Methane fermentation method and system

Abstract

This invention provides a methane fermentation method and system that can produce biogas with a high methane concentration using energy efficiency. [Solution] The methane fermentation system 50 comprises a first methane fermentation tank 51A equipped with an anode 52, a second methane fermentation tank 51B equipped with a cathode 53, and a power supply 54 that applies a voltage between the anode 52 and the cathode 53. With the voltage applied, organic matter is subjected to acid fermentation and methane fermentation in the first methane fermentation tank 51A, and the resulting first biogas 58 and first digestate 56 are supplied to the second methane fermentation tank 51B, where methane fermentation is performed with the voltage applied.

Description

TECHNICAL FIELD

[0001] The present invention relates to a methane fermentation method and system.

BACKGROUND ART

[0002] In order to reduce greenhouse gas emissions, there is a strong demand for the development and widespread adoption of renewable energy technologies. As well-known renewable energy sources, solar power generation, offshore wind power generation, woody biomass power generation and the like are widely known, and methane fermentation technology that generates biogas from organic matter has also been subjected to research and development over many years. The biogas produced by methane fermentation currently has a main component methane concentration of about 40 to 70%, although this value greatly varies depending on the organic matter serving as the raw material and the methane fermentation method. To use biogas as fuel for automobiles and the like, it is necessary to further increase the methane concentration.

[0003] For example, Patent Document 1 describes a biogas conditioning method for effectively utilizing carbon dioxide abundantly contained in biogas to stabilize the methane concentration in the biogas at or above a certain value. According to the method, biogas containing methane and carbon dioxide with a time-varying methane concentration is introduced into a biogas conditioning apparatus main body provided with a conditioning section having a conductive microorganism carrier impregnated with a methanogen-containing liquid, a potential that shifts the carbon dioxide-methane equilibrium system toward the methane production side is applied to the conductive microorganism carrier in the conditioning section, and the methanogen-containing liquid and the biogas are brought into gas-liquid contact in the conditioning section, so that by utilizing the phenomenon that the methane production rate increases in response to a decrease in the methane concentration in the biogas, conditioning is performed to stabilize the methane concentration in the biogas at or above a certain value.

[0004] Furthermore, biometanation technology is known in which, by adding hydrogen during methane fermentation, methane is synthesized from carbon dioxide and hydrogen in the biogas produced by the action of microorganisms, thereby increasing the methane concentration in the biogas (for example, Patent Document 2). This technology has the problem of low hydrogen solubility in liquid. To overcome this problem, for example, Non-Patent Documents 1 and 2 describe how a biogas with a methane concentration of around 90% was obtained by foaming the input gas containing hydrogen with sludge and filling it into a reactor, and reacting it at a temperature of about 37°C in the presence of anaerobic microorganisms. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-211976 [Patent Document 2] Japanese Patent Publication No. 2024-002180 [Non-patent literature]

[0006] [Non-Patent Document 1] Ding Han-Han, Furuzaki Yasutetsu, "A biometanation reactor that maintains the liquid phase in a foamy state," Proceedings of the 34th Annual Meeting of the Japan Society of Waste Management and Resource Recycling, 2023, Presentation Manuscript pp. 251-252. [Non-Patent Document 2] Ding Han-Han and Furuzaki Yasutetsu, "Continuous Operation of High-Load Biometanation Using a Foam-Filled Reactor," Proceedings of the 57th Annual Meeting of the Japan Society on Water Environment, 2023, p.240. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Since cow manure releases methane, which has 28 times the greenhouse effect of carbon dioxide, collecting this manure and artificially producing biogas through methane fermentation, then refining and using it as fuel for automobiles, can reduce the release of methane into the atmosphere. Furthermore, because such biogas is a carbon-neutral fuel, its future use is highly anticipated. However, the methane concentration of biogas produced by methane fermentation is only about 40-70%, and there is a problem in that the refining cost for using such biogas as fuel for automobiles is very high.

