Method for producing methane through nano Fe3O4 reinforced anaerobic digestion

Abstract

The invention discloses a method for producing methane through nano Fe3O4 reinforced anaerobic digestion, which comprises the following steps: fully mixing nano Fe3O4 particles with inoculation mud, adding the mixture into an anaerobic digestion reactor, and participating in anaerobic digestion reaction of tempered organic solid waste to produce methane. According to the method for producing methane through nano Fe3O4 reinforced anaerobic digestion, the problems that particles are easy to agglomerate and the reinforcement effect is influenced when nano Fe3O4 particles are directly added into an anaerobic digestion reactor can be effectively solved, and in addition, the biogas residues containing the nano Fe3O4 particles after the anaerobic digestion reaction are sequentially subjected to pyrolysis and ball milling treatment, so that the methane production efficiency is improved. And returning to the anaerobic digestion reactor as an exogenous additive, so that the anaerobic digestion performance of the organic matters can be continuously enhanced, and efficient cyclic utilization of the biogas residues containing Fe3O4 particles is realized.

Description

Technical Field

[0001] The invention belongs to the field of solid waste treatment and resource utilization, and in particular provides a method for producing methane by nano-Fe3O4-enhanced anaerobic digestion. Background Art

[0002] In recent years, anaerobic digestion technology has attracted widespread attention for its unique advantages in integrating waste treatment, energy recovery, and a virtuous ecological cycle. Anaerobic digestion technology can utilize microorganisms to convert organic solid waste into methane, addressing solid waste pollution while also realizing its energy utilization. This technology is of great significance for addressing solid waste pollution and alleviating energy shortages. However, many large-scale biogas projects using organic solid waste (such as food waste, kitchen waste, and livestock and poultry manure) currently face problems such as poor process stability, low methane recovery rates, and poor operational performance. One reason is that the process by which syntrophic methanogenic microorganisms degrade fatty acids and alcohols into methane primarily relies on indirect electron transfer via hydrogen / formic acid. This interspecies electron transfer is limited by the low solubility of hydrogen and the low diffusion rate of formic acid, resulting in low electron transfer efficiency. This, in turn, can lead to slow fatty acid metabolism and accumulation within the anaerobic digestion system, inhibiting the activity of methanogens. Therefore, enhancing interspecies electron transfer during syntrophic methanogenesis is a key breakthrough for improving the performance of anaerobic digestion systems.

[0003] Recent studies have shown that some exogenous conductive materials can act as carriers for direct electron transfer between symbiotic methanogens, transferring electrons generated by symbiotic bacteria directly to electron acceptor methanogens. This direct interspecies electron transfer overcomes the constraints of hydrogen or formic acid diffusion rates and offers higher electron transfer efficiency, significantly enhancing anaerobic digestion reaction rates and increasing methane production. Among these, nano-Fe₃O₄ particles, with their small size and high conductivity, act as nanowires for direct electron transfer between symbiotic methanogen communities. This significantly improves interspecies electron transfer efficiency, promotes the dominant growth of symbiotic methanogens, increases fatty acid decomposition rates, and boosts methane production. However, numerous studies have reported that nano-Fe₃O₄ particles are less effective in enhancing anaerobic digestion methanogenesis due to their tendency to aggregate in anaerobic reactors.

[0004] Therefore, how to effectively improve the enhancement effect of nano-Fe3O4 particles in the anaerobic digestion methanogenesis process has become an urgent problem to be solved. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for enhancing anaerobic digestion and methane production by nano-Fe3O4, so as to solve the problem that nano-Fe3O4 particles are easily agglomerated when directly added to the anaerobic digestion reactor, thereby affecting the enhancement effect.

[0006] The technical solution provided by the present invention is: a method for producing methane by anaerobic digestion enhanced by nano-Fe3O4, comprising: fully mixing nano-Fe3O4 particles with inoculated mud and adding the mixture into an anaerobic digestion reactor, participating in the anaerobic digestion reaction of conditioned organic solid waste to produce methane.

[0007] Preferably, the nano-Fe3O4 particles are mixed with the seeding mud by ultrasonic dispersion treatment, and the ultrasonic dispersion treatment time is 30-60 minutes.

[0008] More preferably, the particle size of the nano Fe3O4 particles is 20-100 nm.

