Organic Matter Treatment System

Abstract

To provide an organic matter treatment system capable of keeping the pH of methane fermentation liquid lower than a certain value and enabling more stable and efficient operation than the conventional system.SOLUTION: An organic matter treatment system 1 comprising a methane fermentation tank 10 to which organic matter is supplied, and hydrogen supply means 20 for supplying hydrogen into the methane fermentation tank 10, and configured to perform methanation in the methane fermentation tank 10, is provided with pH measuring means 30 for measuring pH of the methane fermentation liquid in the methane fermentation tank 10. The hydrogen supply means 20 is configured to adjust the hydrogen supply flow rate according to the measured pH measured by the pH measuring means 30, supply hydrogen at a standard flow rate when the measured pH is below a predetermined threshold value, and supply hydrogen at a flow rate less than the standard flow rate when the measured pH is above the threshold value.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an organic matter treatment system that treats organic matter by utilizing methane fermentation. [Background technology]

[0002] There is a known technology for converting organic matter contained in sludge and food waste into biogas using methane fermentation. Biogas obtained through methane fermentation of organic matter mainly contains methane and carbon dioxide. In recent years, development of so-called biomethanation technology, which converts carbon dioxide in biogas into methane using microorganisms, has been progressing with the aim of reducing carbon dioxide emissions and using biogas as city gas.

[0003] As a biomethanation technology, for example, a method described in Patent Document 1 has been proposed. The method described in Patent Document 1 is a so-called in situ biomethanation method in which hydrogen gas is added to the methane fermentation process to increase the methane / carbon dioxide ratio in the biogas produced. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special publication 2019-525888 Summary of the Invention [Problem to be solved by the invention]

[0005] In in situ biomethanation, as the conversion of carbon dioxide to methane by methanation progresses, the carbon dioxide concentration in the fermentation broth decreases and the pH of the fermentation broth increases. The increase in pH of the fermentation broth also increases the free ammonia concentration in the fermentation broth. As a result, the activity of methanogens decreases, making it difficult to convert carbon dioxide to methane by methanation. In other words, in a treatment system that uses in situ biomethanation, if the pH of the methane fermentation broth increases as methanation progresses, the operating efficiency of the system decreases, and ultimately the system itself may cease to function.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and its object is to provide an organic matter treatment system that can maintain the pH of the methane fermentation liquid below a certain value and operate more stably and efficiently than conventional systems. [Means for solving the problem]

[0007] The organic matter treatment system according to the present invention for achieving the above object has the following characteristic configuration: a methane fermentation tank to which organic matter is supplied; a hydrogen supply means for supplying hydrogen into the methane fermentation tank; An organic matter treatment system configured to enable methanation in the methane fermentation tank, a pH measuring means for measuring the pH of the methane fermentation liquid in the methane fermentation tank; The hydrogen supply means is configured to adjust the supply flow rate of the hydrogen in accordance with the measured pH measured by the pH measurement means, and supplies the hydrogen at a standard flow rate when the measured pH is less than a predetermined threshold, and supplies the hydrogen at a flow rate less than the standard flow rate when the measured pH is equal to or greater than the threshold. The standard flow rate is a flow rate necessary and sufficient for converting carbon dioxide into methane. It's at the point.

[0008] According to the above characteristic configuration, when the measured pH is less than the threshold value, hydrogen is supplied to the methane fermentation tank by the hydrogen supply means. , which is the flow rate necessary and sufficient for the conversion of carbon dioxide to methane.Therefore, while methane fermentation is carried out in the methane fermentation liquid in the methane fermentation tank, the carbon dioxide in the biogas generated by the methane fermentation can be methanated by the methanogens. On the other hand, as methanation of carbon dioxide by methanogens progresses, the amount of carbon dioxide in the methane fermentation tank decreases, and the pH of the methane fermentation liquid rises. According to the above-described characteristic configuration, when the measured pH is equal to or higher than the threshold, the hydrogen supply means supplies hydrogen into the methane fermentation tank at a flow rate lower than the standard flow rate. Therefore, while carbon dioxide is generated by methane fermentation in the methane fermentation liquid in the methane fermentation tank, the methanation of carbon dioxide by methanogens is intentionally made more difficult to proceed (the amount of carbon dioxide lost by methanation is intentionally reduced), increasing the amount of carbon dioxide in the methane fermentation tank and lowering the pH of the methane fermentation liquid. That is, according to the above-described characteristic configuration, when the pH of the methane fermentation liquid rises above a threshold value as methanation progresses, the flow rate of hydrogen supplied to the methane fermentation tank is reduced, and the pH of the methane fermentation liquid is lowered to below the threshold value. This makes it possible to maintain the pH of the methane fermentation liquid below a certain value, thereby preventing situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to malfunction. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0009] Further, a further characteristic configuration of the organic matter treatment system according to the present invention is as follows: a recovery means for recovering biogas generated in the methane fermentation tank; a biogas circulation means for supplying at least a portion of the biogas recovered by the recovery means into the methane fermentation tank, The biogas circulation means supplies the biogas at a predetermined flow rate into the methane fermentation tank when the measured pH is equal to or higher than the threshold value, and stops the supply of the biogas when the measured pH is lower than the threshold value.

