A method for treating rural domestic sewage

Abstract

The application discloses a rural domestic sewage treatment method and belongs to the technical field of sewage treatment. The method comprises the following steps: sedimentation treatment, anaerobic and aerobic treatment, and adsorption treatment. The composite adsorbent used in the adsorption treatment comprises modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material. The modified biochar is obtained by modifying biomass raw materials with acid or alkali, high-temperature calcination, and then compounding polyvinyl alcohol and maleic anhyay grafted polyvinyl alcohol. The iron-manganese oxide modified bentonite is obtained by modifying bentonite with alkali and then combining with iron-manganese oxide by chemical bonding. The modified chitosan-activated carbon composite material is obtained by cross-linking chitosan and activated carbon with sodium alginate and then compounding with polyethyleneimine. The chemical oxygen demand, total phosphorus and total nitrogen in the rural domestic sewage can reach the national first-level A discharge standard only by the three steps of sedimentation, anaerobic and aerobic treatment and adsorption treatment, and the process is simple and efficient.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically a method for treating rural domestic wastewater. Background Technology

[0002] Existing rural domestic sewage treatment technologies have significant shortcomings in practical applications. On the one hand, traditional treatment processes (such as simple activated sludge processes and membrane bioreactors) often require complex pretreatment procedures, multi-stage reaction units, and precise equipment control, resulting in cumbersome operation, high maintenance costs, and difficulty in promoting their application in rural areas. For example, some processes require multiple chemical precipitations combined with biological treatment to remove pollutants, which not only makes it difficult to control the dosage of reagents but also requires a complex sludge treatment system. Other processes use membrane separation technology, which has a good treatment effect, but the membrane modules are prone to fouling, have high replacement costs, and require stringent operation and management.

[0003] On the other hand, existing technologies are not ideal for the comprehensive removal of major pollutants such as organic matter, total nitrogen, and total phosphorus in wastewater. Some processes remove pollutants through a single treatment unit (such as simple anaerobic or aerobic treatment), lacking synergistic effects, resulting in low removal efficiency for complex pollutants. Other processes, although using combined treatment methods, fail to achieve efficient removal of pollutants due to unreasonable connections between units, making it difficult to meet actual treatment needs.

[0004] Therefore, there is an urgent need to develop a rural domestic sewage treatment method that is simple in process, easy to operate, and has excellent treatment effect, in order to solve the problems of complex process and low treatment efficiency in the existing technology, and to achieve efficient purification and resource utilization of sewage. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for treating rural domestic sewage. This method can achieve the national Class A discharge standard for chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) in rural domestic sewage through only three steps: sedimentation, anaerobic-aerobic treatment, and adsorption treatment. The process is simple and efficient.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A method for treating rural domestic sewage includes the following steps:

[0008] S1. Precipitation treatment;

[0009] S2. Anaerobic-aerobic treatment;

[0010] S3. Adsorption treatment; The composite adsorbents used in the adsorption treatment include modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material.

[0011] The modified biochar is obtained by first modifying biomass raw materials with acid or alkali, then calcining them at high temperature, and finally compounding them with polyvinyl alcohol.

[0012] The iron-manganese oxide modified bentonite is obtained by first modifying bentonite with alkali and then combining it with iron-manganese oxide through chemical bonding.

[0013] The modified chitosan-activated carbon composite material is obtained by first cross-linking chitosan and activated carbon with sodium alginate, and then compounding them with polyethyleneimine.

[0014] As some possible implementations of this application, the mass ratio of the modified biochar, iron-manganese oxide modified bentonite, and chitosan-activated carbon composite material is 4:(3-6):(2-5).

[0015] Furthermore, as some possible embodiments of this application, the mass ratio of the modified biochar, iron-manganese oxide modified bentonite, and chitosan-activated carbon composite material is 4:5:3.

[0016] As one possible implementation method of this application, maleic anhydride-grafted polyvinyl alcohol is added during the preparation of modified biochar.

[0017] As one possible implementation of this application, the surface of iron-manganese oxide modified bentonite is coated with a nano-silica coating.

[0018] As one possible implementation of this application, the silica coating is replaced with a silica-alumina composite coating.

[0019] As some possible implementations of this application, the bentonite modified with iron-manganese oxide is first modified with polyacrylic acid before being coated with a silica-alumina composite coating or a silica coating.

[0020] As some possible embodiments of this application, in the silica-alumina composite layer, the mass ratio of silica to alumina is 1:(0.5~2).

[0021] As one possible implementation of this application, in step S1, 5-60 mg / L of flocculant is added.

[0022] As some possible implementations of this application, in step S2, the dissolved oxygen concentration in the anaerobic zone is ≤0.2mg / L, the dissolved oxygen concentration in the aerobic zone is 2-5mg / L, and the pH is 6.5-9.

[0023] The beneficial effects of this invention are:

[0024] 1. This invention first treats rural domestic sewage through sedimentation, using gravity to settle suspended solids and remove larger particulate impurities. Next, it proceeds to an anaerobic-aerobic treatment stage. In the anaerobic zone, anaerobic microorganisms decompose large organic molecules into smaller molecules, while denitrifying bacteria in the anaerobic zone remove nitrogen. In the aerobic zone, organic matter degradation, ammonia nitrification, and phosphorus uptake by polyphosphate-accumulating bacteria are completed. Finally, residual pollutants are removed through adsorption. This multi-stage treatment effectively removes major pollutants such as chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) from rural domestic sewage (after treatment, COD ≤ 16.8 mg / L, TP ≤ 0.15 mg / L, and TN ≤ 2.2 mg / L), effectively improving the rural water environment and reducing sewage pollution of surrounding water bodies, soil, and other ecological environments.

