Mesophilic methane fermentation treatment device and method for operating the same

Abstract

To provide a technology that can suppress the foaming of sludge more easily than before, especially inside a first-stage methane fermentation tank, in a medium-temperature methane fermentation processing system that performs anaerobic fermentation in two stages, while suppressing adverse effects on the sludge in anaerobic fermentation.SOLUTION: A medium-temperature methane fermentation processing apparatus 101 includes a first methane fermentation tank 1, a second methane fermentation tank 2, a sludge transfer pipe 5, a first circulation pipe 4, a first circulation pump 11, first heating means (first heat exchanger 9), a second circulation pipe 6, a second circulation pump 13 and a sludge return pipe 8. In this medium-temperature methane fermentation processing apparatus 101, the temperature of sludge in the first methane fermentation tank 1 is set higher than that of sludge in the second methane fermentation tank 2, and the temperature of sludge in the first methane fermentation tank 1 is set at 45°C or lower.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a mesophilic methane fermentation treatment apparatus and a method for operating the same. [Background technology]

[0002] In methane fermentation tanks that perform anaerobic fermentation of organic waste, the sludge inside the tank may foam during treatment. If foaming is severe, the foam surface may rise and become mixed into the biogas recovery pipe. If foam gets mixed into the recovery pipe, it may damage the biogas equipment in the downstream stage. For example, Patent Documents 1 and 2 describe techniques to deal with foaming in methane fermentation tanks.

[0003] The technology described in Patent Document 1 relates to an antifoaming agent supplying device. An antifoaming agent storage tank is provided above a methane fermentation tank where anaerobic treatment is performed. The antifoaming agent in the antifoaming agent storage tank is supplied to the methane fermentation tank.

[0004] In Patent Document 2, a pipe for circulating hot water from a boiler or a pipe for circulating warm water generated by utilizing the exhaust heat of a biogas utilization device is provided in the gas phase of a methane fermentation tank. The hot water from the boiler or the warm water generated by utilizing the exhaust heat of the biogas utilization device heats the bubbles that have formed in the gas phase, thereby reducing the viscosity of the bubbles and making them easier to break down. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6378957 [Patent Document 2] Patent No. 5166337 Summary of the Invention [Problem to be solved by the invention]

[0006] When only an antifoaming agent is used as a foaming countermeasure using an antifoaming agent injection device such as that described in Patent Document 1, there is a problem that the cost of chemicals (antifoaming agent cost) increases. In addition, when a large amount of antifoaming agent is used, there is another problem that the fermentation liquid is diluted by the antifoaming agent.

[0007] On the other hand, the technology described in Patent Document 2 makes it possible to reduce the amount of antifoaming agent used. However, the technology described in Patent Document 2 requires the installation of piping through which hot water or warm water flows in the gas phase section of the methane fermentation tank, which makes the gas phase section of the methane fermentation tank complicated. In addition, measures to prevent corrosion of the piping are also required.

[0008] One method of anaerobic fermentation is a two-stage process using two methane fermentation tanks. In this two-stage process, sludge (raw sludge) is fed into the first-stage methane fermentation tank, so foaming is more likely to occur in the first-stage methane fermentation tank than in the second-stage methane fermentation tank.

[0009] The object of the present invention is to provide a technology that can more easily suppress foaming of sludge, particularly inside the first-stage methane fermentation tank, in a mesophilic methane fermentation treatment device that performs anaerobic fermentation treatment in two stages, while suppressing the adverse effects of anaerobic fermentation on sludge. [Means for solving the problem]

[0010] The mesophilic methane fermentation treatment apparatus disclosed in the present application comprises a first methane fermentation tank into which sludge containing organic waste is introduced, a second methane fermentation tank disposed downstream of the first methane fermentation tank, a sludge transfer pipe for transferring the sludge in the first methane fermentation tank from the first methane fermentation tank to the second methane fermentation tank, a first circulation pipe for circulating the sludge in the first methane fermentation tank, the first circulation pipe being for withdrawing the sludge from the bottom of the first methane fermentation tank and returning the withdrawn sludge to the top of the first methane fermentation tank, and The mesophilic methane fermentation treatment apparatus includes a first circulation pump disposed in a first circulation piping, a first heating means for heating the sludge in the first methane fermentation tank, a second circulation piping for circulating the sludge in the second methane fermentation tank, the second circulation piping for withdrawing sludge from the bottom of the second methane fermentation tank and returning the withdrawn sludge to the top of the second methane fermentation tank, a second circulation pump disposed in the second circulation piping, and a sludge return pipe for returning the sludge in the second methane fermentation tank from the second methane fermentation tank to the first methane fermentation tank. In this mesophilic methane fermentation treatment apparatus, the temperature of the sludge in the first methane fermentation tank is set higher than the temperature of the sludge in the second methane fermentation tank, and the temperature of the sludge in the first methane fermentation tank is set to 45°C or lower.

[0011] Causes of foaming include an increase in the viscosity of sludge, the mixing in of foaming substances from organic waste, and the secretion of foaming substances from microorganisms. When the viscosity of sludge increases or when there is a large amount of foaming substance in the sludge, the sludge is more likely to foam. Here, when the temperature of the sludge is high, the viscosity of the sludge decreases and the foaming substance disperses more easily. As a result, the sludge is less likely to foam. Furthermore, when the viscosity of sludge is low, the bubbles disappear more easily. An example of a foaming substance is oil or fat. With the mesophilic methane fermentation treatment device configured as described above, by setting the temperature of the sludge in the first methane fermentation tank higher than the temperature of the sludge in the second methane fermentation tank, it is possible to easily prevent sludge from foaming inside the first-stage methane fermentation tank (first methane fermentation tank), which is prone to foaming.

