Methane production device

Abstract

To provide a methane production apparatus capable of increasing a methane concentration in a biogas and increasing a methane production rate.SOLUTION: The methane production device 1A includes a storage tank 2 for storing a bacteria-containing liquid FL containing hydrogen-assimilating methane-producing bacteria, a carbon source-supplying means 6 for supplying at least carbon dioxide to the storage tank 2, a hydrogen dissolution-promoting means for promoting dissolution of a hydrogen-containing gas into the bacteria-containing liquid FL, and a concentration-increasing means for increasing the concentration of the hydrogen-assimilating methane-producing bacteria in the bacteria-containing liquid FL. The hydrogen dissolution promoting means includes a fine bubble supply part 5 for supplying the hydrogen-containing gas as fine bubbles to the bacteria-containing liquid FL. The concentration improving means includes a recycling device 8 for filtering the bacteria-containing liquid FL in which the hydrogen-containing gas is dissolved, discharging the filtrate, and returning the filtered substance to the storage tank 2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a methane production device that utilizes methane fermentation. [Background technology]

[0002] The use of biogas has been attracting attention from the viewpoints of preventing environmental pollution and regenerating energy.

[0003] Anaerobic fermentation of organic resources (biomass) derived from living organisms ultimately produces combustible biogas, primarily composed of methane and carbon dioxide. Anaerobic fermentation of biomass can be broadly divided into two biochemical reactions: a solubilization process using hydrolytic and acidogenic bacteria, and a methane fermentation process using methanogenic bacteria. High molecular weight organic compounds such as proteins, carbohydrates, and fats are broken down into lower molecular weight compounds by hydrolytic bacteria, resulting in higher fatty acids, amino acids, and sugars. The lower molecular weight organic compounds are then broken down by acidogenic bacteria into hydrogen, carbon dioxide, and organic acids (acetic acid, butyric acid, propionic acid, pyruvic acid, formic acid, lactic acid, succinic acid, etc.). The organic acids are then subjected to methane fermentation by methanogenic bacteria, producing methane.

[0004] As a methane generation apparatus for producing biogas containing methane and the like by anaerobic fermentation as described above, an apparatus equipped with a single-tank fermenter that performs a solubilization step and a methane fermentation step is known (see Patent Document 1). Also known is an apparatus equipped with a two-tank fermenter in which an acid fermenter is provided in the first stage to perform the solubilization step, and a methane fermenter is provided in the second stage to perform the methane fermentation step (see Patent Document 2).

[0005] Meanwhile, methane is produced by installing a bioreactor that performs the biomethanation reaction and supplying a carbon dioxide-containing gas and a hydrogen-containing gas to the liquid phase in which microorganisms exist in this bioreactor (see Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-183481 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-289946 [Patent Document 3] Special Publication No. 2023-525958 Summary of the Invention [Problem to be solved by the invention]

[0007] In the methane generation devices disclosed in Patent Documents 1 and 2, carbon dioxide is inevitably produced by the decomposition of low-molecular-weight organic matter by acid-producing bacteria, so the methane concentration in the generated biogas is low at approximately 50 to 60%, and the methane generation rate is slow, leaving room for improvement before the devices can be put into practical use.

[0008] In Patent Document 3, a hydrogen-containing gas is supplied to a bioreactor together with a carbon dioxide-containing gas. However, because hydrogen is difficult to dissolve in water, it is difficult to efficiently dissolve the hydrogen-containing gas in the liquid phase in the bioreactor, resulting in a slow methane production rate and leaving room for improvement for practical application.

[0009] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a methane generator that can increase the methane concentration in biogas and increase the methane production rate. [Means for solving the problem]

[0010] The characteristic configuration of the methane generation device according to the present invention for solving the above problems is as follows: a storage tank for storing a bacteria-containing liquid containing hydrogen-utilizing methanogens; a carbon source supply means for supplying at least carbon dioxide to the storage tank; a hydrogen dissolution promoting means for promoting dissolution of a hydrogen-containing gas into the bacteria-containing liquid; a concentration increasing means for increasing the concentration of the hydrogen-utilizing methanogens in the bacteria-containing liquid; The purpose is to provide the following.

[0011] In the methane generator of this configuration, methane is produced by a carbon dioxide reduction methane production reaction (hereinafter simply referred to as the "methane production reaction") by hydrogen-assimilating methanogens using carbon dioxide derived from, for example, a carbon dioxide-containing gas dissolved in a bacteria-containing liquid and hydrogen derived from a hydrogen-containing gas dissolved in the bacteria-containing liquid as substrates. According to the methane generator of this configuration, the hydrogen dissolution promoting means promotes the dissolution of the hydrogen-containing gas into the bacteria-containing liquid, and the concentration increasing means increases the concentration of hydrogen-assimilating methanogens in the bacteria-containing liquid. As a result, the hydrogen required for the methane production reaction by the hydrogen-assimilating methanogens using hydrogen and carbon dioxide as substrates can be rapidly supplied, and methane fermentation can be carried out with high efficiency. Therefore, the methane concentration in the biogas can be increased, and the methane production rate can be increased.

[0012] Next, the characteristic configuration of the methane generation device according to the present invention for solving the above problems is as follows: a storage tank for storing a bacteria-containing liquid obtained by fermenting biomass, the bacteria containing hydrogen-utilizing methanogens; a hydrogen dissolution promoting means for promoting dissolution of a hydrogen-containing gas into the bacteria-containing liquid; a concentration increasing means for increasing the concentration of the hydrogen-utilizing methanogens in the bacteria-containing liquid; The purpose is to provide the following.

[0013] In this methane generator, methane is produced by a methanogenic reaction catalyzed by hydrogen-utilizing methanogens using carbon dioxide, which is inevitably produced in the bacteria-containing liquid due to the decomposition of low-molecular-weight organic matter by acid-producing bacteria, and hydrogen derived from a hydrogen-containing gas dissolved in the bacteria-containing liquid as substrates. In this methane generator, the hydrogen dissolution promoting means promotes the dissolution of the hydrogen-containing gas into the bacteria-containing liquid, and the concentration increasing means increases the concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid. As a result, the hydrogen required for the methane production reaction catalyzed by the hydrogen-utilizing methanogens using hydrogen and carbon dioxide as substrates can be rapidly supplied, and methane fermentation can be carried out with high efficiency. This increases the methane concentration in the biogas and the methane production rate.