[0010] If the amount of biogas generated by methane fermentation in the methane fermentation tank is small, simply reducing the flow rate of hydrogen supplied to the methane fermentation tank will take a long time for the carbon dioxide in the methane fermentation tank to increase. However, according to the above-described characteristic configuration, when the measured pH is equal to or higher than the threshold value, at least a portion of the biogas recovered from the methane fermentation tank is supplied to the methane fermentation tank at a predetermined flow rate by the biogas circulation means. Because the biogas contains carbon dioxide, supplying at least a portion of the biogas to the methane fermentation tank results in carbon dioxide being supplied to the methane fermentation tank. Therefore, according to the above-described characteristic configuration, the rate of increase of carbon dioxide in the methane fermentation tank can be increased, and the pH of the methane fermentation liquid can be reduced more quickly, compared to when the flow rate of hydrogen supplied to the methane fermentation tank is simply reduced. Then, when the pH of the methane fermentation liquid decreases and the measured pH becomes less than the threshold value, the supply of biogas by the biogas circulation means is stopped. That is, according to the above-described characteristic configuration, when the pH of the methane fermentation liquid rises above the threshold value as methanation proceeds, the biogas (carbon dioxide) recovered from the methane fermentation tank is supplied to the methane fermentation tank, and the pH of the methane fermentation liquid can be lowered below the threshold value more quickly than when the flow rate of hydrogen supplied to the methane fermentation tank is simply reduced. This makes it easier to maintain the pH of the methane fermentation liquid below a certain value, thereby preventing situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to malfunction. Furthermore, the time during which methanation is stopped can be shortened compared to when the flow rate of hydrogen supplied to the methane fermentation tank is simply reduced. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0011] Further characteristic configurations of the organic matter treatment system according to the present invention include: a biogas supply means for supplying the biogas stored in the storage means into the methane fermentation tank; The biogas supply means supplies the biogas at a predetermined flow rate into the methane fermentation tank when the measured pH is equal to or higher than the threshold value, and stops the supply of the biogas when the measured pH is lower than the threshold value.

[0012] As described above, if the amount of biogas produced by methane fermentation in the methane fermentation tank is small, simply reducing the flow rate of hydrogen supplied to the methane fermentation tank will take a long time to increase the carbon dioxide in the methane fermentation tank. However, according to the above-described characteristic configuration, when the measured pH is equal to or higher than the threshold value, the biogas stored in the storage means is supplied to the methane fermentation tank at a predetermined flow rate by the biogas supply means. As described above, because biogas contains carbon dioxide, supplying the biogas to the methane fermentation tank results in carbon dioxide being supplied to the methane fermentation tank. Therefore, according to the above-described characteristic configuration, the rate of increase of carbon dioxide in the methane fermentation tank can be increased, and the pH of the methane fermentation liquid can be reduced more quickly, compared to when the flow rate of hydrogen supplied to the methane fermentation tank is simply reduced. Then, when the pH of the methane fermentation liquid decreases and the measured pH becomes less than the threshold value, the supply of biogas by the biogas supply means is stopped. That is, according to the above-described characteristic configuration, when the pH of the methane fermentation liquid rises above the threshold value as methanation proceeds, biogas (carbon dioxide) stored in the storage means is supplied to the methane fermentation tank, and the pH of the methane fermentation liquid can be lowered below the threshold value more quickly than if the flow rate of hydrogen supplied to the methane fermentation tank were simply reduced. This makes it easier to maintain the pH of the methane fermentation liquid below a certain value, and more effectively prevents situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to malfunction. Furthermore, the time that methanation is stopped can be shortened compared to if the flow rate of hydrogen supplied to the methane fermentation tank were simply reduced. Therefore, according to the above characteristic configuration, the pH of the methane fermentation liquid can be kept lower than a certain value, enabling more stable and efficient operation than conventional methods.

[0013] Further characteristic configurations of the organic matter treatment system according to the present invention include: The biogas supplying means supplies the biogas into the methane fermentation tank via a hydrogen supply line in the hydrogen supplying means.

[0014] According to the above-described characteristic configuration, there is no need to install a separate biogas supply line to supply biogas to the methane fermentation tank, which avoids an increase in costs and a complicated system.

[0015] Further characteristic configurations of the organic matter treatment system according to the present invention include: The organic matter is contained in sludge.

[0016] The inventors of the present application have confirmed that in treating sludge, the pH of the methane fermentation liquid can be maintained below a certain value, enabling more stable and efficient operation than conventional methods.

[0017] Further characteristic configurations of the organic matter treatment system according to the present invention include: The threshold value is set within the range of pH8 to pH10.

[0018] According to the above characteristic configuration, when the pH of the methane fermentation liquid is around pH 9, at which point the activity of methanogens is likely to decrease, the pH of the methane fermentation liquid can be lowered.

[0019] Further characteristic configurations of the organic matter treatment system according to the present invention include: The hydrogen supply means stops supplying the hydrogen when the measured pH is equal to or greater than the threshold value.

[0020] According to the above characteristic configuration, compared to when the flow rate of hydrogen supplied to the methane fermentation tank is reduced, the methanation of carbon dioxide by methanogens is more difficult to proceed, which increases the rate at which carbon dioxide increases in the methane fermentation tank and allows the pH in the methane fermentation tank to be reduced more quickly. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram showing a schematic configuration of an organic matter treatment system according to a first embodiment. [Figure 2] FIG. 3 is a diagram for explaining switching between supplying and stopping hydrogen depending on pH in the organic matter treatment system according to the first embodiment. [Figure 3] FIG. 10 is a diagram showing a schematic configuration of an organic matter treatment system according to a second embodiment. [Figure 4] FIG. 10 is a diagram for explaining switching between supply and stop of hydrogen and biogas depending on pH in an organic matter treatment system according to a second embodiment. [Figure 5] FIG. 10 is a diagram showing a schematic configuration of an organic matter treatment system according to a third embodiment. [Figure 6] FIG. 10 is a diagram for explaining switching between supply and stop of hydrogen and biogas depending on pH in an organic matter treatment system according to a third embodiment. [Figure 7] 1 is a graph showing the relationship between the amount of biogas injected and pH. DETAILED DESCRIPTION OF THE INVENTION

[0022] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An organic matter treatment system according to an embodiment of the present invention will be described below with reference to the drawings. In the following, an organic matter treatment target is a sludge-containing organic matter.

