A method for methane production from anaerobic digestion of sludge based on direct interspecies electron transfer

Abstract

The present invention provides a method for anaerobic digestion of sludge to produce methane based on direct interspecies electron transfer. A conductive material is added to the sludge, and an anaerobic digestion reaction is carried out under stirring; the conductive material is an SC / FeS composite material. In the present invention, the SC / FeS composite material is added to the sludge as the conductive material. Due to the good biocompatibility, fast mass transfer ability and electron transfer ability of SC; as well as the FeS particles with good conductivity, the anaerobic digestion efficiency of microorganisms can be improved, the resource utilization of wastewater treatment can be realized, and the methane production can be increased by 1.2 - 3.75 times.

Description

Technical Field

[0001] The present invention relates to the technical field of sewage treatment and resource utilization, particularly to the technical field of microbial anaerobic fermentation, and specifically to a method for anaerobic digestion of sludge to produce methane based on direct interspecies electron transfer. Background Art

[0002] Anaerobic digestion mainly occurs under anaerobic conditions, where anaerobic bacteria degrade organic matter and convert it into carbon dioxide, methane, etc. During this process, harmful substances in the sludge are decomposed, the total amount of sludge is reduced, and energy gases such as hydrogen and methane are obtained, achieving sludge stabilization and resource utilization simultaneously. Most organic matter exists within sludge flocs, so appropriate pretreatment is required to promote the release of intracellular organic matter and accelerate subsequent acidogenesis and methanogenesis processes. However, there are significant differences between the acidogenesis stage and the methanogenesis stage in terms of microbial ecology, metabolic rate, environmental adaptability, etc. This makes changes in operating conditions, such as temperature, organic load, pH, etc., extremely likely to disrupt the metabolic balance between acidogenesis and methanogenesis, leading to methanation imbalance and stagnation. Therefore, maintaining the metabolic balance between acidogenesis and methanogenesis is the key to enhancing anaerobic digestion. Acidogenic bacteria and methanogenic bacteria use H2 and formic acid as electron carriers to achieve effective electron exchange between each other, thereby realizing the transformation of organic matter into methane, that is, indirect interspecies electron transfer (MIET). MIET is easily affected by fluctuations in hydrogen partial pressure, resulting in acid imbalance.

[0003] In recent years, research has confirmed that in a syntrophic system composed of electroactive bacteria and methanogenic bacteria, the former can transfer electrons generated by intracellular metabolism to methanogenic bacteria through its conductive flagella and cytochrome C, achieving methane production, and this electron transfer process that does not require the participation of external electron carriers is called direct interspecies electron transfer (DIET). Currently, DIET is widely used in the anaerobic digestion treatment of high-concentration organic wastewater such as beer wastewater and aquaculture wastewater. Compared with MIET, DIET does not involve the generation, diffusion, and utilization of electron carriers, improving the acid imbalance problem, and thus has become a research hotspot and frontier. However, due to the accessibility limitations of the conductive flagella and cytochrome C of electroactive bacteria in anaerobic environments, and the relatively limited construction process of DIET, the efficiency and proportion of its actual application in anaerobic digestion are both small.

[0004] Some people have studied the introduction of conductive materials in DIET to solve the problem of its limited accessibility and enhance DIET efficiency. Currently, metallic iron is mostly used as the conductive material. For example, CN112250271A discloses a method for promoting methane production from excess sludge anaerobic digestion by combining a metallic conductive material with sludge pretreatment, in which a metal mesh and sludge are mixed and then put into an anaerobic digestion reactor for anaerobic digestion to produce methane. In the anaerobic digestion system, Fe ions provide electrons and reduce the redox potential, promoting the methane production rate. However, Fe ions are prone to agglomeration and have poor stability, which inhibits electron transfer and reaction activity, thus limiting their application. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for methane production from sludge anaerobic digestion based on direct interspecies electron transfer, so as to improve the efficiency of the microbial anaerobic digestion system.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] A method for methane production from sludge anaerobic digestion based on direct interspecies electron transfer, adding a conductive material to the sludge and carrying out anaerobic digestion reaction under stirring; the conductive material is SC / FeS composite material. In this application, adding the conductive material can increase the methane production by 1.2 - 3.75 times.

