Methane fermentation treatment method and method for producing methane-containing gas

Abstract

The present invention addresses the problem of providing a technique for improving reaction efficiency in a methane fermentation system using an oil cake.　The present invention provides a methane fermentation treatment method for generating a methane-containing gas by using a fermentation raw material containing an oil cake, the treatment method comprising: a sludge removal step for removing, when the total amount of sludge in a methane fermentation tank is defined as 1, sludge in an amount of more than 0 but less than 1 / 4 of the total amount of sludge per day from the methane fermentation tank; a sodium reduction step for reducing the sodium content in the sludge removed in the sludge removal step to obtain a sodium-reduced sludge; and a sludge-feeding step for feeding the sodium-reduced sludge into the methane fermentation tank.

Description

Methane fermentation treatment method and method for producing methane-containing gas

[0001] This invention relates to a methane fermentation treatment method and a method for producing methane-containing gas.

[0002] Methane fermentation is a reaction in which organic matter is broken down by the metabolic action of anaerobic bacteria, producing biogas (methane, carbon dioxide, etc.), and is widely used in waste treatment and other applications.

[0003] For example, Patent Document 1 proposes an efficient methane fermentation method that does not require recalcitrant solids. This method includes a sludge withdrawal step of withdrawing at least a portion of the sludge obtained from methane fermentation from the methane fermentation tank, a dewatering step of dewatering the sludge withdrawn in the sludge withdrawal step, and a dewatered sludge return step of returning at least a portion of the dewatered sludge obtained in the dewatering step to the methane fermentation tank.

[0004] Japanese Patent Publication No. 2004-017024

[0005] On the other hand, the inventors have found that oil residue is a useful raw material in methane fermentation. Furthermore, the inventors have explored conditions to further improve the reaction efficiency in a methane fermentation system using oil residue.

[0006] This invention has been made in view of the above circumstances, and aims to provide a technology for improving reaction efficiency in a methane fermentation system using oil residue.

[0007] The inventors of the present invention have found that the above problem can be solved by removing sludge from the methane fermentation tank in a methane fermentation system using oil residue under predetermined conditions, and returning it to the reaction system after sodium reduction treatment, thereby completing the present invention. Specifically, the present invention provides the following.

[0008] (1) A methane fermentation treatment method for generating methane-containing gas using fermentation raw materials including oil residue, the treatment method comprising: a sludge removal step of removing sludge from the methane fermentation tank at a rate of more than 0 and less than 1 / 4 of the total amount of sludge in the methane fermentation tank per day, with the total amount of sludge in the methane fermentation tank being 1; a sodium reduction step of reducing the sodium content in the sludge removed in the sludge removal step to obtain sodium-reduced sludge; and a sludge input step of introducing the sodium-reduced sludge into the methane fermentation tank.

[0009] (2) The methane fermentation treatment method according to (1), further comprising an oil residue addition step of adding oil residue to the methane fermentation tank.

[0010] (3) The amount of oil residue added at the start of methane fermentation and the amount of oil residue added in the additional oil residue addition process shall each be equivalent to 1 m³ of total sludge per day in terms of organic matter. ３ (2) The methane fermentation treatment method described in (2), wherein the amount is 20 kg or less.

[0011] (4) The methane fermentation treatment method according to any one of (1) to (3), wherein the method for reducing the sodium content in the sodium reduction step is a method using an inorganic flocculant and / or an organic flocculant.

[0012] (5) A methane fermentation treatment method according to any one of (1) to (4), comprising a gas recovery step for recovering methane-containing gas generated in the methane fermentation tank.

[0013] (6) A method for producing methane-containing gas by methane fermentation treatment using fermentation raw materials including oil residue, the method comprising: a sludge removal step of removing sludge from the methane fermentation tank in an amount greater than 0 and less than 1 / 4 of the total amount of sludge in the methane fermentation tank per day, with the total amount of sludge in the methane fermentation tank being 1; a sodium reduction step of reducing the sodium content in the sludge removed in the sludge removal step to obtain sodium-reduced sludge; a sludge input step of introducing the sodium-reduced sludge into the methane fermentation tank; and a gas recovery step of recovering the methane-containing gas generated in the methane fermentation tank.

[0014] (7) A method for producing methane-containing gas according to (6), comprising an oil residue addition step of adding oil residue to the methane fermentation tank.

[0015] (8) The amount of oil residue added at the start of methane fermentation and the amount of oil residue added in the additional oil residue addition process shall each be equivalent to 1 m³ of total sludge per day in terms of organic matter. ３ A method for producing methane-containing gas as described in (7), wherein the amount is 20 kg or less.

[0016] (9) The method for producing methane-containing gas according to any one of (6) to (8), wherein the method for reducing the sodium content in the sodium reduction step is a method using an inorganic coagulant and / or an organic coagulant.

[0017] According to the present invention, a technology is provided for improving reaction efficiency in a methane fermentation system using oil residue.

[0018] This figure shows the measurement results of the sodium ion concentration in the fermented sludge in the example. This figure shows the measurement results of the cumulative flow rate in the example. This figure shows the measurement results of the sodium ion concentration in the fermented sludge in the example. This figure shows the measurement results of the cumulative flow rate in the example.

[0019] The following describes specific embodiments of the present invention in detail. However, the present invention is not limited in any way to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention. The preferred and more preferred embodiments exemplified below can be used in appropriate combinations with each other, regardless of expressions such as "preferred" and "more preferred." The numerical ranges are illustrative examples, and regardless of expressions such as "preferred" and "more preferred," the upper and lower limits of each range, as well as ranges obtained by appropriately combining these values ​​with the numerical values ​​of the examples, can also be used. Terms such as "contains" or "includes" may be read as "essentially" or "consists only" as appropriate. In the present invention, "A (numerical value) to B (numerical value)" means "A or more and B or less."

[0020] (1) Methane Fermentation Treatment Method A methane fermentation treatment method according to one aspect of the present invention, which generates methane-containing gas using fermentation raw materials including oil residue (hereinafter also referred to as "the treatment method of the present invention") satisfies the following requirements: - A sludge removal step in which, when the total amount of sludge in the methane fermentation tank is set to 1, an amount of sludge that is more than 0 and less than 1 / 4 of the total amount of sludge removed from the methane fermentation tank per day is removed - A sodium reduction step in which the sodium content in the sludge removed in the sludge removal step is reduced to obtain sodium-reduced sludge - A sludge input step in which the sodium-reduced sludge is put into the methane fermentation tank

[0021] As described above, the inventors have found that oil residue is a useful raw material in methane fermentation. As a result of their investigations, the inventors have found that in a methane fermentation system using fermentation raw materials including oil residue, if a predetermined amount of sludge is removed from the methane fermentation tank, and the sludge is subjected to a sodium content reduction treatment before being added to the methane fermentation tank, the methane fermentation efficiency is increased.