[0023] [Configuration of the organic matter treatment system according to the first embodiment] First, the configuration of an organic matter treatment system 1 according to the first embodiment will be described. Fig. 1 is a diagram showing a schematic configuration of the organic matter treatment system 1 according to the first embodiment. As shown in Fig. 1, the organic matter treatment system 1 includes a methane fermentation tank 10 to which sludge is supplied, a hydrogen supply unit 20 (an example of a hydrogen supply means) that supplies hydrogen into the methane fermentation tank 10, and a pH measuring device 30 (an example of a pH measuring means) that measures the pH of the methane fermentation liquid in the methane fermentation tank 10.

[0024] The organic matter treatment system 1 also includes a biogas recovery unit 40 (an example of recovery means) that recovers the biogas generated in the methane fermentation tank 10, and a control device (not shown) that controls the operation of each unit.

[0025] As shown in FIG. 1, the methane fermentation tank 10 is composed of a housing. The methane fermentation tank 10 is configured to form a methane fermentation space 11 in which sludge supplied from outside the housing is biodegraded through methane fermentation by methane bacteria. A heat exchanger (not shown) is provided in this methane fermentation space 11, and the methane fermentation liquid in the methane fermentation space 11 is maintained by the heat exchanger at a temperature suitable for efficient methane fermentation (for example, 30 to 37°C or 50 to 60°C). In this embodiment, a sludge supply port 15 for supplying sludge is provided on one of the two opposing inner walls of the housing, facing the methane fermentation space 11. A treated water discharge port 16 for discharging treated water to the outside is provided on the other of the two inner walls, facing the methane fermentation space 11.

[0026] The space above the methane fermentation space 11 (the space above the liquid level of the methane fermentation liquid in the methane fermentation tank 10) forms a biogas collection space 12 that collects biogas such as methane and carbon dioxide produced in the methane fermentation space 11.

[0027] In this embodiment, a hydrogen supply port 23 for supplying hydrogen from the hydrogen supply unit 20 is provided at the bottom of the housing, facing the methane fermentation space 11. In addition, a biogas outlet 43 for extracting biogas to the biogas recovery unit 40 is provided at the inner wall of the housing, on the inner wall where the treated water outlet 16 is provided, facing the biogas collection space 12.

[0028] In this embodiment, the hydrogen supply unit 20 is made up of a hydrogen cylinder 21 in which hydrogen is stored, a hydrogen supply port 23, and a hydrogen supply channel 22 (hydrogen supply line) through which hydrogen flows between the hydrogen cylinder 21 and the hydrogen supply port 23. The hydrogen supply unit 20 supplies hydrogen from the hydrogen cylinder 21 into the methane fermentation space 11 via the hydrogen supply channel 22 and the hydrogen supply port 23. In this embodiment, the operation of an on-off valve (not shown) provided on the hydrogen cylinder 21 can be controlled by a control device. Therefore, the hydrogen supply unit 20 can supply hydrogen to the methane fermentation space 11 in any amount and at any timing. The hydrogen supply unit 20 is not particularly limited as long as it is configured to be able to supply hydrogen to the methane fermentation space 11. For example, a hydrogen production device may be used instead of the hydrogen cylinder 21.

[0029] In this embodiment, the biogas collection unit 40 is made up of a tank 41 connected to a pump (not shown), a biogas outlet 43, and a biogas extraction path 42 through which biogas flows between the tank 41 and the biogas outlet 43. The biogas collection unit 40 sucks biogas from the biogas collection space 12 through the biogas outlet 43 and the biogas extraction path 42 and stores the biogas in the tank 41. In this embodiment, the biogas extraction path 42 is provided with an on-off valve and a flow rate adjustment valve whose operation can be controlled by a control device. Therefore, the biogas collection unit 40 can collect biogas from the biogas collection space 12 in any amount and at any timing. The biogas collection unit 40 is not particularly limited as long as it is configured to collect biogas from the biogas collection space 12.

[0030] The pH measuring device 30 is configured to measure the pH of the methane fermentation liquid in the methane fermentation tank 10 at regular intervals and to be able to transmit the measurement results (measured pH) to the control device.

[0031] In the methane fermentation tank 10 having the above configuration, methane fermentation is carried out in the methane fermentation space 11 to generate biogas, and the carbon dioxide in the biogas is methanated by the methane bacteria in the methane fermentation liquid using the hydrogen supplied into the methane fermentation space 11 (methanation).

[0032] [Treatment flow of the organic matter treatment system according to the first embodiment] Next, we will explain the process of treating sludge using the organic matter treatment system 1 having the above configuration. First, we will explain the effect on the operation of the system of the conversion of carbon dioxide to methane by methanation in the methane fermentation tank 10.