[0009] Further preferably, in the anaerobic digestion reactor, the addition concentration of nano-Fe3O4 particles is 2-10 g / L, the organic load is 25-30 g VS / L, and the reaction temperature is 36-40°C.

[0010] Further preferably, the conditioning method of organic solid waste is as follows: the organic solid waste is crushed and homogenized to form a slurry and sent to a regulating tank, wherein the pH value of the material in the regulating tank is adjusted to be near neutral, and then the material in the regulating tank is sent to an anaerobic digestion reactor.

[0011] Further preferably, the nano-Fe3O4 enhanced anaerobic digestion and methane production method further comprises the following step: returning the biogas slurry produced by the anaerobic digestion reaction to the regulating tank.

[0012] Further preferably, the method for enhancing anaerobic digestion and methane production by nano-Fe3O4 also includes the following steps: pyrolysis and ball milling of the biogas residue containing nano-Fe3O4 particles after the anaerobic digestion reaction are performed in sequence to obtain biogas residue biochar containing Fe3O4 particles with good dispersion and uniform particle size; and then, the biogas residue biochar is returned to the anaerobic digestion reactor as an exogenous additive to continuously enhance the anaerobic digestion performance of organic matter.

[0013] It is further preferred that before the biogas residue containing nano-Fe3O4 particles is subjected to pyrolysis treatment, the biogas residue containing nano-Fe3O4 particles is sequentially dehydrated, dried and crushed, and after the pyrolysis treatment, biogas residue biochar containing nano-Fe3O4 particles is obtained, wherein the pyrolysis temperature is 500-700°C.

[0014] It is further preferred that the biogas residue biochar is washed and dried in sequence before being ball milled, and after ball milling, biogas residue biochar containing nano-Fe3O4 particles with good dispersibility and uniform particle size is obtained.

[0015] More preferably, the organic solid waste is food waste, kitchen waste or livestock and poultry manure.

[0016] The nano-Fe3O4 enhanced anaerobic digestion methane production method provided by the present invention can effectively solve the problem that the nano-Fe3O4 particles are prone to agglomeration and affect the enhancement effect when directly adding the nano-Fe3O4 particles to the anaerobic digestion reactor by fully mixing the nano-Fe3O4 particles with inoculated mud and then adding them to the anaerobic digestion reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments:

[0018] Figure 1 Flow chart of the nano-Fe3O4 enhanced anaerobic digestion and methane production method provided by the present invention;

[0019] Figure 2 is the methane yield data of the anaerobic digestion reaction in Example 1;

[0020] Figure 3 is the cumulative methane production data of the anaerobic digestion reaction in Example 1;

[0021] Figure 4 is the pH value data of the anaerobic digestion reaction in Example 1;

[0022] Figure 5 is the organic acid concentration of the anaerobic digestion reaction in Example 1;

[0023] Figure 6 is the methane yield data of the anaerobic digestion reaction in Example 2;

[0024] Figure 7 is the cumulative methane production data of the anaerobic digestion reaction in Example 2;

[0025] Figure 8 is the pH value data of the anaerobic digestion reaction in Example 2;

[0026] Figure 9 is the organic acid concentration of the anaerobic digestion reaction in Example 2;

[0027] Figure 10 This is the archaeal community structure in Example 1;

[0028] Figure 11 The bacterial community structure in Example 1;

[0029] Figure 12 This is the archaeal community structure in Example 2;

[0030] Figure 13 The bacterial community structure in Example 2;

[0031] Figure 14 This is the apparent morphology of the digestate biochar containing Fe3O4 particles in Example 1;

[0032] Figure 15 This is the apparent morphology of the digestate biochar containing Fe3O4 particles in Example 2;

[0033] Figure 16 This is a comparison chart of the cumulative methane production of the anaerobic digestion reactions in Example 1 and Example 2. DETAILED DESCRIPTION

[0034] The present invention will be further explained below with reference to specific embodiments, but the present invention is not limited thereto.

[0035] like Figure 1 As shown, the present invention provides a method for producing methane by anaerobic digestion enhanced by nano-Fe3O4, comprising:

[0036] The nano-Fe3O4 particles are fully mixed with the inoculated mud and added into the anaerobic digestion reactor to participate in the anaerobic digestion reaction of the conditioned organic solid waste (such as restaurant kitchen waste, kitchen waste, livestock and poultry manure, etc.) to produce methane.

