A biochar adsorption and electrochemical treatment method for treating biogas slurry

Abstract

This invention relates to the field of agricultural waste resource utilization technology, specifically to a biogas slurry treatment method based on biochar adsorption and electrochemical treatment. First, biochar is mixed with biogas slurry to selectively adsorb most of the ammonia nitrogen and some organic matter, yielding nitrogen-loaded biochar and high-phosphorus tailwater. The tailwater is then introduced into a sacrificial anode electrochemical reactor, where metal cations released at the anode react with phosphate to form precipitates, while residual organic matter is flocculated. After treatment by this process, the ammonia nitrogen removal rate is 64%–91%, total nitrogen is reduced by 46%–54%, phosphorus removal rate is 89%–94%, and COD removal rate is 96%–98%, all meeting the "Emission Standards for Pollutants from Livestock and Poultry Farming". This invention achieves targeted treatment and resource recovery through "nitrogen removal followed by phosphorus removal." The nitrogen-loaded biochar can be used as slow-release nitrogen fertilizer, and the phosphorus precipitate can be recycled. It has advantages such as low cost and no secondary pollution, and is suitable for biogas slurry treatment in large-scale biogas projects.

Description

Technical Field

[0001] This invention relates to the field of agricultural waste resource utilization technology, specifically to a biogas slurry treatment method based on biochar adsorption and electrochemical treatment. Background Technology

[0002] Driven by the global energy transition, biogas engineering, as a core method for the resource utilization of organic waste, has been widely applied in livestock and poultry farming. Currently, wet anaerobic fermentation technology is commonly used in this field, producing a large amount of biogas slurry along with the biogas. Biogas slurry is a complex, high-concentration organic wastewater containing high concentrations of chemical oxygen demand (COD), ammonia nitrogen (AN), total phosphorus (TP), and potentially residual antibiotics, heavy metals, and other emerging pollutants. Direct discharge without proper treatment can easily lead to eutrophication, algal blooms, and ecological imbalance in receiving water bodies. However, biogas slurry treatment faces bottlenecks such as high transportation costs, significant seasonal demand variations, and immature deep processing technologies, which have become key issues restricting the sustainable development of biogas engineering.

[0003] Currently, commonly used technologies for treating biogas slurry both domestically and internationally mainly include aerobic / anaerobic biological treatment, natural treatment methods (oxidation ponds, constructed wetlands), and physicochemical methods (flocculation sedimentation, advanced oxidation). Aerobic biological treatment has a good removal effect on COD and AN, but it requires a large amount of aeration when treating high-concentration biogas slurry, resulting in high energy consumption, and its removal efficiency for P is limited. Although natural treatment methods have low operating costs, they require a large area, have long treatment cycles, and are significantly affected by climate conditions, making it difficult to meet the continuous emission requirements of large-scale biogas projects. Physicochemical methods such as flocculation sedimentation can quickly remove P and some COD, but the dosage of reagents is large, the sludge production is high, and it is easy to cause secondary pollution.

[0004] Biochar adsorption (e.g., Wu X, Quan W, Chen Q, et al. Efficient Adsorption of Nitrogen and Phosphorus in Wastewater by Biochar[J]. Molecules, 2024, 29(5):1005 or Wu L, Xu D, Li B, et al. Enhanced removal efficiency of nitrogen and phosphorus from swine wastewater using MgO modified pig manure biochar[J]. Journal of Environmental Chemical Engineering, 2024, 12(1): 111793.) shows great potential due to its wide availability of raw materials (such as straw, sawdust, and biogas residue), low cost, and simple operation. However, the adsorption performance of biochar is determined by its own physicochemical properties, and there are obvious differences in adsorption selectivity: it has a good adsorption effect on nitrogen and phosphorus, but its adsorption capacity for phosphorus is limited, resulting in asynchronous nitrogen and phosphorus recovery and the generation of high-phosphorus tailwater that requires further treatment. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a biogas slurry treatment method based on biochar adsorption and electrochemical treatment. First, biochar is used to adsorb some nitrogen, organic matter, and phosphorus from the biogas slurry, yielding nitrogen-loaded biochar and phosphorus-containing tailwater. The phosphorus-containing tailwater is then electrochemically treated to obtain a mixed liquid. After separation, purified water and phosphorus-containing precipitate are obtained, thus achieving low-cost, high-efficiency, simultaneous recovery of nitrogen and phosphorus and purification of the biogas slurry. The method of this invention has the advantages of novel process design, high treatment efficiency, outstanding cost-effectiveness, and high resource utilization.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The purpose of this invention is to provide a biochar adsorption and electrochemical treatment method for treating biogas slurry, comprising the following steps: S1. Using biochar as an adsorbent, the biogas slurry is adsorbed at room temperature. Through the selective adsorption of nitrogen in the biogas slurry by biochar, nitrogen-loaded biochar and phosphorus-containing tailwater are separated.

