Methane fermentation treatment method, and methane fermentation treatment facility

Abstract

To easily adjust a temperature of an organic waste supplied to a methane fermentation tank to be within an optimal temperature range of methane fermentation treatment while achieving simplification of a configuration and energy saving.SOLUTION: A methane fermentation treatment method includes: a heating step of heating a first organic waste C1, and a second organic waste C2 more hardly decomposable than the first organic waste C1; a cooling step of cooling the second organic waste C2; a supplying step of supplying the first organic waste C1 not through the heating step to a methane fermentation tank 32, and supplying the second organic waste C2 which has been heated in the heating step to the methane fermentation tank 32; and a cooling step of cooling a methane fermentation liquid containing the organic waste supplied to the methane fermentation tank 32.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a methane fermentation treatment method and a methane fermentation treatment facility for producing biogas by subjecting organic waste to methane fermentation treatment. [Background technology]

[0002] For example, methane fermentation (anaerobic fermentation) is widely used to reduce the volume of organic waste such as sludge and biomass generated during wastewater treatment and to convert it into energy. Methane fermentation is a technology in which organic waste is stored under anaerobic conditions for a certain period of time, and the organic waste is decomposed by anaerobic microorganisms to produce biogas such as methane gas and carbon dioxide. This technology is widely used in wastewater treatment facilities and wastewater treatment facilities in Japan.

[0003] A methane fermentation facility that performs methane fermentation is primarily equipped with a methane fermentation tank in which organic waste is stored and methane fermented, an agitator that agitates the methane fermentation liquid in the methane fermentation tank, and a heating device that heats the methane fermentation liquid.

[0004] In order to increase the efficiency of biogas recovery, a method is used in which organic waste for methane fermentation is heated and solubilized before undergoing methane fermentation treatment. By heating the organic waste for methane fermentation, the difficult-to-decompose substances in the organic waste are solubilized, and the amount of biogas generated increases. However, if the organic waste that has been heated to a high temperature during the solubilization treatment is directly supplied to a methane fermentation tank, the temperature of the organic waste may be too high, which may have a negative effect on the microorganisms that perform methane fermentation.

[0005] Therefore, a system has been known in the past that includes a circulation line for circulating organic waste from a methane fermentation tank and a temperature control means for heating and cooling the organic waste circulated in the circulation line (see, for example, Patent Document 1).

[0006] In some systems, organic waste heated for thermal solubilization is supplied to a methane fermentation tank without being cooled (see, for example, Patent Document 2). In Patent Document 2, organic waste is separated into excess sludge after aerobic treatment and primary settling sludge, the excess sludge is heated for thermal solubilization, and the heated excess sludge is mixed with the unheated primary settling sludge and supplied to a methane fermentation tank. The excess sludge and primary settling sludge are not simply mixed, but the flow rate of the excess sludge to be heated is adjusted so that when the heated excess sludge and the unheated primary settling sludge are mixed, the temperature range is within the optimum temperature range for methane fermentation treatment. This eliminates the need for new energy for heating in the methane fermentation treatment, thereby enabling energy savings. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2006-150252 A [Patent Document 2] JP 2012-192351 A Summary of the Invention [Problem to be solved by the invention]

[0008] Patent Document 1 is based on the premise that the organic waste is cooled after heating in order to effect thermal solubilization, whereas Patent Document 2 is based on the premise that the organic waste is not cooled after heating in order to effect thermal solubilization. Therefore, it is difficult to combine Patent Document 1 and Patent Document 2, which have different premise configurations.

[0009] In Patent Document 2, although new energy for heating is not required in the methane fermentation treatment, when the heated excess sludge is mixed with the unheated primary settling sludge, it is necessary to adjust the flow rate of the heated excess sludge so that the temperature range is within the appropriate temperature range for the methane fermentation treatment. However, adjusting the flow rate of the excess sludge is itself difficult, and a complex configuration must be added for that purpose, which leads to a complication of the configuration and an increase in cost. Moreover, since the temperature of the mixed sludge changes depending on the mixing conditions such as the degree of mixing of the excess sludge and the primary settling sludge and the temperature change of the primary settling sludge due to seasonal fluctuations, it becomes difficult to adjust the temperature of the sludge supplied to the methane fermentation tank without performing temperature adjustment such as cooling. Therefore, depending on the mixing conditions and the temperature change of the primary settling sludge, the temperature of the sludge supplied to the methane fermentation tank may not be sufficiently lowered, which may adversely affect the microorganisms performing methane fermentation, and the methane fermentation treatment may not be performed properly.

[0010] In contrast, in Patent Document 1, the organic waste circulated in the circulation line can be heated and cooled, so that the organic waste can be thermally solubilized by first circulating the organic waste in the circulation line and heating the organic waste. Next, the heated organic waste can be circulated in the circulation line and cooled in the circulation line after heating, so that the temperature can be sufficiently lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation. Therefore, in Patent Document 1, it is easier to adjust the temperature of the organic waste supplied to the methane fermentation tank to within an appropriate temperature range than in Patent Document 2.

