Preparation method of biochar capable of enhancing methane production efficiency of anaerobic digestion of sludge

Abstract

The invention provides a preparation method of biochar capable of enhancing the efficiency of producing methane by anaerobic digestion of sludge. The biochar comprises the following components: straws and a 30% H2O2 solution. The preparation method comprises the following steps: drying straw particles, and performing pyrolysis to prepare primary biochar; washing the prepared primary biochar, drying, grinding, adding into a 30% H2O2 solution, stirring, and heating; and finally, centrifuging, cleaning and drying to obtain the final biochar. According to the method, the functional group structure on the surface of the primary biochar is changed by utilizing the oxidability of H2O2, so that the electron transfer capacity is improved, and when the primary biochar is added into a sludge anaerobic digestion treatment system, the accumulative methane yield can be increased by 42.4%-53.5%, and the maximum methane production rate is increased by 85.5%-98.2%.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid waste resource processing, and particularly relates to a method for preparing biochar capable of enhancing the methanogenic efficiency of sludge anaerobic digestion. BACKGROUND

[0002] With the rapid development of China's economic construction and the continuous improvement of social productivity, the amount of sewage in China is increasing, and the sewage treatment capacity has also been greatly improved, resulting in an increasing amount of sludge. Under normal circumstances, municipal sludge generated by municipal wastewater treatment plants contains nutrients such as nitrogen and phosphorus in wastewater, as well as toxic and harmful substances such as polycyclic aromatic hydrocarbons, halides, heavy metals, parasites, pathogenic bacteria, and infectious viruses. Therefore, these sludges are polluting and must be treated by harmless, stabilization, and reduction processes before they can be returned to the natural environment through sanitary landfill, land use, and other means to reduce the harmfulness of secondary pollution. On the other hand, sludge collects the energy contained in municipal wastewater and has resource properties. Recycling energy from sludge has great environmental value and economic benefits for reducing the net energy consumption of municipal wastewater treatment plants.

[0003] Anaerobic digestion is the process of converting organic matter into methane, carbon dioxide, and water through the metabolic activity of anaerobic microorganisms under anaerobic or anoxic conditions. Through the anaerobic digestion process, the total amount of sludge is reduced, and harmful substances in the sludge are decomposed. The mixture gas composed of methane and carbon dioxide is called biogas, which is converted into biomethane after purification and upgrading, and is a renewable energy source. Therefore, anaerobic digestion achieves sludge stabilization and resource utilization at the same time. In the anaerobic digestion process, the substrate available to methanogens is limited, and hydrolytic fermenting bacteria and hydrogen-producing acetogenic bacteria convert substrates that methanogens cannot directly utilize into acetate, hydrogen, and carbon dioxide, which can be directly utilized by methanogens. This interdependent nutritional relationship is called methanogenic syntrophic metabolism, and the essence is electron transfer between microorganisms, i.e., interspecies electron transfer process. The efficiency of interspecies electron transfer and syntrophic relationship between acid-producing and methanogenic microorganisms is generally considered to be the key limiting factor for enhancing the efficiency of sludge anaerobic digestion.

[0004] In the past half century, H2 and formic acid were considered to be the main electron transfer carriers between acid-producing bacteria and methanogenic bacteria, but this electron transfer mode has a low mass transfer rate and is prone to electron loss during mass transfer. At the same time, the fluctuation of hydrogen partial pressure in the system easily destroys the interspecies electron syntrophic relationship between acid-producing bacteria and methanogenic bacteria, ultimately leading to the collapse of the methanogenic system of anaerobic digestion. Since 2012, it has been found that a direct interspecies electron transfer mechanism (DIET) does not require external electron transfer carriers, and by adding conductive materials to the anaerobic digestion system, electrons can be directly transferred to methanogenic bacteria, significantly shortening the startup time of the anaerobic digestion system and greatly improving the methane production efficiency and the resistance of methanogenic bacteria to inhibitory conditions.

