Methane fermentation method and methane fermentation apparatus

Abstract

To provide a methane fermentation method for generating methane gas from an organic substance that can increase a limit load amount.SOLUTION: In a dry methane fermentation method for fermenting a methane fermentation raw material under anaerobic conditions in a methane fermentation tank 11, magnesium carbonate of 1% or more by mass ratio with the methane fermentation raw material supplied from a raw material storage tank 12 is supplied from an agent addition part 13, these are mixed by a raw material mixer 14, thereby increasing a limit load amount in dry methane fermentation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a methane fermentation method and a methane fermentation apparatus for generating methane gas from organic matter. [Background technology]

[0002] Methane fermentation is a process in which organic matter is decomposed by microorganisms under anaerobic conditions to obtain methane gas. It can produce energy and resources while decomposing and processing organic matter in waste, and is one of the technologies that contribute to the global challenge of achieving "carbon neutrality," which does not increase CO2 in the atmosphere, and to the diversification of energy sources. Methane fermentation is classified into wet type (solids concentration: about 10% or less) and dry type (solids concentration: about 15-40%) depending on the solids concentration of the material to be treated. These methods are similar in that they use anaerobic methanogens to break down organic matter and generate methane gas, but in the dry type, methane is generated in a semi-solid material with low fluidity filled in a fermentation tank, whereas in the wet type, methane is generated in a liquid material in a fermentation tank, and there are considerable differences in the mechanical equipment, so in practical terms, they can be said to be mutually different technologies. Conventional techniques relating to such methane fermentation are disclosed in Patent Documents 1 and 2. Patent Document 1 is a technology related to a dry methane fermentation method, and is said to be compatible with both mesophilic fermentation methods (around 35°C) and thermophilic fermentation methods (around 55°C). Patent Document 2 is considered to be a wet methane fermentation method, since it targets sugar liquid, although it is not specified. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2007-098228 A [Patent Document 2] JP 2011-239715 A Summary of the Invention [Problem to be solved by the invention]

[0004] In methane fermentation, if the treatment conditions fall into a state that reduces the activity of methanogenic bacteria, fermentation is inhibited and eventually stops. Factors that inhibit fermentation include a low pH caused by the accumulation of organic acids (low fatty acids) and ammonia inhibition caused by an excessively high ammonia concentration. If too much organic matter is added, this type of fermentation inhibition occurs, so there is a limit (critical load) to the amount of organic matter that can be added. Patent Document 1 describes reducing the supply rate of organic waste to a fermentation tank in order to prevent ammonia inhibition. Patent Document 2 describes that in methane fermentation of a sugar liquid, the sugar liquid is diluted with water to adjust the carbon concentration of the sugar liquid to 0.15 to 0.3 mol / L and the pH of the sugar liquid during methane fermentation is maintained at 6.4 to 7.0. Both technologies limit the amount of organic matter added so as to prevent fermentation inhibition. Thus, the conventional idea is to limit the amount of organic matter input so as not to inhibit fermentation, but in this case, the amount of methane fermentation raw material (organic matter such as waste) that can be processed is limited, which leads to the problem that the operating efficiency of methane fermentation facilities (biogas plants) cannot be improved.

[0005] In view of the above, the present invention relates to a methane fermentation method for generating methane gas from organic matter, and an object of the present invention is to provide a methane fermentation method capable of increasing the limit load. [Means for solving the problem]

[0006] (Configuration 1) A dry methane fermentation method in which a methane fermentation raw material is fermented under anaerobic conditions, the method comprising mixing 1% or more magnesium carbonate by mass ratio with the methane fermentation raw material to suppress the generation of lower fatty acids.

[0007] (Configuration 2) The methane fermentation method according to configuration 1, characterized in that magnesium carbonate is mixed in an amount of 3% by mass or more of the methane fermentation raw material.

[0008] (Configuration 3) The methane fermentation method according to configuration 1 or 2, characterized in that magnesium carbonate is mixed in an amount of 10% by mass or less of the methane fermentation raw material.

[0009] (Configuration 4) 4. The methane fermentation method according to any one of claims 1 to 3, wherein magnesium carbonate is mixed with the methane fermentation raw material and then the mixture is charged into a fermenter.

