Organic waste methane fermentation treatment method

Abstract

To provide a technique that enables methane fermentation in a small fermentation tank without relying on heating energy for keeping a temperature in the fermentation tank constant.SOLUTION: A treatment method for methane fermentation of organic waste in a fermentation tank having a volume of 300 m3 or less includes: a process 1 in which the organic waste is supplied into the fermentation tank to perform methane fermentation; a process 2 in which a temperature T of a process liquid in the fermentation tank is monitored; a process 3 in which when the temperature T becomes a temperature Td below a prescribed range, a supply amount of the organic waste is derived on the basis of a hydrological retention time HRTA calculated from the temperature Td; and a process 4 in which a supply amount of the organic waste is controlled to the derived amount.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for treating organic waste by methane fermentation in a small fermentation tank. [Background technology]

[0002] Methane fermentation is one of the methods to generate energy from biomass resources. Methane fermentation is a decomposition reaction by microorganisms using waste biomass as a raw material, and the reaction is maintained by maintaining the optimum temperature for the microorganisms (specifically, 37°C for medium temperature and 55°C for high temperature). The methane gas generated by the reaction can be used to generate electricity or heat, and the by-product, the liquid matter after fermentation, can be used as liquid fertilizer or dry fertilizer.

[0003] In the technology of methane fermentation, ideas for improving the efficiency of energy generation have been proposed. For example, Patent Document 1 discloses a method for operating a digestion tank comprising a digestion tank body, an agitation device for agitating organic waste put into the digestion tank body, and a heating device for heating the organic waste, characterized in that when the ratio of the amount of digestion gas generated in the digestion tank body to the amount of organic waste put into the digestion tank body becomes lower than a predetermined value and the temperature difference of the organic waste in the digestion tank body is equal to or greater than a predetermined value, the agitation device is operated to increase the agitation strength of the organic waste. Patent Document 2 also discloses a methane fermentation treatment method in which organic wastewater with a temperature higher than the methane fermentation treatment temperature is used as raw water to be treated, and a part of the raw water to be treated is supplied to the methane fermentation tank through a cooling process, and the remaining part of the raw water to be treated is supplied to the methane fermentation tank without a cooling process, and the temperature of the digested sludge in the methane fermentation tank is maintained at a predetermined temperature. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-171669 A [Patent Document 2] JP 2018-1102 A Summary of the Invention [Problem to be solved by the invention]

[0005] Conventional methane fermentation apparatuses such as those described in Patent Documents 1 and 2 basically include a large fermentation tank. Large fermentation tanks are inherently excellent in temperature maintenance due to their size, and therefore the energy required to maintain the fermentation tank at the optimum temperature for the microorganisms can be fully covered by the energy obtained by fermentation.

[0006] On the other hand, in order to further popularize methane fermentation equipment, it is necessary to develop small-scale equipment (specifically equipment with a capacity of 300 m3) that can be used for processing at small-scale dairy farms, for example. 3 It is desirable to have a fermenter (described below). However, when the fermenter of a methane fermentation apparatus becomes small, its ability to maintain temperature significantly deteriorates. For this reason, it is thought that if a control means based on the premise of keeping the fermenter temperature constant is incorporated into a methane fermentation apparatus equipped with a small fermenter, it will be possible to reproduce temperature phenomena similar to those of a conventional methane fermentation apparatus equipped with a large fermenter. However, if such a control means is incorporated, a large amount of heating energy will be required that is not commensurate with the energy available for the scale of the small fermenter, and a problem will arise in which the essence of the methane fermentation apparatus, which is an energy conversion system, will be compromised.

[0007] Therefore, an object of the present invention is to provide a technique that enables methane fermentation in a small fermenter without relying on heating energy to keep the fermenter temperature constant. [Means for solving the problem]

[0008] As a result of intensive research, the present inventors have found that in a method of treating organic waste in a small fermentation tank, by monitoring at least one of the temperature of the treatment liquid and the gas production rate, and controlling the supply amount of organic waste based on the hydraulic retention time calculated from the monitored value when the monitored value falls below a predetermined range, fermentation can be carried out at a temperature lower than the optimum temperature for the methane fermentation microorganism without relying on heating energy to keep the fermentation tank temperature constant. The present invention was completed based on this finding and through further research.

