ort a
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An environmental protection publication in the solid waste management
series (SW-159). Mention of commercial products does not constitute
endorsement by the U.S. Government. Editing and technical content of
this report were accomplished by the Systems Management Division of the
Office of Solid V/aste Management Programs.
Single copies of this publication are available from Solid Waste Information,
U.S. Environmental Protection Agency, Cincinnati, Ohio ^5268.
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ANAEROBIC DIGESTION OF SOLID WASTE AND
SEWAGE SLUDGE TO METHANE
By Steven J. Hitte*
Objectives
The primary objective of this report is to evaluate the potential
for processing organic wastes (solid waste and sewage sludge) using
a controlled anaerobic digestion process for the purpose of producing
methane. Controlled anaerob'ic digestion is a biological process
whereby organic matter decomposes in a regulated oxygen-deficient
environment. This report is intended: (1) to present a way in which
the national energy shortage can be reduced by producing methane
from anaerobic digestion of municipal solid waste and sewage sludge;
(2) to compare and describe this biological process with other
resource recovery concepts; (3) to summarize the current research
being performed in anaerobic digestion; (*») to present an estimated
cost analysis of a 1,000-ton-per-day (TPD) solid waste and sludge
digestion faci1ity.
Energy Demand
Anaerobic digestion for the conversion of waste materials to
methane is one possible means to offset the increasing shortage of
natural gas. The total United States energy demand in 1972 was
approximately 72 quadrillion (lo'->) Btu and is projected to exceed
96 quadrillion Btu by 1980. Natural gas (methane) supplies 32 percent
of this total energy demand (23 quadrillion Btu).1^'2 Yet the nation
reserves of energy, particularly natural gas, will be able to provide
only a decreasing fraction of projected energy supplies. New develop-
ments in technology can help to develop new supplies of energy.
Production of natural gas through anaerobic digestion of solid waste
and sewage sludge is one such new technology that can increase the
nation's supply of energy.
Potential Market
The potential market for a process which converts solid waste
and sludge to methane is significant. There is a potential market
for over 200 1,000-TPD solid-waste and sludge-to-methane facilities
* Mr. Hitte is a mechanical engineer in the Systems Management
Division of EPA's Office of Solid Waste Management Programs.
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in the urban areas of the United States. With the current municipal
solid 'waste generation rate of 3 to 5 pounds per person per day and
a sewage sludge generation rate of 0.3 to 0.5 pounds per person per
day, a population of approximately 500,000 could provide enough waste
to supply one 1,000-TPD facility. According to the 1970 United States
Census, there were 26 cities in the country with populations in excess
of one-half million. More significantly, there are 65 Standard
Metropolitan Statistical Areas (SMSAs) in the United States with
populations in excess of 500,000. The aggregate population of these
SMSAs is in excess of 100 million, half of the nation's' population.
Bioconversion of solid waste and sewage sli/dge is one energy
conversion option. Based on data from bench-scale experiments, a
1,000-TPD bioconversion facility could produce approximately 3-6
million cubic feet of methane per day based on a conservative value of
1.8 cubic feet of methane generated per pound of municipal solid waste
and sewage sludge. '**' The 65 SMSAs with populations in excess of
500,000 therefore have a potential for methane production in excess
of 720 million cubic feet per day. Based on figures published in the
1973 edition of Browns Directory of North American Gas Companies,
this process, if implemented in those 65 SMSAs, could supply a small,
supplementary percentage of the total natural gas consumed in the
United States. In addition, animal, crop, and some industrial wastes
represent the potential of an additional 13 billion cubic feet per day
of methane (20 percent oft, the natural gas demand), although the,eco=
nomics of collection and transportation may restrict their use. '^
These wastes are not considered to be a viable potential for purposes
of this paper.
On a local basis, natural gas produced from municipal solid ;
waste can supply higher percentages of total gas consumption. For
example, if all the waste in the Cleveland SMSA (Cuyahoga County)
(1'970 population:1 2,064,000) could be utilized, 5.3 billion cubic:
feet per year of methane'could be produced, approximately 2.8 percent
of Cleveland's natural gas demands.*
These projections show that methane produced from solid waste
can contribute as a supplemental source of energy. This comes at'
a time when energy shortages and rising solid waste disposal costs
are forcing many major communities to reevaluate their refuse
disposal practices.
