Biogas production method and food or feed production method

Abstract

To provide a biogas production method and its application that can produce biogas with higher combustion efficiency by fixing carbon dioxide contained in biogas obtained from sludge. [Solution] A method for producing biogas includes the steps of producing biogas in a first treatment tank containing sludge and anaerobic methane-producing microorganisms, transporting the biogas produced in the first treatment tank to a second treatment tank containing microalgae that live at a pH of 8.0 to 12.0 and are capable of immobilizing carbon dioxide, and immobilizing the carbon dioxide contained in the biogas in the microalgae, and recovering the biogas with a reduced carbon dioxide content, wherein the methane-producing microorganisms in the first treatment tank and the microalgae in the second treatment tank comprise at least one species selected from a specific group of methane-producing microorganisms and a specific group of microalgae.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing biogas and a method for producing food or feed. [Background technology]

[0002] Activated sludge processes are commonly used to treat sewage from households and industries. The activated sludge process involves adding aerobic microorganisms to the sewage and aerating it to decompose dissolved and residual suspended matter contained in the sewage. During this decomposition process, the aerobic microorganisms proliferate significantly while utilizing nutrients such as organic matter, nitrogen, and phosphorus in the sewage. However, since the decomposition efficiency is improved when the concentration of suspended matter (i.e., aerobic microorganisms) in the sewage is controlled within a certain range, excessively proliferated aerobic microorganisms must be discharged from the system as excess sludge. Sewage treatment plants operate day and night throughout the year, meaning that excess sludge is constantly being generated across the country. As a result, securing disposal sites for excess sludge and increasing disposal costs have become problems. As a method of effectively utilizing excess sludge, some sewage treatment plants use anaerobic digestion (also known as methane fermentation) with methane-producing microorganisms to produce biogas containing about 60% methane from the excess sludge, thereby reducing the volume of the excess sludge. Attempts have been made to reduce the volume of waste by using methane gas production by anaerobic microorganisms, not only for sludge generated from sewage treatment plants, but also for waste containing biological components discharged from food factories, agricultural waste, and livestock waste.

[0003] Regarding the reduction of excess sludge volume, the present inventors have previously proposed a microbial mixture and a methane production method that further decomposes insoluble polysaccharides, peptidoglycans, cellulose, proteins, etc. that are difficult to biodegrade and remain after excess sludge treatment (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-193884 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the microbial mixture and methane production method described in Patent Document 1, it is possible to produce a larger amount of biogas from excess sludge than conventional methods, and the volume of sludge has been successfully reduced.

[0006] However, even with the above method, the methane content of biogas is only about 60%, with the remaining 40% being carbon dioxide and trace amounts of hydrogen, hydrogen sulfide, etc., and the calorific value of biogas is only about 60% of that of known natural gas. Chemical adsorption is a known method for removing carbon dioxide from biogas. Chemical adsorption utilizes the alkali-solubility of carbon dioxide to adsorb carbon dioxide in an alkaline solution, such as an aqueous solution of an amine compound, to remove carbon dioxide from biogas. The carbon dioxide adsorbed in the alkaline solution is generally regenerated into pure carbon dioxide in a regeneration tower. However, from an environmental perspective, releasing carbon dioxide into the atmosphere is undesirable. Therefore, a new method for removing carbon dioxide from biogas and a new method for producing biogas with higher combustion efficiency from which carbon dioxide has been removed are desired.

[0007] An object of one embodiment of the present disclosure is to provide a method for producing biogas that can produce biogas with higher combustion efficiency by immobilizing carbon dioxide contained in biogas obtained from sludge. Another object of another embodiment of the present disclosure is to provide a method for producing food or feed using, as a raw material, microalgae in which carbon dioxide contained in biogas is immobilized. [Means for solving the problem]

[0008] Specific means for solving the problems include the following aspects. <1> The method includes the steps of: producing biogas in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms; transporting the biogas produced in the first treatment tank to a second treatment tank containing a dispersion of microalgae that live at a pH of 8.0 to 12.0 and are capable of fixing carbon dioxide dispersed in a culture solution; and fixing the carbon dioxide contained in the biogas to the microalgae; and recovering the biogas with a reduced carbon dioxide content. A method for producing biogas, wherein the methane-producing microorganisms in the first treatment tank comprise at least one species selected from the following microorganism group A, and the microalgae in the second treatment tank comprise at least one species selected from the following microalgae group B. -Microbial group A- Methanosaeta, Methanobacterium, Methanospirillum, Methanosarcina, Methanobrevibacter, Methanoculleus, Methanomassiliicoccus, Microorganisms belonging to the families Methanosphaera, Methanomethylovorans, and Methanobacteriaceae, and microorganisms belonging to the family Methanosarcinaceae. -Microalgae Group B- Algae in the genus Desmodesmus armatus, algae in the genus Tetradesmus obliquus, Algae in the genus Micractinium inermum, algae in the genus Desmodesmus abundans, Algae belonging to the genus Chlorella sorokiniana and algae belonging to the genus Desmodesmus spinosus.

[0009] <2> The first treatment tank and the second treatment tank are disposed in the same space. <1> The method for producing biogas according to claim 1. <3> The mixture contained in the first treatment tank further contains at least one microorganism selected from the group consisting of microorganisms belonging to the Enterobacteriaceae family, microorganisms belonging to the Pseudomonadaceae family, and microorganisms belonging to the Clostridiaceae family, which decomposes at least one of cellulose, chitin, and protein, and takes in glucose, which is a decomposition product of cellulose, glucosamine, which is a decomposition product of chitin, or amino acids, which are decomposition products of protein, to produce hydrogen, and at least one oxygen-consuming microorganism selected from the group consisting of microorganisms belonging to the Aspergillaceae family and microorganisms belonging to the Arthopyreniaceae family. <1> or <2> The method for producing biogas according to claim 1.