[0007] S2. Electrochemical treatment of phosphorus-containing wastewater is carried out. After the metal cations are released by anodic dissolution, they react with the phosphate ions in the phosphorus-containing wastewater to form phosphate precipitates. At the same time, metal hydrate flocs are generated to capture, flocculate and co-precipitate residual organic matter, resulting in a mixed solution.

[0008] S3. After separating the mixed liquid, purified water and phosphorus-containing precipitate are obtained, thereby purifying the biogas slurry.

[0009] In a preferred embodiment of the present invention, the adsorption time is 20h~25h and the adsorption speed is 150rpm~210rpm.

[0010] In a preferred embodiment of the present invention, the biochar preparation process is as follows: agricultural waste is subjected to anaerobic pyrolysis treatment at a pyrolysis temperature of 450℃~550℃.

[0011] In a preferred embodiment of the present invention, the pH of the phosphorus-containing tailwater is 6.2 to 6.6.

[0012] In a preferred embodiment of the present invention, during the electrochemical treatment process, the anode electrode is a stainless steel electrode, the controlled voltage is 8V~12V, and the time is 3h~4h.

[0013] In a preferred embodiment of the present invention, both the anode and the cathode are made of stainless steel during the electrochemical treatment process.

[0014] In a preferred embodiment of the present invention, the pH of the biogas slurry is 8.9 to 9.7.

[0015] In a preferred embodiment of the present invention, nitrogen-loaded biochar is dried to obtain nitrogen-containing organic fertilizer or soil conditioner; the drying temperature is 60℃~80℃.

[0016] In a preferred embodiment of the present invention, the removal rate of chemical oxygen demand in the purified biogas slurry is 96%~98%, the removal rate of ammonia nitrogen is 64%~91%, the removal rate of total nitrogen is 46%~54%, and the removal rate of total phosphorus is 89%~94%.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention provides a biochar adsorption and electrochemical treatment method for treating biogas slurry. Biochar is used as an adsorbent to adsorb biogas slurry at room temperature. Through the selective adsorption of nitrogen by biochar, some nitrogen, organic matter, and phosphorus in the biogas slurry are removed, resulting in nitrogen-loaded biochar and phosphorus-containing tailwater. The phosphorus-containing tailwater is then electrochemically treated. After the metal cations are released through anodic dissolution, they react with the phosphate ions in the phosphorus-containing tailwater to form phosphate precipitates. Simultaneously, metal hydrate flocs are generated to trap and co-precipitate residual organic matter, resulting in a mixed solution. After separation of the mixed solution, purified water and phosphorus-containing precipitates are obtained, thereby removing pollutants from the biogas slurry. After treatment by this process, the ammonia nitrogen removal rate is 64%~91%, total nitrogen is reduced by 46%~54%, phosphorus removal rate reaches 89%~94%, and COD removal rate reaches 96%~98%. All indicators meet the "Emission Standard of Pollutants for Livestock and Poultry Breeding Industry" (GB 18596-2001).

[0018] 2. The method provided by this invention achieves highly efficient synergistic removal and resource recovery of nitrogen and phosphorus pollutants. First, nitrogen removal and recovery: Through selective adsorption by biochar, the system achieves an AN removal rate of 64%–91%, successfully enriching a large amount of nitrogen in the biogas slurry into the biochar, transforming it into directly usable slow-release nitrogen fertilizer, thus realizing the resource recovery of nitrogen. Second, highly efficient phosphorus removal: The subsequent electrochemical flocculation unit exhibits extremely strong TP removal capability, with a removal rate as high as 93.33%, reducing the phosphorus concentration in the effluent from 115.33 mg / L to 7.69 mg / L, ultimately recovering it in the form of ferric phosphate precipitation. Third, deep purification of organic matter: The entire system achieves a total COD removal rate of up to 97.42%, reducing COD from 4712.5 mg / L to 121.78 mg / L, indicating that this method is extremely effective in degrading organic pollutants and significantly improving the effluent quality.