[0012] In addition, because the upper limit of the sludge temperature in the first methane fermentation tank, in which the sludge temperature is set high, is set to 45°C, adverse effects on the sludge during anaerobic fermentation can be suppressed in both the first methane fermentation tank and the second methane fermentation tank. Furthermore, the sludge can be circulated between the first methane fermentation tank and the second methane fermentation tank using the sludge transfer pipe and the sludge return pipe. Therefore, the sludge does not undergo anaerobic fermentation only in the first methane fermentation tank, which has a relatively high temperature, but also in the second methane fermentation tank, which has a relatively low temperature. From this perspective, adverse effects on the sludge during anaerobic fermentation can be suppressed.

[0013] When the distance between the liquid level of the sludge in the first methane fermentation tank and the bottom surface of the first methane fermentation tank is L, the suction port of the sludge transfer pipe may be located at a position 1 / 4 L to 1 / 2 L below the liquid level.

[0014] This configuration makes it possible to prevent sludge that tends to foam from being extracted from the first methane fermentation tank, and to prevent sludge that tends to foam from flowing into the second methane fermentation tank. As a result, it is possible to prevent sludge from foaming inside the second-stage methane fermentation tank (second methane fermentation tank). It is also possible to prevent sludge that tends to settle from being extracted.

[0015] The first heating means may be a first heat exchanger disposed in the first circulation pipe.

[0016] This configuration makes it easier to maintain the temperature of the sludge in the first methane fermenter at an appropriate temperature.

[0017] The mesophilic methane fermentation treatment device may further include a second heating means for heating the sludge in the second methane fermentation tank.

[0018] With this configuration, it is easier to maintain the temperature of the sludge in the second methane fermentation tank at an appropriate temperature compared to when the sludge is heated only by the first heating means that heats the sludge in the first methane fermentation tank.

[0019] The second heating means may be a second heat exchanger disposed in the second circulation pipe.

[0020] This configuration makes it easier to maintain the temperature of the sludge in the second methane fermentation tank at an appropriate temperature.

[0021] The temperature of the sludge in the second methane fermentation tank may be set to 37°C±3°C, and the temperature of the sludge in the first methane fermentation tank may be set to 40°C±3°C.

[0022] This configuration can further reduce the adverse effects on sludge during anaerobic fermentation.

[0023] When foaming of the sludge in the first methane fermentation tank is detected, the temperature of the sludge in the first methane fermentation tank may be temporarily increased.

[0024] With this configuration, bubbles in the first methane fermenter are easily eliminated.

[0025] The sludge transfer pipe may be arranged so as to supply the sludge in the first methane fermentation tank to the vertical center of the second methane fermentation tank or to a position above the center of the second methane fermentation tank, and the sludge return pipe may be arranged so as to withdraw the sludge from the bottom of the second methane fermentation tank.

[0026] Sludge is circulated between the first methane fermentation tank and the second methane fermentation tank via a sludge transfer pipe and a sludge return pipe. With the above configuration, the sludge supplied to the second methane fermentation tank can be prevented from being immediately returned to the first methane fermentation tank, making it easier to ensure the retention time of the sludge in the second methane fermentation tank.

[0027] The sludge return pipe may branch off from the second circulation pipe.

[0028] With this configuration, the sludge return pipe can be easily installed.

[0029] The sludge return pipe may be connected to the first circulation pipe.

[0030] With this configuration, the sludge return pipe can be easily installed.

[0031] The present application also discloses a method for operating a mesophilic methane fermentation treatment apparatus. This method includes a first methane fermentation tank into which sludge containing organic waste is introduced, a second methane fermentation tank disposed downstream of the first methane fermentation tank, a sludge transfer pipe for transferring the sludge in the first methane fermentation tank from the first methane fermentation tank to the second methane fermentation tank, a first circulation pipe for circulating the sludge in the first methane fermentation tank, the first circulation pipe for withdrawing sludge from the bottom of the first methane fermentation tank and returning the withdrawn sludge to the top of the first methane fermentation tank, a first circulation pump disposed in the first circulation pipe, a first heating means for heating the sludge in the first methane fermentation tank, and a second methane fermentation tank. A method for operating a mesophilic methane fermentation treatment device comprising: a second circulation pipe for circulating sludge in the tank, the second circulation pipe for withdrawing sludge from the bottom of the second methane fermentation tank and returning the withdrawn sludge to the top of the second methane fermentation tank; a second circulation pump arranged on the second circulation pipe; and a sludge return pipe for returning the sludge in the second methane fermentation tank from the second methane fermentation tank to the first methane fermentation tank, characterized in that the temperature of the sludge in the first methane fermentation tank is set higher than the temperature of the sludge in the second methane fermentation tank, and the temperature of the sludge in the first methane fermentation tank is set to 45°C or lower.

[0032] In the above-described operating method, when the distance between the liquid level of the sludge in the first methane fermentation tank and the bottom surface of the first methane fermentation tank is defined as L, the sludge in the first methane fermentation tank may be extracted from a position 1 / 4 L to 1 / 2 L below the liquid level using the sludge transfer pipe and transferred to the second methane fermentation tank. [Effects of the Invention]

[0033] According to the present invention, in a mesophilic methane fermentation treatment device that performs anaerobic fermentation treatment in two stages, it is possible to suppress the adverse effects of anaerobic fermentation on sludge, while more easily preventing sludge from foaming, particularly inside the first-stage methane fermentation tank, than in the past. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a diagram showing a mesophilic methane fermentation treatment apparatus according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0035] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0036] As shown in FIG. 1, a mesophilic methane fermentation treatment device 101 according to one embodiment of the present invention is a two-stage anaerobic fermentation treatment device comprising a first methane fermentation tank 1 and a second methane fermentation tank 2 arranged downstream of the first methane fermentation tank 1.