[0010] (Configuration 5) This methane fermentation apparatus is used for high-temperature dry methane fermentation and is characterized by comprising: a methane fermentation tank for carrying out fermentation under anaerobic conditions; a raw material storage tank for holding raw material for methane fermentation; a raw material mixing device for mixing the raw material supplied from the raw material storage tank with seed sludge; and an agent addition section for adding magnesium carbonate at a mass ratio of 1% or more to the raw material for methane fermentation. Effect of the Invention

[0011] According to the methane fermentation method of the present invention, it is possible to increase the limit load amount and increase the amount of methane fermentation raw material (organic matter such as waste) that can be treated. [Brief description of the drawings]

[0012] [Figure 1] Schematic diagram of a methane fermentation apparatus according to an embodiment of the present invention. [Diagram 2] Graph showing the results of an experiment on pH buffer capacity (Experiment 1) [Diagram 3] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Figure 4] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Diagram 5] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Figure 6] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Figure 7] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Figure 8] Graph showing the results of an experiment (Experiment 2) on the effect of increasing the limit input load in dry methane fermentation [Figure 9] Graph showing the results of an experiment (Experiment 3) on the effect of material addition on sludge pH [Figure 10] Graph showing the results of an experiment comparing magnesium carbonate and sodium bicarbonate [Figure 11] Graph showing the results of an experiment comparing magnesium carbonate and sodium bicarbonate [Figure 12] Graph showing the results of an experiment comparing magnesium carbonate and sodium bicarbonate [Figure 13] Graph showing the results of an experiment comparing magnesium carbonate and sodium bicarbonate [Figure 14] Graph showing the results of an experiment comparing magnesium carbonate and sodium bicarbonate [Figure 15] A graph showing the change in VFA composition (mass ratio) of digested sludge when the sodium bicarbonate addition rate is changed. [Figure 16] A graph showing the change in VFA composition (mass ratio) of digested sludge when the sodium bicarbonate addition rate is changed. [Figure 17] A graph showing the change in VFA composition (mass ratio) of digested sludge when the sodium bicarbonate addition rate is changed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. Note that the following embodiment is one form for embodying the present invention, and is not intended to limit the scope of the present invention.

[0014] FIG. 1 is a schematic diagram of a methane fermentation apparatus according to an embodiment of the present invention. The methane fermentation apparatus 1 is an apparatus used for high-temperature dry methane fermentation. a methane fermentation tank 11 for carrying out fermentation under anaerobic conditions; A raw material storage tank 12 stores raw materials for methane fermentation, such as food waste, food residue, paper waste (or grass, wood, cellulosic materials, etc.), livestock waste, and sewage sludge, and supplies a predetermined amount of raw materials to a raw material mixer 14; A chemical adding section 13 that holds magnesium carbonate and adds a predetermined amount of it to the raw material; a raw material mixer 14 for mixing the raw material supplied from the raw material storage tank 12, the seed sludge (part of the digested sludge) supplied from the methane fermentation tank 11, and magnesium carbonate supplied from the chemical addition unit 13; It is equipped with: Although not specifically shown, the methane fermentation tank 11, the raw material mixer 14, etc. are provided with a heat retaining device for maintaining the treatment conditions at about 55°C.

[0015] When the methane fermentation apparatus 1 is in operation, high-temperature dry methane fermentation is carried out in the methane fermenter 11, and the biogas (methane gas) generated thereby is collected and digested sludge is discharged. A portion of the digested sludge is transferred to the raw material mixer 14 as seed sludge, and mixed with the methane fermentation raw material supplied from the raw material storage tank 12 and magnesium carbonate added from the chemical addition section 13. The raw materials mixed in the raw material mixer 14 are transferred to the methane fermentation tank 11, where the above-mentioned process is repeated. The high-temperature dry methane fermentation process in the methane fermentation apparatus 1 is conceptually similar to conventional high-temperature dry methane fermentation, including the process of extracting methane gas from the generated gas and the process of treating digested sludge, except for the magnesium carbonate addition process by the chemical addition section 13, so further detailed explanation will be omitted here.

