Integrated anaerobic digester system

η>>”ν»ίΐ n’anNJN nabw» ns*»»»

Integrated anaerobic digester system

<img img-format="tif" img-content="drawing" file="IL161031AD00021.tif" id="idf0001" />

Jack L. AINSWORTHDan ATWOODTom RIDEOUT C. 152906 WO 03/042117 PCT/US01/45224 -1 -

TITLE

INTEGRATED ANAEROBIC DIGESTER SYSTEM

FIELD OF THE INVENTION

The present invention relates generally to an Improved process and equipment forconverting feedstock mto useful materials, and more specifically, to an anaerobic fermentativeprocess for bioconverting animal waste, sewage sludge or other biodegradable feedstock into 5 methane gas, carbon dioxide gas, ammonia, carbon black, organic acid, charcoal, a fertilizerand/or an insecticidal mixture.

BACKGROUND OF THE INVENTION

Animal waste poses a significant problem in the poultry, swine and cattle industries.Animal waste from animal raising or processing operations is responsible for a significant 10 amount of underground water contamination and methods are continually being developed forhandling animal wastes. One known method is the bioconversion of animal waste into usefulproducts.

Methods for the anaerobic digestion or treatment of sludge, animal waste, synthesis gasor cellulose-containing waste are disclosed in U.S. Patents No. 5,906,931 to Nilsson et al., No. 15 5,863,434 to Masse et al., No. 5,821,111 to Grady et al. No. 5,746,919 to Dague et al., No. 5,709,796 to Fuqua et al., No. 5,626,755 to Keyser et al., No. 5,567,325 to Townsley et al., No.5,525,229 to Shih, No. 5,464,766 to Bruno, No. 5,143,835 to Nakatsugawa et al., No. 4,735,724to Chynoweth, No. 4,676,906 to Crawford et al., No. 4,529,513 to McLennan, No. 4,503,154 toPaton, No. 4,372,856 to Morrison, No. 4,157,958 to Chow, and No. 4,067,801 to Ishida et al. 20 These patents disclose different processes and equipment for the bioconversion, either by WO 03/042117 PCT/US01/45224 .-2- microbial digestion or enzymatic conversion, of those materials into methane and other usefulmaterials.

The equipment used for the anaerobic digestion or fermentation of waste into fuel, suchas methane, varies greatly and is generally tailored to specific applications. Equipment that is 5 suitable for a first type of feedstock generally has to be modified before it can be used for asecond different type of feedstock

Chemical and biochemical reactions that create a gas are generally conducted at low tosub-atmospheric pressures due to the tendency of the product gas to function as feedbackinhibitor that inhibits further formation of the gas. The art recognizes that variations in the 10 pressure of an anaerobic digester can be used to effect different biochemical and productivityresults. U.S.’Patent No. 4,409,102 to Tanner discloses an anaerobic digestion conducted at sub-atmospheric pressures that unexpectedly affect an increase in methane gas production. U.S.Patent No. 3,994,780 to Klass et al. discloses the high pressure rupture of cells in an anaerobicdigester to render cellular components available to other intact cells in the digester. U.S. Patent 15 No. 3,981,800 to Ort discloses a process for preparing high quality methane (about 98% wt.)with an anaerobic digester operated at 1-5 atm. above atmospheric pressure provided that thesludge is degassed by a recirculator and passed between two digesters connected serially toremove carbon dioxide in the sludge that is then fed back into the digester. Therefore, unlike thepresently claimed system, the system or Ort requires that each batch of feedstock under go a 20 two-stage digestion, wherein the feedstock is predigested in a first anaerobic digester and thencompletely digested in a second anaerobic digester that is connected serially with the firstanaerobic digester. U.S. Patent No. 4,100,023 to McDonald discloses that the internal pressureofthe anaerobic digester should be kept at about 1 to 3 inches of water column to ensure properperformance. U.S. Patent No. 4,568,457 to Sullivan discloses a two-stage anaerobic digester 25 system having an acid forming stage and a methane gas forming stage, wherein the pressure ofthe gas in the headspace of the two stages can be slightly above atmospheric pressure.

Methanogenic microbes that create methane from carbon and hydrogen containingfeedstocks, such as cellulose, animal waste, food processing waste, and sludge, are well known.These microbes have been used in the waste processing industry and are available in their native WO 03/042117 PCT/US01/45224 -3 - forms from natural sources or in genetically altered or manipulated forms, which can producegreater amounts of useful materials per unit weight of waste than· can unaltered methanogenicbacteria.

To date, no equipment containing the required components as described herein has been5 disclosed. Further, the improved equipment design and layout of the present invention providesa higher yield of methane and other useful materials than other comparable equipment. Stillfurther, the improved process and equipment of the invention can be used in the poultry, swine,dairy or cattle industries to convert cellulose-containing animal waste into methane which isused to operate farm or ranch equipment thereby reducing operating costs and the volume of 10 waste produced.

SUMMARY OF THE INVENTION

The present invention provides a system for converting cellulose-containing feedstockinto useful materials, wherein the system comprises: a feedstock slurry feeder; 15 a plurality of conduits connecting various components of the system; a single pressurizable anaerobic digester comprising agitation means, one or more feed ports, one or more discharge ports, an optional pressure regulator, and a reaction vessel forholding a reaction solution comprising an anaerobic microbe which converts an aqueous slurryof cellulose-containing feedstock into at least methane and an enriched effluent; 20 apressurizer; and one or more gas processors directly or indirectly connected to the anaerobic digester;wherein the headspace of the anaerobic digester is pressurized to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobicdigestion ofthe feedstock slurry. 2 5 Depending on the feedstock sluny used, the anaerobic digester will also form a fertilizer, sludge, scum, ammonia, charcoal, carbon black, an organic acid and/or an insecticidal mixture.The anaerobic digester is preferably operated at pressures between 10 to 265 psi, morepreferably 10 to 100 psi, and even more preferably 25-75 psi. In preferred embodiments, the WO 03/042117 PCT/USO 1/45224 -4 - system also comprises one or more of the following: one or more gas scrubbers, one or moreheaters for heating or preheating the slurry being digested in the anaerobic digester, one or morewater storage tanks, one or more feedstock slurry tanks, one or more feedstock grinders, one ormore supernatant storage tanks, one or more sludge storage tanks, one or more sludge dryers, 5 one or more scum storage tanks, one or more CO2 tanks, and/or one or more produced gasstorage tanks.

Other preferred embodiments include those wherein the system does not require a waterlagoon, a foam trap, and/or a water vapor trap. Still other preferred embodiments include thosewherein: (1) the system is operated in a batch, semi-continuous, or continuous mode; (2) the 10 feedstock slurry comprises from about 1-90% wt. solids, more preferably about 1- 60% wt.solids, or even more preferably about 1-40% wt. solids; (3) the agitation means comprises a gas . bubbler, an aerator, a sparger bar, a fluid stream, a mechanical agitator, or a combination thereof;(4) the feedstock slurry is gravity fed or fed under pressure to the anaerobic digester; (5) thepressurizer pressurizes the anaerobic digester with gas or a liquid; (6) the pressurizer is the 15. feedstock slurry feeder, which is preferably a pump, gravity feed system, or a gas compressor;(7) the anaerobic digester does not require aerobic digestion of the feedstock; (8) the anaerobicdigester does not require multiple discrete zones of environmentally incompatible waste-digestive microorganisms; (9) the anaerobic microbe is a methanogenic bacterium; (10) theanaerobic microbe is mesophilic or thermophilic; (11) methane produced by the anaerobic 20 digester is used to operate an internal combustion engine, an electrical current generator, anelectric engine, a water heater, a furnace, an air conditioning unit, a ventilation fan, a conveyor, apump, a heat exchanger, fuel cell, or various components of the system itself and/or to rechargepower cells; (12) the gas processor comprises a gas scrubber and/or a gas separator; (13) a gasrecirculator is used to recirculate gas from the headspace of the reactor to the sluny in the 25 reactor; (14) a gas recirculator adds methane-depleted or carbon dioxide enriched biogas back tothe reactor; and/or (15) a fluid recirculator recycles the scum, supernatant, effluent, or sludge ofthe reactor.

