Method for improving biogas biological purification efficiency based on nano zero-valent iron composite material

Abstract

The application discloses a method for improving biopurification efficiency of marsh gas based on nano zero-valent iron composite material, which comprises the following steps: adding the nano zero-valent iron composite material into kitchen wastewater, adding anaerobic granular sludge into the mixed system, keeping the pH of the whole reaction system as 7.2+ / -0.2, blowing off for 5 minutes by using nitrogen to keep the anaerobic environment, and collecting the generated marsh gas by using a gas collection bag; wherein the nano zero-valent iron composite material comprises nano zero-valent iron coated by extracellular polymeric substance and / or nano zero-valent iron coated by sodium dodecyl sulfate. The application utilizes the kitchen wastewater to produce methane by anaerobic fermentation, adds the nano zero-valent iron composite material into the anaerobic system, provides H2 for hydrogenophilic methane-producing bacteria by corrosion and hydrogen evolution of the nano zero-valent iron composite material, and further improves the calorific value of the marsh gas.

Description

Technical Field

[0001] This invention relates to a method for improving biogas biopurification efficiency based on nano-zero-valent iron composite materials, belonging to the fields of organic wastewater treatment and utilization, and environmental protection and energy technology. Background Technology

[0002] Anaerobic digestion not only solves the ecological and environmental problems caused by wastewater but also generates biogas and other energy substances to alleviate the current tight energy supply and demand contradiction, featuring low cost and low energy consumption. However, traditional anaerobic digestion technologies generally suffer from incomplete hydrolysis and low methanogenesis efficiency. CO2, as an inert gas present in biogas, reduces the calorific value of biogas, hindering its conversion into renewable natural gas for use in natural gas pipelines. To overcome the problem of low biogas calorific value, biogas purification is a key measure to improve anaerobic digestion. Methods such as physical absorption, chemical absorption, and pressure swing adsorption (PSA) to remove CO2 from the system have drawbacks including high energy consumption, cumbersome operation, and high operating costs. Biogas biological purification, by directly converting CO2 into usable energy within the anaerobic system, can not only reduce CO2 emissions but also improve energy efficiency.

[0003] When CO2 is reduced by H2 and used for hydrogenation, methane is produced, offering advantages such as low operating costs, mild reaction conditions, and environmental friendliness. However, the low gas-liquid mass transfer rate of H2 limits the utilization of H2 by methanogenic bacteria. Nano-zero-valent iron (nZVI) can provide in-situ hydrogen supply within anaerobic systems through hydrogen evolution via corrosion with water, fully utilizing the conversion of H2 and CO2 into CH4 by hydrogen-producing methanogens. Nano-zero-valent iron provides a hydrogen source for microorganisms through corrosion, lowers the redox potential (ORP) to create a favorable environment for microorganisms, stimulates key enzymes to promote hydrolysis and acidification, enhances interspecies direct electron transfer (DIET), and alters the community structure of dominant bacteria in the anaerobic digester, thereby increasing methane production. However, due to the aggregation and passivation characteristics of nZVI, its hydrogen evolution performance is lower than theoretical values. On the other hand, because hydrogen-producing methanogens have low utilization rates of H2 and CO2, the contribution of nano-zero-valent iron to methanogenesis in anaerobic digestion remains low, thus limiting the efficiency of biogas biopurification. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a method for improving the biogas biopurification efficiency based on nano-zero-valent iron composite materials. The method involves physically coating the surface of nZVI with a macromolecular protective layer to prepare the nano-zero-valent iron composite material. The use of this composite material reduces the toxicity of nanoparticles to microorganisms, provides sufficient H2 for hydrogen-methanogenic bacteria, and converts CO2 in the system into CH4, thereby reducing the CO2 content, increasing the methane content, and ultimately improving the biogas biopurification efficiency.