[0022] In methane fermentation, sodium in the sludge is known to act as a fermentation inhibitor. Therefore, it was initially thought that the improvement in methane fermentation efficiency described above was simply due to a decrease in the amount of sodium in the reaction system. However, it was found that rapidly reducing the sodium content in the sludge through sodium reduction treatment actually decreased the methane fermentation efficiency. As a result of further investigation by the inventors, it was discovered that treating the sludge in a way that satisfies the above requirements stably increases the methane fermentation efficiency.

[0023] In one aspect of the present invention, methane fermentation is a method of producing methane gas by fermenting a fermentation raw material containing oil residue in a methane fermentation tank using sludge in which the solid matter concentration is typically maintained at 12% by mass or more (preferably 12% to 30% by mass, more preferably 12% to 25% by mass).

[0024] In one embodiment of the present invention, "increased methane fermentation efficiency" includes, in a methane fermentation system using oil residue as a substrate, an increase in the amount of methane-containing gas generated per unit time or the amount of methane gas generated per unit time compared to a case where the requirements of the present invention are not met.

[0025] In one aspect of the present invention, "sludge" means fermented sludge.

[0026] The configuration of the processing method of the present invention will be described in detail below.

[0027] (1-1) Fermentation raw materials: Fermentation raw materials include oil residue.

[0028] "Fermentation raw material" refers to the material used in the processing method of the present invention, and functions as a substrate in methane fermentation.

[0029] (1-1-1) Oil residue "Oil residue" refers to by-products generated during the manufacturing process of oils and fats (especially vegetable oils and fats).

[0030] The form of the oil residue is not particularly limited and includes raw oil residue, dried oil residue, and mixtures thereof. Examples of mixtures of raw oil residue and dried oil residue include intermediates produced in the process of manufacturing dried oil residue from raw oil residue.

[0031] "Raw oil sludge" is a by-product generated in the conventional refining process of vegetable oils and fats. Since raw oil sludge usually contains a large amount of moisture and free fatty acids, it can be highly viscous and pasty. For example, in the refining process of vegetable oils and fats, when phosphoric acid and caustic soda are added to crude vegetable oil and refining (deacidification step) is performed by centrifugation or the like, raw oil sludge is obtained as a residue after removing the refined vegetable oil and fat.

[0032] "Dried oil sludge" is obtained by adding an acid (such as sulfuric acid) to raw oil sludge for neutralization, and then drying the product. Since the fluidity of dried oil sludge is adjusted, it can be easier to handle than raw oil sludge.

[0033] As the oil sludge, those recited in Japanese Unexamined Patent Application Publication No. 2023-150946 may be used.

[0034] (1-1-2) Other Components The fermentation raw material may or may not contain materials other than oil sludge within a range that does not inhibit the effect of the present invention.

[0035] Examples of materials other than oil sludge include any material that functions as a substrate for methane fermentation. Examples of such materials include food waste, residues of food raw materials, soybean hulls, and paper waste.

[0036] In one embodiment of the present invention, the fermentation raw material consists solely of oil sludge.

[0037] (1-2) Fermented Sludge In the present invention, the term "fermented sludge" includes anaerobic sludge capable of promoting methane fermentation of a fermentation raw material. Fermented sludge is also simply referred to as "sludge". The fermented sludge added at the start of methane fermentation is also referred to as "seed sludge".

[0038] The origin of the fermented sludge is not particularly limited, and it may be sludge obtained from a methane fermentation facility, for example.

[0039] In a preferred embodiment of the present invention, the fermented sludge may contain methanogenic archaea (bacteria capable of producing methane and carbon dioxide from acetic acid or hydrogen), other microbial groups capable of decomposing organic substances into acetic acid or hydrogen, and the like. Further, the TS (solid content) of the fermented sludge is preferably 12% by mass or more, more preferably 12% by mass to 30% by mass, and still more preferably 12% by mass to 25% by mass.

[0040] (1-3) Methane Fermentation Methane fermentation is carried out by charging the fermentation raw material containing the aforementioned oil cake and fermented sludge (seed sludge) into a methane fermentation tank.

[0041] The treatment method of the present invention incorporates a sludge removal step, a sodium reduction step, and a sludge charging step, which will be described later, into a methane fermentation system. Except for including these three steps, the treatment method of the present invention can utilize conventionally known conditions and apparatuses for methane fermentation systems.

[0042] The amount of oil cake charged into the methane fermentation tank at the start of methane fermentation (also referred to as "initially charged oil cake" or the like) is not particularly limited, but from the viewpoint of promoting sufficient and appropriate methane fermentation, the amount of initially charged oil cake, in terms of organic matter conversion amount, is per day for 1 m of fermented sludge ３ , preferably an amount of 20 kg or less, more preferably 0.1 kg or more and 20 kg or less, still more preferably 0.1 kg or more and 5 kg or less, and even more preferably 0.1 kg or more and 3 kg or less.

[0043] In the present invention, "per 1 m ３ of fermented sludge" refers to the amount per 1 m ３ of the total amount of fermentation raw material and fermented sludge in the methane fermentation tank. The organic matter conversion amount of oil cake is calculated based on the following formula from the measured values of input amount (g) of oil cake and ignition loss (% by mass) (in accordance with Sewage Test Method 5.1.8). Ignition loss (% by mass) = Evaporation residue (% by mass) − Ignition residue (% by mass) in wet sample Organic matter conversion amount of oil cake (g) = Input amount of oil cake (g) × Ignition loss (% by mass)

[0044] The timing for charging the initial oil cake is not particularly limited, but it is preferable to charge the oil cake after adjusting the temperature of the contents (such as fermented sludge) in the methane fermentation tank to 30°C or higher and 65°C or lower.

[0045] (1-3-1) Methane Fermentation Conditions In methane fermentation, conventionally known methane fermentation conditions and apparatuses can be employed.

[0046] The temperature conditions for methane fermentation are preferably 30°C to 65°C, and more preferably 30°C to 57°C. The "temperature conditions for methane fermentation" may correspond to the temperature conditions for methane fermentation. In this case, the temperature for methane fermentation is preferably 30°C to 65°C, and more preferably 30°C to 55°C. For mesothermal fermentation, the temperature is preferably 30°C to 37°C, and for high-temperature fermentation, it is preferably 50°C to 55°C.

[0047] Methane fermentation is preferably carried out under controlled conditions where oxygen is absent (e.g., anaerobic conditions).