[0005] Biochar is a kind of green functional material with high carbon content and excellent stability, and its surface is rich in functional groups with redox activity. It is usually prepared by pyrolysis under limited oxygen conditions using municipal solid waste, agricultural solid waste and other materials as raw materials. Research reports show that biochar as an additive in sludge anaerobic digestion process can effectively improve the stability of the system and increase the methane yield. At the same time, biochar is low in price, greatly reduces the demand for regeneration in sludge treatment, and can be reserved in the digestion liquid to be directly used as a soil conditioner without separation. Therefore, biochar, which is compatible with the ecological environment, is a good choice for the additive in the sludge anaerobic digestion tank rich in organic matter. The main mechanisms of biochar promoting sludge anaerobic digestion include relieving stress effect, enriching functional microorganisms as carriers, and improving the efficiency of interspecies electron transfer of microorganisms.

[0006] It should be noted that the raw materials, pyrolysis temperature, pyrolysis time and other factors will affect the physicochemical properties of the final prepared biochar. In the past, most of the technologies that apply biochar to the sludge anaerobic digestion process only use biochar as an exogenous additive directly, or mix it with sludge or other conductive materials, without fully utilizing the advantages of the electrochemical properties of biochar itself. The existing patent with publication number CN112938963B introduces a method for preparing magnetic carbon from straw and Fenton sludge. The Fenton sludge is first concentrated, and the straw is cleaned and filtered. Then, the two are mixed in a certain dry weight ratio, soaked for a period of time, and then subjected to drying, grinding, sieving and pyrolysis steps to obtain straw-Fenton sludge-based biochar. The patent CN112938963B provides a low-cost and relatively simple method for preparing magnetic carbon. However, since the raw material Fenton sludge is a hazardous waste containing a large amount of heavy metals, there is a risk of secondary pollution during the preparation process. On the other hand, the components of Fenton sludge produced by different wastewater treatment plants are different, which will lead to unstable performance of the magnetic carbon in large-scale production, thereby reducing the feasibility of the material in practical application.

[0007] Therefore, it is necessary to find a biochar preparation method that is simple to operate, sustainable, low in cost and free of secondary pollution. Through simple processing of the biochar obtained by pyrolysis, the surface functional group structure of the biochar is optimized and the electron transfer ability is improved, which opens up new ideas and practical methods for further efficient use of biochar to strengthen the efficiency of sludge anaerobic digestion. SUMMARY

[0008] In view of the deficiencies in the prior art, the present application aims to provide a biochar preparation method which is simple to operate, sustainable, low in cost and free of secondary pollution. The biochar obtained by pyrolyzing straw particles is added to an H2O2 solution, and stirring and heating treatment are performed, so as to modify the functional group structure on the surface of the biochar by using the oxidizing property of H2O2, and further improve the electron transfer capacity. The modified biochar is simultaneously applied to the anaerobic digestion process of sludge, so as to accelerate the electron transfer rate in the anaerobic digestion system, promote the hydrolysis acidification and methane generation of organic matter, and finally improve the anaerobic digestion performance of sludge. The technical scheme provides a more competitive new idea for the application of biochar in the anaerobic digestion process.

[0009] In order to achieve the above-mentioned application purposes, in the first aspect of the present application, the present application provides a biochar preparation method capable of strengthening the methane production efficiency of sludge anaerobic digestion, which comprises the following steps: Step 1: preparing primary biochar by crushing, drying and pyrolyzing straw; Step 2: treating the primary biochar prepared in step 1 with a 30% H2O2 solution, and then centrifuging and washing to obtain modified biochar material.

[0010] In the step 1, the following steps are specifically included: Step 101: cutting the preliminarily dried straw into pieces with a length of 1-3 cm to obtain straw segments; Step 102: crushing the cut straw and passing it through a 30-60 mesh sieve to obtain straw particles; Step 103: drying the straw particles, the drying temperature is 100-110 DEG C, and the drying time is 24-48 h; Step 104: pyrolyzing the dried straw particles, the pyrolysis temperature is 300 DEG C-350 DEG C, the heating rate is 10-15 DEG C / min, the pyrolysis time is 2 h, and the primary biochar is obtained.

