A physical-chemical and biochemical coupling process for removing hardness from coal gasification wastewater

Abstract

The present invention provides a physical-chemical and biochemical coupling process for removing hardness from coal gasification wastewater, belonging to the technical field of coal gasification wastewater treatment. Aiming at the characteristics of high hardness, high organic matter and high ammonia nitrogen in coal gasification wastewater, part of the pollutants are removed through pretreatment first. Then, a modified chitosan-nanocellulose composite template is added to the pretreated wastewater. The iron ions pre-loaded are used to provide nucleation sites, the biochar doping enhances adsorption and conductivity, and the three-level pore structure optimizes mass transfer, so as to efficiently adsorb calcium and magnesium ions and induce the formation of aragonite calcium carbonate precipitate. The precipitate is used as a biofilm carrier and filled into the anaerobic and aerobic two-stage biochemical reactors, and the supernatant is subjected to nitrification / denitrification treatment. Finally, the sludge is deeply treated and converted into a calcium ferrite photocatalyst to achieve resource utilization. The present invention can effectively remove COD, total hardness and ammonia nitrogen in coal gasification wastewater, and has a high sludge resource utilization rate, providing an efficient, economical and environmentally friendly solution for the treatment of coal chemical wastewater.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal gasification wastewater treatment, and particularly relates to a physical-chemical and biochemical coupling hard removal process for coal gasification wastewater. Background Art

[0002] With the rapid development of the coal chemical industry, the discharge of coal gasification wastewater is increasing day by day. Its characteristics of high hardness, high organic matter, and high ammonia nitrogen pose a serious threat to the environment.

[0003] Currently, the commonly used technologies for treating coal gasification wastewater include chemical precipitation, biological treatment, membrane separation, etc. However, the traditional chemical precipitation method has problems such as low removal efficiency and large sludge production; biological treatment has poor adaptability to high-hardness wastewater and is easily inhibited; membrane separation faces challenges such as membrane fouling and high operating costs.

[0004] Therefore, there is an urgent need to develop a coal gasification wastewater treatment technology that can efficiently remove hardness substances, organic pollutants, and realize resource utilization. Summary of the Invention

[0005] The present invention overcomes the deficiencies of the prior art and provides a physical-chemical and biochemical coupling hard removal process for coal gasification wastewater.

[0006] To achieve the above object, the technical solution adopted by the present invention is: a physical-chemical and biochemical coupling hard removal process for coal gasification wastewater, comprising the following steps:

[0007] S1. Pretreat the wastewater;

[0008] S2. Add a modified chitosan-nanocellulose composite template to the pretreated wastewater to adsorb calcium and magnesium ions and induce the formation of aragonite calcium carbonate precipitate;

[0009] S3. Gradiently stir the wastewater containing precipitate in S2, and stand to separate the supernatant and the precipitate;

[0010] S4. Treat the precipitate in step S3 to make a biofilm carrier, fill it into anaerobic and aerobic two-stage biochemical reactors, and pass the supernatant in S3 into the anaerobic and aerobic two-stage biochemical reactors for nitrification / denitrification treatment, and filter to obtain treated water and sludge;

[0011] S5. Apply a periodic high-voltage electric field to dehydrate the sludge in S4, and calcine the dehydrated sludge to generate a calcium ferrite photocatalyst;

[0012] In step S2, the modification method of the chitosan-nanocellulose composite template includes preloading iron ions, doping biochar, and constructing a hierarchical pore structure. Among them, the method of preloading iron ions is to immerse the composite template in an iron ion solution for adsorption; the method of doping biochar is to mix and grind the biochar material with the composite template; the method of constructing a hierarchical pore structure is to use silica nanospheres as a template to construct hierarchical pores on the composite template.

[0013] Preferably, the pretreatment in S1 includes:

[0014] S11. Add calcium hydroxide solution to the coal gasification wastewater, adjust the pH and stir.

[0015] S12. Coagulate and flocculate the wastewater treated in S11, and filter the wastewater through an ultrafiltration membrane.

[0016] S13. Precipitate and separate the wastewater treated in S12, and perform gas stripping on the separated supernatant.

[0017] Preferably, in step S2, the preparation method of the chitosan-nanocellulose composite template includes:

[0018] S210. Add chitosan powder to acetic acid solution, stir until completely dissolved to obtain a chitosan solution.

[0019] S220. Disperse nanocellulose in water to obtain a nanocellulose suspension.

[0020] S230. Mix and stir the chitosan solution and the nanocellulose suspension to form a mixed solution.

[0021] S240. Add glutaraldehyde crosslinking agent to the mixed solution to initiate a crosslinking reaction.

[0022] S250. Dry the crosslinked mixed solution under vacuum to obtain a chitosan-nanocellulose composite template.

[0023] Preferably, the method of preloading iron ions is further:

[0024] S211. Dissolve ferric chloride in water to obtain an iron ion solution.

[0025] S221. Immerse the chitosan-nanocellulose composite template in the iron ion solution, stir to allow the iron ions to be fully adsorbed onto the material surface.

[0026] S231. Wash the soaked composite template with water to remove the unadsorbed iron ions.

[0027] S241. Dry the washed composite template under vacuum to obtain an iron ion-loaded chitosan-nanocellulose composite template.

