Biomethane production method and biomethane production apparatus

Abstract

To provide a biomethane production method and a biomethane production apparatus by which an amount of biomethane to be obtained can be increased, and operation can be performed stably.SOLUTION: A biomethane production method includes a methane fermentation process of processing organic waste 1 containing hardly decomposable solid matters in a methane fermentation tank 11 where methane bacteria exist. In the methane fermentation process, when supplying at least one of organic acid 2 and biodegradable resin and hydrogen 3 into the methane fermentation tank 11, a ratio of a supply amount of the hydrogen 3 to a supply amount of the organic acid 2 and the biodegradable resin is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less such that pH inside the methane fermentation tank 11 becomes 7.0 or more and 9.0 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing biomethane and a biomethane production device. [Background technology]

[0002] In recent years, there has been a great deal of research into how to efficiently generate methane from organic waste. Specifically, the organic waste is placed in a methane fermentation device together with anaerobic methanogens, and the temperature inside the device is set to a temperature range where the methanogens are active, causing methane fermentation to proceed and producing methane gas.

[0003] Patent Document 1 discloses a waste treatment system comprising a decomposition tank that heats resin-containing waste in the presence of an amine compound to hydrolyze it into monomers, and a methane fermentation section that performs methane fermentation on the monomers obtained in the decomposition tank and organic waste containing non-degradable solids in a fermenter that receives the monomers and organic waste containing non-degradable solids. This method produces methane without passing through a mixed gas that primarily contains hydrogen and carbon monoxide, as opposed to chemical recycling, which recycles waste plastics into gases that can be used as chemical raw materials or fuels. Furthermore, by supplying monomers to a fermenter that is performing methane fermentation on organic waste containing non-degradable solids, both the methane fermentation of the organic waste containing non-degradable solids and the methane fermentation of the monomers can be achieved in the fermenter, enabling efficient methane fermentation.

[0004] Non-Patent Document 1 describes an in-situ biomethanation technology in which hydrogen is introduced into a methane fermentation tank where methane fermentation of organic waste such as sewage sludge is carried out, thereby methanating the CO2 in the organic waste and obtaining biogas containing a high concentration of methane. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-170390 [Non-patent literature]

[0006] [Non-Patent Document 1] Characteristics of in-situ hydrogen biomethanation at mesophilic and thermophilic temperatures(Bioresource Technology,Volume337, Jun 2021, 125455) Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the biomethane production method described in Patent Document 1, adding a large amount of monomers obtained in the decomposition tank causes the pH to become acidic, which has a negative effect on methanogens and limits the ability to increase biomethane production.

[0008] Furthermore, the biomethane production method described in Non-Patent Document 1 has the problem that the pH of the organic waste in the methane fermentation tank becomes alkaline, which reduces the biomethane production capacity.

[0009] An object of the present invention is to realize a biomethane production method and biomethane production apparatus that can increase the amount of biomethane obtained and can be operated stably. [Means for solving the problem]

[0010] The characteristic configuration of the biomethane production method of the present invention is as follows: A biomethane production method that performs a methane fermentation process in which organic waste containing hard-to-decompose solids is treated in a methane fermentation tank where methane bacteria are present, In the methane fermentation step, when at least one of an organic acid and a biodegradable resin and hydrogen are supplied into the methane fermentation tank, the ratio of the supply amount of the hydrogen to the supply amount of the organic acid and the biodegradable resin is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH in the methane fermentation tank is 7.0 or more and 9.0 or less.

[0011] The characteristic configuration of the biomethane production apparatus of the present invention is as follows: A biomethane production device that treats organic waste containing hard-to-decompose solids in a methane fermentation tank where methane bacteria exist, a hydrogen supply unit that supplies hydrogen to the methane fermentation tank; an organic acid / resin supply unit that supplies at least one of an organic acid and a biodegradable resin to the methane fermentation tank; a pH measurement unit that measures the pH in the methane fermentation tank; a control unit that controls the operation, The control unit controls the hydrogen supply unit and the organic acid / resin supply unit to supply at least one of the organic acid and the biodegradable resin and the hydrogen into the methane fermentation tank, and sets the ratio of the supply amount of the hydrogen to the supply amount of the organic acid and the biodegradable resin to be 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH measured by the pH measurement unit is 7.0 or more and 9.0 or less.

[0012] According to the above characteristic configuration, by simultaneously carrying out methane fermentation of organic waste and methane fermentation of organic acids or biodegradable resins in a methane fermentation tank containing methane bacteria, methane can be produced more efficiently than when only organic waste is methane fermented. Furthermore, by methanating the carbon dioxide produced by methane fermentation of organic waste and present in the methane fermentation tank using hydrogen supplied to the methane fermentation tank, methane can be produced more efficiently than when organic waste is subjected to methane fermentation.

