Abstract:
Improved methods for anaerobic digestion of organic matter to produce biogas. Among the improvements given are including ferric iron in a hydrolysis reactor to increase the rate and efficiency of anaerobic hydrolysis to provide substrates for methanogenesis. A solids separation step is added after hydrolysis and before methanogenesis to improve the efficiency of the methanogenesis step. Other improvements involve using separate tanks for the hydrolysis and methanogenesis stages and using two (or more) methanogenesis tanks in sequence, and switching the order of the two (or more) methanogenesis tanks periodically.

Description:
BACKGROUND 
     Anaerobic Digestion (AD) is a way of converting organic biomass to biogas. Hazardous chemical waste can also often be digested by microorganisms and disposed of by fermentation. 
     AD produces biogas, a mixture of primarily methane and carbon dioxide, which can be used for energy. This is an advantage over landfilling waste. 
     AD has three distinct phases: hydrolysis, acetogenesis and methanogenesis. Hydrolysis and acetogenesis often occur in the same tank and are thus sometimes considered one phase. 
     In AD, anaerobic microorganisms ferment carbon substrates to products in the absence of oxygen or oxygen surrogates. For instance, some organisms transform hexose sugars to ethanol and CO 2 . Other common fermentation products include lactic acid, acetic acid, butyric acid, H 2 , and methane. Many of these compounds are themselves substrates for further anaerobic metabolism by other microorganisms. However, two fermentation products cannot be further fermented—methane and CO 2 . Thus, all anaerobic decomposition can ultimately lead to methane and carbon dioxide. 
     AD biologically is generally considered to occur in two phases: (1) Breakdown of sugars and carbohydrates to smaller molecules, particularly organic acids such as acetic acid and butyric acid. This is known as hydrolysis or sometimes as acid production. And (2) production of methane and CO 2  from the smaller organic molecules produced in phase (1), known as methanogenesis. Different organisms catalyze phases (1) and (2). Acid forming microorganisms and other microorganisms, that include both facultative and obligate anaerobic microorganisms catalyze phase (1), the hydrolysis phase. Organisms that produce methane are called methanogens. Methanogens can produce methane from acetate by the reaction acetate+H 2 O-&gt;methane+HCO 3   − . Methanogens can also produce methane from hydrogen and CO 2  by the reaction 4H 2 +HCO 3   − +H + -&gt;CH 4 +3H 2 O. The hydrogen for methanogenesis from carbon dioxide in nature comes from fermentation of reduced carbon substrates. Only methanogens—which are obligate anaerobes—produce methane, hydrogen and carbon dioxide through the cleaving of acetate and formate and remove protons from the cytoplasm. 
     Methanogenesis is the most essential part of AD as it is the only stage that removes protons from the cytoplasm to allow the preceding steps to proceed. A well-functioning methanogenesis stage is key to maintaining a functional and efficient AD systems. Without it AD systems become septic and fail. 
     Of all metabolic pathways, methanogenesis yields the least amount of energy as the end product—methane—has a high enthalpy. Consequently there is minimal energy yield for the organisms involved causing them to grow very slowly. System design to retain biomass is critical. Traditional AD systems accomplish stability through large tanks that provide adequate time for a stable methanogen population to be maintained. This results in large tanks that are expensive to build and are not optimum. 
     Even in such tanks the process is always somewhat incomplete: some portion of the substrate is not digested all the way to methane and CO 2 . And the speed of the process is a limiting factor that determines the size of reactors needed. Thus, more efficient and faster methods of anaerobic digestion are needed, and digester systems that facilitate or allow more efficient and faster anaerobic digestion are needed. 
     SUMMARY 
     The inventors have developed improved methods of fermentation that ferment organic substrates to biogas more quickly and more completely than typical current methods. 
     One embodiment provides a method of producing biogas comprising: 
     (a)(i) grinding an organic substrate comprising solids to produce smaller solids particles (thus allowing for more intimate contact of the substrate with the microorganisms); 
     (a)(ii) transferring the substrate with smaller solids particles to a hydrolysis tank; 
     (a)(iii) hydrolyzing the substrate in an anaerobic condition in a hydrolysis tank comprising Fe 3+  particles; 
     (a)(iv) transferring effluent from the hydrolysis tank to a solids separator 
     (a)(v) separating solids from the hydrolysis effluent to produce a solids fraction and a screened hydrolysis effluent; 
     (b)(i) transferring the screened hydrolysis effluent for a period of time  1  to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     (b)(ii) transferring the methanogenesis tank  1  liquid effluent to a methanogenesis tank  2  comprising fixed film methanogens or granular methanogens during period of time  1  and producing biogas and methanogenesis tank  2  liquid effluent in methanogenesis tank  2 ; 
     (b)(iii) discharging methanogenesis tank  2  liquid effluent during period of time  1 ; 
     (c) and then switching the order of methanogenesis tanks  1  and  2  and 
     (c)(i) transferring the screened hydrolysis effluent for a period of time  2  to said methanogenesis tank  2  and producing biogas and methanogenesis tank  2  liquid effluent; 
     (c)(ii) transferring the methanogenesis tank  2  liquid effluent to said methanogenesis tank  1  during period of time  2  and producing biogas and methanogenesis tank  1  liquid effluent in methanogenesis tank  1 ; 