[0011] In the step 104, before pyrolyzing the dried straw particles, nitrogen gas is introduced into the pyrolysis device at a flow rate of 300-400 mL / min for 15 min.

[0012] In the step 2, the following steps are specifically included: Step 201: washing the primary biochar with deionized water until its pH is neutral, filtering, and then drying at 60 DEG C, the drying time is 24-48 h; Step 202: adding the primary biochar treated in step 201 into a 30% H2O2 solution, stirring at room temperature for 48 h, and then heating to 90 DEG C and maintaining for 2 h; Step 203: Cool the product obtained in step 202 to room temperature, and then centrifuge at 14,000 rpm for 15 min.

[0013] Step 204: Let the product obtained in step 203 stand for 24 hours and then filter it.

[0014] Step 204: Wash the product obtained in step 204 with deionized water until its pH is neutral, filter it, and dry it at 60°C for 24-48 hours to obtain the modified biochar material.

[0015] In step 202, the relationship between biochar BC300 and 30% H2O2 solution is as follows: 1g of biochar BC300 is added to 40ml of 30% H2O2 solution.

[0016] In a second aspect of this application, a method for sludge treatment using modified biochar as described in the first aspect of this application is provided. The method involves: using excess sludge from a municipal wastewater treatment plant as a substrate, inoculating digested sludge from a stably operating mesophilic anaerobic digestion reactor, adding the biochar BC300-H2O2, and carrying out an anaerobic digestion reaction of the sludge.

[0017] In an embodiment of the second aspect of this application, the anaerobic digestion reaction of the sludge is carried out at a temperature of 35~35℃ and a stirring speed of 80~100r / min.

[0018] In an embodiment of the second aspect of this application, the amount of biochar BC300-H2O2 added is 1 g / gTS (dry matter).

[0019] In an embodiment of the second aspect of this application, nitrogen (99.99% purity) is used to purge oxygen from the inside of the reactor for 5 minutes before the reaction begins, and then all reactors are sealed after the reaction is completed.

[0020] The present invention has the following beneficial effects: This application utilizes agricultural waste straw as a single raw material to prepare biochar, which is widely available and inexpensive, achieving high-value utilization of solid waste and aligning with the concept of a circular economy. The prepared biochar is conveniently added to anaerobic sludge digestion systems and can be used as a soil conditioner along with the digestate without additional separation, reducing secondary disposal costs.

[0021] This invention is the first to optimize the electrochemical performance of straw-based biochar through simple treatment with hydrogen peroxide solution. By utilizing its oxidizing properties to precisely regulate the functional group structure on the surface of biochar, the electron transfer capacity of the material, especially the electron supply capacity, is significantly improved. This provides an innovative and economical technical approach to enhance the efficiency of anaerobic digestion of sludge, achieving both environmental and economic benefits.

[0022] The present invention provides a simple method for preparing biochar to enhance the anaerobic digestion efficiency of sludge. It involves only simple steps such as crushing, pyrolysis, and hydrogen peroxide solution treatment. It does not require complex equipment or precise control. Each process parameter is clear and controllable, which can ensure the stable performance of the product and is suitable for industrial mass production. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the preparation process of biochar according to the present invention.

[0024] Figure 2 This is a comparison chart of the electron transfer capacity (electron loss capacity and electron gain capacity) of straw biochar prepared in Example 1 and Comparative Example 2.

[0025] Figure 3 The images are scanning electron microscope (SEM) images of the straw biochar prepared in Example 1 and Comparative Example 2.

[0026] Figure 4 This is a comparison chart of the electron transfer capacity (electron capacity to lose electrons) of the straw biochar prepared in Example 2, Comparative Example 3, and Comparative Example 4. Detailed Implementation

