con-
tinued at the Jay, Florida Station. Here Dr. Lutrick Grows Corn
in Cylinders of Two Soil Types, Red Bay and Troup. Sludge Is
Being Applied at Rates of 0, 3, 6, 9 and 12 Inches Total from the
Right to the Left. Liquids from These Applications Are Col-
lected at the Bottoms of the Cylinders and Are Tested for NO3,
K and Chlorides.
BIBLIOGRAPHY
1. Cropper, J. B. "Greenhouse Studies on Nutri-
ent Uptake and Growth of Corn on Sludge Treated
Soil," M. S. Thesis, University of Illinois, Urbana,
79 pp, 1969.
2. Hinesly, T. D., O. C. Braids and J. E. Molina.
"Agricultural Benefits and Environmental Changes
Resulting from the Use of Digested Sewage Sludge
on Field Crops," An interim report on a solid waste
-------
182 MUNICIPAL SLUDGE MANAGEMENT
demonstration project. U.S. Printing Office SW- 16080 GWF, 1972.
2Faie!?:tH• j-1weth^r R CA -~--^-• "a=i*&
Duffer. Soil Systems for Municipal Effluents/' A Water, and Plants." USD A Agriculture Handbook
workshop and selected references E.P.A. project No. 473, 1971.
-------
INSTITUTIONAL PROBLEMS ASSOCIATED WITH
SLUDGE DISPOSAL
KERRY J. BROUGH
Washington Suburban Sanitary Commission
Hyattsville, Maryland
ABSTRACT
The institutional problems of sludge disposal are
a result of the opinions and attitudes of the public
which are translated into political policy and regula-
tion. Incineration was chosen as the method of
sludge handling at the Piscataway (Md.)
Wastevyater Treatment Plant in a 1970 engineering
study for the Washington Suburban Sanitary Com-
mission (WSSC). However, citizen allegations that
incinerator test operations had caused high blood
lead levels in area children resulted in a County
Council resolution to temporarily halt incineration
at Piscataway. Despite County Health Officer and
EPA findings that there wal no evidence of a health
hazard at Piscataway, a second resolution was pass-
ed that terminated incineration until EPA stan-
dards were established and there was a conclusive
determination that no health hazard would exist in
the Piscataway area.
This decision will seriously affect the WSSC
sludge disposal program and could cost millions of
dollars. It also implies that institutional problems
are expected to increase across the nation with
growing citizen awareness and activity.
INTRODUCTION
Sludge treatment and disposal is rapidly becom-
ing the most sophisticated and costly portion of
sewage treatment in many municipal plants, often
accounting for more than half of the capital and
operating costs of treatment. The increasing quan-
tities ajiid types of municipal sludge, along with the
numerous combinations of treatment and disposal
alternatives, present major engineering and
management problems both in design and opera-
tion of treatment plants. Although the engineer's
application of available technology can lead to
decisions on these problems, much more attention
must be given to improving existing techniques and
the development of new technology. However,
while this improved technology will lead to more
effective decisions on treatment and disposal, the
institutional problems associated with sludge dis-
posal are increasing in scope and magnitude to a
position of overriding importance to the technical
selection of a most "cost-effective" alternative in
many areas.
An institution, as used here, is defined as a
significant and persistent concept in the life of a
society that centers on a fundamental human need,
activity, or value, and is usually maintained and
stabilized through social regulatory agencies. In-
stitutional problems can be considered to originate
from the social-political-aesthetic values, often
referred to as the intangible aspects of engineering
analyses, which are translated into public utility
policy and regulation.
The role of decision making on many sludge
related problems today often rests on the
cumulative expressed attitudes of the public being
affected by those problems and decisions. This
public, increasingly, is selecting sludge disposal
methods and approaches which do not represent
monetary or technical considerations. Rather, they
are making decisions based on their view of the
relationship these disposal methods have with the
environment in which they live. It is these subjec-
tive public opinions and values that form the basis
of the institutional problems of sludge disposal
183
-------
184 MUNICIPAL SLUDGE MANAGEMENT
which seem to be a major source of frustration and
difficulty to many officials involved with municipal
sludge management.
The Piscataway Problem
Institutional problems have particularly plagued
the Washington Suburban Sanitary Commission in
recent years. Established by the Maryland State
Legislature in 1918, the WSSC provides water and
sewer service to Montgomery and Prince George's
counties, which make up the Maryland portion of
the suburban area surrounding Washington, D.C.
Extremely rapid population rise of the suburban
Washington area in the last 15 years has resulted in
numerous growth problems, particularly for public
service agencies. This area is also highly politicized
as a result of its proximity to the Federal Govern-
ment enclave. It is marked by many moderate and
upper-income, well-established neighborhoods
maintaining strong, active local citizen
associations. These citizen groups exert con-
siderable political pressure in the several local
jurisdictions the WSSC serves.
There is an abundance of public and political
agencies dealing with planning and environmental
problems on both a local and regional level. These
numerous agencies and local governments have
created a proliferation of opinions on what is the
best method of handling sewage treatment and the
sludge generated from that treatment. Therefore,
attempts at regional cooperation have been marked
by bitter political battles and court suits. At the
same time, many of these agencies have a direct in-
fluence or control over the WSSC.
Until 1967, almost all of the sewage generated
within the Sanitary District was treated at the
regional Blue Plains Sewage Treatment Plant in
Washington, D.C. At that time, the WSSC began a
program of construction which has resulted in
three major treatment facilities. All three facilities
are undergoing expansion and upgrading to ad-
vanced waste treatment and will have a combined
capacity of almost 70 mgd by 1976.
The plant at Piscataway, ten miles south of
Washington, D.C., was built in 1967 with a capacity
of 5 mgd. It serves the southern Prince George's
County area which is currently undergoing rapid
growth, following the pattern of the rest of the
D.C. area. The Piscataway area is characterized as a
rather exclusive neighborhood by its residents who
would prefer to restrict growth to maintain the
area's present quality. The plant effluent is dis-
charged to Piscataway Bay on the Potomac River
while the sludge undergoes anaerobic digestion,
vacuum filtration, and then landfill or land
spreading. Since 1967, the plant has been expanded
in stages to its present capacity of 30 mgd, with
fluidized-bed incineration facilities the phase just
being completed. The final phase of the expansion
will be conversion of the anaerobic digesters to
holding tanks to support the operation of the in-
cinerators. Currently under design are facilities to
upgrade the 30 mgd secondary plant to advanced
waste treatment for removal of phosphorus,
nitrogen, and residual solids.
The decision to utilize incineration for sludge dis-
posal at Piscataway was based on a 1970 engineer-
ing study on sludge management for all WSSC
sewage treatment facilities2. This study was under-
taken by the WSSC to determine the most feasible
and economical method of sludge treatment and
disposal for the three existing, and one proposed,
plants through the year 2000. At that time, the
general uncertainty surrounding the use and accep-
tance of county controlled landfills for sludge dis-
posal led to the conclusion that the "WSSC should
adopt a sludge management concept that would
provide maximum flexibility and control of the
situation, even under adverse conditions. Such a
concept is sludge combustion." The concluding
recommendation of the study was that sludge in-
cineration should be implemented as the most
"cost-effective" solution at all the plants. With the
decision to build incinerators came the correspon-
ding action to utilize the existing anaerobic
digesters as sludge holding and blending tanks.
Although there was substantial redundancy with a
standby incinerator, we were thus left with no
alternative method of treating raw sludge for land
disposal.
However, in 1970, all indications were that in-
cineration would be the best method of future dis-
posal, particularly for densely populated
metropolitan areas. This view was reinforced by
Federal, State and local health and environmental
agencies. In fact, in early 1974, state and local
health departments reinforced the opinion that in-
cineration was the preferable method-of sludge dis-
posal for the suburban D.C. area. There was no
foreseeable need for any sludge treatment process
other than incineration.
Meanwhile, there had been slowly growing op-
position to expansion, and the associated incinera-
tion, of the Piscataway Plant by area citizen groups.
The WSSC recognized this citizen opposition as
well as the unanswered questions about sewage
sludge incineration, and in the summer of 1973 in-
itiated a contract with Battelle-Columbus
Laboratories to perform an impact analysis on the
operation of the fluidized-bed incinerators at
-------
INSTITUTIONAL PROBLEMS 185
Piscataway3. The study would involve analyzing all
incinerator process streams for all possible
detrimental components—particulates, vapors,
aerosols and trace metals. This data would then be
evaluated in light of existing standards as well as
pertinent supportive or refutive data where ex-
isting standards were questionable, or areas not
identified by existing or proposed standards.
This impact analysis was further expanded to in-
clude ambient and environmental analyses, to be
performed by the University of Maryland. This
data on present and historical conditions in the
Piscataway area would provide a better basis from
which to evaluate the impact of the. incineration
operation. However, by early 1974, citizen opposi-
tion was gaining in strength and momentum and
becoming more vocal. The fluidized bed in-
cinerators at Piscataway were nearing completion
and had operated about 70 hours for curing and
startup testing when, in late March, two area
families claimed that the incinerator operation at
Piscataway had caused high levels of lead in the
blood of their children. The citizens carried their
case to the Prince George's County Council who
immediately passed a resolution which prohibited
incineration at Piscataway until the cause of the
high blood lead levels was determined by the Coun-
ty Health Officer. The County Council resolution
was further reinforced by a telegrarh from Region
III, EPA Headquarters halting incineration until it
could be determined no health hazard existed.
As a result, the County Health Officer per-
formed nearly 200 blood tests on area children
within ten miles of the Piscataway Plant as well as
testing all 80 plant personnel. Of all the tests per-
formed for lead, all levels were normal except for
one plant operator who apparently had an abnor-
mality in his blood from an as yet unknown cause.
The children who originally had high lead levels
were retested twice and found to have normal
blood lead levels both times. In addition, en-
vironmental tests by the Prince George's County
Health Department showed no elevated lead levels
in the air or soil near the plant. The Health Officer
concluded that the incinerators posed no significant
health hazard and there was potentially more
danger of increased lead levels in the Piscataway
environment from auto exhaust emissions.
During the brief period of incinerator testing at
Piscataway, engineers from the EPA Standards
Development Branch in North Carolina were able
to obtain a stack gas sample. This work was per-
formed as part of their larger effort to obtain data
for the establishment of sewage sludge incinerator
stack emission standards for the nation. These
were the only tests that were allowed to be per-
formed on the incinerators before the operation
was halted. Although only preliminary data is
available from these tests, the maximum lead quan-
tity that would be emitted from the incinerator
stacks at a sludge loading rate of about 5,000
pounds per hour was found to be four grams per
day.
The Piscataway area citizens had introduced to
the County Council a new resolution which would
halt incineration at Piscataway until establishment
of EPA standards on sludge incinerator stack
emissions and a conclusive determination that the
sludge burning would pose no health hazard to the
particular topography of the area. On April 28, this
resolution was brought before the Prince George's
County Council. The findings of the County
Health Officer, and the EPA Standards Develop-
ment Branch data were presented along with the
WSSC request to allow the incinerators to be
operated to conduct the Battelle/University of
Maryland stack emission studies to determine if a
health hazard did exist. The EPA had rescinded
their earlier restriction and recommended in-
cinerator operation be allowed in order to conduct
the proposed tests.
The Piscataway area citizens argued against any
operation of the incinerators in that there was a
possible health hazard which could not be absolute-
ly refuted. They did not want to be used as "guinea
pigs" for incinerator tests considering the many
questions about the alleged dangers of stack
emissions that could not be answered.
The resolution to halt incineration passed the
Prince George's County Council unanimously. Im-
plicit with this resolution was that incineration of
sewage sludge in Prince George's County was an
unacceptable alternative until it was conclusively
proven no public health hazard would exist. Thus,
the cumulative opinion of private citizens on the
real or imagined dangers of incineration had been
successfully translated into public law. This in-
stitutional decision had no apparent relation to
technical or economic feasibility, but rather was
based almost exclusively on the attitudes of those
citizens in the area affected by the sludge treatment
and disposal.
CONCLUSION
Unfortunately, much of the public maintains
preconceived ideas about sewage sludge and the
associated disposal techniques. These ideas maybe
based on hearsay, false or inaccurate reports, and
experience with poorly designed or operated dis-
posal facilities. However, for whatever underlying
reasons, it is public opinion which often influences
-------
186 MUNICIPAL SLUDGE MANAGEMENT
and forms policies which affect sludge manage-
ment. The institutional problems of sludge disposal
are thus a result of the subjective values of the
different sectors of the public being affected by the
disposal.
Because different public and private groups and
agencies have different, constantly changing
preferences and values on environmental quality,
they exert differing pressures on municipal agen-
cies. A sludge management decision acceptable to-
day may not be socially or politically acceptable two
or three years from now. Also, as public awareness
and concern for the quality of the environment in
which they wish to live grows, this awareness will
increasingly reflect itself in public attitude and
political decisions under which the public utility
must operate. Therefore, the institutional
problems associated with sludge disposal should be
expected to increase across the nation.