[0010] <4> The second treatment tank is configured to contain a material having a total light transmittance of 80% or more. <1> ~ <3> 10. The method for producing biogas according to claim 9, wherein the biogas is <5> The second treatment tank is equipped with an exposure device that irradiates the microalgae with light. <1> ~ <3> 10. The method for producing biogas according to claim 9, wherein the biogas is <6> When the carbon dioxide fixation ability of the microalgae decreases, a portion of the grown microalgae is recovered and a microalgae culture solution is added. <1> ~ <5> 10. The method for producing biogas according to claim 9, wherein the biogas is

[0011] <7> The method includes the steps of: producing biogas in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms; transporting the biogas produced in the first treatment tank to a second treatment tank containing a dispersion of microalgae that live at a pH of 8.0 to 12.0 and are capable of fixing carbon dioxide dispersed in a culture solution; and fixing the carbon dioxide contained in the biogas in the microalgae; and recovering a portion of the microalgae that have grown in the second treatment tank; and producing food or feed using the recovered microalgae as a raw material. A method for producing food or feed, wherein the methane-producing microorganisms in the first treatment tank comprise at least one species selected from the following microorganism group A, and the microalgae in the second treatment tank comprise at least one species selected from the following microalgae group B. -Microbial group A- Methanosaeta, Methanobacterium, Methanospirillum, Methanosarcina, Methanobrevibacter, Methanoculleus, Methanomassiliicoccus, Microorganisms belonging to the families Methanosphaera, Methanomethylovorans, and Methanobacteriaceae, and microorganisms belonging to the family Methanosarcinaceae. -Microalgae Group B- Algae in the genus Desmodesmus armatus, algae in the genus Tetradesmus obliquus, Algae in the genus Micractinium inermum, algae in the genus Desmodesmus abundans, Algae belonging to the genus Chlorella sorokiniana and algae belonging to the genus Desmodesmus spinosus. [Effects of the Invention]

[0012] According to one embodiment of the present disclosure, it is possible to provide a biogas production method that can produce biogas with higher combustion efficiency by immobilizing carbon dioxide contained in biogas obtained from sludge. According to another embodiment of the present disclosure, it is possible to provide a method for producing food or feed using microalgae as a raw material in which carbon dioxide contained in biogas is immobilized. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a phylogenetic tree of the estimated genera and species of microalgae group B used in the biogas production method of the present disclosure. [Figure 2] FIG. 2 is a schematic diagram showing an example of a first embodiment of a biogas production apparatus used in the biogas production method of the present disclosure. [Figure 3] FIG. 3 is a schematic diagram showing an example of a second embodiment of a biogas production apparatus used in the biogas production method of the present disclosure. [Figure 4] FIG. 4 is a schematic diagram showing another example of the second embodiment of the biogas production apparatus used in the biogas production method of the present disclosure. [Figure 5] FIG. 5 is a graph showing the composition of biogas in glass vials in Examples 1 to 10 and Comparative Examples 1 to 3, which were obtained using digested sludge. [Figure 6] FIG. 6 is a graph showing the composition of biogas in glass vials in Examples 1 to 10 and Comparative Examples 1 to 3, which were obtained using excess sludge. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present disclosure will be described in detail. However, the present disclosure is not limited to the following embodiments. In the following disclosure, components are not essential unless specifically stated. The same applies to numerical values ​​and their ranges, and do not limit the present disclosure. In the present disclosure, the term "process" includes not only a process that is independent of other processes, but also a process that cannot be clearly distinguished from other processes as long as the purpose of the process is achieved. In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit of the numerical range may be replaced with the value shown in the examples. In the present disclosure, when a composition contains multiple substances corresponding to each component, the content of each component in the composition means the total content of the multiple substances present in the composition, unless otherwise specified.

[0015] <Biogas production method> The biogas production method of the present disclosure (hereinafter also referred to as the production method of the present disclosure) includes the steps of: producing biogas in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms (step A); transporting the biogas produced in the first treatment tank to a second treatment tank containing a dispersion of microalgae that live at a pH of 8.0 to 12.0 and are capable of immobilizing carbon dioxide in a culture solution (step B); and immobilizing the carbon dioxide contained in the biogas in the microalgae (step C); and recovering the biogas with a reduced carbon dioxide content.

[0016] <Process A> In step A of the production method of the present disclosure, biogas is produced in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms. Anaerobic methane-producing microorganisms that can be used in step A will be described.

[0017] (methane-producing microorganisms) The methane-producing microorganisms used in the first treatment tank in step A can be any known anaerobic microorganism known as a methane-producing microorganism, and in the present disclosure, a representative example thereof is at least one species selected from the following microbial group A. -Microbial group A- Methanosaeta, Methanobacterium, Methanospirillum, Methanosarcina, Methanobrevibacter, Methanoculleus, Methanomassiliicoccus, Microorganisms belonging to the families Methanosphaera, Methanomethylovorans, and Methanobacteriaceae, and microorganisms belonging to the family Methanosarcinaceae. The first treatment tank may contain at least one species selected from the above-mentioned group A of microorganisms as the methane-producing microorganism, and may contain two or more species.

[0018] When two or more types of microorganisms included in microbial group A are contained, there are no particular limitations on the mixing ratio. A mixture of microorganisms containing two or more types of microorganisms is cultured at room temperature, i.e., by co-culturing the microorganisms contained in the microbial mixture, methane production from biological compositions such as excess sludge becomes possible.

[0019] In addition to the methane-producing microorganisms described in microbial group A, the first treatment tank preferably further contains at least one microorganism selected from the group consisting of microorganisms belonging to the Enterobacteriaceae family, microorganisms belonging to the Pseudomonadaceae family, and microorganisms belonging to the Clostridiaceae family that decomposes at least one of cellulose, chitin, and protein, takes up the decomposed glucose, glucosamine, or amino acids, and produces hydrogen, and at least one oxygen-consuming microorganism selected from the group consisting of microorganisms belonging to the Aspergillaceae family and microorganisms belonging to the Arthopyreniaceae family. The microorganisms in the first treatment tank may further include at least one microorganism selected from the group consisting of microorganisms belonging to the Enterobacteriaceae family, microorganisms belonging to the Pseudomonadaceae family, and microorganisms belonging to the Clostridiaceae family that decomposes at least one of cellulose, chitin, and protein, ingests the decomposed glucose, glucosamine, or amino acids, and produces hydrogen. This decomposes the resistant cellulose, chitin, etc. remaining in the sludge to be treated, producing hydrogen. Furthermore, by including at least one oxygen-consuming microorganism selected from the group consisting of microorganisms belonging to the Aspergillaceae family and microorganisms belonging to the Arthopyreniaceae family, the produced hydrogen and carbon dioxide can be ingested to further produce methane under room temperature conditions. This makes it possible to reduce the volume of materials to be treated, such as sludge, and to produce a larger amount of biogas. These microorganisms are described in detail in Patent Document 1, the disclosure of which is incorporated herein by reference.