[0014] In the methane generation apparatus according to the present invention, It is preferable that the storage tank further comprises a carbon source supply means for supplying at least carbon dioxide to the storage tank.

[0015] With this methane generation device, even if the amount of carbon dioxide generated in the bacteria-containing liquid due to the decomposition of low-molecular-weight organic matter by acid-producing bacteria becomes insufficient for some reason, carbon dioxide can be supplied to the storage tank from the carbon source supply means, so the shortage can be made up, and methane can be stably produced through the methane production reaction by hydrogen-utilizing methanogens using carbon dioxide and hydrogen as substrates.

[0016] In the methane generation apparatus according to the present invention, The hydrogen dissolution promoting means preferably includes a fine bubble supplying unit that supplies hydrogen-containing gas as fine bubbles to the bacteria-containing liquid.

[0017] According to the methane generation device of this configuration, the gas-liquid contact area between the hydrogen-containing gas and the bacteria-containing liquid can be increased, thereby increasing the dissolution rate of the hydrogen-containing gas into the bacteria-containing liquid, and as a result, the methane production reaction by hydrogen-utilizing methanogens that use hydrogen and carbon dioxide as substrates can be promoted.

[0018] In the methane generation apparatus according to the present invention, The hydrogen dissolution promoting means preferably includes a pressure adjusting valve that maintains the inside of the storage tank in a pressurized state at a predetermined pressure.

[0019] According to the methane generation device of this configuration, the inside of the storage tank is maintained under pressure, which prevents hydrogen and carbon dioxide that have been dissolved in the bacteria-containing liquid from being released from the bacteria-containing liquid, and further improves the rate at which the hydrogen-containing gas dissolves in the bacteria-containing liquid.

[0020] In the methane generation apparatus according to the present invention, The concentration improving means preferably includes a recycling device that filters the bacteria-containing liquid in which the hydrogen-containing gas is dissolved, discharges the filtrate, and returns the filtered matter to the storage tank.

[0021] With this methane generator, the filtrate discharged from the bacteria-containing liquid can be effectively used, for example, as liquid fertilizer. The filtered liquid, which contains a large amount of hydrogen-utilizing methanogens, is returned to the storage tank for reuse. This maintains a high concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid in the storage tank, allowing for efficient fermentation.

[0022] In the methane generation apparatus according to the present invention, The concentration-enhancing means preferably includes a carrier that supports the hydrogen-utilizing methanogens.

[0023] With this methane generation device, the hydrogen-utilizing methanogens supported on the carrier grow, maintaining a high concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid in the storage tank, allowing fermentation to be carried out efficiently.

[0024] In the methane generation apparatus according to the present invention, It is preferable that a return passage be provided for returning a portion of the biogas produced by methane fermentation by the hydrogen-utilizing methanogens to the storage tank.

[0025] With this configuration of the methane generation device, a portion of the biogas is returned to the storage tank via the return path, and methane is produced through a methanogenic reaction by hydrogen-utilizing methanogens using the hydrogen and carbon dioxide remaining in the biogas as substrates, thereby further increasing the methane concentration in the biogas. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of a methane generator according to a first embodiment. [Figure 2] FIG. 2 is a schematic diagram showing a schematic configuration of a methane generator according to the second embodiment. [Figure 3] FIG. 3 is an explanatory diagram of alternative embodiments 1 to 3. [Figure 4] FIG. 4 is an explanatory diagram of fourth and fifth alternative embodiments. [Figure 5] FIG. 5 is an explanatory diagram of the sixth and seventh alternative embodiments. [Figure 5A] FIG. 5A is an explanatory diagram of an eighth alternative embodiment. [Figure 6] FIG. 6 is a graph showing the change in the amount of methane produced by the methane generator of Example 1. [Figure 7] FIG. 7 is a graph showing the changes in the amount of methane produced by the methane generators of Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described below with reference to the drawings. However, the present invention is not intended to be limited to the embodiments described below or the configurations shown in the drawings.

[0028] First Embodiment <Overall structure> Fig. 1 is a schematic diagram showing the general configuration of a methane generator 1A of a first embodiment. The methane generator 1A shown in Fig. 1 is an ex-situ type apparatus that produces methane by methane fermentation using hydrogen, carbon dioxide, and other nutrients necessary for the growth of hydrogen-utilizing methanogens as raw materials, and is equipped with a storage tank 2, a nutrient solution supply unit 3, an agitator 4, a fine bubble supply unit 5, a carbon source supply means 6, a biogas recovery unit 7, and a recycling unit 8.

[0029] <Storage tank> The storage tank 2 has a cylindrical body 2a with an axis (not shown) oriented in the vertical direction, and is a tank with the upper and lower sides of the body 2a closed. The storage tank 2 stores a bacteria-containing liquid FL containing hydrogen-utilizing methanogens, and is configured so that the space above the liquid level of the bacteria-containing liquid FL is filled with biogas BG.

[0030] <Nutrient solution supply unit> The nutrient liquid supply unit 3 includes a pressure-transfer unit 11 and a supply pipe 12. Although detailed explanation using drawings is omitted, the pressure-transfer unit 11 includes a pressure-transfer pump that pressure-transfers the nutrient liquid containing nutrients necessary for the growth of hydrogen-utilizing methanogens (for example, a liquid inorganic salt medium, a methane fermentation digested liquid, etc.), a control valve that controls the pressure-transfer amount and pressure, etc. The supply pipe 12 connects the lower part of the storage tank 2 (the part where the bacteria-containing liquid FL is stored) to the pressure-transfer unit 11. In this way, the nutrient liquid supply unit 3 supplies the nutrient liquid to the lower part of the storage tank 2.

[0031] <Stirring device> The agitator 4 may be, for example, a mechanical agitator, a gas agitator, or a circulation agitator, and is not particularly limited, but in this example, a mechanical agitator is used.

[0032] The mechanical agitator 4 employed in this example is configured to include a rotating shaft 15 extending vertically through the ceiling of the storage tank 2 at the center thereof and disposed within the tank, a drive unit 16 connected to the upper end of the rotating shaft 15 so as to be able to transmit rotational power, and a rotor 17 disposed below the storage tank 2 and fixed to the lower end of the rotating shaft 15. In the agitator 4, the rotational power from the drive unit 16 is transmitted to the rotor 17 via the rotating shaft 15 to rotate the rotor 17, thereby agitating the bacteria-containing liquid FL at the bottom of the storage tank 2. Note that, in this example, the rotating shaft 15 is illustrated as extending vertically (vertically), but is not limited thereto and may extend horizontally or obliquely at a predetermined angle relative to the vertical.

