Abstract:
An improved digester apparatus converts wet carbonaceous biomass materials to biogas in a digestion unit operating at a controlled temperature and having a concentrator component and a pressure swing component each containing anaerobic bacteria. The apparatus conveys slurried aqueous biomass from a biomass source to the concentrator component and removes the biogas from the concentrator component and conveys concentrated aqueous biomass from the concentrator component to the pressure swing component and conveys digested aqueous biomass from the pressure swing component to the concentrator component. The apparatus removes waste solids from the pressure swing component and a pressure swing pump controls the pressure within the pressure swing component in a cycle comprising a sub-atmospheric first pressure phase and a second pressure phase at or above atmospheric pressure. Included in the apparatus is a programmable computer provided with a database relating previously measured biomass-biogas conversion data for biomass materials of varying compositions to values for the first and second pressure phases. The computer operates to continuously monitor the pressure of biogas in the pressure swing component and adjust the cycle of the pressure swing pump to optimize biogas conversion of biomass from the biomass source.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the anaerobic biological conversion of biomass such as industrial sludges, slurried refuse and agricultural residues to combined heat and power (CHP). More particularly, this invention relates to a system, a digestion apparatus, and a process for effecting the rapid and complete anaerobic hydrolysis of biomass comprising wet organic matter, and the ultimate conversion of soluble and gaseous byproducts to methane and, ultimately, to useful thermal or electrical energy. 
     BACKGROUND OF THE INVENTION 
     Anaerobic digestion (AD) is a technology having three main disadvantages that result in its rarely being considered for energy production. The principal disadvantage stems from the fact that the reduction in volatile organic solid material is frequently far from complete. The results depend on the substrate, but incomplete conversion is typical of systems in which water is the sole plasticizing agent and hydrolytic pretreatment is not employed. 
     The second disadvantage is the long hydraulic residence time or the length of time the liquid must stay in the digestion system to complete the transformation of the slowest metabolizing materials. Unless the digester is heated, the residence time in the digester can be very long, as much as 40 to 60 days. The third disadvantage is that conventional digestion is prone to failure caused by three types of overloads—organic, hydraulic, and toxic—that can result in the disruption of gas production. In this case, the digestion system must be taken out of service, the digester tank(s) cleaned out, and the system restarted. The environmental effects of such failures can be serious, particularly when other facilities for treatment are unavailable and raw sewage or untreated sludge must be disposed directly to the environment. 
     Incomplete solids hydrolysis is caused by two problems. First, biomass is a mixture of colloidal and particulate constituents that have very different hydrolysis rates. Some organic constituents are metabolized and degrade more rapidly than others. For example, common soluble chemical intermediates such as acetic acid and glucose, as contained in sugar waste waters, are constituents that degrade rapidly. On the other hand, constituents that degrade slowly or not at all include particulate and colloidal materials, such as proteins, fats, vegetable oils, tallow, bacterial and yeast cell walls, lignin and cellulose. Accordingly, the hydrolysis of the most resistant organic fraction becomes the efficiency limiting step since complete degradation can take place only after hydrolysis of all the insoluble constituents&#39; has occurred. In conventional AD, the accumulation of unwanted digestion products wastes reactor space. The economic use of reactor space dictates that the diverse symbiotic bacterial mass and undigested material be efficiently captured and the spent materials be efficiently removed to ensure the hydrolysis of the slowest metabolizing materials. Incomplete hydrolysis and solids accumulation in conventional AD systems is generally responsible for the poor performance for these systems. 
     Conventional AD systems are prone to failure, and operational control has been problematic. Different biomass substrates can have very different degradative characteristics, or different ratios of easily degradable material to refractory organic material. This limits AD systems to one particular substrate and to a small loading range to insure continuous uninterrupted operation. The loading limits are determined by trial and error experimentation. To insure that the operation stays within the limits of digestion, AD operators monitor total gas production supplemented with intermittent analysis of pH, alkalinity and volatile solids testing of the mixed liquor. Error correction is accomplished by manually adjusting the flow rate and solids loading. However, the warning signs of imminent failure usually come too late. This haphazard process control methodology is insufficient to guarantee uninterrupted operation needed for energy production purposes. 
     Various approaches have been proposed for overcoming the shortcomings of conventional AD systems. The disclosures of all of the patents discussed in the paragraphs that follow are incorporated herein by reference. 
     Many investigators have shown the value in recycling solids between two reactors to maintain high substrate and bacterial enzyme concentrations. For example, in U.S. Pat. No. 4,559,142 to Morper, it is recognized that it is economically advantageous to process the more slowly hydrolyzable material in a second reactor, separate from a reactor where the more rapidly hydrolyzed material is treated. Other investigators have recognized the benefit of maintaining the second reactor at a higher temperature to increase hydrolysis of slower hydrolyzing materials. However, these patents do not teach the control of temperature and pressure cycling on the second digester to improve both the rate and completeness of the digestion process. 
     In U.S. Pat. Nos. 5,015,384 and 5,670,047 to Burke, mechanical or chemical enhanced mechanical means are used to thicken and separate the partially digested particulate constituents from the effluent stream and recycle the particulates back to the digester, saving substrate and bacterial enzymes to further the hydrolysis. In subsequent U.S. Pat. No. 6,113,786, Burke recognized the advantage of mechanically removing inorganic solids from the reacting medium in order to preserve reactor space for the partially digested organic solids. The Burke patents, however, do not suggest a process design that promotes in-reactor thickening while digestion and advanced hydrolysis is ongoing, nor do they teach the intermittent separation and removal of inorganic solids via short term gas expulsion during reactor blowdown. 
     Investigators have described the need to improve the digestibility or hydrolysis of wet biomass. In U.S. Pat. No. 5,785,852 to Rivard et al. is proposed an elaborate pretreatment scheme using a pressurized thermochemical and mechanical processes to liquefy 44-66% of the sludge solid prior to digestion. The resulting soluble mixture is then amenable to conventional AD and the inorganic solids washout with the system effluent. The Rivard patent does not suggest the use of in-reactor pressure swing digestion technique to improve the digestibility and hydrolysis of biomass, nor does it teach the reaction of gas plasticization via pressure cycling to disrupt the sludge integrity and enable the advanced hydrolysis of refractory particles. 
     In U.S. Pat. No. 4,642,187 and related U.S. Pat. Nos. 4,401,565 and 4,375,412, the inventor of the present application described a system for separating and routing the slowly hydrolyzable material into a second reactor gas-solid suspension. Anaerobic bacteria are contacted with an influent slurry containing solid organic material, refractory organic material, and undissolved inorganic material, which are captured and recycled between the reactors in a closed loop to achieve a high conversion of the organic material to gaseous products, including methane. It was observed that pressure cycling in this second reactor facilitates the rapid anaerobic breakdown of refractory particulates, as measured by maximum volatile solids reduction and total gas production. 