[0011] However, in Patent Document 1, when the organic waste is heated and cooled in the circulation line, the entire amount of the organic waste is the object to be heated and cooled, which increases the volume, leading to an increase in the size of the equipment and requiring a large amount of energy.

[0012] In view of this situation, the main object of the present invention is to provide a methane fermentation treatment method and methane fermentation treatment equipment that can easily adjust the temperature of organic waste supplied to a methane fermentation tank to within the optimum temperature range for methane fermentation treatment while simplifying the configuration and saving energy. [Means for solving the problem]

[0013] A first characteristic configuration of the present invention is a methane fermentation treatment method for producing biogas by subjecting organic waste to methane fermentation treatment in a methane fermentation tank, comprising: The organic waste includes a first organic waste and a second organic waste that is more difficult to decompose than the first organic waste, a heating step of heating the second organic waste; A supply step of supplying the first organic waste that has not been subjected to the heating step to the methane fermentation tank, and supplying the second organic waste that has been heated in the heating step to the methane fermentation tank; and a cooling step of cooling the methane fermentation liquid containing the organic waste that has been supplied to the methane fermentation tank.

[0014] According to this configuration, the second organic waste, which is more difficult to decompose than the first organic waste, is heated in the heating step and then supplied to the methane fermentation tank. In contrast, the first organic waste is supplied to the methane fermentation tank without going through the heating step. As a result, the object to be heated in the heating step is only the second organic waste, excluding the first organic waste, and the volume of the object to be heated can be reduced, thereby making it possible to miniaturize the device and save energy.

[0015] Since the first organic waste and the second organic waste are supplied to the methane fermentation tank, the mixture (organic waste) of the first organic waste, the second organic waste, and the methane fermentation liquid is cooled in the cooling process. This allows the temperature of the mixture to be sufficiently lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation, and can be easily adjusted to within the appropriate temperature range for methane fermentation treatment. Furthermore, when the first organic waste, the second organic waste, and the methane fermentation liquid are mixed in the methane fermentation tank, a drop in temperature is expected due to the mixing of the first organic waste that has not been subjected to the heating process and the methane fermentation liquid, so even if cooling is performed in the cooling process, the amount of energy required in the cooling process can be reduced, resulting in energy conservation.

[0016] A second characteristic configuration of the present invention is that a first supply path for supplying the first organic waste to the methane fermentation tank and a second supply path for supplying the second organic waste to the methane fermentation tank are separately provided.

[0017] According to this configuration, the first supply path and the second supply path are provided separately. Therefore, even if, for example, the first location where the first organic waste is obtained and the second location where the second organic waste is obtained are separate locations, it is only necessary to arrange the first supply path from the first location to the methane fermentation tank and the second supply path from the second location to the methane fermentation tank. This allows each supply path to be arranged along an optimal route with an optimal path length, thereby simplifying the configuration.

[0018] A third characteristic configuration of the present invention is that the supply process includes a mixing process in which the first organic waste that has not been subjected to the heating process is mixed with the second organic waste that has been heated in the heating process, and a mixture supply process in which the mixture of the first organic waste and the second organic waste mixed in the mixing process is supplied to the methane fermentation tank.

[0019] According to this configuration, in the mixing step, the first organic waste that has not been subjected to the heating step is mixed with the second organic waste that has been heated in the heating step, so that the temperature of the mixture can be lowered by mixing the first organic waste. In the mixture supply step, the mixture of the first organic waste and the second organic waste is supplied to the methane fermentation tank, so that the supply path for supplying the mixture to the methane fermentation tank can be a common supply path for the first organic waste and the second organic waste, and the configuration can be simplified. Moreover, since the first organic waste and the second organic waste can be mixed in advance before being supplied to the methane fermentation tank, the first organic waste and the second organic waste can be sufficiently mixed, and the effect of lowering the temperature by mixing the first organic waste can be easily obtained.

[0020] A fourth characteristic configuration of the present invention is a method for producing a wastewater treatment system comprising the steps of: mixing the first organic waste that has not been subjected to the heating step with the second organic waste that has been heated in the heating step in a mixing tank; In the mixture supplying step, a mixture of the first organic waste and the second organic waste is supplied from the mixing tank to the methane fermentation tank.

[0021] According to this configuration, in the mixing process, the first organic waste is mixed in the mixing tank with the second organic waste that has been heated in the heating process. Therefore, the first organic waste and the second organic waste can be thoroughly mixed using the mixing tank, and the effect of lowering the temperature due to the mixing of the first organic waste can be fully obtained.