[0019] 3. The method provided by this invention leverages the synergistic advantages of "biochar adsorption-electrochemical flocculation". The pre-treatment biochar unit not only removes most of the ammonia nitrogen but also reduces COD by 63.09%, which greatly reduces the processing load on the subsequent electrochemical unit. It avoids the interference of high concentrations of organic matter and ammonia nitrogen on the electrode reaction when the electrochemical unit directly treats the raw biogas slurry, thereby significantly improving current efficiency and reducing energy consumption and anode material consumption.

[0020] 4. The method provided by this invention has low operating costs and is environmentally friendly. It uses unmodified biochar and inexpensive stainless steel electrodes throughout the process, eliminating the need for expensive chemical modifiers, magnesium sources, or precious metal electrodes, resulting in extremely low raw material costs. There is no secondary pollution during the treatment process, and the final products (nitrogen-loaded biochar and phosphorus-containing precipitate) can be utilized as resources, conforming to the circular economy concept of "treating waste with waste and turning waste into treasure."

[0021] 5. The method of this invention cleverly utilizes the selective adsorption of ammonia nitrogen by biochar, decomposing the complex biogas slurry treatment problem into two clearly targeted stages: "prioritized nitrogen removal" and "focused phosphorus removal," resulting in a novel process design. Secondly, it exhibits extremely high removal rates for the main pollutants (nitrogen, phosphorus, and COD) in the biogas slurry, producing high-quality effluent with the potential for compliant discharge or reuse. Thirdly, the core treatment media (biochar and iron electrodes) are inexpensive, the process flow is simple, and it is easy to operate and maintain, with expected low investment and operating costs for large-scale future applications. Finally, it truly realizes the transformation from "treatment" to "recycling," converting pollutants into valuable fertilizer products, creating additional economic benefits, and enhancing the project's sustainability. Attached Figure Description

[0022] Figure 1 This is a diagram of the stainless steel plate electrode processing of the present invention.

[0023] Figure 2 This is a diagram illustrating the stainless steel mesh electrode processing of the present invention.

[0024] Figure 3 This is a diagram of the ferric phosphate precipitate recovered on the electrode plate of this invention.

[0025] Figure 4 This is a diagram of the laboratory filtration and recovery of nitrogen-loaded biochar according to the present invention. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased from the market or prepared by existing methods.

[0028] Biochar adsorption and electrochemical methods are two technologies that have attracted much attention in the field of biogas slurry treatment. On the one hand, biochar adsorption shows great potential due to its wide availability of raw materials (such as straw, sawdust, and biogas residue), low cost, and simple operation. However, the adsorption performance of biochar is determined by its own physicochemical properties, resulting in significant differences in adsorption selectivity: it has a good adsorption effect on ammonia (AN), but its adsorption capacity for phosphorus (P) is limited, leading to asynchronous nitrogen and phosphorus recovery and the generation of high-phosphorus tailwater requiring further treatment. On the other hand, electrochemical methods have shown unique advantages in biogas slurry treatment, especially in degrading recalcitrant organic matter and efficiently recovering phosphorus. However, when directly treating raw biogas slurry, its application is limited by high energy consumption and electrode contamination. Therefore, developing a combined process that leverages the strengths of both methods and avoids their weaknesses, achieving synergistic efficiency, first using biochar to selectively adsorb ammonia nitrogen, and then using electrochemical methods to specifically treat phosphorus-rich tailwater and recover phosphorus, has become a highly innovative solution.

[0029] Based on this, the present invention provides a biochar adsorption and electrochemical treatment method for treating biogas slurry, comprising the following steps: S1. Adsorption pretreatment: Biochar is mixed with raw biogas slurry for adsorption reaction. Utilizing the strong selective adsorption of nitrogen by biochar, most of the nitrogen and some organic matter and phosphorus in the biogas slurry are removed. After solid-liquid separation, nitrogen-loaded biochar and high-phosphorus tailwater are obtained.

[0030] S2, Electrochemical treatment: The high-phosphorus tailwater obtained from S1 is introduced into an electrochemical reactor equipped with a sacrificial anode. After energizing, the anode plate dissolves and releases metal cations (such as Fe). 3+ The metal cations react with phosphate ions in the effluent to form insoluble phosphate precipitates, while the resulting metal hydrate flocs trap and co-precipitate residual organic matter, achieving efficient removal and deep purification of phosphorus.

[0031] S3. Resource utilization: The effluent after S2 treatment is subjected to solid-liquid separation to obtain purified water and phosphorus-containing precipitate; the nitrogen-loaded biochar obtained in S1 can be directly used as a slow-release nitrogen fertilizer or soil conditioner for resource utilization after drying.