[0037] The first methane fermenter 1 is a first-stage methane fermenter into which thickened sludge (raw material sludge) containing organic waste is fed. The second methane fermenter 2 is a second-stage methane fermenter into which the sludge in the first methane fermenter 1 is fed from the first methane fermenter 1. The basic flow of sludge is that raw material sludge is fed into the first methane fermenter 1, and the fed raw material sludge is then fed (supplied) from the first methane fermenter 1 to the second methane fermenter 2. Therefore, the position of the second methane fermenter 2 relative to the first methane fermenter 1 is specified by describing it as the "second methane fermenter 2 located subsequent to the first methane fermenter 1."

[0038] Here, for example, the guidelines of the Japan Sewage Works Association state that in mesophilic fermentation treatment, which is one of the anaerobic fermentation treatments, a methane fermenter is operated at a temperature of approximately 30 to 37°C with a retention time of approximately 20 to 30 days. In thermophilic fermentation treatment, it is operated at a temperature of approximately 50 to 55°C with a retention time of approximately 10 to 15 days. As will be described later, in the disclosure of the present application, the upper limit setting for the temperature of the sludge in the first methane fermenter 1, which is set at a higher temperature than the second methane fermenter 2, is 45°C. When the upper limit setting is 45°C, the temperature of the sludge in the first methane fermenter 1 reaches 45°C. Here, a temperature of 45°C does not fall within the temperature range of mesophilic fermentation treatment in the above guidelines. However, it is a temperature lower than the minimum temperature of thermophilic fermentation treatment, which is 50°C, in the above guidelines. Furthermore, the guidelines are merely guidelines, and do not necessarily mean that a temperature of approximately 30 to 37°C is not considered to be mesophilic fermentation treatment. In other words, the first methane fermentation tank 1, in which the sludge temperature is set to 45°C or below, and the second methane fermentation tank 2, in which the temperature is set lower than this, are both tanks that perform anaerobic fermentation treatment, particularly mesophilic fermentation treatment (mesophilic methane fermentation treatment).

[0039] The first methane fermenter 1 is a tank for anaerobic fermentation of organic waste such as sewage sludge and food waste. The first methane fermenter 1 is, for example, a concrete tank. However, the first methane fermenter 1 may also be a steel plate tank.

[0040] A raw material sludge pipe 7 is connected to the first methane fermentation tank 1. The raw material sludge pipe 7 is a pipe for introducing thickened sludge (raw material sludge) into the first methane fermentation tank 1. The thickened sludge (raw material sludge) is introduced into the upper part of the first methane fermentation tank 1 via the raw material sludge pipe 7.

[0041] The first methane fermentation tank 1 is provided with an agitator 3. The agitator 3 is for agitating the sludge (thickened sludge) introduced into the first methane fermentation tank 1. In this embodiment, the agitator 3 is configured to agitate the sludge with multiple stages of blades 3a (impellers) that rotate horizontally. The rotation of the blades 3a generates a downward flow in the center of the first methane fermentation tank 1. The downward flow spreads and reverses at the bottom of the first methane fermentation tank 1, becoming an upward flow. Note that instead of the impeller-type agitator 3 as in this embodiment, other types of agitators, such as a screw-type agitator, may be used.

[0042] The first methane fermentation tank 1 is provided with a first circulation pipe 4. The first circulation pipe 4 is a pipe for circulating sludge in the first methane fermentation tank 1. The first circulation pipe 4 is provided outside the first methane fermentation tank 1. The first circulation pipe 4 is a pipe for withdrawing sludge from the bottom of the first methane fermentation tank 1 and returning the withdrawn sludge to the top of the first methane fermentation tank 1.

[0043] A first circulation pump 11 is disposed in the path of the first circulation piping 4. The first circulation pump 11 circulates the sludge in the first methane fermentation tank 1. The first circulation pump 11 extracts sludge from the bottom of the first methane fermentation tank 1 and returns the extracted sludge to the top of the first methane fermentation tank 1.

[0044] A first withdrawal valve 12 is arranged in the first circulation pipe 4 at the outlet of the first methane fermenter 1. The first withdrawal valve 12 may be an automatic valve that can be remotely controlled, or may be a manual valve.

[0045] A first heat exchanger 9 is disposed in the path of the first circulation piping 4. The first heat exchanger 9 is an example of a first heating means for heating the sludge in the first methane fermentation tank 1. The first heat exchanger 9 is an indirect heat exchanger for heating the sludge flowing through the first circulation piping 4. Hot water is supplied to the first heat exchanger 9 from a boiler (not shown). The sludge withdrawn from the bottom of the first methane fermentation tank 1 is heated by indirect contact with the hot water in the first heat exchanger 9, and then returned to the first methane fermentation tank 1 from the top of the first methane fermentation tank 1. The first circulation piping 4 and the first circulation pump 11 contribute to the agitation of the sludge in the first methane fermentation tank 1 in addition to circulating the sludge in the tank.

[0046] The heat source for heating the sludge is not limited to hot water from a boiler. For example, hot water may be obtained from the exhaust heat of a biogas generator (not shown) or a sludge incineration facility (not shown), and the hot water may be used as the heat source for heating the sludge. Furthermore, instead of the first heat exchanger 9 (heat exchanger), a direct heating means for directly heating the sludge in the tank with steam may be used as the first heating means for heating the sludge in the first methane fermentation tank 1. Furthermore, heat transfer oil may be used as a heat transfer medium other than hot water or steam.

[0047] A crusher 19 is disposed in the path of the first circulation pipe 4. The crusher 19 crushes impurities contained in the sludge flowing through the first circulation pipe 4. However, the provision of the crusher 19 is not essential.

[0048] The sludge in the first methane fermentation tank 1 is heated by a first heat exchanger 9 and stirred by an agitator 3. Biogas generated by anaerobic fermentation in the first methane fermentation tank 1 is extracted from the first methane fermentation tank 1 through a gas recovery pipe (not shown). The gas recovery pipe is connected to the top surface of the first methane fermentation tank 1. The biogas is, for example, a gas composed of approximately 60% by volume of methane and approximately 40% by volume of carbon dioxide. The biogas extracted from the first methane fermentation tank 1 is used as fuel for a boiler or as fuel for a power generation facility (not shown).