[0027] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are merely to help those skilled in the art further understand the present invention, but do not limit the scope of the present invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0028] To address the problem of low electron transfer efficiency between microorganisms in existing anaerobic sludge digestion technologies, which limits methanogenesis efficiency, this invention provides a method for preparing biochar that enhances the efficiency of anaerobic sludge digestion. Based on straw biochar prepared by pyrolysis of straw raw materials, it is modified using H2O2 solution. The oxidizing properties of H2O2 solution alter the functional group structure on the surface of the biochar, increasing its electron transfer capacity during anaerobic digestion, especially its electron-donating capacity. Applying the modified biochar to the anaerobic digestion of sludge increases the number of terminal electron donors in the system, promotes interspecies electron transfer between acetic acid / VFAs oxidizing bacteria and hydrogenotrophic methanogenic archaea, and improves the metabolic efficiency of methanogenesis. Simultaneously, H2O2 treatment increases the hierarchical structure of the biochar, resulting in more pores, which is beneficial for the enrichment of anaerobic microorganisms, improves microbial diversity and system stability in the anaerobic system, and increases methane production.

[0029] Example 1 The collected rice straw was rinsed with tap water and placed in a ventilated area for 48 hours to preliminarily dry it. The preliminarily dried rice straw was chopped to a length of 2 cm, then crushed and passed through a 60-mesh sieve. The resulting rice straw particles were dried at 105℃ for 24 hours. Nitrogen gas was introduced into a tube furnace for 15 minutes at a flow rate of 300–400 mL / min. The dried rice straw particles were placed in the nitrogen-filled tube furnace, and the temperature was raised to 300℃ for pyrolysis at a rate of 10℃ / min for 2 hours to obtain primary biochar BC300. The primary biochar BC300 was washed with deionized water until its pH was neutral, then filtered. The filtered biochar BC300 was dried at 60℃ for 24 hours. Following a ratio of 1g of BC300 to 40ml of 30% H2O2 solution, the dried BC300 biochar was added to the 30% H2O2 solution and stirred at room temperature for 48h, then heated to 90℃ and maintained for 2h. The resulting product was cooled to room temperature and then centrifuged at 14000rpm for 15min. After centrifugation, the mixture was allowed to stand for 24h and then filtered. The filtered product was washed with deionized water until its pH was neutral, then filtered again. The filtered biochar was dried at 60℃ for 24h to obtain BC300-H2O2 biochar material.

[0030] Using excess sludge from the Qingpu Wastewater Treatment Plant in Shanghai as a substrate, digested sludge (inoculum: substrate 20%, calculated as TS) was inoculated into a stably operating mesophilic anaerobic digestion reactor. Biochar BC300-H2O2 (dosage 1 g / g TS) was added to initiate the anaerobic digestion reaction. The digestion reaction was conducted using an automated methane potential testing system AMPTSⅡ (Bipu Huarui Environmental Technology (Beijing) Co., Ltd.). A 500 mL serum bottle was used as the anaerobic digestion reactor, with a working volume of 400 mL per reactor. Before the reaction began, nitrogen (99.99% purity) was used to purge oxygen from the reactor for 5 minutes, after which all reactors were sealed. The headspace exhaust port of the reactor was connected to a serum bottle containing 3 mol NaOH solution to fix CO2 in the biogas. Subsequently, a real-time gas measurement unit was used to measure and record the volume of CH4 produced daily by each reactor under standard conditions (273 K, 1 atmosphere), and the TS and VS contents during the anaerobic digestion process were also determined. During the reaction, the reactor was placed in a constant-temperature water bath at 37 °C and a stirring rate of 100 rpm. The amount of methane produced was recorded every 24 hours until the daily methane production volume was less than 1% of the cumulative methane production volume. The cumulative methane production is shown in Table 1.

[0031] Comparative Example 1 The difference from Example 1 is that there is no biochar preparation process; instead, commercial glass particles are added to the sludge anaerobic digestion system in place of biochar for the experiment.

[0032] Comparative Example 2 The difference from Example 1 is that the prepared biochar BC300 was added as the final additive to the sludge anaerobic digestion system for the experiment.