     This basic approach has enabled rapid refractory solids hydrolysis with the continuous maintenance of a high bacterial concentration within the system reactors to attain nearly full solids conversion. These factors have allowed a significant reduction in digestion tankage and thus have improved the economic factors involved in energy production. U.S. Pat. No. 4,642,187 and its related patents do not teach the method of tuning the pressure swing program, the nature of the second stage reaction, or a variety of modifications needed to enable a wide array of bioenergy applications. Neither do they teach or suggest an apparatus or process of driving the hydrolysis of refractory solids substantially to completion via gas plasticization. 
     Furthermore, in U.S. Pat. No. 4,642,187, the inventor of the subject application described a method and baffle assembly in the first stage concentrator tank that diverts the flow to assist in stripping the gas, thereby facilitating solid-liquid separation and sedimentation. Solids undergoing active digestion often have gas bubbles adhered to them, which changes their specific gravity and settling characteristics. However, if a gasified particle contacts the gas phase during a free-fall cascade, the gases easily transfer from the solid to the gas phase, essentially degassing the solid and enabling the solid to concentrate by gravitational settling. Passing the gasified particles through a submerged gas volume is a minimum cost method of accomplishing effective solids-liquid separation and thickening in anaerobic environments. U.S. Pat. No. 4,642,187, however, does not teach or suggest the additional use of submerged gas volume to collect a gas volume needed to offset volume losses occurring in the second stage vacuum digester. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved digester apparatus for converting wet carbonaceous biomass materials to biogas comprising gaseous organic fuel. The apparatus comprises a digestion unit operating at a controlled temperature and having a concentrator component and a pressure swing component each containing anaerobic bacteria and means for conveying aqueous slurried biomass from a biomass source to the concentrator component, and for removing the biogas from the concentrator component. The apparatus also includes means for conveying concentrated aqueous biomass from the concentrator component to the pressure swing component and for conveying digested aqueous biomass from the pressure swing component to the concentrator component, thereby forming a closed loop between the concentrator and pressure swing components. The apparatus further includes means for removing waste solids from the pressure swing component and a pressure swing pump for controlling the pressure within the pressure swing component in a cycle comprising a first phase having a first time duration at a sub-atmospheric first pressure and a second phase having a second time duration and a second pressure at or above atmospheric pressure. 
     The improvement comprises: inclusion in the apparatus of a programmable computer provided with a database comprising data relating previously determined biomass-biogas conversion for biomass materials of varying compositions to values for the first and second, respectively, pressures and time durations of the first and second phases, wherein the computer operates to continuously monitor the pressure of biogas in the pressure swing component and adjust the cycle of the pressure swing pump to optimize biogas conversion of biomass from the biomass source. 
     This invention, which extends the capability the anaerobic digestion (AD) to substantially complete the transformation of wet biomass to methane gas and energy products, makes use of an ordinary mixed culture of anaerobes, or a pure culture or a genetically modified organism(s) (GMO) to extract the maximum amount of energy from biomass immersed in water. Fully recovering the mass energy content under water is dependent on completing microbial mass hydrolysis. A unique pressure swing method that cycles between pressurization-depressurization and is computer controlled to optimize system efficiency and reliability is capable of superior hydrolysis performance in anaerobic digestion, particularly of difficultly degradable materials such as cellulose. 
     Accordingly, it is an object of the present invention to provide an improved anaerobic cyclic digestion system for converting wet organic biomass materials such as sewage sludge to useful energy by optimizing the process through computer control. 
     It is a further object of the invention to utilize a programmable computer to control the cyclic digestion process, monitor the process for conditions leading to reduced performance, and take corrective action to maintain optimum operation. 
     It is a still further object of the invention to utilize gas plasticization to improve the second stage hydrolysis and digestion in the influent material fed to the system. 
     It is a further object of the invention to utilize a computer to monitor the anaerobic sludge digestion process for indication of insufficient plasticization and take corrective action to improve gas adsorption and penetration and optimize the second reactor hydrolysis rate. 
     It is a further object of the invention to provide an improved method for continuously and automatically correcting for loss or gain in system hydraulic volume to accommodate in-reactor biomass thickening during active continuous digestion by providing a submerged gas volume that is filled with gas production in the first reactor and subsequently used to offset the volume loss in the second reactor. 
     It is a further object of the invention to provide an improved method for continuous and automatic solids capture by providing a submerged gas volume at the top of the first reactor. 
     It is a further object of the present invention to provide an improved method for removing inorganic solids, thereby preventing the buildup of inorganic materials and maintaining system digestion capacity during active continuous digestion. 
     It is a further object of the present invention to provide means for routing the ammonia-containing gas stream to the pressure side of the digestion system and thereby scrub the ammonia gas from the gas phase. 
     It is a still further object of the present invention to provide a means for scrubbing hydrogen sulfide gas, an undesirable combustion fuel contaminant that corrodes metals and produces sulfur dioxide, whose combustion leads to photochemical smog, from the biogas stream. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art anaerobic biomass digestion system described in U.S. Pat. Nos. 4,375,412; 4,401,565; and 4,642,187. Like reference numbers in FIG.  1  and the other figures refer to like elements. 
     FIG. 2 a  is a schematic illustration of a two stage preferred embodiment of an apparatus for carrying out the process of the present invention, and FIG. 2 b  is a schematic illustration of a one stage preferred embodiment of an apparatus for carrying out the process of the present invention. 
     FIG. 3 is a schematic diagram of the timing sequence of a simple pressure swing control program applied to the second reactor of the apparatus of the present invention. 
     FIGS. 4 a - 4   d  are graphs illustrating the real time correction of the biogas output of the pressure swing program for the second reactor. 
     FIGS. 5 a  and  5   b  are sectional plan views of a first reactor  18  in the system view taken along the lines S a — 5   a  and  5   b — 5   b  in FIGS. 5 c  and  5   e.    
     FIGS. 5 c - 5   f  are fragmentary views in elevation of the upper portion of the first reactor  18  in the system of FIGS. 2 a - 2   b , illustrating either a fixed or floating submerged gas volume in a raised or a lowered position. 
     FIGS. 6 a-d  schematically illustrate four modes of a process for removal of the concentrated inorganic solids from the digester apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is an efficient and reliable biological method of converting organic matter submerged in water to useful forms of energy and accomplishes the conversion much faster and more complete than any previous Anaerobic Digestion (AD) art. The first reactor in the loop is used to convert easily degradable organic solids, primarily to grow and maintain a viable mesophilic or thermophilic methanogenic culture. The second reactor of the process is capable of completing volatile solids destruction via a unique gas plasticization mechanism and is especially useful for converting primarily refractory organic solids such as cellulose. 