[0020] In step A, the function of the methane-producing microorganism can be confirmed, for example, by culturing the microorganism and confirming the growth of the microorganism, the production of methane, and the like. In the first treatment tank, the mixing ratio of the methane-producing microorganisms to the sludge to be treated is not particularly limited, but from the viewpoint of facilitating the growth of the methane-producing microorganisms, the ratio is preferably 0.1 to 10 parts by mass, more preferably 1 to 5 parts by mass, of sludge to 1 part by mass of the methane-producing microorganisms.From the viewpoint of facilitating the growth of the methane-producing microorganisms, it is preferable to further add 10 to 1,000 parts by mass, more preferably 100 to 500 parts by mass of water.

[0021] Generally, the optimum pH for methane production by methanogenic microorganisms is 6.8 to 7.6. Therefore, methane production can be suppressed by lowering the pH below 6.8. Furthermore, hydrogen-producing microorganisms are generally tolerant to low pH and can produce hydrogen at a pH of 4.5 or higher. Methanogenic microorganisms can survive in high pH ranges. Methanogenic microorganisms can produce 10 ml or more of hydrogen when cultured for one month at room temperature, for example, at a pH of 4.5 to 6.8, using 1 g of a biologically derived composition as a substrate. The amount of hydrogen produced is preferably 10 ml to 2000 ml, more preferably 20 ml to 1800 ml, even more preferably 40 ml to 1600 ml, and particularly preferably 80 ml to 1200 ml.

[0022] The methane-producing microorganism is not particularly limited as long as it is a composition that can produce methane from a biologically derived composition, and may be a living microorganism or a microorganism in a suspended animation state.

[0023] The mixture containing methanogenic microorganisms and sludge may further contain a solid medium (e.g., gelatin, lactose), a liquid medium (e.g., water, physiological saline), a solubilizer, a stabilizer, an isotonic agent, etc. The amounts of these components to be added are not particularly limited as long as the conditions are such that the methanogenic microorganisms can grow and proliferate.

[0024] Step A can include seeding methane-producing microorganisms in sludge or the like and culturing the seeded methane-producing microorganisms under room temperature conditions. In the present disclosure, room temperature encompasses a range of 20°C to 35°C, which is a temperature condition that is not particularly controlled, and refers to 25°C unless otherwise specified. In the present disclosure, there are no particular limitations on the method for inoculating the mixture containing sludge and any optional components with methanogenic microorganisms.

[0025] <Process B> In step B of the manufacturing method of the present disclosure, the biogas produced in step A, i.e., the first treatment tank, is transported to a second treatment tank containing a dispersion liquid in which microalgae that live at a pH of 8.0 to 12.0 and can fix carbon dioxide are dispersed in a culture solution, and the carbon dioxide contained in the biogas is fixed in the microalgae. The microalgae used in step B will be described.

[0026] (microalgae) In step B, the microalgae used to fix carbon dioxide are microalgae that live under conditions of pH 8.0 to 12.0, i.e., alkaline conditions where carbon dioxide is highly soluble, and are capable of fixing carbon dioxide, and the microalgae in the second treatment tank include at least one species selected from the algae group B described below. The pH of the culture solution containing microalgae in the second treatment tank is 8.0 or higher, preferably 8.0 to 12.0, from the viewpoint of good carbon dioxide solubility, and a pH of 10.0 to 12.0 is more preferable from the viewpoint of reducing the risk of wild algae proliferation in outdoor culture and achieving both an algae growth environment and carbon dioxide fixation. Among algae that have chlorophyll, which can adsorb and fix carbon dioxide through photosynthesis, common algae have difficulty surviving in extremely high pH ranges such as pH 10.0 to 12.0. The present inventors have previously succeeded in selecting and culturing alkali-tolerant microalgae, and in the production method of the present disclosure, this microalgae is applied to step B to fix carbon dioxide.

[0027] -Microalgae Group B- Algae in the genus Desmodesmus armatus, algae in the genus Tetradesmus obliquus, Algae in the genus Micractinium inermum, algae in the genus Desmodesmus abundans, Algae belonging to the genus Chlorella sorokiniana and algae belonging to the genus Desmodesmus spinosus.

[0028] FIG. 1 shows a phylogenetic tree of the estimated genera and species of microalgae group B used in the biogas production method of the present disclosure. Among these microalgae group B, the genus and species names, strain names, pH values ​​at which they can grow, and carbon dioxide concentrations at which they can grow are shown in Table 1 below for microalgae that have been confirmed to be usable in the biogas production method of the present disclosure. In Table 1, carbon dioxide is abbreviated as "CO2."

[0029] [Table 1]

[0030] As shown in Table 1, the algae included in Microalgae B can grow in a high pH range where carbon dioxide has good solubility. The second treatment tank preferably contains a culture solution for the microalgae in addition to the microalgae. From the viewpoint of improving the growth environment for the microalgae, the culture solution preferably contains water and at least one selected from organic components and inorganic components that can generally be supplied to plants having chlorophyll. Inorganic components suitable for plant growth include Na, K, Cl, Fe, Mn, Zn, and Cu. These inorganic components may be present in the form of a salt. Examples of salts of inorganic components include NaNO3, K2HPO4, K2SO4, MgSO4·7H2O, FeSO4·7H2O, NaCl, CaCl2·2H2O, H3BO4, MnSO4·5H2O, ZnSO4·7H2O, CuSO4·5H2O, and Na2MoO4·2H2O. Furthermore, the culture medium may contain a known pH adjuster, such as a chelating agent such as ethylenediaminetetraacetate (EDTA), for the purpose of adjusting the pH. The culture medium functions as a dispersion medium for the microalgae. The pH of the culture medium is adjusted to 8.0 to 12.0 by adding, for example, 1.0 M NaOH.