[0033] <Fine bubble supply unit> The fine bubble supply unit 5 supplies hydrogen-containing gas as fine bubbles into the bacteria-containing liquid FL in the storage tank 2. The hydrogen-containing gas refers to a gas containing at least 10% (dry) hydrogen by volume relative to the total hydrogen-containing gas. The hydrogen content in the hydrogen-containing gas is preferably 80% or more, more preferably 90% or more, and most preferably 95% or more. There is no particular upper limit to the hydrogen content in the hydrogen-containing gas, and it may be 100%.

[0034] The fine bubble supply unit 5 includes a hydrogen gas supply source 21, a gas supply pipe 22, and a fine bubble generator 23, which are arranged in this order from the upstream side to the downstream side of the gas flow.

[0035] Although detailed description using drawings is omitted, the hydrogen gas supply source 21 is configured to include a hydrogen storage unit that stores hydrogen-containing gas under high pressure, a pressure reducing valve that reduces the pressure of the high-pressure hydrogen-containing gas from the hydrogen storage unit, etc. The hydrogen used here may be, for example, hydrogen obtained by electrolyzing water using electricity generated by renewable energy such as solar, wind, or geothermal power, or surplus hydrogen from a petrochemical plant, etc. In the gas supply pipe (supply path for hydrogen-containing gas) 22, the upstream end of the gas flow is connected to the hydrogen gas supply source 21, and the downstream end of the gas flow is connected to a fine bubble generator 23 disposed in the bacteria-containing liquid FL in the storage tank 2.

[0036] The fine bubble generator 23 is located below the rotor 17 near the bottom of the storage tank 2 and emits hydrogen-containing gas supplied from the hydrogen gas supply source 21 via the gas supply pipe 22 as fine bubbles. The emitted fine bubbles are dispersed throughout the bacteria-containing liquid FL by the stirring action of the rotor 17. Here, "fine bubbles" includes "microbubbles" and "ultrafine bubbles," with "microbubbles" being bubbles with a diameter of less than 100 μm but 1 μm or greater, and "ultrafine bubbles" being bubbles with a diameter of less than 1 μm. Examples of fine bubble generators 23 that can be used include a swirling flow method in which bubbles are generated by mixing a gas (hydrogen-containing gas) with a liquid (bacteria-containing liquid FL) and swirling the mixture at high speed; a pressure dissolution method in which bubbles are generated by applying pressure to a gas, dissolving it in the liquid, and then releasing the pressure all at once; and a micropore method in which bubbles are generated by applying pressure to a gas passing through a micropore such as an orifice. Other examples include those that generate bubbles by blowing gas through a tubular microbubble-generating membrane (e.g., a porous ceramic membrane) having many micropores, those that generate bubbles by blowing gas through a straw-shaped hollow fiber porous body (hollow fiber membrane), and those that generate bubbles by blowing gas through a porous so-called air stone.

[0037] <Carbon Source Supply Means> The carbon source supplying means 6 supplies at least carbon dioxide to the bacteria-containing liquid FL in the storage tank 2, and in this example, a gas containing at least carbon dioxide (hereinafter referred to as "carbon dioxide-containing gas") is supplied as normal bubbles to the bacteria-containing liquid FL in the storage tank 2. The carbon dioxide-containing gas means a gas that contains at least 10% (dry) carbon dioxide by volume relative to the total carbon dioxide-containing gas. The carbon dioxide gas content in the carbon dioxide-containing gas is preferably 40% or more, more preferably 90% or more, and most preferably 95% or more. There is no particular upper limit to the carbon dioxide gas content in the carbon dioxide-containing gas, and it may be 100%.

[0038] The carbon source supply means 6 includes a carbon dioxide gas supply source 25 and a gas inlet pipe (carbon dioxide supply path) 26, and is configured to supply carbon dioxide-containing gas to the bacteria-containing liquid FL in which the hydrogen-containing gas has been dissolved and / or to the bacteria-containing liquid FL before the hydrogen-containing gas has been dissolved.

[0039] Although detailed explanation using drawings is omitted, the carbon dioxide gas supply source 25 is configured to include a carbon dioxide storage unit that stores carbon dioxide-containing gas under high pressure, a pressure reducing valve that reduces the pressure of the high-pressure carbon dioxide-containing gas from the carbon dioxide storage unit, etc. Examples of the carbon dioxide-containing gas used here include gas containing carbon dioxide recovered by chemical absorption, membrane separation, physical absorption, solid absorption, etc. from combustion exhaust gas generated during the combustion of waste, etc., and biogas containing carbon dioxide generated during the fermentation of biomass.

[0040] In the gas blowing pipe 26, the upstream end of the gas flow is connected to a carbon dioxide gas supply source 25, and the downstream end of the gas flow is arranged so that carbon dioxide-containing gas can be blown into the bacteria-containing liquid FL in the storage tank 2.

[0041] <Biogas Recovery Section> The biogas recovery section 7 is equipped with a gas discharge pipe 27. The upstream side of the gas flow of the gas discharge pipe 27 is connected to a gas vent port 28 provided on the top surface of the storage tank 2 so that the biogas BG can be discharged outside the tank. A gas holder 29 is connected to the downstream side of the gas flow of the gas discharge pipe 27. The biogas BG extracted from the storage tank 2 through the gas discharge pipe 27 is temporarily stored in the gas holder 29 and then effectively utilized. A gas purification device may be connected to the downstream side of the gas flow of the gas holder 29. Examples of gas purification devices include a device that removes hydrogen sulfide from the biogas BG, a moisture removal device, a device that further reduces carbon dioxide from the biogas BG by pressure swing adsorption (PSA), membrane separation, high-pressure water absorption, chemical absorption, physical absorption, etc., and a siloxane removal device.