[0028] Preferably, the method of doping biochar is further:

[0029] S221, pyrolyzing rice husk under a nitrogen atmosphere to obtain rice husk biochar, grinding and sieving to obtain rice husk biochar nanosheets;

[0030] S222, the chitosan-nanocellulose composite template is mixed with the rice husk biochar nanosheets, and fully ground to make the biochar evenly dispersed in the composite template.

[0031] Preferably, the method for constructing the tertiary pore structure is further as follows:

[0032] S231, preparing silica nanospheres by sol-gel method;

[0033] S232, mixing the chitosan-nanocellulose composite template and the silica nanospheres uniformly;

[0034] S233, drying the mixture in a vacuum to fix the silica nanospheres on the composite template;

[0035] S234, soaking the cured composite material in a sodium hydroxide solution to remove the silica nanospheres, thereby obtaining a composite template having a tertiary pore structure.

[0036] Preferably, step S3 comprises:

[0037] S31, the wastewater containing precipitate in S2 is subjected to gradient stirring; in the first stage, stirring is performed at 120-180 rpm for 15-25 min; in the second stage, stirring is performed at 40-60 rpm for 8-12 min;

[0038] S32, subjecting the wastewater treated in S31 to sedimentation separation, and dehydrating the sediment.

[0039] Preferably, step S4 comprises:

[0040] S41, processing the precipitate in S3 to prepare a biofilm carrier, and filling the biofilm carrier into an anaerobic and aerobic two-stage biochemical reactor;

[0041] S42, introducing the supernatant in S3 into an anaerobic reactor for anaerobic biological treatment;

[0042] S43, introducing the effluent from the anaerobic reactor in S42 into the aerobic reactor for aerobic biological treatment.

[0043] The present invention solves the defects existing in the background technology and has the following beneficial effects:

[0044] (1) The present invention uses a chitosan-nanocellulose composite template, which is modified by preloading iron ions, doping biochar, and constructing a three-level pore structure, and is applied to the physicochemical and biochemical coupling hard removal process of coal gasification wastewater, achieving efficient removal of hardness substances, organic pollutants, and promoting the resource utilization of sludge.

[0045] (2) The modified composite template provided by the present invention has abundant calcium and magnesium ion adsorption sites and excellent mass transfer performance. Iron ions, as Lewis acid centers, promote the nucleation of calcium carbonate, biochar provides adsorption sites and electron transfer channels, and the three-level pore structure optimizes the mass transfer efficiency, enabling calcium and magnesium ions to precipitate rapidly and efficiently from the wastewater to form aragonite calcium carbonate precipitate. Compared with the traditional chemical precipitation method, the present invention avoids the use of a large amount of chemical agents, reduces secondary pollution, and effectively reduces the scaling risk of subsequent biochemical treatment units.

[0046] (3) A large number of microorganisms are attached to the surface of the modified composite template provided by the present invention to form a highly active biofilm. Biochar provides microbial attachment sites and nutrients, and the iron ion / ferrous ion redox pair promotes electron transfer and improves microbial metabolic activity, enabling organic pollutants to be efficiently degraded by the microorganisms in the biofilm and reducing the COD in the wastewater. Compared with traditional biological treatment, the present invention improves the adaptability of biological treatment to high-hardness wastewater and reduces the sludge treatment and disposal cost.

[0047] (4) The main components of the dehydrated sludge are aragonite calcium carbonate, precipitated iron-calcium-carbon composite, and a small amount of organic matter. By calcining the dehydrated sludge, a catalytically active calcium ferrite photocatalyst is generated, which can be used for photocatalytic degradation of organic pollutants. Compared with traditional sludge landfill or incineration treatment, the present invention realizes the resource utilization of sludge, reduces environmental pollution, and achieves a win-win situation of economic and environmental benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings;

[0049] Figure 1 is a flowchart of a physicochemical and biochemical coupling hard removal method for coal gasification wastewater;

[0050] Figure 2 is a flowchart of a preparation method of a chitosan-nanocellulose composite template;

[0051] Figure 3 is a flowchart of a method for preloading iron ions;

[0052] Figure 4 is a flowchart of a method for doping biochar;

[0053] Figure 5 is a flowchart of a method for constructing a hierarchical pore structure. Detailed implementation manners

[0054] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0055] In the following description, many specific details are set forth in order to fully understand the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0056] In the description of the present application, it should be understood that the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the protection scope of the present application. In addition, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0057] In the description of the present application, it should be noted that unless otherwise clearly specified and limited, the terms "mounted", "connected", "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the internal communication of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0058] The raw materials involved in the present invention can be obtained through commercial purchase or prepared by the preparation methods well known to those skilled in the art without special declaration.

[0059] Exemplary process:

[0060] As Figure 1 shown, a physicochemical and biochemical coupling process for removing hardness from coal gasification wastewater includes the following steps:

[0061] S1. Pretreat the wastewater;

[0062] S2. Add a modified chitosan-nanocellulose composite template to the pretreated wastewater to adsorb calcium and magnesium ions and induce the formation of aragonite calcium carbonate precipitate;

[0063] S3. Gradiently stir the wastewater containing the precipitate in S2, and let it stand to separate the supernatant and the precipitate;

[0064] S4. Process the precipitate in step S3 into a biofilm carrier, fill it into anaerobic and aerobic two-stage biochemical reactors, and pass the supernatant in S3 into the anaerobic and aerobic two-stage biochemical reactors for nitrification / denitrification treatment, and filter to obtain treated water and sludge;

[0065] S5. Apply a periodic high-voltage electric field to dehydrate the sludge in S4, and calcine the dehydrated sludge to generate a calcium ferrite photocatalyst;

[0066] The modification method of the chitosan-nanocellulose composite template in step S2 includes preloading iron ions, doping with biochar, and constructing a three-level pore structure; among them, the method of preloading iron ions is to immerse the composite template in an iron ion solution for adsorption; the method of doping with biochar is to mix and grind the biochar material with the composite template; the method of constructing a three-level pore structure is to use silica nanospheres as a template to construct a multi-level pore channel on the composite template.