[0049] The second methane fermentation tank 2 is a tank for anaerobic fermentation treatment of organic waste, similar to the first methane fermentation tank 1. The second methane fermentation tank 2 is also a tank for settling sludge. The second methane fermentation tank 2 is, for example, a concrete tank. The second methane fermentation tank 2 may also be a steel plate tank.

[0050] A sludge transfer pipe 5 is connected to the second methane fermentation tank 2. The sludge transfer pipe 5 is a pipe for transferring sludge in the first methane fermentation tank 1 from the first methane fermentation tank 1 to the second methane fermentation tank 2. The sludge in the first methane fermentation tank 1 flows down through the sludge transfer pipe 5 by gravity and flows into the second methane fermentation tank 2. A telescope valve (not shown) is arranged at the outlet of the sludge transfer pipe 5 from the first methane fermentation tank 1. This telescope valve adjusts the amount of sludge extracted from the first methane fermentation tank 1. The sludge transfer pipe 5 is preferably arranged so as to supply the sludge in the first methane fermentation tank 1 to the vertical center of the second methane fermentation tank 2 or above the center of the second methane fermentation tank 2. In this embodiment, the sludge transfer pipe 5 is arranged so as to supply the sludge in the first methane fermentation tank 1 to the upper part of the second methane fermentation tank 2. The discharge port 5b of the sludge transfer pipe 5 is disposed in the upper part of the second methane fermentation tank 2. The discharge port 5b is an end portion (an opening at the end of the sludge transfer pipe 5) of the sludge transfer pipe 5. The upper part of the second methane fermentation tank 2 is an example of a portion higher than the center portion inside the second methane fermentation tank 2.

[0051] The second methane fermentation tank 2 is not provided with an agitator 3 .

[0052] The second methane fermentation tank 2 is provided with a second circulation pipe 6, as in the case of the first methane fermentation tank 1. The second circulation pipe 6 is a pipe for circulating sludge in the second methane fermentation tank 2. The second circulation pipe 6 is provided outside the second methane fermentation tank 2. The second circulation pipe 6 is a pipe for withdrawing sludge from the bottom of the second methane fermentation tank 2 and returning the withdrawn sludge to the top of the second methane fermentation tank 2.

[0053] A second circulation pump 13 is disposed in the path of the second circulation piping 6. The second circulation pump 13 circulates the sludge in the second methane fermentation tank 2. The second circulation pump 13 extracts sludge from the bottom of the second methane fermentation tank 2 and returns the extracted sludge to the top of the second methane fermentation tank 2.

[0054] A second withdrawal valve 14 is arranged in the second circulation pipe 6 at the outlet of the second methane fermenter 2. The second withdrawal valve 14 may be an automatic valve that can be remotely controlled, or may be a manual valve.

[0055] A second heat exchanger 10 is disposed in the path of the second circulation piping 6. The second heat exchanger 10 is an example of a second heating means for heating the sludge in the second methane fermentation tank 2. The second heat exchanger 10 is an indirect heat exchanger for heating the sludge flowing through the second circulation piping 6. Hot water is supplied to the second heat exchanger 10 from a boiler (not shown). The sludge withdrawn from the bottom of the second methane fermentation tank 2 is heated by indirect contact with the hot water in the second heat exchanger 10, and then returned to the second methane fermentation tank 2 from the top of the second methane fermentation tank 2. The second circulation piping 6 and the second circulation pump 13 contribute to the agitation of the sludge in the second methane fermentation tank 2 in addition to circulating the sludge in the tank.

[0056] The heat source for heating the sludge is not limited to hot water from a boiler. For example, hot water may be obtained from the exhaust heat of a biogas generator (not shown) or a sludge incineration facility (not shown), and the hot water may be used as the heat source for heating the sludge. Furthermore, as the second heating means for heating the sludge in the second methane fermentation tank 2, a direct heating means for directly heating the sludge in the tank with steam may be used instead of the second heat exchanger 10 (heat exchanger). Furthermore, heat transfer oil may be used as a heat transfer medium other than hot water or steam.

[0057] Furthermore, the second heat exchanger 10 does not have to be provided. In other words, the second heating means for heating the sludge in the second methane fermentation tank 2 does not have to be provided. Since the sludge can be circulated between the first methane fermentation tank 1 and the second methane fermentation tank 2 by the sludge transfer pipe 5 and the sludge return pipe 8, it is possible to heat the sludge in the second methane fermentation tank 2 by only the first heat exchanger 9 (first heating means).

[0058] However, when the second heating means is provided, it is easier to maintain the temperature of the sludge in the second methane fermentation tank 2 at an appropriate temperature compared to when the sludge in the first methane fermentation tank 1 and the second methane fermentation tank 2 is heated only by the first heating means.

[0059] A sludge return pipe 8 is provided for returning sludge in the second methane fermentation tank 2 from the second methane fermentation tank 2 to the first methane fermentation tank 1. The sludge return pipe 8 is piped so as to withdraw sludge from the bottom of the second methane fermentation tank 2. Sludge is circulated between the first methane fermentation tank 1 and the second methane fermentation tank 2 by the sludge transfer pipe 5 and the sludge return pipe 8. If the sludge transfer pipe 5 is piped so as to supply the sludge in the first methane fermentation tank 1 to the center in the vertical direction within the second methane fermentation tank 2 or to a position above the center within the second methane fermentation tank 2, and the sludge return pipe 8 is piped so as to withdraw sludge from the bottom of the second methane fermentation tank 2, it is possible to prevent the sludge supplied to the second methane fermentation tank 2 from being immediately returned to the first methane fermentation tank 1, and therefore it is easy to ensure the retention time of the sludge in the second methane fermentation tank 2.