[0033] As the conversion of carbon dioxide to methane by methanation progresses in the methane fermentation tank 10, the carbon dioxide concentration in the methane fermentation liquid decreases, the pH of the methane fermentation liquid increases, and the free ammonia concentration in the methane fermentation liquid increases. Because free ammonia reduces the activity of methanogens, as the conversion of carbon dioxide to methane by methanation progresses, the activity of the methanogens decreases, making it difficult for methanation to proceed. Therefore, if the pH of the methane fermentation liquid increases as methanation progresses, the operating efficiency of the system decreases, and ultimately the system itself may cease to function.

[0034] To enable stable and efficient operation of the organic matter treatment system, it is effective to take measures to adjust the pH of the methane fermentation liquor so as not to generate so much free ammonia that the activity of methanogens is significantly reduced. Specifically, when the pH of the methane fermentation liquor rises, it is effective to reduce the amount of hydrogen supplied to the methane fermentation tank 10 to suppress the reduction in carbon dioxide due to methanation, or to supply carbon dioxide to the methane fermentation tank 10. In this way, the increased pH of the methane fermentation liquor can be lowered, and the reduction in the activity of methanogens can be suppressed.

[0035] Therefore, in the organic matter treatment system 1 according to this embodiment, the pH of the methane fermentation liquid is measured by the pH measuring device 30, and the flow rate of hydrogen supplied by the hydrogen supply unit 20 is adjusted according to the measured pH (measured pH). Specifically, as shown in FIG. 2, when the measured pH is below a threshold value (pH 9 in this example), hydrogen is supplied into the methane fermentation tank 10 at a flow rate (standard flow rate) necessary and sufficient for converting carbon dioxide to methane. On the other hand, when the measured pH is equal to or greater than the threshold value, the supply of hydrogen into the methane fermentation tank 10 is stopped. Furthermore, in this embodiment, if the supply of hydrogen is stopped, the supply of hydrogen at the standard flow rate is resumed when the measured pH subsequently drops to 8.5 or less. In FIG. 2, areas where hydrogen or biogas flows are indicated by thick lines, and areas where they do not flow are indicated by dashed double-dashed lines.

[0036] Therefore, in the organic matter treatment system 1 according to this embodiment, first, sludge is supplied to the methane fermentation tank 10, and hydrogen is supplied at a standard flow rate into the methane fermentation tank 10 by the hydrogen supply unit 20. As a result, the sludge supplied to the methane fermentation tank 10 is fermented into methane fermentation tank 10, generating biogas.

[0037] Next, in the methane fermentation space 11, methanogens in the methane fermentation liquid methanate part of the carbon dioxide in the generated biogas using hydrogen supplied from the hydrogen supply unit 20. The biogas from which part of the carbon dioxide has been methanated is collected in the tank 41 via the biogas outlet 43. Meanwhile, the treated water produced by methane fermentation is discharged to the outside from the treated water outlet 16.

[0038] As methanation progresses in the methane fermentation space 11 and the amount of carbon dioxide contained in the biogas decreases, the pH of the methane fermentation liquid in the methane fermentation tank 10 increases. When the pH measured by the pH meter 30 reaches 9 or higher, the supply of hydrogen by the hydrogen supply unit 20 is stopped.

[0039] As a result, the decrease in carbon dioxide due to methanation stops, and biogas containing carbon dioxide is generated by methane fermentation, so the amount of carbon dioxide in the methane fermentation tank 10 gradually increases and the pH of the methane fermentation liquid gradually decreases.

[0040] Thereafter, when the pH measured by the pH measuring device 30 becomes 8.5 or less, the hydrogen supply unit 20 starts supplying hydrogen at a standard flow rate into the methane fermentation tank 10. This causes the carbon dioxide in the biogas to be methanated again in the methane fermentation space 11.

[0041] Thus, according to the organic matter treatment system 1 of this embodiment, when the pH of the methane fermentation liquid rises as methanation progresses and exceeds a threshold value, the supply of hydrogen to the methane fermentation tank 10 is stopped and the pH of the methane fermentation liquid is lowered to below the threshold value. This makes it possible to maintain the pH of the methane fermentation liquid below a certain value (pH 9 in this example), preventing situations in which an increase in the pH of the methane fermentation liquid reduces the activity of methanogens, reduces operating efficiency, and causes the system itself to stop functioning. Therefore, the organic matter treatment system 1 can be operated stably and efficiently.

[0042] [Configuration of organic matter treatment system according to the second embodiment] Next, the configuration of an organic matter treatment system 50 according to a second embodiment will be described. FIG. 3 is a diagram showing a schematic configuration of the organic matter treatment system 50 according to the second embodiment. As shown in FIG. 3, the organic matter treatment system 50 according to the second embodiment differs from the first embodiment mainly in that it includes a biogas circulation unit 60 (biogas circulation means). The organic matter treatment system 50 according to the second embodiment will be described below, but a description of the same configuration as the organic matter treatment system 1 according to the first embodiment will be omitted.

[0043] As shown in FIG. 3, the organic matter treatment system 50 according to the second embodiment includes a biogas circulation unit 60 that supplies at least a portion of the biogas recovered in the biogas recovery unit 40 into the methane fermentation tank 10.