[0022] In order to supply the first organic waste that has not been subjected to the heating process and the second organic waste that has been heated in the heating process to the methane fermentation tank, a transport device for transporting the organic waste is required. According to this configuration, the mixing process can be handled by providing a transport device having a transport capacity sufficient to supply the first organic waste to the mixing tank, and a transport device having a transport capacity sufficient to supply the second organic waste from the heating device of the heating process to the mixing tank. The mixture supply process can be handled by providing a transport device having a transport capacity sufficient to supply the mixture from the mixing tank to the methane fermentation tank. As a result, although three transport devices are required, a large transport capacity is not required for each of the three transport devices, and a transport device with a small transport capacity can be used, which simplifies the configuration and reduces costs.

[0023] A fifth characteristic configuration of the present invention is that when the cooling requirement in the cooling process is excessive, that is, when the cooling requirement is in an excessive cooling requirement state, a heating amount adjustment process is performed to adjust the heating amount of the second organic waste in the heating process to a reduced amount.

[0024] By cooling the methane fermentation liquid containing organic waste supplied to the methane fermentation tank in the cooling process, the temperature of the organic waste is sufficiently lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation, and is adjusted to within the temperature range suitable for methane fermentation processing.

[0025] However, depending on the temperature of the organic waste, for example, if the temperature of the organic waste supplied to the methane fermentation tank is too high, the cooling requirement in the cooling process may become excessive. In this excessive cooling requirement state, an excessive load is placed on the cooling device in the cooling process, and in some cases, it may not be possible to sufficiently lower the temperature of the methane fermentation liquid containing the organic waste supplied to the methane fermentation tank.

[0026] Therefore, according to this configuration, when the cooling requirement is excessive, the temperature of the organic waste supplied to the methane fermentation tank can be lowered by performing a heat amount adjustment process in which the amount of heat applied to the second organic waste in the heating process is adjusted to a lower amount. This reduces the burden on the cooling device and the like in the cooling process, and allows the temperature of the methane fermentation liquid containing the organic waste supplied to the methane fermentation tank to be sufficiently and appropriately lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation.

[0027] A sixth characteristic configuration of the present invention is a methane fermentation treatment facility for producing biogas by subjecting organic waste to methane fermentation treatment in a methane fermentation tank, The organic waste includes a first organic waste and a second organic waste that is more difficult to decompose than the first organic waste, a heating unit that heats the second organic waste; A supply unit that supplies the first organic waste that has not passed through the heating unit to the methane fermentation tank, and supplies the second organic waste that has been heated in the heating unit to the methane fermentation tank; and a cooling section for cooling the methane fermentation liquid containing organic waste that is supplied to the methane fermentation tank.

[0028] According to this configuration, as in the first characteristic configuration, the object to be heated in the heating section is only the second organic waste, excluding the first organic waste, and the volume of the object to be heated can be reduced, so that the device can be made smaller and energy-efficient. Since the mixture (organic waste) of the first organic waste, the second organic waste, and the methane fermentation liquid is cooled in the cooling section, the temperature of the mixture can be sufficiently lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation, and can be easily adjusted to within the appropriate temperature range for methane fermentation treatment. Moreover, since a decrease in temperature is expected due to the mixing of the first organic waste that has not passed through the heating section and the methane fermentation liquid, even if cooling is performed in the cooling section, the amount of energy required in the cooling section can be reduced, and energy can be saved. [Brief description of the drawings]

[0029] [Figure 1] FIG. 1 is a diagram showing an overall schematic configuration of a wastewater treatment facility according to a first embodiment. [Diagram 2] FIG. 1 shows a sludge treatment system according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wastewater treatment facility to which the methane fermentation treatment method and the methane fermentation treatment equipment according to the present invention are applied will be described with reference to the drawings.

[0031] [First embodiment] This wastewater treatment facility 1 is a facility that treats organic wastewater such as sewage and wastewater that contain organic matter. As shown in Fig. 1, the wastewater treatment facility 1 is equipped with a wastewater treatment system (sewage treatment system) 2, a sludge treatment system 3 (corresponding to a methane fermentation treatment facility) that subjects organic waste to methane fermentation treatment (anaerobic digestion treatment), and a biogas utilization system 4.

[0032] The wastewater treatment system 2 is equipped with a primary sedimentation tank 21, a biological treatment tank 22, and a final sedimentation tank 23 in the order of the flow direction of raw organic wastewater water A1. The raw water A1 first flows into the primary sedimentation tank 21, where suspended matter and solids that tend to settle are settled and removed. Next, in the biological treatment tank 22, organic matter and other contaminants in the raw water A1 are decomposed and removed by the action of microorganisms. Finally, activated sludge is settled in the final sedimentation tank 23, and the clean supernatant water is discharged as treated water A2.