[0031] The biogas produced in step A is supplied to a second treatment tank containing a dispersion liquid in which microalgae capable of immobilizing carbon dioxide are dispersed in a culture solution, and the carbon dioxide contained in the biogas is immobilized in the microalgae. The carbon dioxide fixation reaction by the microalgae is photosynthesis, so it is preferable that the second treatment tank irradiates the microalgae with visible light in an amount sufficient for photosynthesis to promote photosynthesis. From the viewpoint of improving the efficiency of fixation of carbon dioxide in microalgae, the second treatment tank is preferably made of a material having a total light transmittance of 80% or more. In the present disclosure, a material with a total light transmittance of 80% or more refers to a material that is transparent to the naked eye, and examples thereof include glass (not containing colorants), acrylic resin, etc. When the second treatment tank is configured to contain a material with a total light transmittance of 80% or more, visible light necessary for photosynthesis can be supplied from external light to the microalgae contained in the second treatment tank.

[0032] Furthermore, when the second treatment tank is provided in an environment where it is difficult to take in external light, or when continuous fixation of carbon dioxide is required, such as at night, the second treatment tank may be equipped with an exposure device that irradiates light onto the microalgae. There are no particular restrictions on the exposure device, and any known exposure device can be used without particular restrictions as long as it is equipped with a light source that can irradiate light including visible light, such as an LED (light emitting diode), a fluorescent lamp, etc. The exposure device may be provided with a switch that can be driven as needed, and may perform operations such as turning exposure on and off or adjusting the amount of exposure depending on the amount of light received by an optical sensor located near the bottom of the second treatment tank, for example.

[0033] <Process C> In step C, the biogas with a reduced carbon dioxide content obtained in step B is recovered. In step B, carbon dioxide is fixed in the microalgae, thereby reducing the carbon dioxide content in the biogas produced in step A, and a purer biogas, in other words, methane gas with good combustion efficiency, is obtained. The biogas obtained through steps A and B is recovered in step C and stored, for example, in a gas holder.

[0034] (Biogas production apparatus used in the biogas production method of the present disclosure: First embodiment) The manufacturing method of the present disclosure will be described below with reference to the drawings. FIG. 2 is a schematic diagram showing an example of a first embodiment of a biogas production apparatus used in the biogas production method of the present disclosure. The biogas production apparatus 10 shown in FIG. 2 includes a first treatment tank 12, a second treatment tank 18, and a gas holder 30 for storing biogas. The first treatment tank 12 contains a mixture 14 of sludge, which is the material to be treated, and anaerobic methane-producing microorganisms, and the inside of the first treatment tank 12 has an anaerobic atmosphere. The biogas produced in the first treatment tank 12 is supplied to the second treatment tank 18 via a pipe 16. When biogas is produced, the inside of the first treatment tank becomes pressurized, and therefore the biogas produced in the first treatment tank 12 is supplied to the second treatment tank 18 via the pipe 16 without any special pressurization or suction. The second treatment tank 18 contains a dispersion liquid 20 containing a culture medium with a pH of 8.0 to 12.0 in which microalgae are dispersed. When the biogas supplied from the bottom of the second treatment tank 18 passes through the dispersion liquid 20, carbon dioxide is immobilized in the microalgae through a photosynthetic reaction. The carbon dioxide is fixed in the microalgae, and the biogas with a reduced carbon dioxide content is collected in the gas holder 30 via the pipe 22. The inside of the first treatment tank 12 becomes pressurized by the produced biogas, and the produced biogas moves to the second treatment tank 18 without any particular suction or the like. From the viewpoint of efficiency, it is preferable that the biogas in which the carbon dioxide has been immobilized in the second treatment tank 18 and the carbon dioxide content has been reduced is transported to the gas holder 30 by a pump provided in the pipe 22. As a method for quantifying the methane content in the biogas recovered in step C, it is preferable to quantify the gaseous biogas as the measurement object. For example, there is a method in which the gas phase recovered in the second treatment tank 18 or the gas holder 30 is sampled and the gas contained in the gas phase is quantified using a gas chromatograph.

[0035] (Other processes) The production method of the present disclosure may further include other steps in addition to the above steps A, B, and C. Other steps include a step (step D) of recovering a portion of the grown microalgae and adding a culture medium for the microalgae when the carbon dioxide fixation ability of the microalgae decreases. In step A, by adding sludge to the first treatment tank 12, biogas can be produced continuously.

[0036] On the other hand, in the second treatment tank 18, which is step B, photosynthesis progresses, and the microalgae in the dispersion liquid 20 in the second treatment tank 18 carry out photosynthesis to fix carbon dioxide, thereby growing and increasing the density of the microalgae in the dispersion liquid 20. For this reason, for example, visible light necessary for photosynthesis may be less able to reach the microalgae present in the dispersion liquid 20 near the bottom of the second treatment tank 18, among the microalgae dispersed in the dispersion liquid 20, and the efficiency of fixation of carbon dioxide may decrease. In this way, when the efficiency of fixation of carbon dioxide decreases due to the difficulty of light reaching the microalgae, it is preferable to perform an operation to recover some of the microalgae. As a guideline for when to start recovery, for example, when a light sensor is attached to the bottom surface of the second treatment tank 18 and the reach of visible light drops to 20% to 60% of the reach of the treatment start time, or further when the reach of visible light at the bottom surface drops to 30% to 50% of the reach of the treatment start time, etc. For example, in the case where the top and bottom surfaces of the second treatment tank 18 are constructed of a material with a total light transmittance of 80% or more and external light is taken into the second treatment tank 18, if a certain decrease in visible light transmittance is observed, the amount of light reaching the bottom surface of the second treatment tank 18 will decrease due to the proliferation of microalgae, making it impossible for photosynthesis to occur sufficiently and reducing the efficiency of carbon dioxide fixation, and in this case it is preferable to recover the microalgae. In addition, if the carbon dioxide content in the second treatment tank 18 is measured and the amount of carbon dioxide reduction falls below a certain level, or if the carbon dioxide content in the second treatment tank 18 becomes higher than a predetermined value, this can also be used as an indicator for recovering microalgae. It has been confirmed that the alkali resistance and carbon dioxide fixation ability of the proliferated microalgae do not decrease even after replacement by proliferation.