[0016] In high-temperature dry methane fermentation, the accumulation of organic acids (low-methane fatty acids) tends to be a significant inhibitor of fermentation. Methane fermentation is roughly divided into two stages: acid production and methane production. In high-temperature fermentation (around 55°C), acid fermentation in the acid production stage proceeds quickly, and low-methane fatty acids tend to accumulate. The methane fermentation apparatus 1 is configured to mix magnesium carbonate into the methane fermentation raw material in order to suppress the generation of lower fatty acids during high-temperature dry methane fermentation and to increase the limit load in the dry methane fermentation. The magnesium carbonate added from the chemical adding section 13 is added so that the mass ratio of the methane fermentation raw material supplied from the raw material storage tank 12 to the raw material mixer 14 is 1% or more and 10% or less. The metering of the magnesium carbonate to be added may be automatic or manually controlled by the device. An example of automatic control by an apparatus is to provide a raw material measuring unit that measures the amount (weight, volume, etc.) of methane fermentation raw material fed from the raw material storage tank 12 to the raw material mixing device 14, and to provide an MgCO3 measuring unit that measures the amount of magnesium carbonate, calculated directly or indirectly from the raw material, which is 1% or more and 10% or less in mass ratio to the raw material. The addition of magnesium carbonate is not limited to the raw material mixer 14, but may be added to the raw material storage tank 12 or the methane fermentation tank 11, or may be added on the transport path between them. However, since magnesium carbonate is added at a mass ratio relative to the methane fermentation raw material, it is easier to operate by mixing magnesium carbonate with the methane fermentation raw material and then adding this to the fermentation tank.

[0017] Next, various experiments for evaluating the effect of adding magnesium carbonate and examining the amount of addition will be described. <Experiment 1> Experiment 1 was conducted to examine the pH buffering capacity of materials. Purpose of the Test: Verification of pH buffering capacity when a material (reagent) with pH buffering capacity is added to sludge. Test conditions: Digested sludge from high-temperature dry methane fermentation using grass and corn silage as raw materials was diluted 5-fold with distilled water, and 1 ml of 0.1N hydrochloric acid was added dropwise. Three types of additives that have been reported to have buffering properties, magnesium carbonate (MgCO3), calcium carbonate (CaCO3), and phosphate (Na2HPO4·12H2O:KH2PO4=6:1), were added to the digested sludge at a mass ratio of 10%, as well as calcium oxide (CaO) as a control. Experiments were also conducted under no additive conditions. 0.1N hydrochloric acid was also added dropwise to distilled water.

[0018] Test Results: Figure 2 shows the change in pH under each material addition condition. Compared to distilled water, the pH of digested sludge without added materials changed more slowly when hydrochloric acid was added dropwise, especially at around pH 6. The control material, CaO, changed in a similar manner to the no-addition condition. For the three materials for which buffering effects have been reported, the pH changed slowly within a specific pH range, confirming the buffering effect. Of the three materials, magnesium carbonate showed the greatest buffering effect, changing slowly in a pH range closer to 7.

[0019] <Experiment 2> Experiment 2 was conducted to verify the effect of increasing the critical input load by adding magnesium carbonate to the raw material in dry methane fermentation. Purpose of the Test: A continuous methane fermentation (high-temperature dry methane fermentation) experiment was conducted to verify whether or not the addition of magnesium carbonate has an effect of increasing the limit organic matter load and its extent. Test conditions: A 2L glass separable flask was filled with 1.3 kg of the digested sludge described above, which had been acclimated for about one month at a low load using the input raw material, and high-temperature dry methane fermentation was carried out at a fermentation temperature of 55°C using a mixture of food waste and paper waste in a mass ratio of 8:2 as the raw material. The input amount of raw material (input organic matter load) was increased over time from 1.5 g of organic matter per kg of sludge per day (1.5 gVS / kg sludge / d) to 9.5 gVS / kg sludge / d). When the generation of gas (biogas) accompanying fermentation was no longer observed, it was considered that fermentation had been inhibited and stopped, and the organic matter load at that point was taken as the limit input organic matter load. When the raw materials were added, magnesium carbonate (MgCO3) was added at a mass ratio of 0.1, 1, 3, and 10% to the input raw materials, and the limit organic matter load was compared with that when no magnesium carbonate was added.