[0048] The duration of methane fermentation is not particularly limited and can be set for any period. A acclimatization period may be provided before full-scale operation, if necessary.

[0049] (1-4) Sludge Removal Process The sludge removal process is a process in which a portion of the sludge is removed from the methane fermentation tank. The amount of sludge removed in the sludge removal process is an amount that is greater than 0 and less than 1 / 4 of the total amount of sludge in the methane fermentation tank per day, when the total amount of sludge in the methane fermentation tank is set to 1. By subjecting the sludge removed in this amount to the sodium reduction treatment described later and then putting it back into the methane fermentation tank, the amount of sodium in the reaction system can be appropriately reduced, and the methane fermentation efficiency can be stably increased.

[0050] The amount of sludge removed in the sludge removal process is, when the total amount of sludge in the methane fermentation tank is set to 1, more than 0 and less than 1 / 4 per day, preferably 1 / 70 or more and less than 1 / 4, preferably 1 / 50 or more and less than 1 / 4, preferably 1 / 40 or more and less than 1 / 4, preferably 1 / 30 or more and less than 1 / 4, preferably 1 / 24 or more and less than 1 / 4, preferably more than 0 and 1 / 6 or less, preferably 1 / 70 or more and 1 / 6 or less, preferably 1 / 50 or more and 1 / 6 or less, preferably 1 / 40 or more and 1 / 6 or less, preferably 1 / 30 or more and 1 / 6 or less, preferably 1 / 24 or more and 1 / 6 or less, preferably more than 0 and 1 / 8 or less, preferably 1 / 70 or more and 1 / 8 or less, preferably 1 / 50 or more and 1 / 8 or less, preferably 1 / 40 or more and 1 / 8 or less, preferably 1 / 30 or more and 1 / 8 or less, preferably 1 / 24 or more and 1 / 8 or less.

[0051] The amount of sludge removed in the sludge removal process is not limited to the number of times, etc., as long as the amount per day meets the above requirements. For example, the following are examples of operations in the sludge removal process: - Remove an amount of sludge that is greater than zero and less than 1 / 4 of the total amount of sludge in one go per day. - Remove an amount of sludge that is greater than zero and less than 1 / 4 of the total amount of sludge in 10 separate steps per day (for example, removing an amount that is 1 / 50 of the total amount of sludge each time, 10 times). - Remove an amount of sludge that is greater than zero and less than 1 / 4 of the total amount of sludge from midnight to midnight the following day (a total of 24 hours).

[0052] In one embodiment of the present invention, "total amount of sludge in the methane fermentation tank" means the total amount of contents (water, solids, slurry, etc.) in the methane fermentation tank.

[0053] The sludge removal step can be performed at any time in the treatment method of the present invention. From the viewpoint of ensuring sufficient and appropriate methane fermentation, the sludge removal step is preferably performed once or more, preferably twice or more, after the start of methane fermentation.

[0054] When removing sludge from the methane fermentation tank, the inside of the methane fermentation tank may be stirred as needed.

[0055] (1-5) Sodium Reduction Process The sodium reduction process is a process in which the sludge removed in the sludge removal process is subjected to a treatment to reduce the sodium content, thereby obtaining sodium-reduced sludge.

[0056] The "sodium" separated in the sodium reduction process includes any sodium compounds or sodium salts.

[0057] The method for reducing the sodium content is not particularly limited, and any method that can separate some or all of the sodium from the sample can be employed. One method for reducing the sodium content is to add an amount of inorganic flocculant (such as polyferric sulfate or ferric chloride) to the sludge, preferably in an amount equivalent to 0.1 to 10% by mass, more preferably 2 to 8% by mass, and even more preferably 4 to 6% by mass relative to the amount of sludge, stir, and then recover the solids. Another method for reducing the sodium content is to add an aqueous solution of organic flocculant (such as a high-molecular-weight polymer) to the sludge, preferably in an amount equivalent to 1 to 10 times the amount of sludge, more preferably 1.5 to 8 times, and even more preferably 2 to 5 times the amount of sludge, stir, and then recover the solids. The above reduction methods may be carried out individually or in combination of two or more. Examples of methods for recovering the solids include centrifugation, filter pressing, and decantation.

[0058] Whether the sodium content in the sludge has been reduced can be determined by measuring the sodium content before and after the reduction treatment. Methods for such measurement include ion electrode methods and flame atomic absorption spectrometry.

[0059] (1-6) Sludge Input Process The sludge input process is the process of inputting the sodium-reduced sludge obtained in the sodium reduction process into the methane fermentation tank.

[0060] The sodium-reduced sludge obtained in the sodium reduction process is either entirely or partially added to the methane fermentation tank. This allows for an appropriate reduction in the sodium content of the methane fermentation tank, thereby improving the efficiency of methane fermentation.

[0061] After introducing the sodium-reduced sludge into the methane fermentation tank, stirring or other methods may be performed as needed.

[0062] (1-7) Method of implementing the sludge removal process, sodium reduction process, and sludge input process. The individual processes of the sludge removal process, sodium reduction process, and sludge input process, as well as the time intervals between them, are not particularly limited. These processes may be carried out continuously or discontinuously.

[0063] When the sludge removal process, sodium reduction process, and sludge input process are carried out continuously, it is preferable to perform all processes without interrupting work between each process.

[0064] When the sludge removal process, sodium reduction process, and sludge input process are performed discontinuously, other processes may be inserted between each process, or the work may be temporarily stopped at the end of each process before being resumed.

[0065] (1-8) Additional oil residue addition step The processing method of the present invention may optionally include an additional oil residue addition step. The additional oil residue addition step is a step of adding more oil residue to the methane fermentation tank after the start of methane fermentation.

[0066] The amount of oil residue added in the oil residue addition process is not particularly limited and can be adjusted as appropriate depending on the progress of methane fermentation. From the viewpoint of ensuring sufficient and appropriate methane fermentation, the amount of oil residue added in the oil residue addition process should be, in terms of organic matter equivalent, 1 m³ of fermented sludge per day. ３ The amount is preferably 20 kg or less, more preferably 0.1 kg to 20 kg, even more preferably 0.1 kg to 5 kg, and even more preferably 0.1 kg to 3 kg.

[0067] In addition, in this invention, "fermented sludge 1 m ３ "Quantity relative to" refers to the total amount of fermentation raw materials and fermentation sludge in the methane fermentation tank. ３ This refers to the amount relative to [amount]. The organic matter equivalent amount of oil slag is calculated based on the following formula using the amount of oil slag added (g) and the measured value of loss on ignition (mass%) (according to Sewerage Testing Method 5.1.8): Loss on ignition (mass%) = Evaporation residue (mass%) - Residue on ignition in wet sample (mass%) Organic matter equivalent amount of oil slag (g) = Amount of oil slag added (g) × Loss on ignition (mass%)

[0068] The frequency of adding oil residue is not particularly limited. The oil residue may be added once a day, multiple times a day, or continuously, after adjusting the amount of oil residue added per day (in terms of organic matter) so as not to exceed the amount described above.