[0037] The nano-Fe3O4 enhanced anaerobic digestion methane production method can effectively solve the problem that the nano-Fe3O4 particles are easily agglomerated and the enhancement effect is affected when the nano-Fe3O4 particles are directly added to the anaerobic digestion reactor by fully mixing the nano-Fe3O4 particles with the inoculated mud.

[0038] The nano-Fe3O4 particles are mixed with the seeding mud by ultrasonic dispersion treatment to prepare the seeding mud with good dispersion and uniform mixing of the nano-Fe3O4 particles. Preferably, the ultrasonic dispersion treatment time is 30-60 minutes.

[0039] Among them, the particle size of the nano Fe3O4 particles is 20-100nm, the particle size is small and the particles are relatively uniform, which can act as nano wires for directly transferring electrons between mutually symbiotic methanogenic microbial communities, thereby increasing the microbial electron transfer rate and efficiently enhancing the anaerobic digestion of organic matter to produce methane.

[0040] In the anaerobic digestion reactor, the addition concentration of nano-Fe3O4 particles is 2-10 g / L, the solid content is 8-10%, the organic load is 25-30 g VS / L, and the temperature is 36-40°C.

[0041] Among them, the conditioning methods of organic solid waste are as follows:

[0042] Organic solid waste is crushed and homogenized to form a slurry and sent to a regulating tank. In the regulating tank, the pH value of the material is adjusted to near neutrality (about 7.0). After that, the material in the regulating tank is sent to an anaerobic digestion reactor. The method for adjusting the pH value of the material to near neutrality is to add sodium hydroxide and / or recirculate the biogas slurry produced by the anaerobic digestion reaction. Taking food waste as an example, the pH value of the biogas slurry is about 8.5. After recirculation, the acidity of the food waste can be neutralized, and the rich microorganisms in the biogas slurry can improve the richness and diversity of microorganisms in the anaerobic digestion reaction.

[0043] The nano-Fe3O4 enhanced anaerobic digestion and methane production method further comprises the following steps:

[0044] The biogas residue containing nano-Fe3O4 particles after anaerobic digestion reaction is pyrolyzed and ball-milled to produce biogas residue biochar containing Fe3O4 particles with good dispersion and uniform particle size. The biochar is returned to the anaerobic digestion reactor as an exogenous additive to continuously enhance the anaerobic digestion performance of organic matter and realize the efficient recycling of biogas residue containing Fe3O4 particles.

[0045] The specific steps are as follows:

[0046] (1) Separating the biogas residue and biogas liquid after the anaerobic digestion reaction to obtain the biogas residue containing nano-Fe3O4 particles, wherein the biogas liquid can be returned to the organic solid waste regulating tank for mixing materials and replenishing some microorganisms;

[0047] (2) The biogas residue containing nano-Fe3O4 particles is sequentially dehydrated, dried, and crushed, and then put into a pyrolysis reactor for pyrolysis treatment to obtain biogas residue biochar containing nano-Fe3O4 particles; preferably, the dehydration is mechanical dehydration, and a screw extruder can be used to reduce the moisture content of the biogas residue to 70%; the drying is thermal drying, and the thermal drying temperature is 70-90°C and the time is 3-5h; the crushing is a mechanical crusher, and the particle size of the biogas residue obtained is 0.1-0.15mm; the biogas residue is heated in the pyrolysis reaction. A pyrolysis reaction occurs in the reactor, wherein the pyrolysis temperature is 500-700°C and the residence time is 1-3 hours. During the pyrolysis reaction, the nano-Fe3O4 particles in the biogas residue act as a catalyst, catalyzing the in-situ pyrolysis of the biogas residue, thereby improving the pyrolysis efficiency and the yield of pyrolysis products, thereby producing synthesis gas, pyrolysis oil, and biogas residue biochar containing nano-Fe3O4 particles. The pyrolysis temperature (500-700°C) is lower than the reaction temperature of the biochar and the nano-Fe3O4, thereby avoiding the loss of Fe3O4 due to side reactions.