[0067] Next, each step will be introduced in detail.

[0068] The pretreatment in step S1 includes:

[0069] S11. Add calcium hydroxide solution to the coal gasification wastewater, adjust the pH and stir;

[0070] S12. Coagulate and flocculate the wastewater treated in S11, and filter the wastewater through an ultrafiltration membrane;

[0071] S13. Perform precipitation separation on the wastewater treated in S12, and perform gas stripping on the separated supernatant.

[0072] In step S11, the coal gasification wastewater with a chemical oxygen demand of 2000 - 3000 mg / L and a total hardness of 1000 - 1500 mg / L is input into the reaction tank, and a calcium hydroxide solution with a mass concentration of 15 - 25% is added, with an addition amount of 8 - 12 g / L. Through the interlock control of the pH on-line monitor and the dosing pump, the pH of the wastewater is adjusted to 9.0 - 9.5 to promote the conversion of bicarbonate ions into carbonate ions, increase the concentration of carbonate ions, stabilize the pH environment, and thus improve the subsequent physico-chemical treatment efficiency. The stirring speed is controlled at 60 - 80 rpm, the reaction temperature is 50 - 70 °C, and the reaction time is 30 - 60 min.

[0073] By promoting the conversion of bicarbonate ions into carbonate ions, the pH environment is stabilized, and the subsequent physico-chemical treatment efficiency is improved. At the same time, part of the calcium and magnesium ions are precipitated, the concentration of calcium and magnesium ions in the wastewater is reduced, and the scaling risk of the subsequent treatment unit is reduced.

[0074] In step S12, the wastewater treated in S11 is quickly mixed with a polyaluminum chloride coagulant with a dosing amount of 50 - 80 mg / L through a pipeline static mixer and then enters the slow flocculation zone, with a flocculation time of 10 - 30 min. Then it enters a polyvinylidene fluoride ultrafiltration membrane module with a cut-off molecular weight of 72000 - 88000 Da, the operating pressure is 0.25 - 0.35 MPa, and the membrane flux is 20 - 30 L / (m²·h). The concentration of oil pollutants in the ultrafiltration produced water is controlled to be ≤25 mg / L, and the concentration of suspended solids is ≤50 mg / L.

[0075] By adding polyaluminum chloride (PAC) coagulant, the suspended solids and colloidal particles in the wastewater are coagulated into flocs, and separated through the ultrafiltration membrane to remove the suspended solids and colloidal particles in the wastewater, reduce the turbidity and SDI (pollution index), and prevent the subsequent reverse osmosis membrane from being blocked.

[0076] In step S13, the wastewater treated in S12 is introduced into the inclined plate sedimentation tank. Through the mechanical stirring device at a rotation speed of 60 - 80 rpm and the steam heating system to maintain a temperature of 50 - 70 °C, combined with air stripping in the air stripping tower, the ammonium ions are promoted to be converted into volatile ammonia water. The ammonia nitrogen concentration of the effluent is controlled to be ≤300 mg / L, and the tail gas is treated by the acid absorption tower. The solubility at 50 °C decreases by 60% compared with that at 25 °C, and the increase in temperature reduces the solubility of ammonia gas.

[0077] By heating the wastewater and air stripping in the air stripping tower, the ammonium ions in the wastewater are converted into ammonia gas, and the ammonia gas is absorbed by the acid absorption tower to reduce the ammonia nitrogen concentration in the wastewater.

[0078] In step S2, the wastewater treated in S1 is input into the physicochemical reaction tank, and a modified chitosan-nanocellulose composite template is added, with the addition amount being 1.0 - 2.0 g / L of wastewater; the stirring speed is controlled at 100 - 200 rpm, the reaction temperature is 30 - 40 °C, and the reaction time is 20 - 40 min; calcium and magnesium ions are selectively adsorbed through the chelation of carboxyl groups and calcium ions, inducing the formation of aragonite calcium carbonate precipitate.

[0079] The chitosan-nanocellulose composite template can provide adsorption sites for calcium and magnesium ions, and the carboxyl groups (-COOH) on chitosan can chelate with calcium and magnesium ions. Nanocellulose, as a carrier, increases the specific surface area and provides a template for crystal nucleus growth.

[0080] Specifically, as Figure 2 shown, the preparation method of the chitosan-nanocellulose composite template includes:

[0081] S210. Dissolve chitosan powder in a 1 - 2% acetic acid solution to prepare a chitosan solution with a mass concentration of 1 - 2%, and stir until completely dissolved;

[0082] S220. Disperse nanocellulose in deionized water and make it evenly dispersed through ultrasonic treatment to prepare a nanocellulose suspension with a mass concentration of 0.4 - 0.6%;

[0083] S230. Mix the chitosan solution and the nanocellulose suspension according to a mass ratio of 3:1 to 4:1, and stir for 15 - 30 min to fully mix the two materials;

[0084] S240. Add glutaraldehyde cross-linking agent to the mixed solution, with the dosage of glutaraldehyde being 0.8 - 1.2% of the mass of chitosan, and continue to stir for 1 - 2 h to cause a cross-linking reaction between chitosan and nanocellulose;

[0085] S250. Pour the cross-linked mixed solution into a petri dish and vacuum dry it at 50 - 60 °C for 18 - 24 h to obtain the chitosan-nanocellulose composite template.