[0008] Methods for increasing methane concentration by adding hydrogen, as described in Non-Patent Documents 1 and 2, have several drawbacks, including the low solubility of hydrogen in the liquid phase, the high cost of hydrogen production, and the enormous amount of energy required to procure it.

[0009] Therefore, in view of the above problems, the present invention aims to provide a methane fermentation method and system that can produce biogas with a high methane concentration in an energy-efficient manner. [Means for solving the problem]

[0010] To achieve the above objective, the present invention, in one aspect, provides a methane fermentation system comprising: a first tank provided with an anode; a second tank for methane fermentation provided with a cathode; a power supply for applying a voltage between the anode and the cathode; a digestate piping for sending digestate to the second tank; a first biogas piping for sending biogas to the second tank; and a second biogas piping for recovering high-concentration biogas produced in the second tank, which contains methane at a higher concentration than the first biogas.

[0011] In another aspect, the present invention provides a methane fermentation method, a methane fermentation system comprising a first tank provided with an anode and a second tank for methane fermentation provided with a cathode, comprising the steps of supplying digestate and biogas to the second tank, applying a voltage between the anode and the cathode, and performing methane fermentation in the second tank while the voltage is applied to produce high-concentration biogas containing methane at a higher concentration than the biogas. [Effects of the Invention]

[0012] As described above, according to the present invention, in a methane fermentation system comprising a first tank equipped with an anode and a second tank for methane fermentation equipped with a cathode, by supplying digestate and biogas to the second tank and performing methane fermentation in the second tank while applying a low voltage between the anode and the cathode, the electrons generated in the first tank can be used for methane fermentation in the second tank, thereby promoting the reaction in which hydrogen-capitalizing archaea produce methane and water from hydrogen and carbon dioxide, and thus biogas with a higher methane concentration can be produced with energy savings. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram showing the flow from generating biogas from organic matter such as livestock manure to utilizing it, including the methane fermentation system according to the present invention. [Figure 2] This is a schematic diagram showing one embodiment of the methane fermentation system according to the present invention. [Figure 3] This is a schematic diagram showing another embodiment of the methane fermentation system according to the present invention. [Figure 4] This graph shows the amount of methane fermentation in the examples and comparative examples. [Figure 5] This graph shows the estimated number of archaeal cells in the examples and comparative examples. [Modes for carrying out the invention]

[0014] Hereinafter, an embodiment of the methane fermentation method and system according to the present invention will be described with reference to the accompanying drawings.

[0015] First, as shown in Figure 1, livestock manure 1 such as cow dung generated in a livestock barn 10 for cattle or the like is temporarily collected in a storage tank 10. Water 2 may be added to the livestock manure 1 to achieve a preferable organic matter concentration or volatile suspended solid (VS) concentration. The livestock manure 1 to be treated in the present invention contains biodegradable high-molecular organic substances 3 such as carbohydrates, proteins, and lipids, which are decomposed into low-molecular organic substances 4 such as monosaccharides, amino acids, and higher fatty acids through the solubilization and hydrolysis process 30. Next, the low-molecular organic substances 4 pass through volatile fatty acids and organic acids such as valeric acid, butyric acid, and propionic acid in the acid fermentation process 40, to become an acid fermentation liquid 5 containing acetic acid, hydrogen, and carbon dioxide.

[0016] Furthermore, in the methane fermentation process 50, acetic acid in the acid fermentation liquid 5 is decomposed into methane and carbon dioxide, and methane and water are synthesized from hydrogen and carbon dioxide, thereby producing biogas 6 containing methane and carbon dioxide, and digested liquid 7 containing water and fermentation residues. The biogas 6 can be used as fuel for automobiles 70 or the like as high-concentration methane gas 8 by removing contained carbon dioxide and hydrogen sulfide in the purification step 60. In addition, the digested liquid 7 can be used as an organic fertilizer.