[0005] To achieve the above objectives, this invention provides a method for improving biogas biopurification efficiency based on nano-zero-valent iron composite materials, comprising the following steps: adding nano-zero-valent iron composite materials to kitchen wastewater, adding anaerobic granular sludge to the mixed system, maintaining the pH of the entire reaction system at 7.2±0.2, stripping with nitrogen for 5 minutes to maintain an anaerobic environment, and collecting the generated biogas using a gas collection bag; wherein, the nano-zero-valent iron composite material comprises nano-zero-valent iron coated with extracellular polymer and / or nano-zero-valent iron coated with sodium dodecyl sulfate.

[0006] In one embodiment of the present invention, the concentration of the nano-zero-valent iron composite material in the mixed system of nano-zero-valent iron composite material and kitchen wastewater is 10 g / L.

[0007] In one embodiment of the present invention, the concentration of SCOD in the kitchen wastewater is 13066±123 mg / L, and the kitchen wastewater and anaerobic granular sludge are inoculated at an inoculation ratio of 1 to 1.5:1 (COD:VSS).

[0008] In one embodiment of the present invention, a buffer is used to adjust the pH of the entire reaction system, wherein the buffer is NaHCO3.

[0009] In one embodiment of the present invention, the reaction temperature of the entire reaction system is 35-37°C.

[0010] In one embodiment of the present invention, the preparation method of the nano-zero valent iron composite material includes the following steps: adding nano-zero valent iron to a polymer precursor solution to obtain a dispersion, adjusting the pH of the dispersion to 7.0±0.2, removing air from the system by nitrogen blowing, stirring under anaerobic conditions, and separating the obtained solid to obtain the nano-zero valent iron composite material.

[0011] In one embodiment of the present invention, the polymer precursor is an extracellular polymer and / or sodium dodecyl sulfate, and the concentration of the polymer precursor solution is 6-14 mg / L.

[0012] In one embodiment of the present invention, the concentration of the polymer precursor solution is 10-12 mg / L.

[0013] In one embodiment of the present invention, when the polymer precursor is an extracellular polymer, the concentration of the polymer precursor solution is 10 mg / L.

[0014] In one embodiment of the present invention, when the polymer precursor is sodium dodecyl sulfate, the concentration of the polymer precursor solution is 12 mg / L.

[0015] In one embodiment of the present invention, the concentration of the nano-zero valent iron in the dispersion is 10 g / L.

[0016] In one embodiment of the present invention, the stirring includes mechanical stirring at a speed of 200–400 rpm for 30 minutes, causing the polymer to coat the surface of the nano-zero valent iron, forming a nano-zero valent iron composite material. The polymer is coated on the surface of the nano-zero valent iron in the form of microparticles, thereby forming a slow-release protective layer.

[0017] The present invention also provides a nano-zero-valent iron composite material prepared according to the above preparation method.

[0018] The present invention also provides an application of the above-mentioned nano-zero-valent iron composite material in the field of biogas biopurification.

[0019] Beneficial effects of the present invention

[0020] (1) This invention uses kitchen wastewater as a substrate and employs nano-zero-valent composite material (EPS). 10 -nZVI 10 SDS 12 -nZVI 10 In-situ corrosion for hydrogen supply improves biogas quality. The addition of nano-zero-valent iron composite materials increases the content and stability of methane (CH4) during anaerobic digestion, particularly in EPS. 10 -nZVI 10 SDS 12 -nZVI 10 With the addition of the nano-zero-valent iron composite material, the CH4 content increased, reaching a maximum of 93.33%, which was 22.83% higher than the control group without the addition. The maximum hydrogen enrichment (HE) after adding the nano-zero-valent iron composite material was 561 ppb, indicating improved hydrogen mass transfer efficiency. Due to the addition of the nano-zero-valent iron composite material, the ORP decreased from -101 to -190 mV to -108 to -298 mV, providing a more favorable environment for anaerobic fermentation.