[0037] When the carbon dioxide fixation efficiency decreases, it is preferable to recover a portion of the grown microalgae. A recovery port with a cock 26 is provided at the bottom of the second treatment tank 18 in Figure 2, and a portion of the grown microalgae can be recovered from here. According to the inventors' investigations, the atmosphere in the first treatment tank 12 containing the sludge is anaerobic, and the atmosphere in the second treatment tank 18 is aerobic, so there is no possibility that contaminants such as bacteria attached to the sludge will be transported to the second treatment tank 18. Furthermore, the recovered microalgae contain essential amino acids, unsaturated fatty acids, carotenoids, etc., and are therefore expected to be used as health foods or fattening feed for livestock and aquatic animals.

[0038] When a portion of the microalgae is recovered from the second treatment tank 18, the concentration of the culture solution decreases, which may affect the growth of the microalgae. For this reason, it is preferable to supply a culture solution for the microalgae when a portion of the microalgae is recovered from the second treatment tank 18. In FIG. 2, the culture solution is supplied as needed from the culture solution tank 24. The culture solution can be supplied as needed by opening and closing a cock 26 attached to a pipe provided between the culture solution tank 24 and the second treatment tank 18.

[0039] (Biogas production apparatus used in the biogas production method of the present disclosure: Second embodiment) In the first embodiment described above, the first treatment tank 12 and the second treatment tank 18 were provided separately, but the first treatment tank 12 and the second treatment tank 18 may also be arranged in the same space. FIG. 3 is a schematic diagram showing an example of a second embodiment of a biogas production apparatus used in the production method of the present disclosure. In the biogas production apparatus of Figure 3, the first treatment tank 32 is placed inside the second treatment tank 34. A mixture 14 of sludge and anaerobic microorganisms is accommodated in the first treatment tank 32, and biogas is produced in the first treatment tank 32 (step A). ​​The produced biogas is supplied via a pipe 16 to a dispersion liquid 20 containing microalgae and a culture solution accommodated in the second treatment tank 34, and the carbon dioxide contained in the biogas is immobilized in the microalgae, thereby reducing the carbon dioxide content in the biogas (step B). From the viewpoint of efficiency, it is preferable that the biogas in which the carbon dioxide has been immobilized in the second treatment tank 34 and the carbon dioxide content has been reduced is transported to the gas holder 30 by a pump provided in the pipe 22 and recovered (step C). In Figure 3, methane gas is schematically represented by a shape made up of four circles joined together, and carbon dioxide is schematically represented by a capsule-like shape made up of three circles joined together. According to the second embodiment of the manufacturing method of the present disclosure shown in FIG. 3, the first treatment tank 32 and the second treatment tank 34 are arranged in a partitioned state within the same space, thereby making it possible to further reduce the size of the biogas production apparatus.

[0040] FIG. 4 is a schematic diagram showing another example of the second embodiment of the biogas production apparatus used in the production method of the present disclosure. The biogas production apparatus shown in FIG. 4 is an apparatus that can be used when treating a small amount of material to be treated, or when screening and evaluating the performance of anaerobic microorganisms, microalgae, and the like. Figure 4 shows a state in which a glass tube 36 (corresponding to a first treatment tank) containing a mixture 14 of sludge and anaerobic microorganisms is placed in a vial bottle 38 (corresponding to a second treatment tank) containing a dispersion liquid 20 in which microalgae are dispersed in a culture solution. Within the sealed space of the vial 38, the biogas produced within the glass tube 36 is released from the opening at the top of the glass tube 36, filling the vial 38. The released biogas dissolves in the alkaline dispersion liquid 20 filled near the bottom of the vial 38, and the microalgae in the dispersion liquid 20 fix the carbon dioxide, thereby reducing the carbon dioxide contained in the biogas.

[0041] When treating sludge discharged from sewage treatment plants, factories, etc., the biogas production apparatus of the first embodiment is preferably used because continuous treatment is possible, but when performing batch treatment, the biogas production apparatus of the second embodiment can also be suitably applied.

[0042] According to the production method of the present disclosure, by immobilizing the carbon dioxide contained in the biogas obtained from sludge, it is possible to produce biogas with higher combustion efficiency, and the range of applications is wide.

[0043] <Food or feed manufacturing method> The method for producing food or feed of the present disclosure includes a step (Step A) of producing biogas in a first treatment tank containing sludge and anaerobic methane-producing microorganisms, a step (Step B) of transporting the biogas produced in the first treatment tank to a second treatment tank containing microalgae that live at a pH of 8.0 to 12.0 and are capable of immobilizing carbon dioxide, and immobilizing the carbon dioxide contained in the biogas in the microalgae, and a step (Step E) of recovering a portion of the microalgae that has grown in the second treatment tank and producing food or feed using the recovered microalgae as a raw material, wherein the methane-producing microorganisms in the first treatment tank include at least one species selected from microorganism group A, and the microalgae in the second treatment tank include at least one species selected from algae group B.

[0044] Steps A and B in the food or feed production method of the present disclosure are the same as steps A and B, respectively, in the biogas production method of the present disclosure described above, and therefore will not be described here.

[0045] <Process E> The food or feed manufacturing method of the present disclosure includes, after carrying out steps A and B, step E of recovering a portion of the microalgae that has grown in the second treatment tank and manufacturing food or feed using the recovered microalgae as a raw material.

[0046] In step E, first, a portion of the microalgae that have grown in the second treatment tank is collected. In step E, the grown microalgae are usually collected in the form of a dispersion in a culture medium. First, liquid components such as the culture medium are removed from the dispersion containing the collected microalgae, and solid components are separated. The liquid components can be removed by known methods such as filtration or centrifugation. The separated solid component is mainly composed of microalgae, and may contain trace amounts of components from the culture medium. The solid component containing the microalgae is washed and, if desired, dried to obtain algae that can be used as a raw material for food or feed.