[0032] The biochar is obtained by anaerobic pyrolysis of agricultural waste at 500°C, without any modification by acids, alkalis, or metal salts. Agricultural waste includes, but is not limited to, corn cobs, reed stalks, and apple branches. In a preferred embodiment of the invention, when biochar is obtained from corn cobs and apple branches, the particle size of the biochar is 0.4 mm to 2 mm. When biochar is obtained from reed stalks, the biochar is in the form of strip-shaped powder.

[0033] It should be noted that during the electrochemical treatment of this invention, the anode continuously dissolves and releases Fe. 2+ Fe 2+ Partially oxidized to Fe 3+ It reacts with phosphate ions in the solution to form ferric phosphate precipitate. Simultaneously, the generated Fe(OH)3 and other hydroxyl metal flocs adsorb and trap residual colloidal and dissolved organic matter in the water, forming flocs that settle.

[0034] The adsorption time was 24 hours, and the adsorption rotation speed was 150 rpm to 210 rpm. The core function of the oscillation rate is to overcome the mass transfer resistance of the liquid film and control the adsorption rate, thereby affecting the adsorption efficiency and the time to reach equilibrium. Appropriately increasing the oscillation rate can enhance solution turbulence, thin the liquid boundary layer, and promote the diffusion of pollutant molecules from the bulk liquid phase to the surface of biochar particles, thus shortening the time required for adsorption to reach equilibrium. However, excessively high oscillation speeds may lead to increased wear between biochar particles, or even particle breakage, which may conversely reduce the adsorption effect or make it unstable. The optimal oscillation rate is related to factors such as the biochar dosage, particle size, and initial pollutant concentration. For example, when the dosage is large and the particles are fine, the required rotation speed may be lower.

[0035] The pH of the phosphorus-containing wastewater needs to be adjusted to 6.4. The form of phosphorus in the wastewater varies under different pH conditions. When pH < 2, the dominant form of phosphorus is H3PO4; when pH is 2-7, the dominant form is H2PO4; and when pH is 7-12, the dominant form is HPO4. 2-When pH > 12, the dominant form of phosphorus is PO4. 3- The present invention has shown through testing that the optimal pH value for phosphorus removal is 6.4.

[0036] In the electrochemical treatment process, the anode electrode is a stainless steel electrode, and the controlled voltage is 8V~12V; the time is 3h~4h. The effect of voltage is essentially the effect of current density. Voltage means a strong driving force: both anodic oxidation (direct / indirect) and cathodic reduction reactions will be intensified. Too low a voltage results in a slow reaction, while too high a voltage leads to side reactions dominating, both of which affect phosphorus removal. For example, insufficient voltage results in slow anodic dissolution or cathodic alkali precipitation, leading to flocculant (Fe)... 3+ / Al 3+ Insufficient production of precipitant (OH⁻) or precipitant leads to low phosphorus removal rates; excessively high voltage causes the removal rate to plateau, and the oxygen evolution side reaction further reduces current efficiency and increases wasted energy. Excessive H₂ or O₂ bubbles may break up existing flocs, affecting sedimentation. Furthermore, the duration of energization determines the duration of the electrochemical reaction, directly impacting the total amount of pollutants removed and system energy consumption. Phosphorus removal rates increase over time, typically following a rapid initial increase followed by a slower increase, eventually reaching equilibrium. Once most reactive phosphates have been removed, newly generated metal ions or OH⁻ cannot find enough phosphates to react, and the reaction nears completion. Extending the energization time at this point only wastes electrode and electrical energy, contributing little to phosphorus removal, and may even prevent some fine flocs from settling due to continued reaction.

[0037] In the electrochemical treatment process, both the anode and cathode are made of stainless steel. Preferably, the anode and cathode are stainless steel plates or stainless steel mesh. The difference between stainless steel plates and stainless steel mesh lies in the density of the electrode plates. Furthermore, stainless steel mesh allows the biogas slurry to pass directly through, increasing the contact area between the electrode plates and the biogas slurry, thus enhancing the mass transfer process. Therefore, the initial reaction efficiency of stainless steel mesh is higher than that of stainless steel plates (at the same current density). However, as the reaction continues, the pores will become clogged by sediment, leading to a decrease in reaction efficiency. Higher reaction efficiency also means greater wear and tear, requiring frequent replacement and resulting in higher costs. When the anode and cathode are stainless steel plates, the electrode treatment diagram is as follows... Figure 1 As shown. When the anode and cathode are stainless steel mesh, the electrode treatment diagram is as follows. Figure 2 As shown.