[0033] The electron-donating ability of the biochar materials finally obtained in Example 1 and Comparative Example 2 was tested. Figure 2 The results showed that the electron-donating capacity of straw biochar prepared solely through pyrolysis was 0.6279 mmol / g, while the electron-donating capacity of biochar prepared by pyrolysis and modified with H2O2 solution was 0.7535 mmol / g. This indicates that the oxidizing properties of H2O2 solution altered the functional group structure on the surface of biochar, thereby increasing its electron-donating capacity. This type of biochar can increase the number of terminal electron donors in sludge anaerobic digestion systems, promote interspecies electron transfer between symtrophic acetic acid / VFAs oxidizing bacteria and hydrogen-trophic methanogenic archaea, and improve the metabolic efficiency of the methanogenesis process.

[0034] When this biochar material was applied to anaerobic digestion of sludge for methanogenesis, it was found that compared to the cumulative methanogenesis of Comparative Example 1 (111.55 mL / gVS), the cumulative methanogenesis of the anaerobic digestion systems corresponding to Comparative Example 2 and Example 1 increased by 30.2% (145.23 mL / gVS) and 45.6% (162.41 mL / gVS), respectively. The maximum daily methanogenesis rate increased from 15.45 mL / (gVS / d) in Comparative Example 1 to 29.15 mL / (gVS / d) in Comparative Example 2 and 29.86 mL / (gVS / d) in Example 1. The efficiency of biochar production was increased by 88.7% and 93.3% respectively (mL / (gVS / d)). This indicates that compared to straw biochar prepared solely through pyrolysis, the biochar modified with H2O2 solution in this invention has a better electron-donating capacity, further enhancing the electron transfer efficiency during anaerobic sludge digestion and thus having a stronger promoting effect on methanogenesis in anaerobic sludge digestion. Simultaneously, H2O2 treatment increases the hierarchical structure of the biochar, resulting in a greater number of pores, which is beneficial for the enrichment of anaerobic microorganisms, improving microbial diversity and system stability in the anaerobic system, and increasing methane production.

[0035] Example 2 The difference between this embodiment and Embodiment 1 is that corn straw is replaced with wheat straw in the biochar preparation process and added to the sludge anaerobic digestion system for experimentation.

[0036] Comparative Example 3 The difference from Example 1 is that the pyrolysis temperature in the process of preparing biochar from straw pellets is 500°C.

[0037] Comparative Example 4 The difference from Example 1 is that the pyrolysis temperature in the process of preparing biochar from straw pellets is 700°C.

[0038] The electron transfer capacity of the biochar materials prepared in Example 2, Comparative Example 3, and Comparative Example 4 was compared. Figure 3 The results showed that the electron-donating capacity of the biochar prepared at a pyrolysis temperature of 300℃ (0.7610 mmol / g) was higher than that of the biochar prepared at 500℃ (0.4034 mmol / g) and 700℃ (0.2502 mmol / g). This is mainly because at low temperature pyrolysis, numerous reducing functional groups, mainly hydroquinone, can be formed on the surface of the biochar. As the pyrolysis temperature increases, the redox activity of the biochar is gradually dominated by the electron-donating capacity.

[0039] Compared with Comparative Examples 3 and 4, the cumulative methanogenic production of the sludge anaerobic digestion system in Example 2 increased from 147.01 mL / gVS in Comparative Example 3 and 125.29 mL / gVS in Comparative Example 4 to 166.49 mL / gVS, representing increases of 13.2% and 32.9% respectively. The maximum daily methanogenic rate increased from 28.81 mL / gVS in Comparative Example 3 and 22.03 mL / gVS in Comparative Example 4 to 30.59 mL / gVS, representing increases of 6.2% and 38.8% respectively. Compared to Comparative Example 1, the pyrolysis temperature range of 300℃ to 700℃ in the straw biochar preparation process can improve the anaerobic digestion efficiency of sludge. Furthermore, the biochar obtained at a pyrolysis temperature of 300℃ has the best promoting effect on the anaerobic digestion system. This is mainly because the quinones on the surface of the biochar can replace H2 or formic acid as electron carriers to mediate interspecies electron transfer between acid-producing bacteria and methanogenic bacteria. The biochar material finally prepared at a pyrolysis temperature of 300℃ has a stronger electron-donating capacity, which can significantly increase the number of terminal electron donors in the anaerobic digestion system. After losing electrons, it can be reduced again to restore its electron-donating capacity. As the pyrolysis temperature increases, the redox activity of the functional groups on the biochar surface gradually becomes dominated by electron-gaining capacity, while the electron-donating capacity gradually decreases. In the anaerobic digestion system, the strong electron-gaining capacity of biochar may compete with the reduction reactions of organic matter for electrons. Moreover, since biochar participates in thousands of redox cycles, it may significantly reduce the number of electrons that should have been provided to the metabolic reactions of organic matter, thus affecting the metabolic efficiency of the methanogenic pathway.