[0008] Further, the weight of the added SC / FeS composite material accounts for 0.3 - 0.8 g / L of the volume of the sludge mixture.

[0009] Further, the stirring conditions are: 30 - 40 °C, 100 - 150 rpm.

[0010] Further, the preparation method of the SC / FeS composite material includes the following steps:

[0011] (1) Heat pyrolyze the dried sodium citrate to obtain a carbon material with a honeycomb structure;

[0012] (2) Add the ferrous chloride solution to the carbon material prepared in step (1), stir and perform ultrasonic dispersion; then centrifuge the mixture to obtain a pyrolytic carbon solid adsorbed with iron ions;

[0013] (3) Add the sodium sulfide solution to the pyrolytic carbon solid in step (2) for mixing;

[0014] (4) Expose the mixture in step (3) to nitrogen and then seal it for aging, and then wash it with anaerobic water and freeze-dry it to obtain the SC / FeS composite material.

[0015] In a further embodiment, in step (1), the temperature-raising pyrolysis is carried out by heating to 600 °C at a heating rate of 5 °C / min and pyrolyzing for 2 - 4 h; then cooling to room temperature at a cooling rate of 5 °C / min.

[0016] In a further embodiment, in step (1), the carbon material is obtained by first washing the pyrolysis product with a hydrochloric acid solution having a concentration of 0.8 - 1.2 mol / L, then washing with deionized water until neutral, and then drying.

[0017] In a further embodiment, the ferrous chloride solution is formed by dissolving ferrous chloride tetrahydrate in deionized water, and the sodium sulfide solution is formed by dissolving sodium sulfide nonahydrate in deionized water; the molar ratio of ferrous chloride tetrahydrate to sodium sulfide nonahydrate is 1 - 2:2, and the mass ratio of sodium sulfide nonahydrate to pyrolytic carbon is 16 - 24:1.

[0018] In a further embodiment, in step (2), the ultrasonic dispersion time is 2 - 4 h to ensure that divalent iron ions are completely adsorbed on the surface of the pyrolytic carbon.

[0019] In a further embodiment, in step (4), the nitrogen purging time is 30 - 60 min, and after sealing, it is placed in a shaker and shaken evenly, and aged for at least 24 h. Then it is washed with anaerobic water and separated by centrifugation or filtration to obtain a solid material, which is freeze-dried in a freeze dryer to obtain the SC / FeS composite material.

[0020] Compared with the prior art, the present invention has the following advantages:

[0021] In the present invention, the SC / FeS composite material is added to the sludge as a conductive material, which can improve the efficiency of microbial anaerobic digestion, realize the resource utilization of wastewater treatment, and increase the methane production by 1.2 - 3.75 times.

[0022] The carbon material in the SC / FeS composite material has a rich specific surface area and excellent chemical stability, which is helpful for the attachment and growth of microorganisms; while iron sulfide (FeS) has high electron mobility and excellent biocompatibility, and can effectively collect and transfer electrons. Therefore, the combination of the two in the present application can give full play to the advantages of both and improve the comprehensive performance of the composite material. Therefore, using the SC / FeS composite material as the conductive material for anaerobic digestion can enhance DIET and promote the efficiency of the anaerobic digestion system.