[0069] In the additional oil cake feeding step, not only oil cake but also other components (for example, those that can be used as a fermentation raw material for methane fermentation) may be added within a range that does not inhibit the effect of the present invention.

[0070] (1-9) Step of generating methane-containing gas and gas recovery step As methane fermentation progresses, for example, the following reactions proceed to generate methane-containing gas. According to a preferred embodiment of the present invention, methane-containing gas with high methane concentration can be efficiently generated. 4H ２ +CO ２ →CH ４ +2H ２ O CH ３ COOH→CH ４ +CO ２

[0071] The "methane-containing gas" is a gas containing at least methane (CH ４ ). For example, the methane-containing gas obtained by the treatment method of the present invention may be a gas containing 50% by volume or more, more preferably 55% by volume or more and 85% by volume or less, even more preferably 60% by volume or more and 85% by volume or less, particularly preferably 70% by volume or more and 85% by volume or less of methane, relative to the total gas generated by methane fermentation.

[0072] In addition to methane, the methane-containing gas may contain carbon dioxide, ammonia, hydrogen sulfide and the like.

[0073] The gas generation amount in methane fermentation can be specified by measuring the gas generation amount and the unit gas generation amount by the method shown in the examples.

[0074] The methane-containing gas generated in the methane fermentation system may be recovered from the methane fermentation tank as needed. Such a step is also referred to as a "gas recovery step".

[0075] The method for recovering the methane-containing gas is not particularly limited, and examples thereof include a method in which a gas-solid-liquid separator is provided at the upper part of a tank for performing methane fermentation (fermentation tank) to separate and recover the methane-containing gas.

[0076] If necessary, the recovered methane-containing gas may have its sulfur content removed by a desulfurization device and then stored in a gas holder.

[0077] The recovered methane-containing gas may be used as is or subjected to further treatment. Treatment of methane-containing gas includes methane concentration. Methods for concentrating methane gas include washing the methane-containing gas with water and selectively separating methane by utilizing the difference in solubility in water.

[0078] The recovered methane-containing gas can be used as fuel for power generation, boilers, and other purposes.

[0079] (2) Method for producing methane-containing gas A method for producing methane-containing gas by methane fermentation treatment using fermentation raw materials including oil residue according to one aspect of the present invention (hereinafter also referred to as "the method of production of the present invention") includes the following steps: - A sludge removal step in which, when the total amount of sludge in the methane fermentation tank is set to 1, an amount of sludge that is more than 0 and less than 1 / 4 of the total amount of sludge per day is removed from the methane fermentation tank. - A sodium reduction step in which the sodium content in the sludge removed in the sludge removal step is reduced to obtain sodium-reduced sludge. - A sludge input step in which the sodium-reduced sludge is put into the methane fermentation tank. - A gas recovery step in which the methane-containing gas generated in the methane fermentation treatment is recovered.

[0080] Each component in the manufacturing method of the present invention can be the same as that in the processing method of the present invention described above.

[0081] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0082] <Test 1: Methane Fermentation Treatment Test Using Oil Launder as Fermentation Raw Material (Sodium Reduction Treatment Once per Cycle)> It is known that sodium in sludge acts as a fermentation inhibitor in methane fermentation. Therefore, a test was conducted to confirm whether sodium in sludge acts as a fermentation inhibitor in methane fermentation using oil launder as the fermentation raw material. Normally, when a fermentation raw material containing sodium is added to seed sludge, the amount of sodium in the fermented sludge increases with each addition, inhibiting methane fermentation and reducing gas production. Therefore, in this test, we investigated whether gas production could be increased by washing a portion of the fermented sludge to reduce the amount of sodium. One cycle was 7 days, and the details of the test cycle will be described later.

[0083] The fermentation raw material (consisting solely of oil residue) was subjected to methane fermentation using the method described below, and the amount of gas produced was measured. Tables 1 and 2 show the test details for each test day.

[0084] (1) The following four test sections were established based on the difference in the amount of sludge removed from the methane fermentation tank. In this test, "total sludge amount" refers to the total amount of contents in the methane fermentation tank. ・Test section 1: No sludge is removed (control section). ・Test section 2: 1 / 10 of the total sludge amount in the methane fermentation tank is removed. ・Test section 3: 1 / 6 of the total sludge amount in the methane fermentation tank is removed. ・Test section 4: 1 / 4 of the total sludge amount in the methane fermentation tank is removed.

[0085] (2) Preparation of raw materials for fermentation In this example, raw oil residue was used as the raw material for fermentation. The raw oil residue used was generated in the refining process (deacidification treatment) of vegetable oils manufactured by Nisshin Oillio Group Ltd. in accordance with Japanese Patent Application Publication No. 2023-150946 (soybean raw oil residue generated in the soybean oil manufacturing process: rapeseed raw oil residue generated in the rapeseed oil manufacturing process = 1:1).

[0086] The sodium (sodium ion) content in the raw oil residue used in this example was approximately 4.05% by mass. The sodium (sodium ion) content was measured using flame atomic absorption spectrometry.

[0087] (3) Preparation of seed sludge (fermented sludge) Seed sludge capable of methane fermentation using food waste as the main fermentation raw material was obtained at a methane fermentation facility and used in this example. This seed sludge contains methanogenic archaea (which can produce methane and carbon dioxide from acetic acid or hydrogen) and other microorganisms that can decompose organic matter into acetic acid or hydrogen.

[0088] At the time the seed sludge was obtained from the methane fermentation facility, its sodium (sodium ion) content was approximately 1948 mg / kg. This seed sludge was cultured for about 7 months to increase its sodium (sodium ion) content to 15436 mg / kg, and then subjected to the following processes.

[0089] (4) Preparation of seed sludge (fermented sludge) Seed sludge was prepared in the methane fermentation tank, and the total amount of seed sludge prepared was measured by subtracting the mass of the fermentation tank from the mass of the fermentation tank after the preparation of the seed sludge.

[0090] (5) Methane fermentation Four methane fermentation tanks (capacity: 500 mL) were prepared, and cultivation was carried out in each test plot under the same conditions except for the amount of sodium reduction treatment. In this example, the solid content concentration of the methane fermentation tanks was maintained at 12% to 30% by mass.