Another aspect of the invention provides a system for converting cellulose-containingfeedstock into useful materials, wherein the system comprises: WO 03/042117 PCT/US01/45224 -5 - one or more feedstock slurry feeders; two or more pressurizable anaerobic digesters connected in parallel, each anaerobicdigester comprising agitation means, one or more feed ports, one or more discharge ports, anoptional pressure regulator, and a reaction vessel for holding a reaction solution comprising an 5 anaerobic microbe which converts an aqueous slurry of cellulose-containing feedstock into atleast methane and an enriched effluent; one or more pressurizers; one or more gas processors directly or indirectly connected to each anaerobic digester;and 10 a plurality of conduits connecting various components of the system; wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or more to form the enriched effluent and a discharge gas comprising at least methane during anaerobicdigestion of the feedstock slurry.

Specific embodiments of this aspect of the invention include those wherein: 1) a major15 portion of the discharge gas is methane; 2) the system comprises a single feedstock slurry feeder connected to each of two or more pressurizable anaerobic digesters connected in parallel; 3) theenriched effluent from a first pressurizable anaerobic digester is not fed into a secondpressurizable anaerobic digester; 4) the system further comprises one or more receiving tanks that receive the enriched effluent from each pressurizable anaerobic digester. 2 0 Another aspect of the invention provides an integrated system for converting cellulose- containing feedstock into useful materials, wherein the integrated system comprises: a feedstock slurry feeder system that forms an aqueous slurry of cellulose-containing feedstock; an anaerobic digester system directly or indirectly connected to the feeder system and25 comprising two or more pressurizable anaerobic digesters connected in parallel, wherein eachanaerobic digester receives the aqueous slurry of cellulose-containing feedstock and converts it into a discharge gas and an enriched effluent; and a discharge gas processing system that is directly or indirectly connected to the anaerobic digester system and that at least separates methane from the discharge gas; WO 03/042117 PCT/US01/45224 - 6 - wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or moreto form the enriched effluent and a discharge gas comprising at least methane during anaerobicdigestion of the feedstock slurry.

Specific embodiments include those wherein: 1) the integrated system further comprises5 an enriched effluent processing system; 2) the integrated system further comprises a pressurizersystem; 3) the feedstock slurry feeder system comprises one or more mixing vessels and one ormore pumps; 4) the discharge gas processing system comprises a dehydrator, separator, andscrubber; 5) each anaerobic digester comprises agitation means, one or more feed ports, one ormore discharge ports, an optional pressure regulator, and a reaction vessel for holding a reaction 10 solution comprising an anaerobic microbe that converts the aqueous sluny of cellulose-containing feedstock into at least methane and the enriched effluent; 6) the discharge gasprocessing system comprises one or more separators for separating CO2 or ammonia gas fromthe discharge gas; 7) the integrated system further comprises a gas recirculator to recirculate gasfrom the headspace of an anaerobic digester to the slurry of the digester; 8) a gas recirculator 15 adds methane-depleted or carbon dioxide enriched discharge gas back to an anaerobic digester;and/or 9) the integrated system further comprises a fluid recirculator system that recycles thescum, supernatant, effluent, or sludge of an anaerobic digester.

Another aspect of the invention provides an integrated anaerobic digester systemcomprising: 20 a single-stage anaerobic digester system comprising two or more pressurizable anaerobic digesters connected in parallel, wherein each anaerobic digester receives an aqueous slurry ofcellulose-containing feedstock and converts it into a discharge gas and an enriched effluent; a discharge gas processing system that is directly or indirectly connected to the anaerobicdigester system and that at least separates methane from the discharge gas; and 25 an enriched effluent processing system that is directly or indirectly connected to the anaerobic digester system; wherein the headspace of each anaerobic digester is pressurizable to about 10 psi or moreto form the enriched effluent and a discharge gas comprising at least methane during anaerobicdigestion of the aqueous slurry of cellulose-containing feedstock. WO 03/042117 PCT/US01/45224 - 7 -

Other features, advantages and embodiments of the invention will be apparent to thoseskilled in the art by the following description, accompanying examples and appended claims. BRIEF DESCRIPTION OF THE DRAWINGSThe following drawings are part of the present specification and are included to further 5 demonstrate certain aspects of the invention. The invention may be better understood byreference to one or more of these drawings in combination with the detailed description of thespecific embodiments presented herein.

Figs, la and lb are process flow schematics of a first preferred embodiment of theanaerobic digester system according to the invention. 10 Fig. 2 is a process flow diagram of a second preferred embodiment of the anaerobic digester system of the invention.

Fig. 3 is a chart depicting the temperature, pH, pressure and methane gas volumeproduction oflan exemplary digester according to the invention.

Fig. 4 depicts a process flow diagram of a third embodiment of the anaerobic digester15 system, wherein the system comprises two or more pressurizable anaerobic digesters connected in parallel. DETAILED DESCRIPTION OF THE INVENTIONThe present invention is different than known anaerobic digester system primarily in that it is conducted at elevated pressures' of at least about 10 psi up to about 265 psi, more preferably20 10 to 100 psi, and even more preferably 25-75 psi, during anaerobic digestion of a feedstock slurry and the system requires only a single stage of anaerobic digestion. The anaerobic digestersystem also includes an advantageous combination of known and unknown features thatunexpectedly provides a very efficient system for converting biomass into methane gas, a nutrient enriched solution, and optionally an insecticidal mixture. 25 As used herein, the phrase “single stage of anaerobic digestion” is taken to mean that anaerobic digestion of the aqueous feedstock slurry is accomplished by placing a charge of anaqueous feedstock slurry into a pressurizable anaerobic digester until sufficiently digested andpassing the formed gas onto the discharge gas processing system and the remaining fluid WO 03/042117 PCT/USO1/45224 -8 - (liquid/solids) onto the enriched effluent processing system such that a partially charge offeedstock is not passed from a first anaerobic digester to a second anaerobic digester.Accordingly, a single-stage anaerobic digester system can have one or two or more anaerobicdigesters connected in parallel but not in series. A parallel anaerobic digester system is unlike 5 the serial anaerobic digester system of Ort, wherein the charge from a first anaerobic digester ispassed onto a second anaerobic digester before it is passed onto the enriched effluent processingsystem. A serial anaerobic digester system is a two-stage or multi-stage system, whereas thesystem of the present invention is a single-stage anaerobic digester system.