* The natural gas sales volume of the Cleveland Division of the
East Ohio Gas Company in 1973 was 190 billion cubic feet. The popu-
lation of Cleveland in 1970 was 751,000. The potential for gas
production from wastes in the City of Cleveland is 1.9 billion cubic
feet per yeac.approximately 1 percent of Cleveland's natural gas
demands.2'P-'25 y
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Energy Products from Resource Recovery Concepts
Municipal solid waste is one raw material currently being dis-
carded that can be "mined" for its energy content. Presently, many
different approaches to recovering this energy are being examined.
Included in these resource recovery concepts are: (!) shredded and
classified solid waste as a supplemental fuel, (2) pyrolysis,
(3) waterwall incineration, (A) hydrogasification, (5) methane
production. All of these technologies enable solid waste to be con-
verted into a number of energy forms, including solid, liquid, and
gaseous fuels, and steam and electricity; they also offer the oppor-
tunity of front-end recovery of valuable materials.
To be marketable, these energy products must be produced at a
cost competitive with the fossil fuels they supplement or replace.
This cost is indirectly related to the energy recovery yield of the
system. The equation for this yield is as follows:
ERY
Eo - Ec
EA
Where E^y = percent of energy recovery yield
Eo • = usable energy out of the system measured in Btu
EC = total energy consumed by the system, measured in Btu
E^ - energy available in solid waste based on ^,500 Btu/
pound of waste
For most of these systems, the energy recovery yield ranges from
20 to 30 percent.
Anaerobic digestion is the only known process that produces
an energy form (methane gas) in large quantities that can be used
directly by the consumer for home heating, cooking, and other such
purposes." When the gas produced is cleansed to pipeline quality
(1,000 Btu per cubic foot), it could be easily marketed because it
can be injected directly into a local utility pipeline system. The
indications from a telephone survey are that utilities are very
positive about purchasing even small quantities of this high Btu
gas, as long as the price is competitive with other sources of gas.
Current Research
Presently, there are a number of groups studying the bioconversion
process. Dr. Perry McCarty, Stanford University, and Dr. Clarence
Goluecke, University of California in Berkeley, are two such researchers
* All other energy recovery processes require conversion to
steam or electricity.
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who have performed studies in bioconversion of organic waste^to methane.
Much of their work has been published in scientific journals and pro-
ceedings from various conferences. »p>'>?° Dr. John Pfeffer, University
of Illinois, has also done substantial research in the temperature
ranges and the various dewatering processes to Increase the efficiency
of this process. The Dynatech Corporation, Cambridge, Massachusetts,
has concluded a paper study on the economics of the anaerobic digestion
process. Computer models were designed to incorporate all parameters
of this process to determine the economy of scale. '"'
In Franklin, Ohio, research work Is underway to investigate the
feasibility of combining solid waste with sewage sludge, having the
mixture digested and capturing the energy value through the production
of methane for use as a fuel. In this process, wet-processing may
be advantageous because small particle sizes and large quantities
of water are needed to create optimum conditions for decomposition.
This work is being done in conjunction with a project funded formerly
by EPA with the Black Clawson Company.
Dr. S. Ghosh and Dr. D. Klass, Institute of Gas Technology, have
performed various experiments on varying the particle size of the
solid waste fed into a digester to increase gas production. Their
findings indicate that the finer the particle size, the higher the gas
yield.3'P'7
The engineering department at the University of Arizona has
performed bench-scale work (100 gallons) on digesting combined raw
sewage and solid waste. Scaled-up work for a 20,000 gallon in-ground
digester heated by solar energy has recently been completed. The
cleaned methane will be used for local needs with the remaining C0£ /•
supporting a greenhouse and the residue acting as a soil conditioner.
In the summer of 1975, the Energy Research and Development Administration
(ERDA) awarded a multimillion dollar contract to construct and demon-
strate the feasibility of producing methane gas from the solid waste
stream. This will be a four-year study incorporating a design capacity
of 50 to 100 TPD.
Other industries and universities are studying this process and
dispersing their findings through conferences and publications. With
all this interest in waste digestion, the near future might provide
some sound conclusions and technology on whether this process is viable.
Biological Process
Anaerobic digestion of complex organic wastes is a two-stage
process (Figure 1). In the first stage, the acid-forming bacteria
act upon complex organics and change the form of complex fats, proteins,
and carbohydrates to simple soluble organic materials, commonly known
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as organic or volatile acids. The second stage involves the fermenta-
tion or gas-generation phase which produces the desired methane gas.
In this step, the methane-forming bacteria use the organic acids
produced in the first stage as substrate and produce the end products;
carbon dioxide (C0«), methane ((%) and traces of hydrogen sulfide
(^S) . The quantifies of these off-gases can vary but the mixture
consists of roughly 50 percent CO^ and 50 percent CH^.