     The apparatus of the present invention includes one or two tanks, a combination of pipelines, valves, and pumps, including pump means for repetitively cycling pressure in the apparatus between sub-atmospheric pressure and pressure at or above atmospheric pressure, and a programmable controller that is interfaced to a local personal computer or remotely controlled by an accessible file server of a larger computer. The present invention enables the generation of distributive heat and power at industries, wastewater treatment plants, and farms. 
     The applicant has observed that, when properly seeded biomass is treated with pressurized biogas, the entire substance rapidly swells and becomes softened. For the purposes of this disclosure, this process of swelling and softening is known as “gas plasticization”, which can be generally defined as the use of the anaerobic digestion byproducts, biogas and, to a lesser extent, volatile acids and alcohols, to naturally soften and chemically alter solid surfaces. This process produces a substrate that is more easily degradable by anaerobic bacteria. Gas plasticization can be accomplished while the solids are undergoing digestion. The action is similar to plasticization by water, but, unlike water, biogas can penetrate into the native cellulose areas (crystallites). The plasticizing gases form weak covalent bonds at cellulose hydroxyl groups, effectively modifying the original crystalline structure. By continuously repeating the process, both the rate and degree of hydrolysis increases dramatically at relatively low temperatures and pressures. The applicant has observed that both the rate and extent of hydrolysis is a complex function of the contact time, the innate substrate quality, the type and amount of bacterial population present, the amount of accumulated inhibitors present, and the degree of plasticization that the biomass substrate has sustained. The observed exceptional volatile solids reduction achievable by this method includes significant protein destruction, as evidenced by nitrogen and ammonia evolution from the second reactor. 
     The applicant has devised subsystem improvements and companion computer feedback control to provide the following structural and computer control corrections, thereby insuring optimum combined heat and power (CHP) reliability and efficiency: 
     (a) structural corrections 
     (1) solids capture correction: capturing bacterial enzymes and organic solids and preventing them from leaving the system helps protect against deficient gas production 
     (2) volume correction: the system must provide a correction for loss or gain in system reactor volume caused by thickening and plasticization processes, thereby protecting against deficient gas production 
     (3) ammonia gas correction: ammonia and hydrogen sulfide gas removal improves the biogas quality for combustion. Also, removal of ammonia in the gas phase can help protect against deficient gas production caused by microbial culture inhibition. For bioenergy applications, the presence of ammonia and nitrogen in the gas phase is undesirable because they dilute the BTU content of the fuel stream and form nitrous oxide, an air pollutant that causes photochemical smog on combustion. For waste treatment applications, ammonia nitrogen can be removed in the gas phase. 
     (4) inhibition correction: allowing dissolved inhibitory substances to continuously leave the system in the process effluent to help protect against process inhibition that causes deficient gas production 
     (5) inorganic accumulation correction: as solids conversion progresses over time, the byproducts of refractory degradation inevitably accumulate and occupy an ever increasing proportion of the reactor volume. The continuous or intermittent separation and removal of inorganic materials to prevent buildup helps protect against reducing the conversion efficiency, as observed by the slowing of gas production. 
     (b) computer control corrections 
     (1) toxicity correction: the computer control system provides a correction for toxic overload events and thereby helps protect against deficient gas production 
     (2) plasticization and thickening optimization: the computer control system optimizes the in-reactor thickening-plasticization process to protect against deficient gas production 
     (3) hydraulic correction: the computer control system protects against deficient gas production by correcting flow fluctuations that diminish effective residence time 
     (4) load correction: the computer control system helps protect against deficient gas production by providing a correction for organic overload 
     (5) capacity correction. The computer control system provides a correction for diminishing volumetric capacity, timing the removal of grit and inorganic materials to maintain active digester space and optimum gas production. 
     Continuously reliable fuel delivery is of key importance to practical biomass-to-energy conversion systems. Computers can be used to control process variables and thereby optimize energy conversion. Real time sampling of sensor data (feedback) coupled to computerized corrective action in a closed loop enables a “smart digestion” system. Continuous monitoring of the dynamic biogas stream produced from the slowest metabolizing particulates in the second reactor provides data that define the rate limiting conversion reaction for the digestion system. An applied low pressure period effectively resets the over concentration of all volatile materials derived from the slowest metabolizing materials contained in the second reactor. Each pressure cycle is reset, starting the process from the same equilibrium point. Any change in gas production observed (derivative) early on in the pressure phase of the cycle is directly related to a change in the volatile solids destruction rate. Because a shift can be recognized almost immediately, immediate remedial action can be implemented through a feedback control loop. Optimally, four elements are desirable in an effective and scalable cyclic digestion control system: 
     (1) recognition: the controller must be capable of monitoring feedback signals from gas flow measuring, pressure transducer, and thermal sensing devices 
     (2) understanding: the controller must reliably decipher the change in the rate of the feedback signal with respect to other information that is available or recognized 
     (3) analysis: the controller must be able to deduce a diagnosis from the acquired information and be further be able to distinguish between the type of corrective action needed and a process or device error 
     (4) response: the controller must be able take corrective action by changing pump flow rates and applied pressure duration times while the process is ongoing. To maintain optimum operating conditions, the response signal(s) must adjust the process pump appropriately, thereby varying feed and recycle flow rates, pressure amplitudes, and cycle frequencies and periods. 
     The present invention provides a substantial improvement to the anaerobic treatment process described in U.S. Pat. Nos. 4,375,412; 4,401,565; and 4,642,187, which disclose an anaerobic process wherein an influent stream containing organic particulate and soluble material is converted to gaseous products and soluble products. Referring to FIG. 1 of U.S. Pat. No. 4,375,412 (which is also FIG. 1 in each of U.S. Pat. Nos. 4,401,565 and 4,642,187), a schematic illustration is provided of an anaerobic pump (TAP) apparatus for carrying out an anaerobic treatment process that includes a biomass source, a first anaerobic reactor  18  and a second anaerobic reactor  16 . The output from the biological sludge source is fed continuously into a closed loop system  15  containing two tanks  16  and  18 , in which volatile solids conversion takes place. The influent is delivered to the first anaerobic reactor  18  along line  74  and contacted with anaerobic bacteria contained in the upflow partially fluidized bed  80 . 