As used herein, the term “feedstock” is taken to mean any animal or plant derived10 material that contains one or more components that can be converted, bioconverted orbiodegraded into a useful material by the anaerobic digester of the invention. Animal tissue,biomass, fish tissue or parts, plant parts, fruits, vegetables, plant processing waste, animalprocessing waste, animal manure or urine, mammalian manure or urine solids isolated fromfermentation cultures, and combinations thereof are included in the term feedstock. Particular 15 examples of feedstock include bovine, poultry, equine or porcine manure or urine, woodshavings or chips, slops, mostos, shredded paper, cotton burrs, grain, chaff, seed shells, hay,alfalfa, grass, leaves, sea shells, seed pods, com shucks, weeds, aquatic plants, algae and fungusand combinations thereof. Combinations of poultry, bovine, equine or porcine urine and/ormanure with wood shavings, wood chips, shredded paper, cotton burrs, seed shells, hay, alfalfa, 2 0 grass, leaves, seed pods, or com shucks are particularly preferred and are generally referred to ascellulose-containing feedstock. A feedstock slurry is prepared by suspending a feedstock in an aqueous solution to forma slurry comprising less than about 90% wt. solids, preferably about 0.1- 60% wt. solids, or evenmore preferably about 1-40% wL solids. The particle size of the feedstock can be reduced either 25 prior to or during preparation of the feedstock slurry by employing an in-line or immersedabrader, classifier, mill, high shear mixer, grinder, homogenizer or other particle size reducerknown to those of ordinary skill in the art. No particular particle size is required for thefeedstock; however, smaller particle sizes are preferred as smaller particles are generallybioconverted more quickly than larger particles. WO 03/042117 PCT/US01/45224 .-9-

Grit, such as dirt, sand, soil, stones, pebbles, rocks, feathers, hair and other suchmaterials, is preferably removed prior to addition of the feedstock slurry to the anaerobicdigester; however, grit can be removed at any point along the process. Equipment such asclassifiers, settling tanks, multiphase tanks, and/or or filters can be used to remove the grit. 5 As used herein, the term “useful material” is taken to mean methane gas; hydrogen gas; carbon dioxide; hydrogen sulfide; nitrogen rich fertilizer; protein, amino acid, carbohydrateand/or mineral rich solution or slurry; insecticidal mixture; charcoal; carbon black; insectrepellant mixture; combinations thereof and other such materials which can be prepared byanaerobic digesters from a feedstock. Methane, a nitrogen rich fertilizer, charcoal and an 10 insecticidal slurry are particularly preferred useful materials.

The anaerobic microbe used in the anaerobic digester is any anaerobic bacterium, fungus, mold or alga, or progeny thereof, which is capable of converting the feedstock to auseful material in the anaerobic digester of the invention. Preferred anaerobic microbes areisolated from decaying or composted feedstock, are endogenous to the area in which the 15 feedstock was first obtained, are obtained from bacterial or fungal collections such as those ofthe American Type Culture Collection (ATCC) or have been genetically altered or engineered toconvert a feedstock to a useful material. Particularly preferred anaerobic microbes are those thatwill convert a cellulose-containing feedstock into methane, a nitrogen rich fertilizer, charcoal,humus and an insecticidal slurry. The anaerobic microbe can be a psychrophile, mesophile or 20 thermophile. Generally, a mesophile will prefer operating temperatures in the range of about60o-120° F, and a thermophile will prefer operating temperatures in the range of about 120°-160° F.

Examples of an anaerobic microbe which is useful in the anaerobic digester of theinvention include yeast, a methanogenic bacterium, methanobacterium, acetobacterium, 25 acetogenic bacterium, liquefaction bacterium, Clostridium spp. (methane), Bacillus spp.,Escherichia spp., Staphylococcus spp., Methanobacter spp., Methanobacter (Mb.) omlianskii(methane), Mb. formicicum (methane), Mb. soehngenii (methane), Mb. thermoautrophicum(methane), Mb. ruminatium (methane), Mb. mobile (methane), Mb. methanica (methane),Methanococcus (Me.) mazei (methane), Me. vannielii (methane), Ms. mazei (methane), Mb. WO 03/042117 PCT/USO 1/45224 -10 - suboxydans (methane), Mb. propionicum (methane), Methanosarcina (Ms.) bovekeri (methane),Ms. methanica (methane), Ms. alcaliphilum (methane), Λ&amp;amp;. acetivorans (methane), Ms.thermophilia (methane), Ms. barkeri (methane), Ms. vacuolata (methane), Propionibacteriumacidi-propionici (methane), Saccharomyces cerevisae (ethanol), S. ellipsoideus (ethanol), 5 Clostridium propionicum (propanol), Clostridium saccharoacetoper-butylicum (butanol),Clostridium butyricum (hydrogen), wherein the chemical in parentheses indicates a usefulmaterial which that microbe produces.

Other microbes and/or enzymatic catalysts can be added to the anaerobic digester tofacilitate breakdown of the feedstock into components which are usable by the anaerobic 10 microbe as either nutrients or starting materials for useful materials made by the anaerobicmicrobe. Such other microbes and/or enzymes include, fpr example, amylases, proteases,cellulases, hydrolases, lipid hydrolyzing enzymes, lysozymes, phosphatases, esterases, amidases,and lipases.

The conditions inside the anaerobic digester will vary according to the useful material 15 being produced, the anaerobic microbe being used, the configuration of the anaerobic digester,the feedstock being converted, the desired productivity of the anaerobic digester, and the form ofmicrobe (immobilized or free-flowing) used. Immobilized microbes can be prepared using anymethods known by the artisan of ordinary in the arts. The conditions used to culture theanaerobic microbe and maintain it viable in the anaerobic digester can be varied. Conditions 2 0 which can be controlled include solids content, reaction solution composition, temperature, gascontent, digestion rate, anaerobic microbe content, agitation, feed and effluent rates, gasproduction rate, carbon/nitrogen ratio of the feedstock, pressure, pH, and retention time in thedigester, among other things.

The amount of solids in the digester will generally range from about 1 to about 60% wt., 25 preferably from about 20 to about 50% wL, or more preferably from about 40 to 50% wt. basedupon the total solution weight.

The particle size of solids in the digester affects the rate of digestion. Generally, thesmaller the particle size, the faster the rate of digestion. WO 03/042117 PCT/USO 1/45224 -11 -

The temperature of the reaction solution is generally in the range of about 60° F to about160°F, about 90° F to about 118° F, about 90° F to about 115° F, about 90° F to about 110° F, orabout 90°-95° F. The optimum operating temperature will depend upon the anaerobic microbeused, the product being produced, the pressure under which the digestion is conducted, the 5 carbon to nitrogen ratio of the feedstock, and/or the contents of the feedstock. For Clostridiumspp., the preferred temperature is in the range of about 70-100° F, about 70-95° F, or about 75-95° F. For a mesophilic microbe, the highest level of productivity and highest purity methane isgenerally attained at a temperature in the range of about 90°-l 18° F.

When the digester is operated as described herein, the type of product gas formed will10 depend upon the operating pressure of the digester and the components of the discharge gastreatment system used to process and purify the gas. The digester system can be made toproduce predominantly methane or carbon dioxide. Higher pressure generally promotes theconversion of carbon dioxide to methane and leads to the formation of high purify methane;therefore, lower pressure generally leads to the formation of more carbon dioxide and less 15 methane. Accordingly, when the optimal operating temperature for a particular combination ofanaerobic microbe and feedstock slurry components is identified, the composition of the biogasproduced can be altered by changing the pressure at which the digestion is conducted.