     (c)(iii) discharging methanogenesis tank  1  liquid effluent during period of time  2 . 
     In this embodiment, ferric iron is included in the hydrolysis tank. Typically it is fed daily to the hydrolysis tank. We have found that the presence of Fe 3+  particles increases the rate and efficiency of the hydrolysis stage, broadly defined as producing substrates for methanogenesis, which may include sugars, organic acids, H 2 , and other organic molecules and inorganic molecules that are substrates for methanogenesis. The ferric iron functions as an electron sink or electron acceptor. Other metallic electron acceptors may be used in place of ferric iron. 
     The above embodiment also includes a solids separation step after the hydrolysis step to produce a screened hydrolysis effluent that we have found is more suitable for the methanogenesis stage. We have found that this also improves the speed and efficiency of the methanogenesis step. The term “screened” hydrolysis effluent is used to mean that it is depleted in solids and particles as compared to the hydrolysis effluent before the solids separator. The screened hydrolysis effluent may not be totally clear or totally free of solids and particles. 
     It further includes using separate tanks for the hydrolysis and methanogenesis stages and using two (or more) methanogenesis tanks in sequence, and switching the order of the two (or more) methanogenesis tanks periodically. This is also found to improve the speed and efficiency of the process. 
     Another embodiment provides a method of producing biogas comprising: 
     (a)(i) transferring an organic substrate to a hydrolysis tank; 
     (a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; 
     (b)(i) transferring the hydrolysis effluent for a period of time  1  to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     (b)(ii) transferring the methanogenesis tank  1  liquid effluent to a methanogenesis tank  2  comprising fixed film methanogens or granular methanogens during period of time  1  and producing biogas and methanogenesis tank  2  liquid effluent in methanogenesis tank  2 ; 
     (b)(iii) discharging methanogenesis tank  2  liquid effluent during period of time  1 ; 
     (c) and then switching the order of methanogenesis tanks  1  and  2  and 
     (c)(i) transferring the hydrolysis effluent for a period of time  2  to said methanogenesis tank  2  and producing biogas and methanogenesis tank  2  liquid effluent; 
     (c)(ii) transferring the methanogenesis tank  2  liquid effluent to said methanogenesis tank  1  during period of time  2  and producing biogas and methanogenesis tank  1  liquid effluent in methanogenesis tank  1 ; 
     (c)(iii) discharging methanogenesis tank  1  liquid effluent during period of time  2 ; 
     (d) passing the biogas produced in methanogenesis tanks  1  and  2  through a foam trap to trap foam and separate it from the biogas; and 
     (e) collecting the biogas downstream of the foam trap. 
     The biogas is collected from both methanogenesis tanks  1  and  2 . It is ordinarily collected from the dome of the tanks, allowing the gas to pass through a foam trap prior to the pressure equalizing tanks in preparation for gas conditioning. 
     The process of anaerobic digestion often produces surfactants that cause foam to form. Foam in the biogas often causes many problems including corrosion and plugging of downstream gas units, especially when foam and vapor condenses at cooler temperatures. Including the foam trap in the process minimizes or eliminates these problems. 
     Another embodiment provides a method of producing biogas comprising: 
     (a)(i) transferring an organic substrate to a hydrolysis tank; 
     (a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; 
     (b)(i) transferring the hydrolysis effluent for a period of time  1  to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     (b)(ii) transferring biogas from the methanogenesis tank  1  through a foam trap to trap foam and separate it from the biogas; and 
     (c) collecting the biogas downstream of the foam trap. 
     Another embodiment provides a method of producing biogas comprising: 
     (a)(i) transferring an organic substrate to a hydrolysis tank; 
     (a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; 
     (b)(i) transferring the hydrolysis effluent for a period of time  1  to a methanogenesis tank  1  comprising granulated and fixed film methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     (b)(ii) recycling liquid in methanogenesis tank  1  to promote growth of granulated and attached biomass in the methanogenesis tank  1  reactors to improve retention and reduce volume required for the methanogenesis tank; 
     (b)(iii) transferring biogas from the methanogenesis tank  1  through a foam trap to trap residual foam and separate it from the biogas; and 
     (c) collecting the biogas downstream of the foam trap. 
     Another embodiment provides a method of producing biogas comprising: 
     (a)(i) transferring an organic substrate to a hydrolysis tank; 
     (a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; 
     (b)(i) transferring the hydrolysis effluent for a period of time  1  to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     wherein the hydrolysis tank and methanogenesis tank  1  each have a recirculation loop and one or more instrument detectors for monitoring one or more parameters of the liquid in the tank in the recirculation loop, and the method comprises recirculating liquid from each tank back into the same tank through the recirculation loop for that tank, and monitoring one or more parameters in each tank with the one or more instrument detectors in the recirculation loop of that tank, wherein the parameters are selected from the group consisting of pH, ORP, ionic strength, chemical oxygen demand, and dissolved methane concentration; wherein each recirculation loop has at least one valve separating the recirculation loop from its tank, and the at least one valve can be closed to allow the one or more instrument detectors to be removed for cleaning or service without allowing air to contact liquid in the tank connected to the recirculation loop. 