[0042] <Reuse equipment> The recycling device 8 includes a reflux pipe 31, a pressure pump 32, and a solid-liquid separator 33, and is configured to filter the bacteria-containing liquid FL in which the hydrogen-containing gas is dissolved, discharge the filtrate, and return the filtered material to the storage tank 2. The reflux pipe 31 is disposed in a manner that connects an outlet 34 provided near the liquid level of the bacteria-containing liquid FL stored in the storage tank 2 with an inlet 35 provided at the bottom of the storage tank 2. The pressure pump 32 is disposed in the reflux pipe 31 at a position close to the outlet 34. The solid-liquid separator 33 is disposed in the reflux pipe 31 so as to be located between the pressure pump 32 and the inlet 35. For the solid-liquid separator 33, for example, a filtration membrane such as a microfiltration membrane (MF membrane) or an ultrafiltration membrane (UF membrane) can be used, and the shape of the membrane can be, for example, a flat membrane, a hollow fiber membrane, or the like. In the recycling device 8, the bacteria-containing liquid FL remaining in the upper part of the storage tank 2 after the biogas BG is removed is sent to the solid-liquid separator 33 by the operation of the pressure pump 32. The solid-liquid separator 33 filters the bacteria-containing liquid FL sent from the pressure pump 32 to separate it into a filtrate and a filtrate residue. The filtrate is returned to the lower part of the storage tank 2 via the reflux pipe 31 and reused. Meanwhile, the filtrate (filtered digested liquid) is discharged via a filtrate discharge pipe 36 connected to a filtrate outlet of the solid-liquid separator 33. The discharged filtered digested liquid is recovered in a digested liquid recovery section 37 and effectively utilized, for example, as liquid fertilizer. In this embodiment, an example has been shown in which the solid-liquid separator 33 uses a device that separates solids and liquids using a filtration membrane such as an MF membrane or an UF membrane. However, the solid-liquid separator 33 is not limited to this, and devices such as a rotary drum, a drum screen, a belt press, a screw press, a belt concentrator, a flotation separator, a sedimentation separator, a filter press, and a centrifugal separator can also be used.

[0043] In the methane generator 1A configured as described above, hydrogen-containing gas is supplied as fine bubbles from the fine bubble supply unit 5 into the bacteria-containing liquid FL in the storage tank 2. Furthermore, carbon dioxide-containing gas is supplied from the carbon source supply means 6 to the bacteria-containing liquid FL in the storage tank 2. Then, methane is produced by a methanogenesis reaction caused by hydrogen-assimilating methanogens using carbon dioxide derived from the carbon dioxide-containing gas dissolved in the bacteria-containing liquid FL and hydrogen derived from the hydrogen-containing gas dissolved in the bacteria-containing liquid FL as substrates. The biogas BG produced by methane fermentation accumulates above the liquid surface of the bacteria-containing liquid FL stored in the storage tank 2 and is collected by the biogas collection unit 7 for effective use.

[0044] In the methane generator 1A, the fine bubble supply unit 5 supplies the hydrogen-containing gas to the bacteria-containing liquid FL as fine bubbles, thereby increasing the gas-liquid contact area between the hydrogen-containing gas and the bacteria-containing liquid FL and increasing the rate at which hydrogen dissolves in the bacteria-containing liquid FL. Furthermore, the bacteria-containing liquid FL remaining after biogas BG is removed from the top of the storage tank 2 is sent to the solid-liquid separator 33 by the pressure pump 32, where it is separated into a filtrate and a residue, and the residue is returned to the storage tank 2. The residue contains a large amount of methane bacteria, which is reused by returning it to the storage tank 2, thereby maintaining a high concentration of methane bacteria in the bacteria-containing liquid. This increases the rate at which hydrogen dissolves in the bacteria-containing liquid FL and maintains a high concentration of methane bacteria in the bacteria-containing liquid. This allows for a rapid supply of hydrogen needed for the methane production reaction by hydrogen-utilizing methanogens that use hydrogen and carbon dioxide as substrates, and allows for highly efficient methane fermentation. Therefore, the methane concentration in the biogas BG can be increased, and the methane production rate can be increased. The filtrate (filtered digested liquid) is recovered in the digested liquid recovery section 37 and is effectively utilized, for example, as liquid fertilizer.

[0045] Second Embodiment 2 is a schematic diagram showing the general configuration of a methane generator 1B according to a second embodiment. In the second embodiment, components that are the same as or similar to those in the first embodiment are denoted by the same reference numerals in the drawing, and detailed descriptions thereof will be omitted. The following description will focus on the components unique to the second embodiment.

[0046] The methane generator 1B shown in FIG. 2 is an in-situ type apparatus that stores a bacteria-containing liquid containing hydrogen-utilizing methanogens, which is obtained by fermenting biomass, in a storage tank 2, and uses the storage tank 2 as a methane fermenter in which acid fermentation of the biomass is followed by methane fermentation. Here, biomass refers to organic resources derived from living organisms. Examples of biomass include organic waste, resource crops, and their waste. Examples of organic waste include food waste, manure, sludge, food processing residues, livestock waste, waste oil, animal fats and oils, agricultural crop residues, and organic wastewater from the food industry, paper industry, or livestock industry. Examples of sludge include sewage treatment sludge, human waste treatment sludge, septic tank sludge, and treated sludge from industrial wastewater generated from food factories and the like. Examples of resource crops include potatoes, sugar beets, rapeseed, sunflowers, wheat, chlorella, water hyacinth, corn, sugarcane, and waste generated in the processing of these crops. In the following embodiment, an in-situ methane generator that ferments organic waste to generate biogas (methane) will be described as an example.

[0047] When using the storage tank 2 as a wet fermentation tank, the solids concentration in the tank is adjusted to around 10% by weight or less, and organic waste is fermented in a mesophilic environment (30-45°C) or a high-temperature environment (45-65°C) to produce biogas. When using the storage tank 2 as a dry fermentation tank, the solids concentration in the tank is adjusted to around 8-40% by weight, and organic waste is fermented by methane fermentation in a high-temperature environment (45-65°C) to produce biogas. Dry fermentation equipment includes horizontal dry fermentation equipment and vertical dry fermentation equipment.

[0048] In the methane generator 1B of the second embodiment, methane is produced by a methanogenesis reaction caused by hydrogen-assimilating methanogens using carbon dioxide, which is inevitably produced in the bacteria-containing liquid FL due to the decomposition of low-molecular-weight organic matter by acid-producing bacteria, and hydrogen derived from the hydrogen-containing gas dissolved in the bacteria-containing liquid FL as substrates. Therefore, the methane generator 1B of the second embodiment does not require the carbon source supply means 6 required in the methane generator 1A of the first embodiment. Furthermore, a biomass supply unit 40 is used instead of the nutrient liquid supply unit 3 in the methane generator 1A of the first embodiment.