[0047] The food or feed production method of the present disclosure is a method for producing food or feed using microalgae recovered as a raw material during the production of biogas, which is the production method of the present disclosure described above. According to the biogas production method of the present disclosure, the dispersion containing the microalgae and culture solution contained in the second treatment tank is photosynthetic algae, and therefore the second treatment tank has an aerobic atmosphere, so there is no risk of the anaerobic microorganisms contained in the first treatment tank being mixed into the second treatment tank. Therefore, by fixing carbon dioxide and recovering the grown microalgae from the second treatment tank 18, the microalgae can be used for various purposes without the risk of being contaminated by microorganisms.

[0048] The obtained microalgae were analyzed by the following method, and it was confirmed that the microalgae contained carotenoids such as lutein, β-carotene, astaxanthin, and zeaxanthin, proteins, essential amino acids, and the like.

[0049] <Carotenoid analysis> Freeze-dried algae cells are collected and dispersed using a medium such as beads, and the resulting extract is evaporated to dryness using a nitrogen stream to obtain a sample, which is then dissolved in a solvent such as methanol. The resulting solution is subjected to a treatment to saponify the carotenoid ester, and the saponified sample is quantified by liquid chromatography analysis using, for example, a YMC Carotenoid-C30 column (inner diameter 4.6 mm × 250 mm, 5 μm, YMC Corporation). The liquid chromatography apparatus is preferably a high performance liquid chromatography analyzer equipped with a UV / VIS detector, a photodiode array detector, a PU-1580 pump, and a CO-1560 column thermostat.

[0050] <Proteins, amino acids> Freeze-dried algae cells are collected and mixed with 500 mL of 0.1 M NaOH in a screw-cap tube. The cells are then heated at 80°C for 1 hour to lyse the algae and extract proteins. The extract can be analyzed according to the method developed by Matsui et al. (Matsui, H et al. Effects of phosphorous deficiency of a microalga Nannochloropsis oculata on its fatty acid profiles and intracellular structure and the effectiveness in rotifer nutrition. Algal Res. 2020, 49, 101905.) Alternatively, the amino acids in proteins can be analyzed by hydrolyzing lyophilized cells in 6.0 M HCl in a vacuum-sealed hydrolysis tube at 110°C for 24 hours, neutralizing the hydrolysate to pH 7.0 with 1.0 M NaOH, and centrifuging the resulting hydrolysate. The supernatant is freeze-dried in a freeze dryer, and the lyophilized extract is resuspended in 1.0 mL of 0.1 M HCl and analyzed using an H-Class amino acid analyzer (e.g., Waters, Milford, Massachusetts, USA).

[0051] <Fatty acid content> The fatty acid content can be extracted from freeze-dried cells of microalgae based on the method of Bligh et al. (Bligh, E. G.; Dyer, W. J. A rapid method of total lipid extraction and purification. Can. J. Biochem. Physiol. 1959, 37, 911-917.). Specifically, freeze-dried algal cells were mixed with chloroform, methanol, and deionized water in a screw-cap tube and dispersed using a bead-beating homogenizer. The resulting cell dispersion was centrifuged, and the chloroform layer was stored in a glass vial as the total lipid fraction. The extracted lipid fraction was evaporated to dryness in a nitrogen stream and esterified using a fatty acid methylation kit. The resulting fatty acid methyl esters can be purified and measured using a GC-2014 gas chromatograph equipped with a flame ionization detector (Shimadzu) and a Precision SL100 hydrogen generator (Peak Scientific Instruments, Glasgow, UK). The analytical method is described in detail in the following document, and the description in this document can be referenced in the present disclosure. Literature (Yuri Kikuchi, Daichi Kanai, Kenjiro Sugiyama and Katsuhiko Fujii Biogas Upgrading by Wild Alkaliphilic Microalgae and the Application Potential of Their Biomass in the Carbon Capture and Utilization Technology Fermentation 2024, 10, 134.)

[0052] Table 2 shows the analysis results of the active ingredients contained in microalgae. Microalgae are known to contain a variety of carotenoids, proteins, essential amino acids, and fatty acids. In Table 2 below, the carotenoid and protein contents are shown as the contents per 1 g of dry cells (mg / g-dry cell), the essential amino acid content is shown as the content of essential amino acids relative to the total amount of amino acids contained in the algae (mol%), and the unsaturated fatty acid content is shown as the content of unsaturated fatty acids relative to the total amount of fatty acids (mass%).

[0053] [Table 2]

[0054] As shown in Table 2, microalgae grown by fixing carbon dioxide contain various active ingredients. Active ingredients include, for example, carotenoids such as lutein and zeaxanthin, which are antioxidant carotenoids synthesized exclusively by plants and microalgae and are known to have beneficial effects on animal eye health. Astaxanthin is an antioxidant carotenoid produced by bacteria, yeast, microalgae, etc., and is known to be useful as a UV protection agent and skin protectant in the cosmetics industry and as a natural coloring agent in the aquaculture industry. Furthermore, plant-derived proteins, essential amino acids, etc. are also useful in health foods, etc. Therefore, the microalgae obtained by the food or feed production method of the present disclosure can be expected to be used not only in food and feed, but also as active ingredients in external skin preparations such as cosmetics.

[0055] The microalgae collected through the above-mentioned step E contain various effective nutrients. Therefore, according to the method for producing food or feed using microalgae as a raw material disclosed herein, by recovering microalgae that are a by-product of the biogas production method and using appropriately treated microalgae as a raw material, food or feed such as health foods and livestock feed can be easily produced at low cost. Furthermore, since it contains various nutrients, it is expected to be applicable to a wide range of foods and feeds. [Example]

[0056] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples as long as it does not deviate from the gist of the disclosure. Unless otherwise specified, "parts" are based on mass. "%" is also based on mass.

[0057] In the following, molecular biological reagents were purchased from Toyobo Co., Ltd. (Osaka, Japan), Thermo Fisher Scientific (Waltham, Massachusetts, USA), or MP Biomedical (Santa Ana, California, USA). All other chemicals were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. (Kyoto, Japan). Glass and plastic laboratory ware used for culture was purchased from Maruemu Co., Ltd. (Osaka, Japan) or AS ONE Corporation (Osaka, Japan).

[0058] Examples 1 to 10 and Comparative Examples 1 to 3 (Collection of microbial mixture) A soil sample was collected from the surface sediment of the Kawaguchi River in Hachioji, Tokyo, Japan. 1 g of the soil sample was suspended in 20 ml of sterilized water using a vortex mixer, and this was used as the inoculum.