     Another embodiment provides a method of producing biogas comprising: 
     (a)(i) transferring an organic substrate to a hydrolysis tank; 
     (a)(ii) hydrolyzing the substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; 
     (b)(i) transferring the hydrolysis liquid effluent for a period of time  1  to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; 
     (b)(ii) transferring the methanogenesis tank  1  liquid effluent to a methanogenesis tank  2  comprising fixed film methanogens or granular methanogens during period of time  1  and producing biogas and methanogenesis tank  2  liquid effluent in methanogenesis tank  2 ; 
     (b)(iii) discharging methanogenesis tank  2  liquid effluent during period of time  1 ; 
     (c) and then switching the order of methanogenesis tanks  1  and  2  and 
     (c)(i) transferring the liquid hydrolysis effluent for a period of time  2  to said methanogenesis tank  2  and producing biogas and methanogenesis tank  2  liquid effluent in methanogenesis tank  2 ; 
     (c)(ii) transferring the methanogenesis tank  2  liquid effluent to said methanogenesis tank  1  during period of time  2  and producing biogas and methanogenesis tank  1  liquid effluent in methanogenesis tank  1 ; 
     (c)(iii) discharging methanogenesis tank  1  liquid effluent during period of time  2 . 
     In all these methods, the liquid in the hydrolysis tank is preferably intermittently or continuously mixed. Mixing keeps any solids suspended so they can be more efficiently digested and broken down. It is also important to periodically remove undigestible solids from the hydrolysis tank. This can be accomplished by having the solids evenly suspended when effluent is removed from the hydrolysis tank. 
     Another embodiment provides a mobile system for digesting organic matter and producing biogas, the system comprising: a shipping container or trailer adapted for carriage on a truck or train; the shipping container or trailer containing: (a) a digester comprising: (i) a pump for pumping a liquid containing organic material into (ii) a hydrolysis tank; the hydrolysis tank hydraulically connected to (iii) a methanogenesis tank comprising fixed film methanogens and an outlet for liquid effluent; (iv) a heater adapted for heating liquid in the container or liquid fed into the hydrolysis tank or methanogenesis tank; (v) a plurality of instruments having detectors in contact with liquid or gas in the hydrolysis tank and methanogenesis tank; the instruments linked to a (b) computer for receiving data; and the computer linked to a (c) modem for transmitting data from the shipping container or trailer to a remote computer. 
     Another embodiment provides a method of producing biogas comprising: heating clean water to steam or hot water; mixing the steam or hot water with an organic substrate; and fermenting the organic substrate to biogas. 
     Heaters for anaerobic systems typically involve metal heating elements in contact with the liquid containing substrate being digested. The organic matter in anaerobic digesters, including organic solids being digested, hydrolytic microorganisms, and methanogenic microorganisms, and other biomass, all can corrode heating elements and shorten the lives of heating elements. By heating clean water to produce hot water or steam and mixing the hot water or steam with the an organic substrate to be digested, instead of heating the organic substrate directly, we have been able to extend the life of the heating element. 
     Another embodiment provides a method of producing biogas comprising: (a) hydrolyzing an organic substrate in anaerobic condition in a hydrolysis tank to produce a hydrolysis liquid effluent; (b) transferring the hydrolysis liquid effluent to a methanogenesis tank  1  comprising fixed film methanogens or granular methanogens and producing biogas and methanogenesis tank  1  liquid effluent; wherein the methanogenesis tank  1  has a liquid substrate and a recirculation loop for recirculating liquid substrate in the methanogenesis tank, the recirculation loop comprising a heating element in contact with liquid substrate in the recirculation loop; (c) recirculating the liquid substrate through the recirculation loop and returning the liquid substrate to remainder of the methanogenesis tank; and (d) heating the liquid substrate in the recirculation loop with the heating element; wherein the liquid substrate in the recirculation loop is depleted in the fixed film methanogens or the granular methanogens compared to liquid substrate in the remainder of the methanogenesis tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the components of a digester system and a method of producing biogas. 
         FIG. 2  shows an anaerobic digestion tank with a recirculation loop and a monitoring instrument in the loop. 
         FIG. 3  shows methanogenesis tanks with a foam trap and components for collecting biogas. 
         FIG. 4  shows a mobile system for digesting organic material and producing biogas. The mobile system can be shipped to a distant site and used to test and demonstrate the methods of the invention on specific substrates at distant sites, while still being controlled and monitored at a base location. 
         FIG. 5  shows use of a heater to heat feed water for mixing with the feed organic substrate in the hydrolysis tank. 
         FIG. 6  shows use of a recycle flow and a heater element with a methanogenesis tank. 
     