[0025] 2. This invention employs a regeneration process combining elution and microwave methods. By leveraging the synergistic effect of the components in the composite adsorbent, it exhibits excellent regeneration stability. After five regeneration cycles, the composite adsorbent achieves a COD removal rate ≥90.5%, a TN removal rate ≥93.7%, and a TP removal rate ≥95.5%. This demonstrates that the composite adsorbent provided by this invention can achieve long-term stability in an "adsorption-regeneration" cycle, reducing the replacement cost of consumables in wastewater treatment and making it suitable for long-term operation in rural areas.

[0026] 3. This invention significantly improves the mechanical strength of the composite adsorbent through multiple modification processes, effectively reducing the breakage rate (≤3.2%). The low breakage rate can effectively reduce the risk of adsorption column blockage and extend the equipment operation cycle.

[0027] 4. The method for treating rural domestic sewage in this invention is simple, relying on only three steps: sedimentation, anaerobic and aerobic treatment (each stage is separate) and adsorption treatment. It is easy to operate, suitable for promotion and application in rural areas, and has good economic and environmental benefits. Attached Figure Description

[0028] Figure 1 : A cross-sectional view of the sedimentation device from the front view;

[0029] Figure 2 : A schematic diagram of the scraper blade from the front view;

[0030] Among them, 1-first rectangular trough, 2-bamboo grid, 3-stainless steel grid, 4-second rectangular trough, 5-oil scraper, 6-oil collection trough, 7-composite layer, 8-fourth rectangular trough, 9-foam board, 10-liquid guide pipe, 11-third rectangular trough. Detailed Implementation

[0031] A method for treating rural domestic sewage includes the following steps:

[0032] S1. Precipitation treatment;

[0033] S2. Anaerobic-aerobic treatment;

[0034] S3. Adsorption treatment; The composite adsorbents used in the adsorption treatment include modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material.

[0035] The modified biochar is obtained by first modifying biomass raw materials with acid or alkali, then calcining them at high temperature, and finally compounding them with polyvinyl alcohol.

[0036] The iron-manganese oxide modified bentonite is obtained by first modifying bentonite with alkali and then combining it with iron-manganese oxide through chemical bonding.

[0037] The modified chitosan-activated carbon composite material is obtained by first cross-linking chitosan and activated carbon with sodium alginate, and then compounding them with polyethyleneimine.

[0038] This invention first treats rural domestic sewage through sedimentation, using gravity to settle suspended solids and remove larger particulate impurities. Next, it proceeds to an anaerobic-aerobic treatment stage. In the anaerobic zone, anaerobic microorganisms decompose large organic molecules into smaller ones, while denitrifying bacteria in the anaerobic zone remove nitrogen. In the aerobic zone, organic matter degradation, ammonia nitrification, and phosphorus uptake by polyphosphate-accumulating bacteria are completed. Finally, residual pollutants are removed through adsorption. This multi-stage treatment effectively removes major pollutants such as chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) from rural domestic sewage, bringing the water quality up to the national Class A discharge standard. This effectively improves the rural water environment and reduces sewage pollution of surrounding water bodies, soil, and other ecological environments.

[0039] The synergistic effects of the components of the composite adsorbent are as follows:

[0040] ① Modified biochar is composited with polyvinyl alcohol (PVA). PVA, as a high-molecular-weight polymer, fills the pores of the biochar and forms a continuous phase on its surface, enhancing its mechanical strength. Modified chitosan-activated carbon composites are composited with polyethyleneimine (PEI). PEI molecular chains are interwoven within the chitosan-activated carbon network, providing reinforcement and toughening. Alkali modification of bentonite increases the hydroxyl content on its surface, densifying its layered structure. The subsequent Si-O-Fe / Si-O-Mn covalent bond network formed with iron and manganese oxides further enhances the modified bentonite's resistance to breakage. The synergistic effect of these three components significantly improves the overall mechanical strength of the composite adsorbent, reducing breakage and pulverization during wastewater flow and effectively extending its service life.

[0041] ② Modified biochar, after acid or alkali modification and high-temperature calcination, increases porosity and surface functional groups, exhibiting strong adsorption capacity for organic matter in domestic sewage; iron-manganese oxide-modified bentonite, with the iron-manganese oxides chemically bonded to the bentonite, shows special adsorption effects on heavy metals and phosphorus in domestic sewage; in modified chitosan-activated carbon composite materials, the amino and other functional groups of chitosan synergistically work with the high specific surface area of ​​activated carbon to efficiently adsorb nitrogen and some organic matter in domestic sewage. The synergistic effect of these three materials significantly enhances the adsorption effect on pollutants in domestic sewage.

[0042] To further improve wastewater treatment efficiency, as one of the possible implementation methods of this application, the dosage of each component in the composite adsorbent is further limited, specifically, the mass ratio of the modified biochar, iron-manganese oxide-modified bentonite, and chitosan-activated carbon composite material is 4:(3-6):(2-5). This invention clarifies the range of the mass ratio of the modified biochar, iron-manganese oxide-modified bentonite, and modified chitosan-activated carbon composite material, enabling optimal adsorption of pollutants in rural domestic wastewater.

[0043] When modified biochar is combined with polyvinyl alcohol, the functional groups on the surface of biochar and the molecular structure of polyvinyl alcohol differ significantly, resulting in poor interfacial compatibility. This can easily lead to phase separation during use, affecting the overall performance and lifespan of the adsorbent. Therefore, as one possible implementation method of this application, maleic anhydride-grafted polyvinyl alcohol is added during the preparation of modified biochar.