The Piscataway situation is an institutional
problem which will obviously have a serious impact
on the WSSC sludge disposal program. If the in-
cinerators cannot be operated, the Battelle/Univer-
sity of Maryland studies on the incinerator impact
cannot be performed. Without these tests, it would
be highly doubtful if the potential health hazard of
incineration could be conclusively proven or dis-
proven at Piscataway. It also affects the two other
major WSSC plants where incinerators are under
construction. The costs of this institutional deci-
sion to restrict sewage sludge incineration in Prince
George's County could be millions of dollars for
facilities already constructed and replacement
facilities.
If institutional problems are expected to increase
and introduce potentially high economic risk for
sludge management decisions, what can be done to
reduce the impact of such problems? In the past,
there has often been a lack of understanding and a
resultant lack of consideration by officials in public
utilities and regulatory agencies of the social-
political values associated with sludge disposal.
Analysis of sludge problems has tended to concen-
trate on those areas that are easily quantifiable;
that is, capable of being expressed in readily
measured units such as dollars or tons of solids.
Therefore, the most important consideration in
reducing institutional problems will be recognition
of public attitudes on sludge disposal. This requires
maximum exposure of the public to sludge manage-
ment decisions well in advance of their implemen-
tation. Full public participation in the preliminary
planning stages of sludge disposal programs should
be invited. The values and attitudes of citizens
affected by the sludge disposal should then be
weighed accordingly in any sludge management
decision. Although such techniques may slow the
decision process or seem to lead to an alternative
that does not appear to be the most cost-effective,
consideration of public values in the planning
stages will minimize the impact of institutional
problems after disposal methods are being
developed or in operation. At the same time, ex-
posure to the public provides the municipal utility
the opportunity to educate the citizens on the true
merits of various sludge disposal alternatives.
Because public attitudes are constantly changing
and reflected in changing political regulations,
municipal officials should recognize the economic
risks that can be caused by institutional problems in
selecting a single sludge treatment and disposal
method. Careful consideration must be given to all
alternatives in the initial planning stages and then
narrowed to the several most feasible methods.
The decision on the best alternative or combination
of alternatives should be made with the commit-
ment of appropriate political regulatory agencies
and with full awareness of the affected public. The
other feasible methods should be continuously
evaluated in comparison with the original choice to
insure the best disposal method is being utilized, as
well as to provide a contingent sludge disposal
method if needed.
Thus, while institutional problems of municipal
sludge disposal are expected to increase, increased
attention to public attitude and proper analysis of
disposal alternatives by utility officials will result in
a greatly reduced impact from these problems.
REFERENCES
1. DiNovo, S.T. and Maase, D.L. "Topical
Report on Sludge and Flyash Disposal/Utilization
Options to Washington Suburban Sanitary Com-
mission," Battelle-Columbus Laboratories, May
1974.
2. "A Sludge Management Study prepared for
the Washington Suburban Sanitary Commission,"
Engineering-Science, Inc., Washington, D.C., June
1970.
3. "Development of an Impact Analysis for the
WSSC Piscataway Fluidized Bed Incinerator and In-
vestigation of Uses for Piscataway Sewage Sludge
and Incinerator Ash," Battelle-Columbus
Laboratories, June 1973.
4. Mar, Brian W. "Sludge Disposal
Alternatives/Socio-Economic Considerations,"
Water Pollution Control Federation Journal, 41, 4, 553
(April 69).
-------
ENERGY CONSERVATION AND RECYCLING
PROGRAM OF THE METROPOLITAN SEWER
BOARD OF THE TWIN CITIES AREA
DALE C. BERGSTEDT
Director of Solids Processing
St. Paul, Minnesota
PURPOSE
^ The purpose of this report is to describe recent
actions of the Metropolitan Sewer Board of the
Twin Cities Area in meeting the energy crisis that
engulfed the nation in 1973. As regional
wastewater treatment authority for the seven-
county area focusing on Saint Paul and
Minneapolis, Minnesota, the Board operates all in-
terceptors and the 24 treatment plants in the area.
ABSTRACT
The Board has instituted ten major programs
oriented to process and operational changes in their
major treatment plants relating primarily to energy
saving and recycle, without diminishing the quality
of treatment of wastewater. In addition, many
minor improvements have been made in operations
to conserve materials and energy sources. Among
the major approaches are:
• Phyrolysis of municipal refuse to furnish heat
value for sludge drying.
• Recovery and beneficial use of heat in exhaust
gases from incinerators.
• Continuous pressure dewatering.
• Flameless odor control.
• Improved results by modifying existing sludge
processing equipment.
• Manufacture of fuel-grade sludge cake and
char.
• Preparation of fertilizer, soil conditioner and
recyclable metal scrap.
• Use of solid fossil fuel . . . coal ... to aug-
ment combustion in incinerator furnaces.
• Manufacture of activated carbon for in-plant
use.
• Pulping of refuse fibers for use as filter aid.
Population Involved
In serving a population of almost two million
people, the Metropolitan Sewer Board is serving
one of the more environmentally aware and
ecologically motivated populations in the country.
Past events such as concern for preservation of
wilderness areas in the state, avoiding potential
hazards from industrial and public utility
operations, and active citizen involvement in the
political process have made the Twin Cities Area a
significant focal point for environmental concerns.
Thus, in analyzing present operations and propos-
ing expansions that are more conservative energy-
wise and material-wise, the Metropolitan Sewer
Board is working in behalf of interested citizens
who comprise half the population of the State of
Minnesota.
Urgency
Although finding of alternatives to usual fuels is
critical at this time because of demonstrated short-
ages, the Chairman of the Metropolitan Sewer
Board had urged consideration of alternatives as
long ago as 1970, at which time the likelihood of
shrinking supply of fossil fuels became apparent.
This concern was certainly well justified. In July
of 1973 the Metropolitan Sewer Board opened bids
on fuel oil supply for the coming operating year. Of
seven responding suppliers, none offered a contrac-
tual bid on the Board's fuel requirements of 3.4
million gallons.
This proved to be the first indicator of the
seriousness of the problem. At that time the Board
187
-------
188 MUNICIPAL SLUDGE MANAGEMENT
expedited the finalizing of plans and specifications
for supplemental oil storage capacity that would
allow handling of heavier oils than the present
system can accommodate and also commenced a
series of discussions within the staff on what could
be done in all ways to reduce fuel usage. Economy
of operational equipment and emergency steps that
might have to be taken in the event the fuel supply
was cut off were considered.
As was well documented a few months later,
changes in the world supply of oil products
precipitated a drastic price rise (Figure 1) and also a
shortage which caused the Board to have serious
concern about maintaining full treatment of the
wastewater during the winter months. Costs rose
from 92 cents per million Btu to a peak of $2.30 per
million Btu, settling back to a present figure of
$2.15 per million Btu. Fortunately, building heating
requirements proved moderate in this past winter
and as a result we did not have to interrupt the high
level of treatment because of supply.
NATURAL GAS SUPPLY CUTOFFF
OIL COST TO MSB
15
10
J A S O N D J
1973
M A
1974
Figure 1: Oil Cost to MSB.
However, we were also informed within the past
few months that the natural gas supply will be cur-
tailed stepwise, so that by 1978 (Figure 2) we must
prepare to operate with no .natural gas at all.
Presently, on interruptible service, we are aware
that there are times when the gas supply is tight,
but the duration of the period during which in-
terruptible rules apply has been increased. Gas is no
longer going to be available as the preferred low-
priced, high-grade fuel, for the relatively mundane
task of drying water out of sludge in order to make
it combustible.
A second point in the urgency pattern is the
pressure by the regulatory agencies, both federal
and state, that we comply with the public will for
higher grade wastewater treatment. This calls for
adoption of technology for wastewater treatment
that has the end result of producing larger amounts
of sludge, and more troublesome, sludges that are
801
40 >
201
PERCENT
OF
PRESENT
VOLUME
USAGE
1974
77
Figure 2: Natural Gas Supply Cutoff.
more difficult to dewater. We see more of the same
coming in the future, because of the Federal re-
quirements calling for "best practicable technol-
ogy" to be in effect by mid-1983. We estimate that
adopting presently known technology would raise
our energy consumption by fourfold at least.
Another factor of concern and urgency is that
alternatives to using more of present fuels have
long lead times, including engineering, bidding,
construction, and start-up. Typically, for systems
of this type, the usual span is three to five years
from the start of initial engineering to turning the
system over to the owner.
For example, one of the more attractive and in-
teresting possibilities is pyrolysis of a thermally
balanced mixture (Figure 3) of sludge cake with
other combustible matter, such as shredded refuse
from domestic and commercial sources in the com-
munity. This system will be somewhat more com-
plex than present combustion of sludge in the mul-
tiple hearth incinerators now in use, and will re-
quire some consideration of the performance levels
that must be achieved in order for the process to be
considered a satisfactory technical and economic
success.
Another way in which energy can be conserved is
by heat recovery from the incineration gases. This
seems simple at first glance, but will require major
modifications to the existing (Figure 4) furnaces
and expansion of the building to house the heat
recovery boilers and the necessary ducting and con-
trol systems. Presently, we quench the excess heat
in the furnace gases by spraying water into the
ducting before the scrubbers, and reduce the
temperature by evaporating the water spray, from
a temperature of around 800° F to 200° F. This
temperature reduction is in effect a wastage of i
billion 400 million Btu on a daily basis, or the net
energy equivalent of 15,000 gallons of fuel oil.
-------
ENERGY CONSERVATION AND RECYCLING PROGRAM 189
Cooling Walir
IWTP ifHuint)
Ammonia
Liquors
Carbon Char
To Activation
And Us*/Or$ab
F'rllllar Pmluetlon IPatmllall
-\ _
g) "»<»">"'"'
*^
Cj Aqueous Flow Return Ta WTP
-^
£) Separation For Sole
?) Refine For Sale Or Burn As Fail
Figure 3: Flow Sheet for Demonstration of Combined Disposal of Sludge and Refuse.
M H F
INCINERATOR
WASTE
HEAT
RECOVERY f
I
Kl "
Figure 4: Installation of Waste Heat Recovery and Control
Bypass In Between Breeching of Furnace and Scrubber
Assemble.
It is known that, for heat recovery from sludge
incinerators to be operationally practical, the
temperature must actually be raised higher than
present operating practice of 800° F, to perhaps
1200° or slightly more. This latter temperature is
compatible with recent Minnesota Pollution Con-
trol Agency regulations that require 1200° F or
equivalent for gas deodorizing, so by operating in
this way we will comply and also eliminate the
problem of ash residues depositing on heat ex-
change surfaces that can occur when temperatures
are under 1100°.
Another alternative that can be considered as a
technically viable approach is digestion of the
sludge instead of thermal oxidation. This method
has a long history of both successes and troubles in
wastewater plants around the country, and is a
substantial design challenge within the limits of the
site that the main Metropolitan Plant occupies. It
has the advantage, of course, of producing a stable
soil conditioning residue and an excess of combusti-
ble gases that can be used for other beneficial pur-
poses such as running compressors within the plant
or operating vehicles. However ultimate disposal
on this magnitude poses a real problem. We would
need almost an entire rural township for adequate
operation of land disposal.
The final point of urgency is that we now have
plant expansion design under way for the main
Metro plant. The decisions on technology changes
must be made now so that the effect on the ex-
panded capacity will be accomplished.
Legislative Direction
The Metropolitan Sewer Board has been
stimulated in its consideration of conservation
measures and recycling approaches by the wording
in Public Law 92-500, The Water Pollution Control
Act (Figure 5) Amendments of 1972. This legisla-
tion encourages the integration of wastewater
-------
190 MUNICIPAL SLUDGE MANAGEMENT
TITLE II
Sec. 201 Congressional Record, September 28, 1972 H8865
(d) 'The Administrator shall encourage waste treatnent management
which results in the construction of revenue producing facilities providing
for-
ll) the recycling of potential sewage pollutants through
the production of agriculture, silviculture, or aqua-
culture products, or any combination thereof;
(2) the confined and contained disposal of pollutants not
recycled;
(3) the reclamation of wastewater; and
(4) the ultimate disposal of sludge in a manner that will
not result in environmental hazards."
(e) "The Administrator shall encourage waste treatment management
which results in integrating facilities for sewage treatment and recycling
with facilities to treat, dispose of, or utilize other industrial and municipal
wastes, including but not limited to solid waste and waste heat and thermal
discharges. Such integrated facilities shall be designed and operated to
produce revenues in excess of capital and operation and maintenance costs
and such revenues shall be used by the designated regional management
agency to aid in financing other environmental improvement programs."
(f) "The Administrator shall encourage waste treatment management
which combines open space and recreational considerations with such
management."
Figure 5: Excerpts from Federal Water Pollution Control Act
Amendments of 1972.
treatment processes with other urban waste handl-
ing and also encourages generation of profitable
products that can be derived from the waste. Early
in 1973, the Minnesota Legislature passed an
amendment to the Metropolitan Area legislation to
enable us to comply with this act, thereby endors-
ing it as a state objective. Finally, in the most re-
cent session of the legislature, the name of
Metropolitan Waste Control Commission, effec-
tive January 1, 1975. Once again, this indicates the
legislative intent that the Sewer Board broaden its
scope of activity and encompass the treatment of
solid waste materials as well.