    
    
     DETAILED DESCRIPTION 
     The inventors have developed new methods and systems for anaerobic digestion of organic material to biogas. 
     One method and a system of the invention is shown in  FIG. 1 . It involves transferring an organic substrate  1  to a hydrolysis tank  11 . The organic substrate  1  can be any suitable organic substrate. In some cases it is chemical manufacturing waste. In other embodiments, it is agricultural or food waste, for instance, potato skins, banana skins, food grease or food oil, corn stover, etc. In other embodiments it is municipal garbage, municipal waste, sewage, or livestock manure. In other embodiments, it is slaughterhouse waste. It also may be, of course, a combination of wastes, which may include these wastes or others. 
     In  FIG. 1 , the organic substrate  1 , where it includes solids, may be processed by grinding in grinder  2  to produce smaller solids to enhance hydrolysis. In a specific embodiment, the organic substrate  1  is processed by grinding to a particle size of 6 mm or less. In some cases, the organic substrate is or includes solids, in others it may be or include a liquid. 
     The organic substrate  1  is transferred to hydrolysis tank  11 , where it is hydrolyzed in anaerobic conditions. The hydrolysis tank may optionally include ferric iron (Fe 3+ ). The ferric iron can be in the form of magnetite or other forms of solid or dissolved ferric iron. The ferric iron is believed to improve the efficiency of anaerobic hydrolysis because it functions as an electron sink. It is believed that other metallic electron sinks (electron acceptors) could replace the ferric iron. These would include, for instance, Mn, Co, Ni, Cu, and Zn cations. In other embodiments, the hydrolysis tank does not include ferric iron or any other metallic cation electron sink. 
     In the hydrolysis tank the substrate is hydrolyzed and fermented to smaller molecules that are substrates for methanogenesis. The content of the hydrolysis tank is preferably intermittently or continuously mixed. A hydrolysis effluent  12  is produced and is transferred to a first methanogenesis tank  21 . The hydrolysis effluent  12  is preferably processed by a solids separator  17  to separate out a solids fraction  14 , which is typically removed from the system, and a screened hydrolysis effluent  13 . We have found that placement of a solid separator at this stage improves the efficiency and speed of the methanogenesis stage. 
     The solids separator  17  in one embodiment is a screw type solids separator. Other types of solids separators may also be used. 
     In all the methods described herein, the liquid in the hydrolysis tank is preferably intermittently or continuously mixed. Mixing keeps any solids suspended so they can be more efficiently digested and broken down. It is also important to periodically remove undigestible solids from the hydrolysis tank. This can be accomplished by having the solids evenly suspended when effluent is removed from the hydrolysis tank. Selection of mixing intensity, frequency, and equipment used for mixing can be optimized to effect conversion of the substrate or organic acids and other soluble organic compounds. 
     The screened hydrolysis effluent is transferred to methanogenesis tank  21 . This is an anaerobic tank holding methanogens. Methanogens are obligate anaerobic microorganisms classified as archaea. The methanogenesis tank preferably contains a substrate  41  on which methanogens can grow to form a fixed film  51 . A good substrate is JAEGER SURFPAC. The methanogenesis tank  21  may contain granular bed methanogens  52  in addition to fixed film methanogens  51  or instead of fixed film methanogens  51 . 
     Methanogenesis tank  21  produces methanogenesis tank liquid effluent  21   e.    
     The biogas from methanogenesis tank one ( 21 ) accumulates in a headspace  25 . 
     Methanogenesis tank one liquid effluent  21   e  passes to methanogenesis tank two ( 22 ). We have found that using two methanogenesis tanks in sequence gives more complete digestion of the substrate and more complete conversion to biogas. Having a second methanogenesis tank also helps trap granules of methanogens to prevent loss of the methanogens. It helps trap the granules because the gas production is slower in the second methanogenesis tank since the substrate is depleted by the first methanogenesis tank. Less gas production means less bubbles trapped in the granules and therefore the granules are less buoyant in the second methanogenesis tank and less likely to rise and be carried out in the liquid effluent  22   e.    
     Having recycle loops in each of the methanogenesis reactors (tanks) also serves to favor the growth of granulated and attached film biomass. Over time, poorly settling fluffy biomass is removed while heavier granules and film attached to the substrate for fixed film biomass is retained. 
     A liquid effluent  22   e  is withdrawn from the second methonagenesis tank  22  and biogas comprising CH 4  and CO 2  is also produced and collected. 
     The biogas in methanogenesis tank  22  also accumulates in a headspace  25  before being collected or vented. 
     We have found that it is beneficial to switch the order of methanogenesis tank one ( 21 ) and methonagenesis tank two ( 22 ). This improves the functioning of both tanks and improves the speed and efficiency of biogas production from the substrate. The second tank in order receives less food (substrate for methanogens), so the microorganisms would gradually die back if the tank remained in the second position permanently. By switching the order periodically, we can maintain a high biomass density in the second tank that completes the digestion of the feedstock to biogas with high yield. It also, as mentioned above helps to trap the granular bed methanogens in the second methanogenesis tank before they escape the system. 
     The methanogenesis tanks one and two ( 21  and  22 ) can be switched in order at fixed periods of times, for instance every 24 hours or every 12 hours. Alternatively, we have found good results by switching based on parameters that are easily measured using instruments. One such measurement is the oxidation-reduction potential (ORP) of either or both tanks. For example, the first methanogenesis tank in sequence (the lead methanogenesis tank) should have an ORP of −300 to −550 mV. The ORP in the lead methanogenesis tank rises over time, and it can be switched when the ORP in it rises to a present value. For instance, the switch may be made when the ORP in the lead methanogenesis tank rises to an ORP in the range of −300 to −400 mV. For instance, the preset value to trigger the switch may be a specific ORP between −300 and −400 mV, for instance −350 mV. The second methanogenesis tank (the lag methanogenesis tank) in sequence should have an ORP of about −300 to −550 mV. The ORP in the lag methanogenesis tank falls over time (becomes more negative). We have in some cases executed the switch when the ORP in the lag methanogenesis tank falls to an ORP in the range of −450 mV to −550 mV. When the second reactor is moved to the first position, its ORP will rise because it will begin receiving hydrolysis tank effluent, which has a higher ORP. 
     The specific ORP ranges listed in the foregoing description are presented for illustration purposes and may vary from feedstock to feedstock. 