[0044] The maleic anhydride-grafted polyvinyl alcohol molecular structure contains both functional groups that react with the functional groups on the surface of biochar and segments that are compatible with polyvinyl alcohol. This effectively improves the interfacial compatibility between the two, allowing polyvinyl alcohol to be uniformly coated on the surface of biochar, forming a stable composite structure. This enhances the mechanical strength and adsorption stability of the composite adsorbent, while the introduction of new functional groups can strengthen its adsorption capacity for pollutants.

[0045] Although iron-manganese oxide-modified bentonite fixes the iron-manganese oxides through chemical bonding, in complex rural domestic sewage environments, various chemical components in the sewage (such as humic acid and inorganic ions) may react with these chemical bonds, disrupting the bond between the iron-manganese oxides and bentonite, leading to the loss of the iron-manganese oxides. Therefore, as one possible implementation method of this application, a nano-silica coating is applied to the surface of the iron-manganese oxide-modified bentonite. Nano-silica exhibits good chemical stability and mechanical strength, protecting the iron-manganese oxides and preventing their loss. Simultaneously, its porous structure does not affect the adsorption of pollutants by the bentonite, maintaining the stability of the adsorbent performance.

[0046] In wastewater treatment, composite adsorbents are subjected to impacts and friction from water flow. A single silica coating has relatively low mechanical strength and is prone to damage and peeling during long-term use, resulting in limited protection against iron and manganese oxides (IMOs) and allowing some IMO loss, thus affecting the lifespan and adsorption efficiency of the composite adsorbent. Therefore, as one possible implementation method of this application, the silica coating is replaced with a silica-alumina composite coating. Compared to silica, alumina has higher hardness and better wear resistance. The composite coating formed by combining alumina and silica significantly enhances the overall mechanical strength, wear resistance, and chemical stability of the coating, better resisting damage from external physical forces, effectively preventing IMO loss, enhancing the protection of IMO-modified bentonite, and maintaining the long-term stability of the adsorbent performance. Furthermore, microorganisms easily attach and grow on the surface of a single silica layer, forming a biofilm that blocks pores, reduces the mass transfer efficiency of the adsorbent, and affects the adsorption effect. The silica-alumina composite coating, by altering the surface chemical composition and microstructure, reduces the adhesion tendency of microorganisms and organic matter, thus improving its anti-pollution capability.

[0047] To further prevent coating peeling, as one possible implementation of this application, the bentonite surface is first modified with polyacrylic acid before being coated with a silica-alumina composite coating or a silica coating. The carboxyl groups of the polyacrylic acid molecules form ionic or coordinate bonds with metal ions on the bentonite surface, and the long-chain structure entangles with the subsequent coating, enhancing the interfacial adhesion between the coating and bentonite, preventing coating peeling during use and regeneration, and ensuring the coating provides long-term protection and adsorption.

[0048] To further improve the sedimentation effect in step S1, as one possible implementation method of this application, 5-60 mg / L of flocculant is added in step S1. Adding flocculant in step S1 can cause fine suspended solids to aggregate into larger flocs, improving sedimentation efficiency, reducing the amount of suspended solids entering subsequent treatment units, and ensuring the effectiveness of subsequent treatments.

[0049] To further improve the treatment effect of pollutants in step S2, as some possible implementation methods of this application, in step S2, the dissolved oxygen concentration in the anaerobic zone is ≤0.2 mg / L, the dissolved oxygen concentration in the aerobic zone is 2-5 mg / L, and the pH is 6.5-9. The low dissolved oxygen in the anaerobic zone is conducive to the growth, metabolism, and decomposition of organic matter by anaerobic microorganisms, the appropriate dissolved oxygen in the aerobic zone meets the respiration and metabolic needs of aerobic microorganisms, and the appropriate pH range maintains the enzyme activity in the microorganisms, ensuring that the anaerobic and aerobic treatment efficiently removes organic matter, nitrogen, and phosphorus.

[0050] The following examples will provide a more detailed description of the rural domestic sewage treatment method.

[0051] Example 1

[0052] S1. Sedimentation Treatment: 100L of rural domestic sewage [Chemical Oxygen Demand (COD) 410mg / L, Biochemical Oxygen Demand (BOD5) 160mg / L, Ammonia Nitrogen (NH3-N) 38mg / L, Total Nitrogen (TN) 42mg / L, Total Phosphorus (TP) 6mg / L, Suspended Solids (SS) 200mg / L, heavy metals meeting the national Class A discharge standard] is introduced into a sedimentation tank. 0.6kg of polyaluminum chloride is added as a flocculant and stirred thoroughly to ensure complete dispersion of the flocculant in the sewage, promoting the coagulation of suspended solids. The sedimentation time is controlled at 2 hours. After sedimentation, the supernatant is drawn off and proceeds to the next stage.

[0053] S2. Anaerobic-Aerobic Treatment: The supernatant after sedimentation is introduced into the anaerobic reactor. The dissolved oxygen concentration in the anaerobic zone is controlled to be ≤0.2 mg / L, and the pH is 8. The reaction is carried out under anaerobic conditions for 10 hours. In the anaerobic reactor, the metabolic action of anaerobic microorganisms (acid-producing bacteria, denitrifying bacteria, and methanogenic bacteria mixed in a mass ratio of 5:3:2) decomposes large organic molecules in the wastewater into smaller organic molecules, while removing some nitrogen and phosphorus pollutants. Subsequently, the wastewater is introduced into the aerobic reactor. The dissolved oxygen concentration in the aerobic zone is controlled to be 4 mg / L, and the pH is 8. Aerobic microorganisms (composed of nitrifying bacteria, polyphosphate-accumulating bacteria, and organic matter-degrading bacteria in a mass ratio of 7:5:7, added at a dosage of 8.5 g / m³ of total dry weight of aerobic microorganisms) are introduced into the reactor. 3 Under the action of [addition], small molecule organic matter is further oxidized and decomposed into carbon dioxide and water. At the same time, nitrogen pollutants are removed through nitrification and denitrification, and phosphorus pollutants are removed through the excessive phosphorus uptake of polyphosphate bacteria. The aerobic reaction time is 15 hours. At this time, the wastewater quality is significantly improved, with COD of 67 mg / L, BOD5 of 12 mg / L, ammonia nitrogen of 6 mg / L, total nitrogen of 18 mg / L, and total phosphorus of 0.5 mg / L.