[0048] (3) The biochar is washed and dried, and then ball-milled to obtain biochar containing nano-Fe3O4 particles with good dispersion and uniform particle size; preferably, the washing is divided into two steps of alcohol washing and water washing, and ethanol and deionized water are used for repeated washing 2-3 times; the drying temperature is 90-105°C, and the time is 3-5h; the ball milling is carried out using a planetary ball mill with an orbital speed of 300-400r / min, a rotation speed of 600-800r / min, a forward rotation of 1.5-2h, and a reverse rotation of 1.5-2h; during the ball milling process, the high-energy shear force can further squeeze the Fe3O4 particles scattered on the surface of the biochar into the pores of the biochar, thereby improving the dispersibility of the Fe3O4 particles in the biochar and making the two fully compounded;

[0049] (4) The sludge biochar containing nano-Fe3O4 particles with good dispersibility and uniform particle size is used as an additive and returned to the anaerobic digestion reactor to continuously enhance anaerobic digestion and methane production, thereby achieving efficient recycling of the sludge containing Fe3O4 particles. Preferably, the dosage of the additive is 2-10 g / L. Adding the sludge biochar containing nano-Fe3O4 particles with good dispersibility and uniform particle size into the anaerobic digestion reactor is conducive to the formation of the effect of synergistic enhancement of electron transfer between biochar and Fe3O4 and mutualistic methanogenic microorganisms, further improving the methane production in the anaerobic digestion reactor, and at the same time achieving efficient resource utilization of the sludge containing Fe3O4.

[0050] The above-mentioned method for enhancing anaerobic digestion and methane production by nano-Fe3O4 can enhance anaerobic digestion and methane production by inoculating inoculum mud uniformly mixed with nano-Fe3O4 into the organic solid waste anaerobic digestion system; by pyrolysis and ball milling the biogas residue containing nano-Fe3O4 particles after the reaction, biogas residue biochar containing nano-Fe3O4 particles with excellent performance can be produced; by returning the biogas residue biochar containing nano-Fe3O4 particles as an exogenous additive to the anaerobic digestion system, the anaerobic digestion performance of organic matter can be continuously enhanced, and the efficient recycling of Fe3O4-containing biogas residue can be achieved.

[0051] Example 1

[0052] Food waste was collected from a school cafeteria and manually sorted to remove biodegradable materials (such as bones, plastic, and napkins). The sorted food waste was then crushed in a pulper to create a slurry. The slurry was then fed into a conditioning tank, where the pH was adjusted to a neutral pH of 7.0 before being added to an anaerobic digester. The food waste had a moisture content of 73.18%, a total solids content of 268.24 g / kg, and an organic matter content (VS) of 244.63 g / kg. The inoculum sludge was obtained from an anaerobic digester at a sludge treatment plant and used after a 30-day acclimatization period. The inoculum sludge had a moisture content of 94.51%, a total solids content of 54.92 g / kg, and a biomass of 23.39 g / kg. The nano-Fe₃O₄ particles had a particle size of 20 nm.

[0053] Fe3O4 particles with a particle size of 20 nm were mixed with the inoculum mud, and after ultrasonic dispersion treatment for 30 minutes, they were put into the anaerobic digestion reactor together with the pH-adjusted food waste slurry to start the anaerobic digestion reaction. In the anaerobic digestion reactor, the organic load was 30 g VS / L and the reaction temperature was 37°C.

[0054] Blank control group: no nano-Fe3O4 particles were added.

[0055] Experimental group 1: The added concentration of nano-Fe3O4 particles was 2g / L.

[0056] Experimental group 2: The added concentration of nano-Fe3O4 particles was 5g / L.

[0057] Experimental group three: The added concentration of nano-Fe3O4 particles was 10 g / L.