[0017] The solubilization and hydrolysis process 30, acid fermentation process 40, and methane fermentation process 50 in Figure 1 show the processes from the biodegradable high-molecular organic substances 3 to the generation of methane. Although not illustrated, these three processes may, for example, proceed sequentially in a single methane fermentation tank, or a plurality of fermentation tanks may be used, such as allowing the solubilization and hydrolysis process 30 and the acid fermentation process 40 to proceed in an acid fermentation tank, and allowing the methane fermentation process 50 to proceed in a methane fermentation tank.

[0018] In the acid fermentation process 40, acid-fermenting bacteria, such as bacteria, primarily produce acetic acid, hydrogen, and carbon dioxide from low-molecular-weight organic matter 4 via organic acids. In the methane fermentation process 50, methane-producing bacteria, including acetic acid-assimilating archaea and hydrogen-assimilating archaea, produce methane, carbon dioxide, and water from acetic acid, hydrogen, and carbon dioxide. To explain methane fermentation in more detail, acetic acid-assimilating archaea produce methane and carbon dioxide from acetic acid as follows: CH3COO - / H + →CH4+CO2

[0019] Furthermore, hydrogen-capsulating archaea produce methane and water from hydrogen and carbon dioxide, as follows. At this time, electrons (e) are generated when organic matter is decomposed by bacterial mediators. - ) contributes to the formation of hydrogen molecules. (8H + →) 4H2 + CO2 → CH4 + 2H2O

[0020] Typically, in the methane fermentation process, methane production by acetate-assimilating archaea accounts for about 70%, while methane production by hydrogen-assimilating archaea accounts for about 30%. The methane production reaction by hydrogen-assimilating archaea requires electron transfer via hydrogen diffusion and is an inefficient reaction, making it the rate-determining reaction.

[0021] Furthermore, while the acid-producing bacteria in acid fermentation process 40 have a high growth rate and strong resistance to external factors, the methane-producing bacteria in methane fermentation process 50 have a low growth rate and are susceptible to external factors (temperature changes, toxins, etc.). When both acid fermentation process 40 and methane fermentation process 50 are carried out in a single methane fermentation tank, there is a problem in that acid fermentation process 40 progresses quickly while methane fermentation process 50 progresses slowly.

[0022] As shown in Figure 2, the methane fermentation system 50 of this embodiment mainly comprises a first methane fermentation tank 51A equipped with an anode 52, a second methane fermentation tank 51B equipped with a cathode 53, a power supply 54 that applies voltage between the anode 52 and the cathode 53, a supply pipe 41 that supplies a liquid to be treated 5 containing livestock manure such as cow dung to the first methane fermentation tank 51A, a first digestate pipe 56 that sends the first digestate produced in the first methane fermentation tank 51A to the second methane fermentation tank 51B, a second digestate pipe 57 that discharges the second digestate produced in the second methane fermentation tank 51B, a first biogas pipe 58 that sends the first biogas produced in the first methane fermentation tank 51A to the first digestate pipe 56, and a second biogas pipe 59 that discharges the second biogas produced in the second methane fermentation tank 51B and supplies it back to the second methane fermentation tank 51B to circulate the biogas.

[0023] The first biogas piping 58 and the second biogas piping 59 are installed in the upper portions, preferably on the ceiling, of the first methane fermentation tank 51A and the second methane fermentation tank 51B, respectively, to recover biogas generated around the anode 52 of the first methane fermentation tank 51A and around the cathode 53 of the second methane fermentation tank 51B. The supply piping 41, the first digestate piping 56, and the second digestate piping 57 are installed in the lower portions of the first methane fermentation tank 51A and the second methane fermentation tank 51B.