[0038] The phosphorus-containing precipitate is a phosphate precipitate, and its physical form is as follows: Figure 3 As shown.

[0039] The pH of the biogas slurry needs to be adjusted to 8.9-9.7. Different pH values ​​affect the abundant oxygen-containing functional groups (such as carboxyl-COOH and hydroxyl-OH) on the surface of biochar. The ionization state of these functional groups is controlled by pH, determining the charge state of the biochar surface, thus affecting the adsorption of ammonia nitrogen by the biochar. The adsorption effect of biochar on nitrogen is optimal when the pH is 8.9-9.7. Specifically, the adsorption mechanism is as follows: under alkaline conditions, a high concentration of OH⁻ in the solution promotes the ionization of functional groups. -COOH becomes -COO⁻, and -OH becomes -O⁻, resulting in an overall negative charge on the biochar surface, which interacts with the positively charged NH₄⁺. + Electrostatic attraction is generated.

[0040] The drying temperature of the nitrogen-loaded biochar is 70°C, and the process is as follows: Figure 4 As shown.

[0041] In the purified biogas slurry, the removal rate of chemical oxygen demand is 96%~98%, the removal rate of ammonia nitrogen is 64%~91%, the removal rate of total nitrogen is 46%~54%, and the removal rate of total phosphorus is 89%~94%.

[0042] The method provided by this invention has the following advantages: 1. Highly innovative: It cleverly utilizes the selectivity of biochar for ammonia nitrogen adsorption, decomposing the complex biogas slurry treatment problem into two clearly targeted stages: "prioritized nitrogen removal" and "focused phosphorus removal", resulting in a novel process design.

[0043] 2. High treatment efficiency: It exhibits extremely high removal rates for the main pollutants (nitrogen, phosphorus, COD) in biogas slurry, resulting in good effluent quality with the potential to meet discharge standards or be reused.

[0044] 3. Excellent cost-effectiveness: The core treatment media (biochar, iron electrode) are inexpensive, the process is simple, easy to operate and maintain, and the investment and operating costs are expected to be low when it is applied on a large scale in the future.

[0045] 4. High degree of resource utilization: It truly realizes the transformation from "treatment" to "recycling", turning pollutants into valuable fertilizer products, creating additional economic benefits and improving the sustainability of the project.

[0046] The following specific examples will provide further explanation.

[0047] In this invention, the abbreviation for chemical oxygen demand is COD, the abbreviation for ammonia nitrogen is AN, the abbreviation for total nitrogen is TN, and the abbreviation for total phosphorus is TP.

[0048] Example 1 A biogas slurry treatment method based on biochar adsorption and electrochemical treatment includes the following steps: S1. Adsorption pretreatment: Take 500 mL of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) into a 1 L beaker, adjust the pH to 9.4, and add 11 g of unmodified biochar (particle size: 0.5 mm–2 mm) prepared from corn cobs at 500 °C. Stir at a constant temperature and speed of 210 rpm for 24 hours for adsorption. After the reaction, filter using quantitative filter paper to separate the nitrogen-loaded biochar and adsorbed wastewater. Repeat the above process three times.

[0049] S2, Electrochemical treatment: The adsorbed wastewater obtained in S1 was transferred to a polypropylene electrolytic cell (30cm×20cm×10cm). Both the anode and cathode were made of stainless steel plates (190mm×120mm×0.5mm), with an effective electrode area of ​​247cm². 2 The electrolytic cell was divided into three equal parts with a plate spacing of 10cm. The pH of the effluent was adjusted to 6.4, the DC power supply was connected, the voltage was controlled at 8V, and the reaction time was 180 minutes.

[0050] During the reaction, the anode continuously dissolves and releases Fe. 2+ Fe 2+ Partially oxidized to Fe 3+ The phosphate ions react with the phosphate ions in the solution to form ferric phosphate precipitate. Simultaneously, the generated Fe(OH)3 and other hydroxyl metal flocs adsorb and trap residual colloidal and dissolved organic matter in the water, forming flocs that settle. After the reaction is complete, the mixture is allowed to settle for 30 minutes to obtain a final solution.