[0054] Subsequently, the wastewater that had undergone aerobic treatment was subjected to adsorption treatment as described in Examples 2-6 and Comparative Examples 1-3.

[0055] Example 2

[0056] After anaerobic and aerobic treatment, the wastewater is pumped into the distributor at the top of the adsorption column using a peristaltic pump, with the wastewater flow rate controlled at 6 BV / h (bed volume / h). The empty bed contact time of the adsorption column is set to 50 min, and the peristaltic pump frequency is 25 rpm.

[0057] The preparation method of the composite adsorbent is as follows:

[0058] S31. Preparation of the three components in the composite adsorbent.

[0059] Methods for preparing modified biochar:

[0060] A. Select straw as biomass raw material, wash it with clean water to remove impurities, crush it to a particle size of 2-5mm, and then dry it to a moisture content of less than 10% to obtain pretreated biomass raw material;

[0061] B. The pretreated biomass feedstock was immersed in a 1 mol / L sodium hydroxide solution, with the solid-liquid ratio controlled at 1:7 (g / mL), and the reaction was stirred at room temperature for 3 hours. After the reaction was completed, the feedstock was washed with deionized water until neutral to obtain the alkali-modified feedstock.

[0062] C. The alkali-modified raw material is calcined at high temperature under a nitrogen protective atmosphere. The heating rate is controlled at 5-10℃ / min, the calcination temperature is set at 600℃, and the calcination time is 2h to obtain calcined biochar;

[0063] D. Calcined biochar was added to a 4% (w / w) aqueous solution of polyvinyl alcohol, with a biochar to polyvinyl alcohol mass ratio of 1:0.2, and the mixture was stirred and mixed at 55°C for 2.5 h. The mixture was then dried using spray drying technology to obtain modified biochar.

[0064] Method for preparing modified bentonite:

[0065] A. Sodium-based bentonite was mixed with an 8% sodium hydroxide solution at a solid-liquid ratio of 1:10 (g / mL) and stirred at 75°C for 4 hours. After the reaction was complete, the mixture was filtered and washed to obtain alkali-modified bentonite.

[0066] B. A certain amount of ferrous sulfate and manganese sulfate were dissolved in deionized water at a Fe:Mn molar ratio of 2:1 to prepare a mixed metal salt solution. Under stirring conditions, 12% ammonia water was slowly added dropwise to adjust the pH of the solution to 8-9, causing the iron and manganese ions to precipitate out as hydroxides. Then, alkali-modified bentonite was added to the precipitation system, and the reaction was continued with stirring for 2.5 hours, allowing the iron and manganese hydroxides to precipitate on the surface of the bentonite. After the reaction was completed, the mixture was subjected to a hydrothermal reaction at 135℃ for 16 hours. After the reaction was completed, the product was cooled, filtered, washed, and vacuum dried at 80℃ to constant weight to obtain iron and manganese oxide modified bentonite.

[0067] Preparation method of modified chitosan-activated carbon composite material:

[0068] A. Weigh a certain amount of chitosan and dissolve it in a 2% (w / w) acetic acid solution to prepare a 3% (w / w) chitosan solution. Then add 200-mesh activated carbon, with a mass ratio of activated carbon to chitosan of 1.5:1, and stir until homogeneous. Next, slowly add a 2.5% (w / w) sodium alginate solution dropwise to the mixed solution while stirring. The mass ratio of sodium alginate to chitosan is 1:2. After the addition is complete, continue stirring at room temperature for 1.5 hours to obtain the cross-linked product.

[0069] After filtering and washing the above crosslinking product, it was added to a 4% (w / w) aqueous solution of polyethyleneimine. The mass-to-volume ratio of the crosslinking product to the aqueous solution of polyethyleneimine was 1:6. The mixture was stirred and reacted at 45°C for 3.5 h. After the reaction was completed, the product was filtered and washed again, and then dried at 75°C to constant weight to obtain the modified chitosan-activated carbon composite material.

[0070] S32. A cylindrical adsorption column with an inner diameter of 100 mm and a height of 1500 mm is selected. A 20 mm thick layer of quartz sand (particle size 2-4 mm) is laid at the bottom, and a composite adsorbent [modified biochar (particle size controlled to 2-5 mm after grinding), modified bentonite (particle size controlled to 1-3 mm after grinding), and modified chitosan-activated carbon composite material (particle size controlled to 1-2 mm after grinding) mixed in a mass ratio of 4:5:3] is filled to a height of 1000 mm, with a filling density of 0.7 g / cm³. 3 Water distributors and collectors are installed at the top and bottom of the adsorption column.

[0071] Example 3

[0072] Compared to Example 2, maleic anhydride-grafted polyvinyl alcohol was added in the preparation of modified biochar. Except for the adjustment of the modified biochar preparation method, the other steps and parameters were the same as in Example 2.

[0073] The specific preparation method for modified biochar is as follows:

[0074] A. Same as Example 2.

[0075] B. Same as Example 2.

[0076] C. Same as Example 2.

[0077] D. Maleic anhydride-grafted polyvinyl alcohol (PVA) was mixed evenly with a 4% (w / w) aqueous solution of PVA. Calcined biochar was then added and stirred. The mass ratio of biochar to PVA was 1:0.2, and the mass ratio of biochar to maleic anhydride-grafted PVA was 10:1.5. The mixture was stirred at 55°C for 2.5 hours. The solution was then dried using spray drying technology to obtain modified biochar.