[0023] The present invention uses gradient temperature pyrolysis technology to convert sodium citrate into a carbon material (SC) with a honeycomb structure, and uses it to adsorb divalent iron ions to synthesize an SC / FeS composite material. When added to the sewage anaerobic treatment system, it can significantly increase the methane production in the microbial anaerobic digestion system. The main reason is attributed to the good biocompatibility, fast mass transfer ability and electron transfer ability of SC; secondly, on this basis, FeS particles with good conductivity are loaded on the surface of SC, making the maximum methane production rate of the microbial anaerobic digestion system double compared with the addition of single SC. This is mainly attributed to the fact that the loading of FeS significantly enhances the conductivity of SC, and the synergistic effect between SC and FeS effectively promotes the microbial anaerobic digestion process. Description of the Drawings

[0024] Figure 1 It is the scanning electron microscope image of the material, where a is the scanning electron microscope image of SC prepared in step (1) of Example 1, b is the scanning electron microscope image of single FeS, and c and d are the scanning electron microscope images of the SC / FeS composite material prepared in Example 1 at different sizes;

[0025] Figure 2 It is the Fourier transform infrared spectroscopy (FTIR) spectra of SC, FeS and the SC / FeS composite material prepared in Example 1;

[0026] Figure 3 It is the X-ray photoelectron spectroscopy (XPS) spectra of SC, FeS and the SC / FeS composite material prepared in Example 1;

[0027] Figure 4 It is the high-resolution XPS spectra of the elements. Among them, a1 is the high-resolution XPS spectra of the C element in the SC material, and a2 is the high-resolution XPS spectra of the C element in the SC / FeS composite material prepared in Example 1.

[0028] b1 is the high-resolution XPS spectra of the S element in the FeS material.

[0029] b2 is the high-resolution XPS spectra of the S element in the SC / FeS composite material prepared in Example 1.

[0030] c1 is the high-resolution XPS spectra of the Fe element in the FeS material.

[0031] c2 is the high-resolution XPS spectra of the Fe element in the SC / FeS composite material prepared in Example 1;

[0032] Figure 5 It is the electrochemical impedance spectroscopy (EIS) of SC, FeS and the SC / FeS composite material prepared in Example 1;

[0033] Figure 6 Line graph of the cumulative methane production of Group B with the addition of SC / FeS composite material and the control group in Example 2;

[0034] Figure 7 Bar graph of the change in methane production of different dosing materials in the anaerobic digestion system after experiencing shock loads in Example 3 of the present invention. Detailed implementation manners

[0035] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention; the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] Example 1:

[0037] A preparation method of an SC / FeS composite material, comprising the following steps:

[0038] (1) Dry sodium citrate particles in an oven at 120 °C for 12 h; then, place the dried sodium citrate in a tubular furnace and heat it to 600 °C at a heating rate of 5 °C / min, and pyrolyze for 2 h; then cool it to room temperature at a cooling rate of 5 °C / min;

[0039] Wash the product obtained after pyrolysis first with 200 mL of hydrochloric acid solution with a concentration of 1 mol / L, then wash it with deionized water until neutral, and finally dry it in an oven at 60 °C for 24 h to obtain a honeycomb-structured carbon material (SC) with a large specific surface area;

[0040] (2) Dissolve 3.43 g (0.017 mol) of ferrous chloride tetrahydrate in deionized water, then add 300 mg of the carbon material prepared in step (1), stir and perform ultrasonic dispersion for 2 h to fully load divalent iron ions on the surface of SC; then centrifuge and filter the mixed solution, and remove the excess ferrous solution to prevent the divalent iron ions adsorbed on the surface of SC from being oxidized by air; obtain a pyrolytic carbon solid adsorbed with iron ions;

[0041] (3) Weigh 4.76 g (0.02 mol) of sodium sulfide nonahydrate and dissolve it in 36 mL of deionized water, and add it to the pyrolytic carbon solid prepared in step (2) for mixing;

[0042] (4) Expose the mixture in step (3) to nitrogen for 30 min, then seal it and place it in a shaker to shake evenly, age for at least 24 h, then wash it 3 times with anaerobic water, centrifuge, filter, and separate to obtain a solid material, and finally put it into a freeze dryer for freeze-drying to obtain an SC / FeS composite material.

[0043] As shown in Figure 1 Figure (a) shows the SEM image of the SC prepared in step (1) of Example 1, (b) is the SEM image of single FeS, and (c) and (d) are the SEM images of the SC / FeS composite materials prepared in Example 1 at different sizes, indicating that the prepared SC / FeS composite materials have higher porosity and larger specific surface area.