[0021] (2) The material of this invention is simple to synthesize. By modifying the aggregation and passivation characteristics of nZVI, its corrosion and hydrogen evolution performance is further improved, providing a hydrogen source for the bio-methanation process. Biogas bio-purification converts kitchen wastewater into high-purity methane products, realizing the treatment of organic wastewater and the acquisition of renewable energy. This invention has good application prospects. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the reaction apparatus;

[0023] Figure 2 The corrosion and hydrogen evolution of nano-zero valent iron coated with extracellular polymeric materials at different concentrations in aqueous solution;

[0024] Figure 3 The corrosion and hydrogen evolution of nano-zero valent iron coated with sodium dodecyl sulfate at different concentrations in aqueous solution;

[0025] Figure 4 The percentage of gaseous components in kitchen wastewater with different added materials: (a) CH4; (b) CO2; (c) H2;

[0026] Figure 5 The cumulative biogas production of kitchen wastewater varies with the addition of different materials: (a) changes in cumulative biogas production and BMP; (b) cumulative methane and biogas production.

[0027] Figure 6 The changes in pH, HE, and ORP of kitchen wastewater with the addition of different materials: (a) pH; (b) HE; (c) ORP;

[0028] Figure 7 The changes in SCOD and VFA yields of kitchen wastewater with different additives: (a) SCOD; (b) VFA;

[0029] Figure 8 Enzyme activity in kitchen wastewater with different added materials: (a) acid-producing enzymes; (b) methanogenic enzymes;

[0030] Figure 9 TEM images of microbial communities in kitchen wastewater with different added materials: (a) CK; (b) EPS 10 -nZVI 10 (c)SDS 12 -nZVI 10 ;(A)nZVI;(B)EPS;(C)SDS;

[0031] Figure 10 The changes in microbial diversity in kitchen wastewater with the addition of different materials: (left) Bacteria level; (right) Archaea level. Detailed implementation method:

[0032] Example 1: Improving biogas biopurification efficiency using nano-zero-valent iron coated with extracellular polymeric polymer (EPS).

[0033] 1. Determination of the optimal amount of extracellular polymer in nano-zero valent iron composite material (EPS-nZVI)

[0034] Prepare 400 mL of extracellular polymeric material (EPS) solution in the reaction flask, adjust the pH of the solution to 7.0 ± 0.2, and purge with nitrogen gas for 5 min to maintain an anaerobic environment in the reaction apparatus; uniformly disperse nano-zero valent iron into the EPS solution to achieve a concentration of 10 g / L in the dispersion; mechanically stir the dispersion under anaerobic conditions to obtain nano-zero valent iron composite material, and collect the generated hydrogen gas using a gas collection bag; the stirring speed is 200-400 rpm, and the stirring time is 30 min.

[0035] Extracellular polymeric solutions with concentrations of 6 mg / L, 8 mg / L, 10 mg / L, 12 mg / L, and 14 mg / L were prepared. The hydrogen production during the corrosion and hydrogen evolution process of nano-zero valent iron coated with extracellular polymeric solutions of different concentrations was tested using the water displacement method. The results are as follows: Figure 2 As shown, from Figure 2 It can be seen that the hydrogen production is highest when the extracellular polymeric polymer concentration is 10 mg / L.

[0036] 2. Improving biogas biopurification efficiency by utilizing nano-zero-valent iron coated with extracellular polymeric substances (EPS).

[0037] The nano-zero-valent iron composite material (EPS) prepared in step 1 10 -nZVI 10 The nano-zero-valent iron composite material was added to 400 mL of kitchen wastewater diluted 6 times (SCOD = 13066 ± 123 mg / L) to achieve a concentration of 10 g / L and a COD of 13066.7 mg / L in the wastewater. Anaerobic granular sludge was inoculated into the mixed system at an inoculation ratio of 1.2:1 (COD:VSS), using NaHCO3 as a buffer, maintaining the pH at 7.2 ± 0.2, and the reaction temperature at 35-37℃. Nitrogen was used to purge the sludge for 5 minutes to maintain an anaerobic environment. Biogas was collected using a gas collection bag.

[0038] The composition and content of gases were determined using gas chromatography; the levels of VFAs, SCOD, dehydrogenase activity, alkaline protease activity, and acetate kinase activity in the reaction system were also analyzed. 420 The activity and TEM images of microorganisms and their diversity were measured; all measurement methods adopted national standard methods for analysis, and the specific test methods are shown in Table 1.