Alternatives
A number of alternative fuels could be considered
in substitution for the fossil fuels presently used:
1. The combustible fraction of municipal refuse.
2. Heavier oils, that might be in greater supply
although more difficult to burn.
3. Coal, and lower grades of solid fuels such as
lignite and peat which are found west and
north of the Twin Cities.
4. Dried sludge, that has been brought to a point
of being autogenous by thermal drying.
5. Wood chips from urban area tree trimming
carried out by the city and county forestry
departments and by utilities such as Northern
States Power Company.
t>. Industrial combustible wastes that are
presently being disposed of in landfills.
Methods Chosen
The methods chosen in our program have as
their overall objective that the Board's operations
be as self-sufficient and thermally balanced as
possible, with minimum use of electric power, and
at the same time obtaining by-products that have
value for use or sale. A desirable goal would be to
minimize or eliminate the need to incinerate sludge
cake in the future.
Each of the major plants was viewed as a site for
improvements in technology. The following listing
gives programs that are under way or planned for
each. These are, of course, in addition to the normal
housekeeping economies of removing excess light
bulbs, turning down thermostats, and similar ob-
vious energy conserving methods.
The Metropolitan Wastewater Treatment Plant,
the "flagship of the fleet" is the most attractive area
for energy conservation because it is the location
where almost 90 percent of the wastewater
generated in our area is treated. This plant has a
present average daily flow rating of 218 mgd, and is
now being expanded to a 290 mgd rating.
At this plant, the interesting projects are the
following:
a. Pyrolysis of a mixture of sludge cake with
shredded combustible refuse. This process, a
novel approach to wastewater solids disposal,
has the advantage of disposing of refuse
simultaneously with the sludge. In the process
of preparing the refuse, by shredding, the
potential for recovery of metallic scrap is
created. The process utilizes the gases and
fuels developed in the pyrolytic oven to sus-
tain itself, and therefore it does not require
purchased fuel for operation. In addition,
there is an output of char material, a fixed
carbon, that has sufficient activation to be of
interest for wastewater treatment in the main
plant flow. This would then permit an in-plant
use of a recycled material that will aid
conformance to higher water quality
requirements.
b. Another interesting project under design is a
heat recovery system for recovering thermal
energy from the burning of primary sludge.
These heat recovery units would be fitted
onto the existing furnaces. Hot flue gases are
presently quenched with water sprays, thus
wasting the heat as useless steam plume. We
would be using the existing furnaces to the
best advantage. The flow sheet for the use of
this heat calls for a three-stage step down of
the furnace off-gas, for maximum heat utiliza-
tion. In the first stage, the gases (Figure 6)
-------
ENERGY CONSERVATION AND RECYCLING PROGRAM 191
WASTE
HEAT
BOILER
WASTE HEAT RECOVERY
700 °F
BOILER
FEEDWATER
PREHEAT
600 °F
SLUDGE
DRYING
250 °F
Figure 6: Waste Heat Recovery.
would be cooled from 1250° to 700° F in waste
heat boilers capable of generating high
,-pressure steam that can supply heat for the
new thermal conditioning (heat treatment) of
sludge, a process that improves dewater-
ability.
c. Another program of interest is just about to
commence. This is an experiment with a roll
press that has been used successfully in the
paper and pulp industries for waste sludge
dewatering. This press will be installed in the
filter building at the Metro Plant and dis-
charge cake to conveyors feeding the in-
cinerators. By squeezing and developing a
drier cake, the water burden carried to the fur-
nace will be minimized and that way fuel con-
sumption will be reduced or possibly
eliminated. Pressure filters are the best way of
accomplishing this, although there have been
few outstandingly successful applications of
pressure filters in municipal treatment plants
in this country.
d. We also have great interest in determining the
most efficient means of removing odors from
off-gases. The obvious old standby is to use an
afterburner as a means of destroying odors,
most of which have a combustible nature.
However, this is not always foolproof, and
also is tremendously expensive in terms of
fuel usage. We intend to experiment with a
wet scrubber that uses chemical solutions, to
determine its capability to absorb and retain
odor factors in the gases produced by drying
and burning sludge. This will then be the basis
for a design decision in either direction for our
major sludge drying facility that is now being
planned.
Another area in which we have made signifi-
cant improvements in the past year is in utiliz-
ing our existing gravity thickeners more effec-
tively. In the past, it was the practice to blend a
high ratio of raw waste activated sludge solids
with primary sludge. Typical ratio was 70:30.
We find it more effective to (l) use a much
higher amount of primary solids, 50:50, thus
lowering the ratio of biological solids, and
(2) aerobically digest all waste activated
sludge for a minimum of ten days. By allowing
the overflow to return to the primary settling
tank from which primary sludge is being
drawn for this process, an "overload" condi-
tion can be tolerated. Even though a lot of in-
put solids report in the overflow from the
thickener, 20 to 50 percent, there is in effect a
closed loop on the overflow (Figure 7) which
retains the solids that are carried out. The
applied rates, 20 to 30 pounds per square foot
per day, would normally be unacceptable for a
mixed sludge thickener, but such operation is
satisfactory to us, provided that the biological
solids are aerobically digested. This process
maintains a more dense underflow, in the
-------
192 MUNICIPAL SLUDGE MANAGEMENT
DILUTION
WATER
(II EFFLUENT)
THICKENER OPERATION
OVERFLOW
MIXED
SLUDGE
THICKENER
TO THICKENED SLUDGE
HOLDING TANKS
PST UNDERFLOW SLUDGE
Figure 7: Thickener Operation.
magnitude of four to five percent, versus
previous experience where we frequently
were in the 2Vi percent to 3 percent range.
Partially as a result of this operating practice,
we have materially improved the operations
and dewatering and subsequent burning as
described below.
Improvements in vacuum filtering and burn-
ing have encompassed recommendations
which are considered good practice in other
locations. We have changed the bridging in the
filter valves and reduced the slurry level in the
vacuum vats, maintained a differential
between form and dry sections of the vacuum
filter to get maximum permeability cake and
maximum dryness of the cake, and have in-
sisted that the operators run to a maximum '[/i
inch cake thickness. This has resulted in sub-
stantial improvements in ca'ke dryness, which
together with other changes, are partly
responsible for going from 19 percent solids
average in the month of December to a 23 per-
cent solids content in March. Now, these few
percentage points may not sound like much
but they represent a reduction from eleven to
below eight million Btu fuel burned per ton of
dry solids (Figure 8). At our present costs for
fuel oil, 30 cents per gallon, this represents a
saving of $6.50 per dry ton and a very substan-
tial amount of scarce fuel. Daily saving at this
rate is approximately $800 at present
operating levels, or almost $300,000 annually
if oil becomes our only fuel.
15 *
14 i
11
101
9i
81
EFFECT OF FILTER CAKE SOLIDS
ON FUEL CONSUMPTION
106 BTU
BOUGHT FUEL
PER TON
DRY SOLIDS
PERCENT
SOLIDS
IN CAKE
25
• 24
23
• 22
• 21
• 20
• 19
• 18
17
• 16
OCT NOV DEC JAN FEB MAR APR
1973 1974
Figure 8: Effect of Filter Cake Solids on Fuel Consumption.
-------
ENERGY CONSERVATION AND RECYCLING PROGRAM 193
g. In the future, there are additional projects
that will be carried out at the Metro Plant that
relate to recycling efforts, First, we intend to
take char from the pyrolysis project to
manufacture granular activated carbon, for
tertiary treatment absorber columns as in our
Rosemount Plant. Second, we will be
manufacturing fertilizer type products from
the dried sludge, these products to be for-
mulated for specialty purposes with predic-
table chemical and physical qualities. Finally,
the solid waste shredding operation required
for the pyrolysis system gives us the option of
recovering metals. In addition, there will be
manual separation of all re-pulpable kraft
fiber products such as corrugated board.
Another plant in which we have programs under
way for energy conservation and fuel substitution
is the Seneca Wastewater Treatment Plant, which
was brought into service about a year and a half
ago. It has a design flow of 24 mgd and includes
vacuum filtration and incinerators for sludge com-
bustion. At this plant we have been experiencing a
higher fuel requirement than we would have ex-
pected, and this has been traced partially to inter-
mittent operation. Such operation appeared ap-
propriate in view of the sludge quantity presently
being lower than design loading. However, analysis
of the operating figures shows clearly that we
should run continuously at a somewhat lesser
production rate and avoid the fuel that is burned
during the holding period such as over weekends.
For example, we found that in the whole year of
1973 and early 1974, when we were running inter-
mittently, the total combustible material burned
per pound of water evaporated was 3400 Btu per
pound. In continuous operation in the month of
March 1974 this figure dropped to 2900.
Because of the high fuel consumption at this
plant and because of the physical arrangements
that make such an experiment easy, we have set up
to add coal to the sludge cake as it travels up the
conveyor belt, to add calorific value to the sludge.
The coal and sludge cake are mixed in the horizon-
tal ribbon conveyor that feeds the furnaces, and
further mi-xed by the rabbling action that occurs
within the first two hearths of the furnace. This
prevents premature ignition of the coal and any
likely smoke that would result. The coal being used
is sub-bituminous Western coal from Montana,
with a thermal rating of 8400 Btu per pound and a
fairly substantial moisture content, in the neigh-
borhood of 20 percent. However, because it is
brought into this area in unit trains for electric
power generation, it appears to be our least costly
source of fuel for this operation; estimated cost is
75 percent per million Btu.
Other operating steps that have been taken at
the Seneca Plant in order to conserve on fuel
without sacrificing materially on end results art
these:
• We have had the operators trained to use the
Ohaus moisture balance as a routine control
measure for the purpose of monitoring
vacuum filter performance.
• We have studied the air flotation thickening
operation in order to determine if improve-
ments could result in a lesser sludge disposal
cost by production of thicker sludge.
• We have experimented with the addition of
wood chips to the sludge in a manner similar to
that described above for coal. The wood chips
have burned effectively within the furnace and
without difficulty and we will continue this
work on an expanded scale.
• We have set up a program of experimentation
with pulverized coal added directly to the
sludge either to aid in the thickening or in con-
ditioning of the sludge. The sludge cake would
have a higher calorific value than otherwise,
and thus allow combustion with a minimum of
fuel oil or natural gas.
The newest of our plants, just dedicated last fall,
is our Blue Lake Plant, with a design flow of 20 mgd.
Here we have a minimum of energy requirement
because there is no solids disposal process installed;
sludge is hauled to Seneca, the plant you just saw.
We have under design a system of anaerobic diges-
tion. This will allow the utilization of the digested
sludge as a soil conditioner in landspreading
programs and also produce excess gas that can be
used for motor vehicle fuel or blower drive.
In addition to these three major plants, the
Metropolitan Sewer Board operates 21 other treat-
ment plants in the service areas of the seven county
area, and these operations are being analyzed to
determine what energy savings could be ac-
complished by increasing concentration of sludge.
Regarding other possible energy conversion
processes, there are a couple of questions that often
arise in the discussions on utilizing municipal
refuse as a fuel. One of these is the possible genera-
tion of electric power by raising steam. We do not
consider at this time that an electric power
generating plant is desirable because the thermal
efficiency of conversion of heat from the refuse to
electric power is only about 35 percent whereas
converting the fuel into a usable source for plant
operations has a thermal efficiency of 65 to 70 per-
cent. In addition, a high flow of warmed water must
-------
194 MUNICIPAL SLUDGE MANAGEMENT
be handled in some way. If usable to benefit plant
operations, that may be desirable. However, it also
may be environmentally damaging.
Another question that arises is the generation of
utility steam and distribution to users off-site.
Although this process has an indicated thermal ef-
ficiency of around 65 percent, and thus is superior
to electric power production, the requirement of
steam mains to the points of use raises cost of in-
vestment. Also, the load factors of the users must
be reasonably stable for the process to be operated
on a steady basis. That is, users who only need
steam in the wintertime for building heating or in
the summertime for building cooling are poor can-
didates. On the other hand an industrial process
user who demands steam around the clock seven
days a week all year long is a very good candidate. A
long-term contract with users is a necessity.
In summary, The Metropolitan Sewer Board is
pursuing a wide number of alternatives in the
energy corservation, recovery, and substitution ef-
fort described in the foregoing. This effort will con-
tinue to occupy a significant portion of the staff of
45 engineering and scientific professionals in its
various departments. This program, which began
in earnest over a year before the oil crunch in late
1973, is already bearing fruit. The important factor
in any choice is the reliability of a chosen method.
We hope to establish this by the testing and
development program now underway.
-------
SLUDGE DISPOSAL AT A PROFIT?
W. MARTIN FASSELL
Resource Recovery Systems Division
Barber-Colman Company
Irvine, California
ABSTRACT
* Sludge disposal is a costly operation. With the in-
creasing concern over land or ocean disposal of
heavy metal-containing sludges and the shortage of
energy, alternate methods warrant evaluation. Us-
ing a wet oxidation process operating at 600 psi and
450-465°F in mildly acid conditions, 75-85 percent
COD destruction is achieved. In the process, sub-
stantial quantities of surplus thermal energy are
generated, the nitrogen compounds are converted
to recoverable ammonia, and the metals are
rendered extractable. The predominant organic
compound in the resultant solution is acetic acid,
which may be used • as a carbon source for
denitrification.
