N-P-O-metal biochar and biogas slurry resourceful treatment system and method thereof

Abstract

The invention relates to the field of breeding wastewater treatment, and discloses N-P-O-metal biochar and a biogas slurry resourceful treatment system and method.The preparation method of the N-P-O-metal biochar comprises the steps that straw, a phosphoric acid solution, wood vinegar and biogas slurry are mixed and subjected to a hydrothermal reaction, and hydrothermal carbon is obtained; dipping the hydrothermal carbon in an activating agent solution, and then sequentially activating, washing and drying to obtain one-step activated carbon; soaking and activating the one-step activated carbon in a heteroatom doping agent, and performing oxidation treatment under oxygen-containing gas to obtain N-P-O activated carbon; the preparation method comprises the following steps: mixing a ferric salt solution and a magnesium salt solution to obtain a mixed solution, putting N-P-O activated carbon into the mixed solution, stirring and dipping, and then performing pyrolysis to obtain the N-P-O-metal biochar. The N-P-O-metal biochar disclosed by the invention can be used for treating a clear biogas slurry obtained by filtration through a capacitive deionization device, and efficient enrichment of nutritional ingredients is realized while pollutants are effectively degraded.

Description

Technical Field

[0001] The present invention relates to the field of aquaculture wastewater treatment, and specifically relates to N-P-O-metal biochar and its biogas slurry resource treatment system and method. Background Art

[0002] In recent years, the rapid development of the aquaculture industry has generated a large amount of waste such as livestock and poultry manure. Biogas slurry, as a by-product obtained by anaerobic fermentation of organic wastes such as livestock and poultry manure, its treatment method has become an important issue in the fields of modern agriculture and environmental protection. If biogas slurry is not properly treated, it may cause problems such as water eutrophication and soil salinization, polluting the environment. At the same time, biogas slurry is rich in organic matter and nutrient elements such as nitrogen, phosphorus, and potassium. After application, it can improve soil structure, increase soil fertility, and improve crop yield, having high utilization value. As a green and environmentally friendly material, biochar has received extensive attention in wastewater treatment, soil improvement, carbon sequestration, etc. It is prepared by pyrolyzing biomass. The excellent pore structure of biochar can be used to remove pollutants such as organic matter, heavy metals, and nitrogen compounds in water. Therefore, biochar shows good application prospects in water pollution treatment.

[0003] Pure biochar has problems such as poor surface hydrophilicity and insufficient specific surface area, which limit its actual application effect. In order to improve the performance of biochar, many researchers have begun to try to functionalize and modify biochar in recent years, especially by introducing elements such as nitrogen and phosphorus to endow biochar with more diverse functions. For example, N-P biochar can show good effects in treating nitrogen and phosphorus pollutants in water due to its rich nitrogen, phosphorus and other elements. However, most of the existing N-P biochars use single raw materials or simple doping methods, resulting in limited types and quantities of surface functional groups, making it difficult to meet the complex requirements in actual applications. The introduction of oxygen elements can further optimize the biochar structure, increase the types of functional groups, improve the specific surface area and catalytic ability of biochar, which is one of the main ways to improve the performance of biochar. Different types of metal elements have great differences in the influence on the biochar structure. The introduction of Fe and Mg elements can not only further enrich the pore structure of biochar, increase its specific surface area, but also increase surface active sites and improve the capacitance performance. Through composite modification in various ways, biochar can have more application directions. In capacitive deionization, the modified biochar can not only degrade pollutants through catalytic performance, but also further adsorb and purify, and transfer charged particles by electric field force to achieve the purpose of water purification.

[0004] The multi-stage filtration technology is a simple and effective method for treating biogas slurry. It can effectively reduce pollutants such as suspended solids, organic matter, and nutrient salts in biogas slurry through multiple filtration steps, thereby reducing the environmental impact of biogas slurry and contributing to the resource utilization of biogas slurry. Compared with other complex treatment methods, the multi-stage filtration technology is relatively simple to operate and has a lower cost, so it has been widely applied in practice. Chinese Patent Application CN 105233537 A provides a method for treating biogas slurry by combining biological filter media filtration. Through multi-stage filtration of biogas slurry, biogas slurry clear liquid with low suspended solids and biogas residue rich in nutrients are obtained. However, the suspended solids (SS) in the biogas slurry clear liquid obtained by this scheme are still about 1400 mg / L, and the chemical oxygen demand (COD) is about 5900 mg / L, still containing a large amount of nutrient components, which greatly increases the pressure on the capacitive deionization device, and at the same time does not meet the requirements of the capacitive deionization clear liquid, and the filtered biogas slurry clear liquid is not subject to subsequent treatment.

[0005] Capacitive deionization technology (CDI) is an efficient and environmentally friendly water treatment technology. It adsorbs cations and anions in the solution by applying a low voltage on the electrodes and using the electric double layer formed on the electrode surface, so as to achieve the purpose of purifying water quality. This not only further creates economic value, but also provides a sustainable development direction for the aquaculture industry. Due to its advantages of high efficiency, low energy consumption, and convenient operation, capacitive deionization technology has a wide application prospect in the field of biogas slurry treatment. In addition, by collecting the ions adsorbed on the electrodes, the resource recovery of nutrient components in biogas slurry can be realized. Therefore, capacitive deionization technology is expected to become one of the mainstream technologies for future aquaculture wastewater treatment. In the prior art, Patent Application CN111253005 A successfully prepared struvite from fermentation sewage through flow-through capacitive deionization technology, but the treated water still cannot meet the discharge standards.

[0006] The electrode is one of the important components of the capacitive deionization system. Currently, the commonly used capacitive deionization electrode materials are mainly carbon materials, which usually have a high specific surface area, good electrical conductivity, and high stability. However, the commonly used carbon electrodes have a high cost and limited adsorption capacity, which limits their application in treating high-concentration salt water bodies. Chinese Patent Application CN 115650378 A directly loads Mg and Fe elements on activated carbon for biogas slurry treatment, but the experimental results show that the removal rates of phosphorus resources and metal ions are insufficient. Its removal rate of phosphorus is between 29% and 61%, and at the same time, the removal of refractory organic matter and harmful inorganic components is not considered. Such treatment effects are difficult to meet the discharge standard requirements.

[0007] Chinese Patent Application CN 118324354A provides a cascade treatment system and method for aquaculture wastewater based on photocatalytic degradation. Although this solution combines the advantages of multiple technologies and successfully purifies the aquaculture wastewater to meet the national discharge standards, its structure is complex, the materials are diverse, the energy consumption is high, and the material recyclability is poor, making it difficult to meet the actual application requirements. Summary of the Invention

[0008] The object of the present invention is to overcome the above problems existing in the prior art and provide N-P-O-metal biochar and its biogas slurry resource treatment system and method. This solution uses straw as a raw material and adopts hydrothermal treatment to prepare hydrochar, which can not only be used as a filter element of a solid-liquid separation device to filter biogas slurry, but also be made into N-P-O-metal biochar after activation and other treatments. The metal biochar can be used in a capacitive deionization device to treat the clarified biogas slurry obtained by filtration, thereby effectively enriching the nutrient components.

[0009] To achieve the above object, on the one hand, the present invention provides a preparation method of N-P-O-metal biochar, and the preparation method includes the following steps:

[0010] Mix straw, phosphoric acid solution, wood vinegar liquid and biogas slurry, stir to obtain a mixture, carry out hydrothermal reaction on the mixture, and after the hydrothermal reaction ends, carry out solid-liquid separation to obtain hydrochar and hydrothermal liquid respectively;

[0011] Immerse the hydrochar in an activator solution to obtain the impregnated hydrochar, and then sequentially carry out activation, washing and drying on the impregnated hydrochar to obtain primary activated carbon;

[0012] Immerse the primary activated carbon in a heteroatom dopant to obtain the impregnated primary activated carbon, activate the impregnated primary activated carbon to obtain N-P biochar, then carry out oxidation treatment on the N-P biochar under an oxygen-containing gas, and then sequentially carry out washing and drying to obtain N-P-O activated carbon;

[0013] Mix an iron salt solution and a magnesium salt solution to obtain a mixed solution, immerse the N-P-O activated carbon in the mixed solution and stir to obtain the impregnated N-P-O activated carbon, and then pyrolyze the impregnated N-P-O activated carbon to obtain N-P-O-metal biochar.