[0039] Table 1 Test methods for each parameter

[0040]

[0041] Example 2: Improving biogas biopurification efficiency using sodium dodecyl sulfate (SDS)-coated nano-zero valent iron.

[0042] The difference between Example 2 and Example 1 is that in step 1, the extracellular polymeric substance (EPS) in Example 1 was replaced with sodium dodecyl sulfate (SDS). Sodium dodecyl sulfate solutions with concentrations of 6 mg / L, 8 mg / L, 10 mg / L, 12 mg / L, and 14 mg / L were prepared. The hydrogen production during the corrosion and hydrogen evolution process of nano-zero valent iron coated with sodium dodecyl sulfate at different concentrations was tested using the water displacement method. The results are as follows: Figure 3 As shown, from Figure 3 It can be seen that the hydrogen production is highest when the extracellular polymeric substance concentration is 12 mg / L, therefore the optimal concentration of sodium dodecyl sulfate (SDS) can be determined to be 12 mg / L.

[0043] Comparative Example 1: Biogas Biopurification Using Nano-Zero-Valence Iron

[0044] The difference between Comparative Example 1 and Example 1 is that step 1 is omitted and the nano-zero valent iron composite material in step 2 is replaced with nano-zero valent iron.

[0045] Comparative Example 2: Biogas biological purification using EPS

[0046] The difference between Comparative Example 2 and Example 1 is that step 1 is omitted, and the nano-zero-valent iron composite material in step 2 is EPS. 10 -nZVI 10 .

[0047] Comparative Example 3: Biogas Biopurification Using SDS

[0048] The difference between Comparative Example 3 and Example 1 is that step 1 is omitted, and the nano-zero-valent iron composite material in step 2 is SDS. 12 -nZVI 10 .

[0049] Comparative Example 4: Blank Control (CK)

[0050] The difference between Comparative Example 4 and Example 1 is that step 1 is omitted, and step 2 is a system containing only kitchen wastewater and anaerobic granular sludge.

[0051] Experimental results

[0052] (1) The proportion of gaseous components in kitchen wastewater with different added materials

[0053] The biogas biopurification process and the in-situ hydrogen supply of each material in the anaerobic system in Examples 1-2 and Comparative Examples 1-4 are as follows: Figure 4 As shown. Experimental results show that the nano-zero-valent iron composite materials in Examples 1 and 2 significantly improve the biogas purification efficiency, with methane contents reaching 92.56% and 93.37% in Examples 1 and 2, respectively. Figure 4 (a)). From Figure 4 It can be seen that in the anaerobic system, hydrogenotrophic methanogens utilize H2 to convert CO2 into CH4, consistent with the trend of gradually increasing CH4 content and gradually decreasing CO2 and H2 content. Compared to Examples 1 and 2 with added nano-zero-valent iron composite material, the comparative example showed a higher residual CO2 content and a lower H2 content, indicating that the nano-zero-valent iron composite material improved H2 production and the utilization rate of CO2 and H2, which is beneficial for biogas biopurification in the anaerobic system. The composite material formed by EPS and SDS can reduce the passivation of the nZVI surface, thereby releasing more active centers and increasing the hydrogen supply of the system. In addition, the Fe released by the composite material 2+ It can promote methane enrichment to a certain extent, which is more conducive to the growth of microorganisms. Experimental results show that both nano-zero-valent iron composite materials in the examples can increase the CH4 content in biogas. Moreover, compared with uncoated nano-zero-valent iron, the two nano-zero-valent iron composite materials have higher methane content in the anaerobic system, indicating that the composite materials can improve the gas-liquid mass transfer rate and utilization rate of H2 in biogas.