[0024] Furthermore, the second biogas piping 59 is equipped with a methane meter 67 for measuring the methane concentration in the second biogas and a biogas branching device 62. The biogas branching device 62 is equipped with a biogas recovery piping 61 for recovering the second biogas. The biogas branching device 62 has a branching valve and control function that allows the second biogas to flow to the biogas recovery piping 61 if the methane concentration measured by the methane meter 67 is above a predetermined concentration, and to flow the second biogas through the second biogas piping 59 to the second methane fermentation tank 51B if the concentration is below the predetermined concentration.

[0025] Pumps 63 and 65 for pressurizing the liquid are provided in the supply piping 41 and the second digestate piping 57, respectively. Gas pumps 66, 68, and 69 for pressurizing the gas are provided in the first biogas piping 58, the second biogas piping 59, and the biogas recovery piping 61, respectively. The gas pumps 66 and 68 in the first biogas piping 58 and the second biogas piping 59 need to be high-pressure pumps because they are sending biogas into the liquid. In addition, check valves (not shown) are provided on the discharge side of the air pumps 66 and 68 to prevent digestate from flowing back into the piping. The first biogas piping 58 may also be configured to directly send the first biogas produced in the first methane fermentation tank 51A into the second methane fermentation tank 51B, as shown by reference numeral 58a.

[0026] The first methane fermentation tank 51A and the second methane fermentation tank 51B are both airtight, as the methane fermentation reaction must be carried out in an anaerobic environment without oxygen. Although not shown in the diagram, the first methane fermentation tank 51A and the second methane fermentation tank 51B are also equipped with a stirring device for agitating the liquid inside the tank and a constant temperature device for controlling the temperature inside the tank. Furthermore, multiple granular activated carbon 55 may be provided inside the tanks of the first methane fermentation tank 51A and the second methane fermentation tank 51B.

[0027] The anode 52 and cathode 53 are positioned to be immersed in the liquid to be treated 5 supplied to the tanks of the first methane fermentation tank 51A and the second methane fermentation tank 51B. While a wide range of electrode materials can be used for the anode 52 and cathode 53, as long as they are corrosion-resistant to the liquid to be treated 5, carbon fiber is particularly preferred.

[0028] In a methane fermentation system 50 with this configuration, first, the liquid to be treated 5, which contains livestock manure such as cow dung, is introduced from the supply pipe 41 to the first methane fermentation tank 51A, and then further introduced to the second methane fermentation tank 51B via the first digestate pipe 56. As a result, the anode 52 in the first methane fermentation tank 51A is immersed in the liquid to be treated 5, and the cathode 53 in the second methane fermentation tank 51B is immersed in the liquid to be treated 5a. However, in order to collect and recover the biogas produced in the first methane fermentation tank 51A and the second methane fermentation tank 51B, the liquid to be treated 5 is supplied so that space remains above each fermentation tank 51. It is preferable that the liquid to be treated 5 is adjusted to a predetermined organic matter concentration by adding water to the livestock manure.

[0029] After filling both the first methane fermentation tank 51A and the second methane fermentation tank 51B with the liquids to be treated 5 and 5a, the liquid to be treated 5 is again introduced into the first methane fermentation tank 51A from the supply pipe 41, and a voltage is applied between the anode 52 and cathode 53 using the power supply 54 to start the methane fermentation process of the liquid to be treated 5.

[0030] First, the liquid to be treated 5 supplied to the first methane fermentation tank 51A is broken down into low-molecular-weight organic matter through the solubilization and hydrolysis process described above. Next, the low-molecular-weight organic matter is subjected to an acid fermentation process by acid-fermenting bacteria, such as bacteria, which produces acetic acid, hydrogen, and carbon dioxide through organic acids. In this acid fermentation process, electrons are generated when the low-molecular-weight organic matter is broken down into organic acids by bacteria, and these electrons are transported via the anode 52 to the cathode 53 in the second methane fermentation tank 51B.