[0058] Figure 2 The methane yield data for anaerobic digestion reactions show that the blank control anaerobic system began producing methane after a 24-day gas production lag phase, reaching peak methane production (44.2 mL / g VS day) on day 57 and ceasing gas production on day 83. Experimental groups 1, 2, and 3 had gas production lag phases of 11, 5, and 5 days, respectively, reaching peak methane production of 65.53, 57.13, and 45.83 mL / g VS day on days 33, 18, and 14, respectively, and ceasing gas production on days 59, 61, and 56. Compared with the blank control, the gas production lag phases in experimental groups 1, 2, and 3 were shortened by 13, 19, and 19 days, respectively, and peak methane production was achieved earlier by 24, 39, and 43 days, respectively. The reaction time was shortened by 24, 22, and 27 days, respectively. Adding 2-10 g / L of 20 nm Fe₃O₄ nanoparticles significantly increased the anaerobic digestion reaction rate and shortened the reaction cycle. Figure 3The cumulative methane production data of the anaerobic digestion reaction showed that the cumulative methane production of the blank control group was 606.2 mL / g VS, while the cumulative methane production of experimental groups one, two, and three were 744.62, 740.06, and 719.22 mL / g VS, respectively, which were 22.83%, 22.08%, and 18.64% higher than that of the blank control group. Nano-Fe3O4 particles significantly increased methane production. Figure 4-5 The data of pH value and organic acid concentration of anaerobic digestion reaction show that the maximum organic acid concentration of the blank control group is 20465.60 mg / L, and the pH value recovers to above 7.0 on the 30th day. The maximum organic acid concentrations of experimental groups one, two and three are 17076.28, 14824.99 and 13578.82 mg / L, respectively, which are 16.56%, 27.56% and 33.65% lower than those of the blank control group, and the pH value recovers to above 7.0 on the 20th, 14th and 14th days, respectively, which are 10, 16 and 16 days earlier than those of the blank control group. The addition of nano-Fe3O4 particles accelerates the degradation of organic acids and improves the stability of the system. Figure 10-11 The bacterial and archaeal community structures revealed that the relative abundances of the electroactive bacteria Syntrophomonas and Methanosarcina were 2.26% and 43.23% in the blank control group during peak gas production, respectively. In experimental groups 1, 2, and 3, the relative abundances of Syntrophomonas ranged from 8.59% to 14.11%, and Methanosarcina ranged from 79.13% to 86.55%, both significantly higher than in the blank control group. This suggests that the addition of nano-Fe₃O₄ particles induced the enrichment of a symbiotic methanogenic community composed of electroactive Syntrophomonas and Methanosarcina, and through direct electron transfer between the two, rapid organic acid degradation and methanogenesis were achieved.

[0059] The biogas slurry and residue after anaerobic digestion are separated and returned to the conditioning tank to adjust the material properties and replenish microorganisms. The pH of the returned biogas slurry is around 8.5, which neutralizes the acidity of the food waste (as low as 4.5) and saves dilution water, saving water costs. The rich microorganisms in the biogas slurry can also improve the richness and diversity of microorganisms in the anaerobic system.

[0060] The biogas residue was mechanically dehydrated to reduce its moisture content to 70%, and dried at 90°C for 3 hours. The dried biogas residue was mechanically crushed to a particle size of 0.15 mm. The biogas residue containing nano-Fe3O4 was pyrolyzed in a pyrolysis reactor at a temperature of 700°C for 2 hours to produce synthesis gas, pyrolysis oil and biogas residue biochar containing nano-Fe3O4. The biogas residue biochar was washed three times with ethanol and deionized water to remove impurities, and dried at 105°C for 3 hours. The dried biogas residue biochar was added to a planetary ball mill for ball milling treatment, with an orbital speed of 400r / min, a rotation speed of 800r / min, forward rotation for 1.5 hours, and reverse rotation for 1.5 hours to obtain biogas residue biochar containing nano-Fe3O4 after ball milling. By Figure 14 The surface morphology of the ball-milled biochar containing nano-Fe3O4 shows that the black base is the biochar skeleton, and the gray spherical particles are nano-Fe3O4 particles. The nano-Fe3O4 particles are not only dispersed and attached to the surface of the biochar, but also densely distributed in the internal pores of the biochar, successfully compounding with the biochar to produce biochar containing nano-Fe3O4 with good dispersion and uniform particles. The ball-milled biochar containing nano-Fe3O4 was added to the anaerobic digestion reactor as an additive at a dosage of 2-10g / L. This facilitated the synergistic enhancement of electron transfer between biochar and Fe3O4, further improving the performance of the anaerobic system and achieving efficient recycling of the Fe3O4-containing biochar.

[0061] Example 2

[0062] Food waste was collected from a school cafeteria and manually sorted to remove biodegradable materials (such as bones, plastic, and napkins). The sorted food waste was then crushed in a pulper to create a slurry. The slurry was then fed into a conditioning tank, where the pH was adjusted to a neutral pH of 7.0 before being added to an anaerobic digester. The food waste had a moisture content of 73.18%, a total solids content of 268.24 g / kg, and an organic matter content (VS) of 244.63 g / kg. The inoculum sludge was obtained from an anaerobic digester at a sludge treatment plant and used after a 30-day acclimatization period. The inoculum sludge had a moisture content of 94.51%, a total solids content of 54.92 g / kg, and a biomass of 23.39 ± 0.18 g / kg. The nano-Fe₃O₄ particles had a particle size of 100 nm.