     The dual purpose of the centrally located partially fluidized bed is to foster rapid bacterial growth at the expense of partial digestion, followed by rapid liquid-solid separation. The upflow velocity (v f ) in the partially fluidized bed  80  is suitably less than the settling rate (v s ) for solid particles through water. For typical sewage sludge, this settling rate is approximately 2.7 centimeters per second. The rising particles exit the partially fluidized bed through baffles  82  and  84  and settle and concentrate in the outer peripheral settling chambers  86  and  88 . Effluent from the first reactor  18  is delivered via recycle line  92  to biological sludge source  12  (or disposed to a sewer), forming a second closed loop. The gravity separated solids exit the first reactor at port  78  and are transmitted to the second reactor  16  through conduit  26  via recycle pump  34 . The concentrated sludge enters the inlet of the second reactor, through the bottom plate  42  into the central passageway  36 . The purpose of the second reactor is to foster a gas-solid suspension in outer passageways  38  and  40  that undergoes pressure cycling. A submerged suspension of solid particles supported by gases generated by anaerobiosis is retained in the passageway  36 ,  38 ,  40  during the pressure phase. During the low-pressure phase of the pressure cycle, the solid flow is downward against the vacuum via gravity, and the gas flow is upward via buoyancy toward the applied vacuum from pump  32 . Pump  54  withdraws mineralized solids from the lower region  30  of the tank  16 ; these solids can be either recycled via line  62  or discharged to a drying bed or sewer. Ammonia and hydrogen sulfide gases can be removed in the biogas outlet stream  62 . Capturing the biogas at the outlet stream  62  is preferred if process inhibition by these two dissolved gases is a likelihood or if removal from the process effluent  92  is needed. 
     The first reactor  18  preferably is sized to allow inlet feed  74  composed of influent feed  14  and recycle products  22  to contact anaerobic bacteria in the partially fluidized upflow reactor  80  for a specified period of time. After degrading a portion of the organic fraction to foster bacterial growth in the internal partially fluidized bed  80  of the first anaerobic reactor  18 , the process concentrates the remaining organic particulate matter by stripping gases through a submerged gas volume formed by baffles  82  and  84 . Solids settle and concentrate via gravity into the bottom zone  78 . The Stage I effluent from the solid-liquid separation, composed principally of a small amount of unmetabolized colloidal and soluble fatty acid products of digestion, is recycled via line  92  to the biomass or sludge source  12 , which can also serve as an aerobic polishing unit. 
     The first reactor  18  has inlet  10  that receives sewage or other biomass. A feed pump  20  feeds sludge from a sludge source  12  via outlet  14  to the reactor  18 . The reactor  18  has outlets  22  and  24  at opposite ends thereof. The tank  18  is preferably maintained with its longitudinal axis vertical. The lower end of the tank may be conical in shape. The tank  18  has another inlet  26  for the concentrated and partially digested sludge from the concentrator tank  18 . Inlet  26  enters the tank  18  between the region  28  at the top of the tank and a region  30  at the bottom of the tank. The pressure in the top region  28  is below atmospheric pressure and is a vacuum. The gauge  21  measures the vacuum in the region  28 . A suitable vacuum pressure depends upon the scale of the system. In a small scale system a vacuum pressure of about −50 centimeters of mercury (gage pressure) has been found suitable. The optimum vacuum range is between −30 to −50 cm Hg gage vacuum. The amount of vacuum needed to maintain the solids in suspension is dependent on the character of the solid (i.e., the solid biodegradiability). A plate  42  with holes  60 , is disposed at the bottom of the passageways and baffles  44  and  46  attached to an upper plate  48  are disposed at the top of the passageways  36 ,  38  and  40 . Gas is drawn by the vacuum in the upper region  28  through openings which form the passageways  36 ,  38  and  40 . The gas has been found to consist essentially of nitrogen (N 2 ) with some methane (CH 4 ) released from time to time in small amounts. This gas may be withdrawn through a gas outlet  70  on the pressure side of the vacuum pump  32 . Supernatant may be recycled back into the activated sludge unit by means of a recycling pump  90  through the feed lines  92 . A gas outlet  94  for the methane and carbon dioxide resulting from the partial digestion of the sludge in the tank  18  is also provided. In the event that the recycling of the supernatant from the top of the concentrator tank  18  is used (such recycling is optional and is desirable when additional dissolved organic contaminant removal is needed), its flow is adjusted by the valve  98 . 
     FIGS. 2 a  and  2   b  are schematic illustrations of systems for carrying out the biomass-to-energy-process in accordance with the process of the present invention. The embodiment depicted in FIG. 2 a  comprises a two-stage digestion unit comprising first reactor  16  and second reactor  18 , a gas storage and cleanup unit  112 , a combustion engine-generator set  114 , and an exhaust gas heat exchanger  116 . The methane-biogas streams  70  and  94  convey biogas produced to a storage unit, for example, a conventional gasbag or water sealed inverted bell gas holder. Gas storage unit  112  may be affixed atop the first reactor  18  or located at the confluence of conveyance lines  70  and  94 . Sulfur gas cleanup can be accomplished by, for example, a solid phase method using Iron Sponge, available from Connelly-GPM Inc., or a liquid phase method using SULFA-CHECK™, as described by Nalco/Exxon Energy Chemicals. Gas sweetening is optional and depends on the requirements of air pollution laws and the need to protect the generator set from corrosive gases. 
     The produced biogas may require pressurization prior to transfer to the engine generator set  114 . Conventional combustion engine generator set  114  may be, for example, part of a complete combined heat and power apparatus  114  and  116  such as the Perkins 2000 Series (UK) or Perennial Energy Inc (MO, USA) apparatus. Gas flow measurement  13  can be accomplished by either dry, liquid ultrasonic (Doppler and transit time) or electromagnetic type apparatus, the choice depending on required measurement accuracy over a fluctuating flow rate range. The digestion portion of the system comprises a digestion apparatus similar to that employed in the prior art apparatus depicted in FIG. 1. A prior art digestion apparatus shown in FIG. 6 in each of U.S. Pat. Nos. 4,375,412 and 4,642,187 is identical to that depicted in FIG. 1, except for the inclusion of a timer  200  and a pump drive motor  202 . Timer  200  is used to change the speed of pump  32  after specified discrete time intervals, e.g., one hour and seven hours. 
     The system of unit processes shown in FIG. 2 a , which forms a very efficient combined heat and power (CHP) process, is the preferred embodiment for weakly concentrated organic solid slurries of 5% w/w solid or less. The continuous flow two-stage process is applicable to large scale wastewater treatment for agricultural and industrial applications. The concentrate from the first reactor  18 , which is composed mostly of undigested colloidal and particulate matter, is pumped to the second reactor  16  via a pump  34 . The mass transformation to gas is completed in the second reactor  16 , which is operated in a pressure swing mode. In accordance with the present invention, the pressure swing operation in the second reactor  16  is controlled by a programmable computer  300  that detects changes in gas production using a measuring device such as a linear pressure transducer  21 , together with pump-controlling means  202 , for example, a variable-speed motor, which adjusts the pump head speed of pump  32  in accordance with the present invention. Computer  300  is further optionally provided with a device  13  for measuring biogas flow in line  70 . 