[0091] On the first day of the experiment (start of the experiment), seed sludge (250 g) and oil residue were added to each methane fermentation tank. After adding the oil residue, the fermentation sludge in the methane fermentation tank was stirred for 1 minute. In this experiment, no additional seed sludge was added after the initial addition. Each methane fermentation tank was placed in a constant temperature bath, and the temperature inside the methane fermentation tank was adjusted to 50°C, and this temperature was maintained throughout all processes in this example. Note that if the amount of fermentation sludge in the methane fermentation tank decreases due to multiple sodium reduction treatments (described later), new seed sludge can be added.

[0092] (6) Sludge Removal Process Every 24 hours after the start of the test (around 13:00), a portion of the contents (sludge) was removed from each methane fermentation tank. In this test, the sludge removal process was performed once per cycle. The contents were simply scooped up as is from a portion of the contents of the methane fermentation tank. However, the sludge removal process was not performed in "Test Section 1" (control section).

[0093] (7) Sodium Reduction Process In the sludge removal process, each sludge sample was promptly treated to reduce its sodium content, thereby obtaining sodium-reduced sludge. Specifically, an amount of ferric polysulfate equivalent to 4-6% by mass of the sludge volume and a high-molecular-weight polymer equivalent to twice the volume of the sludge were added to the sludge. After stirring, the mixture was centrifuged, and only the solids were recovered. During centrifugation, most of the sodium migrated to the aqueous phase, so the resulting solids corresponded to sodium-reduced sludge, which had a reduced sodium content.

[0094] Furthermore, the above treatment removed approximately 67% of the sodium contained in the sludge before treatment. However, the sodium reduction process was not performed in "Test Section 1" (control section).

[0095] The details of the treatment agents used in the sodium reduction process are as follows: • Ferric polysulfate (inorganic flocculant): Manufactured by Nankai Chemical Co., Ltd. • High-molecular-weight polymer (organic flocculant): 0.1% aqueous solution of product "CS-80HH" (manufactured by eneco Co., Ltd.)

[0096] (8) Sludge input process: The solids obtained in the sodium reduction process (sodium-reduced sludge) were immediately and completely added to the methane fermentation tank. However, the sludge input process was not performed in "Test Section 1" (control section).

[0097] (9) Oil residue additional input process From the second day onward of the test, oil residue was added to each methane fermentation tank after the sludge input process. After the addition, the fermentation sludge in the methane fermentation tank was stirred for 1 minute. Stirring of the fermentation sludge was not performed at any other time. The oil residue was added at the timings shown in Tables 1 and 2. The amount of oil residue added each time was set to an amount that resulted in an organic matter load of 0.23 to 1.40 kg-VS / t / day. It is preferable to adjust the amount of oil residue added each day by increasing or decreasing it while checking the amount of biogas generated, in order to increase the amount of biogas generated.

[0098] When adding the oil residue, the lid of the methane fermentation tank was opened, and after the addition, nitrogen purging was performed to maintain anaerobic conditions inside.

[0099] (10) Work Content For each test plot, the work content (cycle) was set as follows: ・Test plots 1 to 3: As described in Tables 1 to 2, the work from the first day of the test (fermentation sludge preparation) to the seventh day was set as "1 cycle (days 1 to 7)", and 10 cycles were performed from the eighth day onward (a total of 11 cycles were carried out). However, on the first day of the second cycle onward, the work was slightly different from the first day of the first cycle (fermentation sludge preparation day). ・Test plot 4: In test plot 4, the cumulative flow rate of methane fermentation was less than that of the control (test plot 1) at the end of the seventh cycle, so the test was terminated at that point.

[0100] (11) Measurement of sodium (sodium ion) content The sodium (sodium ion) content in the fermented sludge at each time point from the start of the test was measured using the ion electrode method. "LAQUA twin-Na-11" (manufactured by HORIBA) was used for the measurement. The results are shown in Figure 1.

[0101] (12) Measurement of gas generation The cumulative flow rate (NmL) of gas generated in the methane fermentation tank from the start of the test to the time of measurement was measured every 24 hours (around 13:00) using a wet gas flow meter and is shown in Figure 2.

[0102] The cumulative flow rate (NmL) measured by the wet gas flow meter includes the amount of nitrogen released during nitrogen purging into the methane fermentation tank. Therefore, the amount of nitrogen-containing gas generated during one cycle (described later) is calculated from this cumulative flow rate, and the amount of nitrogen is subtracted from this value to obtain the gas generation amount (NmL), which is shown in Tables 3 to 6 under "Gas Generation Amount". The gas generation amount calculated as described above serves as an indicator of the reaction efficiency of the methane fermentation system; a higher gas generation amount indicates better reaction efficiency. Furthermore, this gas generation amount corresponds to the amount of methane-containing gas.

[0103] Tables 3-6 show the "Gas Generation Intensity (NL / kg-VS)," which is the amount of gas generated per 1 kg of organic matter in the oil residue added during one cycle.

[0104]

[0105]

[0106] (13) Results The fermentation data for each test plot are shown in Tables 3 to 6 and Figures 1 and 2.

[0107] The following is an explanation of each measurement item and its value in Tables 3 to 6. • Oil residue input (g): This is the mass of oil residue added to the methane fermentation tank, and the total amount of oil residue added during one cycle is recorded. • Organic matter input (g): This is the mass of organic matter added to the methane fermentation tank, and the total amount of organic matter added during one cycle is recorded. • Organic matter load (kg-VS / t / d): This is the amount of organic matter added per day (per cycle) per ton of fermentation sludge during one cycle. • Total fermentation sludge (g): This is the total amount of fermentation sludge, and the average value of the measurements taken during one cycle is recorded. • Sodium concentration (mg / kg): This is the sodium ion concentration in the fermentation sludge, and the measurement value on the first day of each cycle is recorded. • Gas generation amount (NmL): The amount of nitrogen-containing gas generated during one cycle is calculated from the cumulative flow rate, and the total amount of gas generated during one cycle is recorded by subtracting the amount of nitrogen from that value. - Cumulative gas flow rate: The cumulative gas flow rate (NmL) generated in the methane fermentation tank from the start of the test to the time of measurement was measured using a wet gas flow meter. The measured value obtained here is recorded in the column for the cumulative gas flow rate of the previous cycle. - Gas generation rate per unit (NL / kg-VS): The calculated value of the amount of gas generated per 1 kg of organic matter in the oil residue added during one cycle is recorded.

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[0112] The results of Test 1 revealed the following: First, when the fermentation sludge in the methane fermentation tank was not subjected to sodium reduction treatment, the amount of sodium in the fermentation sludge increased (Test Section 1). On the other hand, when sodium reduction treatment was performed, the amount of sodium reduction treatment applied to the fermentation sludge decreased as the amount of sodium in the fermentation sludge decreased (Test Sections 2-4).