[0033] The raw sludge that has been settled and removed in the primary settling tank 21 is sent as primary settling sludge B1 to a first thickener 24 (e.g., a mechanical thickener such as a screw thickener, centrifugal thickener or flotation thickener, or a gravity thickener such as a gravity thickener tank) and is thickened, for example, to a solid concentration of about 2 to 10%. A portion of the activated sludge that has been settled and removed in the final settling tank 23 is returned to the biological treatment tank 22 as returned sludge B2, and the remaining excess sludge B3 is sent to a second thickener 25 (e.g., a mechanical thickener such as a screw thickener, centrifugal thickener or flotation thickener) and is thickened, for example, to a solid concentration of about 2 to 10%.

[0034] The sludge treatment system 3 receives the organic wastes C1 and C2 from the wastewater treatment system 2 and produces methane gas by subjecting the organic wastes C1 and C2 to methane fermentation. The organic wastes include a first organic waste C1 and a second organic waste C2 that is more difficult to decompose than the first organic waste C1. The sludge treatment system 3 receives the primary sludge B1 from the first thickener 24 as the first organic waste C1, and receives excess sludge B3 from the second thickener 25 as the second organic waste C2. In this way, the organic waste from the wastewater treatment system 2 is supplied to the sludge treatment system 3 in a state where it is separated into the first organic waste C1 and the second organic waste C2. The volume ratio of the first organic waste C1 to the second organic waste C2 is not particularly limited and may be set appropriately, but it is preferable to set it to, for example, C1:C2=1.5:1, so that there is more first organic waste C1 than second organic waste C2.

[0035] The sludge treatment system 3 is equipped with a heating section 31, a methane fermentation tank 32, a supply section 35 that supplies the first organic waste C1 and the second organic waste C2 to the methane fermentation tank 32, and a cooling section 34 that cools the methane fermentation liquid C4 containing the organic waste that has been supplied to the methane fermentation tank 32. The supply section 35 supplies the first organic waste C1 and the second organic waste C2 to the methane fermentation tank 32 using a first supply path 36 that supplies the first organic waste C1 and a second supply path 37 that supplies the second organic waste C2.

[0036] The heating section 31 is disposed at the most upstream side of the second supply path 37, and heats the second organic waste C2 supplied from the second concentrating device 25. In this way, the heating target of the heating section 31 is only the second organic waste C2, excluding the first organic waste C1, so that the heating capacity can be reduced, making the device more compact, and the amount of energy required can be reduced, making it possible to conserve energy.

[0037] In the heating section 31, the second organic waste C2, which is more difficult to decompose than the first organic waste C1, is heated to thermally solubilize the second organic waste C2. To this end, in the heating section 31, the second organic waste C2 is heated to a target temperature for thermal solubilization (for example, 70°C).

[0038] A thermostatic bath 38 is disposed downstream of the heating unit 31 in the second supply path 37, in which the second organic waste C2 is retained while maintaining its temperature. The second organic waste C2 heated to the thermal solubilization target temperature by the heating unit 31 is retained in the thermostatic bath 38 for a thermal solubilization time (e.g., 30 minutes). This maintains the temperature of the second organic waste C2 at the thermal solubilization target temperature for the thermal solubilization time, thereby ensuring proper thermal solubilization of the second organic waste C2.

[0039] The target temperature for thermal solubilization may be set to a value between 65°C and 75°C, and may be set within a range, not set to a single temperature, for example, between 70°C and 75°C. Preferably, it is 70°C. Similarly, the time for thermal solubilization may be set to a value between 20 minutes and 40 minutes, and may be set within a range, not set to a single time, for example, between 30 minutes and 40 minutes. Preferably, it is 30 minutes.

[0040] The supply section 35 is equipped with a mixing section 51 that mixes the first organic waste C1 and the second organic waste C2, and a mixture supply section 53 that supplies a mixture C3 of the first organic waste C1 and the second organic waste C2 mixed in the mixing section 51 to the methane fermentation tank 32.

[0041] The mixing section 51 has a mixing tank 52, in which the first organic waste C1 that has not passed through the heating section 31 is mixed with the second organic waste C2 that has been heated in the heating section 31. The mixing section 51 connects the first supply path 36 and the second supply path 37 to the mixing tank 52, and supplies the first organic waste C1 of the first supply path 36 and the second organic waste C2 of the second supply path 37 to the mixing tank 52, thereby mixing the first organic waste C1 and the second organic waste C2 in the mixing tank 52.

[0042] The mixture supply unit 53 serves as a supply path for supplying the mixture C3 from the mixing tank 52 to the methane fermentation tank 32. This mixture supply unit 53 serves as a common supply path that serves both the first supply path 36 and the second supply path 37, and this common supply path simplifies the configuration.

[0043] The methane fermentation tank 32 receives the mixture C3 (a mixture of the first organic waste C1 and the second organic waste C2) supplied by the mixture supply unit 53, and performs methane fermentation treatment (anaerobic digestion treatment) by the action of anaerobic bacteria such as acid-producing bacteria and methanogens. Although not shown, the methane fermentation tank 32 is equipped with an agitator that agitates the methane fermentation liquid containing the mixture C3 in the methane fermentation tank 32. A heat insulating material or the like can be provided on the outer wall portion of the methane fermentation tank 32 to prevent a drop in temperature of the mixture C3 and the like in the methane fermentation tank 32.