[0020] Test Results: 3 to 8 are graphs showing the results of Experiment 2, and are as follows: Figure 3: A graph showing the relationship between organic load and gas generation rate under different magnesium carbonate addition conditions. The gas generation rate is the amount generated per amount of organic matter input. Figure 4: Graph showing the change in pH of digested sludge under different magnesium carbonate addition conditions (Figure 4(a)), and graph showing the change in total organic acid concentration (mass ratio) in digested sludge under different magnesium carbonate addition conditions (Figure 4(b)). Figure 5: Graph showing the change in ammonia nitrogen concentration (mass ratio) in digested sludge when the magnesium carbonate addition rate is changed. Figures 6 to 8 are graphs showing the change in VFA composition (mass ratio) of digested sludge when the magnesium carbonate addition rate is changed, with Figure 6(a) showing 0.1% addition, Figure 6(b) showing 1% addition, Figure 7(a) showing 3% addition, Figure 7(b) showing 10% addition, and Figure 8 showing no addition.

[0021] As shown in Figure 3, without magnesium carbonate addition, gas production dropped sharply after the 8th week, which is thought to be when fermentation inhibition occurred. In other words, the input organic matter load of 7.5gVS / kg sludge / d was the limit load. In contrast, it can be seen that the limit load increases when magnesium carbonate is added. Even with an addition rate of 0.1%, an increase in organic load of approximately 7% (0.5 gVS / kg sludge / d) is obtained, and in particular, with addition rates of 1 to 10%, the decrease in the amount of biogas generated is dramatically reduced, making it possible to apply an organic load that is approximately 30% (2 gVS / kg sludge / d) higher than under no addition conditions. As shown in Figure 4, when the organic matter load was 6.5 gVS / kg sludge / d or less (before the 7th week), the sludge pH remained at about 9 under all conditions. However, after this, in the no-addition and 0.1% MgCO3 addition conditions, the VFA concentration rose rapidly around the decrease in gas generation (Figure 4(b)), and the pH fell (Figure 4(a)). With an MgCO3 addition rate of 1% or more, the VFA concentration began to rise at an organic matter load of 7.5 gVS / kg sludge / d or more (after the 8th week), but the change in pH was gradual. When a load of 9.5 gVS / kg sludge / d was applied (after the 14th week), a decrease in pH was observed in the 1% MgCO3 addition group. In addition, as shown in Figure 5, when a load of 9.5 gVS / kg sludge / d was applied (from the 14th week) in the 1% MgCO3 addition area, the ammonia nitrogen concentration, known as an inhibitor of methane fermentation, increased with an increase in the organic matter load, but the increase was more gradual with the addition of MgCO3 compared to the non-addition area. In the areas where 3% or more MgCO3 was added, the increase in ammonia nitrogen concentration was even more gradual.

[0022] When fermentation inhibition occurred without the addition of MgCO3, an increase in the acetic acid concentration was observed just before fermentation stopped (Figure 8), but when fermentation inhibition was alleviated by the addition of MgCO3, the increase in the acetic acid concentration was relatively small, and instead an increase in the isobutyric acid concentration was observed (Figures 6 and 7). From this, it is estimated that when the isobutyric acid concentration is detected to be higher than the acetic acid concentration in digested sludge from dry methane fermentation, MgCO3 was added at a rate of 1% or more. Furthermore, if the isobutyric acid concentration is detected to be higher than the propionic acid concentration in digested sludge from dry methane fermentation, it can be estimated that MgCO3 had been added at 3% or more. The isobutyric acid referred to here is a peak detected at a retention time (tR) of approximately 11.6 minutes when measured using a high performance liquid chromatograph HPLC LC-2000Plus (JASCO Corporation) under the following conditions. Column: Shodex RSpack KC-811 (Showa Denko K.K.) Column temperature: 60℃ Detector: UV-2070 (JASCO Corporation) Detection wavelength: 445nm Mobile phase: 3mM perchloric acid aqueous solution Reaction solution: BTB aqueous solution Flow rate: 1.2mL / min

[0023] As shown in Figure 8, under conditions where no MgCO3 was added, the total amount of low-molecular-weight fatty acids generated in the 9th week exceeded 10,000 mg / kg. However, under conditions where MgCO3 was added, even at 0.1% addition, the total amount of low-molecular-weight fatty acids generated in the 9th week was well below 10,000 mg / kg (Figure 6(a)). It can be seen that under conditions where MgCO3 was added at 1% or more, the amount of low-molecular-weight fatty acids generated was dramatically suppressed (Figures 6(b), 7(a), and 7(b)).