[0113] In methane fermentation, sodium in the sludge is known to act as a fermentation inhibitor. Therefore, it was expected that test plot 4, which had the greatest reduction in sodium content, would produce the most gas. However, surprisingly, despite having a lower sodium content than test plot 1 (control), test plot 4 produced less gas and had lower reaction efficiency than test plot 1. Gas production is an indicator of the reaction efficiency of the methane fermentation system; a higher gas production indicates better reaction efficiency. The reason for the low reaction efficiency in test plot 4 is unclear, but it is presumed to be because the amount of sodium reduction treatment performed at one time (1 / 4 of the fermented sludge) was larger compared to the other test plots.

[0114] In contrast, in both test plots 2 and 3, the amount of sodium in the fermented sludge decreased, and the amount of gas generated was greater than in test plot 1 (control), indicating improved reaction efficiency.

[0115] These findings indicate that if the amount of sodium reduction treatment performed at one time is increased to 1 / 4 of the amount of fermented sludge, in order to reduce the amount of sodium, which is a factor inhibiting fermentation in fermented sludge, the amount of gas generated decreases and the reaction efficiency declines. On the other hand, it was found that if 1 / 10 or 1 / 6 of the fermented sludge in the methane fermentation tank is treated with sodium reduction treatment and then reintroduced into the methane fermentation tank, the reaction efficiency of the methane fermentation system can be stably increased.

[0116] <Test 2: Methane fermentation treatment test using oil residue as a fermentation raw material (test with increased number of sodium reduction treatments)> In this test, we investigated how the decrease in sodium content in the sludge and the amount of gas generated changed when the interval between sodium reduction treatments was shortened compared to Test 1. Unless otherwise specified, the same conditions as in Test 1 were used below.

[0117] The oil residue was subjected to methane fermentation using the method described below, and the amount of gas produced was measured. Tables 7 to 9 show the work performed on each test day.

[0118] (1) The following seven test sections were established based on the difference in the amount of sludge removed from the methane fermentation tank in each test section: - Test section 1: No sludge is removed (control section). - Test section 2: 1 / 24 of the total amount of sludge in the methane fermentation tank is removed. - Test section 3: 1 / 16 of the total amount of sludge in the methane fermentation tank is removed. - Test section 4: 1 / 12 of the total amount of sludge in the methane fermentation tank is removed. - Test section 5: 1 / 8 of the total amount of sludge in the methane fermentation tank is removed. - Test section 6: 1 / 4 of the total amount of sludge in the methane fermentation tank is removed. - Test section 7: The entire amount of sludge in the methane fermentation tank is removed.

[0119] (2) Preparation of raw materials for fermentation In this example, raw oil residue was used as the raw material for fermentation. The raw oil residue used was generated in the refining process (deacidification treatment) of vegetable oils manufactured by Nisshin Oillio Group Ltd. in accordance with Japanese Patent Application Publication No. 2023-150946 (soybean raw oil residue generated in the soybean oil manufacturing process: rapeseed raw oil residue generated in the rapeseed oil manufacturing process = 1:1).

[0120] The sodium (sodium ion) content in the raw oil residue used in this example was approximately 4.05% by mass.

[0121] (3) Preparation of seed sludge (fermented sludge) Seed sludge capable of methane fermentation using food waste as the main fermentation raw material was obtained at a methane fermentation facility and used in this example. This seed sludge contains methanogenic archaea (which can produce methane and carbon dioxide from acetic acid or hydrogen) and other microorganisms that can decompose organic matter into acetic acid or hydrogen.

[0122] At the time of obtaining the seed sludge, its sodium (sodium ion) content was approximately 1948 mg / kg. This seed sludge was cultured for approximately 12 months to increase its sodium (sodium ion) content to 11501 mg / kg, and then subjected to the following processes.

[0123] (4) Preparation of seed sludge (fermented sludge) The above seed sludge was prepared in a methane fermentation tank (500 mL), and the total amount of seed sludge prepared was measured by subtracting the mass of the fermentation tank from the mass of the fermentation tank after the preparation of the seed sludge.

[0124] (5) Methane fermentation Two methane fermentation tanks (capacity: 500 mL) were prepared, and cultivation was carried out in each test plot under the same conditions except for the amount of sodium reduction treatment. In this example, the solid content concentration of the methane fermentation tank was maintained at 12% by mass to 30% by mass.

[0125] On the first day of the experiment (start of the experiment), seed sludge (250 g) and oil residue were added to each methane fermentation tank. After adding the oil residue, the fermentation sludge in the methane fermentation tank was stirred for 1 minute. In this experiment, no additional seed sludge was added after the initial addition. Each methane fermentation tank was placed in a constant temperature bath, and the temperature inside the methane fermentation tank was adjusted to 50°C, and this temperature was maintained throughout all processes in this example. If the amount of fermentation sludge in the methane fermentation tank decreased due to multiple sodium reduction treatments (described later), new seed sludge was added.

[0126] (6) Sludge Removal Process Every 24 hours after the start of the test (around 13:00), a portion of the contents (sludge) was removed from each methane fermentation tank. In this test, the sludge removal process was carried out at the frequencies listed in Tables 7 to 9. Here, the contents were simply scooped up as a portion of the contents of the methane fermentation tank. However, the sludge removal process was not carried out in "Test Section 1" (control section).

[0127] (7) Sodium Reduction Process In the sludge removal process, each sludge sample was promptly treated to reduce its sodium content, thereby obtaining sodium-reduced sludge. Specifically, an amount of ferric polysulfate equivalent to 4-6% by mass of the sludge volume and a high-molecular-weight polymer equivalent to twice the volume of the sludge were added to the sludge. After stirring, the mixture was centrifuged, and only the solids were recovered. During centrifugation, most of the sodium migrated to the aqueous phase, so the resulting solids corresponded to sodium-reduced sludge, which had a reduced sodium content.

[0128] Furthermore, the above treatment removed approximately 67% of the sodium contained in the sludge before treatment. However, the sodium reduction process was not performed in "Test Section 1" (control section).

[0129] The details of the treatment agents used in the sodium reduction process are as follows: • Ferric polysulfate (inorganic flocculant): Manufactured by Nankai Chemical Co., Ltd. • High-molecular-weight polymer (organic flocculant): 0.1% aqueous solution of product "CS-80HH" (manufactured by eneco Co., Ltd.)

[0130] (8) Sludge input process: The solids obtained in the sodium reduction process (sodium-reduced sludge) were immediately and completely added to the methane fermentation tank. However, the sludge input process was not performed in "Test Section 1" (control section).