[0013] In methane fermentation of organic acids and biodegradable resins, the pH in the methane fermentation tank decreases and approaches acidity as the organic load of the organic acids and biodegradable resins increases, adversely affecting methane fermentation bacteria and reducing biomethane production capacity. Furthermore, in methanation using hydrogen supplied to the methane fermentation tank, the pH in the methane fermentation tank gradually increases as the treatment progresses, becoming alkaline (especially above 9), adversely affecting methane fermentation bacteria and reducing biomethane production capacity.

[0014] However, with this characteristic configuration, when at least one of an organic acid and a biodegradable resin and hydrogen are supplied into the methane fermentation tank, the ratio of the amount of hydrogen supplied to the amount of organic acid and biodegradable resin supplied is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH in the methane fermentation tank is 7.0 or more and 9.0 or less. Therefore, even in a configuration in which at least one of an organic acid and a biodegradable resin and hydrogen are supplied to the methane fermentation tank, the pH in the methane fermentation tank can be maintained in the range of 7.0 to 9.0. This allows for efficient and stable production of biomethane without reducing methane production capacity, even when methane fermentation of organic waste, methane fermentation using at least one of organic acids and biodegradable resins, and methanation using hydrogen supply are carried out in parallel.

[0015] Further characteristic features of the biomethane production method of the present invention are: The organic acid includes at least one of a monocarboxylic acid and a dicarboxylic acid.

[0016] Further characteristic configurations of the biomethane production apparatus of the present invention include: The organic acid includes at least one of a monocarboxylic acid and a dicarboxylic acid.

[0017] As a result, since the organic acid used to adjust the pH of organic waste in the methane fermentation tank has one or two carboxyl groups, which are functional groups that exhibit relatively strong acidity, the pH in the methane fermentation tank can be more easily adjusted by adding the organic acid to the methane fermentation tank.

[0018] Further characteristic features of the biomethane production method of the present invention are: The biodegradable resin is at least one selected from the group consisting of polylactic acid, polybutylene succinate, polyhydroxybutyrate, polybutylene adipate terephthalate, and polyhydroxyalkanoate.

[0019] Further characteristic configurations of the biomethane production apparatus of the present invention include: The biodegradable resin is at least one selected from the group consisting of polylactic acid, polybutylene succinate, polyhydroxybutyrate, polybutylene adipate terephthalate, and polyhydroxyalkanoate.

[0020] As a result, the biodegradable resin used to generate biomethane contains a monomer with a carboxyl group in its molecule, making it a raw material for biomethane gas, and also effective as an organic acid in adjusting the pH in methane fermentation tanks. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic diagram of a biomethane production apparatus according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] A biomethane production method according to an embodiment of the present invention is a biomethane production method that involves a methane fermentation process in which organic waste 1 containing difficult-to-decompose solids is treated in a methane fermentation tank in which methane bacteria are present. In the methane fermentation process, when at least one of an organic acid 2 and a biodegradable resin (not shown) and hydrogen 3 are supplied into the methane fermentation tank 11, the ratio of the amount of hydrogen 3 supplied to the amount of organic acid 2 and biodegradable resin supplied is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH in the methane fermentation tank 11 is 7.0 or more and 9.0 or less.

[0023] Furthermore, a biomethane production apparatus 10 according to an embodiment of the present invention treats organic waste 1 containing hard-to-decompose solids in a methane fermentation tank 11 in which methane bacteria are present, and includes a hydrogen supply unit (not shown) that supplies hydrogen 3 to the methane fermentation tank 11, an organic acid / resin supply unit (not shown) that supplies at least one of an organic acid 2 and a biodegradable resin to the methane fermentation tank 11, a pH measurement unit 12 that measures the pH in the methane fermentation tank 11, and a control unit (not shown) that controls operation. The control unit controls the hydrogen supply unit and the organic acid / resin supply unit to supply at least one of the organic acid 2 and the biodegradable resin and hydrogen 3 into the methane fermentation tank 11, and sets the ratio of the amount of hydrogen 3 supplied to the amount of organic acid 2 and the biodegradable resin supplied to between 0.10 L / gCOD and 1.50 L / gCOD so that the pH measured by the pH measurement unit 12 is between 7.0 and 9.0.