The feed rate of the anaerobic digester is expressed in terms of lbs. of feedstock slurryadded to the digester per unit time. The feed rate can be varied as desired; however, for a 1000 20 gal reactor maintained at approximately 80% of capacity, operated at a temperature of about 95°-105° F, and being used to produce methane, about 50 to about 55 lbs. of poultry waste,containing 25% wt. poultry manure and 75% wt. cotton burrs, about 1/40 of the total weight perhour can be added to the digester,

While not absolutely necessary, the feedstock slurry is generally warmed to a 25 temperature approximating the temperature at which the digester is being operated therebyminimizing temperature fluctuations in the digester that might affect the productivity orefficiency of the system. WO 03/042117 PCT/US01/45224 -12 -

The effluent rate, i.e., the rate at which effluent is drawn from the digester, is related tothe feed rate of the digester. Generally, the effluent rate will not exceed the feed rate when thedigester is operated in a continuous mode. However, the feed rate and effluent rate are generallyindependent of one another when the digester is operated in a batch or semi-continuous mode. 5 During continuous operation, the slurry level in the digester will preferably remain relatively. constant and the feed rate and effluent rate will be kept controlled so as to provide the desiredoverall residence time in the anaerobic digester. Further, the total amount of feedstock slurryadded to the digester will generally exceed the total amount of effluent withdrawn from thedigester, since part of the feedstock is converted to a gas that is also drawn from the digester. 10 During continuous operation, feedstock is continuously added to the reactor at approximately thesame time that gas, effluent, scum, supernatant and/or sludge are removed from the reactor.During semi-continuous operation, feedstock is added to the reactor incrementally and gas,effluent, scum, supernatant and/or sludge are removed incrementally at the same or differenttimes. It is generally preferred that addition of feedstock slurry and removal of digested slurry 15 occur simultaneously, in an overlapping manner or within a short period of time from oneanother. During batch operation, larger portions of feedstock are added to the reactor at giventime intervals and larger portions of gas, sludge, effluent, supernatant and/or sludge are removedfrom the reactor at the same or different time intervals. During continuous operation, theoperating temperature and rate of gas production will be relatively constant. Generally 2 0 continuous operation will provide a greater rate of gas production than batch or semi-continuousoperation.

Gas production rate is expressed in terms of volume of fuel gas produced per given timeinterval of operation, e.g. ft.3 of fuel gas produced per hour or day of operation, in terms ofvolume of fuel gas produced per unit weight of feedstock added to the reactor. In the example 25 described herein, the digester produced approximately 5-8 ft.3 of methane per pound offeedstock.

The quality of the fuel gas produced is generally expressed in terms of the BTU rating ofthe gas as it is removed from the reactor. In the example described herein, the methane collectedfrom the digester had an average rating of about 500-800 BTU without a recirculation loop WO 03/042117 PCT/US01/45224 -13 - installed. Higher ratings in the range of about 800 to about 1000 BTU can be achieved usingone or more of the preferred embodiments described herein. Pure methane, or sweet drymethane, has a rating of 1000 BTU. A gas, such as methane, carbon dioxide, hydrogen, ammonia or hydrogen sulfide; which5 is produced in the anaerobic digester will be present in the reaction solution and headspaceabove the reaction solution. The content of each gas in the headspace and the reaction solutionwill vary according to the conditions, feedstock and/or anaerobic microbe present within theanaerobic digester. The content or percentage of each gas can be monitored using a gaschromatograph or other gas sensing or analyzing equipment used to determine the composition 10 or presence of gases or gaseous mixtures. In preferred embodiments for producing methane, the content of the gas in the headspace will be about 60-100% methane, 0-40% carbon dioxide and0-10% of other gases, such as ammonia, hydrogen or hydrogen sulfide. Since the digester isoperated under approximately or strictly anaerobic conditions, the content of oxygen in thedigester will generally be less than about 5%, less than about 1% or about 0%. When methane 15 production starts, the content of oxygen in the digester will be about 0%. In order to minimizethe introduction of oxygen into the digester, the feedstock slurry may be degassed before or afterloading into the digester. Partial degassing can be done by exposing the feedstock slurry to avacuum or by equilibrating (purging) it with an inert gas. The feedstock slurry will preferablyinclude little to no oxygen, although it can include other gases. 20 The methane, carbon dioxide, or hydrogen produced by the anaerobic digester will generally be cleaned or purified by a scrubber to remove moisture, vapor, droplets, suspendedsolids or other such contaminants. The scrubber can comprise one or more of a filter, desiccant,zeolite, activated carbon, fiber, countercurrent wash solution, mixer, homogenizer, or other suchcomponents typically used in association with or comprised within gas scrubbers. Such 25 components are well known to those of ordinary skill in the art of gas processing. In general,hydrogen sulfide is an undesired by-product or off-gas, which is removed from the desiredproduct gas.

The gases that exit the anaerobic digester or the scrubber are then optionally separatedinto their individual components using conventional gas separation equipment, which is known WO 03/042117 PCT/USO 1/45224 -14 - to those of ordinary skill in the art for separating gas mixtures. The gases may also be processedwith one or more compressor, or dehydration equipment. Alternatively, the gases are stored inpressurized storage vessels or tanks once they have been scrubbed. If the stored gas is purifiedmethane or hydrogen or mixtures of methane or hydrogen with carbon dioxide, it can be used 5 directly to operate the anaerobic digester or one or more of its components or it can be used tooperate additional equipment such as that described above. Ammonia may also be found in theabove-described gases.

The agitation means will agitate the reaction solution in the reaction vessel. Exemplaryagitation means include one or more sparger bars, one or more mechanical agitators, a fluid 10 recirculator, a gas recirculator and combinations thereof.

The sparger bar will bubble a gas through the reaction solution. The gas is generally CO2, that is produced by the anaerobic digester system, and can also be an inert gas such asnitrogen. The gas source can be the gas in the headspace of the anaerobic digester, gas that isdownstream from the anaerobic digester, or a gas cylinder. A preferred sparger bar will 15 recirculate downstream gas, and preferably gas that has had at least some of its methaneremoved therefrom, back into the reaction vessel. By feeding back into the reaction solution, inparticular the sludge layer thereof, a product gas that has had methane removed from it, thereactor will produce more methane per pound of feedstock and the methane will be of higherquality, i.e., it will contain less carbon dioxide and have a higher BTU rating. In a preferred 20 embodiment, a gas recirculator will comprise a sparger bar for adding a methane-stripped orreduced product gas, such as CO2, back into the anaerobic digester, an inlet port for receivinggas from the anaerobic digester, and one or more pumps and/or gas separators.