[0060] The suction port 6a of the second circulation piping 6 is arranged at the bottom of the second methane fermentation tank 2. In this embodiment, the sludge return pipe 8 is arranged so as to branch off from the middle of the second circulation piping 6, thereby drawing out sludge from the bottom of the second methane fermentation tank 2. Note that a sludge return pipe 8 may be provided separately from the second circulation piping 6 so that sludge is directly drawn out from the bottom of the second methane fermentation tank 2 by the sludge return pipe 8.

[0061] Here, the second methane fermenter 2 may not be a newly constructed fermenter, but may be an existing static fermenter that has been modified. In this case, if the sludge return pipe 8 is formed by branching off from the second circulation pipe 6 provided outside the tank, the sludge return pipe 8 can be easily installed when modifying the existing fermenter.

[0062] A switching valve 15 is disposed at a branch point of the sludge return pipe 8 from the second circulation pipe 6. A switching valve 20 is disposed on the side of the second circulation pipe 6. The switching valves 15 and 20 may be automatic valves that can be remotely controlled, or may be manual valves.

[0063] The downstream end of the sludge return pipe 8 is connected to the first circulation piping 4. The sludge from the second methane fermentation tank 2 that flows through the sludge return pipe 8 flows into the first methane fermentation tank 1 via the first circulation piping 4. Note that the downstream end of the sludge return pipe 8 may be directly connected to the top of the first methane fermentation tank 1 so that the sludge is returned directly to the top of the first methane fermentation tank 1 through the sludge return pipe 8.

[0064] Here, the first methane fermenter 1 may not be a newly constructed fermenter, but may be a modified version of an existing complete mixing fermenter. In this case, if the sludge return pipe 8 is formed by connecting it to the first circulation piping 4 provided outside the tank, the sludge return pipe 8 can be easily installed when modifying the existing fermenter.

[0065] The sludge in the second methane fermentation tank 2 is heated by the second heat exchanger 10 and is caused to flow within the tank by the second circulation piping 6 and the second circulation pump 13. The biogas generated by anaerobic fermentation in the second methane fermentation tank 2 is extracted from the second methane fermentation tank 2 through a gas recovery pipe (not shown). The gas recovery pipe is connected to the top surface of the second methane fermentation tank 2. The biogas extracted from the second methane fermentation tank 2 is used as fuel for a boiler or as fuel for a power generation facility (not shown).

[0066] The second methane fermentation tank 2 is provided with an extraction pipe 16 and an extraction pump 17. The extraction pipe 16 is installed so as to extract sludge from the bottom of the second methane fermentation tank 2. In this embodiment, the extraction pipe 16 is installed so as to branch off from the middle of the second circulation pipe 6, and so as to extract sludge from the bottom of the second methane fermentation tank 2.

[0067] As mentioned above, the second methane fermentation tank 2 may not be a newly constructed fermentation tank, but may be an existing static fermentation tank that has been modified. In this case, the withdrawal pipe 16 does not branch off from the route of the newly constructed second circulation pipe 6, but the second circulation pipe 6 may branch off from the route of the existing withdrawal pipe 16 extending from the bottom of the second methane fermentation tank 2.

[0068] An extraction valve 18 is disposed at the branch point of the extraction pipe 16 from the second circulation pipe 6. The extraction valve 18 may be an automatic valve that can be remotely controlled, or may be a manual valve. The fermentation residue in the second methane fermentation tank 2 is extracted from the bottom of the second methane fermentation tank 2 to the outside of the tank by the extraction pipe 16 and the extraction pump 17. Note that the sludge may be extracted by gravity flow and discharged to the outside of the tank, as in the first methane fermentation tank 1.

[0069] Here, because raw sludge is first introduced into the first methane fermenter 1, which is the first-stage methane fermenter, the sludge is more likely to foam in the first methane fermenter 1 than in the second methane fermenter 2, which is the second-stage methane fermenter. If sludge containing components that cause foaming is introduced into the first methane fermenter 1, or if some environmental change occurs while such components are present in the first methane fermenter 1, bubbles may suddenly appear and grow in a short period of time. In particular, if the sludge contains a large amount of oils and fats, highly viscous bubbles are likely to form. These types of bubbles are difficult to break, and they rapidly increase in number in a short period of time, resulting in sudden foaming. If foaming is severe, the foam interface may rise and enter the gas recovery pipe, and the bubbles may then break, causing the fermentation liquid to become liquid and flow out of the gas recovery pipe.

[0070] The causes of foaming include an increase in the viscosity of the sludge and the dispersibility of foaming substances secreted by organic waste and microorganisms. If the viscosity increases or the dispersibility of the foaming substances is low, the sludge is more likely to foam. If the viscosity is low or the dispersibility of the foaming substances is high, the sludge is less likely to foam. Also, if the dispersibility of the foaming substances is low, the bubbles that form are difficult to dissipate, and if the dispersibility of the foaming substances is high, the bubbles that form are more likely to dissipate. To increase the dispersibility of the foaming substances and reduce the viscosity of the sludge, the temperature of the sludge is raised.

[0071] Therefore, in the mesophilic methane fermentation treatment apparatus 101, the temperature of the sludge in the first methane fermentation tank 1 is set higher than the temperature of the sludge in the second methane fermentation tank 2. When the temperature of the sludge in the first methane fermentation tank 1 is set higher than the temperature of the sludge in the second methane fermentation tank 2, the temperature of the sludge in the first methane fermentation tank 1 becomes higher than the temperature of the sludge in the second methane fermentation tank 2. When the temperature of the sludge in the first methane fermentation tank 1 becomes higher, as described above, the sludge becomes less likely to foam and any foam that does form easily disappears. Therefore, it is possible to suppress foaming of the sludge inside the first methane fermentation tank 1, which is prone to foaming. Furthermore, when the temperature of the sludge in the first methane fermentation tank 1 becomes higher, the viscosity of the sludge decreases, and the efficiency of stirring the sludge by the agitator 3 also improves.