[0044] The cooling unit 34 cools the methane fermentation liquid C4 containing organic waste (a mixture C3 of the first organic waste C1 and the second organic waste C2) in the methane fermentation tank 32. The methane fermentation tank 32 is provided with a circulation path 33 that circulates the methane fermentation liquid C4 in the methane fermentation tank 32, and the cooling unit 34 cools the methane fermentation liquid C4 circulated in the circulation path 33. In this way, when cooling the methane fermentation liquid C4 in the methane fermentation tank 32, the methane fermentation liquid C4 is removed from the methane fermentation tank 32 through the circulation path 33, the removed methane fermentation liquid C4 is cooled in the cooling unit 34, and the cooled methane fermentation liquid C4 is returned to the methane fermentation tank 32 through the circulation path 33.

[0045] The digested sludge generated by the methane fermentation process in the methane fermentation tank 32 is dehydrated by a dehydrator 39 (e.g., a screw dehydrator or a belt press dehydrator). The dehydrated sludge is transported by a transport vehicle such as a truck to a treatment plant outside the wastewater treatment facility 1. Meanwhile, the dehydrated separated liquid separated from the digested sludge during the dehydration process is returned to the inlet side of the primary sedimentation tank 21 as return water A3.

[0046] The biogas utilization system 4 utilizes the biogas produced by the methane fermentation process in the sludge treatment system 3, and is equipped with a desulfurization tower 41, a gas holder 42, a gas engine 43, and a generator 44.

[0047] The biogas from the methane fermentation tank 32 is stored in a gas holder 42 after hydrogen sulfide is removed in a desulfurization tower 41. The biogas from the gas holder 42 is supplied as fuel to a gas engine 43 for driving a generator 44. The electricity generated by the generator 44 may be used within the wastewater treatment facility 1, or may be sold to an electric power company.

[0048] The operation of the sludge treatment system 3 will be described below, and the methane fermentation treatment method according to the present invention will also be described.

[0049] In this wastewater treatment facility 1, organic waste from a wastewater treatment system 2 is supplied to a sludge treatment system 3 in a state where it is separated into a first organic waste C1 and a second organic waste C2. In the sludge treatment system 3, a heating step is performed in which the second organic waste C2, which is more difficult to decompose than the first organic waste C1, is heated in a heating section 31, thereby thermally solubilizing the second organic waste C2. In contrast, the first organic waste C1 does not pass through the heating section 31 and does not undergo the heating step in the heating section 31.

[0050] The heating section 31 is provided with a heat exchanger (not shown) to which a heating heat medium D1 such as hot water is circulated and supplied from a heat source 61. The second organic waste C2 is heated by heat exchange between the heating heat medium D1 and the second organic waste C2 in the heat exchanger. The heating section 31 is not limited to one equipped with a heat exchanger, and various heating devices can be applied as long as they are capable of heating the second organic waste C2.

[0051] The hot heat source 61 is not particularly limited, but may recover exhaust heat, etc., generated when driving the gas engine 43 or the generator 44 in the wastewater treatment facility 1, and generate the heating heat medium D1 using the recovered exhaust heat, etc. The hot heat source 61 heats the second organic waste C2 to a target temperature for thermal solubilization (e.g., 70°C) in the heating section 31 by adjusting the temperature and flow rate of the heating heat medium D1 circulated and supplied to the heating section 31.

[0052] The target temperature for heating the second organic waste C2 in the heating unit 31 is set to a thermal solubilization target temperature (e.g., 70°C). Since the exhaust heat etc. recovered from the gas engine 43 and the generator 44 is, for example, about 80°C, it is possible to effectively utilize the exhaust heat etc. while generating the heating heat medium D1 for heating the second organic waste C2 to the thermal solubilization target temperature (e.g., 70°C), and furthermore, it is possible to reduce energy consumption and achieve energy conservation without excessively heating the second organic waste C2.

[0053] The second organic waste C2 that has been heated to a target temperature for thermal solubilization (eg, 70° C.) by the heating unit 31 is retained in the thermostatic bath 38 for a thermal solubilization time (eg, 30 minutes).

[0054] The supply unit 35 performs a supply step of supplying the first organic waste C1 and the second organic waste C2 to the methane fermentation tank 32. In this supply step, the first organic waste C1 and the second organic waste C2 are not simply supplied to the methane fermentation tank 32, but a mixing step and a mixture supplying step are performed.