[0086] The amino and hydroxyl groups on the chitosan molecular chain react with glutaraldehyde to form stable chemical bonds, connecting chitosan and nanocellulose together. Nanocellulose increases the specific surface area and porosity of the composite material, providing more space for the adsorption of calcium and magnesium ions.

[0087] Further, the modification method of the chitosan-nanocellulose composite template in step S2 includes preloading iron ions, doping with biochar, and constructing a hierarchical pore structure. Among them, the method of preloading iron ions is to immerse the composite template in an iron ion solution for adsorption; the method of doping with biochar is to mix and grind the biochar material with the composite template; the method of constructing a hierarchical pore structure is to use silica nanospheres as a template to construct hierarchical pores on the composite template.

[0088] Immerse the chitosan-nanocellulose composite template in an iron ion solution to allow the iron ions to adsorb onto the material surface. Utilize the ion exchange between the iron ions and the carboxyl groups on chitosan to fix the iron ions inside the material. By preloading 0.5 - 1.0 wt% of iron ions in the composite template, the mechanical strength and stability of the precipitate are improved, the electron transfer ability is enhanced, and the nucleation and growth of calcium carbonate are promoted.

[0089] Specifically, as Figure 3 shown, the method of preloading iron ions includes:

[0090] S211. Dissolve ferric chloride in deionized water to prepare an iron ion solution with a concentration of 0.07 - 0.13 mol / L;

[0091] S221. Immerse the chitosan-nanocellulose composite template in the iron ion solution, control the solid-liquid ratio to be 1:20 to 2:20, and stir at room temperature for 1 - 2 h to allow the iron ions to be fully adsorbed onto the material surface;

[0092] S231. Wash the impregnated composite template several times with deionized water to remove the unadsorbed iron ions;

[0093] S241. Vacuum-dry the washed composite template at 50 - 60 °C for 18 - 24 h to obtain the iron ion-loaded chitosan-nanocellulose composite template.

[0094] The iron ions coordinate with the carboxyl or amino groups on chitosan to form stable chemical bonds. The iron ions can act as Lewis acid centers to promote the nucleation and growth of calcium carbonate. In addition, the redox couple of iron ions / ferrous ions can promote electron transfer and improve the electrochemical activity of the material.

[0095] Add biochar powder during the preparation process of the chitosan-nanocellulose composite material to form a composite structure between the biochar and the material. By doping with rice husk biochar nanosheets with a thickness of 10 - 20 nm, the specific surface area and pore volume of the material are further increased, and the adsorption sites are increased.

[0096] Specifically, as Figure 4 shown, the method of incorporating biochar includes:

[0097] S221, pyrolyzing rice husk at 300-700°C in a nitrogen atmosphere to obtain rice husk biochar. Grinding the rice husk biochar through a 180-200 mesh sieve to obtain rice husk biochar nanosheets;

[0098] S222, mixing the iron ion loaded chitosan-nanocellulose composite template with the rice husk biochar nanosheets in a mass ratio of 100:3 to 100:7, and grinding them sufficiently to make the biochar evenly dispersed in the composite material.

[0099] The oxygen-containing functional groups (-COOH, -OH) on the surface of biochar interact with chitosan or nanocellulose to enhance the binding force of the material. The porous structure of biochar provides more adsorption sites, improving the adsorption performance of the material. The conductivity of biochar promotes electron transfer and improves the electrochemical activity of the material.

[0100] On the modified chitosan-nanocellulose composite template, a three-level pore structure is constructed using mesoporous silica nanospheres as a template, forming a three-level structure of micropores less than 2nm, mesopores from 2 to 50nm, and macropores greater than 50nm. Micropores are used to provide a high specific surface area and increase adsorption sites; mesopores are used to provide mass transfer channels and accelerate ion diffusion; macropores are used to reduce mass transfer resistance and facilitate wastewater to enter the material.

[0101] Specifically, Figure 5 As shown, the method for constructing the tertiary pore structure includes:

[0102] S231, preparing mesoporous silica nanospheres with an average particle size of 40-60 nm by a sol-gel method;

[0103] S232, mixing the chitosan-nanocellulose composite template and the mesoporous silica nanospheres in a mass ratio of 4:1 to 6:1;

[0104] S233, drying the mixture in a vacuum drying oven at 50-60° C. for 2-3 h to fix the mesoporous silica nanospheres in the composite material;

[0105] S234. Soak the cured composite material in a sodium hydroxide solution to remove the mesoporous silica nanosphere template to obtain a composite template with a tertiary pore structure.

[0106] Mesoporous silica nanospheres form a skeleton structure in the composite material, leaving mesopores and macropores after removing the template. Chitosan, nanocellulose and biochar fill in between the mesopores and macropores to form micropores.

[0107] Step S3 includes:

[0108] S31. Gradient stirring is carried out on the wastewater containing precipitate in S2; in the first stage, stir at 120 - 180 rpm for 15 - 25 min; in the second stage, stir at 40 - 60 rpm for 8 - 12 min.