[0072] The temperature of the sludge in the first methane fermenter 1 is set to 45°C or lower. Although 45°C is a high temperature for mesophilic fermentation treatment, it is lower than the temperature in a thermophilic fermentation treatment, which is operated at a temperature of approximately 50 to 55°C (guideline value), and is therefore an acceptable temperature for mesophilic fermentation treatment. The temperature of the sludge in the first methane fermenter 1 is 45°C or lower, and the temperature of the sludge in the second methane fermenter 2 is lower than the temperature of the sludge in the first methane fermenter 1. Therefore, adverse effects on the sludge in the mesophilic fermentation treatment (anaerobic fermentation) can be suppressed in both the first and second methane fermenters.

[0073] Furthermore, according to the mesophilic methane fermentation treatment device 101, sludge is circulated between the first methane fermentation tank 1 and the second methane fermentation tank 2 by the sludge transfer pipe 5 and the sludge return pipe 8. Therefore, the sludge does not undergo anaerobic fermentation only in the first methane fermentation tank 1, which has a relatively high temperature, but also in the second methane fermentation tank 2, which has a relatively low temperature. From this perspective, it is possible to suppress adverse effects on the sludge during mesophilic fermentation treatment (anaerobic fermentation).

[0074] Here, it is preferable that the temperature of the sludge in the second methane fermentation tank 2 is set to 37°C ± 3°C, and the temperature of the sludge in the first methane fermentation tank 1 is set to 40°C ± 3°C. "Set to 37°C ± 3°C" means that it is set to be not less than 34°C and not more than 40°C. Furthermore, "set to 40°C ± 3°C" means that it is set to be not less than 37°C and not more than 43°C. As a premise, the temperature of the sludge in the first methane fermentation tank 1 is set to be higher than the temperature of the sludge in the second methane fermentation tank 2, so for example, when the temperature of the sludge in the second methane fermentation tank 2 is set to be 40°C, the temperature of the sludge in the first methane fermentation tank 1 will not be set to be lower than 40°C, but will be higher than 40°C and not more than 43°C. That is, even if the temperature of the sludge in the first methane fermentation tank 1 is "set to 40°C ± 3°C," the temperature of the sludge in the first methane fermentation tank 1 is determined in relation to the temperature of the sludge in the second methane fermentation tank 2. The same applies to the temperature of the sludge in the second methane fermentation tank 2. For example, if the temperature of the sludge in the first methane fermentation tank 1 is set to 38°C, the temperature of the sludge in the second methane fermentation tank 2 will not be set to above 38°C, but will be set to be above 34°C and below 38°C.

[0075] When the temperature of the sludge in the second methane fermentation tank 2 is set to 37°C±3°C and the temperature of the sludge in the first methane fermentation tank 1 is set to 40°C±3°C, the adverse effects on the sludge during mesophilic fermentation treatment (anaerobic fermentation) can be further reduced.

[0076] The temperature of the sludge in the first methane fermenter 1 can be controlled, for example, as follows.

[0077] The temperature of the sludge in the first methane fermentation tank 1 is controlled by adjusting the flow rate of hot water supplied from a boiler or the like to the first heat exchanger 9. For example, when the temperature of the sludge in the first methane fermentation tank 1 is to be set at 42°C, the flow rate of hot water supplied from a boiler or the like to the first heat exchanger 9 is adjusted (set) so that the temperature of the sludge in the first methane fermentation tank 1 becomes 42°C. The flow rate of hot water supplied to the first heat exchanger 9 is adjusted by changing the set flow rate of the boiler or the like. In this case, it is assumed that the set temperature of the boiler or the like (the set temperature of the hot water to be supplied) is constant.

[0078] Instead of or in addition to adjusting the flow rate of hot water supplied to the first heat exchanger 9, the temperature of the sludge in the first methane fermentation tank 1 may be controlled by adjusting the flow rate of sludge flowing through the first circulation pipe 4. The flow rate of sludge flowing through the first circulation pipe 4 is adjusted by changing the rotation speed of the first circulation pump 11, for example.

[0079] Furthermore, instead of or in addition to adjusting the flow rate of the hot water supplied to the first heat exchanger 9, the temperature of the hot water supplied to the first heat exchanger 9 may be adjusted to control the temperature of the sludge in the first methane fermentation tank 1. The temperature of the hot water supplied to the first heat exchanger 9 is adjusted by changing the set temperature of a boiler or the like.

[0080] When the sludge in the tank is heated directly with steam rather than indirectly by the first heat exchanger 9, the temperature of the sludge in the first methane fermentation tank 1 is controlled by adjusting the flow rate of steam.

[0081] The method for controlling the temperature of the sludge in the second methane fermentation tank 2 is the same as the method for controlling the temperature of the sludge in the first methane fermentation tank 1. The method for controlling the temperature of the sludge in the second methane fermentation tank 2 is, for example, as follows.

[0082] The temperature of the sludge in the second methane fermentation tank 2 is controlled by adjusting the flow rate of hot water supplied from a boiler or the like to the second heat exchanger 10. For example, when the temperature of the sludge in the second methane fermentation tank 2 is to be set at 40°C, the flow rate of hot water supplied from a boiler or the like to the second heat exchanger 10 is adjusted (set) so that the temperature of the sludge in the second methane fermentation tank 2 becomes 40°C. The flow rate of hot water supplied to the second heat exchanger 10 is adjusted by changing the set flow rate of the boiler or the like. In this case, it is assumed that the set temperature of the boiler or the like (the set temperature of the hot water to be supplied) is constant.