[0051] S3. Solid-liquid separation and resource utilization: The mixture after the reaction in S2 is filtered and separated. The resulting precipitate is mainly sludge containing iron phosphate, which can be used for phosphorus resource recovery. The nitrogen-loaded biochar obtained in S1 is dried at 70℃ to obtain nitrogen-rich organic fertilizer.

[0052] Example 2 A biogas slurry treatment method based on biochar adsorption and electrochemical treatment includes the following steps: S1. Adsorption pretreatment: Take 500 mL of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) into a 1 L beaker, adjust the pH to 9.7, and add 10 g of unmodified biochar (particle size: 0.4 mm ~ 0.6 mm) prepared from apple branches at 500 °C. Stir at a constant temperature and speed of 150 rpm for 24 hours for adsorption. After the reaction is complete, filter and separate using quantitative filter paper to obtain nitrogen-loaded biochar and adsorbed wastewater.

[0053] S2, Electrochemical treatment: The adsorbed wastewater obtained in S1 was transferred to a polypropylene electrolytic cell (30cm×20cm×10cm). Both the anode and cathode were made of stainless steel plates (190mm×120mm×0.5mm), with an effective electrode area of ​​247cm². 2 The electrolytic cell was divided into three equal parts with an electrode spacing of 10 cm. The pH of the effluent was adjusted to 6.4, a DC power supply was connected, the voltage was controlled at 8V, and the reaction time was 180 minutes. After the reaction was completed, the mixture was allowed to settle for 30 minutes to obtain a mixed solution.

[0054] S3. Solid-liquid separation and resource utilization: The mixture after the reaction in S2 is filtered and separated. The resulting precipitate is mainly sludge containing iron phosphate, which can be used for phosphorus resource recovery. The nitrogen-loaded biochar obtained in S1 is dried at 70℃ to obtain nitrogen-rich organic fertilizer.

[0055] Example 3 A biogas slurry treatment method based on biochar adsorption and electrochemical treatment includes the following steps: S1. Adsorption pretreatment: Take 500 mL of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) into a 1 L beaker, adjust the pH to 8.9, and add 13 g of unmodified biochar (in strip form) prepared from reed straw at 500℃. Stir at a constant temperature and speed of 200 rpm for 24 hours for adsorption. After the reaction is complete, filter and separate using quantitative filter paper to obtain nitrogen-loaded biochar and adsorbed wastewater.

[0056] S2, Electrochemical treatment: The adsorbed wastewater obtained in S1 was transferred to a polypropylene electrolytic cell (30cm×20cm×10cm). Both the anode and cathode were made of stainless steel plates (190mm×120mm×0.5mm), with an effective electrode area of ​​247cm². 2The electrolytic cell was divided into three equal parts with an electrode spacing of 10 cm. The pH of the effluent was adjusted to 6.4, a DC power supply was connected, the voltage was controlled at 8V, and the reaction time was 180 minutes. After the reaction was completed, the mixture was allowed to settle for 30 minutes to obtain a mixed solution.

[0057] S3. Solid-liquid separation and resource utilization: The mixture after the reaction in S2 is filtered and separated. The resulting precipitate is mainly sludge containing iron phosphate, which can be used for phosphorus resource recovery. The nitrogen-loaded biochar obtained in S1 is dried at 70℃ to obtain nitrogen-rich organic fertilizer.

[0058] Example 4 A biogas slurry treatment method based on biochar adsorption and electrochemical treatment includes the following steps: S1. Adsorption pretreatment: Take 500 mL of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) into a 1 L beaker, adjust the pH to 9.4, and add 11 g of unmodified biochar (particle size: 0.5 mm–2 mm) prepared from corn cobs at 500 °C. Stir at a constant temperature and speed of 210 rpm for 24 hours for adsorption. After the reaction, filter using quantitative filter paper to separate the nitrogen-loaded biochar and adsorbed wastewater. Repeat the above process three times.

[0059] S2, Electrochemical treatment: The adsorbed wastewater obtained in S1 was transferred to a polypropylene electrolytic cell (30cm×20cm×10cm). Both the anode and cathode were made of stainless steel mesh (190mm×120mm×0.4mm) with a mesh size of 6mm×6mm and an effective area of ​​217.1cm². 2 The electrolytic cell was divided into three equal parts with an electrode spacing of 10cm. The pH of the effluent was adjusted to 6.4, the DC power supply was connected, the voltage was controlled at 12V, and the reaction time was 240 minutes.

[0060] S3. Solid-liquid separation and resource utilization: The mixture after the reaction in S2 is filtered and separated. The resulting precipitate is mainly sludge containing iron phosphate, which can be used for phosphorus resource recovery. The nitrogen-loaded biochar obtained in S1 is dried at 70℃ to obtain nitrogen-rich organic fertilizer.