[0059] (Preparation of excess sludge and digested sludge) Excess sludge, raw sewage sludge produced during secondary wastewater treatment at a sewage treatment plant in Yokohama, Kanagawa Prefecture, Japan, was obtained. Because polyaluminum chloride, an inorganic flocculant used in dewatering sewage sludge, is toxic to microorganisms, the polyaluminum chloride in the dewatered excess sludge was removed by repeated washing with tap water. Specifically, 300 g of dewatered excess sludge and 600 ml of tap water were mixed in a 1000 ml beaker, and the mixture was passed through a three-layer gauze filter to remove the polyaluminum chloride contained in the supernatant. This removal procedure was repeated five times until the pH of the filter effluent reached 6.0 or higher. The washed residue remaining on the filter was completely dried in a dryer FSP450 (ADVANTEC, Tokyo, Japan) at 60°C for 48 hours, crushed into powder, and passed through a 1 mm diameter sieve. The excess sludge powder that passed through the sieve was used as the excess sludge (substrate) in the following tests. In addition, digested sludge, which is a sludge residue composed of hardly biodegradable components generated when anaerobic microorganisms such as methane-producing microorganisms decompose excess sludge as a substrate, was obtained and similarly treated to obtain digested sludge powder. The digested sludge was obtained from a sewage treatment plant in Tokyo, Japan, as dewatered digested sludge (solids content 17%) discharged from the plant.

[0060] (Inoculation of the seed fungus) The inoculum was cultured in a 17 ml glass vial. Specifically, 100 μl of the inoculum was inoculated into a mixture of 10 ml of sterilized water and 100 mg of excess sludge powder or digested sludge powder. Nitrogen gas was introduced into the 7 ml gas phase at the top of the vial for 1 minute, and the vial was sealed with a butyl rubber stopper and aluminum cap. The inoculum was cultured at 30°C, 40°C, or 50°C for 1 month. Hereinafter, the inoculum cultured at 30°C, 40°C, or 50°C for 1 month is referred to as biogas-producing microorganisms. The digested sludge assimilating and biogas-yielding soil microflora is also referred to as DABYS.

[0061] A mixture 14 of 15 mg of dried excess sludge or 15 mg of dried digested sludge, 150 μL of the pre-cultured sludge-decomposing bacterial solution of biogas-producing microorganisms obtained above, and 1.35 mL of pure water was placed inside a first treatment tank 36 in a glass vial 38 having a shape as shown in FIG. In an area of ​​the vial 38 separated from the first treatment tank 36, i.e., the bottom of the vial 38 corresponding to the second treatment tank, the following microalgae, which have been confirmed to live in an environment of pH 8.0 to 12.0 and to grow by fixing carbon dioxide, were placed in the form of dispersion liquid 20 in the culture solution (pH: 9.0) of each microalgae. The strain names of the microalgae contained in dispersion 20 and the pH of dispersion 20 are shown in Table 3. In Comparative Examples 1 to 3, an alkaline aqueous solution with a pH of 9.0, which was a culture medium having the following formulation, was placed in place of the dispersion liquid of microalgae. The composition of the culture medium is as follows: The content of each component in the culture medium is expressed as mass (mg / L) relative to 1 L of pure water. NaNO3:2,500, K2HPO4:500, K2SO4:1,000, MgSO4·7H2O:200, Na2EDTA·2H2O:80, FeSO4·7H2O:10, NaCl:1,000, CaCl2·2H2O:40, H3BO4:2.86, MnSO4·5H2O:2.17, ZnSO4·7H2O:0.222, CuSO4·5H2O:0.079, and Na2MoO4·2H2O:0.021. The above components were dissolved in pure water to obtain an aqueous solution, the pH of which was adjusted to 9.0 by adding an appropriate amount of 1.0 M NaOH, and this solution was used as a culture medium. In Table 3 below, "-" indicates that the component is not included.

[0062] [Table 3]

[0063] When the vial 38 was stored at 25°C for 7 days, methane gas generated from the first treatment tank 36 in the anaerobic atmosphere diffused into the vial 38. Visual observation confirmed that the dispersion liquid 20 in the vial 38 of Examples 1 to 10 became green and turbid, and that microalgae had grown and proliferated in the dispersion liquid 20. According to the evaluation by the present inventors, the density of microalgae in the dispersion liquid was 5 × 10 after treatment at 25°C for 7 days, although it depends on the type of microalgae. 6 cell / mL ~ 34 × 10 6 An increase in cells / mL was observed.

[0064] The contents of hydrogen, methane and carbon dioxide in the gas (i.e., biogas) were quantified under the following conditions. (quantitative determination of hydrogen, methane, and carbon dioxide) After 7 days had passed, the gas phase in the upper part of the vial 38 was sampled, and the amounts of hydrogen, methane, and carbon dioxide were determined. The quantitative determination of hydrogen, methane, and carbon dioxide was carried out using a gas chromatograph GC-8A (Shimadzu Corporation, Kyoto, Japan) equipped with a thermal conductivity detector, a Shincarbon ST column 50 / 80 (2.0 m × 3.0 mm inner diameter, Shinwa Kako Co., Ltd., Kyoto, Japan), an injection volume of 0.5 ml, argon (43.5 ml / min) as the carrier gas, and a column temperature of 80°C. The measurement was carried out three times, and the average value was used as the measured value.

[0065] The results are shown in Figures 5 and 6. Figure 5 is a graph showing the average values ​​of the hydrogen, methane, and carbon dioxide contents measured three times in the gas phase above glass vials 38 of Examples 1 to 10 and Comparative Examples 1 to 3, with digested sludge contained in first treatment tank 36 in glass vials 38 as the evaluation subject. Figure 6 is a graph showing the average values ​​of the hydrogen, methane, and carbon dioxide contents measured three times in the gas phase above the glass vial 38 of Examples 1 to 10 and Comparative Examples 1 to 3, where the evaluation object was the first treatment tank 36 in the glass vial 38 containing excess sludge. 5 and 6, the graph on the left of the vertical axis shows the hydrogen content (mL) contained in the biogas generated per 1 g of dried sludge, the graph in the center shows the methane content (mL), and the graph on the left shows the carbon dioxide content (mL), while the horizontal axis shows the strain name of the microalgae contained in the dispersion liquid. The microalgae used here are algae included in microalgae group B, and the strain names of the microalgae are as listed in Table 3 above. Vials No. 1 to No. 3 represent data in which the dispersion liquid (ie, culture liquid) 20 in the vial bottle 38 does not contain microalgae.