Economic studies are presented which indicate
that substantial cost reductions can result from by-
product recovery utilizing the PURETEC System of
wet oxidation. Where denitrification is practiced, a
net plant operating credit is anticipated.
In the treatment of sanitary waste, the problem
of ultimate disposal of sludge has not been fully
resolved. Sludge handling and disposal is a costly
operation representing 25 to 50 percent of the total
capital and operating cost of a wastewater treat-
ment plant1. Consequently, the selection of the ul-
timate sludge disposal method employed can have a
very significant, long-range economic impact on
the community.
Additional factors must also be considered, in
view of the recent trends of our economy. The
current energy crisis and the long-term prognosis
of general shortages in metals2 require a re-
examination of sewage sludge disposal practices to
select the system with the lowest overall energy
consumption combined with maximum recovery of
non-renewable resources.
Sewage sludge as an energy source is significant.
Based on calorimetric measurement, Brooks3 found
that sludges range from 7,500 to 10,000
BTU/pound of volatile solids. Thus, in the oxidation
of sludge, substantial thermal energy generation is
possible. As will be described later, it is easily possi-
ble to recover four to seven percent of the fuel
value as a "clean" high molecular weight grease that
can be burned directly or worked up to produce
more valuable petrochemical byproducts.
Sewage sludge also serves as a marvelous
concentrator of many of the metals present in the
total waste stream. It is highly likely that the
concentration is due to the presence of HbS as a
result of anaerobic biological activity causing the
precipitation and concentration of the metals and
sulfides. In the Orange County Sanitation District,
it has been reported that approximately $lM/year
of metals are dumped into the sewer. This is not
unusual, and in Table 1, the daily metal values are
TABLE 1
Daily Metal Values Entering Plant
in Pounds Per Day
Orange County
Metal Sanitation
District'1
Silver
Cadmium
Copper
Nickel
Lead
Zinc
11
50
400
200
200
500
22
125
800
400
400
1000
Philadelphia, Pa.s
9
8
300
100
800
1200
10
350
125
900
1400
Rockford, Ill.<>
4
400
30
20
1100
1
10
450
40
25
1300
195
-------
196 MUNICIPAL SLUDGE MANAGEMENT
shown for three municipal/industrial type sewage
treatment plants. The long range environmental
impact of the heavy metals in sewage is of
increasing concern7. Current landfilling and
spreading of sludge is being re-examined and the
desirability of continued ocean dumping is being
questioned.
The preponderance of organic material in sludge,
coupled with the low metal concentration, render
methods such as those proposed by Dean8, and
Cadman and Dellinger9 unsuitable for direct metal
removal from sludge.
Once the organics are destroyed or reduced to
low molecular weight organic acids by wet
oxidation, the resultant sand or ash and solutions
can be easily stripped of metals by using
conventional hydrometallurgical techniques.
Sludge also represents a significant source of
fertilizer, but in a different way than practiced
currently. As will be described later, it is easily
possible to produce either ammonium sulfate or
phosphate concurrent with the sludge disposal
process. Such compounds as (NhUbSCU or
(NHibPGj are in short supply and sufficiently
concentrated to bear the freight to avoid local
supersaturation of the market.
Where the denitrification process is being
practiced, the purified solutions resulting from
sludge disposal can be used as a carbon source
instead of methanol, since the predominant species
remaining is acetic acid.
Where denitrification is not required, this
purified acetate-containing effluent may well be
useful in some new types of flash digesters now
being researched by Professor Perry McCarty at
Stanford University10. Such use could increase the
methane production capabilities and relieve the
plant of ever-rising fuel costs.
Sewage sludge may well be an asset. The
question is, then, how can the above possibilities be
realized?
During the past three years, the Resource
Recovery Systems Division of Barber-Colman
Company has been investigating the application of
hydrometallurgical wet oxidation practice n-'3 to
the destruction of sewage sludge and many other
organic waste materials. The developed system,
termed the PURETEC/WETOX Protess, utilizes an
FiRure 1: 4000 GPD PURETEC System.
-------
DISPOSAL AT A PROFIT? 197
TABLE 2
Technological Comparison of Thermal
Treatment Process for Sewage Sludge3
Process Parameters
Temperature, ° F
Pressure, psi
Retention Time, Minutes
Feed:
PH
02/COD
Performance
COD Reduction, %
Solids
Weight Red., %
Moisture, %
Metals Removal
Problems
Barber- Colman
PURETEC"
450
600
40b
3,
0.7
80
75C
50
Removed or
Insoluble
Agitator Seals
High Pressure
Wet Oxidation
500
1600
80
8
0.7
80
75c
50
Unknown
Recycle Impact
Scaling
Pressure Leaks
Low Pressure
Wet Oxidation
350
350
40
7
0.1
15
15
50
Unknown, but difficult to
remove with organic solids.
Recycle Impact
Scaling
Odors
Thermal
Treatment
350
250
60
7
0
10
10
50 '
Unknown, but difficult to
remove with organic solids.
Recycle Impact
Scaling
Odors
Non-Autogenic
Comparative figures are averages obtained from operating personnel in several plants.
Solids retained 60-80 minutes.
^Remaining solids inorganic.
Vapor Effluent
Liquid Phase ,•
Heat Exchanger j£
Mtrtrt
Vapor Phase
Heat Exchanger
WETOX Reactor
Boi ler
Start-Up
Only
t> Sulfide Concentrate to Smelter
V
To Primary
Treatment
Figure 2: Barber-Colman PURETEC/WETOX System.
-------
198 MUNICIPAL SLUDGE MANAGEMENT
agitated, horizontal,, multiple-compartment
autoclave of the type shown in Figure 1. This
reactor, operating under mildly acid conditions,
consistently achieves 75 to 85 percent destruction
of the COD present in any type of sewage sludge at
440 to 465°F and 600 pounds pressure. We believe
that many significant improvements in the
destruction of sewage sludge are possible using the
above described equipment and process
parameters. In addition, new routes are opened for
the recovery of valuable byproducts which can
substantially reduce sewage sludge destruction
costs.
By way of comparison, the operating parameters
and performance of various wet oxidation and
thermal treatment processes for sludge are
summarized in Table 2.
To more clearly define the PURETEC System,
the basic unit operations we use are shown in
Figure 2. As indicated in Table 2, a mildly acid media
is utilized for the performance of wet oxidation.
This, in part, is responsible for the increased
process efficacy of the system. The acid leachant
has other advantages. It avoids scaling in the heat
exchanger, increases the conversion of residual
organics to acetic acid and solubilizes most of the
metals so that they subsequently can be
precipitated as sulfides in a concentrated form. To
avoid corrosion and erosion, all wetted metallic
parts of the system are fabricated of titanium. This
metal has an unparalleled record of immunity from
attack in acid wet oxidation service. In addition,
titanium is unaffected by NaCl often present in
sewage sludge, especially in coastal regions.
Typical results from continuous pilot plant
operation in our 4-10 System, shown in Figure 3,
are presented schematically in Figures 4 and 5,
One point of significant difference which
deserves further emphasis is the separation of the
liquid and vapor phases prior to discharge from the
VYETOX Reactor. Aside from the improved
performance of the heat exchanger, as much as
one-third of the water present in the sludge is
discharged as vapor phase condensate. This
increases the residence time of the solids within the
reactor, effectively increasing the capacity of the
ure3; PURETEC'WETOX 4-10 System.
-------
DISPOSAL AT A PROFIT? 199
Orange County Sanitation District
Primary Sludge
COD • 59,239 mg/I
H2SO,
Vapor Phase
pH 4.4
A 371 of Flow
Pressure: 600 psig
Temp., Mean: 446°F
COD = 13,626 mg/i
Grease Removal
COD = 8,317 mg/£
190 mg/£ NH?
UETOX COD: 71.6%
OVER-ALL REMOVAL: 81.0%
Liquid Phase
pH 3.0
63S of Flow
COD = 18,350 nig/1
pH 5.0
COD = 9,255 «g/I
NH- = 2,000 mg/£
Figure 4: Continuous Pilot Plant Wet Oxidation Results, Orange
County Sanitation District Primary Sludge.
system. Since no metals are present in the vapor
phase, the metal concentration is increased in the
liquid phase.
The vapor phase condensate also contains an
easily broken emulsion of "grease" in water. While
total identification of this product is not yet
Philadelphia Digester Sludge
North-East Plant
H2S04
70,855 mg/i
pH 3.4
Vapor Phase
pH 4.2
35.7% of Flow
Pressure: 600 psig
Temp., Mean: 446°F
33,375
21,376
18,425
COD = 14,535 mg/i
Grease Removal
COD = 7,130 mg/i
UETOX COD: 76.0*
OVER-ALL REMOVAL: 89.3*
Liquid Phase
COD = 18,425 mg/Jl
64.3% of Flow
pH 6.3
COD = 7,853 mg/4
Figure 5: Continuous Pilot Plant Wet Oxidation Results,
Philadelphia Northeast Digester Sludge.
complete, nuclear magnetic resonance studies as
well as long term saponification tests indicate that
it consists of straight chain aliphatic or paraffin
type organic compounds with a 190 to 250°C
boiling range. Sludges normally yield 35 to 50
pounds of "grease" per dry ton of sludge treated, or
approximately one barrel of "oil" per ten tons of
sludge (dry) treated. This product can be burned
directly or worked up for more valuable
petrochemical feed stock.
The residual organic species in the vapor phase
condensate, as determined by gas phase
chromatography, will run 45 to 55 percent acetic
acid and 15 to 20 percent propionic acid. Trace
quantities of butyric acid, acetaldehyde, acetone,
formic acid, methanol and ethanol are present, as
shown in Figure 6.
In the liquid phase effluent, the residual organic
is 40 to 70 percent acetic acid, depending on the
sludge treated. Propionic acid is present in the 5 to
15 percent range, followed by lesser amounts of
formic acid and acetone. Only trace quantities of
acetaldehyde are present in the liquid, as shown in
Figure 7.
The fate of the metals present in the sludge as a
result of the wet oxidation process is of interest.
The primary sludge (Orange County Sanitation
District) used as an illustration of COD removal is
typical. Wet oxidation, contrary to our original
beliefs, does not completely solubilize all metals. It
was assumed that the presence of acetate, even
though only partially ionized, would leach all
Valeric Acid
Pentanoic Acid
Figure 6: Gas Chromatography Analysis Vapor Phase
Condensate.
-------
200 MUNICIPAL SLUDGE MANAGEMENT
Figure 7: Gas Chromatography Analysis - Liquid Phase Effluent.
metals, since all acetates are very soluble. As shown
in Table 3, as the acid concentration of the influent
sludge is increased, the distribution ratio of the
metals present in the liquid phase increases. With
economically practical levels of sulfuric acid
additions, copper, zinc and cadmium are solubilized
while lead and silver remain in the insoluble residue
or ash.
The soluble metals are easily removed as their
sulfides, using either H2S or preferably calcium
polysulfide. The sulfide precipitate formed settles
rapidly and is easily filtered. Sulfide precipitation is
unexcelled in the completeness of metals removal
and operational simplicity. The resultant
precipitate is a high grade concentrate ready for
shipment to the smelter. In the case of the Orange
County Sanitation District, this "concentrate" will
run about 3% Cd, 23% Cu, 11% Ni, 28% Zn and
22% S.
Based upon laboratory scale tests, the lead, silver
and perhaps gold present in the ash appear
TABLE 3
Percent of Metals Present in
Wet Oxidation Ash Versus Acid Addition
Metal
\\'l. c"c
0
HiSOj Addition to Sludge Feed
grams per liter
6 12 18
Copper
Lead
Zinc
Cadmium
Sil\er
Iron
Titanium
PH
93
100
100
97
98
98
100
5.0
66
90
66
50
94
91
100
53
77
44
38
87
92
100
1.8
7
77
0
0
88
65
100
amenable to chlorinated brine leaching, utilizing a
process invented in 1923 for leaching lead sulfate.
Lead and silver are recovered as metals ready for
smelter shipment.
Acid wet oxidation of sewage sludge also results
in the complete conversion of the organic nitrogen
to ammonia. No oxides of nitrogen or organic
amines have been detected even when oxidizing
such compounds as nitrate esters or trinitrotoluene
under acid conditions in the WETOX Reactor. This
point is clearly evident in Figures 4 through 7.
For primary sludge, approximately 45 pounds of
NHa are generated per dry ton of sludge treated.
(See Figure 4.) This ammonia, present at a concen-
tration of 1.32 g/L in the combined vapor phase
condensate and liquid phase effluent (after metals
removal) can easily be lime stripped. Because of the
limited stream volume and elevated temperature of
this stream (140 to 160°F), it appears theoretically
practical to use an enclosed, agitated,
multicompartment stripper. Based on modest mas
transfer coefficients, 80 percent of the NHa can be
stripped per stage with space tirne of 15 minutes per
stage, resulting in approximately 99 percent
removal of the ammonia. Based on a conservative
recovery of 90 percent of the NHa at a price
equivalent to anhydrous NHa at $180.00 per ton,
this product has a minimum value of $3.65 per ton
of sludge treated. Conversion to (NH4)zSO4 or
preferably (NHibPCU will enhance its value and
provide a highly concentrated fertilizer to satisfy
today's requirements.