[0014] In the present invention, phosphoric acid and wood vinegar are selected because an acidic environment is beneficial to the hydrolysis and carbonization processes of the hydrothermal reaction of straw. Biogas slurry contains organic matter and phosphorus-containing substances, and no other impurities are introduced. The salts in the biogas slurry (such as sodium chloride) can promote the formation of hydrothermal carbon from straw, and the organic matter is beneficial to the formation of bio-oil. At the same time, phosphoric acid can help form a better carbon structure. The components in the biogas slurry are complex and diverse. Without introducing other impurities, it can improve the yield of hydrothermal carbon, fertilize the hydrothermal solution, and facilitate the treatment and application of the biogas slurry.

[0015] In the preferred case, straw, phosphoric acid solution, wood vinegar, and biogas slurry are mixed and stirred at a rotation speed of 350 - 450 r / min for 35 - 65 min to obtain a mixture.

[0016] Among them, the concentration of the phosphoric acid solution is 12 - 21 wt%; the volume ratio of the phosphoric acid solution, wood vinegar, and biogas slurry is 3 - 4:1:1.

[0017] Preferably, the weight ratio of the straw to the total weight of the phosphoric acid solution, wood vinegar, and biogas slurry is 1:8 - 10.

[0018] In the present invention, the conditions of the hydrothermal reaction include: the temperature is 180 - 220 °C, and the time is 15 - 20 h.

[0019] In the above operations of the present invention, through the optimized parameter conditions, less raw materials are used, the economic cost can be further reduced, and at the same time, the performance of the prepared product is higher. Hydrothermal carbon with excellent pore structure is obtained under the conditions of lower hydrothermal temperature and hydrothermal time.

[0020] Among them, after the solid-liquid separation operation, the liquid obtained after solid-liquid separation is the hydrothermal solution, and the solid obtained after solid-liquid separation needs to be further washed with deionized water until neutral, and then dried to obtain hydrothermal carbon.

[0021] In the present invention, the method of solid-liquid separation is preferably filtration.

[0022] The hydrothermal solution obtained above can be used as a liquid fertilizer.

[0023] In a specific embodiment of the present invention, after the hydrothermal carbon is mixed with an activator, deionized water is added and stirred until the solid-liquid mixture is uniform. The activator is dissolved in water to obtain an activator solution, and the hydrothermal carbon is immersed in the activator solution.

[0024] Furthermore, the hydrothermal carbon is immersed in the activator solution, and the activator is adsorbed on the surface of the hydrothermal carbon to obtain the impregnated hydrothermal carbon. Then, the impregnated hydrothermal carbon is activated. During the activation process, through the action of the activator, mesopores and micropores are generated in the hydrothermal carbon. Then, washing and drying are carried out to obtain one-step activated carbon.

[0025] In the preferred case, the hydrothermal carbon is impregnated in the activator solution for 6 to 9 hours; after the impregnation ends, the material together with the liquid is directly dried at 95 to 105 °C for 22 to 24 hours. After the drying ends, all the liquid volatilizes, and the activator is further fixed at the surface position of the hydrothermal carbon, obtaining the impregnated hydrothermal carbon.

[0026] Preferably, the activator in the activator solution is ZnCl2 and KHCO3. The concentration of KHCO3 in the activator solution is 0.7 to 1 mol / L, and the weight ratio of ZnCl2 to KHCO3 is 1 to 1.5:1.

[0027] Specifically, during the activation process, KHCO3 will generate CO2, thereby increasing the pore structure of the hydrothermal carbon through gas etching, and more mesopores can be generated. And ZnCl2 can create more microporous structures for the hydrothermal carbon through ways such as cellulose dissolution and carbon deposition, which helps to further improve the adsorption performance.

[0028] Among them, the weight ratio of the hydrothermal carbon to the activator is 1:1 to 1.5.

[0029] Further, the specific operations of successively activating, washing, and drying the impregnated hydrothermal carbon include: activating the impregnated hydrothermal carbon at 585 to 615 °C for 55 to 75 minutes under a protective gas, then cooling, followed by pickling and water washing, and then drying at 95 to 105 °C for 24 to 26 hours.

[0030] In the present invention, a hydrochloric acid solution with a concentration of 0.8 to 1.1 mol / L is used for pickling.

[0031] Preferably, the one-step activated carbon is impregnated in the heteroatom dopant, and the one-step activated carbon is in full contact with the heteroatom dopant. The ammonium dihydrogen phosphate molecules in the heteroatom dopant adhere to the surface of the one-step activated carbon, obtaining the impregnated one-step activated carbon. The impregnated one-step activated carbon is activated. During the activation process, N and P elements are combined with the one-step activated carbon through heating to form new functional groups, obtaining N-P biochar. Then the N-P biochar is subjected to oxidation treatment under an oxygen-containing gas. During the oxidation treatment, at medium and low temperatures, C, N, and P elements are all oxidized to a certain extent to form oxygen-containing functional groups, finally completing the loading process of N, P, and O on the one-step activated carbon. Then washing and drying are successively carried out to obtain N-P-O activated carbon.

[0032] Further, the one-step activated carbon is impregnated in the heteroatom dopant for 5 to 8 hours.

[0033] Preferably, the weight ratio of the one-step activated carbon to the heteroatom dopant is 1.5:1 to 2.

[0034] Among them, the heteroatom dopant can be various acid-base reagents and salts. In the present invention, it is preferably an ammonium dihydrogen phosphate solution with a concentration of 45-60 wt%, because the selection of ammonium dihydrogen phosphate is simple in operation, and its components coincide with those in biogas slurry, without the influence of impurity elements, and it can provide P element and N element for hydrothermal carbon at the same time.

[0035] Similarly, after the one-step activated carbon is impregnated in the heteroatom dopant, the material including the liquid is directly dried at 75-95 °C for 24-36 h. After drying, all the liquid volatilizes, and the position of ammonium dihydrogen phosphate is fixed, thereby obtaining the impregnated one-step activated carbon.

[0036] Then the impregnated one-step activated carbon is activated at 585-615 °C for 55-75 min under a protective gas.

[0037] In the present invention, the protective gas can be selected from one or more of nitrogen, helium, neon, argon, krypton and xenon, and is preferably nitrogen.

[0038] In the present invention, the N-P biochar is subjected to oxidation treatment in an oxygen-containing gas, and then the specific operations of washing and drying are carried out in sequence: the N-P biochar is oxidized at 330-370 °C for 45-65 min in an oxygen-containing gas, then cooled, and then pickled and washed with water, and then dried at 95-105 °C for 24-26 h.

[0039] In a specific case, the oxygen-containing gas is preferably air.

[0040] Preferably, an iron salt solution and a magnesium salt solution are mixed to obtain a mixed solution. The N-P-O activated carbon is placed in the mixed solution and stirred and impregnated. The pores of the N-P-O activated carbon fully adsorb metal ions (iron ions and magnesium ions) to obtain the impregnated N-P-O activated carbon. Then the impregnated N-P-O activated carbon is pyrolyzed to fix the metal particles, promote the dispersion of the metal particles and form a stable loading structure, and at the same time optimize the pore distribution to complete the loading process and obtain N-P-O-metal biochar.

[0041] Preferably, the concentration of the iron salt solution is 0.4-0.7 mol / L; the concentration of the magnesium salt solution is 0.4-0.7 mol / L.

[0042] In the present invention, the iron salt solution can be FeCl3 solution and / or Fe(NO3)3 solution, preferably FeCl3 solution; the magnesium salt solution can be MgCl2 solution and / or Mg(NO3)2 solution, preferably MgCl2 solution.