[0109] S32. The wastewater treated in S31 is subjected to precipitation separation, and the precipitate is dehydrated.

[0110] In step S31, the wastewater containing precipitate in S2 is introduced into a gradient stirrer. In the first stage, high - speed stirring increases the liquid - phase mass transfer rate, promotes the diffusion of calcium and magnesium ions to the surface of the composite template, and accelerates the formation of crystal nuclei. The high shear force brought by high - speed stirring helps to disperse the crystal nuclei and prevent amorphous precipitation caused by local supersaturation. In the second stage, low - speed stirring reduces the liquid - phase mass transfer rate, slows down the crystal growth rate, enables the crystals to grow slowly under thermodynamic control, and is conducive to the formation of more stable and higher - crystallinity vaterite - type calcium carbonate. Low - speed stirring reduces the collision frequency between crystals and inhibits crystal aggregation.

[0111] In step S32, the wastewater after gradient stirring in S31 is introduced into a static sedimentation tank. After static sedimentation for 1 - 2 h, the upper clear liquid and the lower precipitate are separated by a stratified suction device; the precipitate is dehydrated by screw extrusion, the pressure gradient is controlled at 0.5 - 2.0 MPa, and the moisture content of the dehydrated sludge is ≤70%. By applying pressure, the water in the precipitate is forced to drain from the pores. The shear force generated by screw extrusion helps to break the flocculation structure of the sludge and release water.

[0112] The main components of the precipitate are vaterite - type calcium carbonate, precipitated iron - calcium - carbon complex, and a small amount of organic matter.

[0113] Step S4 includes:

[0114] S41. The precipitate in S3 is processed into a biofilm carrier and filled into an anaerobic and aerobic two - stage biochemical reactor;

[0115] S42. The supernatant in S3 is introduced into an anaerobic reactor for anaerobic biological treatment;

[0116] S43. The effluent from the anaerobic reactor in S42 is introduced into an aerobic reactor for aerobic biological treatment.

[0117] In step S41, the precipitate generated in S3 is crushed into particles with a size of 1 - 5 mm, used as a biofilm carrier and filled into an anaerobic and aerobic two - stage biochemical reactor, and the filling rate is 20% - 40%; the surface conductivity of the carrier is ≥5 S / m, and electron transfer is mediated by the iron ion / ferrous ion redox pair.

[0118] The precipitated iron-calcium-carbon composite is a composite structure formed by the interaction of iron ions / ferrous ions, calcium carbonate, and biochar. It has good electrical conductivity, can promote electron transfer between microbial cells, form an extracellular electron transfer network, and improve the degradation efficiency of organic matter. Biochar has a porous structure and a large specific surface area, can adsorb organic matter and nutrients, promote the attachment and growth of microorganisms on the carrier surface, and form a biofilm. The iron ion / ferrous ion redox pair can mediate electron transfer, promote the metabolic activities of microorganisms, and increase the degradation rate of organic matter.

[0119] In step S42, the dissolved oxygen in the reactor is controlled to be close to 0 mg / L (anaerobic environment), the temperature is 30 - 35 °C, and the hydraulic retention time is 12 - 24 h. Under anaerobic conditions, anaerobic microorganisms are used to decompose refractory organic matter into intermediate products such as small molecular organic acids and alcohols, and denitrification is carried out to convert nitrate into nitrogen. Among them, anaerobic microorganisms include but are not limited to methanogens, fermentative bacteria, and denitrifying bacteria.

[0120] In step S43, the dissolved oxygen in the reactor is controlled to be 1.5 - 2.5 mg / L, the temperature is 30 - 35 °C, and the hydraulic retention time is 6 - 12 h. Under aerobic conditions, electroactive microbial communities are enriched to further degrade organic matter, and nitrification reaction is carried out to convert ammonia nitrogen into nitrate. Among them, electroactive microbial communities include but are not limited to Geobacter and Shewanella; nitrifying bacteria include but are not limited to nitrite bacteria and nitrate bacteria.

[0121] Step S5 includes:

[0122] S51. Dehydrate the sludge produced in S4;

[0123] S52. Calcinate the dehydrated sludge to generate a catalytically active material.

[0124] In step S51, a periodic high-voltage electric field is applied to the sludge in S4, the electric field strength is 3 - 6 kV / cm, the pulse width is 20 - 50 μs, the pulse interval is 0.5 - 1.5 s, and the dehydration time is 15 - 25 min; selective dehydration is achieved by using the low dielectric constant property of aragonite calcium carbonate.

[0125] Under the action of the high-voltage electric field, water molecules are polarized, generating electroosmosis and electrophoresis phenomena, which promote the migration of water from the sludge. Aragonite calcium carbonate has the property of low dielectric constant, which can reduce the loss of electric field energy and improve the dehydration efficiency. At the same time, calcium carbonate crystals can act as a skeleton to enhance the structural strength of the sludge and reduce the breakage of sludge particles during dehydration. The pulsed electric field can destroy the sludge cell wall and release the water inside the cells, improving the dehydration efficiency.

[0126] In step S52, the dewatered sludge is placed in a muffle furnace and calcined at 250-350 °C for 1-3 h to produce a calcium ferrite photocatalyst.