[0024] Furthermore, the biomethane production apparatus 10 according to an embodiment of the present invention includes an organic waste supply unit (not shown) that supplies organic waste 1 to the methane fermentation tank 11, a temperature control unit (not shown) that can control and keep the temperature inside the methane fermentation tank 11 warm, a methane bacteria supply unit (not shown) that supplies methane bacteria into the methane fermentation tank 11, an agitation unit (not shown) that mixes and agitates the organic waste 1, organic acid 2, etc. in the methane fermentation tank 11, and a pressure adjustment unit that adjusts the pressure inside the methane fermentation tank 11.

[0025] <Organic waste> The organic waste 1 is, for example, digested sludge obtained at a sewage treatment plant. The digested sludge contains persistent solid matter containing one or more types of solid organic matter, and the organic matter obtained by solubilizing the persistent solid matter is subjected to acid fermentation, and methane is produced from the organic acids obtained thereby.

[0026] The organic waste 1 can be supplied to the methane fermentation tank 11 using a known method, but it may also be a fed-batch method in which the organic waste 1 is added appropriately as needed, or a continuous method in which the organic waste 1 is supplied continuously. The amount of organic waste 1 supplied by the organic waste supply unit to the methane fermentation tank 11 can be controlled by the control unit.

[0027] <Organic acid> The organic acid 2 may be any organic compound that exhibits acidity in an aqueous solution. Specific examples of fatty acids include acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, and derivatives thereof. Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, decanedicarboxylic acid, cyclohexanedicarboxylic acid, anthracenedicarboxylic acid, and derivatives thereof. Examples of hydroxycarboxylic acids include glycolic acid, L-lactic acid, D-lactic acid, hydroxypropionic acid, hydroxybutyric acid, hydroxyvaleric acid, hydroxycaproic acid, mandelic acid, hydroxybenzoic acid, and derivatives thereof. These may be used alone or in combination of two or more. In this embodiment, the organic acid 2 used is lactic acid, 3-hydroxybutyric acid, 1,4-butanediol, succinic acid, adipic acid, or terephthalic acid. The amount of organic acid 2 supplied by the organic acid / resin supply unit to the methane fermentation tank 11 can be controlled by the control unit.

[0028] <Biodegradable resin> The biodegradable resin may be any resin that is biodegradable, such as a chemically synthesized resin, a microbial resin, or a resin derived from natural products. Specific examples include aliphatic polyesters, polyvinyl alcohol (PVA), celluloses, and starches. Examples of aliphatic polyesters include polylactic acid (PLA) resin and its derivatives, polybutylene succinate (PBS) resin and its derivatives, polycaprolactone (PCL), polyhydroxybutyrate (PHB) and its derivatives, polyethylene adipate (PEA), polyethylene terephthalate succinate (PETS), polyhydroxyalkanoate (PHA), polyglycolic acid (PGA), polytetramethylene adipate, and condensates of diols and dicarboxylic acids. Examples of celluloses include methyl cellulose, ethyl cellulose, and acetyl cellulose. These may be used alone or in combination. In this embodiment, polyhydroxybutyrate (PHB) and 3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBH) are used as the biodegradable resin. The amount of biodegradable resin supplied by the organic acid / resin supply unit to the methane fermentation tank 11 can be controlled by the control unit.

[0029] The biodegradable resin may also be a mixture of the above-mentioned biodegradable resin with a general-purpose chemical resin and additives, such as plasticizers, heat stabilizers, light stabilizers, antioxidants, UV absorbers, flame retardants, colorants, pigments, fillers, inorganic fillers, release agents, antistatic agents, fragrances, lubricants, foaming agents, antibacterial and antifungal agents, and nucleating agents.

[0030] The biodegradable resin preferably contains a decomposition accelerator, preferably in an amount of 0.10 to 20 parts by weight, for example, 1.0 to 20 parts by weight, per 100 parts by weight of the biodegradable resin. If the amount of decomposition accelerator used is too small, it may be difficult to accelerate the decomposition of the biodegradable resin, and if an unnecessarily large amount is used, decomposition of the biodegradable resin may begin during the preparation stage of the resin composition or during use as a molded article.

[0031] Here, the molded article of biodegradable resin may be a molded article formed by a known molding method. Examples of known molding methods include injection molding, extrusion molding, sheet molding, vacuum molding, pressure molding, compression molding, and cast molding. The layer structure of the obtained molded article is not limited to a single-layer structure, but may also be a multi-layer structure, and the layers constituting the multi-layer may be a blend of two or more resins or a mixture with additives. The multi-layer molded article may be formed by co-extrusion molding or co-injection molding using a number of extruders or injectors corresponding to the number of resins, or a film or the like may be attached to the single-layer or multi-layer molded article in a subsequent process using an adhesive, thermocompression bonding, extrusion coating, or the like.