[0043] The reasons for using iron salts and magnesium salts in the above operations are as follows: The N-P-O activated carbon loaded with Fe and Mg metals has an excellent pore structure, an increased specific surface area and pore volume, and a strong adsorption capacity. The functional groups formed can provide a higher adsorption capacity through mechanisms such as surface complexation, hydrogen bonding, π-π stacking, and pore filling effects. At the same time, metal oxides (such as Fe3O4 and MgO) in the Fe-Mg co-modified biochar can reduce the leaching concentration by forming stable complexes, avoiding secondary pollution. Through the combined action of metal loading and the introduction of inorganic substances, biochar can have better catalytic performance and capacitance performance, meeting the degradation requirements of aquaculture wastewater.

[0044] Further, the Fe in the iron salt solution 3+ and the Mg in the magnesium salt solution 2+ have a molar ratio of 0.9 - 1.1:1.

[0045] In the preferred case of the present invention, the N-P-O activated carbon is placed in the mixed solution and stirred and impregnated for 7 - 9 h.

[0046] More preferably, the solid-liquid ratio of the N-P-O activated carbon to the mixed solution is 1 g:6 - 10 mL.

[0047] Similarly, after the stirring and impregnation are completed, the material including the liquid is directly dried at 95 - 105 °C for 22 - 24 h. After drying, all the liquid volatilizes, and the impregnated N-P-O activated carbon is obtained.

[0048] In the preferred case, the impregnated N-P-O activated carbon is pyrolyzed at 585 - 615 °C for 55 - 75 min under a protective gas, and the metal thermal loading process is completed.

[0049] The second aspect of the present invention provides an N-P-O-metal biochar prepared by the above preparation method.

[0050] The entire preparation process of the N-P-O-metal biochar obtained in the present invention adopts a hydrothermal treatment and carbonization activation method, and has a developed pore structure and a large specific surface area, and can be used as a key electrode material for capacitive adsorption deionization.

[0051] The third aspect of the present invention provides an application of the above N-P-O-metal biochar in the resource treatment of biogas slurry.

[0052] The fourth aspect of the present invention provides a biogas slurry resource treatment system, and the biogas slurry resource treatment system includes: a solid-liquid separation device and a capacitive deionization device;

[0053] The solid-liquid separation device is used to perform solid-liquid separation on the biogas slurry to obtain a clear biogas slurry;

[0054] The capacitive deionization device is used to catalytically degrade the clarified biogas slurry from the solid-liquid separation device to obtain the treated clarified biogas slurry;

[0055] The capacitive deionization device includes a cathode and an anode, and the surfaces of the current collectors of the cathode and the anode are both coated with the above-mentioned N-P-O-metal biochar;

[0056] The cathode and the anode are used to catalytically decompose the nitrogen-containing, phosphorus-containing and other organic substances in the clarified biogas slurry during the catalytic degradation treatment, and at the same time degrade ammonia nitrogen and nitrite nitrogen compounds (decompose these refractory organic substances into nitrates and phosphates), generate more charged ions, and adsorb these charged ions through the action of an electric field.

[0057] In a specific embodiment, the preparation method of the cathode includes: ultrasonically mixing the above-mentioned N-P-O-metal biochar with a conductive agent, a binder and an organic dispersant to obtain a biochar electrode slurry, coating the biochar electrode slurry on the surface of the cleaned current collector, and then performing vacuum drying to obtain the cathode.

[0058] The preparation method of the anode is the same as that of the cathode, and specifically includes: ultrasonically mixing the above-mentioned N-P-O-metal biochar with a conductive agent, a binder and an organic dispersant to obtain a biochar electrode slurry, coating the biochar electrode slurry on the surface of the cleaned current collector, and then performing vacuum drying to obtain the anode.

[0059] Wherein the conductive agent is Ketjen black, the binder is polyvinylidene fluoride (PVDF), the organic dispersant is N-methylpyrrolidone; the current collector is a titanium plate.

[0060] In a specific embodiment, the weight ratio of the N-P-O-metal biochar, the conductive agent and the binder is 8:1:1; the role of the organic dispersant is to mix these things and then coat them on the titanium plate, and then the organic dispersant volatilizes, so that the materials are firmly bonded to the titanium plate. Therefore, the dosage of the organic dispersant can mix the N-P-O-metal biochar with the conductive agent and the binder evenly and stably bond them to the titanium plate.

[0061] In a preferred case, the time of ultrasonic mixing is 3-5 min; the conditions of vacuum drying include: the temperature is 55-75 °C and the time is 22-24 h.

[0062] Further, the solid-liquid separation device includes a biogas slurry container, a primary filter column, a secondary filter column, a tertiary filter column and a clarified biogas slurry container connected in sequence;

[0063] The primary filter column is filled with a first filter material, the secondary filter column is filled with a second filter material, and the tertiary filter column is filled with a third filter material;

[0064] The biogas slurry in the biogas slurry container is transported to the first - stage filtration column, and undergoes the first filtration through the first filter material to obtain the first filter residue (the first filter residue is large - particle filter residue, deposited above the first - stage filtration column) and the first filtrate. Then, the first filtrate is transported to the second - stage filtration column, and undergoes the second filtration through the second filter material for further filtration and adsorption to obtain the second filter residue and the second filtrate. Next, the second filtrate is transported to the third - stage filtration column, and undergoes the third filtration through the third filter material for more complete adsorption and separation to obtain the clarified biogas slurry and the third filter residue. The clarified biogas slurry is transported to the clarified biogas slurry container for storage;

[0065] The first filter material is crushed straw; the second filter material is a mixture of hydrochar and crushed straw, where the weight ratio of hydrochar to crushed straw is 1 - 1.5:1 (this ratio is determined according to the filtration requirements. In the second - stage filtration column, it is necessary to completely remove small - particle size particles, and at the same time, let the hydrochar adsorb some macromolecular substances, so this ratio range is more suitable); the third filter material is hydrochar.

[0066] Furthermore, the particle size of the crushed straw is 1.5 - 3 mm; if the particle size is too small, it will lead to a slow filtration speed and even cause blockage, and if the particle size is too large, it will be difficult to meet the filtration requirements and small particles will be difficult to filter out. Therefore, the optimized selection is 1.5 - 3 mm.

[0067] Among them, the crushed straw can filter out impurities with larger particle sizes; the mixture of hydrochar and crushed straw can further filter out impurities with smaller particle sizes and adsorb a certain amount of substances such as ammonia nitrogen and organic matter; the hydrochar can further filter impurities to clarify the biogas slurry, and at the same time further adsorb ammonia nitrogen and organic matter, reducing the adsorption and degradation pressure of subsequent capacitive deionization.

[0068] In the preferred case, the packing density of the crushed straw in the first - stage filtration column is 0.87 - 0.99 g / cm 3 , the packing density of the mixture of hydrochar and crushed straw in the second - stage filtration column is 0.95 - 1.07 g / cm 3 , and the packing density of the hydrochar in the third - stage filtration column is 1.14 - 1.38 g / cm 3 .

[0069] In a specific embodiment, the diameters of the first - stage filtration column, the second - stage filtration column, and the third - stage filtration column are all 10 cm, the heights are all 1 m, and the packing heights of the first - stage filtration column, the second - stage filtration column, and the third - stage filtration column are all 80 cm.

[0070] In a specific embodiment, the hydrochar used in the solid - liquid separation device is the hydrochar prepared by the above - mentioned preparation method.

[0071] The diameter of the hydrothermal carbon prepared by the above preparation method is 0.074 - 0.125 mm.

[0072] Furthermore, a peristaltic pump is provided on the pipeline connecting the biogas slurry container and the first - stage filtration column for pumping the biogas slurry in the biogas slurry container into the first - stage filtration column.