[0040] In summary, the biochar obtained by pyrolyzing straw particles at 300℃ and treating them with H2O2 solution, as described in this invention, possesses relatively excellent electron transfer capabilities and is more suitable for promoting methanogenesis through anaerobic digestion of sludge. The preparation method of this invention is relatively simple, requiring no complex equipment or precise control; the parameters of each step are clearly defined and controllable, ensuring stable product performance and making it suitable for large-scale industrial production.

[0041] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and various modifications and variations can be made within the scope of the claims by those skilled in the art. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be within the protection scope of the present invention.

Claims

1. A method for preparing biochar that can enhance the methanogenic efficiency of anaerobic digestion of sludge, characterized in that... Includes the following steps: Step 1: Crush, dry, and pyrolyze the straw to prepare primary biochar; Step 2: Treat the primary biochar with a 20-40 wt% H2O2 solution, then centrifuge and wash to obtain modified biochar.

2. The method for preparing biochar that enhances the methanogenic efficiency of anaerobic digestion of sludge according to claim 1, characterized in that, Step 1 specifically includes the following steps: Step 101: Chop the pre-dried straw into pieces 1-3 cm in length to obtain straw segments; Step 102: Crush the chopped straw and pass it through a 30-60 mesh sieve to obtain straw pellets; Step 103: Dry the straw pellets at a temperature of 100-110℃ for 24-48 hours. Step 104: Pyrolyze the dried straw pellets at a temperature of 300℃~350℃, a heating rate of 10~15℃ / min, and a pyrolysis time of 2h to obtain primary biochar.

3. The method for preparing biochar that enhances the methanogenic efficiency of anaerobic digestion of sludge according to claim 1, characterized in that, Step 2 specifically includes the following steps: Step 201: Wash the primary biochar with deionized water until its pH is neutral, filter and dry it; Step 202: The primary biochar treated in step 201 is added to an H2O2 solution and stirred at room temperature for 48–96 h, then heated to 90 °C and maintained for 2–4 h; Step 203: Cool the product obtained in step 202 to room temperature, and then centrifuge; Step 204: Let the product obtained in step 203 stand and filter; Step 204: Wash the product obtained in step 204 with deionized water until its pH is neutral, filter and dry to obtain modified biochar material.

4. The method for preparing biochar that enhances the methanogenic efficiency of anaerobic digestion of sludge according to claim 2, characterized in that, In step 104, before pyrolyzing the dried straw pellets, nitrogen gas is introduced into the pyrolysis device for 10-20 minutes at a flow rate of 300-400 mL / min.

5. The method for preparing biochar that enhances the methanogenic efficiency of anaerobic digestion of sludge according to claim 3, characterized in that, In step 202, the correspondence between primary biochar and H2O2 solution is as follows: 1g of primary biochar is added to 40ml of 30wt% H2O2 solution.

6. A sludge treatment method, characterized in that, Using the excess sludge from a municipal wastewater treatment plant as a substrate, digested sludge from a stably operating mesophilic anaerobic digestion reactor is inoculated, and modified biochar as described in any one of claims 1-5 is added to carry out the anaerobic digestion reaction of the sludge.

7. The method according to claim 6, characterized in that, The anaerobic digestion of the sludge was carried out at a temperature of 35-35℃ and a stirring speed of 80-100 r / min.

8. The method according to claim 6, characterized in that, The dosage of primary biochar is 1 g / g dry matter.

9. The method according to claim 6, characterized in that, Nitrogen gas is used to purge oxygen from the inside of the reactor before the reaction begins, and all reactors are sealed after the reaction is completed.

Images