[0031] Furthermore, within the first methane fermentation tank 51A, the methane fermentation process 50 generates methane and carbon dioxide from acetic acid by acetic acid-assimilating archaea, and methane and water from hydrogen and carbon dioxide by hydrogen-assimilating archaea. At this time, in the first methane fermentation tank 51A, the electrons generated in the acid fermentation process are sent to the second methane fermentation tank 51B by the application of voltage, thus further suppressing the methane production reaction by hydrogen-assimilating archaea, which require electrons. The first digestate, containing water and unfermented organic matter, organic acids, acetic acid, etc., is sent to the second methane fermentation tank 51B via the first digestate piping 56, and the first biogas, containing methane and carbon dioxide, is sent to the second methane fermentation tank 51B via the first biogas piping 58.

[0032] In this way, the liquid to be treated 5a, which contains the first digestate and first biogas supplied to the second methane fermentation tank 51B, contains unfermented organic matter and organic acids, so the acid fermentation process proceeds, but since it mainly contains acetic acid, carbon dioxide, and hydrogen, the methane fermentation process proceeds mainly. Electrons are supplied to the cathode 53 of the second methane fermentation tank 51B from the anode 52 of the first methane fermentation tank 51A, thus promoting the reaction by hydrogen-utilizing archaea, which is the rate-limiting reaction, to produce methane and water from hydrogen and carbon dioxide. Therefore, the carbon dioxide obtained in the reaction by acetic acid-utilizing archaea to produce methane and carbon dioxide in the second methane fermentation tank 51B can also be converted to methane and water by the methane production reaction by hydrogen-utilizing archaea, thus making the methane concentration of the second biogas produced in the second methane fermentation tank 51B significantly higher than that of the first biogas.

[0033] The voltage applied between the anode 52 and the cathode 53 can be a low voltage (e.g., 1.2V or less) sufficient to send electrons from the anode 52 to the cathode 53. This is significantly lower than the voltage used in electrolysis for hydrogen production.

[0034] The second biogas is sent to the biogas branching device 62 via the second biogas piping 59 and a methane meter 67. The methane meter 67 measures the methane concentration of the second biogas. If the methane concentration is above a predetermined level, it is sent to the biogas recovery piping 61 by the biogas branching device 62 and recovered for use as automotive fuel. On the other hand, if the methane concentration is below the predetermined level, the second biogas is supplied directly to the second methane fermentation tank 51B via the second biogas piping 59, where it undergoes the same methane fermentation process as the digestate in the tank. As the second biogas is circulated and processed in the second methane fermentation tank 51B, an optimal fermentation state is gradually created, and biogas with the desired high methane concentration can be recovered.

[0035] The digestate containing the fermentation residue after methane fermentation in the second methane fermentation tank 51B is discharged from the second methane fermentation tank 51B via the second digestate pipe 57 and can be used for purposes such as organic fertilizer. In this way, the liquid to be treated 5, which contains livestock manure, is supplied from the supply pipe 41 to the first methane fermentation tank 51A, and biogas containing a high concentration of methane can be obtained. The liquid to be treated 5 is supplied intermittently with time intervals required for the acid fermentation process and the methane fermentation process in the first methane fermentation tank 51A and the second methane fermentation tank 51B, and the digestate and biogas are similarly discharged intermittently from the first methane fermentation tank 51A and the second methane fermentation tank 51B.

[0036] In addition, the first digestate is supplied from the first methane fermentation tank 51A to the second methane fermentation tank 51B via the first digestate pipe 56. However, to supply the first digestate more quickly, a pump may be provided to pressurize the first digestate. In this case, as shown in Figure 3, for example, the first methane fermentation tank 51A and the second methane fermentation tank 51 may be connected by another first digestate pipe 56a, and a pump 64 may be placed in this other first digestate pipe 56a. By providing two first digestate pipes 56 and 56a between the first methane fermentation tank 51A and the second methane fermentation tank 51B in this way, and providing a pump 64 in one of them, even if the pump 64 obstructs the flow of electricity between the first methane fermentation tank 51A and the second methane fermentation tank 51B, the other pipe can bypass the flow of current.