[0078] The preparation method of maleic anhydride-grafted polyvinyl alcohol is as follows:

[0079] ① Weigh 100g of polyvinyl alcohol (degree of polymerization 1750), put it into a 1000mL three-necked flask, add 800mL of deionized water, and stir in a 95℃ water bath to dissolve until a uniform and transparent polyvinyl alcohol solution is formed.

[0080] Weigh 15g of maleic anhydride and place it in a 200mL beaker. Add 50mL of acetone to dissolve it and stir until the maleic anhydride is completely dissolved to obtain a maleic anhydride acetone solution.

[0081] Prepare 2g of benzoyl peroxide (BPO) as an initiator, dissolve it in 20mL of acetone, and prepare an initiator solution.

[0082] ② The maleic anhydride acetone solution was slowly added dropwise to the polyvinyl alcohol solution while stirring. After the addition was complete, stirring was continued for 30 minutes. Then, the initiator solution was slowly added dropwise. After the addition was complete, the three-necked flask was placed in an oil bath at 100°C, and the reaction was carried out under a nitrogen atmosphere with a stirring speed of 250 rpm for 3.5 hours. After the reaction was completed, the reaction solution was cooled to room temperature, precipitated in anhydrous ethanol, filtered, washed, and dried under vacuum at 65°C to constant weight to obtain maleic anhydride-grafted polyvinyl alcohol.

[0083] Example 4

[0084] Compared to Example 3, a nano-silica coating was applied to the surface of the iron-manganese oxide modified bentonite. Except for adjustments to the preparation method of the modified bentonite, all other steps and parameters were the same as in Example 3.

[0085] The preparation method of the modified bentonite in this embodiment is as follows:

[0086] Weigh 20g of tetraethyl orthosilicate and pour it into a 500mL beaker. Add 100mL of anhydrous ethanol and stir until homogeneous to form the first solution. In another beaker, measure 50mL of deionized water and add 2mL of concentrated hydrochloric acid (37% by mass). Stir until homogeneous to obtain the second solution. While continuously stirring, slowly add the second solution dropwise to the first solution over a period of 25 minutes. After the addition is complete, continue stirring for 2.5 hours to form a uniform and transparent nano-silica sol.

[0087] The iron-manganese oxide modified bentonite prepared in Example 3 was immersed in nano-silica sol for 15 min, and then the bentonite was slowly and vertically lifted from the sol surface at a speed of 8 cm / min. The iron-manganese oxide modified bentonite coated with the wet coating was then dried in a constant temperature drying oven at 60°C for 2.5 h, and then transferred to a muffle furnace and heated to 300°C at a heating rate of 5°C / min, and held at this temperature for 2 h to obtain the modified bentonite.

[0088] Example 5

[0089] Compared to Example 3, a silica-alumina composite coating was applied to the surface of the iron-manganese oxide modified bentonite. Except for adjustments to the preparation method of the modified bentonite, all other steps and parameters were the same as in Example 3.

[0090] The preparation method of the modified bentonite in this embodiment is as follows:

[0091] Weigh 15g of tetraethyl orthosilicate and pour it into a 300mL beaker; weigh 15g of aluminum nitrate nonahydrate and pour it into another 300mL beaker, labeling them beaker one and beaker two respectively.

[0092] Add 80 mL of anhydrous ethanol to the first beaker and stir to dissolve the tetraethyl orthosilicate. Add 80 mL of deionized water to the second beaker and stir to completely dissolve the aluminum nitrate nonahydrate. Prepare 1.5 mL of concentrated hydrochloric acid (37% by mass) as a catalyst, divide it evenly into the two beakers, and stir well.

[0093] Slowly pour the aluminum nitrate solution from the second beaker into the first beaker while stirring, until the two solutions are fully mixed.

[0094] The mixed solution was allowed to stand at room temperature for 4 hours, with stirring every 30 minutes. Then, the iron-manganese oxide modified bentonite prepared in Example 3 was completely immersed in the prepared silica-alumina sol for 13 minutes. The bentonite was then slowly and vertically lifted from the sol surface at a speed of 7 cm / min. The iron-manganese oxide modified bentonite, coated with a wet coating, was placed in a constant temperature drying oven at 65°C for 3 hours. It was then transferred to a muffle furnace and heated to 400°C at a heating rate of 4°C / min, and held at this temperature for 2.5 hours to obtain the modified bentonite.

[0095] Example 6

[0096] Compared to Example 5, the bentonite modified with iron-manganese oxide was first modified with polyacrylic acid before being coated with a silica-alumina composite coating. Except for adjustments to the preparation method of the modified bentonite, the other steps and parameters were the same as in Example 5.

[0097] In this embodiment, the modified bentonite is prepared as follows:

[0098] Weigh 1.5g of polyacrylic acid, pour it into a 200mL beaker, add 150mL of deionized water, and stir at 45℃ for 1.5h to obtain a polyacrylic acid solution.

[0099] The iron-manganese oxide-modified bentonite prepared in Example 3 was completely immersed in a polyacrylic acid solution and soaked in a constant temperature water bath at 60°C for 2 hours. After soaking, the bentonite was removed, washed, and dried to obtain the polyacrylic acid-modified iron-manganese oxide-modified bentonite.

[0100] The polyacrylic acid-modified iron-manganese oxide-modified bentonite was completely immersed in the prepared silica-alumina sol (preparation method as in Example 5), with the immersion time controlled at 13 min. Then, the bentonite was slowly and vertically lifted from the sol surface at a speed of 7 cm / min. The iron-manganese oxide-modified bentonite coated with the wet coating was then placed in a constant temperature drying oven at 65°C and dried for 3 h. Afterward, it was transferred to a muffle furnace and heated to 400°C at a heating rate of 4°C / min. It was then held at this temperature for 2.5 h to obtain the modified bentonite.