The low molecular weight acids, acetic and
propionic, represent potentially one of the most
valuable by-products of sewage sludge wet
oxidation. Acetic acid has proved to be an entirely
satisfactory carbon source for denitrification14.
Because of cost, methyl alcohol has been the
preferred carbon source15.
Based upon the combined acetate concentration
of the clarified liquid phase and vapor phase
effluents, 250 pounds of acetate are produced per
dry ton of sludge processed. Based upon the current
methanol price of $0.086 per pound, the acetic acid
by-product has a value of $21.60 per ton of sludge
processed. ^
Thermal energy is also a useful by-product of wet
oxidation. Detailed studies of the energy balance of
the PURETEC/WETOX System, which are beyond
the scope of this paper, show that under our
operating condition, five to six million BTU's of
energy are available over that needed to preheat the
influent sludge. This value is based on six percent
solids with 70 percent volatile acids with a fuel
value of 6,000 BTU's per pound of dry sludge. This
-------
DISPOSAL AT A PROFIT? 201
energy can most economically be utilized as 180°F
water or low pressure steam for heating waste
water to accelerate biological activity or space
heating in plant or , adjacent communities.
Considering fuel oil with a higher heating value of
19,000 BTU's per pound or 6.9 gallons per million
BTU's and a fuel-fired boiler efficiency of 65
percent the thermal energy produced by wet
oxidation is equivalent to 50 to 60 gallons of oil per
ton of sludge processed. With current fuel oil costs
ranging between $8.00 to $14.00 per barrel, this
energy has an equivalent economic value of $10.00
to $18.00 per ton of sludge processed.
The potential recoverable by-products for a
typical domestic/industrial sludge are summarized
in Table 4.
In order to evaluate the economics of wet
oxidation destruction of sewage sludge, relatively
detailed equipment costs and operating costs have
been calculated for two major wastewater
treatment facilities. The first, shown in Table 5, is
for the City of Philadelphia. This analysis uses the
existing digester with wet oxidation for
destruction of the digester sludge with metal
recovery. The cost analysis is based on the current
cost-effective analysis guidelines16.
As is shown in Table 5, the total cost of sludge
disposal will be $46 to $47/ton, with a plant
operating credit of $36-$37/ton. These data
suggest that sludge disposal costs may be reduced
to approximately $10/ton.
Current sludge disposal costs are approximately
$25-$30/ton, based on lagooning the digester
sludge and ocean dumping. Assuming the proposed
method of sludge disposal can be demonstrated on a
plant basis, it appears technically and economically
possible to reduce current costs.
TABLE 4
By-Product Value from
Sludge Wet Oxidation Destruction
Bv- Product
Value per Ton of Sludge
(Dry Basis)
"Grease"
Metals*
Ammonia
Thermal Energy
Acetate/Methanol
Equivalent
Ex Acetate Credit:
0.80 1.40
12.00 15.00
3.65
10.00 18.00
20.00 - 30.00
$46. - 68. /Ton
26. - 38. /Ton
* Metal credit based on 80 percent of Engineering Mining Journal
quotes of April, 1974.
In Table 6, a similar economic evaluation is
presented for the Blue Plains Wastewater
Treatment Facility. Because the influent sewage is
largely of domestic origin, the metal values are of
no consequence. This facility, however, will include
a denitrification stage and, hence, could utilize the
acetic acid present in the effluent from wet
oxidation.
Because of high power costs in the Washington,
D.C. area, the plant operating costs are $45-
$46/dry ton of sludge. Because of the possibility of
acetate utilization as a carbon source in
denitrification and its economic value, the by-
products produced provide a credit of
approximately $47/ton. Therefore, at least in
theory, it may be possible to achieve a net plant
operating credit from the sludge disposal plant.
It is anticipated that a large scale demonstration
plant utilizing the PURETEC/WETOX System of
sludge destruction will be in operation early in 1975
to verify our pilot plant results.
ACKNOWLEDGMENTS
The author would like to take this opportunity to
thank Mr. Steven Townsend of the Philadelphia
Water Department and Mr. Alan Cassel of the Blue
Plains Wastewater Treatment Plant for providing
plant operating data used in this report.
REFERENCES
1. S. Balakrishnan, W. E. Williamson and R. W.
Okey, State of the Art Review on Sludge
Incineration Practice, U.S. Dept. of Interior,
Federal Water Quality Adm. #17070 Div. 04/70,
April, 1970.
2. Final Report, National Commission on
Materials Policy, June 1973, Library of Congress
Card No. 73-600202, U.S. Government Printing
Office, Stack No. 5203-00005.
3. R. B. Brooks, Water Pollution Control, Paper
No. 2, 1970, pp 221-231.
4. Private Communication, Dr. John E. Sigler,
Resource Management Engineer, Orange County
Sanitation District.
5. State of Pennsylvania.
6. Private * Communication, R. Eick, Sanitary
District of Rockford, Illinois.
7. J. G. Everett, Water Pollution Control, 1973,
pp 428-435.
8. J. G. Dean, F. L. Bosquis and K. L. Landuett,
Environmental Science and Technology, Vol. 6, June 1972,
pp 518-522.
-------
TABLE 5
City of Philadelphia
c
z
n
IKO Mi/lion Gal/ Day
(.'/ M i/ l,,n 2(1(1 Parts/ Million
fj/n I, ni/ Day HUM') 70':; Svilk'ahk' Solids
S 5.S5 Thickener
105 Tons of Sludge
5% Solid
5> 7.25 (Includes Anaerobic
Pumping) Digester
90 Tons of Sludge
8% Solid
$21.90 2 PURETEC Systems
$ 4.50 Metal/ Ammonia
Removal
45 Tons of Residue
50% Solids
S 7.00 Hauling
S46.50
180 Million Gal/ Day
200 Paris /Mi/lion Credits /Ton
70% Settleahle Solids (210 Ton/ Day Base)
Thickener
105 Tons of Sludge
5% Solid
Anaerobic Methane $ 8.00
Digester ( I.40/ Million BTU)
90 Tons of Sludge
8% Solid
2 PURETEC Systems Grease 1.00
Heat 12.00
Metal/ Ammonia Metal 12.00
Removal
45 Tons of Residue NH3 3.50
50%, Solids
Hauling
CREDIT: $36.50
Bui/ding
(15,000 Sq. Ft.)
At $50/Sq. R.
Maintenance
Interest
PURETEC System:
4 PURETEC Systems
Interest (!<",.)
Maintenance
Labor
Power
H2S04
Lime
Cost 120 ».v.
$ 750K
300
525
$I,575K
$3,900 K
5,460
3,600
9,340
9,986
1,971
1,314
,$35,57 IK
Cost/ Yr.
$ 25K
15
26
$ 66K
$ 195K
273
180
466
499
99
66
$1,778K
Cost
(2 10 Ton/ Day)
$ .3
.2
.3
$ 0.8
$ 2.5
3.6
2.0
6.0
6.0
1.0
$ 21.9
t~
a
m
O
m
Z
H
Post WETOX Treatment
Building, 5,000 Ft.2
at S50/Ft.-
Mctal/ Ammonia Equip
Interest
Labor
Maintenance
Reagents
Power
250
. 650
1,260
2,400
650
1,220
460
6,890
12.5
32.5
63.0
120.0
32.5
61.0
23.0
344.5
.2
.4
.8
1.6
.4
.8
.3
4.5
Net Operating Cost: SIO.O.O/Ton at 210 Ton/Day
-------
TABLE 6
Blue Plains (PURETEC System)
Cost/ Ton Credits/ Ton
(432 Ton /Day Base) (432 Ton) Day Base)
$ 6.00 Thickener
432 Tons/ Day of Sludge
8% Solids
30.08 9 PURETEC Systems Grease: 1.00
Thermal
Energy: 21.60
172 Tons of Residue Ammonia: 3.65
20% Solids Acetate: 21.00
Ammonia Recovery
4.12 Acetic Acid Production
172 Tons Residue
60% Solids
5.47 Hauling
$ 45.67 $ 47,25
Plant Operation Credit = $1.58 per Day Ton of Input Solids
PURETEC System
9 PURETEC Systems
Interest at 7%
Maintenance
Labor
Power at 3i/KW-HR
H2SO4
Lime
Post Treatment
Ammonia Stripping/
Acetic Acid Polish
Interest at 7%
Maintenance
Power
Chemicals
Cost/20 Yrs.
$ 9,100
12,740
8,100
6,420
44,939
9,460
3,154
$93,913
2,500
3,500
2,000
2,500
2,523
$13,023
Cost/ Yr.
$ 455
637
405
321
2,247
473
158
$4,696
125
175
100
125
126
$ 651
Cost/ Ton
$ 2.88
4.04
2.88
2.03
14.25
3.00
1.00
$ 30.08
0.79
1.11
0.63
0.79
0.80
$ 4.12
g
on
O
en
O
T)
»—i
H
to
O
OJ
-------
204 MUNICIPAL SLUDGE MANAGEMENT
9. T. W. Cadman and R. W. Dellinger, Chemical
Engineering, April 15, 1974, pp 79-85.
10. Private Communication, Prof. Perry
McCarty, Dept. of Civil Engineering, Stanford
University, Palo Alto, California.
11. W. M. Fassell, Pure and Applied Chemistry, Vol. 5,
1962, pp 683-699.
12. W. H. Dresher, M. E. Wadsworth and W. M.
Fassell, Min. Eng., 1956, pp 738-744.
13. W. M. Fassell, Pressure Leaching of Ores and
Concentrates - Chemical Requirements - American
Chemical Society, May 1958.
14. R. L. Culp and G. L. Culp, Advanced Wastewater
Treatment, Van Nostrand Reinhold, 1971, pp 228-
231.
15. J. N. English, et al., ]our. WPCF, Vol. 46, No. 1,
Jan 1974, pp 28-42.
16. Federal Register, Vol. 39, No. 29, Feb 11,
1974, pp 5269-5270.
-------
SLUDGE MANAGEMENT SYSTEM FOR ST. LOUIS
PETER F. MATTEI
Metropolitan St. Louis Sewer District
St. Louis, Missouri
ABSTRACT
The Metropolitan St. Louis Sewer District has
two types of solids handling systems both based on
the needs of the watershed for which they were de-
signed. Anaerobic digestion with storage basins
was selected for secondary treatment plant at Cold-
water Creek. The plant serves an area of 41 square
miles and has a design average flow of 25 mgd.
Vacuum filtration, incineration and ash storage
basins were selected for the two Mississippi River
treatment plants—Lemay and Bissell Point. Both
plants provide primary treatment for an average
design flow of 424 mgd and serve an area which
contains 196 square miles.
The digestion process was selected for the Cold-
water Creek because an adequate area of land was
available for storage of digested sludge. The
methane gas from the digestion process was avail-
able for engine operation to drive blowers and
pumps at an economical cost for the secondary
process.
Vacuum filtration with incineration was selected
for the large Mississippi River plants because of the
greater volume reduction of solids and the inert
characteristics of the ash material. This signifi-
cantly reduced the land requirement for the two
plants with large flows containing high concentra-
tion of solids.
INTRODUCTION
The Metropolitan St. Louis Sewer District was
created by a vote of the people in February, 1954.
The District serves an area draining 247 square
miles and a population of 1,555,000 people.
Approximately 3,500 industrial and commercial
establishments are located within the District.
Prior to creation of the District, most of the wastes
generated within its boundary entered the
Mississippi and Missouri Rivers without
treatment.
MSD is subdivided into three subdistricts—the
Mississippi subdistrict which serves 196 square
miles and a population of 1,375,000; the Coldwater
Creek subdistrict, which serves 41 square miles and
a population of 165,000; and the Sugar Creek
subdistrict, which serves ten square miles and
13,000 people. Each subdistrict has one major
treatment plant with the exception of the
Mississippi subdistrict which has two treatment
plants. The west and south St. Louis and St. Louis
County portion of the Mississippi subdistrict is
served by the Lemay Plant The drainage area is 118
square miles and the population served is 785,000
persons. The northern portion of the Mississippi
subdistrict is served by the Bissell Point Plant. The
drainage area is 78 square miles and the population
served is 592,000 persons.
Selection and Description of Treatment
Plants and Processes
Coldwater Creek Treatment Plant
The Coldwater Plant is a 25 mgd average design,
conventional activated sludge plant. Coldwater was
the first of three major plants designed and placed
in operation in September, 1965. The plant effluent
enters Coldwater Creek which, for a major part of
the year, is a dry stream. The effluent travels four
and one-half miles in the Creek before entering the
205
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206 MUNICIPAL SLUDGE MANAGEMENT
Missouri River ten miles above the City of St. Louis
Water Intake. Since the effluent enters a dry
stream which discharges above the City water
intake, secondary treatment followed by
chlorination was chosen as the basis for design.