[0037] Granular activated carbon 55 may be provided in the tanks of the first methane fermentation tank 51A and the second methane fermentation tank 51B. By dispersing the granular activated carbon 55 in the liquids to be treated 5 and 5a, electrons generated when bacteria decompose organic matter into organic acids during the acid fermentation process in the tanks can be transferred to the methane production reaction by hydrogen-utilizing archaea via the granular activated carbon 55. This direct interspecies electron transfer can accelerate the methane production reaction by hydrogen-utilizing archaea, which is the rate-limiting reaction described above. The granular activated carbon 55 may be provided in either the first methane fermentation tank 51A or the second methane fermentation tank 51B. It should be noted that as long as such direct interspecies electron transfer occurs, the material is not limited to granular activated carbon 55, and conductive granular materials such as biochar (charcoal) can be used.

[0038] Furthermore, although not shown in the diagram, hydrogen-containing gas, or gas containing hydrogen and carbon dioxide, may be supplied to the first methane fermentation tank 51A and the second methane fermentation tank 51B. As mentioned above, electrons are necessary to produce methane from hydrogen and carbon dioxide by hydrogen-capsulating archaea. In the first methane fermentation tank 51A, the acid fermentation process proceeds, and electrons are generated when organic matter is decomposed into organic acids. These electrons can be effectively utilized to produce methane from the supplied hydrogen. By supplying hydrogen to the first methane fermentation tank 51A in this way, methane is produced by reaction with the carbon dioxide generated in the first methane fermentation tank 51A, or with the carbon dioxide supplied simultaneously, thereby increasing the methane concentration of the resulting first biogas. Similarly, supplying hydrogen to the second methane fermentation tank 51B can also increase the methane concentration of the resulting second biogas.

[0039] While the embodiment described above involves supplying the liquid to be treated 5, which includes livestock manure such as cow dung, to the first methane fermentation tank 51A, the present invention is not limited thereto. Alternatively, an acid fermentation tank or methane fermentation tank (not shown) may be provided separately in the pre-processing step before the first methane fermentation tank 51A. The liquid to be treated 5 may be supplied to this tank to carry out solubilization, hydrolysis, acid fermentation, and part of the methane fermentation process. A portion of the digestate and biogas may then be extracted from this separate acid fermentation tank or methane fermentation tank and supplied to the first methane fermentation tank 51A via the supply pipe 41 and the biogas supply pipe 42. With this configuration, the acid fermentation process of the digestate extracted from the separate acid fermentation tank or methane fermentation tank will proceed in the first methane fermentation tank 51A. As a result, electron transfer by voltage application occurs in the same manner as described above, and the methane concentration of the second biogas produced in the second methane fermentation tank 51B can be increased.

[0040] Furthermore, if a separate acid fermentation tank or methane fermentation tank is provided as described above, a portion of the digestate and biogas may be extracted from this separate acid fermentation tank or methane fermentation tank and supplied to the second methane fermentation tank 51B via the supply pipe 41a and the biogas supply pipe 42a. In this case, the first methane fermentation tank 51A may not be supplied with the liquid to be treated or the digestate, but rather with an electrolytic liquid such as water so that the anode 52 is immersed. Even with this configuration, electron transfer occurs due to the application of voltage, so the methane concentration of the second biogas produced in the second methane fermentation tank 51B can be increased. Furthermore, when supplying an electrolytic liquid such as water to the first methane fermenter 51A, as shown in Figure 3, by arranging a first digestate pipe 56 and a first digestate pipe 56a equipped with a pump 64, and not starting the pump 64, the supply of an electrolytic liquid such as water from the first methane fermenter 51A to the second methane fermenter 51B can be minimized, preventing dilution of the digestate concentration in the second methane fermenter 51B and maintaining high reaction efficiency. In addition, if supply pipe 41a and biogas supply pipe 42a are provided in Figure 2, the first digestate pipe 56 does not need to be provided if electrons are supplied from the anode 52 to the cathode 53 by applying a voltage, such as by grounding both the first methane fermenter 51A and the second methane fermenter 51B.