     In a specific embodiment, the time period between switching the order the two methanogen tanks is between 6 and 24 hours inclusive. In another embodiment, it is between 6 and 48 hours, inclusive. 
     The hydrolysis tank  11 , in addition to preferably containing Fe 3+ , preferably contains a microorganism that that reduces Fe 3+  and produces at least one volatile organic acid from organic substrates. Thus, Fe 3+  is an electron acceptor for anaerobic respiration. This improves the speed and efficiency of anaerobic digestion. The microorganism that reduces Fe 3+  and produces at least one volatile organic acid from organic substrates in one embodiment is or is derived from ATCC 55339. 
     Each of the tanks preferably has a recirculation loop, as shown in  FIG. 2  for hydrolysis tank  11 . Recirculation loop  63  allows recirculation of fluid in the tank. This is one way of maintaining an upward flow of liquid in the main portion of the tank to stir the contents of the tank and maintain good mixing and less settling. This improves the efficiency of the digestion. A recirculation pump  61  is coupled to recirculation loop  63  to pump liquid through the loop. In specific embodiments, the recirculation loop  63  includes one or more probes  62  to measure one or more parameters of the liquid in the loop, such as pH, ORP, total organic carbon, and chemical oxygen demand. The liquid in the recirculation loop  63  of course is the same liquid in the tank  11 , so this allows one to measure these parameters in the tank. The recirculation loop  63  can include valves  65  that can be shut off to separate the recirculation loop. This allows us to remove the probe or probes  62  for cleaning or calibration, without emptying the entire reactor and without exposing the contents of the tank to oxygen. 
     Another embodiment of the invention is the use of a pressure gauge near the bottom of the tank and a gas pressure gauge in the head space of the tank to measure liquid level in the tank. By the difference between pressure in the bottom of the tank and gas pressure above the liquid level, one can calculate the height of the liquid by use of a density of 1.00 kg/L or whatever the density of the liquid in the tank actually is. Thus, another embodiment is a method of calculating height of liquid in a tank, wherein the tank comprises a liquid level and a gas headspace above the liquid level, the method comprising measuring liquid pressure at a known position A near the bottom of the tank and measuring gas pressure in a headspace above the liquid, and calculating the height of the liquid above the known position A by the difference in pressure between the gas pressure in the headspace and the liquid pressure at position A. 
     In yet another embodiment, one or more hydrolysis tanks and/or one or more methanogenesis tanks comprise a gas pressure probe in a headspace of the tank and a liquid pressure probe near the bottom of the tank. 
     The hydrolysis tank  11  and first methanogenesis tank  21  and second methanogenesis tank  22  may each comprise a plurality of vessels, although they are each shown as a single vessel in  FIG. 1 . 
     Each of the reactors—the hydrolysis reactor, the methanogenesis reactor  1  and methanogenesis reactor  2 —has a hydraulic retention time, which is the average time that the liquid is contained in the reactor. This is equal to the inflow flow rate (or outflow flow rate) divided by the liquid volume of the reactor. With the systems and methods described herein, surprisingly small hydraulic retention times can be used while still achieving almost complete conversion of the substrate to biogas. In one typical embodiment, the hydrolysis tank has a hydraulic retention time of 2 days, and methanogenesis tanks  1  and  2  each have a hydraulic retention time of 1 day, and the entire system has a hydraulic retention time of 4 days. 
     In specific embodiments, the hydrolysis tank has a hydraulic retention time of 9 hours or less, 24-96 hours, 24-72 hours, 36-48 hours, or 72 hours or less. 
     In specific embodiments, methanogenesis tank  1  and methanogenesis tank  2  each have a hydraulic retention time of 48 hours or less, 12-48 hours, 24-38 hours, 12-36 hours, 24-36 hours, or 12-24 hours. 
     The total hydraulic retention time is the hydraulic retention time for the whole system, i.e., the sum of the hydraulic retention times of all hydrolysis tanks and methanogenesis tanks connected in series. In specific embodiments, the total hydraulic retention time is 8 days or less, 6 days or less, 4 days or less, about 4 days, 3-6 days, 4-8 days, 3-8 days, or 2-8 days. 
     Referring to  FIG. 3 , the biogas accumulates in headspace  25  of the one or more methanogenesis tanks  21  and  22 . Where there are two or more methanogenesis tanks, the headspace of the two or more tanks is preferably connected, as shown in  FIG. 3 . There may be valves that can be closed to separate the tanks. 
     The headspace biogas in one embodiment is collected through a foam trap  62 . The foam trap  62  has a large amount of surface area, on which foam and other liquids are trapped or condense. 
     The foam trap is advantageously used because foam can plug the gas collection system and can cause corrosion of system hardware. 
     Upstream from the foam trap, in some embodiments, may be a gas/solids separator  61 . This also separates solids from the gas and separates a significant amount of liquids and foam from the gas as well. The gas/solids separator typically has an over-and-under flow pattern with a wide section, then a narrow passageway, then another wide section. Solids and liquids tend to be trapped and fall back down in the narrow passageway. 
     After the foam trap  62 , the biogas is collected in biogas collector  63 . This is preferably expandable to maintain the same gas pressure at all times. 
     In all the methods, systems, and devices described herein, the methanogens may be retained as fixed film methanogens or as granular bed methanogens. In some embodiments, both fixed film and granular bed methanogens are present. 
       FIG. 5  shows a system and method of the invention with hydrolysis tank  11 . Organic substrate  1  is shown being added to the hydrolysis tank  11  for hydrolysis of the organic substrate  1 . In  FIG. 5 , organic substrate  1  also passes through a grinder  2  to grind solids to smaller solids particles for better hydrolysis and digestion of the solids. In  FIG. 5  a heater  101  is shown. Water is heated by the heater  101  to produce hot water or steam, and the hot water or steam is mixed with the organic substrate. The hot water or steam may be mixed with the organic substrate before it is added to the hydrolysis tank, as is shown in  FIG. 5 , or can be mixed with the organic substrate in the hydrolysis tank. Either method accomplishes the goal of heating the organic substrate achieve an optimal temperature to promote faster and more efficient hydrolysis. With the method shown in  FIG. 5 , the heating element of the heater  101  only contacts clean water, not the organic substrate. This prolongs the life of the heater. Also, it may be advantageous to premix the hot water or steam with the organic substrate before adding the organic substrate to the hydrolysis tank  11 . This prevents contacting the microorganisms in the hydrolysis tank with extremely hot water or steam, which would happen if the hot water or steam is mixed directly with the contents of the hydrolysis tank, and which might kill some of the hydrolytic microorganisms. The term “clean water” in this context is intended to mean not that the water is necessarily absolutely pure, but that it has less organic matter and other substances that can damage the heating element than the remainder of the “organic substrate.” 