[0049] <Biomass Supply Department> The biomass supply unit 40 includes a pressure-transfer unit 41 and a supply pipe 42. Although detailed explanation using drawings is omitted, the pressure-transfer unit 41 includes a pressure-transfer pump that pressure-transfers the organic waste, a control valve that controls the pressure-transfer amount and pressure, and the like. The supply pipe 42 connects the lower part of the storage tank 2 (the part where the bacteria-containing liquid FL is stored) to the pressure-transfer unit 41. Before the biomass supply unit 40, the organic waste used as raw material is subjected to pretreatment equipment (not shown) to homogenize the raw material by removing foreign matter and adjust the moisture content to a level suitable for methane fermentation. The organic waste that has undergone such pretreatment is supplied to the lower part of the storage tank 2 by the biomass supply unit 40.

[0050] In the methane generator 1B configured as described above, hydrogen-containing gas is supplied as fine bubbles from the fine bubble supply unit 5 into the bacteria-containing liquid FL in the storage tank 2. As a result, hydrogen derived from the hydrogen-containing gas supplied by the fine bubble supply unit 5 is contained in the bacteria-containing liquid FL, in addition to the hydrogen produced by the decomposition of low-molecular-weight organic matter by the acid-producing bacteria. Methane is then produced by a methanogenic reaction caused by hydrogen-utilizing methanogens, using the hydrogen derived from the hydrogen-containing gas and carbon dioxide inevitably produced by the decomposition of low-molecular-weight organic matter by the acid-producing bacteria as substrates. The biogas BG produced by methane fermentation accumulates above the liquid surface of the bacteria-containing liquid FL stored in the storage tank 2 and is collected by the biogas collection unit 7 for effective use.

[0051] In the methane generator 1B of the second embodiment, as in the methane generator 1A of the first embodiment, the rate at which hydrogen dissolves in the bacteria-containing liquid FL is increased and the concentration of methanogens in the bacteria-containing liquid is maintained at a high level, so that the hydrogen required for the methane production reaction by hydrogen-utilizing methanogens using hydrogen and carbon dioxide as substrates can be quickly supplied and methane fermentation can be carried out with high efficiency. Therefore, the methane concentration in the biogas BG can be increased and the methane production rate can be increased.

[0052] The methane generation apparatus of the present invention has been described above based on several embodiments, but the present invention is not limited to the configurations described in the above embodiments, and the configuration can be changed as appropriate within the scope of the invention.

[0053] Fig. 3 is an explanatory diagram of alternative embodiments 1 to 3. The hydrogen dissolution promoting means is not limited to the embodiments shown in Figs. 1 and 2, and embodiments shown in Figs. 3(a) to 3(c) can also be used.

[0054] (Another embodiment 1) A methane generator 1C according to another embodiment 1 shown in Figure 3(a) is based on the methane generator 1A of the first embodiment, but differs in the configuration of the fine bubble supply unit 5. In the fine bubble supply unit 5 of the methane generator 1C, the fine bubble generator 23 includes a plurality of gas-permeable hollow fiber membranes 50 extending tubularly (cylindrically) inside the storage tank 2 along the axial direction (vertical direction) of the body 2a, a one-side support part 51 that supports one end side (lower end side) of the plurality of hollow fiber membranes 50 inside the storage tank 2, and a other-side support part 52 that supports the other end side (upper end side) of the plurality of hollow fiber membranes 50 inside the storage tank 2. In the fine bubble generator 23, the hydrogen-containing gas supplied from the hydrogen gas supply source 21 to the one side support part 51 via the gas supply pipe 22 is distributed and supplied to the inside of each of the plurality of hollow fiber membranes 50, and is sent in the form of fine bubbles through the membrane pores (not shown) of each hollow fiber membrane 50 into the bacteria-containing liquid FL outside each hollow fiber membrane 50. This configuration can increase the efficiency of dissolving hydrogen into the bacteria-containing liquid FL.

[0055] The hollow fiber membrane 50 may be, for example, a porous gas-permeable membrane made of polypropylene, polyethylene, polysulfone, or the like, with a pore size of 0.01 to 3 μm, preferably 0.1 to 1 μm. To maintain strength against pressure, the membrane thickness is preferably formed to about 10 to 100 μm. The porosity of the gas-permeable membrane is preferably 10 to 40%.

[0056] In the fine bubble supply unit 5 shown in Fig. 3(a), hydrogen can be supplied through the membrane pores of the hollow fiber membrane 50 to a biofilm formed by a membrane-like aggregation of hydrogen-utilizing methanogens attached to the surface of the hollow fiber membrane 50. The hydrogen-utilizing methanogens then produce methane through a methanogenic reaction from the hydrogen supplied through the membrane pores of the hollow fiber membrane 50 and carbon dioxide dissolved in the bacteria-containing liquid FL.

[0057] (Alternative embodiment 2) A methane generator 1D according to another embodiment 2 shown in FIG. 3(b) is based on the methane generator 1A of the first embodiment, and has a pressure regulating valve 60 interposed in the gas exhaust pipe 27. When the pressure inside the storage tank 2 exceeds a predetermined pressure, the pressure regulating valve 60 opens, and when the pressure is equal to or lower than the predetermined pressure, the pressure regulating valve 60 closes, thereby maintaining the inside of the storage tank 2 in a pressurized state at the predetermined pressure. Note that in this example, an example in which the inside of the storage tank 2 can be pressurized by providing the pressure regulating valve 60 has been shown, but the present invention is not limited thereto. For example, the inside of the storage tank 2 may be pressurized by pumping a gas such as a hydrogen-containing gas or a carbon dioxide-containing gas into the storage tank 2 using a pressure pump.