[0073] Even further, a first pressure gauge is provided at the inlet of the first - stage filtration column for monitoring the flow rate of the material at the inlet of the first - stage filtration column, and a second pressure gauge is provided at the outlet of the first - stage filtration column for monitoring the flow rate of the material at the outlet of the first - stage filtration column; a third pressure gauge is provided at the inlet of the second - stage filtration column for monitoring the flow rate of the material at the inlet of the second - stage filtration column, and a fourth pressure gauge is provided at the outlet of the second - stage filtration column for monitoring the flow rate of the material at the outlet of the second - stage filtration column; a fifth pressure gauge is provided at the inlet of the third - stage filtration column for monitoring the flow rate of the material at the inlet of the third - stage filtration column, and a sixth pressure gauge is provided at the outlet of the third - stage filtration column for monitoring the flow rate of the material at the outlet of the third - stage filtration column.

[0074] Preferably, the capacitive deionization device further includes a DC power supply and an electrolytic cell. The negative electrode of the DC power supply is connected to the cathode, and the positive electrode of the DC power supply is connected to the anode.

[0075] Both the cathode and the anode are arranged in the electrolytic cell, and the size of the electrolytic cell depends on the specific working conditions.

[0076] Furthermore, the capacitive deionization device further includes a first water inlet pipeline and a second water inlet pipeline, and one end of the second water inlet pipeline is connected to the first water inlet pipeline, and the other end of the second water inlet pipeline is connected to the inlet of the electrolytic cell.

[0077] The capacitive deionization device further includes a first water outlet pipeline and a second water outlet pipeline, and one end of the first water outlet pipeline is connected to the second water outlet pipeline, and the other end of the first water outlet pipeline is connected to the outlet of the electrolytic cell.

[0078] The capacitive deionization device further includes a third water outlet pipeline. One end of the third water outlet pipeline is respectively connected to the first water outlet pipeline and the second water outlet pipeline, and the other end of the third water outlet pipeline is respectively connected to the first water inlet pipeline and the second water inlet pipeline.

[0079] Furthermore, a first valve is also provided on the first water inlet pipeline, a circulating water pump is provided on the second water inlet pipeline, a second valve is provided on the third water outlet pipeline, and a third valve is provided on the second water outlet pipeline.

[0080] In a specific embodiment, the biogas slurry supernatant from the solid-liquid separation device is transported to the first water inlet pipeline. The first valve and the circulation water pump are opened, and the second valve is closed. The biogas slurry supernatant enters the electrolytic cell along the second water inlet pipeline through the inlet of the electrolytic cell. The DC power supply is turned on, and the cathode and anode catalyze and degrade the biogas slurry supernatant in the electrolytic cell. During the catalytic degradation process, organic substances such as nitrogen and phosphorus in the biogas slurry supernatant are catalytically decomposed, and at the same time, ammonia nitrogen and nitrite nitrogen compounds are degraded, generating more charged ions, and these charged ions are adsorbed through the action of the electric field, thereby obtaining the treated biogas slurry supernatant. The treated biogas slurry supernatant enters the first water outlet pipeline through the outlet of the electrolytic cell. The third valve is opened, and the treated biogas slurry supernatant is discharged from the second water outlet pipeline; after long-term use, the treatment capacity of the cathode and anode will decrease to some extent. At this time, the treated biogas slurry supernatant may not meet the discharge standard. When the treated biogas slurry supernatant does not meet the discharge standard, the first valve and the third valve are closed, and the second valve is opened. The first water inlet pipeline stops transporting the biogas slurry supernatant from the solid-liquid separation device. At this time, under the action of the circulation water pump, the treated biogas slurry supernatant is transported from the first water outlet pipeline through the third water outlet pipeline and the second water inlet pipeline in sequence, enters the electrolytic cell through the inlet of the electrolytic cell, and continues to carry out catalytic degradation treatment on the treated biogas slurry supernatant in a cycle until the finally obtained treated biogas slurry supernatant meets the discharge standard.

[0081] After operating for a period of time, after the cathode and anode have adsorbed a large amount of charged ions (i.e., enriched nutrients), at this time, it is necessary to discharge the enriched nutrients adsorbed on the cathode and anode. The first valve is closed, and the transportation of the biogas slurry supernatant to the capacitive deionization device is stopped. After all the treated biogas slurry supernatant in the electrolytic cell is discharged from the second water outlet pipeline, the DC power supply is turned off, and the electric field effect disappears. The enriched nutrients adsorbed on the cathode and anode enter the first water outlet pipeline through the outlet of the electrolytic cell (at this time, it is necessary to keep the second valve closed and the third valve open), and then are discharged from the second water outlet pipeline.

[0082] Specifically, the introduction of N in N-P-O-metal biochar can improve the pore structure of the carbon material and increase its electrical conductivity; the introduction of O can form oxygen-containing functional groups such as -COOH, C=O, and -OH, improving the surface wettability of the material; the introduction of P can increase the interlayer spacing of carbon atoms, improving the wettability of the material. At the same time, the P element has a strong electron-donating ability. When P-doped biochar is used as an adsorbent, it can coordinate with metal ions. The metal element becomes an active site by attaching to the biochar surface, endowing the material with higher catalytic performance and electrochemical performance. Under the combined action of these heteroatoms, the pore structure, catalytic performance, and electrochemical performance of the biochar material are greatly improved. In the catalytic degradation treatment of the biogas slurry supernatant from the solid-liquid separation device, elements such as N and P can be stripped from it. After converting macromolecular substances into small molecules through catalytic action, they are adsorbed on the electrode by the electric field force, achieving the goal of recovering nutrients and purifying water quality.

[0083] In the present invention, the above-mentioned cyclic treatment for removing charged particles can ensure the removal effect. At the same time, after discharging the adsorbed and enriched nutrients, it also facilitates the replacement of the biochar electrode.

[0084] The fifth aspect of the present invention provides a method for resource treatment of biogas slurry. The method for resource treatment of biogas slurry is implemented in the above-mentioned biogas slurry resource treatment system, and specifically includes the following steps:

[0085] Use a solid-liquid separation device to perform solid-liquid separation on the biogas slurry to obtain the biogas slurry supernatant;

[0086] Transport the biogas slurry supernatant to a capacitive deionization device for catalytic degradation treatment of the biogas slurry supernatant. The cathode and anode are used to catalytically decompose the organic matter in the biogas slurry supernatant during the catalytic degradation treatment, while degrading ammonia nitrogen and nitrite nitrogen compounds, generating charged ions, and adsorbing the charged ions through the electric field action to obtain the treated biogas slurry supernatant.

[0087] Since the capacitive deionization technology has certain requirements for the influent water quality, using the capacitive deionization technology to treat biogas slurry requires that the content of suspended solids, turbidity, and macromolecular organic matter in the water cannot be too high, so as not to affect the adsorption performance and service life of the electrode. Therefore, multi-stage filtration can be combined with capacitive deionization to further deeply treat the biogas slurry supernatant after multi-stage filtration using the capacitive deionization technology.

[0088] Preferably, the flow rate of the biogas slurry supernatant is 25-40 mL / min, the voltage applied across the cathode and anode is 0.5-1.6 V; the operating time is 1.5-6.5 h, that is, after the electrolytic cell operates for such a long time, the DC power supply must be turned off to discharge the enriched nutrients, and at the same time, the nutrients enriched on the electrode can be washed away.

[0089] Furthermore, the above-mentioned first filter residue, second filter residue, and third filter residue can be used as solid fertilizers for returning to the field, and the hydrothermal solution and the enriched nutrients adsorbed on the electrodes can be used as liquid fertilizers.

[0090] The present invention proposes a method for treating biogas slurry by combining multi-stage filtration and capacitive deionization technology. Using straw as a raw material, through hydrothermal treatment, the prepared hydrochar can, on the one hand, be used as a filter material for a multi-stage filtration device to filter biogas slurry, and on the other hand, can be further carbonized and activated to make activated carbon. The obtained activated carbon can be used as a capacitive deionization electrode material to treat the supernatant of the biogas slurry obtained by filtration, thereby achieving effective enrichment of nutrient components. Compared with traditional methods for preparing carbon materials, directly using biomass as a precursor to prepare carbon materials has the advantages of wide sources, high carbon content, and sustainability, providing a new way for preparing high-performance, low-cost, and environmentally friendly electrode materials.

