Gas production by accelerated in situ bioleaching of landfills

A process for improved gas production and accelerated stabilization of landfills by accelerated in situ bioleaching of organic wastes by acid forming bacteria in substantially sealed landfills, passing the leachate of hydrolysis and liquefaction products of microbial action of the microorganisms with the organic material to an acid phase digester to regenerate the activated culture of acid forming microorganisms for recirculation to the landfill, passing the supernatant from the acid phase digester to a methane phase digester operated under conditions to produce methane rich gas. The supernatant from the methane phase digester containing nutrients for the acid forming microorganisms and added sewage sludge or other desired nutrient materials are circulated through the landfill. Low Btu gas is withdrawn from the acid phase digester while high Btu gas is withdrawn from the methane phase digester and may be upgraded for use as SNG. The process of this invention is applicable to small as well as large organic waste landfills, provides simultaneous disposal of municipal solid waste and sewage sludge or other aqueous organic waste in a landfill which may be stabilized much more quickly than an uncontrolled landfill as presently utilized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improved production of gas, particularly methane, 
from sanitary landfills or other confined deposits of organic matter. 
2. Prior Art 
Sanitary landfills formed by filling a land area with successive layers of 
solid waste, principally household waste, and layers of earth or soil are 
well known. The uncontrolled landfill depends upon natural biological 
action, precipitation and climate to effect decomposition. In areas where 
oxygen is present, the decomposition will be aerobic and in areas where 
little oxygen is present, such as at the deeper depths, decomposition will 
be slower and anaerobic producing methane containing gas. Initially, there 
is no methane production from the landfill and it increases very slowly 
with time to an amount representing only about 15 to 20 percent of the 
total potential production after many years. The formed methane is an 
explosion or fire hazard and may migrate to buildings or structures 
several hundred feet from the landfill if not removed from the landfill. 
Further, the natural precipitation draining out of the landfill may carry 
highly toxic contaminated water to contaminate underground water supplies, 
surface streams and wells. Due to the very slow stabilization, the 
uncontrolled landfill is not usable for other purposes for long periods of 
time and thus, particularly near metropolitan areas, represents a large 
waste of land resources. 
One approach to rendering waste disposal landfills safer is suggested by 
U.S. Pat. No. 3,586,624 which teaches a liquid impervious containment of 
the lower portion of the landfill with continuous flow of water through 
the landfill to accelerate the decomposition, decrease the fire hazard and 
flush contaminants from the landfill in a controlled manner. The water 
drained from the landfill may be treated for removal of contaminants and 
recycled to the landfill. 
In the past, methane gas has been frequently vented and flared from 
landfills as a safety precaution. However, in recent years and especially 
in view of energy conservation, the recovery and utilization of methane 
from sanitary landfills and desirability of early utilization of the 
landfill area for other purposes has been recognized. "Methane Production, 
Recovery, and Utilization from Landfills", James, S. C. and Rhyne, C. W., 
Symposium Papers on Energy from Biomass and Wastes, Washington, D. C., 
Aug. 14-18, 1978, pgs. 317-324, and "Recovery and Utilization of Methane 
Gas from a Sanitary Landfill--City of Industry, California", Stearns, R. 
P., Wright, T. D. and Brecher, M., Symposium Papers on Energy from Biomass 
and Wastes, Washington, D. C., Aug. 14-18, 1978, pgs. 325-343. Presently 
methane is most frequently recovered from landfills by pipes extending 
into the landfill and transporting the methane containing gas formed 
within the landfill to a collecting area for further treatment. 
In the United States, about 1151 million tons (dry) of organic wastes are 
generated annually in the form of municipal solid waste, agricultural 
residue, manure, logging and wood manufacturing residues, municipal sludge 
solids, industrial organic wastes and miscellaneous organic wastes 
representing production potential of 11.8 trillion SCF/yr. of substitute 
natural gas (SNG). The most readily available solid waste for energy 
recovery is municipal solid waste estimated to be currently generated at 
about 260 million tons per year in the United States. Additionally, urban 
areas in the United States produce about 25 million tons (dry) per year of 
organic waste solids in sewage sludge. These wastes have presented 
intractable waste management and disposal problems and represent 
continuing loss of energy resources. 