Comp 1 ex
organics
Staqe 1. \
Acid-forming '
Organic
(volatile)
acids
Staae 2 ^
Methane-forming '
CH/i
and
C02
Figure 1. This diagram illustrates anaerobic digestion of complex
organics, which are defined as large molecular chain structures con-
taining carbon, hydrogen, glucose, cellulose, etc.
Parameters Controlling Methane Production
To attain continuous digestion, a proper balance between the
acid-forming bacteria and the methane-forming bacteria is required.
Optimum levels of five environmental parameters are essential to the
establishment and maintenance of this balance; these parameters are:
temperature, anaerobisis, pH, nutrients, and toxicity of input.
Temperature is an important operational parameter in an anaerobic
digestion process. As temperature increases, biological reactions
proceed much faster, and this results in more efficient operation and
lower retention-time requirements which may vary from k to 30 days.
Two temperature levels have been established: in the mesophilic level
the temperatures range from 30 to 45°C
range from k$ to 60°C. Although rates
level are much faster due to increased
in the mesophilic level, the economics
in the thermophilic level they
of reaction in the thermophilic
bacteria formation than those
of most sewage sludge digestion
systems have indicated operation in the mesophilic level.*
* Much controversy over the temperature levels has been voiced
by various researchers. The debate is over the efficiency and
economy of operating a digester at these temperature levels. Some
bench-scale experiments have been performed varying the temperature
and monitoring the gas produced with the retention time, but results
are not consistent and may not be applicable to a full-scale system.
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Another environmental requirement for anaerobic digestion is the
maintenance of atiaerobic conditions (anaerobisis) in the digester.
The methane formers are strict anaerobes and even small amounts of
oxygen can be quite detrimental to them. This necessitates a closed
digestion tank which excludes oxygen while also facilitating collection
of the methane produced.
The third environmental requirement for optimum operation is
proper pH control. Anaerobic digestion can proceed quite wel1 under
slightly acidic conditions, with a pH varying from 6.7 to 7.0./>p>
Beyond these limits, anaerobic digestion proceeds with decreasing
efficiency. Under more acidic conditions, a pH of 6.2 or lower,
waste stabilization ceases. Control of pH is exercised by the addi-
tion of an alkali [sodium bicarbonate (NaCCO-).)], which has recently
been found to control pH better than lime.
The bacteria responsible for waste fermentation in the anaerobic
process require nitrogen, phosphorus, and other materials for optimum
growth. Therefore, another important environmental condition i's the
presence of the required nutrients in adequate quantities. These
nutrients are measured against a carbon-nitrogen (C-N) ratio. The C-N
ratio of solid waste is not sufficient for maximum digestion, hence
the addition of sewage sludge, which adds nitrogen to create a more
favorable ratio, is necessary.
For successful anaerobic treatment, the fifth environmental
parameter, that of toxicity of input, must be at the level where the
waste is free from toxic materials. These inhibitory materials range
from inorganic salts to toxic organic compounds. Control of toxicity
can be achieved by removal of toxic materials by chemical precipitation
within the digester and by dilution of the waste stream below the toxic
threshold of the toxicity-causing material by such means as increasing
the moisture content of the slurry. *
Once these five parameters have been established and maintained
at their optimum levels, then production of gas should occur naturally.
The methane remaining after gas cleansing has a heating value of
1,000 Btu per cubic foot. This is pipeline quality and acceptable
from a gas company's viewpoint.
Process Description (Conceptual Discussion)
As devised by the researchers on their benchrscale experiments,
the physical apparatus for producing methane gas from municipal solid
waste and sewage sludge could be divided into four areas of operation
(Figure 2): (l) waste handling and mixing, (2) digestion, (3) gas
treatment, (k) effluent disposal.
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Market
Receiving
Size reduction
Separation
Rejects
Mixing
Sludge
Recovery
Gas
scrubber
Liquid
disposal
Digestion
Residue
disposal
Nutrients
Figure 2: This diagram shows areas of operation in the anaerobic digestion of municipal
solid waste and sewage sludge.
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The municipal solid waste, after being deposited on the tipping
floor, would be shredded for ease in materials handling. The shredding
operation would fulfill two primary functions: it would allow for
efficient separation of the organic material from the inorganic,
nondigesttble matter (metal cans, bottles, etc.) found in municipal
waste; and it would reduce the feed to a homogeneous size, which would
be more readily digested.