       FIG. 6  shows the use of a heating element in a recirculation loop connected with a methanogenesis tank. In  FIG. 6 , methanogenesis tank  22  is shown with a recirculation loop  66 . Liquid substrate  22   s  of the methanogenesis tank is circulated through the recirculation loop  66 , and it contacts a heating element  102  in the recirculation loop  66 . This allows heating of the liquid substrate  22   s  in the methanogenesis tank to an optimum temperature for methanogenesis. The recirculation loop  66  is shown connected to the methanogenesis tank  22  near the upper level of the liquid substrate  22   s . Since the granules  52  and fixed film methanogens  51  are heavier than water, they will tend to fall in the tank and be depleted in the liquid substrate in the recirculation loop  66  as compared to the liquid substrate in the remainder of the methanogenesis tank  22 . This tends to save the heating element by minimizing its contact with solids and methanogenic granules and microorganisms. 
     The use of a recirculation flow as is shown in  FIG. 6 , with or without a heating element in the recirculation loop, is beneficial. By optimizing the rate of recirculation, growth of granular methanogens is optimized while preventing washout of the granular methanogens. Too rapid a flow can lead to washout of the granular methanogens through liquid effluent  22   e . But lower flow rates promote growth of granular methanogens. 
       FIG. 4  shows a mobile system of the invention for digesting organic matter and producing biogas. The mobile system  70  comprises a shipping container or trailer  71  adapted for carriage on a truck or train. The shipping container or trailer contains: (a) a digester  72  comprising: (i) a pump  73  for pumping a liquid containing organic material into (ii) a hydrolysis tank  11 ; the hydrolysis tank  11  hydraulically connected to (iii) a methanogenesis tank  21  comprising fixed film methanogens. The methanogenesis tank comprises (iv) an outlet  23  for liquid effluent. The digester further comprises (v) a heater  74  adapted for heating liquid contained in or fed into the hydrolysis tank or methanogenesis tank; and (vi) a plurality of instruments  75  having detectors in contact with liquid or gas in the hydrolysis tank and methanogenesis tank. The instruments are linked to (b) a computer  76  for receiving data; and the computer linked to (c) a modem  77  for transmitting data  78  from the shipping container to a remote computer  81 . The remote computer may be at a distance from the shipping container, that is, across the country or the world. 
     The remote computer and the system may be configured to allow the remote computer to control and monitor the digester. 
     The mobile system may also include a solids separator operating between the hydrolysis tank and the methanogenesis tank, as described above. 
     The mobile system allows the digester system to be transported by truck or rail to a distant site to test and demonstrate the performance of the system on a particular feedstock on location and under the conditions found at the location. The digester can be remotely monitored and controlled by persons at a distance so that those persons do not need to travel also to the site where the system is tested. 
     Example 
     The Novus Bio-Catalytic (NBC™) mobile pilot is a trailer-mounted, multi-cell, two stage anaerobic digestion system. The system currently consists of 3 digestion cells, but has been designed to accommodate up to 7 cells. Each digestion cell is 4 feet in diameter (O.D.) and 9 feet tall with a liquid capacity of 3,000 gallons (8′ liquid column) and a gas headspace of 50.3 cubic ft. In the 3 cell configuration, liquid capacity is 9,000 gallons. The pilot is housed in a semi-trailer with a 50′×7′ box, ventilated and with electric baseboard heating. The pilot is designed to digest a variety of solid and liquid organic substrates. In the current 3 cell configuration, the digestion loading capacity of the system is: 
     1. Solids capacity: 1000 lbs (dry wt.) per day 
     2. Liquid Capacity: 500 gallons per day 
     The pilot system consists of the following: 
     1. Subsystem  1000 : Feed Preparation and Pumping 
     2. Subsystem  2000 : Hydrolysis 
     3. Subsystem  3000 : Solids separation and recycle 
     4. Subsystem  4000 : Methanogenesis and Polishing 
     5. Subsystem  5000 : Gas Handling 
     The stages and their modes of operation are described in the following sections. 
     Subsystem  1000 : Feedstock Preparation and Mixing 
     The purpose of Subsystem  1000  is to convert solid and liquid substrate to a slurry suitable for hydrolytic digestion. The subsystem blends solid and liquid substrates with magnetite into a slurry, reducing the particulate size of the solid substrate so that it can pass through a ¼″ opening and blend it with liquid waste and magnetite additive into a slurry. Stage Subsystem  1000  components include a feed pump (P 1000 ) followed by a grinder with an open feed hopper for inlet.loop Subsystem  1000  includes a influent pump to the grinder (40 gpm capacity), a feed water heater (A.O. Smith model ATI 305201, 175,000 Btu/hour capacity), and a feed pump to the hydrolysis tank, and a feed make-up water tank, for water to add to the hydrolysis tank as needed. 
     Subsystem  2000 : Hydrolysis 
     Subsystem  2000  solubilizes the solid particulate in the slurry to liquid organic acids (hereinafter referred to as leachate) through the biochemical and bacteriological catalysis of water and complex molecules (hydrolysis). The slurry is mixed and digested under anaerobic conditions at 35° C. to facilitate the converting the solids into volatile acids. Feedstock slurry is pumped via P 1000  into hydrolysis tank TK 2001 , while leachate/slurry is pumped out of the tank via another pump. The hydrolysis tank (Ace Roto Mold VT1000-64, 1000 gallons). The contents in the hydrolysis tank are mixed by a ½ hp mixer with a variable motor. A discharge pump withdraws slurry from the top of the tank. Digestion parameters—[pH, ORP and Temperature]—are monitored by instrument probes mounted in the recirculation pipe. Liquid level in the tank is monitored through a pressure element PE 2001 , set to maintain the preset level. In addition, a high level alarm LAH  2001  shut down the pumps P 1000 . The pumps are then restarted only after intervention and reset by the operator. 
     Parameters monitored in Subsystem  2000  are: 
                                                             Type of   Parameter           Control Input   Output   output   Measured                           AIT 2001.1   Pump P2000   Digital   pH               On/Off               signal           AIT 2001.1   Pump P2000   Digital   pH               RPM           PE2001   Cut off signal   Digital   Tank Level               to P1000           LSH 2001/   High Level   Digital   Tank Level           PE2001   Alarm LAH           PE2001   Cut off signal   Digital   Tank Level               to P1000,           Temperature   Thermostat   Digital   Reactor           TIT2001   Control       Temperature               Signal           AIT 2001.2   Record ORP   Digital   Reactor ORP                        
Subsystem  3000 : Solids Separation and Recycle
 