[0054] (2) Changes in the cumulative gas production of kitchen wastewater under the addition of different materials

[0055] The effects of Examples 1-2 and Comparative Examples 1-4 on the cumulative gas production in the anaerobic system are as follows: Figure 5 As shown. Experimental results show that EPS 10 -nZVI 10 SDS 12 -nZVI 10 The addition of all of these can increase cumulative methane production. Figure 5 (a) The biogas yield reached 506.93 ml / gVS and 510.96 ml / gVS, respectively, representing increases of 20.98% and 21.94% compared to Comparative Example 4 (CK). The EPS and SDS-encapsulated nano-zero-valent iron composite material not only increased biogas production but also exhibited a relatively high methane content, increasing by 57.89% and 56.04% respectively compared to Comparative Example 4 (CK). Figure 5 (b)). BMP values ​​are used to assess the degradation rate of organic matter during anaerobic digestion; the higher the BMP, the higher the degradation rate. EPS can be obtained through calculation of biomethane potential. 10 -nZVI 10 SDS 12 -nZVI 10 The addition of [a substance] can increase the BMP value to 99% ( Figure 5 (a) suggests that these two composite materials can significantly improve anaerobic digestion efficiency and enhance methane production. Further evidence indicates that the two nano-zero-valent iron composite materials can increase the CH4 content in biogas, which is beneficial for achieving biogas biopurification within anaerobic systems.

[0056] (3) Changes in pH, HE, and ORP of kitchen wastewater with the addition of different materials

[0057] The effects of Examples 1-2 and Comparative Examples 1-4 on changes in pH, HE, and ORP in the anaerobic system are as follows: Figure 6 As shown. Changes in pH within the anaerobic system can affect the activity of anaerobic bacteria. Compared to Comparative Example 4 (CK), the pH change was smaller after adding the nano-zero-valent iron composite materials from Examples 1 and 2. Figure 6 (a) This may be due to the generation of OH during the hydrogen release process of nano-zero valent iron. - At the end of the anaerobic digestion of kitchen wastewater, the maximum pH of the reaction system was 8.13 (nZVI), which could be maintained within the optimal activity range of anaerobic digesting bacteria. The hydrogen enrichment value (HE) reflects the hydrogen content in the liquid phase. The in-situ hydrogen evolution effect of the nano-zero-valent iron composite material can increase the hydrogen content in the liquid phase, i.e., increase the HE value. The HE value reached its maximum on the second day of the reaction. In Examples 1-2 and Comparative Examples 1-4, the HE values ​​were 550, 561, 540, 259, 266, 531, 279, and 278 ppb, respectively. Figure 6 (b) The study found that the gradual consumption of H2 over 2–6 days was consistent with the increasing CH4 content in biogas, indicating that improved biogas biopurification efficiency promoted CH4 formation. ORP can affect the fermentation type of anaerobic digestion systems, and relatively low ORP is conducive to the occurrence of hydrogen-trophic biomethanation. Figure 6 (c) The addition of the nano-zero-valent iron composite materials in Examples 1 and 2 resulted in a significant decrease in ORP, with the largest changes ranging from 0 to -285 and 0 to -295, respectively. This rapid decrease in ORP promotes the conversion of propionic acid to acetic acid and effectively enhances dehydrogenase activity. The decrease in ORP may be one of the reasons for the increased CH4 production after the addition of the nano-zero-valent iron composite materials.

[0058] (4) Changes in SCOD and VFA of kitchen wastewater with the addition of different materials

[0059] In the anaerobic fermentation process of food wastewater to produce VFA, VFA / SCOD can represent the degree of conversion of dissolved organic matrix into VFA and is an important indicator for measuring acidification efficiency. For example... Figure 7 As shown, in the anaerobic digestion treatment of kitchen wastewater, most of the SCOD is easily degraded by anaerobic microorganisms, and the SCOD concentration gradually decreases. On the first day, the SCOD concentration decreased from the initial concentration of 11920 mg / L to about 5900 mg / L, with a degradation rate of about 50%. After 6 days of anaerobic digestion treatment, the final SCOD concentration in the reaction system was all below 200 mg / L. Figure 7 (a)). From Figure 7As can be seen in (a), after adding the nano-zero-valent iron composite materials of Examples 1 and 2, the initial concentration of COD decreased at a faster rate on the first day, indicating that adding the nano-zero-valent iron composite material is beneficial to reducing COD in kitchen wastewater.