Separation systems based on two different principles are currently
being developed; these are the dry and wet separation processes. The.
dry separation process is being demonstrated in St. Louis and the wet
separation process in the City of Franklin, Ohio.°>5 Both presses
provide a waste stream with a high concentration of organic matter
relatively free of metals, glass, and grit. In a .dry separation
process, the shredded material is air classified during which the
organic materials are separated and recovered as the lighter fraction.
The light organic material then would be shredded in a secondary shredder
and conveyed pneumatically to a storage silo where it would be finally
ready for digestion. In the wet system, the shredded waste would be
fed into a hydropulper to be mixed with a large amount of water.
This process is similar to that of a kitchen sink disposal unit.
Fibrous materials are recovered as a dilute aqueous stream which
would be conveyed pneumatically to a storage silo where it would
be finally ready for digestion.
Before the waste material enters tlSe digester, it must be mixed
with nutrients (sewage sludge) and other chemicals (lime, sodium
bicarbonate, phosphorus) necessary for the digester operation. 'At this
stage, animal or agricultural wastes could be blended into the slurry
if these were part of a locality's waste stream. Each digester would
be maintained at constant pressure and temperature and would be pro-
vided with a means for continuously stirring the contents.* Stirring
allows uniform digestion of the,material to proceed in two stages as
described in a previous section. The products of digestion consist
of two streams. One stream would be composed of methane and carbon
dioxide in equal volumes, and the other would be residue that must be
disposed of appropriately.
The methane produced from the digester would contain carbon dioxide
and traces of hydrogen sulfide. These two acid gases must be removed
before the methane is sold. This could be accomplished v.ia one of a
number of gas-cleansing processes. These are the molecular sieve,
Selexol,^ and diglycolamine processes. All three systems are designed
to remove large concentrations of carbon dioxide (50 percent for
digester gas).°
* Ten 60,000 cubic feet digesters would be needed for a 1,000-
TPD plant.
+ Selexol is a registered trademark of Allied Chemical Corporation
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The final operation, the effluent disposal, would be best carried
out by separating the solids from the liquid and returning the liquid
to the sewage treatment plant for subsequent treatment and final dis-
charge. The solids, in the form of the moist sludge obtained from
various dewatering processes such as vacuum filtration, centrif uging ,
and heat drying, could then be disposed of or utilized. Various
methods could be used: incineration, landf i 1 1 ing, use as a soil
conditioner, reclamation of strip mines, or compression to form fiber-
board. This sludge, whose volume would be only 20 percent of the
incoming solid waste, would have a heating value of 4,000 Btu per
pound (25 percent solid) and could be burned to generate usable steam.
Benefits
The potential benefits resulting from the anaerobic digestion
of solid waste are: (l) energy recovery in the form of methane gas
to be used directly in home heating and cooking, (2) reduction of the
municipal solid waste disposal problem on a large scale, (3) reduction
of the sewage sludge disposal problem, (4) materials recovery from the
sale of ferrous metals and other secondary materials (Figure 3).
Waste
Components:
Benefits:
Municipal
wastes
^- — .
^-^
Sewage
sludge
~- •»_
Energy Sol id waste
recovery disposal
__ — -
_ — -—
Combined animal ,
crop & industrial
wastes
. — ' — ~
Sewage Materials
disposal recovery
Figure 3. A simplified block diagram illustrates the waste stream
components and associated benefits resulting from the anaerobic digestion
of organic materials.
Projected Economics of a Conceptual System
The economics of an anaerobic digester plant can only be esti-
mated since the process has not been demonstrated on a full scale
(1,000 TPD) . The following capital and operating costs are based
on'a study done in.July 197^ by the Dynatech Corporation, Cambridge,
Massachusetts. 'p Annual capital cost figures are based on
typical 20-year, 6-percent financing. It would not be advisable to
assume that these figures are automatically applicable to all parts
of the United States without a prior study of pertinent factors such
as site costs, labor and material costs, product marketability, plant
size, etc.
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The capital and operating costs per ton and revenues per ton of
the plant for a 1,000-TPD plant, processing wastes 310 days per year,
producing gas 365 days per year have been projected (Table l). If
a municipality pays $13 per ton or more to dispose of its solid waste
and sewage sludge, serious consideration should be given to imple-
mentation of the anerobic digestion process should full-scale systems
be proven technically feasible.
TABLE 1
PROJECTED ECONOMICS OF 1,000-TPD BYCONVERSION PLANT
Costs
Capital including amortization5'
Operating+
Residue disposal?