[0044] As shown in Figure 2 Figure shows the Fourier transform infrared spectroscopy (FTIR) spectra of SC, FeS, and the SC / FeS composite material prepared in Example 1. The vibration peaks at 1627 cm -1 and 672 cm -1 correspond to C=O and C-H bonds respectively. These bonds may be related to functional groups such as ketones, aldehydes or carboxylic acids and aromatic compounds; the absorption peaks at 1160 cm -1 and 1110 cm -1 correspond to C-O (or C-C) and C-O bonds respectively. These bonds may exist in the form of quinones and hydroquinones. According to the theory of reversible conversion between functional species, phenolic hydroxyl and quinone (C-O) can undergo electron-mediated conversion. Therefore, it is speculated that the SC / FeS composite material may have the function of an electron shuttle, promoting the occurrence of direct interspecies electron transfer (DIET).

[0045] As shown in Figure 3 , Figure 4 Figure shows the X-ray photoelectron spectroscopy (XPS) spectra of SC, FeS, and the SC / FeS composite material prepared in Example 1, as well as the high-resolution XPS spectra of each element, further determining the surface functional groups of the SC / FeS composite material and proving that the SC / FeS composite material has been successfully prepared.

[0046] As shown in Figure 5 Figure shows the electrochemical impedance spectra (EIS) of SC, FeS, and the SC / FeS composite material prepared in Example 1. The results show that the SC / FeS composite material has significantly better conductivity, which helps to enhance the efficiency of the direct electrochemistry reaction (DIET) process.

[0047] Example 2:

[0048] Anaerobic digestion application:

[0049] In this example, the SC / FeS composite material prepared in Example 1 was applied to the anaerobic granular sludge microbial anaerobic digestion methane production system.

[0050] In the experiment, a 500 mL anaerobic serum bottle was used as an anaerobic bioreactor, and the substrate was wastewater with an artificially simulated COD concentration of 4000 mg / L; the anaerobic sludge was taken from the Zhongliang Sewage Treatment Station in Bengbu City, Anhui Province, and its moisture content was 86.29%.

[0051] The experiment was divided into two groups, A and B. Each group had three parallel samples. In each parallel sample, 1.25 g of the above-mentioned anaerobic sludge and 250 mL of wastewater were added. After stirring and mixing, the sludge concentration was 5 g / L.

[0052] Group A: As the control group, no material was added.

[0053] Group B: The SC / FeS composite material prepared in Example 1 was added, and after stirring, the concentration of the SC / FeS composite material in the mixed solution was 0.5 g / L.

[0054] All reaction systems were carried out in a shaking water bath under constant temperature conditions of 35 °C and 120 rpm. The dissolved oxygen in the wastewater was removed by nitrogen stripping to maintain an anaerobic environment, and at the same time, the bottle mouth was sealed. The biogas generated during the reaction process was collected by a syringe, and the relative contents of methane (CH4) and carbon dioxide (CO2) in the biogas were analyzed by gas chromatography, and the air at the top of the reactor was recorded as the gas volume.

[0055] During the reaction process, 5 mL of water samples were taken from the top of the reactor by syringe at the same time point, and then the water samples were extruded and filtered through a 0.45 μm filter membrane to measure the COD in the treated wastewater, and the data was recorded.

[0056] As Figure 6 shows the effect of adding the SC / FeS composite material in Group B on the methane production in the microbial anaerobic digestion system under the initial COD concentration of 4000 mg / L. The addition of the SC / FeS composite material significantly increased the methane production in the microbial anaerobic digestion system, which was 1.5 times higher than that of the control group (Group A) without adding materials. The Gompertz model was used to fit the data (P = P0exp(-exp[Rmaxe(t0 - t) / p0 + 1])), which further confirmed that at the start-up stage of anaerobic digestion, the addition of the SC / FeS composite material increased the maximum methane production rate by 3.75 times compared with the control group. Therefore, the scheme of using the SC / FeS composite material to strengthen the microbial anaerobic digestion system to increase methane production is feasible.