[0024] Furthermore, the magnesium carbonate added in this experiment was confirmed to have a dramatic effect on increasing the limit organic matter load when added at a rate of 1% or more by mass relative to the raw material, and no inhibition of fermentation due to excessive addition was confirmed. Magnesium carbonate is insoluble in water but soluble in acid, so the pH changes depending on the organic acid, and as a result, it falls within the pH range (7-9) for the stable state of methane fermentation, and is thought to have had no effect on fermentation and gas production. In other words, the pH can be expected to be adjusted by simply adding magnesium carbonate at a specified rate, without having to measure the properties of the digested sludge successively. Due to these properties, it is believed that even if the amount of magnesium carbonate added is increased, it will not affect fermentation and gas production, but if the amount added exceeds 10% by mass relative to the raw material, the raw material will lose its fluidity and flexibility and become hard, which may result in adverse effects on the operation of the methane fermentation apparatus. Therefore, it is preferable to add magnesium carbonate in an amount of 10% or less by mass relative to the raw material.

[0025] <Experiment 3> Experiment 3 was conducted to examine the effect of material addition on sludge pH. Purpose of the Test: In order to simply evaluate the effect of the amount of magnesium carbonate added, we investigated the effect on pH when changing the amount of magnesium carbonate added and the organic acid concentration. In particular, we investigated the effect of adding too much. Test conditions: The same digested sludge as in experiment 1 was added with 0.1, 0.5, 1, 3, 5, 10, and 30% by mass of each reagent, and acetic acid equivalent to 5000, 10000, 15000, and 20000 ppm was added to observe the change in pH. The reagents were magnesium carbonate (MgCO3), calcium carbonate (CaCO3), and sodium bicarbonate (NaHCO3), which is used as a pH adjusting material. After adding the materials to the digested sludge, the mixture was diluted 10 times with distilled water and shaken at 200 rpm for 1 hour, after which the pH was measured, and then acetic acid was added and shaken again at 200 rpm for 30 minutes, after which the pH was measured again.

[0026] Test Results: Figure 9 shows the change in pH for each added material and each amount of acetic acid added. For all added materials, the pH decreased as the amount of acetic acid increased. For CaCO3, the change in pH with the amount of acetic acid was similar regardless of the amount of material added, and its buffering effect against acetic acid was small (Figure 9(c)). The more NaHCO3 added, the smaller the change in pH with increasing acetic acid amount, but when the amount added was 5% or less, the buffering effect was small, with the pH falling below 7 at acetic acid amounts of 10,000 ppm or more (Figure 9(b)). Also, although the mechanism is unknown, the more NaHCO3 added, the lower the pH became before the addition of acetic acid. As shown in Figure 9(a), MgCO3 showed the smallest change in pH with the amount of acetic acid. At an addition amount of 1%, the pH remained above 7 up to an acetic acid amount of 10,000 ppm, and at 3% or more, the pH did not fall below 7 even at an acetic acid amount of 20,000 ppm. Furthermore, even when the amount of MgCO3 added was increased to 30%, the sludge pH before the addition of acetic acid did not decrease, indicating that there is no need to adjust the amount added according to the state of the sludge.

[0027] As described above, in the dry methane fermentation method in which the methane fermentation raw material is fermented under anaerobic conditions, the amount of lower fatty acids generated can be suppressed and the limit load can be increased by mixing 1% or more magnesium carbonate in a mass ratio with the methane fermentation raw material. A small amount of magnesium carbonate added in a mass ratio of 1% with respect to the raw material can dramatically increase the limit organic matter input load, which is very useful. Actual biogas plants, which use food waste and paper waste as their main raw materials, operate by treating industrial waste, and the increase in treatment volume due to the improvement of the limit organic matter load is a major benefit in terms of plant operation.

[0028] In addition, magnesium carbonate has an excellent advantage in that its buffering effect eliminates the need to "measure the pH of the sludge every time and adjust the amount of reagent added so that the pH remains at the specified value," thereby simplifying operations. If a strong alkaline material such as sodium hydroxide is added to deal with the drop in pH, the pH will change significantly when the material is added, and the carbon dioxide in the biogas will be absorbed, which may cause negative pressure. If the pressure inside the fermentation tank becomes negative, problems such as the inability to extract biogas will occur. In contrast, due to the buffering effect of magnesium carbonate on the pH, the addition of magnesium carbonate has little effect on the sludge pH (the pH does not become too high), so there is no need for detailed control, such as adjusting the amount added according to the state of the sludge, which can simplify operations.