[0041] When an acid fermentation tank or a methane fermentation tank is provided in the preceding process to the first methane fermentation tank 51A or the second methane fermentation tank 51B, it is preferable to use a large-capacity tank for the acid fermentation tank or methane fermentation tank, and a small-capacity tank for the first methane fermentation tank 51A and the second methane fermentation tank 51B. This allows for efficient voltage application in the first methane fermentation tank 51A and the second methane fermentation tank 51B, making it easier to obtain a second biogas with a high methane concentration, and also increasing the amount of livestock manure that can be processed.

[0042] Furthermore, while embodiments for methane fermentation of livestock manure such as cow dung have been described, the present invention is not limited thereto. In addition to livestock manure, it can also be applied to organic waste such as food scraps and sewage sludge. Moreover, methane fermentation can be similarly carried out on any organic matter, such as resource crops, pre-treated organic matter, and wastewater such as sewage. In such cases, for organic matter that does not contain acid-fermenting bacteria or methane-producing bacteria, it is necessary to add acid-fermenting bacteria or methane-producing bacteria to the material to be treated. [Examples]

[0043] The following describes test examples relating to the present invention, but the present invention is not limited to these.

[0044] Methane fermentation tests were conducted in three test groups: an anode group, a cathode group, and a control group. In all groups, 70 mL of digestate collected from a methane fermentation tank owned by a livestock pig farm was used as seed sludge (final concentration 5 g-VS / L), 70 mL of inorganic salt medium was added to a 250 mL screw-cap bottle, 2 mL of Na2S·9H2O was added as a reducing agent, and 33 mL of phosphate buffer was added as a buffer, for a total volume of 175 mL. 10 g of pig manure (final concentration 2.5 g-VS / g) was used as the substrate. In the anode group, only the anode was introduced into the screw-cap bottle, and in the cathode group, only the cathode was introduced into the screw-cap bottle. Activated carbon fiber felt (length 40 mm × width 10 mm × thickness 2 mm), which has a large surface area, was used for both the anode and cathode electrodes. In the anode and cathode test sections, a DC voltage of 0.7V was continuously applied between the anode and cathode for approximately 30 days under fermentation temperature of 37°C while stirring with a magnetic stirrer. No electrodes were introduced in the control test section.

[0045] As a comparative study of these three methods, methane production and methane concentration were measured over time in each test group. In addition, microbial community structure analysis was performed on the electrode surfaces of the anode and cathode test groups, as well as on the digestate of the control group, using next-generation sequencing (NGS) and quantitative PCR (qPCR).

[0046] Figure 4 shows the measurement results of methane production in each test plot. From day 7 onwards, the methane production in the cathode plot increased significantly more than in the other plots. The anode plot produced more methane than the control plot until day 17, but this reversed after day 17, and the methane production in the anode plot did not increase thereafter. Similarly, the methane production in the cathode plot did not increase after day 17. On the other hand, the methane production in the control plot continued to increase almost constantly until day 30. These results indicate that the reaction efficiency of methane production was greatly improved by applying voltage.

[0047] Furthermore, while the methane concentration of the biogas obtained in the control plot was 64%, the methane concentration of the biogas obtained in the cathode plot was 80%, which was significantly higher than the control plot. On the other hand, the methane concentration of the biogas obtained in the anode plot was 50%, which was lower than the control plot.