[0101] Example 7

[0102] In Examples 1-6, the sedimentation process not only needs to effectively remove various solid impurities such as vegetable leaves, fibers, and residues mixed in rural domestic sewage, but also needs to effectively remove a large amount of animal and vegetable oils originating from kitchen wastewater. Otherwise, it will have an adverse impact on subsequent processes. For example, in the S2 anaerobic-aerobic treatment, solid impurities will entangle anaerobic and aerobic microorganisms, hindering the contact between microorganisms and organic matter in the sewage, and reducing the decomposition efficiency of organic matter. In the S3 adsorption treatment, they will clog the pores of the composite adsorbent, reducing the adsorption capacity for COD and total nitrogen. Animal and vegetable oils will consume a large amount of dissolved oxygen in the S2 anaerobic-aerobic treatment, inhibiting the activity of nitrifying bacteria and polyphosphate-accumulating bacteria, thus reducing the removal rates of COD, total nitrogen, and total phosphorus. In the S3 adsorption treatment, an oil film will form on the surface of the adsorbent, covering the active sites and hindering the physical adsorption and chemical action of pollutants by the adsorbent.

[0103] Based on this, in order to effectively remove solid impurities such as vegetable leaves, residues, and fibers, as well as animal and vegetable oils from wastewater, and to provide qualified influent for subsequent anaerobic and aerobic treatment and adsorption treatment, this application provides a sedimentation device, such as... Figure 1 As shown, it includes a grid unit, an oil separator unit, a sedimentation unit, and a collection unit, which are connected sequentially by pipes.

[0104] The grid unit includes a first rectangular trough 1 and a double-layer grid inclinedly arranged in the rectangular trough. The upper layer is a bamboo grid 2 with a hole diameter of 8-10mm, which is mainly used to intercept large impurities such as vegetable leaves and straw in sewage. The lower layer is a stainless steel grid 3 with a hole diameter of 2-3mm, which can effectively intercept small residues and short fibers and other tiny solid impurities.

[0105] The oil-separating unit includes a second rectangular trough 4, an oil scraper 5 (the length of which matches the width of the second rectangular trough) movably disposed within the second rectangular trough 4, and a third rectangular trough 11 disposed within the second rectangular trough 4 (the outer wall of which is fitted to the inner wall of the second rectangular trough 4, and the bottom wall of the third rectangular trough 4 is provided with multiple holes with a diameter of 3-5mm). The third rectangular trough 11 is filled with an alkali-modified loofah sponge-activated carbon composite layer 7, the filling thickness of which is 15cm (the composite layer 7 is placed in an 80-mesh nylon mesh bag and stacked in layers to avoid material loss and compression; the mesh bag can be removed and replaced as a whole). The top of the third rectangular trough 11 is provided with a perforated cover plate (the perforation diameter is 5-10mm, which can disperse the flowing sewage, prevent the water flow from directly impacting the composite layer 7 and causing material compression, and at the same time prevent the composite layer 7 from floating).

[0106] The composite layer 7 is composed of a 7:3 mass ratio of loofah sponge soaked and dried in sodium hydroxide solution to activated carbon with a particle size of 2-5 mm. The distance between the bottom ends of the third rectangular tank 11 and the second rectangular tank 4 is 10-15 cm.

[0107] In addition, such as Figure 2 As shown, an oil collection groove 6 is provided on the upper side wall of the oil scraper 5.

[0108] The sedimentation unit includes a fourth rectangular tank 8. This unit requires the addition of a mixed flocculant, which is composed of polyaluminum chloride (dosage as in Example 1) and chitosan-bentonite composite additive at a mass ratio of 5:1. The chitosan-bentonite composite additive is prepared as follows: a 3% chitosan acetic acid solution from stage S3 of Example 1 is taken and mixed with iron-manganese oxide modified bentonite (particle size 1-3 mm) at a solid-liquid ratio of 1:8, stirred at 300 rpm for 30 minutes, and then dried.

[0109] The collection unit includes a foam board 9 (density 0.3 g / cm³) floating inside the fourth rectangular trough 8. 3 The system is designed to ensure that the liquid drawn from the sedimentation unit is always supernatant, and is equipped with a liquid guide pipe 10 on the foam plate 9. The bottom end of the liquid guide pipe 10 passes through the foam plate 9 and is provided with a filter funnel 10. The side wall and bottom wall of the filter funnel 10 are provided with 80-mesh filter holes (to further filter fine flocs).

[0110] It is worth noting that all pipes and liquid guides in this embodiment are equipped with valves (existing technology, not shown in the figure).

[0111] Working Principle: 100L of rural domestic sewage (including 25mg / L of animal and vegetable oils in addition to the pollutants described in Example 1, with a floating oil to emulsified oil mass ratio of 6:4) flows into the bar screen interception unit. It first contacts the upper bamboo bar screen 2, where large impurities are intercepted on its surface. As the sewage continues to flow, it passes through the lower stainless steel bar screen 3, where fine residues and short fibers are further trapped. After passing through the two bar screens, the sewage flows into the oil separator unit. Floating oil takes 15-20 minutes to rise to the surface. Therefore, the oil skimming operation only needs to be performed once, 20 minutes after the sewage flows into the oil separator unit. Specifically, during oil skimming, the scraper is manually controlled to move horizontally along the length of the tank. When it reaches the end, it rotates 30° around the bottom, guiding the floating oil to the oil collection tank 6. During this process, the alkali-modified loofah sponge-activated carbon composite layer 7 adsorbs the emulsified oil and some residual small solid impurities. Only after the oil scraper has finished scraping the oil should the valve on the pipe connecting the oil separator unit and the sedimentation unit be opened.