[0058] In the methane generator 1D of Alternative Embodiment 2, the inside of the storage tank 2 is maintained in a pressurized state by the pressure regulating valve 60, thereby making it possible to prevent the hydrogen and carbon dioxide once dissolved in the bacteria-containing liquid FL from being released from the bacteria-containing liquid FL and to further improve the rate at which hydrogen and carbon dioxide dissolve in the bacteria-containing liquid FL. In the methane generator 1D, the pressure regulating valve 60 functions as a hydrogen dissolution promoting means that promotes the dissolution of hydrogen-containing gas into the bacteria-containing liquid FL, and therefore the hydrogen dissolution promoting means in the methane generator 1D is configured to include both the fine bubble supplier 5 and the pressure regulating valve 60.

[0059] (Alternative embodiment 3) A methane generator 1E according to another embodiment 3 shown in FIG. 3(c) is based on the methane generator 1A of the first embodiment and uses a storage tank 65 that is longer than the storage tank 2. The storage tank 65 has a cylindrical body 65a with an axis (not shown) oriented in the vertical direction, and is a vertical single tank with the upper and lower sides of the body 65a closed. Like the storage tank 2, the storage tank 65 also stores the bacteria-containing liquid FL, and is configured so that the space above the liquid level of the bacteria-containing liquid FL is filled with biogas BG. In the storage tank 65, the ratio (H / D) of the height (H: substantially the height of the body 65a) to the inner diameter (D: substantially the inner diameter of the body 65a) is preferably 3.1 to 5.7, more preferably 3.5 to 5.4, and even more preferably 3.9 to 4.8. According to the methane generator 1E of Alternative Embodiment 3, it is possible to set the liquid depth of the bacteria-containing liquid FL larger than in the storage tank 2, and the residence time of the fine bubbles in the bacteria-containing liquid FL can be lengthened, thereby increasing the amount of hydrogen-containing gas dissolved in the bacteria-containing liquid FL. In the methane generator 1E, the vertically long storage tank 65 functions as hydrogen dissolution promoting means that promotes the dissolution of the hydrogen-containing gas in the bacteria-containing liquid FL, and therefore the hydrogen dissolution promoting means in the methane generator 1E is configured to include both the fine bubble supply unit 5 and the vertically long storage tank 65.

[0060] Fig. 4 is an explanatory diagram of alternative embodiments 4 and 5. The concentration improving means is not limited to the embodiments shown in Figs. 1 and 2, and embodiments shown in Figs. 4(a) and (b) can also be adopted.

[0061] (Alternative embodiment 4) A methane generator 1F according to another embodiment 4 shown in Figure 4(a) is based on the methane generator 1A of the first embodiment, and uses a fluidized bed carrier 70 as a concentration improving means instead of the recycling device 8 in the methane generator 1A of the first embodiment. In this example, hydrogen-utilizing methanogens are supported on the cylindrical granular carrier 70 and suspended in the bacteria-containing liquid FL in the storage tank 2.

[0062] (Alternative embodiment 5) A methane generator 1G according to another embodiment 5 shown in Fig. 4(b) is based on the methane generator 1A of the first embodiment, and uses a fixed-bed carrier 75 as a concentration improving means instead of the recycling device 8 in the methane generator 1A of the first embodiment. In this example, a plate-like or mesh-like carrier 75 is fixed inside the storage tank 2, and hydrogen-utilizing methanogens are supported on the fixed carrier 75.

[0063] In both of the fourth and fifth alternative embodiments, the hydrogen-utilizing methanogens supported on the carriers 70, 75 grow, thereby maintaining a high concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid FL in the storage tank 2, and enabling efficient fermentation. Concentration-improving means other than the fluidized bed method and fixed bed method include, for example, the UASB method (upflow anaerobic sludge filter method) and the trickling filter method.

[0064] Fig. 5 is an explanatory diagram of alternative embodiments 6 and 7. As shown in Fig. 5(a) and (b), a configuration in which return paths 80, 90 for biogas are provided can be employed.

[0065] (Alternative embodiment 6) A methane generator 1H according to another embodiment 6 shown in FIG. 5(a) is based on the methane generator 1A of the first embodiment, and has a return pipe 81 (corresponding to the "feed path" of the present invention) branched from the gas exhaust pipe 27 and connected to the gas inlet pipe 26. The gas exhaust pipe 27, the return pipe 81, and the gas inlet pipe 26 form a reflux path 80. As described above, the reflux path 80 is configured to include a feed path (return pipe 81) that feeds a portion of the biogas to the carbon dioxide supply path (gas inlet pipe 26) to the storage tank 2 by the carbon source supply means 6. In this way, by using the carbon source supply means 6, a portion of the biogas is returned to the storage tank 2, and therefore, the device configuration is simplified by sharing the main components of the carbon source supply means 6, and a portion of the biogas can be reliably returned to the storage tank 2.

[0066] In the methane generator 1H of Alternative Embodiment 6, a portion of the biogas BG extracted from the interior of the storage tank 2 through the gas discharge pipe 27 is returned to the interior of the storage tank 2 through the reflux path 80. This allows methane to be produced by a methanogenesis reaction by hydrogen-assimilating methanogens using hydrogen and carbon dioxide remaining in the biogas BG as substrates, thereby further increasing the methane concentration in the biogas BG. Note that a portion of the biogas BG extracted from the interior of the storage tank 2 through the gas discharge pipe 27 may be returned to the interior of the storage tank 2 through the reflux path 80 (gas discharge pipe 27, return pipe 81, and gas inlet pipe 26), and then further extracted from the interior of the storage tank 2 and returned to the interior of the storage tank 2 through the reflux path 80, thereby circulating the biogas. In this case, a circulation circuit is formed by the storage tank 2, the gas discharge pipe 27, the return pipe 81, and the gas inlet pipe 26.

[0067] (Alternative embodiment 7) A methane generator 1I according to another embodiment 7 shown in FIG. 5(b) is based on the methane generator 1A of the first embodiment, and a return pipe 91 arranged in a form branching from the gas exhaust pipe 27 is connected to the middle of the gas supply pipe 22. A reflux path 90 is formed by the gas exhaust pipe 27, the return pipe 91, and the gas supply pipe 22. In this way, the reflux path 90 is configured to include a supply path (return pipe 91) that supplies a portion of the biogas to the supply path (gas supply pipe 22) of the hydrogen-containing gas to the bacteria-containing liquid FL by the fine bubble supply unit 5. In this way, by using the fine bubble supply unit 5, a portion of the biogas is returned to the storage tank 2, and therefore, the device configuration can be simplified by sharing the main components of the fine bubble supply unit 5, and a portion of the biogas can be reliably returned to the storage tank 2.