     The system shown in FIG. 2 b , which comprises a single-stage cyclic pressure digestion unit  16 , a gas storage and cleanup unit  112 , a combustion engine-generator set  114 , and an exhaust gas heat exchanger  116 , represents a very efficient combined heat and power (CHP) process that can be advantageously employed for small scale, high solids (5-10% solid) situations such as farm and MSW applications. Both FIGS. 2 a  and  2   b  illustrate the preferred positioning of energy conversion device  114  and heat recovery apparatus  116  following the reactor(s)  16  (and  18 ) and the connection of the biogas discharge lines  70  and  94 . The waste heat recovered by apparatus  116  can be used to produce steam for industrial applications, or the steam can be directed into a steam turbine to produce additional electricity. Combined cycle plants of this type can increase their efficiencies up to 50-60%. 
     Gas plasticization and thickening represents a method of pressure manipulation that enables the recovery of the maximum amount of energy from an organic solid fraction. Full solids plasticization is necessary to achieve substantially complete conversion of refractory organic materials because otherwise these substances remain too brittle and resistant for anaerobic hydrolysis to occur. Without effective hydrolysis, these substances accumulate, wasting reactor space, require separation from the system and eventually ultimate disposal. 
     The present invention exploits a unique successive gas plasticization and thickening process to enhance anaerobic digestive mechanisms, in particular, the hydrolysis of refractory organic materials. Although gas plasticization has some similarity to plasticization by water, it is well known that water cannot enter the native crystalline structures. Gases, on the other hand, can enter crystalline structures and thereby permanently change the physical properties of biomass. The physical adsorption of biogas on solid surfaces that occurs during the applied pressure phase of the pressure program applied to the second reactor  16  causes surface deformations that are measurable by observing the viscosity hysteresis and are indicative of slurry thixotropic rheology. By repeating the compression-decompression cycle and thereby repeatedly deforming the solid structure, a suspension of semi-elastic or partially gelatinized solids is formed. On decompression, the gas accumulation is rapidly released (blowdown), and the applied low pressure is sufficient to overcome the forces responsible for the poor gas-liquid or mass transfer rates typical of thick digestion environments. The cyclic compression-decompression prepares the solid surface for increased microbial enzyme action and produces a facultative-methanogenic culture in which the anaerobic activity is far superior to that found under normal equilibrium digestion conditions. 
     The conversion efficiency is maximized by sequentially and repetitively executing the steps of the pressure swing adsorption (PSA) cycle depicted in FIG. 3, as follows: rapid pressurization of a low pressure suspension, gas plasticization and thickening of the pressurized suspension, rapid decompression of the suspension, and low pressure stripping of gas. Upon termination of the decompression phase, the slurry is fully degassed. Degasification and thickening are simultaneous processes occurring at low pressure, resulting in settling of the solids to occupy the degassed volume. There is then a transition to high pressure, where the gas plasticization process occurs. During this period of relatively high pressure, the nascent gas bubbles produced by the anaerobic action accumulate in the solid particle voids, creating a suspension held in passageways  38  and  40 . The growing gas volume produced by digestion of the solids displaces the interstitial fluid upward out of the reactor as recycle effluent at  22 . Next, during the applied decompression phase of the pressure program, the gas bubbles rise through the suspension, effectively weight classifying the suspension by gas density differential classification, allowing the lighter volatile organic solids to rise in the column and the heavier mineralized solids to fall downward. The continuous displacement of liquid upward and digesting solids downward effectively accomplishes automatic thickening in the second reactor  16 . In-reactor thickening results in improved process stability with respect to organic overloading by effectively increasing the solid residence time, increasing the solution pH, and increasing the substrate concentration. It is the mechanism of sequential plasticization and thickening that enables the substantially complete solids conversion at room temperature of organic particles having very different hydrolysis rates. 
     Cellulose is an example of a major component of most biomass feedstocks that, because of its crystalline structure, is resistant to degradation by anaerobes. However, digestive byproducts can be used as gas plasticizing agents to initiate the dehydration of cellulose and other complex polysaccharides. Anaerobic plasticizing agents include polyhydric “sugar” alcohols, volatile fatty acids, and the gases of anaerobiosis: ammonia, hydrogen, carbon dioxide, methane, and hydrogen sulfide. Regardless of the chemical differences among the various agents, the overall conversion efficiency is dependent on whether the treatment is sufficient to derivitize native cellulose, presumably by forcing it to form a covalent bond with the plasticizing agent. When cellulose is derivatized, the fibers are chemically modified, and the plasticized material is said to be “partially gelatinized”. The cellulosic solid thus becomes compressible whereas the surrounding liquid is incompressible. While the cellulose derivative is in a “plastic” state, the cellulose is well oriented for bacterial attack from a high density anaerobic population grown on the organic mass surfaces in the first reactor  18 . In the second reactor  16 , these high density fermentation enzymes hydrolyze the plasticized long chain molecules. The moisture level within the reactor needs to be controlled to be at least 50% or more to assure proper expansion for the desired degree of plasticization. A solids residence time of about 2 to 5 hours is sufficient to plasticize the admixture at room temperature and 1.5 atmospheres. The fermentation mixture containing a high concentration of facultative anaerobic bacterial cells is held for the appropriate plasticization time in the second reactor  16 . As the cellulose decomposing enzymes secreted by the facultative anaerobic bacteria convert the fibers into much shorter ones, the particles become much smaller, and the plasticizing operation in the second reactor is greatly facilitated because the increasing surface area can support an ever increasing bacterial population. Fermentation mixtures having lower moisture content require higher pressures, and higher moisture formulations require lower pressures, to achieve the same results. It is beneficial to pre-soak with water or preheat the feed mixture to initiate plasticization under suitable plasticizing conditions. The digestion of refractory solids in cellulosic herbaceous biomass materials is thereby greatly accelerated. 
     Biomass materials frequently contain substantial levels of bacterial pathogens such as, for example, fecal streptococci. The waste solids produced by the process of the present invention are substantially free of bacterial pathogens. 
     The preferred embodiment of the present invention uses programmable computer means  300  with proportional, integral, and differential (PID) capability to control the rate and extent of solid gasification in the second reactor  16 . There are many modified pressure swing adsorption (PSA) cycles, as known in the separation industry and as schematically depicted in FIG. 3, that can be applied to a variety of designs with multiple adsorption beds in parallel or series configurations. PSA configurations, i.e., purge and backfill steps, can be designed to optimize the kinetics of gas transfer for various mixtures and enzyme-catalyzed adsorbent mediums. Thus, for anaerobic systems, various PSA configurations can be applied to mixed culture anaerobic systems as well as to fermentation systems that involve pure or genetically altered organisms (GAO) cultures. 