[0032] The decomposition accelerator preferably releases an acid upon hydrolysis. The acid released has a pH (25°C) of 4.0 or less, particularly 3.0 or less, in an aqueous solution or dispersion at a concentration of 0.0050 g / ml. A polymer that readily hydrolyzes to release an acid upon mixing with water is preferably used. Specific examples include oxalic acid, maleic acid, maleic anhydride, glycolic acid, etc., with oxalic acid and glycolic acid being preferred. Examples of such decomposition accelerators include polyoxalate, polyethylene maleate, and polyglycolic acid. Preferred decomposition accelerators are polyethylene oxalate and polyglycolic acid.

[0033] Furthermore, the above-mentioned decomposition accelerator preferably has a glass transition temperature (Tg) lower than the deactivation temperature of the enzyme used to decompose the biodegradable resin. By using an agent with such a low glass transition temperature, it becomes possible to more rapidly promote the enzymatic decomposition of the biodegradable resin. The glass transition temperature can be measured, for example, using a DSC6220 (differential scanning calorimetry) manufactured by Seiko Instruments Inc. The substances contained in the biodegradable resin other than the biodegradable resin are not particularly limited as long as they do not adversely affect methane fermentation.

[0034] <Hydrogen> Hydrogen is supplied from a hydrogen supply source (not shown) by a hydrogen supply unit to the methane fermentation tank 11. The hydrogen supply unit is configured to supply hydrogen from the bottom of the methane fermentation tank 11. The amount of hydrogen 3 supplied by the hydrogen supply unit to the methane fermentation tank 11 can be controlled by the control unit. <Methane fermentation tank> In the methane fermentation tank 11, organic waste 1 and at least one of a biodegradable resin and an organic acid 2 undergo methane fermentation to produce methane. Specifically, when the target of methane fermentation is sewage sludge, the organic matter obtained by solubilizing the persistent solids undergoes acid fermentation, and methane is produced from the organic acid thus obtained. Also, when the target of methane fermentation is at least one of a biodegradable resin and an organic acid 2, methane is produced from the organic acid 2 present. Furthermore, hydrogen is supplied into the methane fermentation tank 11 from the bottom, and the hydrogen methanates the carbon dioxide in the biogas produced by methane fermentation, producing methane.

[0035] <Methane bacteria> Methanogens are archaea that synthesize methane under anaerobic conditions, and are not particularly limited as long as they are used in the art for methane fermentation. Examples of facultative anaerobic bacteria include hydrocarbon-decomposing bacteria such as Clostridium, Bacillus, and Staphylococcus; fiber-decomposing bacteria such as Plecridium spumarum and Caduceus cellosaehydrogenicus; protein-decomposing bacteria such as Clostridium, Proteus, Bacterium, and Bacillus; and Clostridium kluberii. Examples of essential anaerobic bacteria include fat-decomposing bacteria such as Methanococcus, Methanobacterium, and Methanosarcina. One or more types of these methane fermentation bacteria can be used, and as methane fermentation continues, one or more types of methane fermentation bacteria act depending on the environment, causing the reaction to proceed.

[0036] <Biomethane production method> The biomethane production method according to this embodiment involves a methane fermentation step in which organic waste 1 containing hard-to-decompose solids is treated in a methane fermentation tank 11 in which methane bacteria are present. Specifically, organic waste 1 is placed in a methane fermentation tank 11 containing methanogens, nitrogen is passed through the methane fermentation tank 11 to purge the inside of the methane fermentation tank 11, and the temperature inside the methane fermentation tank 11 is then raised to a temperature at which methane fermentation can be carried out. Thereafter, while stirring the inside of the methane fermentation tank 11, the supply and withdrawal of organic waste 1 is repeated so that the residence time is a predetermined number of days. Once the amount of methane generated stabilizes at a predetermined amount, hydrogen 3 and either an organic acid 2 or a biodegradable resin are supplied into the methane fermentation tank 11 while checking the pH measurement unit 12 provided in the methane fermentation tank 11 so that the pH inside the methane fermentation tank 11 is 7.0 or higher and 9.0 or lower. Once the pH has stabilized within the above range, the temperature inside the methane fermentation tank 11 is maintained at a temperature at which methane fermentation can be carried out, and operation of the methane fermentation tank 11 is commenced.