[0063] Fe3O4 particles with a particle size of 100 nm were mixed with the inoculum mud, and after ultrasonic dispersion treatment for 30 minutes, they were put into the anaerobic digestion reactor together with the food waste slurry after pH adjustment to start the anaerobic digestion reaction. In the anaerobic digestion reactor, the organic load was 30 g VS / L and the reaction temperature was 37°C.

[0064] Blank control group: no nano-Fe3O4 particles were added.

[0065] Experimental group 1: The added concentration of nano-Fe3O4 particles was 2g / L.

[0066] Experimental group 2: The added concentration of nano-Fe3O4 particles was 5g / L.

[0067] Experimental group three: The added concentration of nano-Fe3O4 particles was 10 g / L.

[0068] Figure 6 Anaerobic digestion methane yield data showed that the blank control anaerobic system began producing methane after a 24-day gas production lag phase, reaching peak methane production (44.2 mL / g VS day) on day 57 and ceasing gas production on day 83. Experimental groups 1, 2, and 3 had gas production lag phases of 13, 7, and 4 days, respectively, reaching peak methane production of 50.52, 53.83, and 53.03 mL / g VS day on days 50, 20, and 12, respectively, and ceasing gas production on days 71, 62, and 54. Compared with the blank control, experimental groups 1, 2, and 3 had gas production lag phases shortened by 11, 17, and 20 days, respectively, reaching peak methane production earlier by 7, 37, and 45 days, respectively, and shortening reaction times by 12, 21, and 29 days, respectively. Adding 2-10 g / L of 100 nm Fe₃O₄ nanoparticles significantly increased the anaerobic digestion reaction rate and shortened the reaction cycle. Figure 7 The cumulative methane production data of the anaerobic system showed that the cumulative methane production of the blank control group was 606.2 mL / g VS, while the cumulative methane production of experimental groups one, two, and three were 783.02, 739.11, and 741.63 mL / g VS, respectively, which were 29.17%, 21.93%, and 22.34% higher than that of the blank control group. Nano-Fe3O4 particles significantly increased methane production. Figure 8-9 The data of pH value and organic acid concentration of anaerobic system showed that the maximum organic acid concentration of blank control group was 20465.60 mg / L, and the pH value recovered to above 7.0 on the 30th day. The maximum organic acid concentrations of experimental groups one, two and three were 16187.86, 15921.60 and 13103.14 mg / L, respectively, which were 20.90%, 22.20% and 35.97% lower than those of blank control group, and the pH value recovered to above 7.0 on the 26th, 14th and 14th days, respectively, which were 4, 16 and 16 days earlier than those of blank control group. The addition of nano-Fe3O4 particles accelerated the degradation of organic acid and improved the stability of the system. Figure 12-13The bacterial and archaeal community structures revealed that the relative abundances of the electroactive bacteria Syntrophomonas and Methanosarcina were 2.26% and 43.23% in the blank control group during peak gas production, respectively. In experimental groups 1, 2, and 3, the relative abundances of Syntrophomonas ranged from 12.59% to 25.23%, and Methanosarcina ranged from 81.47% to 98.29%, both significantly higher than in the blank control group. This suggests that the addition of nano-Fe₃O₄ particles induced the enrichment of a symbiotic methanogenic community composed of electroactive Syntrophomonas and Methanosarcina, and through direct electron transfer between the two, rapid organic acid degradation and methanogenesis were achieved.

[0069] The anaerobic digestion process separates the biogas slurry from the residue and returns it to the conditioning tank to adjust the material properties and replenish microorganisms. The pH of the returned biogas is around 8.5, which neutralizes the acidity of the food waste (as low as 4.5) and saves dilution water, reducing water costs. The biogas slurry's rich microbial content also increases the richness and diversity of microorganisms in the anaerobic system.