SUMMARY OF THE INVENTION 
This invention relates to a process for improved gas production providing 
higher gas production rates and yields by accelerated in situ bioleaching 
of organic wastes in substantially sealed landfills. Methane producing 
anaerobic digestion systems utilize acid forming bacteria and methane 
producing organisms as are well known to be employed to produce methane 
from sewage sludge and are suitable for use in the process of this 
invention. Two phase digestion is used under controlled digester 
conditions in the process of this invention, that is, acid phase digestion 
operated at mesophilic or thermophilic conditions to promote the growth of 
the acid forming bacteria and a second methane phase digestion operated at 
mesophilic or thermophilic conditions to promote the growth of the methane 
producing organisms. In the process of this invention organic material in 
a substantially sealed landfill is contacted in situ with an aqueous 
activated culture of hydrolytic and liquefying anaerobic microorganisms 
under growth conditions to produce a bioleachate of hydrolysis and 
liquefaction products of microbial action of the microorganisms with the 
organic material. The bioleachate and the deactivated acid forming 
bacteria are passed from the landfill to an acid phase digester to 
regenerate the activated culture of hydrolytic and liquefying anaerobic 
microorganisms for recirculation to the landfill. The supernatant from the 
acid phase digester is passed to the methane phase digester operated under 
conditions to produce gas rich in methane. The supernatant from the 
methane phase digester, containing nutrients for the acid forming 
microorganisms, is mixed with the activated culture of hydrolytic and 
liquefying anaerobic microorganisms from the acid phase digester and 
recirculated to contacting organic material in situ in the substantially 
sealed landfill. Low Btu gas is withdrawn from the acid phase digester and 
may be also withdrawn from the landfill from time to time while high Btu 
gas is withdrawn from the methane phase digester for direct use or 
upgrading for use as substitute natural gas (SNG). Sewage sludge or other 
organic waste materials may be added to the landfill to increase the 
biological activity in the landfill, improve the nutrient balance and to 
dispose of the organic waste. 
The process of this invention provides rapid onset of landfill 
bioconversion, increased gas production rate and higher concentrations of 
methane resulting in stabilized landfill available for other use much 
sooner than conventional landfills. In conventional landfills, biological 
gasification is severely retarded and there is a long time lag between 
closing of the landfill and onset of active methane fermentation, which 
then continues at very slow rates and in uncontrolled manners for many 
years. Further, the methane gas formed in the landfill migrates both 
vertically and laterally in an uncontrolled fashion causing a very 
hazardous situation. The process of this invention greatly decreases the 
formation of methane in the landfill itself, and enhances overall energy 
production from the landfill by operation of two phase digestion under 
controlled conditions. 
It is an object of this invention to provide a process for improved gas 
production from landfills of solid organic wastes. 
It is another object of this invention to reduce fire and explosion hazards 
and pollution of areas surrounding solid organic waste landfills. 
It is still another object of this invention to increase the methane 
content of gaseous products produced by anaerobic digestion based upon 
solid organic waste landfills. 
It is yet another object of this invention to provide a process which is 
suitable for a wide variety of sizes of landfills, applicable to small as 
well as large organic waste landfills. 
It is still another object of this invention to provide simultaneous 
disposal of municipal solid waste and sewage sludge or other aqueous 
organic waste in a substantially sealed landfill.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of this invention is applicable to landfills of all types of 
organic wastes. By the term "organic waste" as used in this disclosure and 
the appended claims, I mean all types of organic refuse including sewage 
sludge, animal waste, municipal waste, industrial waste, forestry waste, 
agricultural waste, and the like. By forestry waste and agricultural waste 
I mean to include portions of plants after some physical or chemical 
treatment, usually not including the entire plant, for example, stumps 
from logging, sawdust, wood chips, corn stalks, corncobs and bagasse. 
Municipal solid waste landfills constitute an important application of the 
process of this invention. Treatment of municipal solid waste and 
industrial solid waste for removal of undesired material such as glass, 
metals, plastics, stones, and the like, is well known to the art. 
Municipal solid waste contains over 50 weight percent (dry) cellulosics. 
Exemplary average composition of raw municipal solid waste is shown in 
Table I for municipal solid waste collected in Indianapolis, Ind. 