[0037] Although there are no particular restrictions on the organic load in the methane fermentation tank 11, it is preferable to supply the organic waste 1, organic acid 2, biodegradable resin, and hydrogen into the methane fermentation tank 11 so that the organic load is 0.60 g CODCr / L / day or more and 15.0 g CODCr / L / day or less. Note that this embodiment is configured to enable stable generation of biomethane even when the organic load is 5.0 g CODCr / L / day or more. Here, CODCr is an index that indicates the amount of organic matter in water, and is the amount of oxygen required when potassium dichromate oxidizes the target oxidizable substance. For example, when the organic acid is lactic acid, the organic load is calculated using the following formula (1): Organic load = lactic acid input (mL / L / day) x lactic acid concentration (%) x 1.06 (gCODCr / g lactic acid)

[0038] Furthermore, in the methane fermentation process, the fermentation liquid may contain other organic substances for methane fermentation, such as food processing residues or food waste, as well as nutrients for supporting the growth and activity of methanogens. In order to increase the efficiency of methane fermentation of the organic substances, pretreatment such as acid decomposition, alkaline decomposition, thermal decomposition, steam decomposition, enzymatic decomposition, or decomposition by microorganisms other than methanogens may be carried out before methane fermentation, or a combination of these pretreatments may be carried out. Methane fermentation can be carried out while stirring appropriately using a stirring unit as necessary. The temperature of methane fermentation is not particularly limited and can be set appropriately by those skilled in the art to a temperature suitable for fermentation, but can be, for example, 20°C to 80°C, preferably 35°C to 60°C, e.g., 55°C. In this case, it is preferable to carry out fermentation using a thermostatic bath to maintain the appropriate temperature.

[0039] Furthermore, the treatment time for methane fermentation can be appropriately set by a person skilled in the art depending on the amount of organic waste 1, the type of methane bacteria used, and the fermentation temperature, but can be, for example, from 1 day to 30 days, preferably from 1 day to 20 days, and more preferably from 1 day to 10 days.

[0040] Methane fermentation can be performed in conjunction with other organic biodegradation methods, such as hydrogen fermentation, ethanol fermentation, lactic acid fermentation, and acetone-butanol fermentation. For example, when hydrogen fermentation is used in combination, a two-stage hydrogen-methane fermentation system can be implemented, and the organic acids produced by hydrogen fermentation, such as acetic acid, butyric acid, propionic acid, and lactic acid, can be subjected to methane fermentation, allowing for efficient recovery of hydrogen and methane.

[0041] The methane gas produced often contains various impurities, so it can be purified as needed. Gas purification methods include desulfurization and denitrification, and these processes can be carried out using a gas purification tank or the like, which allows for the production of highly pure methane gas.

[0042] After the operation of the methane fermentation tank 11 starts, predetermined amounts of hydrogen, organic acid, and biodegradable resin are fed into the methane fermentation tank 11 at predetermined time intervals. At this time, the ratio of the supply amount of hydrogen to the supply amount of organic acid 2 and biodegradable resin is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH in the methane fermentation tank 11 is 7.0 or more and 9.0 or less. This makes it possible to obtain a larger amount of biogas while maintaining stable operation.

[0043] It is more preferable to set the ratio of the supply amount of hydrogen to the supply amount of organic acid 2 and biodegradable resin so that the pH in the methane fermentation tank 11 is 7.5 or more and 8.5 or less. Furthermore, the ratio of the amount of hydrogen supplied to the amount of organic acid 2 and biodegradable resin supplied is more preferably 0.40 L / gCOD or more and 1.20 L / gCOD or less. Hereinafter, the ratio of the amount of hydrogen supplied to the amount of organic acid 2 and biodegradable resin supplied may be referred to as the hydrogen input amount / organic acid, etc. input amount ratio. If the pH in the methane fermentation tank 11 falls outside the range of 7.0 or more and 9.0 or less, hydrogen 3 and at least one of organic acid 2 and biodegradable resin are additionally added to the methane fermentation tank 11 according to a pH-organic acid supply amount table showing the relationship between the pH in the methane fermentation tank 11 and the required supply amount of organic acid 2 (biodegradable resin), or a pH-hydrogen supply amount table showing the relationship between the pH in the methane fermentation tank 11 and the required supply amount of hydrogen 3.

[0044] The recovered methane gas can be reused for power generation, fuel production, and polymer synthesis. Furthermore, when hydrogen fermentation, ethanol fermentation, lactic acid fermentation, acetone-butanol fermentation, and other processes are used in combination, it is possible to simultaneously obtain the fermentation products of each process, which can also be reused. The resulting methane gas can be recovered using standard methods and reused according to the intended purpose. For example, when used for power generation, it can be stored in a gas holder, or it can be directly fed into a fuel cell or gas generator. Furthermore, the residue and wastewater generated during the fermentation process can also be reused by converting them into fertilizer and animal feed.