[0068] In the methane generator 1I of Alternative Embodiment 7, a portion of the biogas BG extracted from the interior of the storage tank 2 through the gas discharge pipe 27 is returned to the interior of the storage tank 2 through the reflux path 90. This allows methane to be produced by a methanogenesis reaction by hydrogen-assimilating methanogens using hydrogen and carbon dioxide remaining in the biogas BG as substrates, thereby further increasing the methane concentration in the biogas BG. Note that a portion of the biogas BG extracted from the interior of the storage tank 2 through the gas discharge pipe 27 may be returned to the interior of the storage tank 2 through the reflux path 90 (gas discharge pipe 27, return pipe 91, and gas supply pipe 22), and then further extracted from the interior of the storage tank 2 and returned to the interior of the storage tank 2 through the reflux path 90, thereby circulating the biogas. In this case, a circulation circuit is formed by the storage tank 2, the gas discharge pipe 27, the return pipe 91, and the gas supply pipe 22.

[0069] The methane generation apparatuses 1C to 1I according to the above-described alternative embodiments 1 to 7 are configured based on the ex-situ type methane generation apparatus 1A of the first embodiment, but can also be configured based on the in-situ type methane generation apparatus 1B of the second embodiment.

[0070] (Alternative embodiment 8) FIG. 5A is an explanatory diagram of another embodiment 8. The methane generator 1J shown in FIG. 5A is based on the in-situ methane generator 1B of the second embodiment shown in FIG. 2, and is obtained by adding a carbon source supply means 6 to the methane generator 1B. That is, the methane generator 1J according to another embodiment 8 includes a storage tank 2 for storing a bacteria-containing liquid FL containing hydrogen-assimilating methanogens and obtained by fermenting biomass, a carbon source supply means 6 for supplying a carbon dioxide-containing gas to the bacteria-containing liquid FL, a fine bubble supply unit 5 (hydrogen dissolution promotion means) for promoting dissolution of the hydrogen-containing gas into the bacteria-containing liquid FL, and a recycling device 8 (concentration improvement means) for increasing the concentration of the hydrogen-assimilating methanogens in the bacteria-containing liquid FL. Like the methane generator 1B, the methane generator 1J also includes an agitator 4 and a biogas recovery unit 7. In the methane generation apparatus 1J configured as described above, even if the carbon dioxide generated in the bacteria-containing liquid FL due to the decomposition of low-molecular-weight organic matter by acid-producing bacteria becomes insufficient for some reason, the carbon source supply means 6 can supply carbon dioxide-containing gas to the bacteria-containing liquid FL in the storage tank 2, thereby making up for the shortage, and methane can be stably produced through a methane production reaction by hydrogen-utilizing methanogens using carbon dioxide and hydrogen as substrates.

[0071] (Alternative embodiment 9) In the above embodiment, the carbon source supplying means 6 is exemplified as a means for supplying a gas containing at least carbon dioxide to the bacteria-containing liquid FL in the storage tank 2, but is not limited thereto and may be a means for supplying a liquid in which a gas containing at least carbon dioxide is dissolved to the bacteria-containing liquid FL in the storage tank 2. Furthermore, the carbon source supplying means 6 may be a means for supplying carbon monoxide instead of carbon dioxide or a gas containing both carbon monoxide and carbon dioxide to the bacteria-containing liquid FL in the storage tank 2, or a means for supplying a liquid in which the gas is dissolved to the bacteria-containing liquid FL in the storage tank 2. In other words, the "carbon source supplying means" of the present invention may be any means that supplies at least carbon monoxide and / or carbon dioxide to the bacteria-containing liquid FL.

[0072] In the above-mentioned alternative embodiment 6, an example is shown in which the return pipe 81 is arranged in a form in which it branches off from the middle of the gas exhaust pipe 27 and connects to the gas inlet pipe 26, but there are also possible configurations in which the return pipe 81 is not arranged, i.e., in which the reflux path 80 is not formed.

[0073] In the above-mentioned alternative embodiment 7, an example is shown in which the return pipe 91 is arranged in a form in which it branches off from the middle of the gas exhaust pipe 27 and connects to the gas supply pipe 22, but there are also possible configurations in which the return pipe 91 is not arranged, i.e., in which the reflux path 90 is not formed.

[0074] In the above embodiment, a mode is shown in which carbon dioxide-containing gas from the carbon dioxide gas supply source 25 is directly blown into the bacteria-containing liquid FL in the storage tank 2 via the gas blowing pipe 26, but this is not limited to this. For example, there may be a mode in which the downstream end of the gas flow of the gas blowing pipe 26 is positioned in a space above the liquid surface of the bacteria-containing liquid FL in the storage tank 2, the carbon dioxide-containing gas is blown into the space, and the carbon dioxide in the carbon dioxide-containing gas is dissolved in the bacteria-containing liquid FL. [Example]

[0075] Next, an example of the methane generation apparatus of the present invention will be described, but the present invention is not limited to the following example.

[0076] Example 1 Methane was produced using the methane production apparatus 1A of the first embodiment (see FIG. 1).

[0077] (Comparative Example 1) In the methane generator 1A of the first embodiment, the fine bubble generator 23 in the fine bubble supply unit 5 was omitted, and the recycling device 8 was also omitted, but methane was generated using a methane generator having the same configuration as the methane generator 1A. In Comparative Example 1, hydrogen-containing gas was supplied as normal bubbles (bubbles with a diameter of 1 mm or more) from the hydrogen gas supply source 21 via the gas supply pipe 22 to the bacteria-containing liquid FL in the storage tank 2 (the same applies to Comparative Example 2).

[0078] (Comparative Example 2) In the methane generator 1A of the first embodiment, the fine bubble generator 23 in the fine bubble supply unit 5 was omitted, and methane was generated using a methane generator having the same configuration as the methane generator 1A except for that.

[0079] (Comparative Example 3) In the methane generator 1A of the first embodiment, the recycling device 8 was omitted, and methane was generated using a methane generator having the same configuration as the methane generator 1A except for the above.