[0131] (9) From the second day of the oil residue addition process test onward, oil residue was added to the methane fermentation tank after the sludge addition process. After addition, the fermentation sludge in the methane fermentation tank was stirred for 1 minute. Stirring of the fermentation sludge was not performed at any other time. The oil residue was added at the timings shown in Tables 7 to 9. The amount of oil residue added per instance was set to an amount that resulted in an organic matter load of 1.00 to 1.14 kg-VS / t / day. As an equivalent amount, it was set to an amount of 1.00 kg to 1.14 kg per ton of total sludge per day. It is preferable to adjust the amount of oil residue added on a given day by increasing or decreasing it while checking the amount of biogas generated, in order to increase the amount of biogas generated.

[0132] When adding the oil residue, the lid of the methane fermentation tank was opened, and after the addition, nitrogen purging was performed to maintain anaerobic conditions inside.

[0133] (10) Work Procedures For each test plot, the work procedures were set as follows: ・Test plots 1 to 6: Various treatments were carried out for the number of days shown in Tables 7 to 9. Here, the period from day 1 (fermentation sludge preparation) to day 7 of the test was designated as Week 1, the period from day 8 to day 14 as Week 2, and the period from day 15 to day 21 as Week 3. On day 22 of the test, the cumulative flow rate of gas, the total amount of fermentation sludge, and the sodium concentration were measured. ・Test plot 7: Various treatments were carried out for the number of days shown in Tables 7 to 9. In Test plot 7, even on day 14 of Week 2, the cumulative flow rate of methane fermentation was lower than that of Test plot 1 (control), so on day 15, the cumulative flow rate of gas, the total amount of fermentation sludge, and the sodium concentration were measured and the test was terminated.

[0134] (11) Measurement of sodium (sodium ion) content Similar to Test 1, the sodium (sodium ion) content in the fermented sludge was measured based on the ion electrode method. The results are shown in Figure 3.

[0135] (12) Measurement of gas generation amount Similar to test 1, the cumulative flow rate of gas from the start of the test to the time of measurement was measured. The results are shown in Figure 4. In addition, the amount of nitrogen-containing gas generated on each test day was calculated from this cumulative flow rate, and the gas generation amount (NmL) obtained by subtracting the amount of nitrogen from this value is shown in "Gas generation amount" in Tables 10 to 12. It was confirmed that the generated gas was methane-containing gas.

[0136] The gas generation rate per unit (NL / kg-VS) in Tables 10-12 is calculated based on the amount of gas generated per 1 kg of organic matter in the oil residue introduced during each one-week test period.

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[0140] (13) Results The fermentation data for each test plot are shown in Tables 10 to 12 and Figures 3 and 4.

[0141] (13-1) Regarding Test Section 1 For Test Section 1, the measured values ​​for each measurement item are recorded for each test day so that the details of the test measurements can be seen. The following is an explanation of each measured value listed in the table. ・Amount of oil residue added (g): This is the mass of oil residue added to the methane fermentation tank, and the amount of oil residue added on each test day is recorded. ・Amount of organic matter added (g): This is the mass of organic matter added to the methane fermentation tank, and the amount of organic matter added on each test day is recorded. ・Total amount of fermented sludge (g): This is the total amount of fermented sludge, and the average value of the values ​​measured during each test period of one week is recorded. ・Sodium concentration (mg / kg): This is the sodium ion concentration in the fermented sludge, and was measured on the 1st, 8th, 15th, and 22nd day of the test, and the values ​​are recorded in the sodium concentration column. ・Gas generation amount (NmL): The amount of nitrogen-containing gas generated on each test day was calculated from the cumulative flow rate, and the value obtained by subtracting the amount of nitrogen from that value is recorded in the gas generation amount column for each test day. - Cumulative gas flow rate: The cumulative gas flow rate (NmL) generated in the methane fermentation tank from the start of the test to the time of measurement was measured using a wet gas flow meter. The measured value obtained here is recorded in the column for the cumulative gas flow rate on the measurement day. - Gas generation rate per unit (NL / kg-VS): The calculated value of the amount of gas generated per 1 kg of organic matter in the oil residue added during each test period of the week is recorded. - Organic matter load (kg-VS / t / d): The amount of organic matter added per day (per test) per ton of fermentation sludge during each test period of the week is recorded.

[0142] (13-2) Test Sections 1 to 7 For Test Sections 1 to 7, the measured values ​​for each measurement item were recorded in the table for each one-week test period. The following is an explanation of each measurement value recorded in the table. ・Amount of oil residue added (g): The mass of oil residue added to the fermentation tank. The total amount of oil residue added during each one-week test period is recorded. ・Amount of organic matter added (g): The mass of organic matter added to the fermentation tank. The total amount of organic matter added during each one-week test period is recorded. ・Total amount of fermented sludge (g): The total amount of fermented sludge. The average value of the values ​​measured during each one-week test period is recorded. ・Sodium concentration (mg / kg): The sodium ion concentration in the fermented sludge. The value measured on the 8th day was recorded in the sodium concentration column for the 1st week, the value measured on the 15th day was recorded in the sodium concentration column for the 2nd week, and the value measured on the 22nd day was recorded in the sodium concentration column for the 3rd week.・Gas generation amount (NmL): The amount of nitrogen-containing gas generated during the one-week test period was calculated from the cumulative flow rate. The value measured on the 8th day was recorded in the "Gas generation amount for week 1" column, the value measured on the 15th day was recorded in the "Gas generation amount for week 2" column, and the value measured on the 22nd day was recorded in the "Gas generation amount for week 3" column. ・Cumulative gas flow rate: The cumulative flow rate (NmL) of gas generated in the methane fermentation tank from the start of the test to the time of measurement was measured using a wet gas flow meter. The measured value obtained here was recorded in the "Cumulative gas flow rate for the previous week" column. ・Gas generation rate per unit (NL / kg-VS): The calculated value of the amount of gas generated during each test period per 1 kg of organic matter in the oil residue added during each one-week test period is recorded. ・Organic matter load (kg-VS / t / d): The amount of organic matter added per ton of fermentation sludge per day during each one-week test period is recorded.

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[0146] The results of Test 2 revealed the following: First, when sodium reduction treatment was not performed on the fermentation sludge in the methane fermentation tank, the amount of sodium in the fermentation sludge tended to increase (Test Section 1). On the other hand, when sodium reduction treatment was performed, the amount of sodium in the fermentation sludge decreased as the amount of fermentation sludge treated increased (Test Sections 2-7).