[0044] In this embodiment, the biogas circulation unit 60 is composed of a biogas supply port 62 and a biogas circulation path 61. One end of the biogas circulation path 61 is connected to the biogas extraction path 42 between the tank 41 and the biogas outlet 43, and the other end is connected to the biogas supply port 62. The biogas supply port 62 is provided at the bottom of the housing so as to face the methane fermentation space 11. The biogas circulation unit 60 supplies a portion of the biogas flowing through the biogas extraction path 42 to the methane fermentation space 11 via the biogas circulation path 61 and the biogas supply port 62. In this embodiment, the operation of an on-off valve (not shown) and a flow rate adjustment valve (not shown) provided in the biogas circulation path 61 can be controlled by a control device. Therefore, the biogas circulation unit 60 can supply at least a portion of the biogas generated in the methane fermentation tank 10 to the methane fermentation space 11 in any amount and at any timing.

[0045] [Treatment flow of the organic matter treatment system according to the second embodiment] Next, the process of treating sludge using the organic matter treatment system 50 having the above configuration will be described.

[0046] As described above, if the pH of the methane fermentation liquid increases as methanation progresses, the operating efficiency of the system will decrease, and ultimately the system itself may cease to function. In order to enable stable and efficient operation of the organic matter treatment system, it is also effective to take measures to supply carbon dioxide into the methane fermentation tank 10.

[0047] Therefore, in this embodiment, not only the supply and stop of hydrogen by the hydrogen supply unit 20 but also the supply and stop of biogas by the biogas circulation unit 60 are switched depending on the pH of the methane fermentation liquid measured by the pH measuring device 30 (measured pH). Specifically, as shown in FIG. 4 , when the measured pH is below a threshold value (pH 9), the hydrogen supply unit 20 supplies hydrogen at a standard flow rate into the methane fermentation tank 10, and the biogas circulation unit 60 stops supplying biogas into the methane fermentation tank 10. On the other hand, when the measured pH is equal to or higher than the threshold value, the supply of hydrogen by the hydrogen supply unit 20 into the methane fermentation tank 10 is stopped, and the biogas circulation unit 60 supplies a portion of the biogas recovered by the biogas recovery unit 40 into the methane fermentation tank 10. Note that in FIG. 4 , areas where hydrogen and biogas flow are indicated by thick lines, and areas where they do not flow are indicated by dashed double-dashed lines.

[0048] Therefore, in the organic matter treatment system 50 according to this embodiment, the supply of biogas into the methane fermentation tank 10 by the biogas circulation unit 60 is stopped, and then sludge and hydrogen are supplied into the methane fermentation tank 10 to start treatment, as in the organic matter treatment system 1 according to the first embodiment.

[0049] Then, when the pH measured by the pH measuring device 30 reaches 9 or higher, the supply of hydrogen by the hydrogen supply unit 20 is stopped, and the supply of biogas into the methane fermentation tank 10 by the biogas circulation unit 60 is started.

[0050] As a result, the reduction of carbon dioxide due to methanation stops, and biogas containing carbon dioxide is generated by methane fermentation. Furthermore, in this embodiment, the biogas (containing carbon dioxide) recovered in the biogas recovery unit 40 is supplied by the biogas circulation unit 60 to the methane fermentation tank 10. Therefore, compared to the case where the supply of hydrogen is simply stopped, the rate at which carbon dioxide increases in the methane fermentation tank 10 is faster, and the pH of the methane fermentation liquid decreases more quickly.

[0051] Thereafter, when the pH measured by the pH measuring device 30 becomes 8.5 or less, the hydrogen supply unit 20 starts supplying hydrogen at a standard flow rate into the methane fermentation tank 10, and the biogas circulation unit 60 stops supplying biogas into the methane fermentation tank 10. This causes the carbon dioxide in the biogas to be methanated again in the methane fermentation space 11.

[0052] As described above, according to the organic matter treatment system 50 of this embodiment, when the pH of the methane fermentation liquor rises above a threshold value due to the progress of methanation, not only is the supply of hydrogen to the methane fermentation tank 10 stopped, but biogas is also supplied to the methane fermentation tank 10. Therefore, the organic matter treatment system 50 can more quickly lower the pH of the methane fermentation liquor to below the threshold value. Therefore, the organic matter treatment system 50 can more easily maintain the pH of the methane fermentation liquor below a certain value than when only stopping the supply of hydrogen to the methane fermentation tank 10, thereby more effectively preventing problems caused by reduced activity of methanogens (such as reduced operating efficiency or the system's own malfunction). Furthermore, the time during which methanation is stopped can be shortened compared to when only stopping the supply of hydrogen to the methane fermentation tank 10. Therefore, the organic matter treatment system 50 can operate more stably and efficiently.

[0053] [Configuration of organic matter treatment system according to the third embodiment] Next, the configuration of an organic matter treatment system 70 according to a third embodiment will be described. FIG. 5 is a diagram showing a schematic configuration of the organic matter treatment system 70 according to the third embodiment. As shown in FIG. 5, the organic matter treatment system 70 according to the third embodiment differs from the first and second embodiments mainly in that it includes a biogas supply unit 80 (biogas supply means). The organic matter treatment system 70 according to the third embodiment will be described below, but descriptions of configurations similar to those of the organic matter treatment systems 1 and 50 according to the first and second embodiments will be omitted.

[0054] As shown in FIG. 5, an organic matter treatment system 70 according to the third embodiment includes a biogas supply unit 80 that supplies biogas stored in a tank 81 into the methane fermentation tank 10.