     In anaerobic pressure swing, however, the overall kinetics of enzyme catalyzed gas-solid reactions is more complex than simple gas separation via adsorption. There are multiple biochemical pathways complexed with competing mechanisms. The pressure swing amplitudes and phase durations required to drive transformations are nonlinear functions of a combination of matrix characteristics and process physical variables. At a minimum, the kinetic performance is dependent on the mass biodegradability (including percentage cellulose), mass transfer characteristics, solid permeability and adsorptive capacity, gas concentration, substrate (solid) concentration, temperature, and pressure. Assuming reliable isothermal conditions, however, it is only necessary to capture the gases produced and control the critical variable, applied pressure. The multivariate complex problem is thereby reduced to just one process control variable. A record of each applied pressure cycle is measured by monitoring the gas production  13  on line  70  throughout the cycle shown in FIG.  3 . The gas production pattern is digitally logged for each cycle, forming a historical database. The current gas production pattern is compared to the recent pattern records to detect and flag any errors or deficiencies that are recognized. The points of differential change between current and recent historical gas production patterns are calculated in near real time. Significant pattern and differential changes trigger a real-time corrective feedback to improve production, the appropriate response being selected from a subset of allowable responses. The magnitude of the response can be based on some probabilistic rule or be simply proportional to the magnitude of the change. The default response is the “do nothing” response. 
     The preferred embodiment of the present invention uses a programmable computer  300  to monitor biogas production at line  70 , pressure at  21 , process temperature in the water jacket return  120 , pump flow rates  34 ,  54 , and  32 , and valves  56  and  58 . Computer  300  and pump drive motor  202  control vacuum pump  32 , which operates cyclically to produce faster flow for a period of time, then slower flow for a period of time. As the solid volume is reduced due to advanced hydrolysis, the incoming solids from passageway  36  enter the passageways  38  and  40 , displacing liquid upward, continuously replenishing and concentrating the solids in reactor  16 . The rapid decompression step shown in FIG. 3 releases the gases accumulated during the previous plasticization phase. Computer  300  causes valves  56  and  58  to open, exposing the reactor  16  contents to near atmospheric pressure. The compressed gases that have accumulated expand toward the low pressure outlet at valve  56  and  58 . Recycle pump  34 , controlled by computer  300 , runs at a constant rate that corresponds to the rate of volume turnover in the digestion system. The pressure regulating program of the present invention uses computer  300 , shown in FIGS. 2 a  and  2   b  to adjust pump speeds after comparing the time derivative of the feedback variable to the stored historic value. (This function could not be performed by the timer 200 shown in prior art FIG. 6 of U.S. Pat. Nos. 4,375,412 and 4,642,187.) New set-points are calculated for the next cycle; when the elapsed time set-point is reached, the pressure program triggers a change of the pump drive motor  202  voltage to reach the new time or pressure setpoint. A change in pump  32  flow rate causes more or less flow to exit the second reactor  16  at  22  and varies the pressure therein between a pressure below atmospheric and above atmospheric pressure. For room temperature (20° C.) operation, a low cellulose substrate (relatively high biodegradability) program would have, for example, an amplitude and phase of about 0.5 atmosphere of low pressure for one hour and about 1.5 atmospheres of high pressure for one hour. By contrast, a relatively high-cellulose substrate (low biodegradability) program would have, for example, an amplitude and phase of about 0.5 atmosphere of low pressure for 1 hour and about 1.5 atmospheres of high pressure for 3-4 hours. Increasing the digestion temperature will increase the biochemical reaction rate proportionately. A unit increase in reactor temperature enables the pressure amplitude and phase duration to be decreased proportionately to achieve the same results. 
     The process control computer  300  employs pattern recognition capability to continuously track and optimize the gas production and solids conversion in the second reactor  16 . The most important portion of the gas production pattern is acquired during the low pressure decompression phase, which is compared to logged patterns produced at the outlet  70 . As shown in FIGS. 4 a  and  4   c , the discharge of the gas accumulated during the compression phase causes the derivative to oscillate and requires a considerable settling time of approximately ½ hour to reach a consistent production level. The applied low pressure effectively zeros out the gas accumulation in the second reactor  16 . The gas production at line  70  provides a characteristic reproducible gas production pattern. Each pattern is digitally logged, the discrete point measurements being preserved for derivative analysis and for providing a historical record for future comparisons. As a new production segment is being acquired, the previous segment is being scanned for deficiencies. During each scan in background, the process control computer discriminates between properly formed and deficient gas production patterns that are the object derivative analysis. For example, FIG. 4 a  and its integral FIG. 4 b  show a deficient gas production pattern, where production falls off prematurely to almost zero at the 2500 second mark of a one hour low pressure decompression period. In this example, approximately 110 seconds of gas production time is wasted. The accumulative loss over time causes deficient gas production that has a considerable negative effect on performance. In this case, it is unlikely that the premature falloff is due to inadequate bacterial density, since loading to and gas production from the first reactor  16  has remained constant, an indication of good culture activity. Instead, the abrupt falloff is most likely due to insufficient plasticization time during the compression phase of the cycle. Depending on whether the gas production is increasing or decreasing from previous measurements determines whether the time derivatives are positive or negative. For the positive derivative, proportionately increasing the plasticization time will correct the error and yield the improved gas production pattern shown in FIGS. 4 c  and  4   d . The process response specification requires PID computer  300  to correct for the production deficiency over several cycle periods or a few hours. The most recent output pattern at  70  is compared to the historical pattern record to diagnose deficiencies and formulate and execute a corrective action in that time frame. Since the wide range of biomass materials is characterized by greatly differing hydrolysis rates, a relatively short response time is crucial. Continuous process tuning is required to correct for changes in hydrolysis time and enable the conversion of a variety of materials without process slowing and shutdown. This method of continuously adjusting control parameters allows the process to accommodate large changes in feed material characteristics or loading without experiencing weak performance. Switching from one slurried fuel to another becomes seamless. Typical process ailments such as incomplete plasticization, hydraulic overload, organic overload, toxicity and insufficient acclimation time will take only a few cycles, measured in hours, to detect, diagnose, and correct. Unmanned process monitoring and adjustment can routinely prevent anaerobic fermentation failure that generally occurs over a much longer time frame. 
     A well tuned PID-controlled process can ramp up or down to reach pressure set-points within minutes, as shown in FIG.  3 . The response specification is based on overshoot, rise time, decay ratio, and settling time that is initialized during process startup. These initial specifications are established during clean reactor pressure tests. Default PID parameters for process gain, time constant, and time delay are estimated based on the analysis of the transient response of the closed loop system to setpoint changes or load disturbances. These parameters can be calculated using “modified” Ziegler Nichols tuning. During operation, the discrete data are analyzed for error derivatives in background to pinpoint performance changes. The working specification based response adjustment is a continuous process. Because of the superior response of PID control to pressure or time setpoint changes, response adjustments are generally much faster than slow developing process perturbations that affect the gas production patterns illustrated in FIGS. 4 a-d.    