[0045] The methane production method of the present invention simultaneously carries out methane fermentation of organic waste and methane fermentation of organic acids or biodegradable resins in a methane fermentation tank containing methane bacteria, thereby enabling methane to be produced more efficiently than when only organic waste is subjected to methane fermentation. Furthermore, by methanating the carbon dioxide produced by methane fermentation of organic waste and present in the methane fermentation tank using hydrogen supplied to the methane fermentation tank, methane can be produced more efficiently than when organic waste is subjected to methane fermentation. Generally, in methane fermentation of organic acids and biodegradable resins, as the organic load of the organic acids and biodegradable resins increases, the pH in the methane fermentation tank decreases and approaches acidity, which has a negative effect on methane fermentation bacteria and reduces biomethane production capacity. Also, in methanation using hydrogen supplied to the methane fermentation tank, the pH in the methane fermentation tank gradually increases as the treatment progresses, becoming alkaline (especially above 9), which has a negative effect on methane fermentation bacteria and reduces biomethane production capacity. However, with this characteristic configuration, when at least one of an organic acid and a biodegradable resin and hydrogen are supplied into the methane fermentation tank, the ratio of the amount of hydrogen supplied to the amount of organic acid and biodegradable resin supplied is set to 0.10 L / gCOD or more and 1.50 L / gCOD or less so that the pH in the methane fermentation tank is 7.0 or more and 9.0 or less. Therefore, even in a configuration in which at least one of an organic acid and a biodegradable resin and hydrogen are supplied to the methane fermentation tank, the pH in the methane fermentation tank can be maintained in the range of 7.0 to 9.0. This allows for efficient and stable production of biomethane without reducing methane production capacity, even when methane fermentation of organic waste, methane fermentation using at least one of organic acids and biodegradable resins, and methanation using hydrogen supply are carried out in parallel. In addition, since the organic acids used to adjust the pH of organic waste in the methane fermentation tank have one or two carboxyl groups, which are functional groups that exhibit relatively strong acidity, the pH in the methane fermentation tank can be more easily adjusted by adding the organic acid to the methane fermentation tank.

[0046] Examples of the present invention will be described below, but the present invention is not limited to these examples.

[0047] Example 1 A 3.0 L jar fermenter (Mitsuwa Frontech Co., Ltd., NBC-3000) was charged with 2.0 L of high-temperature anaerobic digested sludge collected from a general sewage treatment plant, and the headspace was purged for 5 minutes with a nitrogen flow rate of 1.0 L / min. The lid was then closed and the vessel was heated to 55°C to construct a lab-scale methane fermentation tank. Concentrated sludge from the same wastewater treatment plant was added at a rate of 100 mL / day to the methane fermenter, ensuring a 20-day retention time. The methane fermenter was agitated at 500 rpm using an attached agitator. The methane fermenter was operated under these conditions for one month. After confirming that methane production had stabilized at 0.75 L / L / day, 100 mL / day of concentrated sludge was added. Hydrogen was injected into the bottom of the fermenter at a rate of 7.53 L / L / day using a 6 mm diameter stainless steel pipe. Fermented lactic acid (90% content, remainder water, Fuso Chemical Co., Ltd.) was added to maintain an organic load of 5.14 g CODCr / L / day. Excess sludge withdrawal was adjusted to maintain a digested sludge volume of 2.0 L in the methane fermenter. This methane fermentation tank was operated continuously for one month, and the amount of biogas produced, methane concentration, amount of methane produced, and pH in the methane fermentation tank were measured on the 10th day, and the amount of methane produced and pH in the methane fermentation tank after one month. The pH in the methane fermentation tank on day 0 was 7.8. The amount of biogas produced was measured by collecting biogas in a gas bag using a plastic syringe. The methane concentration was measured using a gas chromatograph (product name: MicroGC990, manufactured by GL Sciences Inc.). A compact pH meter (LAQUAtwin pH-11B, manufactured by Horiba Ltd.) was used.

[0048] Example 2 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 5.79 L / L / day.

[0049] Example 3 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 4.05 L / L / day.

[0050] Example 4 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 2.31 L / L / day.

[0051] Example 5 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 0.57 L / L / day.

[0052] Example 6 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 2.89 L / L / day and the organic acid used to generate biogas was 3-hydroxybutyric acid (content 95%, remainder water, Sigma-Aldrich).