Another preferred embodiment of the invention provides an anaerobic digester systemcomprising a gas recirculation system comprising a gas separator for removing methane from the 25 discharge gas received directly or indirectly from the anaerobic digester to form a methane-reduced gas, or carbon dioxide enriched gas, which is subsequently fed directly or indirectlyback into the anaerobic digester. In this manner, the thermodynamic equilibrium for thedigestion of the feedstock is pushed toward methane production and carbon dioxideconsumption. WO 03/042117 PCT/US01/45224 -15 -

Another preferred sparger bar will recirculate gas from the headspace of the reactor backthrough the reaction solution and preferably the sludge layer to improve conversion of carbondioxide to methane. A fluid recirculator will preferably recirculate reaction solution from a first part of the5 reaction vessel to a second part of the reaction vessel. Alternatively, the fluid recirculator willrecirculate feedstock sluny, scum sludge, supernatant or reaction effluent, or portions thereofthrough the reaction vessel. For example, the recirculator could recirculate either one or more ofthe scum, supernatant or sludge phases of the reaction effluent. A recirculator could alsorecirculate one or more fluids removed from the digester and added to a tank back to the 10 digester. A recirculator could also recycle supernatant into the feedstock feeder to aid inpreparing the feedstock slurry. According to another preferred βιηί^ίιηβηζ a fluid that isrecycled back into the reaction vessel will have been stripped of at least some and preferablymost or all of its methane gas prior to being added back to the reaction vessel.

Mechanical agitators which are useful in the anaerobic digester include all known fluid 15 agitators such as a turbine, propeller, impeller, paddle, wheel, helical bar, stirrer, rotatingreaction vessel, flexible tube or rod, magnetic agitator, tumbler, paddle wheel, and othermechanical agitators known to those of ordinary skill in the art of fluid mixing. The preferredmechanical agitator is a paddle.

By operating the anaerobic digester at higher pressures, higher quality, i.e., purer, 2 0 methane is produced. Generally, the higher the digester pressure, the higher the purity or BTUrating of methane produced by the reaction vessel. The anaerobic digester generally does notrequire pressurization by external means as gas formation in the digester tends to pressurize thereaction vessel sufficiently. However, the reaction vessel can be pressurized with a pressurizer.The pressurizer can be a compressed gas cylinder, pump, or other such equipment, that forces an 25 inert gas, a produced gas, feedstock sluny, or reaction effluent into the reaction vessel toincrease the pressure of the reaction vessel to the desired operating pressure. Accordingly, thefeedstock slurry feeder, gas recirculator, fluid recirculator, sparger bar or combinations thereofcan serve as the pressurizer. In a preferred embodiment, the anaerobic digester system willcomprise one or more pressure relief valves, vents or exhaust valves to reduce pressure within WO 03/042117 PCT/US01/45224 -16 - the reaction vessel. The anaerobic digester will also preferably comprise a pressure controllercapable of controlling pressure within the reaction vessel and/or a pressure monitor capable ofmonitoring pressure within the reaction vessel. The anaerobic digester system can also compriseone or more pressure gauges that indicate the pressure within the system. 5 The feedstock slurry feeder can be a force-feed or gravity-feed system; however, a force- feed system is preferred. Preferred feeders include pumps of all types or gas pressurized feedtubes or chambers. Pumps are generally more preferred and a progressive cavity pump is mostpreferred.

The productivity of the anaerobic digester system, in terms of gas, especially methane,10 production is related to the pressure within the reaction vessel. The present inventors havefound that the anaerobic digester can be operated at pressures exceeding 10 psi up to a pressurethat does not exceed the design operating pressure of the gas handling system or preventmethane gas from flashing out of the liquid slurry in the digester. The increased pressure effectsan increase in the rate of gas, preferably methane, production and feedstock digestion thereby 15 reducing digestion periods and increasing the overall productivity of the anaerobic digestersystem in terms of ft.3 of methane produced per pound of feedstock. Generally, the higher thepressure of the reaction vessel headspace, the higher the BTU rating of the methane gasproduced.

Temperature affects the productivity of the anaerobic digester. Generally, elevating the 2 0 temperature will increase the productivity, e.g. faster or more efficient gas production, of thedigester up to a temperature that is harmful to the microbial flora in the digester, at whichtemperature productivity will decrease. Different microbes have different optimal temperatures.The temperature of the reaction solution can be controlled with a temperature controller thatheats and/or cools the reaction solution. The temperature controller can be a heater, heat 25 exchanger, jacket surrounding the reaction vessel, coil within the reaction vessel or other suchequipment used for controlling the temperature of fluids within reactors. The temperature of thereaction vessel will preferably be monitored with a temperature monitor, such as a thermocoupleor other equipment known to those of ordinary skill in the art. Alternatively, the temperature ofthe reaction solution is controlled by adding a temperature controller to the fluid recirculator, the WO 03/042117 PCT/US01/45224 -17 - sparger bar, or the feedstock slurry feeder. A heating or cooling jacket surrounding the reactionvessel is alternatively used to control the temperature of the reaction vessel contents.

Fluid levels in the reaction vessel are monitored with a fluid level detector and controlledwith a fluid level controller that either increases or decreases the flow of feedstock slurry into or 5 reaction effluent out of the reaction vessel.

Figs, la and lb include a process flow schematic of a first embodiment of the anaerobic digester system according to the invention. In this embodiment, cotton burrs obtained from acotton gin are converted to methane and a nutrient rich effluent. Cotton burrs are separated fromraw cotton in a cotton gin. The processed cotton is baled and the cotton seed is collected. The 10 cotton burrs are collected and sized in a grinder to an acceptable particle size to form afeedstock. The dirt and sand in the feedstock are separated from the cotton burrs in a cleaner. Acalculated amount of cotton burrs and a calculated amount of chicken manure, which togetherprovide a feed mixture having an approximately 30:1 carbon:nitrogen ratio, are placed in a slurrymixer and heated fresh water is added to form a feedstock slurry which is fed directly into the 15 digester. An anaerobic microbe is added to the digester to form a reaction solution that isheated. In batch operation, the digestion period is allowed to extend for 1 to 60 days, preferablyless than 45 days, and more preferably less than 30 days, while forming a biogas containingpredominantly methane and carbon dioxide and possibly other gaseous compounds. The biogasis passed through a scrubber to remove unwanted components to form a raw gas mixture that is 20 then passed through a low-pressure compressor. The raw gas mixture from the low-pressurecompressor is collected in a low-pressure storage tank or passed through a gas treater to removecarbon dioxide from the raw gas and form high purity (&gt;90% wt., or &gt;95% wt., or &gt;98% wt.)methane. The high purity methane is then compressed with a high-pressure compressor anddried with a dehydrator to fonn “sweet-diy” methane. The sludge from the anaerobic digester is 2 5 sent to a collection tank or a diyer to form dried sludge that can be used as landfill, artificial peatmoss, charcoal briquettes, fuel or other similar purpose. The supernatant or effluent from theanaerobic reactor contains ammonia and is sent to an ammonia stripper that removes theammonia from the supernatant. The treated or ammonia-stripped supernatant is then fed backinto the anaerobic digester and used to digest additional feedstock. The ammonia collected from WO 03/042117 PCT/US01/45224 -18 - the supernatant can be used to make a plant fertilizer. Alternatively, a diluted form of theammonia rich supernatant is used as a fertilizer without removing the ammonia therefrom.