[0127] Under high-temperature conditions, the organic matter in the sludge is oxidized and decomposed, and calcium and iron form calcium ferrite crystals. Calcium ferrite is a type of spinel ferrite, where the A-site is divalent metal ions (calcium ions) and the B-site is trivalent metal ions (iron ions). Calcium ferrite has visible-light photocatalytic activity, can absorb light energy and convert it into chemical energy for the degradation of organic pollutants. Example 1

[0128] A physicochemical and biochemical coupling process for removing hardness from coal gasification wastewater includes the following steps:

[0129] S11: Input the coal gasification wastewater into a reaction tank, add a calcium hydroxide solution with a mass concentration of 20%, and the dosage is 10 g / L. Through the interlock control of the pH online monitor and the dosing pump, adjust the wastewater pH to 9.2. Control the stirring speed at 70 rpm, the reaction temperature at 60 °C, and the reaction time at 45 min.

[0130] S12: After the wastewater is quickly mixed with a polyaluminum chloride coagulant with a dosage of 65 mg / L through a pipeline static mixer, it enters the slow flocculation zone, and the flocculation time is 20 min; then it enters a polyvinylidene fluoride ultrafiltration membrane module with a cut-off molecular weight of 80000 Da, the operating pressure is 0.3 MPa, and the membrane flux is 25 L / (m²·h).

[0131] S13: In the inclined plate sedimentation tank, maintain the temperature at 60 °C through a mechanical stirring device at a speed of 70 rpm and a steam heating system, combined with air stripping in the air stripping tower, and the tail gas is treated by an acid absorption tower.

[0132] S2: Input the pretreated wastewater into a physicochemical reaction tank, add a chitosan-nanocellulose composite template modified by preloading iron ions, doping biochar, and constructing a three-level pore structure, and the dosage is 1.5 g / L of wastewater. Control the stirring speed at 150 rpm, the reaction temperature at 35 °C, and the reaction time at 30 min.

[0133] S31: Stir at 150 rpm for 20 min in the first stage to promote the diffusion and nucleation of calcium and magnesium ions. Stir at 50 rpm for 10 min in the second stage.

[0134] S32: After standing and precipitating for 1.5 h, separate the upper clear liquid and the lower precipitate through a layered suction device. The precipitate is dewatered by screw extrusion, and the pressure gradient is controlled at 1.25 MPa.

[0135] S41. Crush the precipitate produced in the S3 precipitation separation step into 3-mm particles, which are used as biofilm carriers and filled into the anaerobic and aerobic two-stage biochemical reactors with a filling rate of 20%.

[0136] S42. Introduce the wastewater into the anaerobic reactor, control the dissolved oxygen in the anaerobic reactor to be close to 0 mg / L, the temperature to be 32.5 °C, and the hydraulic retention time to be 18 h.

[0137] S43. Introduce the effluent of the anaerobic reactor into the aerobic reactor, control the dissolved oxygen in the aerobic reactor to be 2.0 mg / L, the temperature to be 32.5 °C, and the hydraulic retention time to be 9 h.

[0138] S51. Apply a periodic high-voltage electric field to the precipitate with an electric field strength of 4.5 kV / cm, a pulse width of 35 μs, a pulse interval of 1 s, and a dehydration time of 20 min.

[0139] S52. Place the dehydrated sludge in a muffle furnace and calcine it at 300 °C for 2 h to produce a calcium ferrite photocatalyst.

[0140] Among them, the preparation method of the chitosan-nanocellulose composite template includes:

[0141] S210. Dissolve the chitosan powder in a 1% acetic acid solution to prepare a chitosan solution with a mass concentration of 1%, and stir until completely dissolved.

[0142] S220. Disperse the nanocellulose in deionized water and make it evenly dispersed by ultrasonic treatment to prepare a nanocellulose suspension with a mass concentration of 0.5%.

[0143] S230. Mix the chitosan solution and the nanocellulose suspension according to a mass ratio of 3:1 and stir for 30 min.

[0144] S240. Add a glutaraldehyde cross-linking agent to the mixed solution. The dosage of glutaraldehyde is 1% of the mass of chitosan, and continue to stir for 2 h.

[0145] S250. Pour the cross-linked mixed solution into a petri dish and dry it in vacuum at 60 °C for 24 h to obtain the chitosan-nanocellulose composite template.

[0146] The method for preloading iron ions includes:

[0147] S211. Dissolve ferric chloride in deionized water to prepare an iron ion solution with a concentration of 0.10 mol / L.

[0148] S221. Immerse the chitosan-nanocellulose composite template in the iron ion solution, control the solid-liquid ratio to be 1:20, and stir for 2 h.

[0149] S231. Wash the impregnated composite template several times with deionized water.

[0150] S241. Vacuum-dry the washed composite template at 60 °C for 24 h to obtain an iron-ion-loaded chitosan-nanocellulose composite template.

[0151] The method for doping biochar includes:

[0152] S221. Pyrolyze rice husk at 500 °C under a nitrogen atmosphere to obtain rice husk biochar. Grind it through a 200-mesh sieve to obtain rice husk biochar nanosheets.

[0153] S222. Mix the iron-ion-loaded chitosan-nanocellulose composite template and the rice husk biochar nanosheets in a mass ratio of 100:5 and grind them thoroughly.

[0154] The method for constructing a hierarchical pore structure includes:

[0155] S231. Prepare mesoporous silica nanospheres with an average particle size of 50 nm by the sol-gel method.

[0156] S232. Mix the chitosan-nanocellulose composite template and the mesoporous silica nanospheres evenly in a mass ratio of 5:1.