[0057] Example 3

[0058] On the basis of Example 1 and Example 2, experiments were carried out to evaluate the strengthening effect of the SC / FeS composite material on the recovery of shock load (starvation state) in the anaerobic digestion system.

[0059] On the basis of Example 2, two additional groups were set up, with other conditions remaining unchanged. The groups were divided into four groups: A, B, C, and D, with three parallel samples in each group. In each parallel sample, 1.25 g of the above anaerobic sludge and 250 mL of wastewater were added. After stirring and mixing, the sludge concentration was 5 g / L.

[0060] Specifically:

[0061] Group A: As the control group, no material was added.

[0062] Group B: As the control group, the single SC material prepared in Example 1 was added, and after stirring, the concentration of the SC material in the mixed solution was 0.5 g / L.

[0063] Group C: As the control group, a single FeS material was added, and after stirring, the concentration of the FeS material in the mixed solution was 0.5 g / L.

[0064] Group D: The SC / FeS composite material prepared in Example 1 was added, and after stirring, the concentration of the SC / FeS composite material in the mixed solution was 0.5 g / L.

[0065] At each time point, 5 mL of water sample was extracted from the top of the reactor using a syringe, and then 5 mL of simulated artificial wastewater with a COD of 3000 mg / L was added for a 1-month acclimation treatment. Subsequently, a shock load treatment (starvation treatment) was applied for 15 days.

[0066] After the end of this stage, high-concentration synthetic wastewater was added to ensure that the COD concentration of the solution in the anaerobic bioreactor remained at 3000 mg / L. The biogas produced during the reaction was collected through a syringe, and the relative contents of methane (CH4) and carbon dioxide (CO2) in the biogas were analyzed using gas chromatography. The total gas production of the reactor was monitored using the water displacement method.

[0067] As Figure 7 shown, it shows the effect of restoring the addition of exogenous substances on methane production in the anaerobic digestion system after experiencing a shock load (starvation). On the first day of the reaction, the methane production of the group with the addition of the SC / FeS composite material increased by 8.4% compared to the control group; on the sixth day of the reaction, with the recovery of the anaerobic digestion system, the methane production of the SC / FeS composite material group increased by 17.47% compared to the control group, and the cumulative methane production was also 3.8% higher than that of the control group. These results indicate that the addition of the SC / FeS composite material significantly enhanced the shock load resistance of the anaerobic reactor and was beneficial to the recovery of the damaged microbial anaerobic digestion system.

[0068] Example 4:

[0069] A preparation method of an SC / FeS composite material, comprising the following steps:

[0070] (1) Dry the sodium citrate granules in an oven at 100 °C for 15 h; then, place the dried sodium citrate in a tubular furnace and heat it to 600 °C at a heating rate of 5 °C / min, pyrolyze for 4 h; then cool it to room temperature at a cooling rate of 5 °C / min;

[0071] Wash the product obtained after pyrolysis first with 200 mL of hydrochloric acid solution with a concentration of 1.2 mol / L, then wash it with deionized water until neutral, and finally dry it in an oven at 80 °C for 20 h to obtain a carbon material (SC) with a honeycomb structure and a large specific surface area;

[0072] (2) Dissolve 3.94 g (0.02 mol) of ferrous chloride tetrahydrate in deionized water, and then add 300 mg of the carbon material prepared in step (1). Stir and perform ultrasonic dispersion for 4 h to fully load divalent iron ions on the surface of SC; then centrifuge and filter the mixed solution, and remove the excess ferrous solution to prevent the divalent iron ions adsorbed on the surface of SC from being oxidized by air; obtain a pyrolyzed carbon solid adsorbed with iron ions;

[0073] (3) Weigh 7.2 g (0.03 mol) of sodium sulfide nonahydrate and dissolve it in deionized water, and add it to the pyrolyzed carbon solid prepared in step (2) for mixing;

[0074] (4) Expose the mixture in step (3) to nitrogen for 60 min, then seal it and place it in a shaker to shake evenly, age for 28 h, then wash it 3 times with anaerobic water, centrifuge, filter, and separate to obtain a solid material, and finally put it into a freeze dryer for freeze-drying to obtain the SC / FeS composite material.