TABLE I 
______________________________________ 
Components Weight % dry 
______________________________________ 
Paper Products 45.7 
Wood 2.1 
Textiles and Rags 1.9 
Food and Garden Wastes 
10.9 
Total Cellulosics 60.6 
Metallics 13.6 
Glass, Ceramics, etc. 16.6 
Dirt, Ash, Rocks 3.0 
Plastics 2.1 
Bulky Materials 3.0 
Rubber and Leather 1.1 
Total Non-Cellulosics 
39.4 
______________________________________ 
The municipal solid waste is preferably shredded followed by magnetic 
separation of ferrous metals to reduce landfill volume and permit recovery 
of the ferrous metal. It is also preferred to separate glass from the 
waste also to reduce landfill volume and provide recovery for recycling. 
The landfill area to be filled with organic waste is isolated from ground 
water and surrounding soil formations using a linear having sides and a 
floor shown as 11 in FIG. 1. The liner may be any suitable barrier such as 
compacted clay, asphalt or other commercially availably liner materials. 
The floor has collection drain 12 leading to sump 13 for recovery of the 
liquid leachate. The organic waste is spread and compacted in layers, 6 to 
8 foot lifts, by conventional methods of constructing sanitary landfills. 
Collection drain 12 is preferably surrounded with crushed stone to enhance 
the liquid flow to the collection drain. During filling of the landfill 
with organic waste 10, liquid distribution means, shown in FIG. 1 as pipes 
20 and 19, are put into place to assure distribution of liquid throughout 
the landfill volume. The liquid distribution means may include any 
arrangement of pipes or conduits with suitable holes or other means for 
distributing liquid, including liquid-solid slurries, both horizontally 
and vertically throughout the landfill volume. When the landfill is full 
of organic waste, top liner 14 and compact soil 15 is put into place to 
substantially seal the landfill. Gas withdrawal pipe 16 having valve 17 is 
provided to withdraw any low Btu gas collecting at the top of the 
landfill. 
Bioconversion of the organic waste is achieved by contacting the organic 
waste with an aqueous activated culture of hydrolytic and liquefying 
anaerobic microorganisms under growth conditions to produce a bioleachate 
of hydrolysis and liquefaction products of microbial action of the 
microorganisms with the organic waste material in situ. The active culture 
also contains desired nutrients for the hydrolytic and liquefying 
anaerobic microorganisms and by continued application of the activated 
microorganisms, moisture and nutrients, the landfill is transformed into a 
medium supporting the growth of the hydrolytic and liquefying 
microorganisms. Growth and continued supply of the acid forming 
microorganisms may be enhanced by also supplying to the landfill aqueous 
liquid organic waste such as sewage sludge. 
Any active methane producing mesophilic or thermophilic anaerobic digestion 
system may be used in the process of this invention. Methane-producing 
anaerobic systems utilizing acid forming bacteria and methane-producing 
organisms as are well known to be employed to produce methane from sewage 
sludge can be employed in practice of the present invention. A review of 
the microbiology of anaerobic digestion is set forth in Anaerobic 
Digestion, 1. The Microbiology of Anaerobic Digestion, D. F. Toerien and 
W. H. J. Hattingh, Water Research, Vol. 3, pages 385-416, Pergamon Press 
(1969). As set forth in that review, the principal suitable acid forming 
bacteria include species from genera including Aerobacter, Aeromonas, 
Alcaligenes, Bacillus, Bacteroides, Clostridium, Escherichia, Klebsiella, 
Leptospira, Micrococcus, Neisseria, Paracolobactrum, Proteus, Pseudomonas, 
Rhodopseudomonas, Sarcina, Serratia, Streptococcus and Streptomyces. 
Exemplary methane-producing organisms suitable for use in the present 
invention include members of Methanobacterium, Methanococcus and 
Methanosarcina, specific members being Methanobacterium formicicum, 
Methanosarcina barkerii, Methanobacterium omelianskii, Methanococcus 
vannielii, Methanobacterium sohngenii, Methanosarcina methanica, 
Methanococcus mazei, Methanobacterium suboxydans and Methanobacterium 
propionicum. It is usually preferred to use mixed cultures to obtain the 
most complete fermentation action. Nutritional balance and pH adjustments 
may be made to the anaerobic system as is known to the art to optimize 
hydrolytic and liquefying action or methane production from the culture 
used, dependent upon the phase of the process. 