Solid briquettes can be formed from the sludge by a process including the steps of: a)removing the sludge from the digester; b) optionally filtering the sludge in conventional solids 5 filtration equipment to remove the excess fluid from the sludge to form a water-reduced sludge; c) forming the briquettes by pressure molding the water-reduced sludge; and d) optionally dryingthe briquettes in conventional drying equipment. Alternatively, the sludge can be dried aftereither step a) or step b) above. The briquettes and/or sludge need not be, but are preferably,completely dried before use as a fuel; 10 Fig. 2 is a process flow diagram of a second preferred embodiment of the anaerobic digester system according to the invention comprising a feedstock source (16), a feedstockgrinder (18), a feedstock slurry tank (20) and mixer (19), a fresh water source (17), a gas-firedwater heater (6a), a solar water heater (6b), a Jw/Dd-1 hot water heat exchanger (6c), an engineexhaust water heat exchanger (6e), a discharge-gas compressor/water heat exchanger (6d), a 15 feedstock slurry feeder (23), an inlet port (2), an anaerobic digester (1), a reaction solutionagitator (comprising a mechanical agitator (5a) and a sparger bar (5b)), effluent ports (3a-3c), asupernatant storage tank (8), a sludge storage tank (9), a scum storage tank (7), a discharge gasscrubber (10), a discharge gas compressor (11), a discharge gas treater (12), a gas treater outletscrubber (13), a discharge gas aerial cooler (14) and a discharge gas storage vessel (15), a 2 0 temperature controller coil (4), an optional low pressure biogas recirculator (21), and an optionalhigh pressure biogas recirculator (22). The anaerobic digester (1) has a temperature controller,e.g. heating coil, (4) through which water from the heat exchangers or heaters is circulated tocontrol the temperature of the reaction solution. A sparger bar (5b) in the anaerobic digester incombination with the low pressure gas recirculator (21) recirculates gas from the headspace 25 through the reaction solution to provide mild agitation of the reaction solution. The mixer (5a)and the high pressure gas recirculator (22) provide more strenuous agitation if it is needed tomobilize the solids of the reaction solution.

Under standard operating conditions, a feedstock is loaded into the grinder (18) where itis ground to the desired particle size. The ground feedstock and an aqueous solution are then WO 03/042117 PCT/USO 1/45224 - 19 - placed in tank (20) and mixed with the mixer (19) to form a feedstock slurry. An inoculate of ananaerobic microbe and an aqueous solution is loaded into the anaerobic digester (1). Thefeedstock slurry is loaded into the anaerobic digester (1) with the feeder (23) through the inletport (2) until the desired amount of feedstock slurry is added. The digester contents are 5 thoroughly mixed. The anaerobic digester is operated at an elevated pressure, such as thatgenerated by the digester itself, and at a temperature sufficient to promote the digestion of thefeedstock and the formation of the product gas. After a sufficient period of time has passed, thereaction solution is removed from the anaerobic digester as a whole, in portions, or fromdifferent parts of the digester. For example, if the reaction solution is permitted to partition in 10 the digester, the reaction solution can be drawn from the scum, supernatant and/or sludge layersand placed in the respective tanks (7,8,9). The product gas is removed from the headspace of thedigester (1) and passed through a gas scrubber (10). Once the product gas is removed from thedigester, it is also termed the discharge gas. The discharge gas is then compressed by the gas . compressor (11) and passed through the gas treater (12) - gas separation system (13) that 15 removes CO2, and H2S from the discharge gas. The NH3 gas can be removed by passing the gasthrough a contactor and reboiler, regenerator and condenser, and a strong alkaline unit. Thegases removed from the discharge gas can be passed through another gas separation system, notshown, that isolates one or more of the gas components. The gas separation system generallycomprises a series of compressors, condensers, evaporators, pumps, tanks and optionally heating 2 0 and/or cooling coils. The isolated component gas, preferably methane for burning or CO2 forfood grade CO2 production, is then stored in the storage vessel (15) or released to a pipeline (notshown).

The supernatant, sludge and scum solutions and slurries, collectively termed the effluent,which are stored in their respective tanks (7,8,9), are then used as nitrogen rich fertilizer, 25 insecticidal mixture, landfill, anaerobic digester inoculant, or other useful purpose. In addition,the effluent can be dried using conventional equipment to form valuable solid materials that canalso be used as fertilizers, charcoal, carbon black, or other useful materials. The anaerobicdigester tends to form ammonia, which can be removed from the product gas by the scrubber(10), gas treater (12) or gas separation system (13). The ammonia can be removed from the WO 03/042117 PCT/US01/45224 -20 - effluent by evaporation, condensation, precipitation or reaction with an acid source usingmethods well known in the art. The effluent can also be filtered, centrifuged or placed in settlingtanks to separate the solids from the aqueous solution portion.

The product gas, if methane, can be used to operate gas-powered machinery such as the5 hot water heat exchanger (6d), the water heater (6a), and other equipment detailed above.Accordingly, the anaerobic digester system of the invention can be used in a ranch or farmsetting to form a self-sustaining system. For example, the anaerobic digester system can beoperated in conjunction with a chicken (broiler/hens) house to convert waste from the chickenhouse to methane gas, which is used to operate machinery or equipment associated with the 10 chicken house. A cooperative system as described will in effect permit significant wastereduction thereby reducing the harmful effects that excessive hen house waste has on theenvironment. According to another example, the anaerobic digester system of the invention isused cooperatively in conjunction with a swine ranch to convert swine waste to methane and afertilizer solution, wherein the methane is used to operate machinery used in the swine ranch. 15 Other examples include the use of the anaerobic digester system in a cattle feedlot or a dairycattle operation to convert waste material to methane and an insecticidal solution, wherein themethane is used to operate machinery used in the feedlot or dairy ranch.

The anaerobic digester can be operated such that the reaction solution is well mixed or. stratified into the scum, supernatant and sludge zones or layers. When stratified, the scum layer 20 includes materials that float in the reaction solution or are not well digested by the anaerobicmicrobe in the digester. The sludge layer includes materials that are denser than water and mayor may not be digested by the anaerobic microbe. The sludge can also include feedstock solidsthat have not yet been digested. The supernatant layer is between the scum and sludge layersand generally comprises the bulk of the reaction solution. The supernatant layer includes water 2 5 soluble components of the feedstock sluny and water soluble components, such as organic acidsand ammonia, produced by the anaerobic microbe or other microbe or enzyme catalyst present inthe digester.

The anaerobic digester system of the invention can be used to digest feedstockcomprising any farm plant waste or any farm animal manure mixed with the proper additives to WO 03/042117 PCT/US01/45224 -21-. maintain an approximately 30:1 carbon:nitrogen ratio. Exemplary feedstock include cottonburrs; cow manure; cow manure mixed with feed, straw, hay, alfalfa, grass, soil, sand, tumbleweeds and/or wood shavings; and chicken manure and urine containing wood shavings or cottonburrs. The useful materials prepared with these feedstock materials include ammonia, methane, 5 charcoal (briquettes), carbon black, carbon dioxide, a nitrogen rich fertilizer, an insecticidalsolution, and/or an insect repellant.

The supernatant solution taken directly from the reactor was tested as an insecticidalsolution. The supernatant was applied directly to active fire ant mounds in a lawn. Within 24-48 hours, the ant mounds were inactive. In some cases, no ant activity has been seen in the 10 treated mounds for a period of up to three to five months. The surrounding treated lawn is lushand thriving.

Methane was prepared in the anaerobic digester system exemplified herein. The productgas was collected from the headspace of the digester and passed through a scrubber containing amist pad, or a glycol solution. The product gas was then compressed to about 300 psi. The gas 15 was then passed through an amine gas treater to remove CO2. The gas was then pressurizedabove 300 psi and passed through a dehydrator to remove water to form sweet dry methane.Finally, the methane was stored in a pressurized vessel for later use.