[0055] In the mixing step, the mixing section 51 mixes the first organic waste C1 that has not passed through the heating section 31 in the heating step with the second organic waste C2 that has been heated in the heating section 31 in the heating step in the mixing tank 52. This makes it possible to lower the temperature of the mixture C3 by mixing the first organic waste C1. In the mixture supplying step, the mixture supplying section 53 supplies the mixture C3 of the first organic waste C1 and the second organic waste C2 that have been mixed in the mixing step from the mixing tank 52 to the methane fermentation tank 32.

[0056] In the cooling section 34, a cooling step is performed to cool the methane fermentation liquid C4 in the methane fermentation tank 32. In the cooling step, the methane fermentation liquid C4 in the methane fermentation tank 32 is circulated through the circulation path 33, and the methane fermentation liquid C4 in the circulation path 33 is cooled in the cooling section 34. The cooling section 34 is provided with a heat exchanger (not shown) to which a cooling heat medium D2 such as cold water is circulated and supplied from a cold heat source 62. The methane fermentation liquid C4 is cooled by exchanging heat between the cooling heat medium D2 and the methane fermentation liquid C4 in the heat exchanger. As with the heating section 31, the cooling section 34 is not limited to one equipped with a heat exchanger, and various cooling devices can be applied as long as they can cool the methane fermentation liquid C4.

[0057] In the methane fermentation tank 32, the methane fermentation liquid C4 cooled in the cooling section 34 is stirred while undergoing methane fermentation treatment (anaerobic digestion treatment) by the action of anaerobic bacteria such as acid-producing bacteria and methanogenic bacteria.

[0058] In order to carry out the methane fermentation process in the methane fermenter 32, it is required to adjust the temperature of the methane fermentation liquid C4 in the methane fermenter 32 within a suitable temperature range for the methane fermentation process (for example, 34°C to 39°C, preferably 37°C). Therefore, as shown by the dotted line in Fig. 1, a cooling amount adjustment unit 71 is provided that performs a cooling amount adjustment step of adjusting the amount of cooling of the methane fermentation liquid C4 in the cooling unit 34 so that the temperature of the methane fermentation liquid C4 in the methane fermenter 32 falls within the suitable temperature range for the methane fermentation process.

[0059] The optimum temperature range for methane fermentation is, for example, 30° C. to 45° C., preferably 35° C. to 40° C., and more preferably 37° C., and is illustrated as an example of a case where methane fermentation is performed at a medium temperature, but is not limited to this temperature range and is merely shown as an example. For example, the optimum temperature range for methane fermentation can be set to 50° C. to 60° C., preferably 54° C. to 57° C., and more preferably 55° C., and methane fermentation can also be performed at a high temperature.

[0060] The temperature of the methane fermentation liquid C4 in the methane fermentation tank 32 can be detected, for example, by providing a temperature sensor in the methane fermentation tank 32. Alternatively, a temperature sensor that detects the temperature of the methane fermentation liquid C4 after passing through the cooling section 34 in the circulation path 33 can be provided, and the temperature detected by the temperature sensor can be used as the temperature of the methane fermentation liquid C4 in the methane fermentation tank 32.

[0061] Regarding the adjustment of the amount of cooling in the cooling section 34, the cooling amount adjustment section 71 adjusts the temperature and flow rate of the cooling heat medium D2 circulated and supplied from the cold heat source 62 to the cooling section 34, thereby adjusting the amount of cooling of the methane fermentation liquid C4 in the cooling section 34.

[0062] In this way, by adjusting the cooling amount in the cooling section 34 using the cooling amount adjustment section 71, the temperature of the methane fermentation liquid C4 in the methane fermentation tank 32 can be sufficiently lowered to a temperature that does not adversely affect the microorganisms performing methane fermentation, and can be adjusted to within the temperature range suitable for methane fermentation processing.

[0063] However, depending on the temperature conditions of the first organic waste C1 and the second organic waste C2, for example, if the temperature of the mixture C3 supplied to the methane fermentation tank 32 is too high, an excessive cooling requirement state may occur in which the amount of cooling required in the cooling section 34 is excessive. In this excessive cooling requirement state, an excessive load is placed on the heat exchanger and the like in the cooling section 34, and in some cases, it may not be possible to sufficiently lower the temperature of the methane fermentation liquid C4.

[0064] 1, in the case of an excessive cooling demand state where the cooling demand in the cooling section 34 in the cooling step is excessive, a heat quantity adjustment section 72 is provided which performs a heat quantity adjustment step of adjusting the heat quantity of the second organic waste C2 in the heating section 31 in the heating step to the lower side. The heat quantity adjustment section 72 adjusts the heat quantity of the second organic waste C2 in the heating section 31 by adjusting the temperature and flow rate of the heating heat medium D1 circulated and supplied from the heat source 61 to the heating section 31.