[0009] That is, the present invention provides the following aspects. Item 1. Capacity is 300m 3 A method for treating organic waste by methane fermentation in a fermentation tank comprising: A step 1 of supplying the organic waste to a fermentation tank and subjecting it to methane fermentation; A step 2 of monitoring the temperature T of the treatment liquid in the fermenter; When the temperature T is below a predetermined range, the hydraulic retention time HRT calculated from the temperature Td is A A step 3 of deriving the supply amount of the organic waste based on and step 4 of controlling the supply amount of the organic waste to the derived amount. Item 2. Capacity is 300m 3 A method for treating organic waste by methane fermentation in a fermentation tank comprising: A step 1 of supplying the organic waste to a fermentation tank and subjecting it to methane fermentation; A step 2 of monitoring the production rate R of methane or biogas produced from the treatment liquid in the fermenter; When the generation rate R is below a specified range, the hydraulic retention time HRT calculated from the generation rate Rd is B A step 3 of deriving the supply amount of the organic waste based on the above. and step 4 of controlling the supply amount of the organic waste to the derived amount. Item 3. The treatment method according to any one of Items 1 to 4, wherein the temperature T when equal to or higher than the predetermined range is 15°C to 35°C. Item 4. In step 4, the organic matter load is 1.8 kg VS / m 3 / day or less. Item 5. The treatment method according to any one of Items 1 to 6, wherein the organic waste comprises animal feces and urine and food waste. Item 6. A reservoir for organic waste; A fermenter for methane fermentation of the organic waste; a pump for adjusting the amount of the organic waste supplied from the reservoir to the fermenter; a first thermometer for measuring the temperature of the treatment liquid in the fermenter and / or a second thermometer for measuring the temperature of the organic waste in the reservoir; When the temperature T of the treatment liquid indicated by the first thermometer or calculated from the second thermometer becomes a temperature Td below a predetermined range, the hydraulic retention time HRT calculated from the temperature Td is A a control unit that derives a supply amount of the organic waste based on the above-mentioned, and controls the pump so that the supply amount of the organic waste becomes the derived amount; A methane fermentation apparatus comprising: Item 7. A reservoir for organic waste; A fermenter for methane fermentation of the organic waste; a pump for adjusting the amount of the organic waste supplied from the reservoir to the fermenter; A pressure gauge for measuring the flow rate of the biogas produced in the fermenter; When the production rate R indicated by the pressure gauge is below a predetermined range, the hydraulic retention time HRT calculated from the production rate Rd is B a control unit that derives the supply amount of the organic waste based on the above-mentioned, and controls the pump so that the supply amount of the organic waste becomes the derived amount; A methane fermentation apparatus comprising: Effect of the Invention

[0010] According to the present invention, methane fermentation in a small fermenter is possible without relying on heating energy to keep the fermenter temperature constant. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of an embodiment of a methane fermentation apparatus of the present invention. [Diagram 2] FIG. 1 is a schematic diagram of an embodiment of a methane fermentation apparatus of the present invention. [Diagram 3] FIG. 1 is a schematic diagram of an embodiment of a methane fermentation apparatus of the present invention. [Figure 4A] FIG. 2 is a schematic diagram of a methane fermentation apparatus used in a preliminary test example. [Figure 4B] 1 shows the effect of total organic load on biogas yield obtained from a preliminary test example. [Diagram 5] Fermenter temperature, hydraulic retention time HRT, organic loading, and biogas production rate during the treatment of the test examples are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [1. Method for treating organic waste using methane fermentation] In the present invention, in a treatment method for methane fermentation of organic waste in a small fermentation tank, at least one of the treatment liquid temperature and the gas production rate is monitored, and when the monitored value falls below a predetermined range, the supply amount of organic waste is controlled based on the hydraulic retention time calculated from the monitored value.