[0060] The concentration of VFAs in kitchen wastewater first increased and then decreased. Figure 7 (b) On day 1, the concentration of VFAs increased from an initial 0.88–0.90 g / L to 1.42–1.80 g / L, reaching its maximum on day 1. The CK group had the highest concentration at 1.809 g / L, which then gradually decreased. This indicates that under these conditions, organic matter was fully decomposed, leading to increased CH4 production. (Comparison) Figure 7 (b) shows the effect curves of each material on VFA concentration. It can be seen that after adding the nano-zero-valent iron composite material in Examples 1 and 2, the concentration of VFA in kitchen wastewater increases at a slower rate within 1 day, and the concentration of VFA decreases faster as time goes on. This indicates that the addition of nano-zero-valent iron composite material is beneficial to improving the efficiency of organic matter decomposition and further increasing the production of CH4.

[0061] (5) Enzyme activity in kitchen wastewater with different added materials

[0062] Multiple enzymes are involved in the anaerobic fermentation of kitchen wastewater to produce methane, such as α-glucosidase, acetate kinase (ACK), and F420. Figure 8 The table shows the activity of key enzymes in the sludge of each system after the reaction. α-Glucosidase can release glucose from oligosaccharide substrates; alkaline protease can promote the hydrolysis of proteins in organic matter; and acetate kinase is a key enzyme in acetic acid production. In Examples 1 and 2, the enzyme activities were significantly higher than those in Comparative Example 4 (CK). The enzyme activities in Examples 1 and 2 were also higher than those in Comparative Example 1 (the system with added nano-zero valent iron). This indicates that the addition of the nano-zero valent iron composite material increased the activity of related enzymes and promoted the hydrolytic fermentation of kitchen wastewater by microorganisms. Figure 8 (a)). Dehydrogenases and coenzyme F 420 It is an important enzyme in the bio-methanation process. Dehydrogenases participate in the conversion of H2 and CO2 to CH4. In Examples 1 and 2, the activities of dehydrogenases were 5.95 and 5.90 μg / min / gVS, respectively, which were 431.25% and 423.79% higher than those in Comparative Example 4 (CK). Furthermore, due to the low ORP provided by the nano-zero-valent iron composite material, the dehydrogenase maintained high activity under these conditions, which is beneficial for the efficient utilization of H2 and CO2 by microorganisms, thereby increasing methane yield. Coenzyme F 420 It is a methanogenase, mainly involved in the production of CH4. Coenzyme F was used in Examples 1 and 2. 420The activity was increased by 224% and 228% respectively compared with the CK group (Comparative Example 4), indicating that EPS 10 -nZVI 10 SDS 12 -nZVI 10 The addition of coenzyme F increased 420 The activity of the nano-zero-valent iron composite material. Furthermore, Fe produced by the dissolution of the nano-zero-valent iron composite material. 2+ It can also increase the activity of related enzymes in the biogas purification system, which promotes CO2 conversion. In Examples 1 and 2, dehydrogenase and coenzyme F... 420 The high activity indicates that the hydrogen-nutritive methanation activity in the biogas purification system is high, which improves the conversion efficiency of H2 and CO2 to CH4.

[0063] (6) TEM images of microbial communities in kitchen wastewater with different added materials

[0064] TEM images of the microorganisms showed cell integrity, indicating that the added nano-zero-valent iron composite material did not damage the cells. Compared to nano-zero-valent iron uniformly coating the cells, the nano-zero-valent iron composite material surrounds the cells, which can reduce the toxic effects of nanoparticles on microorganisms to some extent. Figure 9 (b) The EPS content around the microbial cells was significantly higher than in other groups. The additional EPS can not only coat the nanomaterials but also act as a protective layer for the microorganisms, preventing nZVI particles from damaging the microbial cell membranes. SDS, as a foaming agent, can prevent the aggregation and adsorption of nanoparticles on the microbial surface, maintaining cell viability. This indicates that at a certain dosage, the coating material can prevent the damage of nano-zero valent iron to microbial cells, thereby enhancing CH4 generation capacity. In the comparative group with added nano-zero valent iron, the EPS decreased slightly ( Figure 9 (A) This may be because the small particle size of nZVI particles allows them to adsorb onto the surface of EPS or microbial cells, thereby promoting EPS degradation. Another possible reason for the slight decrease in EPS is that after contact with nZVI, microorganisms adapt to the environment in which nZVI particles are present, gradually reducing EPS secretion.