[0091] The present invention combines biomass-based activated carbon with a capacitive deionization device to propose a new method for treating biogas slurry, which can effectively treat biogas slurry and achieve the purpose of resource treatment. While realizing the enrichment of nutrient substances, it can efficiently purify aquaculture sewage to meet the national discharge requirements.

[0092] Compared with the prior art, the present invention has the following advantages:

[0093] (1) Enhanced versatility: The N-P-O-metal biochar prepared by the present invention has more functional groups compared with traditional biochar. In particular, the introduction of nitrogen (N), phosphorus (P), and oxygen (O) elements significantly improves its adsorption capacity and selective adsorption capacity for organic matter, nitrogen, and phosphorus pollutants in water. Most biochars in the prior art only have a single adsorption function, while the biochar of the present invention has a wider range of pollutant removal functions.

[0094] (2) Low cost and simple preparation method: The preparation method of the present invention obtains N-P-O-metal biochar by mixing materials such as straw, phosphoric acid solution, wood vinegar liquid, and biogas slurry and performing hydrothermal reaction, followed by simple activation, impregnation, and pyrolysis treatments. Compared with the metal doping or modification methods in the prior art that usually require complex processes and expensive materials, the technology of the present invention is more cost-effective and easy to operate.

[0095] (3) High-efficiency biogas slurry treatment ability: The application of the N-P-O-metal biochar of the present invention in the resource treatment of biogas slurry has significant effects, and can effectively degrade organic pollutants in biogas slurry, especially showing high catalytic activity in removing ammonia nitrogen and nitrite nitrogen. The biogas slurry treatment methods in the prior art often have problems such as low treatment efficiency and high costs, while the selective adsorption performance of the present invention greatly improves the treatment efficiency and effect.

[0096] (4) Enhanced catalytic and selective adsorption performance and stability: In the present invention, the catalytic and selective adsorption ability of biochar is enhanced by introducing metal ions such as iron salts and magnesium salts, and the metal particles are fixed through the pyrolysis process, improving the stability of biochar during long-term use. Compared with the biochar in the prior art, the metal biochar of the present invention can maintain a high catalytic and selective adsorption effect and a long service life in practical applications.

[0097] (5) Environmental protection and sustainability: The present invention utilizes resources such as agricultural waste (such as straw) and biogas slurry to prepare highly efficient N-P-O-metal biochar through green chemical methods, realizing the high-value utilization of waste. Compared with the prior art, the treatment process of the present invention is more environmentally friendly and meets the current requirements of resource recycling and sustainable development. Description of the Drawings

[0098] Figure 1 is a process diagram for the preparation of N-P-O-metal biochar in Example 1;

[0099] Figure 2 is a SEM image of the N-P-O-metal biochar obtained in Example 1;

[0100] Figure 3 is a SEM image of the N-P-O-metal biochar obtained in Example 1;

[0101] Figure 4 is a structural schematic diagram of the solid-liquid separation device;

[0102] Figure 5 is a structural schematic diagram of the capacitive deionization device;

[0103] Figure 6 is a flow chart of the biogas slurry treatment method.

[0104] Description of the Reference Numerals in the Drawings

[0105] 1, solid-liquid separation device; 11, biogas slurry container; 12, primary filtration column; 13, secondary filtration column; 14, tertiary filtration column; 15, biogas slurry clear liquid container; 16, first filter material; 17, second filter material; 18, third filter material; 19, peristaltic pump; 121, first pressure gauge; 122, second pressure gauge; 131, third pressure gauge; 132, fourth pressure gauge; 141, fifth pressure gauge; 142, sixth pressure gauge;

[0106] 2, capacitive deionization device; 21, cathode; 22, anode; 23, electrolytic cell; 24, first water inlet pipeline; 25, second water inlet pipeline; 26, first water outlet pipeline; 27, second water outlet pipeline; 28, third water outlet pipeline; 241, first valve; 251, circulating water pump; 281, second valve; 271, third valve. Detailed implementation manners

[0107] The following will combine the accompanying drawings and embodiments to detail the specific implementation manners of the present invention. It should be understood that the specific implementation manners described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0108] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0109] Example 1

[0110] This example is a preparation method of N-P-O-metal biochar, and the preparation process is as Figure 1 shown, and specifically includes the following steps:

[0111] (1) Put the straw into the mixture prepared by mixing phosphoric acid solution (concentration 20 wt%), wood vinegar liquid and biogas slurry (the volume ratio of phosphoric acid solution, wood vinegar liquid and biogas slurry (the biogas slurry is from the fermented manure biogas slurry of a pig farm in Wuhan, Hubei, and its water quality is shown in Table 1) is 3.5:1:1, and the weight ratio of straw to the total weight of phosphoric acid solution, wood vinegar liquid and biogas slurry is 1:9), stir for 45 min at a rotation speed of 400 r / min to obtain a mixture, transfer the mixture to a high-pressure reaction kettle, and carry out hydrothermal reaction. The conditions of the hydrothermal reaction include: the temperature is 200 °C, the time is 20 h. After the hydrothermal reaction, filter. The liquid obtained after filtration is the hydrothermal liquid, and the solid obtained after filtration is further washed with deionized water until neutral, and then dried to obtain hydrothermal carbon; it is detected that the diameter of the hydrothermal carbon is 0.074 - 0.125 mm; the hydrothermal liquid can be used as liquid fertilizer;

[0112] (2) The hydrothermal carbon obtained in step (1) is mixed with activating agents (ZnCl2 and KHCO3, with a weight ratio of ZnCl2 to KHCO3 of 1:1). After mixing (with a weight ratio of hydrothermal carbon to activating agents of 1:1), deionized water is added and stirred until the solid-liquid mixture is homogeneous. The activating agents dissolve in water to form an activating agent solution with a KHCO3 concentration of 0.7 mol / L in the activating agent solution. The hydrothermal carbon is immersed in the activating agent solution for 8 h, and the activating agents are adsorbed on the surface of the hydrothermal carbon. After the immersion, the material including the liquid is directly dried at 100 °C for 24 h to obtain the impregnated hydrothermal carbon. The impregnated hydrothermal carbon is placed in a nickel boat and put into a horizontal tube furnace, and activated at 600 °C for 60 min under N2 gas. During the activation process, mesopores and micropores are generated in the hydrothermal carbon through the action of the activating agents. Then it is cooled, followed by pickling with a hydrochloric acid solution with a concentration of 1 mol / L, then washed with deionized water, and then dried at 100 °C for 24 h to obtain the one-step activated carbon;

[0113] (3) The one-step activated carbon obtained in step (2) is immersed in a heteroatom dopant (an ammonium dihydrogen phosphate solution with a concentration of 45 wt%). The weight ratio of the one-step activated carbon to the heteroatom dopant is 1.5:1.5, and the one-step activated carbon is in full contact with the heteroatom dopant. The ammonium dihydrogen phosphate molecules in the heteroatom dopant adhere to the surface of the one-step activated carbon. After the immersion, the material including the liquid is directly dried at 80 °C for 24 h to obtain the impregnated one-step activated carbon. The impregnated one-step activated carbon is placed in a nickel boat and put into a horizontal tube furnace, and activated at 600 °C for 60 min under N2 gas. During the activation process, N and P elements combine with the one-step activated carbon through heating to form new functional groups, obtaining N-P biochar. Then the N-P biochar is placed in a nickel boat and put into a horizontal tube furnace, and oxidized at 350 °C for 50 min under air. During the oxidation treatment, C, N, and P elements are all oxidized to a certain extent to form oxygen-containing functional groups, finally completing the loading process of N, P, and O on the one-step activated carbon. Then it is cooled, followed by pickling with a hydrochloric acid solution with a concentration of 1 mol / L, then washed with deionized water, and then dried at 100 °C for 24 h to obtain N-P-O activated carbon;

[0114] (4) Mix the FeCl3 solution and the MgCl2 solution. The Fe in the FeCl3 solution 3+ and the Mg in the MgCl2 solution 2+The molar ratio is 1:1, the concentration of the FeCl3 solution is 0.5 mol / L, and the concentration of the MgCl2 solution is 0.5 mol / L to obtain a mixed solution. The N-P-O activated carbon obtained in step (3) is placed in the mixed solution, and the solid-liquid ratio of the N-P-O activated carbon to the mixed solution is 1 g:8 mL. It is stirred and impregnated on a magnetic stirrer for 8 h. The pores of the N-P-O activated carbon fully adsorb metal ions (iron ions and magnesium ions). After the stirring and impregnation are completed, the material including the liquid is directly dried at 100 °C for 24 h to obtain the impregnated N-P-O activated carbon. Then, the impregnated N-P-O activated carbon is placed in a nickel boat and placed in a horizontal tube furnace. It is pyrolyzed at 600 °C for 60 min under N2 gas to fix the metal particles, promote the dispersion of the metal particles and form a stable loading structure, and at the same time optimize the pore distribution to complete the loading process and obtain N-P-O-metal biochar.