[0055] In this embodiment, the biogas supply unit 80 is composed of a tank 81 (storage means) and a biogas supply channel 82. One end of the biogas supply channel 82 is connected to the tank 81, and the other end is connected to the hydrogen supply channel 22 between the hydrogen cylinder 21 and the hydrogen supply port 23. The biogas supply unit 80 supplies the biogas stored in the tank 81 into the methane fermentation space 11 via the biogas supply channel 82, the hydrogen supply channel 22, and the hydrogen supply port 23. In this embodiment, the operation of an on-off valve (not shown) and a flow rate adjustment valve (not shown) provided in the biogas supply channel 82 can be controlled by a control device. Therefore, the biogas supply unit 80 can supply the biogas stored in the tank 81 into the methane fermentation space 11 in any amount and at any timing.

[0056] The tank 81 is provided separately from the tank 41 that constitutes the biogas recovery unit 40. In this embodiment, the tank 81 stores in advance biogas that has been separately produced using a methane fermentation apparatus or the like and that has not undergone methanation of carbon dioxide (methanation).

[0057] [Treatment flow of the organic matter treatment system according to the third embodiment] Next, the process of treating sludge using the organic matter treatment system 70 having the above configuration will be described.

[0058] As described above, in order to enable stable and efficient operation of the organic matter treatment system, it is also effective to take measures to supply carbon dioxide into the methane fermentation tank 10.

[0059] Therefore, in this embodiment, not only the supply and stop of hydrogen by the hydrogen supply unit 20 but also the supply and stop of biogas by the biogas supply unit 80 are switched depending on the measured pH. Specifically, as shown in FIG. 6 , when the measured pH is below a threshold value (pH 9), the hydrogen supply unit 20 supplies hydrogen at a standard flow rate into the methane fermentation tank 10, and the biogas supply unit 80 stops supplying biogas into the methane fermentation tank 10. On the other hand, when the measured pH is equal to or higher than the threshold value, the supply of hydrogen by the hydrogen supply unit 20 into the methane fermentation tank 10 is stopped, and the biogas supply unit 80 supplies biogas stored in a tank 81 into the methane fermentation tank 10 at a predetermined flow rate (for example, an amount equivalent to four times the amount of biogas generated in the methane fermentation tank 10). Note that in FIG. 6 , areas where hydrogen and biogas flow are indicated by thick lines, and areas where they do not flow are indicated by two-dot chain lines.

[0060] Therefore, in the organic matter treatment system 70 of this embodiment, the supply of biogas into the methane fermentation tank 10 by the biogas supply unit 80 is stopped, and sludge and hydrogen are supplied into the methane fermentation tank 10 to start treatment, as in the organic matter treatment system 1 of the first embodiment.

[0061] Then, when the measured pH reaches 9 or higher, the supply of hydrogen by the hydrogen supply unit 20 is stopped, and the supply of biogas into the methane fermentation tank 10 by the biogas supply unit 80 is started.

[0062] As a result, as in the second embodiment, the rate of increase in carbon dioxide in the methane fermentation tank 10 increases, and the pH of the methane fermentation liquid decreases more quickly, compared to when the supply of hydrogen is simply stopped. Furthermore, in the second embodiment, because biogas recovered in the biogas recovery unit 40 is used, the amount of biogas that can be supplied by the biogas circulation unit 60 depends on the amount of biogas generated in the methane fermentation tank 10. In contrast, in the present embodiment, biogas stored in advance in the tank 81 is used, so it is possible to supply more biogas (i.e., supply more carbon dioxide) than the amount of biogas generated in the methane fermentation tank 10. Therefore, in the organic matter treatment system 70 of this embodiment, the pH of the methane fermentation liquid decreases more quickly than in the organic matter treatment system 50 of the second embodiment.

[0063] Thereafter, when the measured pH becomes 8.5 or less, the hydrogen supply unit 20 starts supplying hydrogen at a standard flow rate into the methane fermentation tank 10, and the biogas supply unit 80 stops supplying biogas into the methane fermentation tank 10. This causes the carbon dioxide in the biogas to be methanated again in the methane fermentation space 11.

[0064] Thus, in the organic matter treatment system 70 according to this embodiment, when the pH of the methane fermentation liquor rises above a threshold value due to the progress of methanation, not only is the supply of hydrogen to the methane fermentation tank 10 stopped, but biogas is also supplied to the methane fermentation tank 10. Therefore, the organic matter treatment system 70 can more quickly lower the pH of the methane fermentation liquor to below the threshold value. Therefore, the organic matter treatment system 70 can more easily maintain the pH of the methane fermentation liquor below a certain value than when only stopping the supply of hydrogen to the methane fermentation tank 10 or circulating the biogas generated in the methane fermentation tank 10, thereby more effectively preventing problems caused by reduced activity of methanogens (such as reduced operating efficiency or system shutdown). Furthermore, the time during which methanation is stopped can be shortened compared to when only stopping the supply of hydrogen to the methane fermentation tank 10 or circulating the biogas generated in the methane fermentation tank 10. Therefore, the organic matter treatment system 70 can operate more stably and efficiently.

[0065] [Verification of pH decrease] Below, we investigated the pH decrease caused by biogas supply using sludge.

[0066] [pH change due to biogas injection] First, we confirmed how the pH of the sludge changes when biogas is injected into the sludge.

[0067] Specifically, biogas (a simulated gas consisting of 60 vol% methane and 40 vol% carbon dioxide) was injected into 160 ml of sludge at a rate of 10 ml / min, and the pH of the sludge was measured 5, 15, 35, and 65 minutes after the start of injection. The results are summarized in Table 1. Figure 7 shows a graph illustrating the relationship between the amount of biogas injected per unit sludge volume and the change in pH. The dotted line in Figure 7 is an approximation curve.