[0065] (7) Changes in the diversity of microbial communities in kitchen wastewater with the addition of different materials

[0066] Changes in microbial community diversity with the addition of different materials, such as Figure 10 As shown, Figure 10(Left) shows the relative abundance of bacteria at the phylum level in the system. After anaerobic digestion of kitchen wastewater, the dominant bacteria in all examples and comparative examples were mainly composed of Chloroflexi, Bacteroidetes, and Firmicutes, accounting for 62%–69% of the total relative abundance. Chloroflexi can promote the decomposition of sugars in wastewater and produce acetic acid, which is beneficial for anaerobic digestion; Bacteroidetes can convert organic matter into acetic acid, playing an important role in the hydrolysis and acidification stages. Firmicutes can effectively degrade macromolecular substrates such as proteins and lipids, providing substrates for methanogens to produce CH4. The relative abundance of Actinobacteria decreased, which is beneficial for reducing propionic acid accumulation and providing a stable anaerobic digestion environment.

[0067] Figure 10 (Right) shows the relative abundance of archaeal communities at the genus level. The dominant genera in all examples and comparative examples were *Methanomassiliicoccus*, *Methanothrix*, and *Methanobacterium*, among others; these dominant archaeal genera have different methanogenic pathways. (Add EPS) 10 -nZVI 10 SDS 12 -nZVI 10 Subsequently, the relative abundance of *Methanomassiliicoccus* increased. The relative abundance of *Methanothrix* increased by 9.61%–22.22% compared to control example 4 (CK). This is because the nano-zero-valent iron composite material improves the utilization of substrates by acid-producing bacteria, promoting the conversion of propionic acid to acetic acid. During this process, *Methanothrix* converts the increased acetic acid in the system into CH4 as a substrate. Compared to the control group, *Methanobacterium* and *Methanolinea*, hydrogenotrophic methanogens, showed increased relative abundance after the addition of the nano-zero-valent iron composite material. Hydrogenotrophic methanogens can convert H2 and maintain a low hydrogen partial pressure, increasing CH4 production.

[0068] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for improving biogas biopurification efficiency based on nano-zero-valent iron composite materials, characterized in that, Includes the following steps: Nano-zero-valent iron composite material was added to kitchen wastewater, and anaerobic granular sludge was added to the mixed system. The pH of the entire reaction system was maintained at 7.2±0.2, and nitrogen was used to strip the mixture for 5 min to maintain an anaerobic environment. The biogas produced was collected using a gas collection bag. The nano-zero-valent iron composite material comprised either extracellular polymer-coated nano-zero-valent iron or sodium dodecyl sulfate-coated nano-zero-valent iron, with a concentration of 10 g / L. The SCOD concentration in the kitchen wastewater was 13066 ± 123. The concentration of the nano-zero-valent iron composite material is 10 mg / L, and the inoculation ratio of COD of kitchen wastewater to VSS of anaerobic granular sludge is 1~1.5:

1. The preparation method of the nano-zero-valent iron composite material includes the following steps: adding nano-zero-valent iron to an extracellular polymer solution or a sodium dodecyl sulfate solution to obtain a dispersion, adjusting the pH of the dispersion to 7.0±0.2, removing air from the system by nitrogen blowing, stirring under anaerobic conditions, separating the obtained solid, and thus obtaining the nano-zero-valent iron composite material. The mass concentration of the extracellular polymer solution is 10 mg / L, the mass concentration of the sodium dodecyl sulfate solution is 12 mg / L, and the concentration of the nano-zero-valent iron is 10 g / L.

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