[0061] Comparative Example 1 A biochar-based biogas slurry treatment method includes the following steps: Take 500 mL of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) into a 1 L beaker, adjust the pH to 9.4, and add 11 g of unmodified biochar (particle size: 0.5 mm–2 mm) prepared from corn cobs at 500 °C. Stir at a constant temperature and speed of 210 rpm for 24 hours for adsorption. After the reaction, filter using quantitative filter paper to separate the nitrogen-loaded biochar and adsorbed wastewater. Repeat the above process three times.

[0062] Comparative Example 2 A biogas slurry treatment method based on electrochemical treatment includes the following steps: 4 L of biogas slurry (initial AN concentration: 156.72 mg / L, TN concentration: 207.17 mg / L, TP concentration: 115.33 mg / L, COD: 4712.5 mg / L) was placed in a polypropylene electrolytic cell (30 cm × 20 cm × 10 cm). Both the anode and cathode were made of stainless steel plates, and the effective electrode area was 247 cm². 2 The electrolytic cell was divided into three equal parts with an electrode spacing of 10 cm. The pH was adjusted to 6.4, the DC power supply was connected, the voltage was controlled at 8V, and the reaction time was 180 minutes. After the reaction was completed, the mixture was allowed to settle for 30 minutes to obtain a mixed solution.

[0063] The removal of biogas slurry pollutants was tested using the methods described in Examples 1-4 and Comparative Examples 1-2. The testing process included the following steps:

[0064] S1. Water Sample Pretreatment: Before the start of all testing steps, effluent samples from different process stages are collected. After collection, the samples are filtered through a 0.45μm aqueous microporous membrane to obtain a clear solution suitable for analysis.

[0065] S2, Ammonia Nitrogen (AN) Determination: Nessler's reagent spectrophotometric method (HJ 535-2009).

[0066] Measurement Procedure: Using the spectrophotometric function of a multi-parameter water quality analyzer, a suitable amount of the filtered clear liquid was placed in a specific colorimetric tube, and Nessler's reagent was added to initiate a colorimetric reaction. After the reaction stabilized, the absorbance of the solution was measured at a wavelength of 420 nm. The instrument automatically calculated and displayed the ammonia nitrogen concentration in the water sample based on the built-in standard curve.

[0067] S3. Determination of total nitrogen (TN): Alkaline potassium persulfate digestion ultraviolet spectrophotometry (HJ 636-2012).

[0068] Measurement procedure: Take an appropriate amount of water sample and add it to a stoppered glass ground glass colorimetric tube. Add alkaline potassium persulfate solution and mix well. Place the colorimetric tube in an autoclave and digest at 120℃ for 30 minutes. After cooling to room temperature, use a multi-parameter water quality analyzer to measure its absorbance at wavelengths of 220nm and 275nm. Calculate the total nitrogen concentration using the instrument's dual-wavelength correction function.

[0069] S4. Determination of total phosphorus (TP): Ammonium molybdate spectrophotometric method (GB 11893-89).

[0070] Determination Procedure: A suitable amount of water sample was placed in a colorimetric tube, and potassium persulfate solution was added. The sample was then digested under high pressure (120℃, 30 minutes) to convert all phosphorus compounds in the water sample into orthophosphate. After digestion and cooling, ammonium molybdate, potassium antimony tartrate, and ascorbic acid solution were added, resulting in the formation of a blue complex. The absorbance of the solution was measured at a wavelength of 700 nm using a multi-parameter water quality analyzer, and the total phosphorus concentration was calculated based on the standard curve.

[0071] S5. Determination of Chemical Oxygen Demand (COD): Determination method: The high-range COD determination reagent method of Hach Corporation (the principle is the potassium dichromate method) was adopted.

[0072] Measurement procedure: Using a dedicated COD digestion tube, add 2 mL of water sample to the tube pre-filled with reagents such as potassium dichromate, tighten the cap, and shake well. Place the digestion tube in a COD digester and digest at 150℃ for 2 hours. After digestion, allow it to cool to room temperature, and then read the COD concentration value using a Hach multi-parameter water quality analyzer or directly.