[0080] In Example 1, the mass transfer capacity coefficient (hydrogen dissolution rate) is increased. Here, the mass transfer capacity coefficient (J) is expressed by the following formula (1). J = K L ·a(C * -C) (1) J: Molar amount transferred per unit time per unit dispersed phase [kmol / (m 3 ·s)] K L : Overall mass transfer coefficient [m / s] a: Gas-liquid interfacial area per unit dispersed phase [m 2 / m 3 ] C * : Equilibrium concentration in the liquid at the gas-liquid interface [kmol / m 3 ] C: Concentration in liquid [kmol / m 3 ]

[0081] Specifically, in Example 1, the fine bubble supply unit 5, which functions as a hydrogen dissolution promoting means, supplies hydrogen-containing gas as fine bubbles to the bacteria-containing liquid FL, thereby increasing the gas-liquid interfacial area (a) per unit dispersed phase in the above formula (1), and increasing the mass transfer capacity coefficient (J) by 10 times compared to Comparative Example 1. Furthermore, in Example 1, the filtered matter containing a large amount of methane bacteria is returned to the storage tank 2 by the recycling device 8, thereby maintaining the concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid FL at a high concentration about 10 times higher than that of Comparative Example 1.

[0082] FIG. 6 is a graph showing the change in the amount of methane produced by the methane generator of Example 1. FIG. 7(a) is a graph showing the change in the amount of methane produced by the methane generator of Comparative Example 1. As shown in FIG. 6, in Example 1, the methane production reaction by hydrogen-utilizing methanogens using hydrogen and carbon dioxide as substrates progressed quickly from the start of the reaction until the elapsed time (T1). On the other hand, as shown in FIG. 7(a), in Comparative Example 1, the progress of the methane production reaction was slower than in Example 1. In Example 1, the average rate of change (methane production rate) of the amount of methane produced per elapsed time (T1) divided by the elapsed time (T1) was V1, whereas in Comparative Example 1, the amount of methane produced decreased with time (T 11 The average rate of change in methane production (methane production rate) divided by the mean methane production rate was approximately V1 / 10.

[0083] FIG. 7(b) is a graph showing the progress of the methane production amount and the like in the methane generation apparatus of Comparative Example 2. In Comparative Example 2, as in Example 1, the concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid FL was maintained at a high concentration of about 10 times that of Comparative Example 1, but the mass transfer capacity coefficient (J) was about the same as that of Comparative Example 1. Therefore, as shown in FIG. 7(b), the methane production reaction proceeded somewhat faster than in Comparative Example 1, but the low mass transfer capacity coefficient (J) was the limiting factor. As a result, in Comparative Example 2, the methane production amount decreased over time (T 22 The average rate of change in the amount of methane produced (methane production rate) divided by the average rate of change in the amount of methane produced (methane production rate) was about twice that of Comparative Example 1 and about 1 / 5 that of Example 1.

[0084] FIG. 7(c) is a graph showing the progress of the methane production amount and the like in the methane generator of Comparative Example 3. In Comparative Example 3, as in Example 1, the mass transfer capacity coefficient (J) was increased by 10 times compared to Comparative Example 1, but the concentration of hydrogen-utilizing methanogens in the bacteria-containing liquid FL was about the same as in Comparative Example 1. Therefore, as shown in FIG. 7(c), although the methane production reaction proceeded somewhat faster than in Comparative Example 1, the limiting factor was that the hydrogen-utilizing methanogens were not maintained at a high concentration. As a result, in Comparative Example 3, the methane production amount decreased over time (T 33 The average rate of change in the amount of methane produced (methane production rate) divided by the average rate of change in the amount of methane produced (methane production rate) was about twice that of Comparative Example 1 and about 1 / 5 that of Example 1.

[0085] From the above, even if either the volumetric mass transfer coefficient (J) or the concentration of hydrogen-utilizing methanogens is increased, the other becomes a limiting factor, and the methane production rate cannot be significantly increased, for example, by about 10 times compared to Comparative Example 1. To significantly increase the methane production rate, it is important to increase both the volumetric mass transfer coefficient (J) and the concentration of hydrogen-utilizing methanogens. [Industrial Applicability]

[0086] The methane generation apparatus of the present invention can be used, for example, to produce fuel for city gas, gas engines, etc. [Explanation of symbols]

[0087] 1A~1J Methane generator 2. Reservoir 5 Fine bubble supply unit (hydrogen dissolution promotion means) 6. Carbon source supply means 8 Reuse equipment (concentration improvement means) 60 Pressure regulating valve (hydrogen dissolution promoting means) 65 Storage tank (means for promoting hydrogen dissolution) 70,75 Carrier (means for increasing concentration) 80,90 Circulation channel

Claims

1. a storage tank for storing a bacteria-containing liquid containing hydrogen-utilizing methanogens; a carbon source supply means for supplying at least carbon dioxide to the storage tank; a hydrogen dissolution promoting means for promoting dissolution of a hydrogen-containing gas into the bacteria-containing liquid; a concentration increasing means for increasing the concentration of the hydrogen-utilizing methanogens in the bacteria-containing liquid; A methane generating device comprising:

2. a storage tank for storing a bacteria-containing liquid obtained by fermenting biomass, the bacteria containing hydrogen-utilizing methanogens; a hydrogen dissolution promoting means for promoting dissolution of a hydrogen-containing gas into the bacteria-containing liquid; a concentration increasing means for increasing the concentration of the hydrogen-utilizing methanogens in the bacteria-containing liquid; A methane generating device comprising:

3. The methane generator according to claim 2 , further comprising a carbon source supplying means for supplying at least carbon dioxide to the storage tank.

4. 3. The methane generator according to claim 1, wherein the hydrogen dissolution promoting means includes a fine bubble supply unit that supplies hydrogen-containing gas as fine bubbles to the bacteria-containing liquid.

5. 3. The methane generation apparatus according to claim 1, wherein the hydrogen dissolution promoting means includes a pressure regulating valve that maintains the inside of the storage tank in a pressurized state at a predetermined pressure.

6. 3. The methane generator according to claim 1, wherein the concentration improving means includes a recycling device that filters the bacteria-containing liquid in which the hydrogen-containing gas is dissolved, discharges the filtrate, and returns the filtered matter to the storage tank.

7. 3. The methane generator according to claim 1, wherein the concentration increasing means includes a carrier that supports the hydrogen-utilizing methanogens.

8. 3. The methane generator according to claim 1, further comprising a return passage for returning a portion of the biogas produced by methane fermentation by the hydrogen-utilizing methanogens to the storage tank.

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