[0066] As shown in Figures 5 and 6, in both cases where digested sludge was used as dried sludge (Figure 5) and where excess sludge was used (Figure 6), it was confirmed that fixation of carbon dioxide progressed in vials No. 4 to No. 13 containing dispersion liquid 20 containing microalgae, and that carbon dioxide was removed from the biogas in vial 38. It was also confirmed that this tendency did not change even when microalgae were replaced due to their proliferation. On the other hand, no decrease in carbon dioxide was observed in the biogas inside vials No. 1 to No. 3. The different carbon dioxide contents in vials No. 1 to No. 3, which correspond to Comparative Examples 1 to 3, indicate that the composition of the biogas generated is not necessarily the same even when excess sludge and methane-producing microorganisms are mixed and cultured under the same conditions. The results of Examples 1 to 10 show that, regardless of the composition of the biogas generated, the biogas production method disclosed herein can produce biogas with good combustion efficiency from which carbon dioxide has been immobilized and removed. [Industrial Applicability]

[0067] According to the biogas production method of the present disclosure, biogas with good combustion efficiency can be produced by immobilizing and removing carbon dioxide from biogas generated from waste containing organic matter such as sludge, and the obtained biogas is useful as fuel. This reduces the amount of waste, and furthermore, because the carbon dioxide in the biogas is immobilized in the microalgae, there is no need to treat the carbon dioxide removed from the biogas, reducing the environmental burden. The food or feed production method disclosed herein can produce microalgae that immobilize carbon dioxide in biogas produced from waste. It has been confirmed that the algae grow when immobilizing carbon dioxide in the biogas. Therefore, the microalgae can be periodically collected and used as a food or feed ingredient. The obtained microalgae do not contain harmful substances and, as described above, contain carotenoids, essential amino acids, and the like, making them useful as food or feed ingredients and expected to be used in a variety of applications, such as processing into health foods and feed.

Claims

1. producing biogas in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms; A step of transporting the biogas produced in the first treatment tank to a second treatment tank containing a dispersion of microalgae that live at a pH of 8.0 to 12.0 and are capable of fixing carbon dioxide dispersed in a culture solution, and fixing the carbon dioxide contained in the biogas to the microalgae; and recovering the biogas having a reduced carbon dioxide content; The methane-producing microorganisms in the first treatment tank include at least one species selected from the following microorganism group A: The microalgae in the second treatment tank include at least one selected from the following microalgae group B: Biogas production method. -Microbial group A- Methanosaeta, Methanobacterium, Methanospirillum, Methanosarcina, Methanobrevibacter, Methanoculleus, Methanomassiliicoccus, Microorganisms belonging to the families Methanosphaera, Methanomethylovorans, and Methanobacteriaceae, and microorganisms belonging to the family Methanosarcinaceae. -Microalgae group B- Algae in the genus Desmodesmus armatus, algae in the genus Tetradesmus obliquus, Algae in the genus Micractinium inermum, algae in the genus Desmodesmus abundans, Algae belonging to the genus Chlorella sorokiniana and algae belonging to the genus Desmodesmus spinosus.

2. The method for producing biogas according to claim 1 , wherein the first treatment tank and the second treatment tank are arranged in the same space.

3. The mixture contained in the first treatment tank is at least one microorganism selected from the group consisting of microorganisms belonging to the family Enterobacteriaceae, microorganisms belonging to the family Pseudomonadaceae, and microorganisms belonging to the family Clostridiaceae, which decomposes at least one of cellulose, chitin, and protein, and takes in glucose, which is a decomposition product of cellulose, glucosamine, which is a decomposition product of chitin, or amino acids, which are decomposition products of protein, to produce hydrogen; and at least one oxygen-consuming microorganism selected from the group consisting of microorganisms belonging to the family Aspergillaceae and microorganisms belonging to the family Arthopyreniaceae; The method for producing biogas according to claim 1 or claim 2, further comprising:

4. The method for producing biogas according to claim 1 , wherein the second treatment tank is configured to contain a material having a total light transmittance of 80% or more.

5. The method for producing biogas according to claim 1 or 2, wherein the second treatment tank is provided with an exposure device that irradiates the microalgae with light.

6. 3. The method for producing biogas according to claim 1, further comprising the step of recovering a portion of the grown microalgae and adding a culture solution for the microalgae when the microalgae grow over time and their carbon dioxide fixation ability is reduced due to the growth of the microalgae.

7. producing biogas in a first treatment tank containing a mixture of sludge and anaerobic methane-producing microorganisms; A step of transporting the biogas produced in the first treatment tank to a second treatment tank containing a dispersion of microalgae that live at a pH of 8.0 to 12.0 and are capable of fixing carbon dioxide dispersed in a culture solution, and fixing the carbon dioxide contained in the biogas to the microalgae; and The method includes recovering a portion of the microalgae grown in the second treatment tank and producing food or feed using the recovered microalgae as a raw material. The methane-producing microorganisms in the first treatment tank include at least one species selected from the following microorganism group A: The microalgae in the second treatment tank include at least one selected from the following microalgae group B: Food or feed manufacturing methods. -Microbial group A- Methanosaeta, Methanobacterium, Methanospirillum, Methanosarcina, Methanobrevibacter, Methanoculleus, Methanomassiliicoccus, Microorganisms belonging to the families Methanosphaera, Methanomethylovorans, and Methanobacteriaceae, and microorganisms belonging to the family Methanosarcinaceae. -Microalgae group B- Algae in the genus Desmodesmus armatus, algae in the genus Tetradesmus obliquus, Algae in the genus Micractinium inermum, algae in the genus Desmodesmus abundans, Algae belonging to the genus Chlorella sorokiniana and algae belonging to the genus Desmodesmus spinosus.

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