Total
Revenues
Sale of natural gas _
Sale of ferrous metal
Credit for sludge disposal""
Total
Costs/Ton
$ 3.60
7.10
2.30
$13.00
Revenues/Ton
$3.60
2.70
1.90
$8.20
* Plant cost: $22 million, 20 years, 6 percent
municipal bonds; includes design, site, equipment,
and construction costs.
+ Includes supplies, chemicals, maintenance,
utilities, labor overhead, taxes; no detailed break-
down is available.
+ Sanitary landfill (SLF) at $5 per ton of
d.igester residue, heavies from air classifier, and
waste water.
i Sell at $1 per mcf ($l/MMBtu).
*" Sell at $AO per ton.
** SLF at $5 per ton.
10
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Env i ronmenta1 Impact
The greatest advantage of an anaerobic digestion system is its
positive environmental impact. Solid waste which would normally
be disposed of in a land disposal site can now be converted into a
useful product (gas) with no adverse impact on the environment.
Because of the absence of air pollutants and with proper control of the
effluent and residue, there will be no adverse environmental effect
from the operation of such a solid waste conversion plant. The posi-
tive contributions of this system in the elimination of the land dis-
posal of wastes and in the recovery of valuable materials and fuel
make this an environmentally desirable approach to solid waste management,
Disadvantages
Because anaerobic digestion of waste materials has not been
demonstrated on a large scale, there is considerable risk that the
system will not perform as predicted. The potential exists that the
digesters will sour from time to time. This potential is supported
by the experience of operating sewage sludge digesters where the bio-
logical process occasionally is inhibited. The addition of air-
classified organic solid waste to sewage sludge in a digester should
maintain the proper chemical balance so as not to inhibit this bio-
logical decomposition of the waste materials. As other resource
recovery concepts, the process is also capital-cost intensive. Other
drawbacks are that it initially has relatively low gasification rates
over a period of time (retention time in days) as compared to other
resource recovery concepts and that the construction of a 1,000-TPD
plant would cover significant acreage (12 acres) if land was at a
premium. If a full-scale system was implemented, almost all of these
disadvantages could be overcome by experience.
Summary
In summary, the anaerobic digestion process if developed to
applicable technology stages could:
0 maximize the conversion of municipal solid waste and sewage
sludge into a usable fuel;
0 facilitate recovery of materials;
0 handle other wastes mixed with the municipal solid waste
and sewage sludge such as animal, crop, and some industrial
wastes;
0 operate without causing pollution to the air.
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So far, the only drawback to implementing an anaerobic digestion
system for a community is the initial capital investment to construct
such a facility. The existing technology for the various system
components such as the shredder, digestion tank, and gas cleansing
unit are available and operating today but all these components must
still be joined together into a fluent, functioning system for solid
waste and sewage sludge. Until this is done, the anaerobic digestion
process will remain1dormant.
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References
1. Bendixen, T. W., and G. L. Huffman. News of environmental
research in Cincinnati; impact of environmental control
technologies on the energy crisis. Cincinnati, U.S.
Environmental Protection Agency, National Environmental
Research Center, Jan. 11, 1974. 8 p.
2. Kispert, R. 6., et al. Fuel gas production from solid waste;
semi-annual progress report. Dynatech Report No. 1207.
Cambridge, Mass., Dynatech Corporation, July 31, 1974.
184 p.
3. Klass, D. L., and S. Ghosh [Institute of Gas Technology].
SNG from biogasification of waste materials. Presented
at SNG Symposium 1, Chicago, Mar. 12-16, 1972. 15 p.
4. Proceedings; Bioconversion Energy Research Conference,
Amherst, University of Massachusetts, June 25-26,
1973. Washington, National Science Foundation. 120 p.
5. Arella, D. G. Recovering resources from solid waste using
wet-processing; EPA's Franklin, Ohio, demonstration project.
Environmental Protection Publication SW-47d. Washington,
U.S. Government Printing Office, 1974. 26 p.
6. Personal communication. S. A. Hoenig, University of Arizona,
to S. J. Hitte, Office of Solid Waste Management Programs,
Mar. 1975.
7. Pfeffer, J. T. Reclamation of energy from organic waste.
U.S. Environmental Protection Agency, 1974. 143 p.
(Distributed by National Technical Information Service,
Springfield, Va., as PB-231176.)
8. Lowe, R. A. Energy recovery from waste; solid waste as
supplementary fuel in power plant boilers. Environmental
Protection Publication SW-36d.ii. Washington, U.S.
Government Printing Office, 1973. 24 p.
9. An examination of several potential landfill gas cleaning
processes. Washington, U.S. Environmental Protection Agency,
1974. 17 p. (Unpublished report.)
VKT1192
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