[0112] After the wastewater undergoes oil separation treatment, it enters the sedimentation unit. First, a mixed flocculant is added. Polyaluminum chloride destabilizes the colloids. In the chitosan-bentonite composite additive, chitosan carries a positive charge, which can neutralize the negative charge on the surface of emulsified oil droplets and destroy the interfacial film of the emulsified oil, causing it to demulsify. Bentonite has a layered structure, which can adsorb fine oil droplets and destabilized colloidal particles. Together with polyaluminum chloride, it forms a "floc-oil droplet" complex, thereby effectively removing suspended solids and oil content from the wastewater.

[0113] After sedimentation, the supernatant (with SS content of 2-5 mg / L and animal and vegetable oil content of 1.5-2 mg / L) is drawn out along the guide pipe for subsequent treatment. During the anaerobic-aerobic stage, acid-producing bacteria secrete lipases, which decompose the residual animal and vegetable oils (triglycerides) in the supernatant into glycerol and long-chain fatty acids, providing substrates for subsequent reactions. Organic matter-degrading bacteria secrete oxidases to further oxidize the long-chain fatty acids produced in the anaerobic stage into CO2 and H2O, further reducing the animal and vegetable oil content. In the adsorption treatment stage, modified biochar adsorbs residual oils through its porous structure and van der Waals forces. Iron-manganese oxide-modified bentonite physically traps fine oil droplets using its layered structure. The modified chitosan-activated carbon composite material utilizes the hydroxyl and amino groups of chitosan molecules to form hydrogen bonds with oils, enhancing the adsorption effect. Finally, after adsorption treatment, the animal and vegetable oil content is ≤0.8 mg / L, meeting the national Class A emission standard for animal and vegetable oils.

[0114] Comparative Example 1

[0115] Compared to Example 2, the modified biochar was replaced with calcined biochar.

[0116] The remaining steps and parameters are the same as in Example 2.

[0117] Comparative Example 2

[0118] Compared to Example 2, the modified bentonite was obtained by directly combining bentonite with iron-manganese oxides via chemical bonding without alkali modification. All other steps and parameters were the same as in Example 1.

[0119] Comparative Example 3

[0120] Compared to Example 2, the modified chitosan-activated carbon composite material is obtained by crosslinking chitosan and activated carbon with sodium alginate, and is not subsequently compounded with polyethyleneimine. The remaining steps and parameters are the same as in Example 1.

[0121] In this comparative example, the preparation method of the modified chitosan-activated carbon composite material is as follows:

[0122] A certain amount of chitosan was weighed and dissolved in a 2% (w / w) acetic acid solution to prepare a 3% (w / w) chitosan solution. Then, 200-mesh activated carbon was added, with a mass ratio of activated carbon to chitosan of 1.5:1, and the mixture was stirred until homogeneous. Next, a 2.5% (w / w) sodium alginate solution was slowly added dropwise to the mixed solution while stirring, with a mass ratio of sodium alginate to chitosan of 1:2. After the addition was complete, the reaction was continued at room temperature with stirring for 1.5 hours to obtain a crosslinked product. The crosslinked product was then filtered, washed, and dried at 50℃ and a vacuum of -0.1 MPa for 5 hours to obtain a modified chitosan-activated carbon composite material.

[0123] Experimental Example

[0124] (a) The pollutant indicators in the water after treatment in Examples 2-6 and Comparative Examples 1-3 were measured, and the results are shown in Table 1.

[0125] Table 1:

[0126]

[0127]

[0128] (II) Adsorbent regeneration experiment.

[0129] A general regeneration method using elution and microwave methods:

[0130] Elution treatment: The saturated adsorbent was eluted twice, first with 0.2 mol / L sodium hydroxide solution and then with ethanol-water solution (volume ratio 7:3).

[0131] Microwave treatment: Place the eluted adsorbent in a microwave oven and treat it at low power (250W) for 5-10 minutes.

[0132] The regenerated adsorbent was refilled into the adsorption column, and the wastewater treatment process was repeated. The treatment capacity of the adsorbent for wastewater pollutants after the fifth regeneration was recorded. The results are shown in Table 2.

[0133] In practice, the adsorbent is operated in a cycle of "40 hours of operation → regeneration → 40 hours of operation".

[0134] Table 2:

[0135]

[0136] (III) Adsorbent breakage rate.

[0137] Based on Experiment (II) above, adsorbent samples were randomly selected from different positions (upper, middle, and lower layers) of the adsorption columns after the fifth regeneration and 10 hours of operation in Examples 2-6 and Comparative Examples 1-3. The selected samples were mixed thoroughly, and 100g was taken as the test sample using the quartering method. These samples were then dried to constant weight at 40℃ and a vacuum of -0.1MPa. The breakage rate was then determined using a sieve method. The samples were sieved (1mm mesh), and the fine particles passing through the sieve were collected, weighed, and their percentage of the total sample mass was calculated. This percentage represents the adsorbent breakage rate, and the results are shown in Table 3.

[0138] Table 3:

[0139] Group Breakage rate (%) Example 2 3.2 Example 3 2.8 Example 4 3.1 Example 5 2.6 Example 6 2.0 Comparative Example 1 7.9 Comparative Example 2 6.8 Comparative Example 3 7.4

[0140] The following conclusions can be drawn from Table 1-3:

[0141] After treatment with the composite adsorbent, the COD in the water in Examples 2-6 was ≤16.8 mg / L, TP ≤0.15 mg / L, and TN ≤2.2 mg / L, meeting the national Class A discharge standard (as shown in Table 1). Specific analysis is as follows:

[0142] In Example 2, modified biochar was combined with polyvinyl alcohol through alkali modification to increase surface functional groups and pore structure, thereby enhancing the physical and chemical adsorption of organic matter; iron-manganese oxide modified bentonite effectively removed phosphorus by utilizing the redox properties and surface active sites of iron-manganese oxide; modified chitosan-activated carbon composite material combined the amino groups of chitosan and the porous structure of activated carbon, and had good adsorption effects on ammonia nitrogen and organic matter.