[0065] The determination of whether or not the cooling requirement is excessive can be made based on the magnitude of the cooling requirement amount in the cooling section 34. For example, the heating amount adjustment section 72 can determine that the cooling requirement is excessive when the cooling requirement amount in the cooling section 34 is equal to or greater than a set amount for an excessive cooling requirement state. Alternatively, the determination can be made based on the temperature of the methane fermentation liquid C4 that has passed through the cooling section 34 in the circulation path 33. In this case, for example, the heating amount adjustment section 72 can determine that the cooling requirement is excessive when the temperature of the methane fermentation liquid C4 is equal to or greater than a reference temperature for an excessive cooling requirement state.

[0066] Specifically, the reference temperature for the excessive cooling requirement state is a value exceeding 45°C (e.g., 46°C) when the methane fermentation process is performed at a medium temperature, or a value exceeding 60°C (e.g., 61°C) when the methane fermentation process is performed at a high temperature, and when the temperature of the methane fermentation liquid C4 becomes equal to or higher than the reference temperature for the excessive cooling requirement state, it is determined that the excessive cooling requirement state exists. When it is determined that the excessive cooling requirement state exists, for example, the set value of the thermal solubilization target temperature is subtracted by 5°C from the set value (if it was set to 70°C, it is set to 65°C) and the second organic waste C2 is heated until the excessive cooling requirement state is released.

[0067] When adjusting the heating amount of the heating section 31 to the reduced side, even if the cooling request is in an excessive cooling request state, the amount of heating to be reduced can be changed according to the degree of the excessive cooling request state. For example, if the cooling request amount in the cooling section 34 is greater than the set amount for the excessive cooling request state by a predetermined amount or more, the excessive cooling request state can be determined to be large, and the amount of reduction in the heating amount can be increased. Also, if the temperature of the methane fermentation liquid C4 that has passed through the cooling section 34 in the circulation path 33 is higher than the reference temperature range for the excessive cooling request state by a predetermined temperature or more, the excessive cooling request state can be determined to be large, and the amount of reduction in the heating amount can be increased.

[0068] Specifically, if the temperature of the methane fermentation liquid C4 is 3°C or more higher than the reference temperature for the excessive cooling requirement state, it is determined that the excessive cooling requirement state is large, and the amount of heating is reduced by a large amount. If it is determined that the excessive cooling requirement state is large, the second organic waste C2 is heated by, for example, a value obtained by subtracting a predetermined value from the set value of the target temperature for thermal solubilization when determining that the excessive cooling requirement state exists (for example, the set value of the target temperature for thermal solubilization is 70°C, and it is determined that the excessive cooling requirement state exists, and 5°C is subtracted to set it to 65°C), and then further subtracting 5°C (in the above example, 65°C is further subtracted to set it to 60°C).

[0069] However, when adjusting the amount of heat in the heating section 31 to the reduced side, if the amount of heat is adjusted without limit, there is a possibility that the second organic waste C2 cannot be thermally solubilized even if it is heated in the heating section 31. Therefore, in the heating section 31, the second organic waste C2 is basically heated to a target temperature for thermal solubilization (e.g., 70°C), but for thermal solubilization, an allowable range for thermal solubilization (e.g., 60°C to 90°C) is set based on the target temperature for thermal solubilization.

[0070] When the cooling request is excessive, the heat amount adjustment unit 72 adjusts the amount of heat of the second organic waste C2 of the heating unit 31 to the reduced side, but the adjustment range of the heat amount is limited so that the temperature of the second organic waste C2 is within the allowable range for thermal solubilization. When the temperature of the second organic waste C2 is outside the allowable range for thermal solubilization, the heat amount adjustment unit 72 does not adjust the amount of heat of the second organic waste C2 of the heating unit 31.

[0071] Three relationships between the magnitude of the excessive cooling requirement in the excessive cooling requirement state, the reduction in the amount of heat of the second organic waste C2 in the heating section 31, and the temperature of the second organic waste C2 heated in the heating section 31 are preset through experiments, theoretical calculations, etc. When the cooling requirement is excessive, the heat amount adjustment section 72 uses the preset relationships to adjust the amount of heat of the second organic waste C2 in the heating section 31 to the reduced side while the temperature of the second organic waste C2 is within the thermal solubilization allowable range.

[0072] When the excessive cooling requirement state occurs, the amount of heat applied to the second organic waste C2 in the heating section 31 is adjusted to a reduced amount, and the temperature of the mixture C3 supplied to the methane fermentation tank 32 drops. This reduces the burden on the cooling section 34, and the excessive cooling requirement state is resolved. When the excessive cooling requirement state is resolved, the heat amount adjustment section 72 stops adjusting the amount of heat to a reduced amount, and the second organic waste C2 can be heated to the target temperature for thermal solubilization (e.g., 70°C).

[0073] As described above, for the purpose of thermal solubilization, an allowable range for thermal solubilization (e.g., 60°C to 90°C) is set based on the target temperature for thermal solubilization. Therefore, assuming that the temperature of the second organic waste C2 heated by the heating section 31 is within the allowable range for thermal solubilization (e.g., 60°C to 90°C), the heat amount adjustment section 72 can not only adjust the heat amount of the heating section 31 to a lower side when the cooling requirement is excessive, but can also adjust the heat amount of the heating section 31 to an increased side depending on various conditions.