[0013] [1-1. Process 1] The organic waste is fed to a fermenter for methane fermentation. Methane fermentation can be initiated by a conventional method. The capacity of the fermenter used in the method for treating organic waste by methane fermentation of the present invention is 300 m 3 Less than or equal to 250m, preferably 3 Less than 100m, preferably 3 Less than 50m, more preferably 3 Less than 30m, more preferably 30m 3Less than 20m, more preferably 3 Less than 10m, particularly preferably 3 The following can be mentioned. As the methane fermentation device, the device described in "2. Methane fermentation device" below can be used.

[0014] As the organic waste, any material used as biomass can be used without any particular limitation, but preferably includes squeezed juice of animal manure and food residues. Specific examples of squeezed juice of animal manure include liquid obtained by solid-liquid separation of animal manure. The animals may be any mammal, typically livestock, more specifically cows, sheep, horses, etc. Examples of food residues include food waste, more specifically waste discharged from food processing facilities, and food waste generated at farms where methane fermentation equipment is installed (e.g., discarded milk, discarded dairy products, coffee grounds, etc.).

[0015] In a preferred embodiment of the present invention, the temperature of the organic waste is not controlled before it is charged in. Examples of such temperature control include heating the organic waste before it is charged in to a temperature higher than the methane fermentation treatment temperature, or cooling the organic waste that has been heated to a temperature higher than the methane fermentation treatment temperature before it is charged in.

[0016] [1-2. Process 2] During methane fermentation, at least one of the temperature T of the treatment liquid in the fermenter and the production rate R of methane or biogas produced from the treatment liquid in the fermenter is monitored. The temperature T may be monitored based on the actual measured value of the temperature of the fermenter, or based on a temperature converted from the actual measured value of the temperature of the organic waste fed in. The production rate R may be monitored based on the actual measured value of a pressure gauge of the gas generated from the fermenter.

[0017] In the present invention, since at least one of the temperature T and the production rate R is monitored, in a preferred embodiment, monitoring by measuring the organic acid concentration of the treatment liquid is not performed.

[0018] [1-3. Process 3] When the temperature T falls below the specified range, the hydraulic retention time HRT calculated from the temperature Td is A In addition, when the generation rate R is a generation rate Rd below a predetermined range, the hydraulic retention time HRT X The supply amount of the organic waste can be calculated based on the above.

[0019] When the temperature T becomes a temperature Td below a predetermined range, the input amount can be derived from the temperature Td through the HRT as follows.

[0020] Methane or biogas production rate γ(m 3 gas / m 3 The hydraulic retention time (HRT) at which the maximum value of ρ / d is reached is expressed by the following equation (1):

number

[0021] B0 is the methane or biogas yield (m 3 B0 is a value specific to the input raw material, and may be calculated while operating the methane fermentation apparatus, or may be obtained in advance by an experiment using the input raw material. S0 is the organic matter concentration of the input organic waste (kg VS / m 3 )

[0022] k is a kinetic constant represented by the following formula (2).

number

[0023] μ m is the maximum specific growth rate (1 / d) expressed by the following formula (3) at temperature Td (℃).

number

[0024] This allows the calculation of the hydraulic retention time (HRT) at which the methane or biogas velocity γ is maximized at the temperature Td. The obtained HRT is multiplied by a safety factor (usually about 1.2) to obtain the HRT corresponding to the temperature change. A can be calculated.

[0025] The supply of organic waste is the daily input amount F(m 3 / d) and the fermenter volume V (m 3 ) and hydraulic retention time (HRT) A It is derived from the following equation (4).

number

[0026] When the production rate R is below a predetermined range, the input amount can be calculated from the production rate Rd through the HRT in the same manner as described above. Specifically, the production rate Rd and the maximum specific growth rate μ obtained from the temperature Td using the above formula (3) can be calculated as follows: m The HRT at which the methane production rate is at its maximum is calculated using the above formula. This HRT is multiplied by a safety factor (usually around 1.2) to calculate the HRT corresponding to the change in the methane production rate. B can be calculated.