The growth of the acid forming bacteria in the organic waste landfill is 
promoted by maintaining a low pH of the introduced culture of about 4 to 7 
and high throughput rates, for example, displacement of the pore volume 
liquid in about 1 to 4 days. As the aqueous culture moves down through the 
organic waste bed, the microbial action extracts and captures the 
hydrolysis and liquefaction products of the organic waste to produce a 
bioleachate. The bioleachate and deactivated acid forming organisms may be 
collected by a system of riser pipes and underdrains within the landfill 
for passage to an acid phase digester. Thus, the principal bioactivity in 
the landfill is the formation of the bioleachate and not the production of 
methane gas. Some low Btu gas and smaller amounts of methane may be 
produced in the landfill volume and may be removed from time to time by 
gas withdrawal pipe 16, but wells as used in the past for gas recovery 
from landfills are not necessary nor desirable. The relatively high rates 
of liquid throughput in the landfill provide removal of reaction products 
and toxicants from the landfill. 
The bioleachate and deactivated acid forming microorganisms collected in 
sump 13, shown in FIG. 1, are transported by pump 18 through conduit 31 to 
acid phase digester 30. The digestion system which serves as a generator 
of activated acid forming microorganisms and gasification of the 
bioleachate may be a conventional single stage, completely mixed digester, 
but is preferably a two phase digester system with a first digester 
operated under conditions promoting the growth of acid forming 
microorganisms and the second digester operated under conditions promoting 
the growth of methane forming microorganisms. Two phase anaerobic 
digestion is known to the art and is further disclosed in U.S. Pat. No. 
4,022,665. The digesters shown in FIG. 1 are closed-loop plug flow 
digesters with built-in settling systems to facilitate the withdrawal of 
activated cultures, the digesters being more fully described with respect 
to FIGS. 3 and 4. 
The acid phase digester 30 is fed the bioleachate and deactivated acid 
forming microorganisms through conduit 31. Anaerobic digestion is carried 
out in the acid phase digester under mesophilic, 15.degree. to 45.degree. 
C., or thermophilic, 45.degree. to 70.degree. C., temperatures and a 
detention time of about 1 to 3 days. Mesophilic conditions are preferred 
when the organic waste is municipal solid waste. The pH of the acid phase 
digester is maintained at about 5 to 7 and loading is maintained at about 
0.4 to 2.0 lb. VS/ft.sup.3 -day. These conditions promote the growth of 
activated acid forming microorganisms. The activated hydrolytic and 
liquefying microorganisms are collected in digester sump 32 and pass from 
the digester in recirculation conduit 33. Low Btu gas, in the order of 150 
to 400 Btu/SCF gas, is formed by the acid forming anaerobic culture and 
may be withdrawn from the acid phase digester by low Btu gas withdrawal 
pipe 34. Such low Btu gas may be used to supply process heat or other 
energy consumed in the process. The supernatant from the acid phase 
digester is rich in volatile fatty acids, alcohols and other solubles and 
is transferred to methane phase digester 40 by supernatant transfer 
conduit 35. 
Methane phase digester 40 is operated under conditions to promote the 
growth and action of methane forming microorganisms. The loading is about 
0.01 to 0.40 lb. VS/ft.sup.3 -day and the digester operated at a 
mesophilic or a thermophilic temperature for a detention time of about 3 
to 30 days. The pH of the methane phase digester is maintained between 
about 6.5 and 8.0. Sludge from the methane phase digester is collected in 
digester sump 42 and is withdrawn through sludge conduit 43 to purge the 
system of inhibitory substances and toxic microbial metabolites. High Btu 
gas, about 500 to 800 Btu/SCF gas, is withdrawn through high Btu gas 
withdrawal pipe 44. The high Btu gas from methane phase digester 40 has 
greater than 50 mole percent methane shortly after initiation of the 
process and increases to 60 to 70 percent methane after a few months of 
operation of the landfill. The methane containing gas produced may be 
upgraded by methods known to the art to provide substitute natural gas 
(SNG). The total digester volume, acid phase and methane phase, is about 3 
to 5 percent of the total volume of the landfill. 