[0068] [Table 1]

[0069] As is clear from Table 1 and Figure 7, the pH of the sludge decreases as the amount of biogas injected increases. The approximate curve in Figure 7 indicates that in order to decrease the pH of the sludge from the initial value of 9.0 to 8.5, the amount of biogas injected per unit sludge volume needs to be approximately 2.4 m 3 / m 3 It can be seen that it is sufficient for the sludge to become sludge. Note that this verification was conducted over a short period of time, about one hour, and it is thought that the amount of biogas generated by the sludge itself through methane fermentation is negligible. Therefore, it is assumed that the decrease in pH is solely due to the injected biogas.

[0070] [Simulation of the time required for pH reduction] Next, the time required to decrease the pH of the methane fermentation broth from 9.0 to 8.5 was simulated.

[0071] Considering the results obtained in the above [pH change due to biogas injection], in order to decrease the pH of sludge from 9.0 to 8.5, 3 per 2.5m 3 Therefore, the amount of biogas injected is sufficient for 1 m of sludge. 3 per 2.5m 3 The time required for this to occur can be considered to be the time required to lower the pH of the sludge from 9.0 to 8.5.

[0072] Therefore, when methane fermentation is carried out in a methane fermentation tank and the carbon dioxide in the biogas generated by methane fermentation is methanated using hydrogen, we simulated the time required for the pH of the sludge to decrease from 9.0 to 8.5 when the pH reaches 9.0 and the following measures 1 to 3 are taken, respectively, under the following prerequisites 1 to 3. Prerequisite 1: The change in pH depends only on the amount of biogas generated by methane fermentation of the sludge itself or the amount of biogas injected. Prerequisite 2: The amount of biogas generated is 1 m of sludge. 3 per 0.5m3 / day. Prerequisite 3: The amount of hydrogen supplied is 1 m of sludge. 3 per 2.0m 3 / day.

[0073] [Response 1] In response 1, the supply of hydrogen is stopped. In this case, by stopping the supply of hydrogen, 1 m of sludge is stored in the methane fermentation tank. 3 per 0.5m 3 Biogas is generated per day. Therefore, the time required for the pH to decrease from 9.0 to 8.5 is 5 days (= 2.5 m 3 ] / 0.5[m 3 / day]).

[0074] [Response 2] In response 2, the supply of hydrogen is stopped and the entire amount of biogas generated is circulated. In this case, as in response 1, 3 per 0.5m 3 / day of biogas is generated, and the total amount of biogas (0.5m 3 / day), so 1m of sludge is circulated in the methane fermentation tank. 3 per 1.0m 3 Therefore, the time required for the pH to decrease from 9.0 to 8.5 is 2.5 days (= 2.5 [m 3 ] / 1.0[m 3 / day]).

[0075] [Response 3] In Response 3, the supply of hydrogen is stopped and pre-stored biogas is supplied through the hydrogen supply line. In this case, as in Response 1, 3 per 0.5m 3 In the case of this method 3, the amount of biogas supplied does not depend on the amount of biogas generated in the methane fermentation tank 10, and the amount of biogas injected can be set arbitrarily. Therefore, the amount of biogas injected is set to the same amount as the amount of hydrogen supplied, i.e., per m3 of sludge. 3 per 2.0m 3 In this case, 1 m of sludge will be stored in the methane fermentation tank.3 per 2.5m 3 Therefore, the time required for the pH to decrease from 9.0 to 8.5 is 1.0 day (= 2.5 [m 3 ] / 2.5[m 3 / day]).

[0076] As described above, the pH of the methane fermentation liquid can be lowered even if the supply of hydrogen is stopped, as in Response 1. However, by supplying biogas into the methane fermentation tank in addition to stopping the supply of hydrogen, the time required to lower the pH can be shortened. This shortens the time that methanation is stopped, making it easier to maintain the pH below a predetermined value. Furthermore, by using biogas that has been stored in advance, as in Response 3, the amount of biogas supplied can be set regardless of the amount of biogas generated in the methane fermentation tank. Therefore, it is possible to significantly shorten the time required to lower the pH by increasing the amount of biogas supplied. For example, in Response 3 above, the amount of biogas injected is set at 1 m sludge. 3 per 2.0m 3 Although the rate is stated as / day, it may be higher, in which case the time required to lower the pH will be shorter than 1.0 day.

[0077] [Another embodiment] [1] In the above embodiment, the hydrogen supply unit 20 stops supplying hydrogen when the measured pH is equal to or higher than the threshold value. However, this is not limiting and the hydrogen supply unit 20 may supply hydrogen at a flow rate lower than the standard flow rate. Even in this case, the amount of carbon dioxide lost due to methanation is smaller than when hydrogen is supplied at the standard flow rate. Therefore, the carbon dioxide in the methane fermentation tank can be increased, and the pH of the methane fermentation liquid can be lowered.

[0078] [2] In each of the above embodiments, sludge is treated, but the present invention is not limited to this and may be used to treat food waste or the like.

[0079] [3] In the above embodiments, a pH of 9 is used as the threshold value, but this is not limiting and any value may be used, although it is preferable to set a value within the range of pH 8 to pH 10. If the threshold value is set too high, the activity of the methanogens may be too low when the supply of hydrogen is stopped, resulting in very little biogas being generated in the methane fermentation tank. As a result, the rate of increase in carbon dioxide in the methane fermentation tank also slows, and it takes longer to lower the pH of the methane fermentation liquid.