     Subsystem  3000  consists of the solids separation and leachate recycle systems. The liquefied slurry from the hydrolysis tank is pumped to the Solids Separator (Vincent Corp. Model KP-10, 10 gpm capacity) using the Centrifugal Pump (American Machine Tool, AMPT-315-95A, 40 gpm capacity). Liquid leachate from the Separator is pumped to Methanogenesis Subsystem  4000  on a continuous basis. 
     Subsystem  4000 : Methanogenesis 
     Subsystem  4000  converts the leachate produced in Subsystem  2000  (Hydrolysis) into biogas. The clarified leachate from the solids separator is collected by gravity into tank TK 4000  that serves as a feed pump station for this Subsystem. The leachate is pumped by pumps P- 4000 . a  or  4000 . b  The pump P- 4000 . a  is oversized to keep the tank TK 4000  mixed. TK 4000  will have 2 chambers connected by an open loop at the bottom. A portion of the flow will be directed into TK 4001  or TK 4002  and P 4000 . b  is a variable speed low flow pump. The leachate is passed though loose fill media under strict anaerobic conditions at 35° C. to facilitate the conversion of the acids into biogas. Leachate is pumped into the tank at the bottom and is removed at the top of the tank by gravity. TK 4001  (methanogenesis tank  1 ) is mixed via recycle pump P 4001 , while TK 4002  (methanogenesis tank  2 ) is mixed using pump P 4002 . 
     Both pumps withdraw leachate from the top and re-inject it at the bottom of their respective tanks. Digestion parameters—[pH, ORP and Temperature]—are monitored by instrument probes mounted on the discharge loop of pumps P 4001  and P 4002 . 
     Liquid level in the tanks is monitored through Pressure elements PE 4001  and P 4002 , set to maintain levels of 8′ in the tank, +/−3,″ by matching the rate of inflow and outflow: flow into the bottom of the tank via pump P 4001 , (and/or P 4002 ) and 3 gph flow out at the top of TK 4001  (and/or TK 4002 ) Outflow is determined by a gravity feed past a pre-set constricted opening. As the liquid levels in TK 4001  (and/or TK 4002 ) rise to or drop below the preset levels as signaled by level sensor PE 4001  (or PE 4002 ), the system starts or stops the pumps P 4000 _B and P 5000  in order to bring the liquid level back within range. 
     In addition, a high levels alarms LAH  4001  and LAH  4002  and a low level alarm LAL  4000  in respective tanks TK 4001  and TK 4002  will shut down pumps P 4000 _B, and P 5000  in the event the pre-set maximum or minimum levels (in TK 4000 , TK 4001  or in TK 4002  is reached. The pumps will be restarted when the levels change. 
     Tanks TK 4001  and TK 4002  are designed to function in a series configuration wherein the lead and lag positions are switched periodically to achieve higher removal efficiencies and maintain more robust bacteria colonies in both tanks. The switching is determined by the ORP level in the lead tank. 
     The switching of the flow order of tanks TK 4001  and TK 4002  is accomplished by controlling valves V 4000 . 5 , V 4000 . 6 , V 4001 . 10  and V 4001 . 26 . The valve configurations for the two flow arrangements are as follows: 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Valve 
                 Valve 
                 Valve 
                 Valve 
               