[0029] As shown in Figure 3, when magnesium carbonate was added at 3% or more, no fermentation inhibition occurred even in the 15th week when the load of 9.5 gVS / kg sludge / d was continued, and as shown in Figure 5, the increase in ammonia nitrogen concentration, which is known as an inhibitor of methane fermentation, is further reduced by adding magnesium carbonate at 3% or more. Therefore, it is more preferable to mix magnesium carbonate at a mass ratio of 3% or more with the methane fermentation raw material. In addition, from the experimental results of magnesium carbonate shown in FIG. 9(a), the addition of 3% or more in mass ratio to the raw material does not cause the pH to fall below 7 even when the amount of acetic acid is 20,000 ppm or more, so from this viewpoint, the addition of 3% or more is more preferable. In Experiment 3, the reagent is added in mass ratio to the digested sludge, and the reagent (MgCO3) is not added in "mass ratio to the raw material" as in Experiment 2, but in actual continuous fermentation, the addition ratio of both is ultimately at the same level. This is because in continuous fermentation, accumulation of the input MgCO3 occurs, so at the point where fermentation inhibition may occur, the concentration of MgCO3 in the digested sludge reaches the same level as the addition ratio of MgCO3 to the raw material. Therefore, the results of Experiment 3 show that the addition of 3% or more MgCO3 in mass ratio to the raw material does not cause the pH to fall below 7 even when the amount of acetic acid is 20,000 ppm or more.

[0030] <Comparative experiment> Next, a comparative experiment was conducted under the same conditions as in Experiment 2 using sodium bicarbonate (NaHCO3), which is used as a pH adjusting material, as a comparison. Purpose of the Test: The usefulness of magnesium carbonate will be examined using sodium bicarbonate as a comparison. Test conditions: The experimental conditions were the same as in Experiment 2, except that sodium bicarbonate was used as an additive for comparison, and it was added at mass ratios of 0.1, 1, 3, and 10% relative to the input raw materials.

[0031] Test Results: 10 to 17 are graphs showing the results of the comparative experiments, and are as follows: Figure 10: A graph showing the relationship between the organic load and the amount of gas generated for magnesium carbonate (results of Experiment 2) and sodium bicarbonate under different addition amounts. The amount of gas generated is the amount generated per amount of organic matter added. Figure 11: A graph showing the change over time in the mass change rate (decrease rate) of the input raw materials under different addition amounts for magnesium carbonate (results of Experiment 2) and sodium bicarbonate. Figure 12: Graph showing the critical organic load at each material addition rate for magnesium carbonate (results of experiment 2) and sodium bicarbonate. Figure 13: A graph showing the change in pH of digested sludge under different addition amounts of magnesium carbonate (results of Experiment 2) and sodium bicarbonate. Figure 14: A graph showing the change in free ammonia concentration in digested sludge under different addition amounts of magnesium carbonate (results of Experiment 2) and sodium bicarbonate. Figures 15 and 16 are graphs showing the change in VFA composition (mass ratio) of digested sludge when the addition rate of sodium bicarbonate is changed, with Figure 15(a) showing 0.1% addition, Figure 15(b) showing 1% addition, Figure 16(a) showing 3% addition, Figure 16(b) showing 10% addition, and Figure 17 showing no addition.

[0032] As shown in FIG. 10, sodium bicarbonate also has the effect of improving the critical organic load, but the critical organic load peaks at an addition rate of 1% and then decreases, resulting in a lower critical organic load at an addition rate of 10% than when no addition was made. As shown in FIG. 11, the mass reduction rate of the input methane fermentation raw material was 40% during the stable period (40% of the input raw material mass was reduced in one week), but the mass reduction rate also decreased when fermentation stopped. As shown in Figure 12, the higher the magnesium carbonate addition rate, the higher the critical load, up to a maximum of 9.5 gVS / kg sludge / d. In contrast, sodium bicarbonate peaked at an addition rate of 1% and then decreased to a maximum of 8.5 gVS / kg sludge / d. With magnesium carbonate, the critical organic matter load improved by up to 30% compared to the no-addition condition, whereas with sodium bicarbonate the improvement was only around 13%.