     Any loss of digestible organic solids in the process effluent  92  from the first reactor  18  reduces process performance. To prevent this, reactor  18  is provided with either a fixed baffle assembly, as shown in FIGS. 5 c-d , or an internal floating roof assembly, as shown in FIGS. 5 e-f , is used to capture enough gas volume to make up the volume lost in the second reactor  16 . In a preferred embodiment for small volume makeup, the first reactor  18  contains an internal fixed baffle assembly, shown in FIGS. 5 c-d , that consists of cylindrical volume forming fixed inverted baffles  82  and  84  attached to a top plate, thereby creating an underwater baffle assembly above and adjacent to the settling passageways  86  and  88 . The submerged gas volume is a sealed continuous gas volume, and internal walls of passageways  86  and  88  are vertically slotted to accommodate the baffle length. The uppermost liquid level is limited by the elevation of outlet port  92  and the lowest most vertical liquid level by the top of the internal upflow partially fluidized reactor  80 . The baffled volume has two purposes. First, the gasified particles rising through the upflow partially fluidized bed  80 , are stripped of their attached gases as they fall through the submerged gas volume, enabling the degasified particles to easily settle in the outer passageways  86  and  88 . Secondly, the gas volume provides for variable reactor liquid volume while maintaining a consistent overhead gas phase pressure. 
     As the liquid level in the first reactor  18  falls due to the loss of volume in the second reactor  16 , the pressure regulator transfers gas from the submerged baffled volume  82  and  84  to the overhead gas space equivalent to the amount of liquid volume lost. The pressure swing operation causes the loss in system volume. Therefore, the two-stage system is in a perpetual state of either increasing or decreasing in digestion volume. For design purposes, the reactor volume lost during the compression phase in the second reactor  16  is used to size the baffled volume in the first reactor  18 . The cyclic operation produces an effluent substantially free of suspended solids exiting at line  92  and a thickened solid concentrate exiting at  78  of the first reactor  18 . Referring to FIG. 5, the tank cover is closed by a fixed cover  102  at the upper end thereof where the gas outlet  94  is located. The clarified supernatant liquid outlet  92  is located in passageway  86  or  88  and the gas is released through outlet  94 . An internal fixed volume with the device shown in FIGS. 5 c-d  is more reliable for volume correction since it has fewer moving parts. However, an internal floating roof, as shown in FIGS. 5 e-f , can be substituted for the fixed baffle assembly depicted in FIGS. 5 c-d  for larger variable volume applications. In the floating roof design shown in FIGS. 5 e-f , the floating pontoon assembly  98  can be weighted to add pressure to the gas submerged volume, which gives the first reactor  18  expandable storage capability needed to accommodate and equalize widely fluctuating volume and pressure applications. 
     The interstitial liquid within the partially fluidized bed  80  flows upward in the first reactor tank  18  in the direction shown by the arrows on the dashed lines shown in FIG.  1 . Tank  18  is maintained at approximately atmospheric pressure. The flow velocity V f  must be significantly less than the settling velocity of the suspended fraction v s ; otherwise the solids liquid separation significantly deteriorates, and solids capture by the system is less than optimum. Gas produced by anaerobiosis collects under baffled region  82  and  84 , forming and holding a submerged gas volume. As the slurry mixture tops the weir elevation and falls over the weir, the contact with the submerged gas volume assists in stripping the biogas from the digesting particles. The clarified effluent flows out through the outlet  94 . Gas stripping enables the solid particles to be concentrated both by bioflocculation and by gravity sedimentation in the outer passageways, and then be withdrawn before they can regasify and rise through the clarified water column. Concentrated and partially digested sludge from the tank  18  is recycled by the pump  34  to the inlet  26  of the second reactor tank  16 . The recycle rate through the pump  32  is sufficient to turn over the complete system volume approximately once per day. 
     The concentrated solids from the first reactor  18  comprise biomass, organic particulate and colloidal solids that have not been hydrolyzed, a high concentration of anaerobic bacteria, and inorganic precipitates such as metal sulfides, struvite, phosphates, and other complex precipitates. The organic particulate and colloidal constituents in line  26  are the more slowly hydrolyzed and degraded materials that composed the influent stream  14 . The organic particulate and colloidal constituents with anaerobic bacteria are confined in a gas suspension created in the second anaerobic reactor  16  via an applied vacuum for a period sufficient to nearly completely hydrolyze and metabolize the constituents. The second anaerobic reactor is constructed to withstand at least a one-atmosphere pressure swing between 0.5 atmosphere and 1.5 atmospheres. The enriched anaerobic microbial culture found there will cause a rapid hydrolysis of more slowly metabolizable constituents. The improvement of plasticization allows the volume of this second reactor  16  to be reduced in size to about one-third the size of the first anaerobic reactor  18 . The quantity of biomass temporarily stored and digesting in the second anaerobic reactor  16  affects the system hydraulic retention time, which is determined by the rate of decomposition of plasticized particulates as well as the mass of anaerobic bacteria required to hydrolyze of influent substrate from the first anaerobic reactor  18 . The hydraulic retention time in the second anaerobic reactor  16  may also be affected by the accumulation of anaerobic decomposition products such as ammonia and sulfide and mineralized solids. 
     Process efficiency can be improved by drawing the vacuum on the second reactor  16  at the position of vacuum pump  32  on line  22 , as shown in FIGS. 2 a-b . At this position, it is easier to control both the vacuum and pressure cycling swings across the second reactor  16  and the flow velocity to the first reactor along line  74 , which feeds the upflow velocity in the partially fluidized bed  80 . The relocation of pump  32  from line  72  to line  22  prevents wasting energy in separating out influent dissolved gases, thus preventing additional nitrogen and ammonia gases from contaminating the biogas at outlet  70 . Decreasing the amount of nitrogen and ammonia improves the methane quality of the biogas stream  70  and reduces the formation of nitrous oxide air pollutants during biogas combustion. Nitrogen and ammonia gases produce undesirable photochemical smog on combustion. 
     The removal of mineralized solids from digestion is important to prevent the accumulation over time of spent solid, which would occupy a portion of digestion space needed for active digestion. If these solids are not removed, the mineralized solids will eventually accumulate to a point where the digestion mechanism and the performance of the system is impaired. The quantity of waste solids is a function of the influent inorganic solids content together with the portion that dissolves during fermentation and plasticization and exits the system via system effluent line  92 . Since the inorganic solids accumulate slowly, they will normally need to be removed from the apparatus only intermittently. 