[0083] Instead of or in addition to adjusting the flow rate of the hot water supplied to the second heat exchanger 10, the temperature of the sludge in the second methane fermentation tank 2 may be controlled by adjusting the flow rate of the sludge flowing through the second circulation pipe 6. The flow rate of the sludge flowing through the second circulation pipe 6 is adjusted by changing the rotation speed of the second circulation pump 13, for example.

[0084] Furthermore, instead of or in addition to adjusting the flow rate of the hot water supplied to the second heat exchanger 10, the temperature of the hot water supplied to the second heat exchanger 10 may be adjusted to control the temperature of the sludge in the second methane fermentation tank 2. The temperature of the hot water supplied to the second heat exchanger 10 is adjusted by changing the set temperature of a boiler or the like.

[0085] When the sludge in the tank is heated directly with steam rather than indirectly by the second heat exchanger 10, the temperature of the sludge in the second methane fermentation tank 2 is controlled by adjusting the flow rate of the steam.

[0086] Here, when foaming of the sludge is detected in the first methane fermentation tank 1, the temperature of the sludge in the first methane fermentation tank 1 may be temporarily raised. Foaming of the sludge is detected, for example, by foaming detection means (not shown). The foaming detection means is an instrument for detecting generated foam. As the foaming detection means, a capacitance type level meter, an electrode type level meter, an ultrasonic type level meter, or the like is used. The foaming detection means is provided in the first methane fermentation tank 1. Note that foaming of the sludge may also be detected visually by an operator without using any particular instrument.

[0087] As the temperature of the sludge in the first methane fermentation tank 1 increases, bubbles in the first methane fermentation tank 1 tend to disappear. If the temperature of the sludge is constantly high, it may cause problems in the mesophilic fermentation treatment (anaerobic fermentation). Therefore, the temperature of the sludge in the first methane fermentation tank 1 is temporarily increased, while taking care not to cause problems in the anaerobic fermentation.

[0088] The control method for increasing the temperature of the sludge in the first methane fermenter 1 is, for example, as follows.

[0089] The temperature of the sludge in the first methane fermentation tank 1 is increased by increasing the flow rate of hot water supplied from a boiler or the like to the first heat exchanger 9 above the flow rate when foaming is not detected.

[0090] Instead of or in addition to increasing the flow rate of hot water supplied to the first heat exchanger 9, the temperature of the sludge in the first methane fermentation tank 1 may be increased by increasing the flow rate of sludge flowing through the first circulation pipe 4. Furthermore, instead of or in addition to increasing the flow rate of hot water supplied to the first heat exchanger 9, the temperature of the sludge in the first methane fermentation tank 1 may be increased by raising the temperature of the hot water supplied to the first heat exchanger 9.

[0091] When the sludge in the tank is heated directly with steam rather than indirectly by the first heat exchanger 9, the temperature of the sludge in the first methane fermentation tank 1 is raised by increasing the flow rate of steam.

[0092] The control of setting the temperature of the sludge in the first methane fermenter 1 and the temperature of the sludge in the second methane fermenter 2 to a certain temperature or within a certain temperature range, or the control of temporarily raising the temperature of the sludge in the first methane fermenter 1, may be performed automatically by a control device (not shown) or manually by an operator. That is, the mesophilic methane fermentation treatment apparatus 101 may be equipped with a control device that controls the temperature of the sludge in the first methane fermenter 1 and the temperature of the sludge in the second methane fermenter 2. When the control is performed automatically, the control device sets the temperature of the sludge in the first methane fermenter 1 to be higher than the temperature of the sludge in the second methane fermenter 2, and sets the temperature of the sludge in the first methane fermenter 1 to be 45°C or lower. When the temperature of the sludge in the first methane fermenter 1 is to be temporarily raised, the control device temporarily raises the temperature of the sludge in the first methane fermenter 1.

[0093] 1, in the mesophilic methane fermentation treatment apparatus 101 of this embodiment, when the distance between the liquid level of the sludge in the first methane fermentation tank 1 and the bottom surface of the first methane fermentation tank 1 is L, the suction port 5a of the sludge transfer pipe 5 is positioned 1 / 4 L to 1 / 2 L downward from the liquid level. When the distance between the liquid level of the sludge in the first methane fermentation tank 1 and the suction port 5a of the sludge transfer pipe 5 is H, the relationship is 1 / 4 L≦H≦1 / 2 L. The suction port 5a is the end portion of the sludge transfer pipe 5 (the opening at the end of the sludge transfer pipe 5).

[0094] Foam is likely to form near the sludge surface in the first methane fermentation tank 1. If the suction port 5a of the sludge transfer pipe 5 is positioned 1 / 4 L to 1 / 2 L below the sludge surface, it is possible to prevent sludge that tends to foam from being drawn out of the first methane fermentation tank 1, and to prevent sludge that tends to foam from flowing into the second methane fermentation tank 2. As a result, it is possible to prevent sludge from forming foam inside the second-stage methane fermentation tank (second methane fermentation tank 2). It is also possible to prevent sludge that has a high tendency to settle from being drawn out of the first methane fermentation tank 1.

[0095] The present invention is not limited to the above-described embodiment. The configurations of the above-described embodiment can be appropriately combined, and various modifications can be made to the above-described embodiment. For example, the above-described embodiment can be further modified as follows.

[0096] For example, the second methane fermenter 2 may be provided with an agitator 3 like that provided in the first methane fermenter 1.