[0157] S233. Dry the mixture in a vacuum drying oven at 60 °C for 3 h to fix the mesoporous silica nanospheres in the composite material.

[0158] S234. Immerse the cured composite material in a sodium hydroxide solution to remove the mesoporous silica nanosphere template. Example 2

[0159] In this example, different from Example 1, the concentration of the ferric chloride solution in step S211 is adjusted to 0.07 mol / L, and the other steps are the same as those in Example 1. Example 3

[0160] Different from Example 1, the concentration of the ferric chloride solution in step S211 is adjusted to 0.13 mol / L, and the other steps are the same as those in Example 1. Example 4

[0161] In this example, different from Example 1, the mass ratio of the iron-ion-loaded chitosan-nanocellulose composite template to the rice husk biochar nanosheets in step S222 is adjusted to 100:3, and the other steps are the same as those in Example 1. Example 5

[0162] In this embodiment, different from Embodiment 1, the mass ratio of the iron ion-loaded chitosan-nanocellulose composite template to the rice husk biochar nanosheets in step S222 is adjusted to 100:7, and the remaining steps are the same as those in Embodiment 1. Embodiment 6

[0163] In this embodiment, different from Embodiment 1, the mass ratio of the chitosan-nanocellulose composite template to the mesoporous silica nanospheres in step S232 is adjusted to 4:1, and the remaining steps are the same as those in Embodiment 1. Embodiment 7

[0164] In this embodiment, different from Embodiment 1, the mass ratio of the chitosan-nanocellulose composite template to the mesoporous silica nanospheres in step S232 is adjusted to 6:1, and the remaining steps are the same as those in Embodiment 1. Embodiment 8

[0165] In this embodiment, different from Embodiment 1, only the chitosan-nanocellulose composite template is added in S2, and no preloading of iron ions, doping of biochar, and modification of constructing a three-level pore structure are carried out. The remaining steps are the same as those in Embodiment 1.

[0166] Experimental Example 1

[0167] Experimental purpose: To verify the synergistic effect of preloading iron ions, doping biochar, and modifying the construction of a three-level pore structure on the physicochemical and biochemical coupling process for hardness removal from coal gasification wastewater.

[0168] Experimental object: The wastewater discharged from a coal gasification plant, with an initial COD of 2500 mg / L, an initial total hardness of 1250 mg / L, and an initial ammonia nitrogen of 300 mg / L.

[0169] Experimental steps:

[0170] i. Pretreat the wastewater according to steps S11, S12, and S13 in Embodiment 1.

[0171] ii. Prepare the chitosan-nanocellulose composite template according to steps S210 - S250 in Embodiment 1 respectively; divide the prepared chitosan-nanocellulose composite template into 7 groups, and preload iron ions, dope biochar, and modify the construction of a three-level pore structure according to the methods in Embodiments 1 - 7 respectively to prepare different modified composite templates. Embodiment 8 directly uses the chitosan-nanocellulose composite template without any treatment.

[0172] iii. Input the pretreated wastewater into 8 physicochemical reaction tanks respectively, and add the corresponding composite templates according to the methods in Embodiments 1 - 8; control the stirring speed at 150 rpm, the reaction temperature at 35 °C, and the reaction time at 30 min.

[0173] iv. Transfer the suspensions in the 8 physicochemical reaction tanks to the sedimentation tank respectively, and perform sedimentation separation according to the steps of S31 and S32 in Example 1.

[0174] v. Crush the 8 groups of precipitates into 3-mm particles respectively, and use them as biofilm carriers to fill 8 anaerobic-aerobic two-stage biochemical reactors with a filling rate of 20%. The anaerobic reactor controls the dissolved oxygen to be close to 0 mg / L, the temperature to be 32.5 °C, and the hydraulic retention time to be 18 h. The aerobic reactor controls the dissolved oxygen to be 2.0 mg / L, the temperature to be 32.5 °C, and the hydraulic retention time to be 9 h.

[0175] vi. Perform pulsed electric field-assisted dewatering on the 8 groups of sludge after biochemical treatment respectively, and operate according to the steps of S51 in Example 1; place the 8 groups of dewatered sludge in a muffle furnace respectively, and calcine according to the steps of S52 in Example 1 to generate calcium ferrite photocatalyst.

[0176] vii. Measure the COD, total hardness, and ammonia nitrogen indexes in the wastewater after S1 treatment, S2 treatment, S3 treatment, and S4 treatment in the 8 groups of experiments respectively. Calculate the removal rates of COD, total hardness, and ammonia nitrogen in the 8 groups of experiments, as well as the sludge reduction rate and resource utilization rate.

[0177] COD removal rate = (raw water COD - COD after S4 treatment) / raw water COD × 100%;

[0178] Total hardness removal rate = (raw water total hardness - total hardness after S4 treatment) / raw water total hardness × 100%;

[0179] Ammonia nitrogen removal rate = (raw water ammonia nitrogen - ammonia nitrogen after S4 treatment) / raw water ammonia nitrogen × 100%;

[0180] Sludge resource utilization rate = amount of sludge for resource utilization / initial sludge amount × 100%;

[0181] The results are shown in the following table:

[0182] Table 1 Experimental data table 1

[0183]

[0184] Table 2 Experimental data table 2

[0185]

[0186] Experimental data show that after the S2 treatment, the COD, total hardness, and ammonia nitrogen concentrations in each experimental group decreased to varying degrees. Among them, the reduction in groups 1-7 was significantly greater than that in group 8 (the unmodified group). After the S3 and S4 treatments, the pollutant concentrations in each group further decreased, and obvious inter-group differences were shown in the effluent concentration and removal rate after the S4 treatment. Among them, Example 1 showed the best treatment effect, reaching the highest values in the removal rates of COD, total hardness, ammonia nitrogen, and sludge resource utilization rate. For other modified groups, due to different modification ratios, the treatment effects showed certain fluctuations, but all were better than the unmodified group.