[0115] Scanning electron microscopy observation was carried out on the N-P-O-metal biochar. The scanning electron microscopy (SEM) images are as Figure 2 and Figure 3 shown, where Figure 2 is the SEM image magnified 2500 times, Figure 3 is the SEM image magnified 10000 times. According to Figure 2 and Figure 3 it can be seen that the N-P-O-metal biochar has a rich pore structure with micropores, mesopores and macropores, a relatively large specific surface area and pore volume. Among them, the macropore structure can provide storage space for ions and shorten the migration distance of ions; the mesopores can act as ion transfer channels; the micropores can provide abundant ion adsorption sites. These diverse pore structures make the capacitance performance and catalytic performance of the material more excellent.

[0116] Example 2

[0117] A biogas slurry resource treatment system includes: a solid-liquid separation device 1 as shown in Figure 4 and a capacitive deionization device 2 as shown in Figure 5 ;

[0118] The solid-liquid separation device 1 is used to separate the solid and liquid of the biogas slurry to obtain the clarified biogas slurry;

[0119] The capacitive deionization device 2 is used to carry out catalytic degradation treatment on the clarified biogas slurry from the solid-liquid separation device 1 to obtain the treated clarified biogas slurry;

[0120] The capacitive deionization device 2 includes a cathode 21 and an anode 22;

[0121] The cathode 21 and the anode 22 are used to catalytically decompose the nitrogen-containing, phosphorus-containing and other organic substances in the biogas slurry supernatant during the catalytic degradation treatment, while degrading ammonia nitrogen and nitrite nitrogen compounds, generating more charged ions, and adsorbing these charged ions through the action of an electric field;

[0122] Among them, the solid-liquid separation device 1 includes a biogas slurry container 11, a primary filter column 12, a secondary filter column 13, a tertiary filter column 14 and a biogas slurry supernatant container 15 connected in sequence; a peristaltic pump 19 is arranged on the pipeline connecting the biogas slurry container 11 and the primary filter column 12;

[0123] The diameters of the primary filter column 12, the secondary filter column 13 and the tertiary filter column 14 are all 10 cm, and the heights are all 1 m. The filling heights of the primary filter column 12, the secondary filter column 13 and the tertiary filter column 14 are all 80 cm;

[0124] The primary filter column 12 is filled with a first filter material 16, the secondary filter column 13 is filled with a second filter material 17, and the tertiary filter column 14 is filled with a third filter material 18;

[0125] The first filter material 16 is crushed straw; the second filter material 17 is a mixture of hydrothermal carbon and crushed straw, where the weight ratio of hydrothermal carbon to crushed straw is 1:1; the third filter material 18 is hydrothermal carbon;

[0126] The particle size of the crushed straw is 1.5 - 3 mm; the hydrothermal carbon used is the hydrothermal carbon prepared in step (1) of Example 1;

[0127] The filling density of the crushed straw in the primary filter column 12 is 0.87 - 0.99 g / cm 3 , the filling density of the mixture of hydrothermal carbon and crushed straw in the secondary filter column 13 is 0.95 - 1.07 g / cm 3 , the filling density of the hydrothermal carbon in the tertiary filter column 14 is 1.14 - 1.38 g / cm 3 ;

[0128] A first pressure gauge 121 is arranged at the inlet of the primary filter column 12, and a second pressure gauge 122 is arranged at the outlet of the primary filter column 12; a third pressure gauge 131 is arranged at the inlet of the secondary filter column 13, and a fourth pressure gauge 132 is arranged at the outlet of the secondary filter column 13; a fifth pressure gauge 141 is arranged at the inlet of the tertiary filter column 14; a sixth pressure gauge 142 is arranged at the outlet of the tertiary filter column 14;

[0129] The capacitive deionization device 2 further includes a DC power supply and an electrolytic cell 23. The negative pole of the DC power supply is connected to the cathode 21, and the positive pole of the DC power supply is connected to the anode 22;

[0130] The cathode 21 and the anode 22 are both arranged in the electrolytic cell 23;

[0131] Among them, the preparation method of the cathode 21 includes: mixing the N-P-O-metal biochar prepared in Example 1 with a conductive agent (Ketjen black) and a binder (polyvinylidene fluoride) (the weight ratio of N-P-O-metal biochar, conductive agent, and binder is 8:1:1), adding an organic dispersant (N-methylpyrrolidone) and ultrasonically mixing for 5 min to obtain a biochar electrode slurry, coating the biochar electrode slurry on the surface of a cleaned conductive current collector (titanium plate), and then performing vacuum drying. The conditions for vacuum drying include: a temperature of 60 °C and a time of 24 h to obtain the cathode 21;

[0132] The preparation method of the anode 22 is the same as that of the cathode, specifically including: mixing the N-P-O-metal biochar prepared in Example 1 with a conductive agent (Ketjen black) and a binder (polyvinylidene fluoride) (the weight ratio of N-P-O-metal biochar, conductive agent, and binder is 8:1:1), adding an organic dispersant (N-methylpyrrolidone) and ultrasonically mixing for 5 min to obtain a biochar electrode slurry, coating the biochar electrode slurry on the surface of a cleaned conductive current collector (titanium plate), and then performing vacuum drying. The conditions for vacuum drying include: a temperature of 60 °C and a time of 24 h to obtain the anode 22;

[0133] The capacitive deionization device 2 further includes a first water inlet pipeline 24 and a second water inlet pipeline 25, and one end of the second water inlet pipeline 25 is connected to the first water inlet pipeline 24, and the other end of the second water inlet pipeline 25 is connected to the inlet of the electrolytic cell 23;

[0134] The capacitive deionization device 2 further includes a first water outlet pipeline 26 and a second water outlet pipeline 27, and one end of the first water outlet pipeline 26 is connected to the second water outlet pipeline 27, and the other end of the first water outlet pipeline 26 is connected to the outlet of the electrolytic cell 23;

[0135] The capacitive deionization device 2 further includes a third water outlet pipeline 28, one end of the third water outlet pipeline 28 is respectively connected to the first water outlet pipeline 26 and the second water outlet pipeline 27, and the other end of the third water outlet pipeline 28 is respectively connected to the first water inlet pipeline 24 and the second water inlet pipeline 25;

[0136] A first valve 241 is further provided on the first water inlet pipeline 24, a circulation water pump 251 is provided on the second water inlet pipeline 25, a second valve 281 is provided on the third water outlet pipeline 28, and a third valve 271 is provided on the second water outlet pipeline 27.