[0070] The biogas residue was mechanically dehydrated to a moisture content of 70% and dried at 80°C for 5 hours. The dried biogas residue was mechanically crushed to a particle size of 0.15 mm. The biogas residue containing nano-Fe3O4 was pyrolyzed in a pyrolysis reactor at a temperature of 600°C for 3 hours to produce synthesis gas, pyrolysis oil and biogas residue biochar containing nano-Fe3O4. The biogas residue biochar was washed three times with ethanol and deionized water to remove impurities, and dried at 95°C for 5 hours. The dried biogas residue biochar was added to a planetary ball mill for ball milling treatment with an orbital speed of 300r / min, a rotation speed of 600r / min, forward rotation for 2 hours, and reverse rotation for 2 hours to obtain biogas residue biochar containing nano-Fe3O4 after ball milling. By Figure 15 The surface morphology of the ball-milled biochar containing nano-Fe3O4 shows that the black base is the biochar skeleton, and the gray spherical particles are nano-Fe3O4 particles. The nano-Fe3O4 particles are not only dispersed and attached to the surface of the biochar, but also densely distributed in the internal pores of the biochar, successfully compounding with the biochar to produce biochar containing nano-Fe3O4 with good dispersion and uniform particles. The ball-milled biochar containing nano-Fe3O4 was added to the anaerobic digestion reactor as an additive at a dosage of 2-10g / L. This facilitated the synergistic enhancement of electron transfer between biochar and Fe3O4, further improving the performance of the anaerobic system and achieving efficient recycling of the Fe3O4-containing biochar.

[0071] By comparing the cumulative methane production of anaerobic digestion reactions in Example 1 and Example 2 ( Figure 16) and found that both 20nm and 100nm nano-Fe3O4 particles can significantly increase the anaerobic digestion reaction rate, shorten the reaction cycle, increase methane production, and enhance system stability. The comprehensive effect is better when the dosage of nano-Fe3O4 with a particle size of 100nm is 2g / L.

[0072] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the scope of knowledge possessed by ordinary technicians in this field without departing from the scope of the present invention.

Claims

1. A method for producing methane by anaerobic digestion enhanced by nano-Fe3O4, characterized in that: include: The nano-Fe3O4 particles are fully mixed with the inoculated mud and added into the anaerobic digestion reactor to participate in the anaerobic digestion reaction of the conditioned organic solid waste to produce methane.

2. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 1, characterized in that: The nano-Fe3O4 particles are mixed with the seeding mud by ultrasonic dispersion treatment, and the ultrasonic dispersion treatment time is 30-60 minutes.

3. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 1, characterized in that: The particle size of the nano Fe3O4 particles is 20-100 nm.

4. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 1, characterized in that: In the anaerobic digestion reactor, the addition concentration of nano-Fe3O4 particles is 2-10 g / L, the organic load is 25-30 g VS / L, and the reaction temperature is 36-40°C.

5. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 1, characterized in that: The conditioning method of organic solid waste is as follows: the organic solid waste is crushed and homogenized to form a slurry and sent to a regulating tank, wherein the pH value of the material in the regulating tank is adjusted to be near neutral, and then the material in the regulating tank is sent to an anaerobic digestion reactor.

6. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 5, characterized in that: The method further comprises the following steps: returning the biogas slurry produced by the anaerobic digestion reaction to the regulating tank.

7. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 1, characterized in that: The method also includes the following steps: pyrolyzing and ball-milling the biogas residue containing nano-Fe3O4 particles after the anaerobic digestion reaction in sequence to obtain biogas residue biochar containing Fe3O4 particles with good dispersion and uniform particle size; and then returning the biogas residue biochar as an exogenous additive to the anaerobic digestion reactor to continuously enhance the anaerobic digestion performance of organic matter.

8. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 7, characterized in that: Before the biogas residue containing nano-Fe3O4 particles is subjected to pyrolysis treatment, the biogas residue containing nano-Fe3O4 particles is sequentially dehydrated, dried and crushed. After the pyrolysis treatment, biogas residue biochar containing nano-Fe3O4 particles is obtained, wherein the pyrolysis temperature is 500-700°C.

9. The method for enhancing anaerobic digestion and methane production by nano-Fe3O4 according to claim 8, characterized in that: Before the biogas residue biochar is subjected to ball milling treatment, the biogas residue biochar is sequentially washed and dried. After the ball milling treatment, biogas residue biochar containing nano-Fe3O4 particles with good dispersibility and uniform particle size is obtained.

10. The method for enhancing methanogenesis by anaerobic digestion with nano-Fe3O4 according to any one of claims 1 to 9, characterized in that: The organic solid waste is food waste, kitchen waste or livestock and poultry manure.

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