[0027] HRT A and HRT B Either one may be used alone or both may be used in combination. A and HRT B When both are used in combination, the supply amount can be derived based on the average value of both values ​​or the longer value.

[0028] HRT A and HRT B Examples of the period include 10 to 70 days, preferably 15 to 60 days, and more preferably 20 to 55 days.

[0029] [1-4.Process 4] The amount of organic waste supplied to the fermenter is controlled to the amount derived in step 3. In this control, the organic load of the treatment liquid in the fermenter is not particularly limited, but from the viewpoint of suppressing the effect on the treatment speed (specifically, a decrease in the treatment speed) due to a change in the temperature of the treatment liquid (specifically, a decrease in the temperature), the organic load is set to 1.8 kg VS / m 3 It is preferable to control the organic matter load of the treatment liquid in the fermentation tank to less than 0.4 kg VS / m. 3 / day or more, or 0.45kg VS / m 3 / day or more.

[0030] In the present invention, since the supply amount of organic waste is controlled in response to changes in the monitored values, a preferred embodiment does not include a step of controlling the stirring strength (stirring speed) in response to changes in the monitored values.

[0031] [1-5.Other] After step 4 is performed, steps 2 to 4 can be repeated. In the method of the present invention, methane fermentation can be maintained even at a temperature below the optimal temperature of the microorganism for methane fermentation, so that step 2 can include a step of performing methane fermentation at a temperature below the optimal temperature of the microorganism for methane fermentation. Examples of temperatures below the optimal temperature of the microorganism for methane fermentation include 15°C or higher and 35°C or lower, preferably 18°C ​​or higher and lower than 30°C, and more preferably 20°C or higher and 27°C or lower. Examples of the duration of the step of performing methane fermentation at a temperature below the optimal temperature of the microorganism for methane fermentation include 2 to 4 months, and preferably 3 to 4 months.

[0032] [2. Methane fermentation equipment] The methane fermentation apparatus of the present invention is an apparatus capable of carrying out the above-mentioned "1. Method for methane fermentation treatment of organic waste".

[0033] An example of a methane fermentation apparatus of the present invention is shown in Fig. 1. The methane fermentation apparatus 100 shown in Fig. 1 includes a reservoir 3 for organic waste, a fermentation tank 1 (equipped with an agitator 2) for methane fermentation of the organic waste, a pump P (equipped with the reservoir 3) for adjusting the amount of organic waste supplied from the reservoir 3 to the fermentation tank 1, a first thermometer t for measuring the temperature of the treatment liquid in the fermentation tank 1, a second thermometer t for measuring the temperature of the organic waste in the reservoir 3, a pressure gauge m for measuring the flow rate of the biogas generated in the fermentation tank 1, and a control unit 7 for deriving the amount of organic waste supplied and controlling the pump p (equipped with the reservoir 3) so that the amount of organic waste supplied becomes the derived amount. The methane fermentation apparatus 100 also includes a dehydration tank 4 for dehydrating the biogas generated in the fermentation tank 1, a desulfurization tank 5 for desulfurizing the biogas, and a biogas bag 6 for collecting the biogas. The thick solid arrow a indicates the route for introducing organic waste into the fermenter 1, and the dashed arrow b indicates the route for discharging the biogas generated in the fermenter 1 (biogas route).

[0034] When the temperature T of the treatment liquid indicated by the first thermometer t or calculated from the second thermometer t falls below a predetermined range, the control unit 7 of the methane fermentation apparatus 100 calculates the hydraulic retention time HRT A and controls the pump p (provided in the reservoir 3) so that the supply amount of organic waste becomes the calculated amount, and / or, when the production rate R indicated by the pressure gauge m becomes a production rate Rd below a predetermined range, the hydraulic retention time HRT calculated from the production rate Rd. B The control unit 7 calculates the supply amount of organic waste based on the calculated amount and controls the pump p (provided in the reservoir 3) so that the supply amount of organic waste becomes the calculated amount. Details of the contents of the control by the control unit 7 are as described above in "1. Method for treating organic waste by methane fermentation."