Supernatant from the methane phase reactor is rich in inorganic nutrients 
and organic growth factors and is withdrawn through supernatant withdrawal 
conduit 45 for mixing with the activated hydrolytic and liquefying 
microorganism culture for recirculation to the landfill. The supernatant 
from the methane phase digester may be passed to a mixing tank (not shown) 
for mixing with the activated acid forming microorganisms withdrawn from 
the acid phase digester for recirculation. This will provide a 
nutrient-rich active culture of acid forming bacteria which may be drawn 
upon for recirculation by recirculation conduits 21 and 22 controlled by 
valves 23 and 29 and necessary pumps and controls (not shown) for 
supplying distributor means 19 and 20, respectively, within organic waste 
landfill 10. Aqueous organic wastes such as sewage, industrial wastes, 
feed lot runoff, sewage or industrial sludge, may be added to the landfill 
through the distributor means. Nutrients, pH adjusting chemicals and other 
desired chemicals may be added in this fashion or separately. Such wastes 
provide nutrients for the microorganisms and accelerated bioleaching in 
addition to simultaneous gasification at enhanced rates. The aqueous 
liquids or slurries may be provided to distributor means 19 and 20 by 
liquid supply conduit 24 and liquid conduits 25 and 26 feeding into 
distributor means 19 and 20, respectively. The flow to each of the 
distributor means may be controlled by valves 27 and 28 and pumps and 
controls (not shown). It is suitable to add to a municipal solid waste 
landfill about 1/2 to 3 weight percent (dry basis) sewage sludge slurry 
based upon weight of municipal solid waste as received. Thus, a unified 
sewage sludge and municipal solid waste disposal system is advantageously 
obtained in a fashion interacting to promote high energy recovery. 
FIG. 2 shows a sectional view through a portion of a landfill cell 
according to one embodiment of this invention and suitable for laboratory 
work. FIG. 2 shows activated acidogenic anaerobic culture and nutrient 
supply conduit 66 controlled by valves 67 and 68 supplying the active 
acidogenic culture together with nutrients and aqueous organic waste, if 
desired, to distributor means 75 and 72, respectively. The distributor 
means may be perforated pipes to allow desired distribution throughout the 
landfill area and may be packed with or surrounded by crushed rock or 
gravel. The horizontal distributor means 72 and 75 are interjoined by 
vertical risers 69 and 70 which are also perforated pipes which may be 
packed with or surrounded by crushed rock. Even distribution of the 
liquids throughout the landfill volume may be enhanced by utilizing 
different sized crushed rock or different sized perforations in the pipes. 
As shown in FIG. 2, risers 69 and 70 may be joined directly with 
underdrain 73 which is also surrounded by gravel or soil 74. Underdrain 73 
collects the leachate and deactivated microorganisms from the landfill for 
transfer to the acid phase digester. The digester is enclosed by 
polyvinylchloride liner 60 and covered with clay 61 and dirt 62. Gas 
collection cover 63 is provided with gas withdrawal pipe 64 and valve 65. 
It is seen that the application of activated acidogenic anaerobic culture 
on a continual basis throughout the landfill serves to moisten the 
landfill bed and promote active acid phase decomposition relatively 
uniformly throughout the landfill. Nutrients for the acid forming 
anaerobes may be distributed throughout the landfill bed on a continuous 
basis to encourage the hydrolytic and liquefying action of the acid 
forming organisms. Further, the removal of the microbial reaction products 
from the reaction zones throughout the landfill further rapid anaerobiosis 
and bioleaching of the organic waste in the landfill. 
Any anaerobic digester and various means for increasing methane yield, gas 
quality and digestion kinetics such as feed pretreatment, residue 
post-treatment and recycling or advanced digestion modes may be used in 
conjunction with the process of this invention. One preferred 
configuration for each of the digesters in the two phase system of a 
preferred embodiment of this invention is shown in FIGS. 3 and 4. The 
digester tank 80 is in oval, or race track, form. The digester is not 
completely filled with liquid but always has a suitable gaseous headspace. 
The digester is supplied liquid by supply conduit 81 just behind cage 
rotor 85. Cage rotor 85 is used for propelling the liquid through the 
digester as well as mixing and gas transfer from the liquid to the 
headspace and is of a type known to the art and now commercially used in 
oxidation ditches. Other aids, such as baffles, may be used to enhance 
transfer of gases from the moving liquid. As the liquid passes around the 
digester tank, supernatant is removed by supernatant conduit 82 and gas 
from the headspace is removed by gas removal conduits 86 to gas collector 
conduit 87. As the liquid moves through the digester, it passes over 
settler section 83 which provides a sump for gravity separation of the 
heavy materials in the digester which may be removed by removal conduit 
84. In the acid phase digester, the activated acidogenic microorganisms 
collect in settler section 83 while in the methane phase digester, sludge 
will collect in settler section 83. Suitable heaters, means for additions 
for pH adjustments and other anaerobic digester features known to the art 
may be readily adapted to this type of digester. The digester of this type 
is proposed for use in the process of this invention in view of its 
effective utilization of tank volume for steady state biological reactions 
with low energy requirements for mixing and gas transfer and low overall 
costs. 