[0053] Example 7 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 1.08 L / L / day and the organic acid used to generate biogas was 1,4-butanediol (content 98%, remainder water, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0054] Example 8 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 5.52 L / L / day and the organic acid used to generate biogas was succinic acid (content 99.5%, remainder impurities, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0055] Example 9 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 3.25 L / L / day and the organic acid used to generate biogas was adipic acid (content 99.5%, remainder impurities, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0056] Example 10 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 4.74 L / L / day and the organic acid used to generate biogas was terephthalic acid (content 95%, the remainder being impurities, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.).

[0057] Example 11 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 2.89 L / L / day and the biodegradable resin used to generate biogas was PHB (polyhydroxybutyrate, powder, manufactured by Sumitomo Pharma Food & Chemical Co., Ltd., Biosoft 915).

[0058] Example 12 Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen input into the methane fermentation tank was 3.07 L / L / day and the biodegradable resin used to generate biogas was PHBH (3-hydroxybutyrate-co-3-hydroxyhexanoate).

[0059] The PHBH used in Example 12 was prepared by the following procedure. A medium (pH 7.0) was prepared by adding 5 g of meat extract, 5 g of peptone, 2 g of yeast extract, 0.5 g of KH2PO4, 1.5 g of K2HPO4, and 0.1 g of MgSO4·7H2O to water to a total volume of 1 liter. Aeromonas hydrophila OL-338 strain was added to this medium and cultured with shaking at 30°C for 48 hours. After the culture was completed, the culture broth was centrifuged to collect the bacterial cells. Next, a medium (pH 7.0) was prepared by adding 25.4 g of β-hydroxycaproic acid, 1.5 g of KH2PO4, 1.5 g of K2HPO4, 0.25 g of MgSO4·7H2O, and 0.5 g of Tween 85 to water to a total volume of 1 liter. The entire amount of the above-mentioned bacterial cells was added to this medium and cultured with shaking at 30°C for 24 hours. After the culture was completed, the bacterial cells were washed with distilled water and methanol and dried under reduced pressure to obtain dried bacterial cells. The dried bacterial cells were extracted with chloroform at 50°C for 2 hours. After the bacterial cells were removed, 10 volumes of methanol were added to the chloroform extract, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (biodegradable resin PHBH) with a 3-hydroxybutyrate:3-hydroxyhexanoate ratio of 51:49 was precipitated and recovered.

[0060] (Comparative Example 1) Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 0.40 L / L / day.

[0061] (Comparative Example 2) Biogas was generated under the same conditions as in Example 1, except that the amount of hydrogen fed to the methane fermentation tank was 8.30 L / L / day.

[0062] [Table 1]

[0063] In Examples 1 to 12, the hydrogen input / organic acid input ratio was 0.10 L / gCOD or more and 1.50 L / gCOD or less, and the pH of the digested sludge was within the range of 7.0 or more and 9.0 or less from 10 days after biogas generation. In contrast, in Comparative Examples 1 and 2, the ratio of hydrogen input amount to organic acid input amount was not in the range of 0.10 L / gCOD or more and 1.50 L / gCOD or less, and the pH of the digested sludge was not in the range of 7.0 or more and 9.0 or less from the 10th day after biogas generation.

[0064] In all of Examples 1 to 12 and Comparative Examples 1 and 2, methane gas was generated at a rate of 2.3 to 4.4 L / L / day on the 10th day after the start of biogas generation. When the amount of methane gas generated one month after the start of biogas generation was checked, it was found that in Examples 1 to 12, the amount of methane gas generated was 2.0 L or more, which was about the same as the amount generated 10 days after biogas generation. In contrast, in Comparative Examples 1 and 2, the amount of methane gas generated was 0.21 L and 0.12 L, respectively, one month after the start of biogas generation, indicating that almost no methane gas was generated. In Comparative Example 1, the amount of methane gas generated one month after the start of biogas generation was significantly low because the amount of organic acid added was excessive, causing the pH of the digested sludge to fall below 7.0. In Comparative Example 2, the amount of methane gas generated one month after the start of biogas generation was significantly low because the pH of the digested sludge was 9.0 or higher due to an excessive amount of hydrogen input.

[0065] In Examples 2 to 4, the ratio of hydrogen input / organic acid, etc. input was 0.4 to 1.2 L / g-COD, and the pH of the digested sludge was in the range of 7.5 to 8.5, so it was found that biomethane generation could be carried out more stably compared to Examples 1 and 5.