[0048] Microbial community structure analysis revealed that the estimated number of archaeal cells in each experimental plot showed a significant increase in the estimated number of hydrogen-assimilating methanogenic bacteria, Methanoculleus and Methanobacterium, compared to the control plot in both the cathode and anode plots, as shown in Figure 5. Furthermore, when comparing the relative abundance of all archaeal organisms, these hydrogen-assimilating methanogenic bacteria accounted for approximately 70% in the cathode plot, which was higher than the approximately 60% in the control plot. On the other hand, in the anode plot, while the relative abundance of these hydrogen-assimilating methanogenic bacteria was low at less than 40%, the relative abundance of Methanosarcina, which assimilates acetic acid and hydrogen, was approximately 50% in the anode plot, which was higher than the approximately 20% in the control and cathode plots, indicating a significant increase.

[0049] The reason for the increased methane production in the cathode test area is thought to be that electrons were supplied from the cathode to hydrogen-assimilating methane-producing bacteria, increasing the proportion of Methanoculleus and Methanobacterium, and thus producing more methane. On the other hand, when the estimated number of bacterial cells in each test area was examined, the number of Clostridium sensu stricto cells, which decompose carbohydrates and amino acids to produce organic acids, was approximately twice as high in the anode test area compared to the cathode test area. This is thought to be the reason why less methane was produced in the anode test area. Although Methanosarcina was abundant in the anode test area, the amount of organic acids produced by the aforementioned bacteria was greater than the amount of organic acids consumed by Methanosarcina, which is thought to have caused fermentation inhibition during the process.

[0050] These test results revealed that Methanobacterium proliferated in the cathode test plot, promoting biomethanation and efficiently synthesizing methane from hydrogen and carbon dioxide in the biogas. On the other hand, it is presumed that fermentation was inhibited in the anode test plot due to the accumulation of organic acids, and methane fermentation did not proceed sufficiently. [Explanation of Symbols]

[0051] 1. Livestock manure 2 water 6. Biogas 7 Digestive juices 8. High concentration of methane gas 9. Automobile 10 Livestock barns 20 tanks 30 Solubilization / hydrolysis process 40. Acid fermentation process 41, 41a Supply piping 42, 42a Biogas supply piping 50 Methane fermentation process 51A First Methane Fermentation Tank 51B Second Methane Fermentation Tank 52, 72 anodes 53, 73 Cathode 54, 74 Power supply 55 Granular activated carbon 56 1st digestive fluid piping 57 Second digestive fluid piping 58, 58a First biogas piping 59 Second Biogas Piping System 60 Purification process 61 Biogas recovery piping 62 Biogas branching device 63, 64, 65 Pumps 66, 68, 69 Gas pumps 67 Methane measuring instrument 70 Screw-top bottles

Claims

1. A methane fermentation system comprising: a first tank having an anode; a second tank for methane fermentation having a cathode; a power supply for applying a voltage between the anode and the cathode; digestate piping for sending digestate to the second tank; first biogas piping for sending biogas to the second tank; and second biogas piping for recovering high-concentration biogas produced in the second tank, which contains methane at a higher concentration than the first biogas.

2. The methane fermentation system according to claim 1, wherein the first tank is for acid fermentation or methane fermentation, the digestate piping is for sending the digestate produced in the first tank to the second tank, and the first biogas piping is for sending the biogas produced in the first tank to the second tank.

3. The methane fermentation system according to claim 1, wherein the anode and the cathode are made of carbon fiber.

4. The methane fermentation system according to claim 1 or 2, wherein at least one of the first tank and the second tank comprises a plurality of granular activated carbons.

5. A methane fermentation system comprising a first tank having an anode and a second tank for methane fermentation having a cathode, comprising the steps of supplying digestate and biogas to the second tank, A step of applying a voltage between the anode and the cathode, The process involves performing methane fermentation in the second tank while the aforementioned voltage is applied, thereby producing high-concentration biogas containing methane at a higher concentration than the aforementioned biogas. A methane fermentation method that includes this.

6. A step of supplying organic matter to the first tank, The process involves applying the aforementioned voltage and then, in the first tank, causing at least acid fermentation of the organic matter to produce the digestate and the biogas. The methane fermentation method according to claim 5, further comprising:

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