[0080] [4] In the above embodiments, after the hydrogen supply by the hydrogen supply unit 20 is stopped or after the biogas supply by the biogas circulation unit 60 and the biogas supply unit 80 is started, the timing for restarting the hydrogen supply or stopping the biogas supply is set to the point where the measured pH becomes 8.5 or less. However, this is not limited to this. The timing for restarting the hydrogen supply or stopping the biogas supply can be set arbitrarily, but the later the timing for restarting the hydrogen supply or stopping the biogas supply, the longer the time for methanation to be stopped. Therefore, it is preferable to restart the hydrogen supply or stop the biogas supply when the measured pH becomes equal to or less than an arbitrarily set value within the range of 7.5 or more and less than a threshold value.

[0081] [5] In each of the above embodiments, the hydrogen supply port 23 and the biogas supply port 62 are provided at the bottom of the housing constituting the methane fermentation tank 10 so as to face the methane fermentation space 11, but this is not limitative. For example, the hydrogen supply port 23 and the biogas supply port 62 may be provided at the ceiling of the housing so as to face the biogas collection space 12.

[0082] [6] In the second embodiment, the biogas circulation unit 60 supplies a portion of the biogas recovered by the biogas recovery unit 40 into the methane fermentation tank 10, but the present invention is not limited to this. The entire amount of biogas recovered by the biogas recovery unit 40 may be supplied into the methane fermentation tank 10.

[0083] [7] In the second embodiment, the biogas recovered by the biogas recovery unit 40 is supplied to the methane fermentation tank 10 by branching it off from the biogas extraction path 42 before being collected in the tank 41. However, the present invention is not limited to this. The biogas may be once collected in the tank 41 and then supplied to the methane fermentation tank 10.

[0084] [8] In the second embodiment, the other end of the biogas circulation path 61 is connected to the biogas supply port 62, but this is not limiting. The other end of the biogas circulation path 61 may be connected to the hydrogen supply path 22 between the hydrogen cylinder 21 and the hydrogen supply port 23, and biogas may be supplied from the hydrogen supply port 23.

[0085] [9] In the third embodiment, the other end of the biogas supply channel 82 is connected to the hydrogen supply channel 22 between the hydrogen cylinder 21 and the hydrogen supply port 23, and biogas is supplied into the methane fermentation tank 10 through the hydrogen supply port 23. However, the present invention is not limited to this. For example, as in the second embodiment, a biogas supply port may be provided in the housing constituting the methane fermentation tank 10, the other end of the biogas supply channel 82 may be connected to the biogas supply port, and biogas may be supplied into the methane fermentation tank 10 through the biogas supply port.

[0086]

[10] In the third embodiment, the biogas supply unit 80 supplies into the methane fermentation tank 10 an amount of biogas equivalent to four times the amount of biogas generated in the methane fermentation tank 10, but the present invention is not limited to this. The amount of biogas supplied by the biogas supply unit 80 can be set as desired.

[0087] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, as long as no contradictions arise. Furthermore, the embodiments disclosed in this specification are examples, and the embodiments of the present invention are not limited to these, and can be modified as appropriate within the scope that does not deviate from the purpose of the present invention. [Explanation of symbols]

[0088] 1, 50, 70: Organic matter treatment system 10: Methane fermentation tank 20: Hydrogen supply unit (hydrogen supply means) 22: Hydrogen supply line (hydrogen supply line) 30: pH measuring device (pH measuring means) 40: Biogas recovery section (recovery means) 60: Biogas circulation section (biogas circulation means) 80: Biogas supply unit (biogas supply means) 81: Tank (storage means)

Claims

1. a methane fermentation tank to which organic matter is supplied; a hydrogen supply means for supplying hydrogen into the methane fermentation tank; An organic matter treatment system configured to enable methanation in the methane fermentation tank, a pH measuring means for measuring the pH of the methane fermentation liquid in the methane fermentation tank; the hydrogen supply means is configured to adjust the supply flow rate of the hydrogen in accordance with the measured pH measured by the pH measurement means, to supply the hydrogen at a standard flow rate when the measured pH is less than a predetermined threshold, and to supply the hydrogen at a flow rate reduced from the standard flow rate when the measured pH is equal to or greater than the threshold, the standard flow rate being a flow rate necessary and sufficient for converting carbon dioxide to methane.

2. a recovery means for recovering biogas generated in the methane fermentation tank; a biogas circulation means for supplying at least a portion of the biogas recovered by the recovery means into the methane fermentation tank, 2. The organic matter treatment system according to claim 1, wherein the biogas circulation means supplies the biogas to the methane fermentation tank at a predetermined flow rate when the measured pH is equal to or higher than the threshold value, and stops the supply of the biogas when the measured pH is lower than the threshold value.

3. a biogas supply means for supplying the biogas stored in the storage means into the methane fermentation tank; 2. The organic matter treatment system according to claim 1, wherein the biogas supply means supplies the biogas into the methane fermentation tank at a predetermined flow rate when the measured pH is equal to or higher than the threshold value, and stops the supply of the biogas when the measured pH is lower than the threshold value.

4. 4. The organic matter treatment system according to claim 3, wherein the biogas supplying means supplies the biogas into the methane fermentation tank via a hydrogen supply line in the hydrogen supplying means.

5. The organic matter treatment system according to claim 1 , wherein the organic matter is contained in sludge.

6. 2. The organic matter treatment system according to claim 1, wherein the threshold value is set within a range of pH8 to pH10.

7. 7. The organic matter treatment system according to claim 1, wherein the hydrogen supply means stops supplying the hydrogen when the measured pH is equal to or higher than the threshold value.

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