[0066] In Example 6, 3-hydroxybutyric acid, which has a longer carbon chain than lactic acid, was used, and the results show that using 3-hydroxybutyric acid is more preferable than using lactic acid, as it generates more methane.

[0067] In Example 7, 1,4-butanediol, which does not have a carboxylic acid in the molecule, was used as the organic acid, and the pH of the digested sludge was 8.6. These results show that when there is no carboxylic acid in the molecule, the organic acid does not have sufficient ability to lower the pH of digested sludge, but the pH in the methane fermentation tank falls within the range of 7.0 to 9.0, so it can be used for methane fermentation.

[0068] In Examples 8 to 10, succinic acid, adipic acid, and terephthalic acid, which are dicarboxylic acids, were used as the organic acids, and the pH of the digested sludge in these cases was 7.2 to 7.4. These results show that, although dicarboxylic acids significantly lower the pH of digested sludge as organic acids in the present invention, they can be used for methane fermentation because the pH in the methane fermentation tank falls within the range of 7.0 to 9.0.

[0069] In Examples 11 and 12, PHB and PHBH, which are plastics that can undergo methane fermentation simply by being placed in a methane fermentation tank without hydrolysis, were used as organic acids. These results indicate that PHB and PHBH can be used as organic acids in biomethane generation, even though the pH of the digested sludge is relatively high at 8.7.

[0070] In Comparative Examples 1 and 2, methane fermentation was carried out when the amount of hydrogen introduced into the methane fermentation tank was too little and too much, respectively, compared to Examples 1 to 5. In Comparative Example 1, the pH of the digested sludge reached 6.1 one month after the start of methane fermentation, and in Comparative Example 2 it reached 9.6. These results show that when an insufficient amount of hydrogen is added to the methane fermentation tank, the pH does not rise sufficiently, and when an excessive amount of hydrogen is added, the pH rises excessively. Under either condition, the pH of the organic waste is not suitable for generating biomethane.

[0071] Although the embodiments and examples of the present disclosure have been described above, it is originally intended that the configurations of the above-described embodiments and examples may be appropriately combined or modified in various ways.

[0072] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined by the claims, not by the embodiments and examples described above, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims. [Industrial Applicability]

[0073] The present invention can be used in a biomethane production method and a biomethane production device. [Explanation of symbols]

[0074] 1. Organic waste 2 Organic acids 3. Hydrogen 10. Biomethane production equipment 11 Methane fermentation tank 12 pH measurement unit

Claims

1. A biomethane production method that performs a methane fermentation process in which organic waste containing hard-to-decompose solids is treated in a methane fermentation tank where methane bacteria are present, In the methane fermentation step, when at least one of an organic acid and a biodegradable resin and hydrogen are supplied into the methane fermentation tank, the ratio of the amount of hydrogen supplied to the amount of the organic acid and the biodegradable resin supplied is set to 0.10 L / g COD or more and 1.50 L / g COD or less so that the pH in the methane fermentation tank is 7.0 or more and 9.0 or less.

2. The method for producing biomethane according to claim 1 , wherein the organic acid comprises at least one of a monocarboxylic acid and a dicarboxylic acid.

3. 3. The method for producing biomethane according to claim 1 or 2, wherein the biodegradable resin is selected from at least one of polylactic acid, polybutylene succinate, polyhydroxybutyrate, polybutylene adipate terephthalate, and polyhydroxyalkanoate.

4. A biomethane production device that treats organic waste containing hard-to-decompose solids in a methane fermentation tank where methane bacteria exist, a hydrogen supply unit that supplies hydrogen to the methane fermentation tank; an organic acid / resin supply unit that supplies at least one of an organic acid and a biodegradable resin to the methane fermentation tank; a pH measuring unit that measures the pH in the methane fermentation tank; a control unit that controls the operation, the control unit controls the hydrogen supply unit and the organic acid / resin supply unit to supply the hydrogen and at least one of the organic acid and the biodegradable resin into the methane fermentation tank, and sets the ratio of the supply amount of the hydrogen to the supply amount of the organic acid and the biodegradable resin to be 0.10 L / g COD or more and 1.50 L / g COD or less so that the pH measured by the pH measuring unit is 7.0 or more and 9.0 or less.

5. The biomethane production apparatus according to claim 4 , wherein the organic acid includes at least one of a monocarboxylic acid and a dicarboxylic acid.

6. 6. The biomethane production apparatus according to claim 4, wherein the biodegradable resin is selected from at least one of polylactic acid, polybutylene succinate, polyhydroxybutyrate, polybutylene adipate terephthalate, and polyhydroxyalkanoate.

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