Each of the sludge or supernatant layers prepared with the anaerobic digester served as anitrogen enriched fertilizer. For example, an effluent solution that had been 80-90% digested 20 was applied to grass at the required rate. The effluent could be diluted with water prior toapplication. Water containing effluent was applied to a nearby patch of grass. Within about oneto four weeks, the treated grass was visibly greener and lusher than nearby untreated grass.

Carbon dioxide is a common product of anaerobic digestion. The carbon dioxide couldbe separated from the methane by use of a gas treater. The isolated carbon dioxide can be used to 25 make dry ice, to pressurize the anaerobic digester or to provide an inert atmosphere in theanaerobic digester. Alternatively, the carbon dioxide can be reacted with caustic in the presenceof heat or a catalyst to form a bicarbonate salt. The carbon dioxide can also be fed back into thedigester to be converted to methane and increase the overall yield of bioconversion to methane. WO 03/042117 PCT/US01/45224 ' . ' -22 -

Ammonia is also a common product of anaerobic digestion. The ammonia can beseparated from the carbon dioxide and methane by using a gas separator. The ammonia can beisolated as liquid, compressed gas, or aqueous solution containing ammonia. For example,when the product gas is treated with water in the scrubber in a countercurrent manner, the water 5 absorbs the ammonia from the product gas thereby generating an aqueous solution containingammonia. Alternatively, the ammonia can be separated from the other gases by treating theproduct gas with an acidic agent that reacts with ammonia to form an ammonium salt or bypassing the product gas through a bed containing a sequestrant of ammonia to sequester theammonia. The ammonia can be used to make fertilizer or other nitrogen-based products. 10 The sludge and scum layers contain relatively high concentrations of solids. When the solids are dried, they can be used in fertilizer, ground fill, pressed board material, solid fuel,pressed fireplace logs, charcoal briquettes, medium for water filters, or as a “peat moss”equivalent. Alternatively, the sludge and scum layers removed from the reactor can be addedback to the reactor. The solids may also be pulverized into a dust and blown into a burner 15 system for a boiler or furnace for use as a fuel. The sludge can be dried by air, sun, and/or heat.

Fig. 4 depicts one embodiment of an integrated anaerobic digester system (25) comprising a feedstock slurry feeder system (26), a single-stage anaerobic digester system (27), adischarge gas processing system (29), an enriched effluent. processing system (28), a gasrecirculation system (31), and an effluent recirculation system (30). The feeder system prepares 20 the aqueous feedstock slurry in one or more vessels (32-34) and charges the feedstock into theanaerobic digester system (27). The vessels (32-34) are connected in parallel via the conduits(38) or each vessel can be connected to a corresponding anaerobic digester (35-37, respectively)via the conduits (41-42). The anaerobic digester system (27) comprises three anaerobic digesters(35-37) connected in parallel, such that the effluent of one digester does not enter another 25 digester by way of the conduits (38) or (39). The flow of mass is indicated by the direction ofthe arrows. Together, the three anaerobic digesters (35-37) represent a single stage of anaerobicdigestion, meaning that the feedstock is converted to discharge gas and enriched effluent withouthaving to go from a first anaerobic digester (35) into a second anaerobic digester (36). This isunlike the system of Ort, which requires that the feedstock be predigested in a first anaerobic WO 03/042117 PCT/TJSOl/45224 -23 - digested and then completely digested in a second anaerobic digester; therefore, the anaerobicdigesters of Ort are connected serially as opposed to being in parallel.

Even though the anaerobic digester system (27) is depicted with three anaerobic digestervessels (35-37) it can comprise two, four or more such vessels. The vessels can be of the same 5 construction and design or they can be different. In other words, each anaerobic digester vesselwithin a system can be the same or different than another digester vessel in the same system.

After digestion has progressed to the desired extent, the effluent is passed onto theeffluent processing system (28) by way of the conduit (39). There it is processed as describedpreviously herein. The effluent processing system comprises the above-described components. 10 The effluent processing system (28) can be used as an effluent recirculation system (as indicatedby the dash-dot-dot arrow (43)), wherein effluent is returned to the same anaerobic digester fromwhich it is taken. Alternatively, a separate effluent recirculation system (30) can be used. Ineither case, effluent is returned to the same anaerobic digester from which it is taken.

The discharge gas is passed onto the discharge gas processing system (29) by way of the 15 conduit (40), where it is processed as described previously herein. The discharge gas processingsystem comprises the above-described components. The discharge gas processing system can beused as a gas recirculation system (as indicated by the dash-dot-dot arrow (44)). Alternatively, aseparate gas recirculation system (31) can be used.

The integrated system can also comprise a pressurizer system (not shown in Fig. 4) that 20 pressurizes the anaerobic digester vessels (35-37). The pressurizer system will comprise theabove-described components.

The connection of the various systems (26-31) of the integrated system (25) permits easeof total system and individual system scaling such that each system and the system as a wholecan be scaled as needed. For large scale production, for example, the feeder system could 2 5 comprise two mixing vessels; the digester system could comprise four digesters; and the entiresystem couid comprise one or more of each an effluent recirculation system, a discharge gasrecirculation system, a discharge gas processing system and an enriched effluent processingsystem. For small scale production, for example, the feeder system would comprise one mixingvessel; the digester system would comprise one digester; and the entire system could comprise WO 03/042117 PC17US01/45224 . -24 - one of each an effluent recirculation system, a discharge gas recirculation system, a dischargegas processing system arid an enriched effluent processing system.

The carbon to nitrogen ratio of the non-water components of the feedstock will affect theperformance of the digester, i.e., it will affect the productivity of the system, the efficiency of the 5 digestion, the conversion of carbon dioxide to methane, the ability to maintain a stable pH andoperating temperature. Generally, the carbon to nitrogen ratio of the feedstock will be about30:1 for the systems exemplified below; however, feedstock having a carbon to nitrogen ratio inthe range of about 25:1 to about 35:1 will still provide suitable results. When thecarbon:nitrogen ratio of the feedstock is too high, the digestion will be less efficient and will .10 leave an amount of undigested cellulose material.

For feedstock material having a low nitrogen content, a suitable feedstock slurry is obtained by adding a high nitrogen content. As detailed in the examples herein, cotton burrs(low nitrogen content feedstock) is mixed with chicken manure (high nitrogen content material)to form a feedstock material having an approximately 31:1 carbon:nitrogen ratio. The ammonia 15 stripped from the digester effluent or discharge gas can be added in salt, liquid or gas form to alow nitrogen containing feedstock in order to raise the nitrogen content of the feedstock beforeaddition to the digester.

In order to maintain a relatively constant pH for the slurry being digested, it may benecessary to add a buffering agent, alkalizing agent or acidifying agent to the slurry. 20 As used herein, the term "buffering agent" is intended to mean a compound used to resist change in pH. Such compounds include, by way of example and without limitation, potassiummetaphosphate, potassium phosphate, monobasic sodium acetate and sodium citrate anhydrousand dihydrate and other materials known to one of ordinary skill in the art

Generally, an acidifying agent will not be needed as the digestion of the feedstock tends 25 to generate organic acids that drop the pH of digestion medium. As used herein, the term“acidifying agent” is intended to mean a compound used to provide an acidic medium tocounteract a rise in pH of the digestion medium. Such compounds include, by way of exampleand without limitation, inorganic acid, acetic acid, amino acid, citric acid, fumaric acid and other WO 03/042117 PCT/USO1/45224 -25 - alpha hydroxy acids, such as hydrochloric acid, ascorbic acid, and nitric acid and others knownto those of ordinary skill in the art.