FIG. 5, by dotted lines, shows total gas production and methane production 
from presently used uncontrolled sanitary municipal solid waste landfill. 
The bioactivity in such landfills takes place as a result of natural 
environmental conditions and the produced gas is withdrawn from the 
landfill by wells distributed throughout the landfill. It is seen that for 
the first several years gas production is very low and only after 9 to 10 
years reaches about one-third of its total potential. Likewise, the 
methane fraction of the gas produced is very low, reaching only 10 percent 
after 5 years and about 45 percent after 10 years. The solid lines show 
calculated gas production from municipal solid waste landfills according 
to the process of the present invention. It is seen that the total gas 
production increases rapidly a short time after the landfill is capped, 
reaching over 50 percent of its total potential within 2 years and up to 
about 90 percent of its total potential in about 5 years. Likewise, the 
concentration of methane in the produced gas is in excess of 50 mole 
percent initially and increases to about 70 percent within the first 3 to 
4 years following closing of the landfill. Practice of the process of this 
invention, therefore, provides a stabilized landfill which may be used for 
other purposes in a fraction of the time that landfills are being returned 
to other uses when present uncontrolled landfill practices are used. 
EXAMPLE I 
A landfill cell as shown in FIG. 2 is constructed from polypropylene or 
polyethylene tanks about 6 feet tall and 31/2 feet in diameter to provide 
50 ft.sup.3 landfill capacity. Coarse-shredded and magnetically separated 
municipal solid waste having an analysis as set forth above for 
Indianapolis municipal solid waste is placed in the cell in lifts of 3 
feet and compacted to produce a bulk density of 20 lbs./ft.sup.3, or a 
total of 1000 lbs. The bottom 3 feet of waste is covered with 3 inches of 
soil and the top portion of waste put in place. The top of the waste is 
covered with a polyvinylchloride cap which is covered with 6 inches of 
compacted montmorillonite clay and 3 inches of dirt. An acid phase 
digester and a methane phase digester of the configuration shown in FIGS. 
3 and 4 are connected to each other and to the landfill cell in the manner 
shown in FIG. 1. The acid phase digester is sized to accommodate an active 
culture volume of 10 liters and the methane phase digester sized to 
accommodate an active culture volume of 40 liters. An active anaerobic 
culture of acid forming microorganisms from an existing culture is 
transferred to the acid phase digester and an active anaerobic culture of 
methane forming microorganisms from an existing culture is transferred to 
the methane phase digester. Pumping of liquid from the digesters through 
the distribution pipes in the landfill is started and continued at the 
rate of 1 l/day from the settler portion of the acid phase digester which 
is rich in activated hydrolytic and liquefying microorganisms; 3 l/day 
supernatant from the methane phase digester which is rich in nutrients for 
the microorganisms; and 0.25 l/day added liquid which may be aqueous 
organic waste or other nutrients or materials for the system adjustment or 
balance. Thus, about 4.25 l/day bioleachate is collected from the landfill 
and passed to the acid phase digester. Supernatant in an amount of 3.25 
l/day is transferred from the acid phase to the methane phase digester 
making a detention time of 2.3 days in the acid phase digester which is 
maintained at 35.degree. C. and pH of 6. The methane phase digester is 
maintained at 35.degree. C. and a pH of 7.5 with 3 l/day supernatant 
withdrawn for recycle to the landfill and 0.25 l/day withdrawn as sludge 
making a detention time of 12.3 days. The landfill cell is stabilized 
after 5 months of continuous operation with 400 ft.sup.3 low Btu gas (30% 
by volume methane) being withdrawn from the acid phase digester and 2100 
ft.sup.3 high Btu gas (70% by volume methane) being withdrawn from the 
methane phase digester. 
While in the foregoing specification this invention has been described in 
relation to certain preferred embodiments thereof, and many details have 
been set forth for purpose of illustration, it will be apparent to those 
skilled in the art that the invention is susceptible to additional 
embodiments and that certain of the details described herein can be varied 
considerably without departing from the basic principles of the invention.