[0143] In Example 3, the anhydride groups of maleic anhydride undergo esterification with the hydroxyl groups on the surface of biochar, while the polyvinyl alcohol segments become intertwined, further increasing the specific surface area and active sites of the biochar and enhancing its adsorption capacity for organic matter, especially for recalcitrant organic matter.

[0144] In Example 4, the nano-silica coating forms a uniform and dense protective layer on the surface of iron-manganese oxide-modified bentonite, reducing the loss of iron-manganese oxides during adsorption. Simultaneously, the abundant silanol groups in the nano-silica can interact with pollutants through hydrogen bonding or electrostatic adsorption, improving the adsorption selectivity for phosphorus and heavy metals. Furthermore, the nano-silica coating maintains mechanical strength while enhancing adsorption selectivity, reducing breakage rate.

[0145] In Example 5, the silica-alumina composite coating has a more complex pore structure and diverse active sites, which can chemically adsorb and exchange pollutants such as ammonia nitrogen and phosphorus. At the same time, the composite coating enhances the mechanical strength of bentonite and reduces the breakage rate of adsorbent.

[0146] In Example 6, polyacrylic acid modification first introduces a large number of carboxyl groups into the surface of bentonite, increasing the surface negative charge and active sites, thereby improving the adsorption capacity for cationic pollutants; then, a silica-alumina composite coating is applied to further optimize the pore structure and distribution of active sites, achieving efficient removal of various pollutants, while significantly improving the regeneration stability and mechanical properties of the adsorbent.

[0147] In Comparative Example 1, the unmodified biochar had a limited specific surface area and a limited number of functional groups, and its adsorption of organic matter mainly relied on physical adsorption. This resulted in a small adsorption capacity and poor selectivity, leading to a high concentration of pollutants after treatment and a significant decrease in adsorption performance after regeneration. Furthermore, high-temperature calcination caused structural embrittlement, increasing the breakage rate.

[0148] In Comparative Example 2, the bentonite was not alkali-modified, had few surface active sites, and was not firmly bound to iron and manganese oxides. It was easy to detach during adsorption and regeneration, which affected the removal effect on phosphorus and heavy metals. In addition, the stability of the adsorbent was reduced and the breakage rate was high.

[0149] In Comparative Example 3, the chitosan-activated carbon composite material was not combined with polyethyleneimine, lacking the cross-linking reinforcement effect of polyethyleneimine. The material structure was loose, with low mechanical strength, and it was easily broken during adsorption and regeneration. At the same time, it had insufficient adsorption sites for ammonia nitrogen and organic matter, resulting in poor treatment effect and regeneration performance.

[0150] It is worth noting that this invention itself has a certain adsorption capacity for heavy metals (such as lead, cadmium, chromium, etc.), and because the concentration of heavy metals in rural domestic sewage is generally low (generally close to or slightly higher than the Class A standard), the water quality after treatment by this invention can meet the national Class A discharge standard for heavy metal limits. Meanwhile, the water obtained in Examples 1-6 and Comparative Examples 1-3 above still needs to be disinfected to ensure that E. coli meets the national Class A discharge standard.

Claims

1. A method for treating rural domestic sewage, characterized by, It comprises the following steps: S1. Precipitation treatment; S2. Anaerobic and aerobic treatment; S3. Adsorption treatment; the composite adsorbent used in the adsorption treatment comprises modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material; The modified biochar is obtained by modifying biomass raw material with acid or alkali, high-temperature calcination, and then compounding with polyvinyl alcohol; The iron-manganese oxide modified bentonite is obtained by modifying bentonite with alkali, and then combining with iron-manganese oxide by chemical bonding; The modified chitosan-activated carbon composite material is obtained by cross-linking chitosan and activated carbon with sodium alginate, and then compounding with polyethyleneimine; The mass ratio among the modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material is 4: (3-6): (2-5); Maleic anhydride grafted polyvinyl alcohol is added in the preparation of modified biochar; The surface of the iron-manganese oxide modified bentonite is coated with a nano-silicon dioxide coating.

2. The method for treating rural domestic sewage according to claim 1, characterized in that, The mass ratio among the modified biochar, iron-manganese oxide modified bentonite, and modified chitosan-activated carbon composite material is 4:5:

3.

3. The method for treating rural domestic sewage according to claim 1, characterized in that, The nano-silicon dioxide coating is replaced with a silicon dioxide-aluminum oxide composite coating.

4. The method for treating rural domestic sewage according to claim 3, characterized in that, The iron-manganese oxide modified bentonite is modified with polyacrylic acid before being coated with the silicon dioxide-aluminum oxide composite coating or the nano-silicon dioxide coating.

5. The method for treating rural domestic sewage according to claim 3, characterized in that, In the silicon dioxide-aluminum oxide composite coating, the mass ratio of silicon dioxide to aluminum oxide is 1: (0.5-2).

6. The method for treating rural domestic sewage according to claim 1, characterized in that, In step S1, 5-60 mg / L of flocculant is added.

7. The method for treating rural domestic sewage according to claim 1, characterized in that, In step S2, the dissolved oxygen concentration in the anaerobic zone is ≤0.2 mg / L, the dissolved oxygen concentration in the aerobic zone is 2-5 mg / L, and the pH is 6.5-9.

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