[0033] As shown in FIG. 13, sodium hydrogen carbonate tended to have a higher pH overall than magnesium carbonate. As with no addition, magnesium carbonate decreased pH under conditions of decreased gas yield, whereas sodium bicarbonate caused pH to remain high and the range of change was small. Because the pH was high, sodium bicarbonate had a high concentration of free ammonia, which inhibits methane fermentation, and this increased rapidly especially when the limit organic matter load was applied (see Figure 14).

[0034] Regarding the organic acid concentrations, as shown in Figures 15 to 17, although the propionic acid concentration was somewhat high, the overall concentration was within the general component range. Furthermore, only when 10% sodium bicarbonate was added did a reaction that appeared to be sodium inhibited occur, and a reaction different from the other conditions was obtained, such as an increase in the malic acid concentration.

[0035] The results of comparative experiments showed that sodium bicarbonate also had a slight inhibition-alleviating effect, and when added at 1%, it had the effect of increasing the limit load by 1.0 gVS / kg sludge / d compared to the no-addition condition (7.5→8.5 gVS / kg sludge / d). However, the effect can only be obtained in a narrow range, with a peak at an addition rate of 1%, and the marginal organic load also decreases as the addition rate is increased. Under conditions other than an addition rate of 1%, the pH remains high as shown in Figure 13, and the free ammonia concentration that inhibits methane fermentation increases as shown in Figure 14. In other words, it is thought that sodium bicarbonate will not be sufficiently effective (and may have the opposite effect in some cases) unless the addition rate is controlled while monitoring the sludge properties (pH, free ammonia concentration, etc.). As with existing technologies, it is shown that the amount of addition must be adjusted according to the sludge pH. In addition to this, sodium bicarbonate shows signs of sodium inhibition (addition rate of 10%) and the free ammonia concentration increases just before fermentation stops, suggesting the possibility that it may promote fermentation inhibition. In contrast, magnesium carbonate has the excellent feature of being more effective in improving the limit organic load (magnesium carbonate improves it by up to about 30%, while sodium bicarbonate improves it by about 13%). Furthermore, compared to the somewhat peaky characteristics of sodium bicarbonate, magnesium carbonate has an extremely high buffering capacity, and the limit load can be increased simply by adding a certain amount or more. In other words, magnesium carbonate has the excellent characteristic of being able to "provide inhibition mitigation effects by adding it at a certain rate, regardless of the sludge condition."

[0036] As described above, according to the present invention, it is possible to improve the operating efficiency of a methane fermentation facility (biogas plant) by using an approach different from conventional approaches. The conventional approach is to maintain the state of methane fermentation by monitoring the properties of the sludge, such as by adding sodium bicarbonate to deal with a drop in pH, and "adjusting" the sludge so that the pH is within the desired range. In contrast, the present invention "suppresses" changes in pH by suppressing the amount of lower fatty acids produced, and this new technical idea can be used to improve the efficiency of operating methane fermentation facilities. [Explanation of symbols]

[0037] 1...Methane fermentation equipment 11...Methane fermentation tank 12...Raw material storage tank 13...Drug addition section 14...Raw material mixing device

Claims

1. A dry methane fermentation method in which a methane fermentation raw material is fermented under anaerobic conditions, A methane fermentation method, comprising mixing magnesium carbonate in an amount of 1% by mass or more with the methane fermentation raw material, thereby suppressing the generation of lower fatty acids.

2. 2. The method for methane fermentation according to claim 1, wherein magnesium carbonate is mixed in an amount of 3% by mass or more of the methane fermentation raw material.

3. 3. The method for methane fermentation according to claim 1, wherein magnesium carbonate is mixed in an amount of 10% by mass or less of the methane fermentation raw material.

4. 4. The method for methane fermentation according to claim 1, wherein magnesium carbonate is mixed with the methane fermentation raw material and then the mixture is charged into the fermentation tank.

5. This is a device used for high-temperature dry methane fermentation. a methane fermentation tank for carrying out fermentation under anaerobic conditions; A raw material storage tank for storing raw materials for methane fermentation; a raw material mixing device for mixing the raw material supplied from the raw material storage tank with seed sludge; A chemical addition unit that adds 1% or more of magnesium carbonate in a mass ratio to the methane fermentation raw material; A methane fermentation apparatus comprising:

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