Sludge treatment employs gravity grit removal,
primary sedimentation, sludge thickening,
anaerobic sludge digestion with digested sludge
lagoon storage on the plant site.
Anaerobic digestion was the only method of
sludge handling considered due to the plant size and
location. At that time the area around the plant site
was undeveloped and primarily of rough wooded
topography. Therefore, adequate and suitable land
was available for disposal of digested sludge by
storage in sludge lagoons, or basins. Since
considerable farmland and pastureland existed,
some thought was also given to land spreading of
liquid digested sludge as a supplement to storage. A
38 acre tract was acquired and developed into four
cells of approximately five acres each. At the
present rate of filling the sludge basins should
provide adequate storage for 25 years. Should
future development result in land for additional
sludge storage being unavailable, the option of
pumping the digested sludge to the Bissell Point
Plant for incineration and disposal could be
considered.
Lemny and Bissell Poini Treatment Plants
The Lemay Plant is a primary treatment plant
designed to treat 173 mgd prior to discharging its
effluent to the Mississippi River. This plant was
placed in operation in May of 1968.
The Bissell Point Plant is the larger of the
Mississippi River subdistrict treatment plants with
a design average flow capacity of 250 mgd. The
Bissell Point Plant was officially placed in operation
in November of 1970. This primary effluent is also
discharged to the Mississippi River. Because of
these direct discharges to the Mississippi River,
which provide an approximate dilution of one part
of wastewater to 1000 parts of river water, and due
to Federal requirements at that time, primary
trea tment was the major consideration at these two
plants.
The liquid wastewater treatment processes at
both the Lemay Plant and the Bissell Point Plant are
similar. Flow is pumped into gravity grit removal
tanks, followed by comminution of solids,
preaeration of the wastewater, and primary
sedimentation for removal of settleable solids and
floating material.
Sludge treatment consists of vacuum filtration of
primary solids followed by incineration. Incinerator
ash is slurry pumped and stored in ash drying
basins located at each plant. The following eight
solids-handling processes were considered for
sludge treatment prior to design of the two plants:
1. Digestion, vacuum filtration and incineration
2. Digestion, vacuum filtration and trucking
cake to fill
3. Digestion, vacuum filtration and production
of soil conditioner
4. Digestion, pumping of liquid sludge to lagoons
5. Digestion, barging of sludge to lagoons
6. Fresh sludge vacuum filtration and
incineration
7. The Laboon Process
8. The Zimmermann Process
Of the above eight alternatives, digestion with
pumping to lagoons and digestion with barging to
lagoons had the lowest total equivalent annual cost
for both plants. These two methods, however,
were considered unsuitable for the Metropolitan
St. Louis Sewer District plants because of the
following:
The combined flow of the Lemay and Bissell
Point Plants was estimated to be 424 mgd for the
design year 1985. The average daily solids removal
was estimated at 273 tons per day or 1,086,000
gallons of sludge per day at six percent solids. After
digestion a total of 183 dry tons of solids had to be
disposed of to the lagoons. This would result in a
flow of 726,000 gallons of digested sludge at six
percent solids.
The land area required to store the above
quantity of sludge is very large. Computations
showed that by 1985 nearly 450 acres of land would
be filled and abandoned, and by year 2000, over 750
acres would be used. To protect the future
operations of the District, it was concluded that the
initial design should involve purchase or option to
purchase at least 1,000 acres of land suitable for
lagooning.
Suitable tracts of land of this size, of low enough
value to be considered for this purpose, can be
found in the vicinity of St. Louis only as flood plain
areas along the Mississippi River. A survey was
made of sites which might be available for sludge
lagoons. The closest site was 18 miles from Bissell
Point with a net usable area of 290 acres. The next
site was an island in the Mississippi River located 23
miles from Bissell Point with a net usable area of
390 acres. The last possible site was located 40 miles
away with a net usable area of 755 acres.
The operation of digested sludge lagoons on such
a scale along the Mississippi River would be subject
-------
SYSTEM FOR ST. LOUIS 207
to a number of "serious difficulties. The most
important of these, perhaps, is the possibility of
contamination of water supplies. The low-lying
islands available for sludge lagooning are composed
of river bottom sand and silt, underlain by
extensive gravel and sand formations. These gravel
formations form an aquifer from which many
nearby wells draw water for both public and private
supplies. The Missouri cities of Crystal City, Festus
and Herculaneum are all located near the proposed
lagoon sites, and all have public water supplies
using wells. The possibility existed that seepage
from the sludge lagoons could enter the porous
formations and find its way into these water
supplies. Because of the large areas involved, there
appeared to be no method available that would be
practical and economically feasible to seal the
lagoons and prevent seepage.
A second problem concerning the public health
would result from the necessity to discharge the
supernatant liquor to the Mississippi River. This
liquor would be relatively small in quantity, but
high in BOD and alkalinity. It was not feasible to
pump the supernatant back to the sewage
treatment plants for treatment. Although the
effect of such a discharge on the river is debatable,
it was possible that the public agencies having
jurisdiction would have voiced an objection. A third
factor involving the public health was the
possibility that flood waters could breach the
lagoon dikes, and release large quantities of sludge
to the river. In this connection it should be
mentioned that the economic evaluation was based
upon lagoon dikes constructed to river stage 35.
Under this plan, it was recognized that the dikes
would be overflowed at about five to seven year
intervals with the consequent inundation of the
lagoon areas. Following such a flood, the excess
flood water would be drained off, and the dikes
repaired. It was found impracticable, from the
standpoint of cost, to provide dikes high enough to
protect against the maximum flood, since the
additional cost of the higher dikes would have
exceeded $1,000,000.
The lagooning of sludge on a scale as large as
required in this case, involved several other
problems as well. It would have been difficult, if not
impossible, to prevent the entry of unauthorized
persons into the large lagoon areas, in spite of
fences, and the hazards of drowning in the 10 to 14-
foot depth of the lagoons would surely have been
present. The prevention of insect and vermin
breeding would have been another problem
encountered, and although the potential odor and
insect problems could have been adequately
controlled in a small sludge lagoon, it would have
been difficult to provide complete control in a
lagoon installation on the order of 1,000 acres in
size.
Another feature which adversely affected the
selection of lagooning was the loss of extensive land
areas from agricultural, wildlife and recreational
uses. Removing such large areas permanently from
more productive uses, and devoting them simply to
storage of sludge could have provoked considerable
adverse reaction from the public.
Because of these considerations, anaerobic
digestion followed by lagoons was not selected even
though it had the lowest equivalent yearly
operating cost.
The next lowest alternative was vacuum
filtration of primary sludge and incineration of
solids. This process has two distinct advantages for
plants that have large flows and large quantities of
solids. These advantages are volume reduction and
solids sterilization.
During vacuum filtration, the solids content is
increased from 6 to 30 percent solids. During
incineration all moisture is evaporated from the
solids and the volatile portion is burned off at
1600°F. The volatile content for the two Mississippi
River plants was estimated to be 63 percent. Based
on this value a total of 101 tons of ash had to be
disposed of daily. Thus the total land requirement
for ash basins for a 20 year design period was 13
acres with fill to depth of 12 feet. Land sites for
disposal of ash of this quantity were available
within a short distance from each plant.
Incinerator ash is non-putrescible,. sterile, inert
material. It may be disposed of without concern of
objectionable odor problems or public health
nuisances. Because of volume reduction and solids
sterilization, the District chose vacuum filtration
and incineration as the method of treating
wastewater solids at the two Mississippi River
plants.
Operational Problems and Solutions
Coldwaier Creek Plant
The Cbldwater Creek Treatment Plant, utilizing
digestion for solids handling, was put on the line in
September,1965. There are six digesters, 100 feet
in diameter with a center depth 25 feet. Each
digester has a volume of approximately one million
gallons. Four of the tanks have fixed covers while
two are equipped with floating gas holders with a
gas storage of 125,000 cubic feet each.
Digester temperatures are maintained by
recirculating sludge through heat exchangers
-------
208 MUNICIPAL SLUDGE MANAGEMENT
employing hot water recovered heat from the
pump and blower engine operations. Digester
mixing is accomplished by two methods, bottom
gas mixing and conventional heat exchanger pump
recirculation applied to the first stage digesters
only.
The digesters were started in a normal manner
by filling them with raw sewage and raising the
contents to an operating temperature of 95°F. Lime
was used to maintain the pH at an operating range
of 6.8 to 7.0 and by early March, 1966, the system
was operating effectively. Upsets have occurred
over the years, primarily due to heavy metals.
Early in 1967, anhydrous ammonia was very
effectively utilized to adjust the pH during one such
upset. In the spring of 1967, the scum line became
plugged. Attempts to open the line by mechanical
means were unsuccessful due to the many
horizontal and vertical bends in the line. Sections
on the line were relaid to reduce the number and
degree of the bends. Subsequent plugging has been
avoided by employing a steam-cleaning process
routinely on an annual basis.
In the fall of 1969, assisted by grant money from
the Environmental Protection Agency, a study was
conducted to identify odors associated with the
operation of a sludge thickener. As part of the
study, the thickener was covered with a styrofoam
dome in order to control the atmosphere
immediately over this unit. The dome continues to
serve as an excellent means of controlling odors
around such a unit.
A unique method of digested sludge elutriation is
practiced utilizing chlorinated plant effluent to
provide a buffering solution in digested sludge
disposal operations. This process has been an
excellent aid in controlling odors from the sludge
lagoons.
In general, the plant is in the ninth year of
continous operations and has not experienced any
serious sludge handling problems. No additional
sludge handling provisions are contemplated or
planned for the near future.
Lemay Plant
The Lemay Plant has been in operation for six
years. During the last fiscal year 1972-73 the
average flow was 117 mgd. The wastewater
received is relatively weak with influent suspended
solids averaging 161 mg/1 and the effluent 69 mg/1.
This is equivalent to a suspended solids removal of
57 percent for the year. A total of 17,314 dry tons of
solids were vacuum filtered and incinerated during
the year and the volatile content of the sludge
averaged 50 percent.
Six vacuum filters with stainless steel coil spring
media are used to dewater solids removed from the
primary tanks. During the first two years of
operation, extremely poor vacuum filter results
were achieved. The vacuum filter yield averaged
only three pounds per square foot per hour coupled
with an extremely high moisture content of
approximately 80 percent. During this period the
raw wastewater contained highly flocculated solids
which produced excellent suspended solids removal
in the primary clarifiers. However, the sludge did
not compact in the clarifiers and sludge depths of
two to four feet were constantly maintained. The
chemical demand for sludge vacuum conditioning
prior to vacuum filtration was extremely high,
producing a chemical cost of approximately $15.00
per ton during the first six months of operation.
From the appearance of the solids, it was evident
that a large quantity of industrial waste material
was causing poor settling of the sludge and high
chemical costs for vacuum filtration. Through a
vigilant sampling program by material was
determined.
A manufacturer of paint pigments was disposing
of large quantities of bentonite clay as a process
waste. The bentonite clay, a flocculent aid,
absorbed large quantities of water which could not
be chemically removed and thus, could not be
properly dewatered on the vacuum filters.
Motivated by discussions held and particularly by
the implementation of the District's Industrial
Waste and Surcharge Ordinances, this company
installed equipment to drastically reduce the
quantity of bentonite clay discharged.
With the reduction of this material, a significant
improvement in the operation of the vacuum filters
was noted. Moisture contents decreased from 80 to
approximately 70 percent. Since the bentonite clay
material is a non-volatile material, the volatile
content of the vacuum filter cake increased from 39
to 50 percent. Vacuum filter yields nearly doubled
and chemical conditioning costs decreased from ten
dollars to about five dollars per ton.
The plant began operation using ferric chloride
and lime as conditioning chemicals for vacuum
filtration. After six months of operation with poor
filtering results, cationic polymers were tried. A
significant reduction in cost was immediately
noted. A short time thereafter, anionic polymers in
combination with the cationic polymers were
investigated and a further reduction in cost was
realized. Over the years, chemical conditioning
-------
SYSTEM FOR ST. LOUIS 209
costs haye steadily decreased from $5.00/ton in
1969 to $1.70/ton in our last fiscal year of 1972-73.
Lemay has three 11-hearth, twenty-to and one-
half foot diameter incinerators. The vacuum filter
cake is belt-conveyed into the top of these
incinerators. Complete combustion of the organic
portion of the solids takes place along with the
complete evaporation of moisture and seven-fold
reduction in solids volume.
As with the vacuum filters, a more efficient
incineration operation has been achieved over the
past six years. During the first two years of
operation, some problems were encountered due to
the high moisture content of the filter cake
discharged to the incinerators. The moisture, along
with the low volatile content, caused excessive gas
consumption for heat of evaporation resulting in
high operating costs. The additional heat
requirements created severe operating problems.
The rabble arms on both inhearths 4 and 6 had to be
replaced more frequently than normal due to the
excessive amount of heat rising through the center
drop hole of the incinerator on these hearths.
With the lower moisture content and the higher
volatile solids content achieved by the elimination
of the bentonite clay, the quantity of gas required
to maintain the incinerators at operating
temperatures between 1400 and 1600°F decreased.