[0137] Example 3

[0138] As Figure 6The shown biogas slurry treatment method is implemented in the biogas slurry resource treatment system of Embodiment 2. The biogas slurry to be treated is the fermented manure biogas slurry from a pig farm in Wuhan, Hubei. The water quality before and after treatment is shown in Table 1 (where the chemical oxygen demand is tested by the potassium dichromate method; the ammonia nitrogen content is tested by the Nessler's reagent spectrophotometry method; the suspended solid content (SS) is tested by the centrifugal sedimentation method; the biochemical oxygen demand (COD) is tested by the dilution and inoculation method; the total phosphorus content is tested by the molybdenum blue spectrophotometry method). The specific biogas slurry treatment method includes the following steps:

[0139] S1. Pump the biogas slurry in the biogas slurry container 11 of the solid-liquid separation device 1 into the first-stage filter column 12 by using a peristaltic pump 19 (monitor the flow rate of the material at the inlet of the first-stage filter column 12 with the first pressure gauge 121 and monitor the flow rate of the material at the outlet of the first-stage filter column 12 with the second pressure gauge 122). After the first filtration by the first filter medium 16, obtain the first filter residue (the first filter residue is large-particle filter residue, deposited above the first-stage filter column) and the first filtrate. Then, transport the first filtrate to the second-stage filter column 13 for the second filtration by the second filter medium 17 (monitor the flow rate of the material at the inlet of the second-stage filter column 13 with the third pressure gauge 131 and monitor the flow rate of the material at the outlet of the second-stage filter column 13 with the fourth pressure gauge 132) for further filtration and adsorption to obtain the second filter residue and the second filtrate. Then, transport the second filtrate to the third-stage filter column 14 for the third filtration by the third filter medium 18 (monitor the flow rate of the material at the inlet of the third-stage filter column 14 with the fifth pressure gauge 141 and monitor the flow rate of the material at the outlet of the third-stage filter column 14 with the sixth pressure gauge 142) for more complete adsorption and separation to obtain the clarified biogas slurry (the suspended solid content in the clarified biogas slurry is lower than 30 mg / L and the chemical oxygen demand is lower than 890 mg / L) and the third filter residue. Transport the clarified biogas slurry to the clarified biogas slurry container 15 for storage;

[0140] S2. Transfer the biogas slurry supernatant in the biogas slurry supernatant container 15 in step S1 to the first water inlet pipe 24 of the capacitive deionization device 2. Open the first valve 241 and the circulation water pump 251, and close the second valve 281. The biogas slurry supernatant enters the electrolytic cell 23 through the inlet of the electrolytic cell 23 along the second water inlet pipe 25. Turn on the DC power supply, and the cathode 21 and the anode 22 catalytically degrade the biogas slurry supernatant in the electrolytic cell 23. In the catalytic degradation process, organic substances such as nitrogen and phosphorus in the biogas slurry supernatant are catalytically decomposed, while ammonia nitrogen and nitrite nitrogen compounds are degraded, generating more charged ions, and these charged ions are adsorbed through the action of the electric field, thereby obtaining the treated biogas slurry supernatant. The treated biogas slurry supernatant enters the first water outlet pipe 26 through the outlet of the electrolytic cell 23. Open the third valve 271, and the treated biogas slurry supernatant is discharged from the second water outlet pipe 27. The water quality of the treated biogas slurry supernatant is shown in Table 1; among them, the flow rate of the biogas slurry supernatant is 30 mL / min; the voltage applied across the cathode 21 and the anode 22 is 1.6 V; the operation time is 5 h.

[0141] After long-term use, the treatment capabilities of the cathode 21 and the anode 22 will decline to some extent. At this time, the treated biogas slurry supernatant may not meet the discharge standard. When the treated biogas slurry supernatant does not meet the discharge standard, close the first valve 241 and the third valve 271, and open the second valve 281. The first water inlet pipe 24 stops transporting the biogas slurry supernatant from the biogas slurry supernatant container 15. At this time, under the action of the circulation water pump 251, the treated biogas slurry supernatant is transported from the first water outlet pipe 26 through the third water outlet pipe 28 and the second water inlet pipe 25 in sequence, enters the electrolytic cell 23 through the inlet of the electrolytic cell 23, and continues to catalytically degrade the treated biogas slurry supernatant in a cycle until the finally obtained treated biogas slurry supernatant meets the discharge standard.

[0142] Among them, after the cathode 21 and the anode 22 adsorb a large amount of charged ions (i.e., enriched nutrients) after operating for a period of time, it is necessary to discharge the enriched nutrients adsorbed on the cathode 21 and the anode 2 at this time. Then close the first valve 241 to stop transporting the biogas slurry supernatant into the capacitive deionization device 2. After all the treated biogas slurry supernatant in the electrolytic cell 23 is discharged from the second water outlet pipe 27, turn off the DC power supply, and the electric field effect disappears. The enriched nutrients adsorbed on the cathode 21 and the anode 22 enter the first water outlet pipe 26 through the outlet of the electrolytic cell 23 (it is necessary to keep the second valve 281 closed and the third valve 271 open at this time), and then are discharged from the second water outlet pipe 27.

[0143] The first filter residue, the second filter residue, and the third filter residue obtained in the above step S1 can be used as solid fertilizer for returning to the field, and the enriched nutrients adsorbed on the electrodes in step S2 can be used as liquid fertilizer.

[0144] Table 1

[0145]

[0146] As can be seen from Table 1, the treated biogas slurry supernatant not only meets the discharge standard requirements but also meets the secondary standard of urban secondary sewage treatment plants, that is, the chemical oxygen demand is less than 120 mg / L, the ammonia nitrogen content is less than 30 mg / L, the suspended solid content is less than 30 mg / L, and the biochemical oxygen demand is less than 30 mg / L, and it can be regarded as clear water. At the same time, the total phosphorus content is lower than the national secondary discharge standard, less than 1.0 mg / L.

[0147] Example 4

[0148] The biogas slurry was treated according to the treatment method of Example 3. The difference is that the biogas slurry to be treated is the fermented manure biogas slurry from a beef cattle farm in Hubei, and its water quality is shown in Table 2. The water quality of the treated biogas slurry supernatant is shown in Table 2. Among them, the suspended solid content in the biogas slurry supernatant obtained after the third filtration through the third filter material 18 is less than 30 mg / L, and the chemical oxygen demand is less than 890 mg / L.

[0149] Table 2

[0150]

[0151] As can be seen from Table 2, the treated biogas slurry supernatant can be regarded as clear water and meets the discharge standard requirements.

[0152] According to the above results, in the present invention, by changing the packing material, the SS residue in the biogas slurry supernatant is less than 30 mg / L, and the chemical oxygen demand (COD) is less than 890 mg / L, which can meet the requirements of the capacitive deionization device; and by optimizing the loading method and the types of loading substances, the capacitance of the material is better, and at the same time, the catalytic effect is higher. After treating the biogas slurry supernatant in multiple experiments, the phosphorus removal rate can reach more than 98%, and basically can be completely removed.

[0153] The treatment process and method of the present invention are reasonable. Straw and biogas slurry are prepared into hydrothermal carbon and modified biochar; then the suspended solids and poorly soluble organic matters in the biogas slurry are filtered; the filtered biogas slurry further concentrates the nutrient components in the biogas slurry while purifying the biogas slurry into clear water. The system described in the present invention can effectively utilize biomass such as straw while purifying the biogas slurry, produce liquid fertilizer, and achieve efficient and energy-saving purification and discharge of biogas slurry sewage. While enriching the nutrient components of the biogas slurry to create economic benefits, the present invention further purifies the biogas slurry to meet the secondary standard of urban secondary sewage treatment plants, and the process flow is more perfect.

[0154] It should be understood that the parts not elaborated in detail in this specification all belong to the prior art.

[0155] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A preparation method of N-P-O-metal biochar, characterized in that, The preparation method includes the following steps: Mix straw, phosphoric acid solution, wood vinegar liquid and biogas slurry, and stir to obtain a mixture. Perform hydrothermal reaction on the mixture, and after the hydrothermal reaction, separate the solid and liquid to obtain hydrothermal carbon; Immerse the hydrothermal carbon in an activator solution to obtain the impregnated hydrothermal carbon, and then sequentially activate, wash and dry the impregnated hydrothermal carbon to obtain one-step activated carbon; Immerse the one-step activated carbon in a heteroatom dopant to obtain the impregnated one-step activated carbon, activate the impregnated one-step activated carbon to obtain N-P biochar, and then perform oxidation treatment on the N-P biochar under an oxygen-containing gas, and then sequentially wash and dry to obtain N-P-O activated carbon; Mix an iron salt solution and a magnesium salt solution to obtain a mixed solution. Immerse the N-P-O activated carbon in the mixed solution and stir to obtain the impregnated N-P-O activated carbon, and then pyrolyze the impregnated N-P-O activated carbon to obtain N-P-O-metal biochar.