               
                   
                 Flow Regime 
                 4000.5 
                 4000.6 
                 4001.26 
                 4001.1026 
               
               
                   
                   
               
             
             
               
                   
                 TK4001-TK4002 
                 Open 
                 Open 
                 Closed 
                 Closed 
               
               
                   
                 TK4002-TK4001 
                 Closed 
                 Closed 
                 Open 
                 Open 
               
               
                   
                   
               
             
          
         
       
     
     Parameters monitored in Subsystem  4000  are: 
                                                         Type of           Control Input   Output   output                           AIT 4000.1   Pump P4000.1   Digital               Status           LSH 4000   High pump   Digital               level alarm           Temperature   Thermostat   Digital           TIT4001.1   Control Signal           pH, AIT   Low pH alarm   Digital           4001.2   &amp; Pump Speed               Control Signal               for pH below               limits           ORP, AIT   High ORP   Digital           4001.3   signal to reverse               flows           PE4001   Start and stop   Digital               signal to P4000_A               and               P4000_B           LSH 4001/   High Level   Digital           PE4001   Alarm LAH           PE4002   Cut off signal to   Digital               P4001           LSH 4002/   Low Level   Digital           PE4002   Alarm LAL           PE4002   Cut off signal to   Digital               P4002           LSH 4002/   High Level   Digital           PE4002   Alarm LAH           PE4002   Cut off signal to   Digital               P4002, Close               Valve ( )           LSH 4002/   Low Level   Digital           PE4002   Alarm LAL           pH, AIT   Low pH alarm   Digital           4002.2   &amp; Pump Speed               Control Signal               for pH below               limits           ORP, AIT   High ORP   Digital           4002.3   signal to reverse               flows           Temperature   Thermostat   Digital           TIT4002.1   Control Signal           pH, AIT   Low pH alarm   Digital           4002.2   &amp; Pump Speed               Control Signal               for pH below               limits           ORP, AIT   High ORP   Digital           4002.3   signal to reverse               flows           FE4000   Flow Signal   Digital                        
Subsystem  5000 : Gas Handling
 
     The gas produced is measured and recorded on a continuous basis. In addition the system has an ambient gas level monitor mounted inside the trailer outside the tanks for safety purposes. The trailer is equipped with a large manometer consisting of 2 gas holding and 2 water balancing tanks to maintain positive pressure on the system as well as allow the tanks to be filled and emptied without introducing air. In essence the manometer functions like a floating cover.