[0147] In methane fermentation, sodium in the sludge is known to act as a fermentation inhibitor. Therefore, it was expected that test plot 7, which had the greatest reduction in sodium content, would produce the most gas. However, surprisingly, despite having a lower sodium content than test plot 1 (control), test plot 7 produced less gas and had lower reaction efficiency than test plot 1. The reason for the low reaction efficiency in test plot 7 is not clear, but it is presumed to be because the amount of sodium reduction treatment (the total amount of fermented sludge) performed at one time was larger compared to the other test plots. Furthermore, in test plot 6, which had the next greatest reduction in sodium content after test plot 7, gas production was also lower and reaction efficiency was low despite having a lower sodium content than test plot 1 (control).

[0148] In contrast, test sections 2 through 5 all produced more gas than test section 1 (control), indicating improved reaction efficiency.

[0149] These findings indicate that sodium reduction treatment of fermentation sludge is more effective when applied to a portion of the sludge (for example, 1 / 24, 1 / 16, 1 / 12, or 1 / 8 of the sludge) rather than the entire sludge at once, resulting in increased gas generation and improved reaction efficiency. In particular, it was found that the reaction efficiency could be further enhanced when 1 / 12 or 1 / 8 of the fermentation sludge was treated for sodium reduction and then reintroduced into the methane fermentation tank.

[0150] <Comparison of Results from Tests 1 and 2> Below, we consider the conditions that result in higher reaction efficiency based on a comparison of the above test results.

[0151] (1) Comparison of Test Section 4 of Test 1 and Test Section 3 of Test 2 Comparing the results of Test Section 4 of Test 1 and Test Section 3 of Test 2, in Test Section 4 of Test 1, 1 / 4 of the fermented sludge was treated for sodium reduction per day, while in Test Section 3 of Test 2, 1 / 16 of the fermented sludge was treated for sodium reduction per day. Here, in Test Section 4 of Test 1, this was done once a week, and in Test Section 3 of Test 2, it was done four times a week. Considering the amount of sodium reduction treatment performed per week, it is the same amount, i.e., 1 / 4 of the fermented sludge. Looking at the amount of gas generated in each test section, Test Section 4 of Test 1 generated less gas and had lower reaction efficiency than Test Section 1 (control) of Test 1. However, Test Section 3 of Test 2, which treated the same amount of fermented sludge for sodium reduction in four separate batches over a week, generated more gas and had improved reaction efficiency than Test Section 1 (control) of Test 2. From the above, it was found that even if the weekly processing volume of fermented sludge is the same, a smaller processing volume per batch results in higher reaction efficiency.

[0152] (2) Comparison of Test Section 4 of Test 1 and Test Sections 4 and 5 of Test 2 Comparing the results of Test Section 4 of Test 1 with Test Sections 4 and 5 of Test 2, in Test Section 4 of Test 1, 1 / 4 of the fermented sludge was treated for sodium reduction per day, while in Test Sections 4 or 5 of Test 2, 1 / 12 and 1 / 8 of the fermented sludge were treated for sodium reduction per day, respectively. Here, in Test Section 4 of Test 1, this was done once a week, while in Test Sections 4 and 5 of Test 2, it was done four times a week. Considering the amount of sodium reduction treatment performed per week, in Test Section 4 of Test 1, 1 / 4 of the fermented sludge was treated for sodium reduction, while in Test Sections 4 and 5 of Test 2, 1 / 3 and 1 / 2 of the fermented sludge were treated for sodium reduction, which is more than in Test Section 4 of Test 1. Furthermore, when examining the amount of gas generated in each test plot, Test Plot 4 of Test 1 generated less gas and had lower reaction efficiency than Test Plot 1 (control) of Test 1. However, Test Plots 4 and 5 of Test 2, which treated a larger amount of fermented sludge than Test Plot 4 of Test 1 in one week, in four separate sodium reduction treatments, generated more gas and had improved reaction efficiency than Test Plot 1 (control) of Test 2. From the above, it was found that when treating fermented sludge for sodium reduction, fermentation efficiency can be improved by treating a smaller amount (1 / 16, 1 / 12, or 1 / 8 of the fermented sludge) in several days, rather than treating the entire amount or 1 / 4 of the fermented sludge in one day.

Claims

1. A methane fermentation treatment method for generating methane-containing gas using fermentation raw materials including oil residue, the treatment method comprising: a sludge removal step of removing sludge from the methane fermentation tank at a rate of more than 0 and less than 1 / 4 of the total amount of sludge in the methane fermentation tank per day, with the total amount of sludge in the methane fermentation tank being set to 1; a sodium reduction step of reducing the sodium content in the sludge removed in the sludge removal step to obtain sodium-reduced sludge; and a sludge input step of introducing the sodium-reduced sludge into the methane fermentation tank.

2. The methane fermentation treatment method according to claim 1, further comprising an oil residue addition step of adding oil residue to the methane fermentation tank.

3. The amount of oil residue added at the start of methane fermentation, and the amount of oil residue added in the additional oil residue addition process, each in terms of organic matter equivalent, per day, per 1 m³ of total sludge volume. ３ The methane fermentation treatment method according to claim 2, wherein the amount is 20 kg or less.

4. The methane fermentation treatment method according to claim 1, wherein the method for reducing the sodium content in the sodium reduction step is a method using an inorganic flocculant and / or an organic flocculant.

5. A methane fermentation treatment method according to any one of claims 1 to 4, comprising a gas recovery step of recovering methane-containing gas generated in the methane fermentation tank.

6. A method for producing methane-containing gas by methane fermentation treatment using fermentation raw materials including oil residue, the method comprising: a sludge removal step of removing sludge from the methane fermentation tank in an amount greater than 0 and less than 1 / 4 of the total amount of sludge in the methane fermentation tank per day, with the total amount of sludge in the methane fermentation tank being set to 1; a sodium reduction step of reducing the sodium content in the sludge removed in the sludge removal step to obtain sodium-reduced sludge; a sludge input step of introducing the sodium-reduced sludge into the methane fermentation tank; and a gas recovery step of recovering the methane-containing gas generated in the methane fermentation tank.

7. A method for producing methane-containing gas according to claim 6, comprising an oil residue addition step of adding oil residue to the methane fermentation tank.

8. The amount of oil residue added at the start of methane fermentation and the amount of oil residue added in the additional oil residue addition process, each in terms of organic matter equivalent, per day, per 1 m³ of total sludge volume. ３ A method for producing methane-containing gas according to claim 7, wherein the amount is 20 kg or less.

9. The method for producing methane-containing gas according to any one of claims 6 to 8, wherein the method for reducing the sodium content in the sodium reduction step is a method using an inorganic coagulant and / or an organic coagulant.

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