Gas consumption was reduced from 125 therms per
dry ton to 40 therms per dry ton. The volatile
content of the solids now contributes 90 percent of
the total heat requirements for the incinerators
operation. The additional ten percent heat is
obtained by burning natural gas. In 1972-73 this
amounted to $2.34/ton incinerated. With the
reduction of gas usage, the problem of warping
arms in hearths 4 and 6 was significantly reduced.
Clinker formation in the incinerator was also
reduced considerably.
Incinerator ash is screw-conveyed into a slurry
tank where primary effluent is added. This ash
slurry is then pumped three-fourths of a mile to
three ash-basins. The ash settles out readily upon
entering the basins and the supernatant is decanted
to the Mississippi River. The ash basins at Lemay
occupy a total land area of 13 acres. At present
loading rates the basins will provide ash storage for
approximately 20 years.
Bissell Point
The Bissell Point Plant provides primary
treatment for a current dry weather flow of 120
mgd which is 60 percent domestic wastewater and
40 percent industrial wastes. Less than 15 percent
of the. flow is fro..n a sanitary system while the
remainder is from an old, combined sewer system
within the City of St. Louis.
The wastes treated are strong with COD that
fluctuate from 600 to 2900 mg/1, BOD which
average 300 mg/1 and suspended solids which
average 335 mg/1. The suspended solids removal in
the primary basins averages 55 percent resulting in
92 dry tons of solids to be treated daily. In addition,
approximately eight wet tons of grit are removed
each day.
The vacuum filters are of the cloth-belt type with
a surface area of 500 square feet designed for a
maximum vacuum of 24 inches of mercury (12 psi).
A total of ten filters can serve the five incinerators.
A common conveyor belt receives the sludge cake
from a pair of filters and delivers it either to an
inclined belt for direct feed to the top of the
incinerator or to a transfer belt for feed to another
incinerator.
Excellect filtration results were obtained during
the spring of 1971, before all the sewage was being
intercepted using lime (about five percent) and
ferric chloride (less than one percent). Sludge cake
moisture contents were on the order of 65 percent
with yields holding at about 5.5 pounds per square
foot of filter surface. Cost per dry ton of
conditioned sludge averaged about $1.50 during
this period.
A problem with the lime unloading system
occurred early in May, however, and the
conditioning system had to be altered to handle a
single liquid cationic polymer that was at that time
being used at the Lemay Plant. This was
supplemented in late July with a dry anionic
polymer. Generally, yields dropped significantly,
averaging below 2.0 pounds in September and
consequently costs rose and held at above ten
dollars per dry ton. The lime-ferric system was back
on line by mid-November and costs dropped to
about three dollars per ton and yields as high as ten
pounds per square foot were achieved.
Since then, lime and ferric chloride have been the
basic sludge conditioning chemicals used at the
plant although there have been several plant-wide
runs with polymers of different manufacture.
These experiences have indicated that the polymers
used could only be successful in the winter months
when there is no chance of septicity, and that
consistency of results with polymer conditioning
was virtually impossible to achieve over even a
modest period of time.
Current experience at Bissell Point with lime and
ferric chloride for sludge conditioning indicates
yields of about nine pounds of dry solids per square
foot per hour with a lime addition of 11 percent and
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210 MUNICIPAL SLUDGE MANAGEMENT
ferric chloride at about 3.5 percent, at a cost of
$5.85 per dry ton of sludge filtered.
Bissell Point is less fortunate than Lemay, in that
two ash basins with a total capacity of only 160,000
cubic yards are available for temporary ash storage.
Approximately 50 tons of ash per day are slurried to
the basins for storage. One basin is presently full
and will require removal of the ash to another fill
site. Approximately twice as much ash as at Lemay,
about 18,000 dry tons per year, must receive
ultimate disposal.
CONCLUSIONS
The Metropolitan St. Louis Sewer District chose
two types of solids handling systems for its two
major subdistricts. Anaerobic digestion with
storage lagoons was selected for the secondary
plant at Coldwater Creek which serves an area of
41 square miles and has a design average flow of 25
mgd.
The digestion process was selected for the
smaller plant because adequate land area was
available for storage of digested sludge. The
methane gas from the digestion process was
available for engine operation to drive blowers and
pumps at an economical cost for the secondary
process.
Vacuum filtration, incineration and ash storage
basins were selected for the two Mississippi River
subdistrict plants—Lemay and Bissell Point. Both
plants provide primary treatment for an average
design flow of 424 mgd and serve a watershed of
196 square miles. Vacuum filtration with
incineration was selected for the large Mississippi
River subdistrict plants because of the greater
volume reduction of solids and the inert
characteristics of the ash material. This
significantly reduced the land requirement for
these two plants with large flows and high
concentrations of solids.
Both of the above solids systems were selected
based on the needs of the individual watersheds.
Both systems have provided an excellent degree of
solids treatment at their specific location.
-------
THE ENVIRONMENTAL PROTECTION AGENCY'S
RESEARCH PROGRAM
WILLIAM A. ROSENKRANZ
Municipal Pollution Control Division
Office of Research and Development
U. S. Environmental Protection Agency
Washington, D.C.
I am pleased to participate in this National Con-
ference on Municipal Sludge Management because
of the importance of providing environmentally
sound alternatives for the handling, disposal, and
utilization of sludges generated at municipal
sewage treatment works.
Sludge processing and utilization are clearly a
most important factor in the design, operation and
costs of wastewater treatment. For example, ap-
proximately 35 percent of the capital costs and 55
percent of the annual operation and maintenance
costs are associated with sludge production.
The Water Pollution Control Act Amendments
of 1972 set deadlines for the implementation of
secondary and best practicable treatment for
municipal wastewater which will require up-
grading of a large portion of the wastewater treat-
ment works in the country. This upgrading of
treatment levels will result in increased volumes of
municipal sludges. The Marine Protection,
Research, and Sanctuaries Act of 1972 (PL 92-532)
additionally limits the practice of ocean disposal of
sludges. These laws will impact the sludge disposal
problems and practices of most metropolitan areas
of the country.
The Environmental Protection Agency's
municipal waste inventory indicates that ap-
proximately 34 percent of the sewered population
is served by less than secondary treatment. An ad-
ditional five percent (7.3 million) is served by treat-
ment systems which, in large measure, do not meet
the performance requirements of secondary treat-
ment. Thus, treatment facilities for 39 percent of
the total U.S. population will require upgrading to
meet new requirements. Upgrading will have a sub-
stantial impact on the total quantity of sludge
generated nationwide.
New and improved technology must be
developed and demonstrated to meet the current
and future needs of municipalities. The task of
achieving an orderly improvement of sludge dis-
posal practices must be a cooperative effort by local
jurisdictions, state agencies, private industry and
the Federal Government. The nine billion dollars to
be made available for the construction grants
program through FY 1975 will assist an estimated
6,000 projects, many of which will be wastewater
treatment facilities generating large volumes of
sludge. The research arm of EPA is deeply involved
in the development and demonstration of new and
improved technology to support the continuing
construction grants program.
Research and development associated with
municipal sludges is a key element in our program.
About 20 percent of our resources are specifically
allocated to sludge problem solving. It should be
noted also that nearly all other technical areas
within the Municipal R&D program are involved in
some way with sludge. Development of new treat-
ment processes nearly always results in the produc-
tion of more sludge or changes the sludge
characteristics. Sometimes both happen. Sludge
aspects, therefore, are considered in everything we
do. One of the reasons we were interested in the
development of pure oxygen activated sludge
systems was that such systems offered the poten-
tial to produce less sludge with improved dewater-
ing characteristics. Phosphorus removal processes
produce more sludge which is difficult to dewate'r.
Control of combined sewer overflows will result in
211
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212 MUNICIPAL SLUDGE MANAGEMENT
greatly increased quantities of sludge to dispose of.
It has been estimated, for example, that control of
combined sewer overflows in Washington, D.C.
will double the amount of sludge to be handled. In
short, all portions of the collection, transport and
treatment system are inter-related and all have an
influence on the volume and characteristics of the
resulting sludges.
One of our major objectives is to demonstrate
developed technology at full-scale in order to
evaluate cost and performance, the information re-
quired by treatment works planners and designers
before new technology can be placed into field prac-
tice.
Examples of such demonstrations include the
Cedar Rapids pressure filter project and the Denver
aerobic digestion project discussed in detail earlier
in the Conference. Two other recent projects
worthy of note include a "top-feed" vacuum filter
developed under an EPA contract and a capillary
suction dewatering device. The "top-feed" filter
will be demonstrated at Milwaukee. Estimates of
cost and performance indicate that sludge de-
watering savings, with full conversion to the new
filters, would amount to about one million dollars
per year in capital and operating costs. Filter yields
and cake solids are expected to be significantly
greater than bottom feed filters.
The unique capillary suction device was
developed for dewatering activated sludge with
minimum use of conditioning chemicals, coupled
with reasonable capital and operation and
maintenance costs. St. Charles, Illinois will conduct
the full-scale demonstration.
New initiatives in the beginning stages include:
a. Investigation of the applicability of pyrolysis
as a sludge disposal technique. This work is
being conducted by the Bureau of Mines
through an interagency agreement with EPA.
The Bureau will apply its past research
expertise on pyrolysis of coal, utilizing pilot
scale units, to thermally degrade differing
sludges and sludge-solid waste mixtures.
Pyrolysis may offer advantages of minimal air
pollution and by-product recovery in the form
of oil or gas. The project will yeild information
on processing conditions nature and amounts
of by-products, air pollution characteristics
and identification of any new water pollution
problems which may develop through use of
the process.
b. An interagency agreement with the Depart-
ment of Agriculture will result in an evalua-
tion of land application and filling procedures
for dewatered sewage sludge. Work will in-
clude identification of the effects of nitrogen
form and movement, pathogen persistence
and movement, metals presence and plant up-
take.
c. Information on thickening and dewatering
rates of sludges generated by phosphate
removal processes will be obtained under a
contract with Envirotech-Eimco Division.
Better selection of process hardware will be
possible once this information is available,
hopefully resulting in lower processing costs.
d. Seattle METRO will demonstrate and
evaluate the application of sludge in a forest
environment. It is expected that the project
will identify the effects of sludge application
on forest growth, establish effective methods
of application, establish application rates
which can maximize forest growth and
minimize impact on ground and surface
waters and establish short-term effects on the
fore§t organisms, physical and chemical
characteristics of forest soil and the chemistry
of soil water.
Technology areas identified for early starts in-
clude demonstration of anerobic or aerobic ther-
mophillic digestion; evaluation of wet oxidation;
disinfection of sludge by pasteurization, sonics and
radiation and further work on sludge incineration
or co-incineration with solid wastes.
Efforts toward investigation of the fate and
effects of heavy metals, pathogens and nitrates for
land application systems should be greatly ex-
panded as soon as possible. The health effects
aspects of sludge utilization and disposal must be
resolved in order to adequately identify the "effec-
tiveness" portion of the cost/effectiveness picture
so important to planning, design and public
relations.
Conference discussions have concentrated
almost exclusively on unit processes. Sludge
thickening and dewatering, anaerobic and aerobic
digestion, heat treatment, land application and
several other subjects have been c'overed. This
seems to be the approach at most conferences. I
don't want to discount the validity of this approach,
since it is the stuff that most research is made of
and a large number of plant operating problems
seem to focus on unit processes. I suggest that we
should include the techniques of system planning
and design in our technical meetings.
When we design and construct treatment works
we are not designing and building unit processes,
we are designing and building systems. Once the
works are in place, the manager must operate a
treatment system. Mr. Garrett alluded to this in his
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EPA'S RESEARCH PROGRAM 213
paper earlier in the program, as did Mr. Cassel a bit
later.
Recent changes in water pollution control re-
quirements, national economics and our national
resource picture, particularly energy, make it ex-
tremely important that more thought be given to
control and treatment systems as opposed to unit
processes. It should be routine to carefully examine
all aspects of the treatment system so as to obtain
lowest system capital cost, efficient and dependable
performance, minimum O&M costs, minimum
energy requirements and full utilization of energy
sources available within the system. Hundreds of
alternatives and trade-offs are possible in es-
tablishing the system of choice. Time does not per-
mit detailed discussion of this subject, but I would
like to encourage discussion of techniques that
have been or could be used for this purpose during
future conferences. Complicated study matrices
are required for the large metropolitan areas and I
know that the Metropolitan Sewer Board of the
Twiri Cities used this approach in selecting their
future system.
In closing, I hope that the modest Federal
research and development efforts will be
supplemented and complemented by the private,
municipal and State sectors. Active participation by
all sectors will be necessary to solve our sludge
problems. The EPA involvement is to a large extent
for the purpose of assistance in assuming the risks
associated with research, development and
demonstration. The private and municipal sectors
must be involved to ensure that technology ad-
vancements will offer truly cost/effective alter-
natives.
Movement of improved technology into practical
field application will not be accomplished with
reasonable speed unless the technology can be
demonstrated in full-scale installations. Such
demonstration is necessary to determine operating
efficiencies, characteristics, costs and design data.