[0035] FIG. 2 shows another example of the methane fermentation apparatus of the present invention. The methane fermentation apparatus 101 shown in FIG. 2 further includes a heating unit in addition to the fermenter 1 of the methane fermentation apparatus 100 shown in FIG. 1. Specifically, the fermenter 1 is provided with a hot water coil (not shown), and includes a biomass boiler 8, a hot water tank 9 that contains water to be heated by the biomass boiler, and a pump p that adjusts the amount of hot water from the hot water tank 9 to the hot water coil. The thin solid arrow c indicates the hot water route. The methane fermentation apparatus 101 shown in FIG. 2 further includes a digestive liquid tank 10 that contains the treated liquid (digestive liquid) generated in the fermenter 1 in addition to the fermenter 1 of the methane fermentation apparatus 100 shown in FIG. 1. The two-dot chain arrow d indicates the digestive liquid route.

[0036] Yet another example of the methane fermentation apparatus of the present invention is shown in Fig. 3. The methane fermentation apparatus 102 shown in Fig. 3 has a plurality of fermenters 1 (two in the illustrated embodiment) of the methane fermentation apparatus 101 shown in Fig. 2 connected in series.

[0037] As a modified example, the methane fermentation apparatuses 100 to 102 may lack either the first thermometer t or the second thermometer t. EXAMPLES

[0038] The present invention will be specifically described below by way of examples, but the present invention should not be construed as being limited to the following examples.

[0039] [Preliminary test example] A demonstration test was conducted at a dairy farm using the methane fermentation apparatus shown in Figure 4A. The spherical fermentation tank had a volume of 9.6 m 3 (liquid phase volume 7.9m 3 ), and two units were installed underground in parallel. The fermentation tank was heated by circulating antifreeze liquid heated in a biomass boiler using felled wood and / or waste oil, etc., inside the fermentation tank. The fermentation tank was stirred using a motor. The main substrates used as input materials were dairy cow manure and liquid biomass from dairy processing plants. Discarded milk and food residues were used as secondary substrates. The test period was one year and four months. During the test period, the fermentation tank temperature (oC), organic matter load (kg VS / m 3 / d) and hydraulic retention time HRT (d) were measured, and the biogas production rate (m 3 / m 3 / d), biogas yield (m 3 The HRT was operated for 25 or 50 d. During the study period, the fermentation tank temperature was 21.3-40.0°C, and the organic load was 2.51 kg VS / m 3 / d, the average biogas production rate was 0.21 m 3 / m 3 / d, and the average biogas yield was 0.19 m 3 / kg VS / d. Figure 4B shows the effect of total organic loading on biogas yield. 3 At temperatures above 32°C, the effect of temperature is large. Specifically, the average biogas yield when the fermenter temperature is below 32°C is 0.12 m, a 65% decrease compared to when the fermenter temperature is 32°C or higher. 3 / kg VS / d, while 1.0 kg VS / m 3 When the fermenter temperature was less than 32°C, the effect of temperature was relatively small. Specifically, the average biogas yield when the fermenter temperature was less than 32°C was 0.20 m, a 29% decrease compared to when the fermenter temperature was 32°C or higher. 3 / kg VS / d.

[0040] [Test example] The device in Figure 3 (in this test example, the volume is 9.6 m 3 The two fermentation tanks are referred to as tank A and tank B, respectively.) were used, and organic waste was treated by methane fermentation using a mixture of dairy manure and liquid biomass from dairy processing plants as the main substrate, and waste milk and food waste as the secondary substrate. The test period was from April 29, 2022 to January 29, 2023. The initial hydraulic retention time (HRT) was 25 d. Figure 5 shows the fermentation tank temperature, hydraulic retention time (HRT), organic load, and biogas production rate during the treatment period.

[0041] As shown in [1] of Figure 5, when the biogas production rate falls below the specified range (June 1 to 14, 2022, the average value of the biogas production rate during this period is 0.05 m 3 / m 3 As shown in [2], the hydraulic retention time (HRT) was controlled to 50 d, which was calculated from the biogas production rate that was below the specified range. As shown in [3], the biogas production rate recovered (July 10-24, 2022, the average biogas production rate during this period was 0.19 m 3 / m 3 After that, the heating was stopped and the fermentation temperature was allowed to go by itself, and as a result, methane fermentation was maintained stably for a long period of time, even though the temperature was below the optimum temperature for the microorganisms.