[0075] Example 5:

[0076] Same as Example 2, the only difference is that the added is the SC / FeS composite material prepared in Example 4, and after stirring at 40 °C and 100 rpm, the concentration of the SC / FeS composite material in the mixed solution is 0.8 g / L.

[0077] After testing, the addition of the SC / FeS composite material significantly increased the methane production in the microbial anaerobic digestion system, which was 3 times higher than that of the control group (Group A) without adding the material.

[0078] Example 6:

[0079] Same as Example 2, the only difference is that the added is the SC / FeS composite material prepared in Example 4, and after stirring at 30 °C and 150 rpm, the concentration of the SC / FeS composite material in the mixed solution is 0.3 g / L.

[0080] After detection, the addition of the SC / FeS composite material significantly increased the methane production in the microbial anaerobic digestion system, which was 1.2 times higher than that of the control group (Group A) without the added material.

[0081] The above content is only an example and explanation of the structure of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar methods for substitution. As long as they do not deviate from the structure of the present invention or exceed the scope defined by this claim book, they should fall within the protection scope of the present invention.

Claims

1. A method for anaerobic digestion of sludge to produce methane based on direct interspecies electron transfer, characterized in that, Add a conductive material to the sludge and carry out an anaerobic digestion reaction under stirring; the conductive material is an SC / FeS composite material; the preparation method of the SC / FeS composite material includes the following steps: (1) Heat the dried sodium citrate by pyrolysis to obtain a carbon material with a honeycomb structure; (2) Add a ferrous chloride solution to the carbon material prepared in step (1), stir and perform ultrasonic dispersion; then centrifuge the mixture to obtain a pyrolytic carbon solid adsorbed with iron ions; (3) Add a sodium sulfide solution to the pyrolytic carbon solid in step (2) for mixing; (4) Expose the mixture in step (3) to nitrogen and then seal it for aging, then wash it with anaerobic water and freeze-dry it to obtain the SC / FeS composite material.

2. The anaerobic digestion method of sludge for methane production according to claim 1, characterized in that The weight of the added SC / FeS composite material accounts for 0.3 - 0.8 g / L of the volume of the sludge mixture.

3. The anaerobic digestion method of sludge for methane production according to claim 1, wherein The conditions for the stirring are 30 - 40 °C and 100 - 150 rpm.

4. The anaerobic digestion method of sludge for methane production according to claim 1, wherein In step (1), the heating by pyrolysis is carried out by heating to 600 °C at a heating rate of 5 °C / min and pyrolyzing for 2 - 4 h; then cooling to room temperature at a cooling rate of 5 °C / min.

5. The anaerobic digestion method of sludge for methane production according to claim 1, characterized in that, In step (1), the carbon material is obtained by first washing the pyrolysis product with a hydrochloric acid solution with a concentration of 0.8 - 1.2 mol / L, then washing it with deionized water until neutral and then drying it.

6. The anaerobic digestion method of sludge for methane production according to claim 1, characterized in that, The ferrous chloride solution is formed by dissolving ferrous chloride tetrahydrate in deionized water, and the sodium sulfide solution is formed by dissolving sodium sulfide nonahydrate in deionized water; the molar ratio of ferrous chloride tetrahydrate to sodium sulfide nonahydrate is 1 - 2:2, and the mass ratio of sodium sulfide nonahydrate to pyrolytic carbon is 16 - 24:

1.

7. The anaerobic digestion method of sludge for methane production according to claim 1, wherein In step (2), the ultrasonic dispersion time is 2 - 4 h.

8. The anaerobic digestion method of sludge for methane production according to claim 1, characterized in that, In step (4), the nitrogen exposure time is 30 - 60 min, and after sealing, it is placed in a shaker and shaken evenly for at least 24 h.

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