Generally, an alkalizing agent will be required since digestion ofthe feedstock tends togenerate organic acids that drop the pH of digestion medium. As used herein, the term 5 “alkalizing agent” is intended to mean a compound used to provide alkaline medium tocounteract a drop in pH of the digestion medium. Such compounds include, by way of exampleand without limitation, lime, lye, ammonia solution or gas, ammonium carbonate, potassiumhydroxide, sodium borate, sodium carbonate, sodium bicarbonate, sodium hydroxide and othersknown to those of ordinary skill in the art. 10 When an anaerobic digester system is constructed according to the invention, loaded with the correct mixture of components in order to form a feedstock having the desired carbon to . nitrogen ratio, operated at the desired pressure and temperature and pH, the system will producehigh purity methane gas.

The anaerobic digester system can be used to purify impure methane gas containing15 carbon dioxide. In this instance, the impure gas is injected into the digester containing adigestion slurry. As the impure gas is added, the methane gas flashes through the digestionslurry while a portion or all of the carbon dioxide is absorbed by the digestion slurry. Theabsorbed carbon dioxide is then converted to methane such that ultimately higher purity methane gas is produced. 2 0. The following examples should not be considered exhaustive, but merely illustrative of only a few of the many embodiments contemplated by the present invention. The methodsdescribed herein can be followed to conduct anaerobic digestion according to the invention.

Example 1

Anaerobic Digestion of Cotton Burrs Mixed with Chicken Manure 25 This process was conducted in a batch-type manner. Fresh well water (318 gal.) was loaded into a digester equipped with an internal mechanical agitator and a heat controller. Thewater was heated until it reached a temperature of about 100°F. Then, anaerobic bacteriuminoculant (10 gal.) and fresh wet chicken manure (14.5 lbs.) were added to the water withmixing. Finally, clean unground cotton burrs (200 lbs.) were added to the digester with mixing WO 03/042117 PCT/US01/45224 -26 - and the digester was sealed. The digester was then purged repeatedly with nitrogen gas to createa substantially anaerobic environment With this loading, the percent solids of the reactionsolution was approximately 4.48%. The digester was run for a period of 45 days with periodicsampling of the headspace. The temperature ranged from about 95°F to about 120°F and 5 averaged about 105°F to about 110°F. The pressure within the digester ranged from about 1 psito about 45 psi. The total amount of gas produced was about 931.5 ft.3. The average rate of gasproduction was about 0.88 ft.3/hr, about 0.004 ft.3/hr/lb. of burrs or about 4.66 ft.3/lb. of burrs.The individual gas components of the product gas ranged from about 49% wt. to about 65% wt.for methane and from about 51% wt. to about 35% wt. for carbon dioxide. The content of 10 ammonia in the product gas was not measured.

The following analytical measures were determined for the cotton burrs and chicken manure used in this procedure. MEASURE BURRS CHICKEN MANURE TOTAL DRIED SOLIDS IN EFFLUENT Carbon (lbs. 16.0789 1.16 17.2389 Nitrogen (lbs.) 0.429 0.1239 0.5529 Carbon/Nitrogen 37.48 9.39 31.18 Energy (BTU/lbs.) 7593 7330 Ash (% wt.) 6.00 10.91 7.22 Volatile Solids (% wt.) 76.30 16.52 69.33 Moisture (% wt.) 17.49 71.5 23.45 Nitrogen (% wt.) 0.21 1.03

For other 200 lbs. batches using approximately the same amounts of ingredients, the total C/N15 ratio ranged from about 30 to about 32. In other batches run according to this procedure, the percent solids ranged from 4.5% to 5.0% wt. WO 03/042117 PCT/US01/45224 -27 -

Example 2

Anaerobic Digestion of Cotton Burrs Mixed with Chicken ManureThis procedure was substantially the same as that of Example 1. The digester produced product gas containing about 63% methane. 5 Example 3

Anaerobic Digestion of Cow Manure

This process was conducted in a batch manner. Fresh well water (150 gal.) was loadedinto a digester equipped with an internal mechanical agitator and a heat controller. The waterwas heated until it reached a temperature of about 100°F. Then, a Clostridium spp. inoculant (1 10 gal.) and cow manure slurry (60 gal.) were mixed in a vessel having a total capacity of 150 gal.,added to the digester and the digester was sealed. The digester was then purged repeatedly withnitrogen gas to create a substantially anaerobic environment. With this loading, the percentsolids of the reaction solution was approximately 25-50%. The pH of the digester slurry wasoptionally adjusted to about 6.5-6.8 with lime. The digester was run for a period of 80 days with 15 periodic sampling of the headspace. The temperature ranged from about 70°F to about 100°Fand averaged about 80°F to about 85°F. The pressure within the digester ranged from about 5 psito about 18 psi. The pH ofthe reaction solution was kept between about 6.5-6.8 by the additionof lime. The methane produced was vented each day and the amount collected ranged fromabout 20-40 ft.3 and averaged about 30-40 ft.3. The total amount of methane produced was about 20 2000 ft.3. About 10-15 lbs. of feedstock were added on a semi-weekly basis. The total amount of feedstock added was about 300 lbs. The amount of sludge, scum and supernatant removedfrom the reactor was about equal to the amount of feedstock added. The average rate of gasproduction was about 6-7 ft.3/hr of methane/lb. of feedstock Fig. 3 depicts a chart of themeasurements obtained for reaction solution pH, amount of methane gas produced (ft.3), 25 reaction solution temperature (° F), and the headspace pressure of the reactor (psi). The BTUrating ofthe methane produced by the digester operated under these conditions ranged fromabout 600 to 850. WO 03/042117 PCT/USO 1/45224 -28 -

Example 4

Anaerobic Digestion of Partially Composted Chicken Mixed with Cotton Gin TrashCompost material, obtained from a plant in Mt. Pleasant, Texas, contained 75% wt. chicken (feathers, blood, bone, organs, flesh, intestines) and 25% wt. gin trash. The compost5 material had been composted for four days in a rotating composter prior to placement in theanaerobic digester. The anaerobic digester was operated within the acceptable ranges of theoperating parameters described herein. The anaerobic digester (55 gal. size drum) included aninoculant obtained from a previous digestion of chicken manure. The particle size of compostmaterial fed into the anaerobic digester was about 1.5 inches initially and was reduced to less 10 than 0.5 inches during the period of digestion. The composted material was slurried with waterto a ratio of about 1 part water to 2 parts compost material prior to sealing the digester. Methaneproduction began about four days after the digester was sealed. Methane gas was released fromthe digester daily. The anaerobic digester was operated for six weeks and produced methane gasat a rate as described herein. 15 A process as described herein can be conducted in a meat, in particular chicken, rendering plant.

The above is a detailed description of particular embodiments of the invention. It isrecognized that departures from the disclosed embodiments may be made within the scope of the 20 invention and that obvious modifications will occur to a person skilled in the art. Those of skillin the art should, in light of the present disclosure, appreciate that many changes can be made inthe specific embodiments which are disclosed herein and still obtain a like or similar resultwithout departing from the spirit and scope of the invention. All of the embodiments disclosedand claimed herein can be made and executed without undue experimentation in light of the 25 present disclosure.