2. The preparation method according to claim 1, wherein Mix straw, phosphoric acid solution, wood vinegar liquid and biogas slurry, and stir at a rotation speed of 350-450 r / min for 35-65 min to obtain a mixture; The concentration of the phosphoric acid solution is 12-21 wt%; The volume ratio of the phosphoric acid solution, wood vinegar liquid and biogas slurry is 3-4:1:1; The weight ratio of the straw to the total weight of the phosphoric acid solution, wood vinegar liquid and biogas slurry is 1:8-10; The conditions of the hydrothermal reaction include: temperature is 180-220 °C, and time is 15-20 h.

3. The preparation method according to claim 1 or 2, characterized in that, Immerse the hydrothermal carbon in an activator solution, and the activator adsorbs on the surface of the hydrothermal carbon to obtain the impregnated hydrothermal carbon. Then activate the impregnated hydrothermal carbon. During the activation process, through the action of the activator, mesopores and micropores are generated in the hydrothermal carbon, and then wash and dry to obtain one-step activated carbon; Immerse the hydrothermal carbon in the activator solution for 6-9 h; The activator in the activator solution is ZnCl2 and KHCO3, the concentration of KHCO3 in the activator solution is 0.7-1 mol / L, and the weight ratio of ZnCl2 to KHCO3 is 1-1.5:1; The weight ratio of the hydrothermal carbon to the activator is 1:1-1.5; The specific operations of sequentially activating, washing and drying the impregnated hydrothermal carbon include: activating the impregnated hydrothermal carbon at 585-615 °C under a protective gas for 55-75 min, then performing pickling and water washing, and then drying at 95-105 °C for 24-26 h.

4. The preparation method according to claim 1, characterized in that, The one-step activated carbon is immersed in a heteroatom dopant, and ammonium dihydrogen phosphate molecules in the heteroatom dopant adhere to the surface of the one-step activated carbon to obtain the impregnated one-step activated carbon. The impregnated one-step activated carbon is activated. During the activation process, N and P elements combine with the one-step activated carbon through heating to form functional groups, obtaining N-P biochar. Then, the N-P biochar is subjected to oxidation treatment under an oxygen-containing gas. During the oxidation treatment, C, N, and P elements are all oxidized to form oxygen-containing functional groups, completing the loading process of N, P, and O on the one-step activated carbon. Then, washing and drying are carried out in sequence to obtain N-P-O activated carbon; The one-step activated carbon is immersed in the heteroatom dopant for 5-8 h; The weight ratio of the one-step activated carbon to the heteroatom dopant is 1.5:1-2; The heteroatom dopant is an ammonium dihydrogen phosphate solution with a concentration of 45-60 wt%; The impregnated one-step activated carbon is activated at 585-615 °C for 55-75 min under a protective gas; The specific operations of subjecting the N-P biochar to oxidation treatment under an oxygen-containing gas and then carrying out washing and drying in sequence include: subjecting the N-P biochar to oxidation treatment at 330-370 °C for 45-65 min under an oxygen-containing gas, then carrying out pickling and water washing, and then drying at 95-105 °C for 24-26 h.

5. The preparation method according to claim 1, characterized in that, An iron salt solution and a magnesium salt solution are mixed to obtain a mixed solution. The N-P-O activated carbon is placed in the mixed solution and stirred and impregnated. Metal ions are adsorbed by the pores of the N-P-O activated carbon to obtain the impregnated N-P-O activated carbon. Then, the impregnated N-P-O activated carbon is pyrolyzed to fix metal particles, obtaining N-P-O-metal biochar; The concentration of the iron salt solution is 0.4-0.7 mol / L; The concentration of the magnesium salt solution is 0.4-0.7 mol / L; The Fe in the iron salt solution 3+ and the Mg in the magnesium salt solution 2+ have a molar ratio of 0.9 to 1.1:1; The N-P-O activated carbon is placed in the mixed solution and stirred and impregnated for 7-9 h; The solid-liquid ratio of the N-P-O activated carbon to the mixed solution is 1 g:6-10 mL; The impregnated N-P-O activated carbon is pyrolyzed at 585-615 °C for 55-75 min under a protective gas.

6. The N-P-O-metal biochar prepared by the preparation method according to any one of claims 1-5.

7. The application of the N-P-O-metal biochar according to claim 6 in the resource treatment of biogas slurry.

8. A biogas slurry resource treatment system, characterized in that, The biogas slurry resource treatment system includes: a solid-liquid separation device (1) and a capacitive deionization device (2); The solid-liquid separation device (1) is used to carry out solid-liquid separation on biogas slurry to obtain biogas slurry clear liquid; The capacitive deionization device (2) is used to carry out catalytic degradation treatment on the biogas slurry clear liquid from the solid-liquid separation device (1) to obtain the treated biogas slurry clear liquid; The capacitive deionization device (2) includes a cathode (21) and an anode (22), and the surfaces of the conductive current collectors of the cathode (21) and the anode (22) are coated with the N-P-O-metal biochar according to claim 6; The cathode (21) and the anode (22) are used to catalytically decompose the organic matter in the clarified biogas slurry during the catalytic degradation treatment, while degrading ammonia nitrogen and nitrite nitrogen compounds, generating charged ions, and adsorbing the charged ions through the action of an electric field.

9. The biogas slurry resource treatment system according to claim 8, characterized in that, The solid-liquid separation device (1) includes a biogas slurry container (11), a primary filtration column (12), a secondary filtration column (13), a tertiary filtration column (14), and a clarified biogas slurry container (15) connected in sequence; The primary filtration column (12) is filled with a first filter medium (16), the secondary filtration column (13) is filled with a second filter medium (17), and the tertiary filtration column (14) is filled with a third filter medium (18); The biogas slurry in the biogas slurry container (11) is transported to the primary filtration column (12), subjected to a first filtration through the first filter medium (16), then transported to the secondary filtration column (13), subjected to a second filtration through the second filter medium (17), then transported to the tertiary filtration column (14), subjected to a third filtration through the third filter medium (18) to obtain clarified biogas slurry, and the clarified biogas slurry is transported to the clarified biogas slurry container (15) for storage; The first filter medium (16) is crushed straw; the second filter medium (17) is a mixture of hydrochar and crushed straw; the third filter medium (18) is hydrochar; The particle size of the crushed straw is 1.5 - 3 mm, and the weight ratio of hydrochar to crushed straw is 1 - 1.5:1; The capacitive deionization device (2) further includes a DC power supply and an electrolytic cell (23). The negative electrode of the DC power supply is connected to the cathode (21), and the positive electrode of the DC power supply is connected to the anode (22); The cathode (21) and the anode (22) are both arranged in the electrolytic cell (23).

10. A method for resource treatment of biogas slurry, characterized in that, The biogas slurry resource treatment method is implemented in the biogas slurry resource treatment system described in claim 9, and specifically includes the following steps: Using the solid-liquid separation device (1) to perform solid-liquid separation on the biogas slurry to obtain clarified biogas slurry; Transporting the clarified biogas slurry to the capacitive deionization device (2) to perform catalytic degradation and deionization treatment on the clarified biogas slurry. The cathode (21) and the anode (22) are used to catalytically decompose the organic matter in the clarified biogas slurry during the catalytic degradation treatment, while degrading ammonia nitrogen and nitrite nitrogen compounds, generating charged ions, and adsorbing the charged ions through the action of an electric field to obtain the treated clarified biogas slurry.

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