[0073] Table 1 shows the removal efficiency of pollutants from biogas slurry using the methods in Examples 1-4 and Comparative Examples 1-2. As shown in Table 1, when biochar adsorption was used (Comparative Example 1), the AN concentration in the effluent after adsorption decreased to 41.73 mg / L, with a removal rate of 73.37%; the TN concentration was 131.14 mg / L, with a removal rate of 36.70%; the TP concentration was 110.53 mg / L, with a removal rate of only 4.16%; and the COD decreased to 1739.14 mg / L. The results confirm the selective adsorption characteristics of unmodified biochar for ammonia nitrogen, and the effluent is characterized by high phosphorus and low carbon-to-nitrogen ratio. When electrochemical treatment was used (Comparative Example 2), the AN concentration in the treated biogas slurry decreased to 134.87 mg / L, with a removal rate of 13.94%; the TN concentration was 168.52 mg / L, with a removal rate of 18.66%; the TP concentration was only 11.93 mg / L, with a removal rate as high as 89.66%; and the COD concentration was 848.28 mg / L, with a removal rate of 82%. When biochar adsorption + electrochemical treatment was used (Example 1), the AN concentration in the effluent was 34.9 mg / L, with a removal rate of 77.73%; the TN concentration was 108.02 mg / L, with a removal rate of 47.86%; the TP concentration was 10.83 mg / L, with a removal rate of 90.61%; and the COD decreased to 135.72 mg / L. Simultaneously, the type of biochar was changed and combined with electrochemical treatment (Examples 2-3). The removal rates of TP and COD remained at around 90% and 97%, respectively. Although the AN removal rate decreased, all pollutant indicators in the final effluent met the "Emission Standard for Pollutants from Livestock and Poultry Farming". In addition, the stainless steel plate was replaced with a stainless steel mesh (Example 4). The results showed that the removal rates of AN and TP increased, while TN and COD decreased, but still met the emission standard requirements.

[0074] Table 1. Removal effect of biogas slurry pollutants on Examples 1-4 and Comparative Examples 1-2. It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended scope of protection is intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of this invention.

[0075] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of protection of this invention and its equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for treating biogas slurry based on biochar adsorption and electrochemical treatment, characterized in that, Includes the following steps: Using biochar as an adsorbent, biogas slurry was adsorbed at room temperature. Nitrogen-loaded biochar and phosphorus-containing wastewater were separated through the selective adsorption of ammonia nitrogen by the biochar. The adsorption time was 20-25 hours, and the adsorption speed was 150-210 rpm. The biochar preparation process involved: anaerobic pyrolysis of agricultural waste at 500℃; drying of the nitrogen-loaded biochar to obtain nitrogen-containing organic fertilizer or soil conditioner at 60-80℃. Electrochemical treatment of phosphorus-containing wastewater releases Fe metal cations through anodic dissolution. 3+ Subsequently, it reacts with phosphate ions in phosphorus-containing tailwater to form phosphate precipitate, while metal hydrate flocs are generated to capture, flocculate, and co-precipitate residual organic matter, resulting in a mixed solution; during the electrochemical treatment process, the anode electrode is a stainless steel electrode, the voltage is controlled at 8V~12V, and the time is 3h~4h; the cathode material is stainless steel. After separating the mixture, purified water and phosphorus-containing precipitate are obtained, thereby purifying the biogas slurry; Biochar is produced by anaerobic pyrolysis of agricultural waste at 500°C without any acid, alkali or metal salt modification treatment; The initial concentrations of AN, TN, TP, and COD in the biogas slurry were 156.72 mg / L, 207.17 mg / L, 115.33 mg / L, and 4712.5 mg / L, respectively. The pre-treatment biochar unit not only removed most of the ammonia nitrogen but also reduced COD by 63.09%, alleviating the processing load on the subsequent electrochemical unit and avoiding interference from high concentrations of organic matter and ammonia nitrogen on the electrode reaction when the electrochemical unit directly treats the raw biogas slurry.

2. The biogas slurry treatment method based on biochar adsorption and electrochemical treatment according to claim 1, characterized in that, The pH of phosphorus-containing tailwater is 6.2~6.

6.

3. The biogas slurry treatment method based on biochar adsorption and electrochemical treatment according to claim 1, characterized in that, The pH of the biogas slurry is 8.9~9.

7.

4. The biogas slurry treatment method based on biochar adsorption and electrochemical treatment according to claim 1, characterized in that, In the purification of biogas slurry, the removal rate of chemical oxygen demand is 96%~98%, the removal rate of ammonia nitrogen is 64%~91%, the removal rate of total nitrogen is 46%~54%, and the removal rate of total phosphorus is 89%~94%.

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