I would like to encourage each of you to help
achieve these objectives. Without your full support
and interest it will be extremely difficult to advance
technology toward more cost/effective solutions to
our many pollution problems. I encourage you to
develop new ideas and concepts to deal with the
sludge problem and keep us informed of your ideas
and needs.
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SUMMARY OF "PRETREATMENT AND ULTIMATE
DISPOSAL OF WASTEWATER SOLIDS
CONFERENCE" - HELD MAY 21-22,
RUTGERS UNIVERSITY
ROBERT W. MASON
Region II
U.S. Environmental Protection Agency
New York, New York
There was a total of 15 papers, the first nine
dealing with characteristics of sludge, its stabiliza-
tion, incineration, thickening, dewatering,
chemistry, and economics. The next six dealt with
various aspects of ocean disposal.
Dr. ].B. Farrell, EPA, presented an "Overview of
Sludge Handling and Disposal", (contents of paper
not summarized here).
Mr. B.V. Salotto, EPA, summarized analytical
results on sludges, principally of the digested type,
taken from 33 wastewater treatment plants in the
United States. They had been analyzed for 20
metals, nitrogen, phosphorus, and sulfur. The BTU
value of some sludges was also determined. Atomic
absorption method was used for the determination
of metals. No detectable amount of beryllium was
found in any sample analyzed.
Mathematical analysis of the data indicated that
the distribution of heavy metals in sludge is ap-
proximately log/normal. This behavior is
characteristic of all sludge types analyzed thus far.
Comparison of the levels of metals in the United
States sludges with corresponding levels in Scan-
danavian sludges show higher levels in the United
States sludges. Variation of any one metal in
sludges of a particular wastewater treatment plant
was much less than in sludge samples taken from
different plants.
Major emphasis of continuing work will be to
more accurately define the composition of sludges
produced by secondary treatment plants processing
municipal wastewaters.
Mr. C.A. Counts, Battelle Northwest, discussed his
work on the stabilization of municipal sludge by
high lime dosage. There were two objectives: (l)
determination of the degree of stability produced
by large lime doses, and (2) the effect of spreading
lime sludges on crop lands.
The conclusions were: (1) the lime dosage re-
quired to raise the pH to 11 or 12 varied as the
sludge character, that is, the higher the solids con-
tent the higher the lime dosage required, (2) the
pHs decayed over time (24 hours being the chosen
time) unless an excess of lime was employed, (3) a
pH of 12 resulted in 99 percent inactivation of the
bacterial population, although fecal strep was fairly
resistant, (4) to prevent regrowth of the bacterial
population it was necessary to maintain a high pH,
(5) the odor of the sludges was pronouncedly
decreased by lime treatment, (6) lime treatment
improved the settling characteristics, and (7) im-
proved the soil productivities measured in both
green house and outdoor test plots.
The treatment is cheap (about ten dollars per dry
ton) and is recommended for small treatment
plants, as an auxiliary when the plant exceeds
capacity, or in the case of emergency.
The next paper by Stephan Hathaway of EPA dealt
with thickening characteristics of treated sludge.
Phosphate removal from municipal wastewater
can be accomplished by adding Al or Fe salts to the
primary clarifier. Sludges produced differ from
conventional primary sludges in thickening and
other dewatering characteristics. The thickening
characteristics of these sludges have been in-
vestigated using bench-scale gravity thickening
equipment and a pilot-scale air flotation thickener.
Both the Al-primary and the Fe-primary sludges
show poorer thickening than do conventional
primary sludges. Gravity thickening of both types
of sludges was ordinarily poor. However, if the
sludges were diluted with effluent, thickening rates
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216 MUNICIPAL SLUDGE MANAGEMENT
and solids content of settled sludge increased sub-
stantially.
Factorial experiment demonstrated that an in-
crease in the level of either Al or Fe in the sludge
produced poorer air flotation results—solids con-
tent of floated sludge was lower and losses to un-
derflow were higher. Polymer addition reduced
losses to the underflow and increased solids con-
tent of the floated sludge.
Mr. Darryl Cook, Eimco Division of Envirotech
talked about dewatering of sludges.
Physical chemical sludges produced by coagula-
tion precipitation in a 100 gpm pilot plant were
thickened and then dewatered. Parallel laboratory
studies were performed with the pilot plant on
identical sludge samples in an effort to improve the
efficiency of the dewatering and obtain a correla-
tion where possible.
Lime, alum, and ferric chloride sludges were
produced, polymer being used with the alum and
ferric chloride studies.
Lime sewage sludges were found to dewater very
well with no chemical addition. Alum sewage
sludge dewatered well when lime was added as a
conditioning chemical. Ferric chloride sewage
sludge contained large amounts of ferrous sulfide
which hindered dewatering. There were some in-
dications that ferric chloride sewage sludge con-
taining ferrous sulfide could be dewatered using a
conditioning pretreatment of ferric chloride
followed by lime.
In a discussion of "Thermal Degradation of
Sludge", R.A. Olexsey, EPA pointed out that in-
cineration is an increasingly important sludge dis-
posal technique and currently accounts for about
25 percent of the sludge disposed of in the United
States. Before a sludge can be incinerated it must be
dewatered to a solids content approaching 30 per-
cent. This dewatering is expensive since mechanical
dewatering costs average twelve dollars per dry ton
of sludge.
The multiple hearth is the rnost common type of
incinerator used for sludge combustion although
the fluidized bed is becoming increasingly popular.
Other types of incinerators used are the flash
dryer, the rotary kiln, and the cyclone furnace.
Sludge incinerators consume considerable
amounts of auxiliary fuel and can contribute to air
pollution if not properly controlled.
Some of the feasible alternatives to sludge com-
bustion are pyrolysis, combined incineration with
solid waste, and wet oxidation. These methods are
largely in experimental and demonstration stages.
R.L. Kaercher in a paper on incineration design
presented a general discussion of incineration
problems and, together with a discussion of the
Federal and State emission regulations, a plea for
more reasonable standards was made.
Some of the problems with each were considered
while the multiple hearth followed by wet gas
scrubbing was presented as the most practical.
D. Derr of Rutgers University outlined the economics
of sludge disposal systems. He suggested
procedures for the estimation and evaluation of
costs for alternative sewage sludge disposal
systems.
On considering the various disposal methods, (l)
incineration, (2) landfill, (3) ocean disposal, and (4)
land disposal other than landfill, he analyzed each
method into its component parts and generalized
these into the following basic processes: transport,
dewatering, and storage.
Solids concentration was confined to two levels,
5 percent and 30 percent.
In the various options the dewater-no-dewater
option is incorporated.
Flow charts were presented for the various dis-
posal methods combining alternate routes.
Employing standard financial procedures the
authors calculate in cost per dry ton the various
steps in the disposal system so that by suitable com-
bination the total cost of the proposed system can
be estimated.
Dr. ].V. Hunter, Rutgers University talked about
"Future Problems in Sludge Production Handling
Systems". The basic chemical reactions in alum,
ferric chloride and lime treatment to remove
phosphate were discussed. While coagulants used
together with alum or ferric chloride produced a
greater weight of sludge, increased size of digestion
facilities will probably not be required. However,
the presence of the aluminum, iron or calcium salts
may interfere with the digestion process.
Anaerobic digestion of sludge containing
aluminum phosphate or iron phosphate does not
result in appreciable dissolution of the phosphate.
The solids content of treated sludges increases in
the order of alum, iron chloride, and lime. A similar
situation prevails for the dewatered sludge.
Some work has been done on regeneration of the
inorganics, the lime offering the most promise of
success. However, a viable process is still a long way
from operational, if indeed it ever will be.
The next six papers were devoted to ocean dis-
posal of sludge.
Dr. W.F. Rittal, Pacific Northwest Environmental
Research Laboratory, EPA, Corvallis, Oregon,
presented a paper on the Koh-Chang model for the
barged release of sewage sludge to coastal waters.
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SUMMARY—RUTGERS CONFERENCE 217
While this model has not been verified under field
conditions, it has been evaluated by several other
agencies. The model has three computer programs,
for barges with puff, jet and wake-plume convec-
tion. The capabilities of the model were
demonstrated by showing the results of the calcula-
tion for disposal of sludge in the New York Bight
under both summer and winter conditions. The
charts and curves have been computed for depth
versus waste-cloud drift, dilution versus time,
depth versus concentration, and other parameters.
There is no other procedure or model available to
the regulatory programs which provides this
degree of analysis. It will therefore be of significant
value to those with the responsibility of safeguar-
ding the marine environment.
Presently, a study to modify the model for es-
tuarine use is being funded.
Dr. Mullenhoff, Oregon State University, described in-
teresting research on laboratory studies of sludge
degradation. Aerated sea water was passed over an
inch deep sludge bed contained in a glass vessel. In
experiments whose duration was up to 80 days the
water and sludge were exposed to both normal and
elevated pressures. Oxygen uptake by and carbon
content decrease in the sludge were determined.
Methods of analysis including a sludge
respirometer for oxygen uptake are described.
Curves of the total carbon content were drawn as a
function of time. Both the surface and the bottom
of the bed showed initial rapid decreases for the
first 40 days and then a plateauing for the next 40.
The reaction was treated as first order and a rate
constant derived. Equipment has been constructed
to carry out similar studies on deeper sludge beds.
He also described some underwater sludge
studies conducted for a week in the Bahamas at a
depth of 150 feet.
Mr. Richard Dewling, Director, Surveillance and
Analysis Division, EPA, Edison, New Jersey described
the dumping problems in the New York Bight and
the Philadelphia areas where large quantities of
dredge spoils, sludge, waste acid and toxic materials
are dumped annually. The problem of sludge will be
exacerbated when, with the completion of secon-
dary treatment plants in New York City, sludge re-
quiring disposal will be tripled.
Although the sludge depth is only five feet deep
in the Bight, the depth of the dredge spoils is about
40 feet. NOAA is presently selecting two alternate
sites for sludge dumping to be used if an emergency
develops. Despite reports of a "sludge monster"
creeping along the bottom towards the New York
beaches, Mr. Dewling believes the dumping is safe
so far as recreational waters are concerned for
three to five years. At that time a solution will have
to be available. Perhaps controlled ocean dumping
in different areas for limited times to allow
recovery of the benthic life will be the answer. Both
incineration and landfilling, as far as New York is
concerned, have definite drawbacks.
Dr. Portmann, Ministry of Agriculture in England
reported the British view on ocean disposal of
sludge. They have been dumping from Manchester,
Glasgow and London increasing quantities of
sludge for the past 50 years until the total is now
some 8,000,000 tons annually. Extensive tests have
shown little damage to benthic fauna or commercial
fisheries. This explains why the authorities con-
tinue to be favorably disposed toward sewage
sludge disposal at sea. For the past five years dump-
ing has been regulated by a "voluntary scheme"
although a new dumping law is pending in Parlia-
ment. The British sludge problem is probably
abated by the large tides, reaching 20 feet in the
Thames, and the consequent swift currents which
disperse the sludge very rapidly. Dr. Portmann
presented data on the sludge heavy.metal content.
He also described the use of a radioisotope of silver
in following the sludge drift after dumping. In-
creases in ocean dumping are largely traceable to
the growing sentiment against land disposal which
is generated by increased distance transport costs
and by the fact that most sewage sludge results
from a mixture of domestic and industrial wastes.
Mr. F.K. Mitchell, University of California at Berkeley.
In Santa Monica Bay about 1.3 million gallons of
sludge diluted with 3 million gallons of secondary
effluents to facilitate pumping are discharged daily
from an outfall seven miles into the bay at the edge
of the Santa Monica Canyon at 320 feet. From a se-
cond outfall five miles long 335 mgd of combined
primary and secondary effluent are discharged.
Both outfalls have been in operation since 1960.
Depth profiles of sedimental heavy metals concen-
trations indicate that the depths of significant
quantities of sludge particulates are greater than
one foot near the outfall and one foot or less at dis-
tances greater than two miles down canyon from
the discharge. Chlorinated hydrocarbons in the
surface sediments present a picture similar to that
of the metals. Total DDT and PCB concentrations
are higher in the canyon in the shallow areas of the
bay and are highest by far at locations closest to the
sludge outfalls. Biological monitoring in the vicinity
of the discharge shows that the area is by no means
a biological desert.
Dr. D. Dorman, Monmouth College, reported on
bioassay methods. A multidisciplinary approach
appears to be the most satisfactory one. Deter-
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218 MUNICIPAL SLUDGE MANAGEMENT
mination of the disposal site far enough in advance
of its use to allow adequate base-line data surveys,
including chemical, physical, and biological
parameters, should be made. Subsequent field and
bioassay monitoring, on a programmed basis,
would then be utilized after waste disposal began.
In addition, no single test organism can provide the
necessary data to determine the impact evaluation
of a disposal site. Extrapolation of the responses of
one species, or of several species, might not include
the response of the most sensitive species if that
organism was not included among the test species.
A multidisciplinary approach with constant
monitoring would possibly provide enough data to
assess the impact on single species and, more im-
portantly, in the food web. Additional safeguards,
as promulgated by bioassays and impact evaluation,
could then be made at the effluent and waste dis-
posal sources to reduce possible toxicants to levels
manageable by marine organisms.
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