[0074] Second Embodiment This second embodiment is another embodiment of the supply unit 35 that supplies the first organic waste C1 and the second organic waste C2 to the methane fermentation tank 32, and supplies the first organic waste C1 and the second organic waste C2 separately to the methane fermentation tank 32. Since other configurations are the same as those of the first embodiment, the same reference numerals are used and the description thereof is omitted, and the description will be centered on the point that the first organic waste C1 and the second organic waste C2 are separately supplied to the methane fermentation tank 32, based on Fig. 2. In Fig. 2, the wastewater treatment system 2 and the biogas utilization system 4 in Fig. 1 are omitted, and only the sludge treatment system 3 is shown.

[0075] 2, a first supply path 36 for supplying the first organic waste C1 to the methane fermentation tank 32 and a second supply path 37 for supplying the second organic waste C2 to the methane fermentation tank 32 are separately provided. The downstream end of the first supply path 36 is connected to the methane fermentation tank 32, and the first organic waste C1 is supplied to the methane fermentation tank 32 by the first supply path 36 without being mixed with the second organic waste C2.

[0076] [Another embodiment] Other embodiments of the present invention will be described below. Note that the configurations of the embodiments described below are not limited to being applied alone, but may also be applied in combination with the configurations of other embodiments.

[0077] (1) In the above embodiment, the first organic waste C1 is the primary settling sludge B1, and the second organic waste C2, which is more difficult to decompose than the first organic waste C1, is the excess sludge B3. However, the first organic waste and the second organic waste can be changed as appropriate. For example, the first organic waste can be food waste such as deli scraps, and the second organic waste can be oxidation ditch sludge or woody or plant biomass such as thinning materials or rice straw.

[0078] (2) In the above embodiment, the methane fermentation liquid C4 circulated through the circulation path 33 is cooled, but it does not have to be cooled all the time. The methane fermentation liquid C4 may be circulated or the circulation may be stopped. In addition, when the temperature of the methane fermentation liquid C4 falls below the temperature range suitable for methane fermentation treatment, the methane fermentation liquid C4 may be heated without cooling. For example, the methane fermentation liquid C4 may be cooled all the time or intermittently in summer, while being cooled and heated or only heated, all the time or intermittently, from autumn to spring. [Explanation of symbols]

[0079] 31 Heating section 32 Methane fermentation tank 34 Cooling section 35 Supply section 36 First Supply Route 37 Second Supply Route 52 Mixing tank C1 First organic waste C2 Secondary organic waste C3 mixture

Claims

1. A methane fermentation method for producing biogas by subjecting organic waste to methane fermentation in a methane fermenter, comprising the steps of: The organic waste includes a first organic waste and a second organic waste that is more difficult to decompose than the first organic waste, a heating step of heating the second organic waste; A supply step of supplying the first organic waste that has not been subjected to the heating step to the methane fermentation tank, and supplying the second organic waste that has been heated in the heating step to the methane fermentation tank; and a cooling step of cooling the methane fermentation liquid containing organic waste that has been supplied to the methane fermentation tank.

2. 2. The methane fermentation treatment method according to claim 1, wherein a first supply path for supplying the first organic waste to the methane fermentation tank and a second supply path for supplying the second organic waste to the methane fermentation tank are separately provided.

3. 2. The methane fermentation treatment method according to claim 1, wherein the supply step includes a mixing step of mixing the first organic waste that has not been subjected to the heating step with the second organic waste that has been heated in the heating step, and a mixture supply step of supplying the mixture of the first organic waste and the second organic waste mixed in the mixing step to the methane fermentation tank.

4. In the mixing step, the first organic waste that has not been subjected to the heating step is mixed with the second organic waste that has been heated in the heating step in a mixing tank, 4. The methane fermentation treatment method according to claim 3, wherein in the mixture supplying step, a mixture of the first organic waste and the second organic waste is supplied from the mixing tank to the methane fermentation tank.

5. The methane fermentation treatment method according to any one of claims 1 to 4, further comprising a heating amount adjustment step for adjusting the heating amount of the second organic waste in the heating step to a reduced amount when the cooling amount required in the cooling step is an excessive cooling requirement state.

6. In a methane fermentation treatment facility that produces biogas by subjecting organic waste to methane fermentation in a methane fermentation tank, The organic waste includes a first organic waste and a second organic waste that is more difficult to decompose than the first organic waste, a heating unit that heats the second organic waste; A supply unit that supplies the first organic waste that has not passed through the heating unit to the methane fermentation tank, and supplies the second organic waste that has been heated in the heating unit to the methane fermentation tank; and a cooling section for cooling the methane fermentation liquid containing organic waste supplied to the methane fermentation tank.

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