[0097] The crusher 19 may be omitted. [Explanation of symbols]

[0098] 1: First methane fermentation tank 2: Second methane fermentation tank 4: 1st circulation piping 5: Sludge transfer pipe 5a: Intake port 6:Second circulation piping 8: Sludge return pipe 9: First heat exchanger (first heating means) 10:Second heat exchanger (second heating means) 11: First circulation pump 13: Second circulation pump 101: Mesophilic methane fermentation treatment equipment

Claims

1. a first methane fermentation tank into which sludge containing organic waste is introduced; a second methane fermenter disposed downstream of the first methane fermenter; a sludge transfer pipe for transferring sludge in the first methane fermentation tank from the first methane fermentation tank to the second methane fermentation tank; a first circulation pipe for circulating sludge in the first methane fermentation tank, the first circulation pipe being for withdrawing sludge from a bottom of the first methane fermentation tank and returning the withdrawn sludge to an upper part of the first methane fermentation tank; a first circulation pump disposed in the first circulation pipe; a first heating means for heating the sludge in the first methane fermentation tank; a second circulation pipe for circulating sludge in the second methane fermentation tank, the second circulation pipe being for withdrawing sludge from the bottom of the second methane fermentation tank and returning the withdrawn sludge to the top of the second methane fermentation tank; a second circulation pump disposed in the second circulation pipe; a sludge return pipe for directly returning the sludge in the second methane fermentation tank from the second methane fermentation tank to the first methane fermentation tank; Equipped with the temperature of the sludge in the first methane fermentation tank is set higher than the temperature of the sludge in the second methane fermentation tank, and the temperature of the sludge in the first methane fermentation tank is set to 37°C or higher and 45°C or lower, and the temperature of the sludge in the second methane fermentation tank is set to 34°C or higher and 40°C or lower; Mesophilic methane fermentation treatment equipment.

2. The mesophilic methane fermentation treatment device according to claim 1, When the distance between the liquid level of the sludge in the first methane fermentation tank and the bottom surface of the first methane fermentation tank is L, the suction port of the sludge transfer pipe is located at a position 1 / 4 L to 1 / 2 L below the liquid level. Mesophilic methane fermentation treatment equipment.

3. The mesophilic methane fermentation treatment device according to claim 1 or 2, The first heating means is a first heat exchanger disposed in the first circulation piping. Mesophilic methane fermentation treatment equipment.

4. The mesophilic methane fermentation treatment device according to claim 1 or 2, The system further includes a second heating means for heating the sludge in the second methane fermentation tank. Mesophilic methane fermentation treatment equipment.

5. The mesophilic methane fermentation treatment device according to claim 4, The second heating means is a second heat exchanger disposed in the second circulation pipe. Mesophilic methane fermentation treatment equipment.

6. The mesophilic methane fermentation treatment device according to claim 1 or 2, The temperature of the sludge in the first methane fermentation tank is 40°C ± 3°C. Mesophilic methane fermentation treatment equipment.

7. The mesophilic methane fermentation treatment device according to claim 1 or 2, When foaming of the sludge is detected in the first methane fermentation tank, the temperature of the sludge in the first methane fermentation tank is temporarily increased. Mesophilic methane fermentation treatment equipment.

8. The mesophilic methane fermentation treatment device according to claim 1 or 2, the sludge transfer pipe is piped so as to supply the sludge in the first methane fermentation tank to a vertical center portion of the second methane fermentation tank or to a portion above the vertical center portion of the second methane fermentation tank, The sludge return pipe is piped so as to draw out sludge from the bottom of the second methane fermentation tank. Mesophilic methane fermentation treatment equipment.

9. The mesophilic methane fermentation treatment device according to claim 8, The sludge return pipe branches off from the second circulation pipe. Mesophilic methane fermentation treatment equipment.

10. The mesophilic methane fermentation treatment device according to claim 8, The sludge return pipe is connected to the first circulation pipe. Mesophilic methane fermentation treatment equipment.

11. The mesophilic methane fermentation treatment device according to claim 9, The sludge return pipe is connected to the first circulation pipe. Mesophilic methane fermentation treatment equipment.

12. a first methane fermentation tank into which sludge containing organic waste is introduced; a second methane fermenter disposed downstream of the first methane fermenter; a sludge transfer pipe for transferring sludge in the first methane fermentation tank from the first methane fermentation tank to the second methane fermentation tank; a first circulation pipe for circulating sludge in the first methane fermentation tank, the first circulation pipe being for withdrawing sludge from a bottom of the first methane fermentation tank and returning the withdrawn sludge to an upper part of the first methane fermentation tank; a first circulation pump disposed in the first circulation pipe; a first heating means for heating the sludge in the first methane fermentation tank; a second circulation pipe for circulating sludge in the second methane fermentation tank, the second circulation pipe being for withdrawing sludge from the bottom of the second methane fermentation tank and returning the withdrawn sludge to the top of the second methane fermentation tank; a second circulation pump disposed in the second circulation pipe; a sludge return pipe for directly returning the sludge in the second methane fermentation tank from the second methane fermentation tank to the first methane fermentation tank; A method for operating a mesophilic methane fermentation treatment apparatus comprising: The temperature of the sludge in the first methane fermentation tank is set higher than the temperature of the sludge in the second methane fermentation tank, and the temperature of the sludge in the first methane fermentation tank is set to 37°C or higher and 45°C or lower, and the temperature of the sludge in the second methane fermentation tank is set to 34°C or higher and 40°C or lower. A method for operating a mesophilic methane fermentation treatment device.

13. The method for operating a mesophilic methane fermentation treatment apparatus according to claim 12, When the distance between the liquid level of the sludge in the first methane fermentation tank and the bottom surface of the first methane fermentation tank is L, the sludge in the first methane fermentation tank is extracted from a position 1 / 4 L to 1 / 2 L below the liquid level using the sludge transfer pipe and transferred to the second methane fermentation tank. A method for operating a mesophilic methane fermentation treatment device.

14. A method for operating a mesophilic methane fermentation treatment apparatus according to claim 12 or 13, When foaming of the sludge is detected in the first methane fermentation tank, the temperature of the sludge in the first methane fermentation tank is temporarily increased. A method for operating a mesophilic methane fermentation treatment device.

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