     As already discussed, the process of the invention entails a simple particle classification by weight, using buoyancy provided by rising gas bubbles discharged during the blowdown phase of the pressure cycle, as shown in FIG.  3 . This method effectively separates heavy residual particles from the lighter volatile particles through the release of temporarily stored biogas accumulated during the pressure plasticization phase. In this way, certain soluble constituents that contain hydrophobic structures such as organic acids, proteins, enzymes and lighter particulate matter can be separated from the heavier inorganic particles by buoyancy provided by the rising gas surge. Increased removal of such constituents can be accomplished by increasing the gas bubble surface area through utilization of fine gas bubbles or increasing gas/solids ratios. It is understood that separation aids or chemicals may be used to help accomplish or improve the separation. If, on the other hand, non-chemically aided micro-bubble floatation is used, the method must be capable of producing a recycle concentrate substantially composed of undigested volatile solids and an underflow principally composed of mineralized or inorganic solids. 
     FIGS. 6 a-d  illustrate the preferred process for removal of the concentrated inorganic solids, wherein valves  58  and  56 , sludge pump  54 , pump bypass line  72 , recycle line  62 , and discharge line  76  comprise the assembly positioned at the bottom of the second reactor  16 . The concentration of spent solids provides an opportunity for effective particle separation by simple density classification using only the rising blowdown gases. This procedure is preferred for its simplicity, but it is recognized that the separation of undissolved inorganic materials can be accomplished by any number of different differential density classification technologies such as centrifuges, hydrocyclones, and cyclones. 
     In the four modes of removing the inorganic solids in accordance with the present invention, illustrated in FIGS. 6 a-d , the operation of the valves and pump are controlled by computer  300  to open or close, turn on or off, at a precise time and for a precise time duration, depending on the operation mode. The first two operational modes, shown in FIGS. 6 a-b , are sequentially used to prevent inorganic solid accumulation in the system. A gas blowdown event, i.e., release of the gas pressure in reactor tank  16 , produces a rapid gas discharge of accumulated digestive gases under pressure. During this event, the heavier inorganic particles fall rapidly through rising bubbles, whereas lighter organic floc will rise via bubble buoyancy. The components of the slurry mixture in line  62  are classified according to their respective specific gravities by the rising gases released when valve  56  opens and valve  58  closes. Eventually, the rising gases exit the system via gas outlet  70 . The result is particulate stratification, whereby the heavier mineralized solids are located at the bottom of line  62  and the lighter solids fraction is carried upward via gas buoyancy. Referring to FIG. 6 b , the hydrostatic pressure in line  62  can be used to expel the heavier mineralized solids at the bottom by opening valve  58  and closing valve  56  following blowdown. The valve discharge is timed to remain open only long enough to discharge the mineralized solids layer. The intermittent discharge of inorganic and mineralized solids via this method is typically conducted once per week for normal sewage sludge feedstocks. The light suspended, colloidal and particulate constituents, bacteria and inorganic precipitates separated from the soluble constituents are recycled, being delivered to the first anaerobic reactor  18  via line  62  when the third mode of operation begins. Referring to FIG. 6 c , the third mode of operation is the recycling of the bottom solids back to the rest of the system to reseed these solids that have undergone repeated cyclic decompression and rapid hydrolysis, as shown in FIG.  3 . Also, these solids effectively weight the gravity separation of solids in the outer separation chambers  86  and  88  that follow the upflow partially fluidized bed  80 . It is preferable to have the solids content of the material influent line  74  as high as possible because the more concentrated the solid feed stream is, the smaller the first anaerobic reactor  18  need be, provided the higher concentration of solids does not inhibit the anaerobic bioactivity from accumulating dissolved byproducts of digestion. Referring to FIG. 6 d , the fourth mode of operation is the hydrostatic removal of stabilized solids from the system in the case where production of stabilized soil conditioning solids for land application is desired or if solids need to be prematurely wasted from the system to reduce the effects of inhibitory conditions. 
     In accordance with the present invention, both reactors of the pressure swing digestion system preferably maintain a consistent fermentation temperature in either the mesophilic or thermophilic range. The temperature is controlled in the range of, preferably, about 20° C. to about 60° C., a temperature of about 35° C. being especially preferred. Both of the reactors shown in FIGS. 2 a-b  have double inner and outer wall insulation to prevent heat loss to the environment. Either heated water or exhaust gas from an engine-generator set can be used as the heat transfer fluid that flows through the jacketed volume to make up the environmental heat loss. Although other heat exchange methods such as external sludge or mixed liquor heat exchange or gas heat exchange can be used, they are less favorable because they require the removal of liquid and/or gases which disturb the preferred dispersive plug flow hydraulic regime. 
     Preferably, the gas outlet line  62  extends to intersect manifold line  14  on the discharge side of pump  20  to scrub the ammonia and sulfur-containing gases from the biogas stream. If ammonia and hydrogen sulfide are produced in small enough quantity, their dissolved and recycled species will not inhibit digestion in first reactor tank  18 . It is particularly desirable to scrub these gas constituents out of the biogas stream on the pressure side of the process, thereby removing them from the combustion fuel mixture. Further reduction in sulfur from the product gas streams  70  and  94  can be accomplished by providing either a soluble or liquid sulfur scavenger system at  112 . This is particularly useful for CHP applications. 
     The preferred embodiment of the two-stage digestion process of the present invention reduces the ammonia and sulfide toxicity problems recognized to exist with conventional methane digesters, either single or multiple stage system designs. The two digestion reactors operate in a closed loop, with pressure swing digester tank  16  passing its recycle effluent to the concentrator digester tank  18 , liquid being separated from the gravity thickened solids, which are then returned to the digester tank  16  while liquid effluent  92  is continuously discharged. This process substantially reduces the concentration of ammonia and sulfide and prevents toxicity problems. If necessary, additional water can be added to recycle line  26  or  74  to dilute the products of digestion and prevent interference with anaerobic bioactivity. 
     The apparatus and process of the present invention improve both the speed of digestion and completeness of conversion, which greatly improves the process economic feasibility. The capital and operating costs are reduced by a decrease in hydraulic retention time, a decrease in system reactor volume, and complete conversion of the feed. Referring to FIGS. 2 a-b , note that the cost of removing the solids in the first reactor  18  is based on the minimum hydraulic detention time of the reactor while maintaining a high solids capture ratio. The quantity of solids recycled from the first reactor  18  dictates the size and capital cost of the second reactor  16 . The preferred economic decision point will be where the easily hydrolyzed solids are successfully removed in the first reactor  18 , while the soluble or nonparticulate constituents discharged through line  92  and the more slowly hydrolyzed solids are completely removed in the second reactor  16 . The combination of great improvements in speed and conversion completeness enables a much smaller system tankage and therefore much improved capital economics and process feasibility. 
     From the foregoing description, it is apparent that the present invention provides an improved apparatus and process for the anaerobic digestion of organic biomass materials such as sewage sludge or biomass. While exemplary forms of the system and the best mode for its operation have been described, it will be appreciated that variations and modifications can be effected within the spirit and scope of the invention, which is defined by the claims that follow.