[0042] In this way, in a treatment method in which organic waste is fermented into methane in a small fermentation tank, it was shown that by controlling the amount of organic waste supplied based on the hydraulic retention time (HRT), which is determined based on the fermentation monitor value (in this test example, the biogas production rate), it is possible to maintain methane fermentation without using heating energy to keep the fermentation tank temperature constant. [Explanation of symbols]

[0043] 100 Methane fermentation equipment 101 Methane fermentation equipment 102 Methane fermentation equipment 1 Fermentation tank 2 Mixer 3. Organic waste reservoir 4 Dehydration bottle 5 Desulfurization cylinder 6 Biogas Bags 7 Control section 8. Biomass boiler 9. Hot Water Tank 10 Digestive fluid tank p Pump t Thermometer (first thermometer provided in fermenter 1, second thermometer provided in organic waste reservoir 3) m Pressure Gauge a (thick solid arrow) Organic waste input route b (dashed arrow) Biogas route c (thin solid arrow) Hot water route d (Dash-dotted arrow) Digestive fluid route

Claims

1. Capacity is 300m 3 A method for treating organic waste by methane fermentation in a fermentation tank comprising: A step 1 of supplying the organic waste to a fermenter and fermenting it into methane; A step 2 of monitoring the temperature T of the treatment liquid in the fermenter; When the temperature T becomes a temperature Td below a predetermined range, the hydraulic retention time HRT calculated from the temperature Td A A step 3 of deriving the supply amount of the organic waste based on the above. and step 4 of controlling the supply amount of the organic waste to the derived amount.

2. Capacity is 300m 3 A method for treating organic waste by methane fermentation in a fermentation tank comprising: A step 1 of supplying the organic waste to a fermenter and fermenting it into methane; A step 2 of monitoring the production rate R of methane or biogas produced from the treatment liquid in the fermenter; When the generation rate R is a generation rate Rd below a predetermined range, the hydraulic retention time HRT calculated from the generation rate Rd is B A step 3 of deriving the supply amount of the organic waste based on the above. and step 4 of controlling the supply amount of the organic waste to the derived amount.

3. The treatment method according to claim 1 or 2, wherein the temperature T when the temperature is equal to or higher than the predetermined range is 15°C to 35°C.

4. In step 4, the organic load is set to 1.8 kg VS / m 3 The method according to claim 1 or 2, wherein the concentration of the oxidized carbon dioxide is controlled to less than 1 / day.

5. 3. The method according to claim 1 or 2, wherein the organic waste comprises animal waste juice and food waste.

6. a reservoir for organic waste; A fermenter for methane fermentation of the organic waste; a pump for adjusting the amount of the organic waste supplied from the reservoir to the fermenter; a first thermometer for measuring the temperature of the treatment liquid in the fermenter and / or a second thermometer for measuring the temperature of the organic waste in the reservoir; When the temperature T of the treatment liquid indicated by the first thermometer or calculated from the second thermometer becomes a temperature Td below a predetermined range, the hydraulic retention time HRT calculated from the temperature Td is A a control unit that derives the supply amount of the organic waste based on the above-mentioned, and controls the pump so that the supply amount of the organic waste becomes the derived amount; A methane fermentation apparatus comprising:

7. a reservoir for organic waste; A fermenter for methane fermentation of the organic waste; a pump for adjusting the amount of the organic waste supplied from the reservoir to the fermenter; A pressure gauge for measuring the flow rate of the biogas produced in the fermenter; When the production rate R indicated by the pressure gauge is a production rate Rd below a predetermined range, the hydraulic retention time HRT calculated from the production rate Rd is B a control unit that derives the supply amount of the organic waste based on the above-mentioned, and controls the pump so that the supply amount of the organic waste becomes the derived amount; A methane fermentation apparatus comprising:

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