[0187] Generally speaking, the pre-loading of iron ions increases the surface charge of the template, stabilizes the biochar adsorption, and enhances the mechanical strength of the template to maintain the three-dimensional pore structure. However, excessive iron ions inhibit the microbial activity, and too little iron ions cannot fully play the nucleation role. Biochar provides a dispersion environment for iron ions, improves their activity, enhances the mechanical strength of the template, and assists in the integrity of the three-dimensional pore structure. However, excessive biochar occupies too much pore space, affecting mass transfer, and too little biochar cannot provide enough adsorption sites. The three-dimensional pore structure provides a larger surface area and porosity for iron ions and biochar, improves the adsorption capacity and reaction rate, and reduces the mass transfer resistance. However, too many three-dimensional pore structures reduce the template strength, and too few three-dimensional pore structures have limited improvement in mass transfer efficiency. The synergistic effect of the three modifications makes the modified group better than the unmodified group, and there is an optimal ratio.

[0188] Specifically, the complex synergistic effect among the pre-loading of iron ions, biochar doping, and the construction of the three-dimensional pore structure is reflected in the charge transfer, surface active sites, and microenvironment regulation.

[0189] The introduction of iron ions changes the electronic structure of the composite material and promotes the charge transfer process. Iron ions are not only excellent Lewis acid centers themselves, capable of adsorbing negatively charged carbonate ions and promoting the nucleation of calcium carbonate, but they also have a significant impact on the electron cloud density distribution on the biochar surface. By changing the electronic structure of the biochar surface, iron ions can improve the adsorption ability of biochar for organic pollutants and enhance the electron transfer efficiency between biochar and microorganisms. This enhanced electron transfer effect is particularly important for the aerobic denitrification process and can promote the reduction of nitrate to nitrogen. However, excessive iron ions form an oxide layer on the biochar surface, reducing its conductivity and inhibiting electron transfer instead. Too few iron ions cannot effectively change the electronic structure of biochar, and the improvement of electron transfer efficiency is limited. In addition, iron ions can also affect the distribution of biochar in the three-dimensional pore channels by forming coordination bonds with the amino or hydroxyl groups of chitosan, controlling the exposure degree of biochar, and thus affecting its adsorption performance.

[0190] The π-electron conjugated structure of biochar can form a charge transfer complex with iron ions, further stabilizing the existence state of iron ions and reducing their loss. More importantly, the porous structure of biochar can effectively regulate the microenvironment of the tertiary pores. By changing the pore size distribution and surface chemical properties of biochar, the humidity, pH value, and redox potential inside the pores can be precisely controlled, thereby optimizing the growth environment of microorganisms and enhancing the activity of biofilms. However, excessive biochar blocks the tertiary pores, hindering mass transfer; too little biochar cannot effectively regulate the pore microenvironment, and the improvement of microbial activity is limited.

[0191] The tertiary pore structure not only provides a larger specific surface area, but more importantly, it creates a multi-scale reaction space. Micropores can enrich pollutants through capillary condensation, increasing the concentration of local reactants. Mesopores can promote the diffusion of reaction intermediates, accelerating the reaction rate. Macropores can reduce the mass transfer resistance and improve the transport efficiency of reactants and products. In addition, the tertiary pore structure can also affect the exposure degree and active site distribution of iron ions and biochar. By controlling the size and shape of the pores, specific types of active sites can be selectively exposed, thereby improving the selectivity and catalytic efficiency of the composite material. However, too many tertiary pores result in too thin pore walls, which are prone to collapse, reducing the structural stability; too few tertiary pores cannot effectively improve mass transfer, and the utilization rate of active sites is low.

[0192] Based on the inspiration of the ideal embodiments of the present invention, through the above description, relevant personnel can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and the technical scope must be determined according to the scope of the claims.

Claims

1. A physical-chemical-biochemical coupling hardness removal process for coal gasification wastewater, characterized in that: The following steps are involved: S1. Pre-treat wastewater; S2, adding a modified chitosan-nanocellulose composite template to the pretreated wastewater to adsorb calcium and magnesium ions and induce the formation of aragonite-type calcium carbonate precipitation; S3, subjecting the wastewater containing precipitate in S2 to gradient stirring, and allowing it to stand to separate the supernatant and precipitate; S4, treating the precipitate in step S3 to prepare a biofilm carrier, filling it into an anaerobic and aerobic two-stage biochemical reactor, and passing the supernatant in S3 into an anaerobic and aerobic two-stage biochemical reactor for nitrification / denitrification treatment, and filtering to obtain treated water and sludge; S5, applying a periodic high voltage electric field to dehydrate the sludge in S4, and calcining the dehydrated sludge to generate a calcium ferrite photocatalyst; The modification method of the chitosan-nanocellulose composite template in step S2 includes pre-loading iron ions, doping with biochar, and constructing a tertiary pore structure; wherein the pre-loading iron ions method is to immerse the composite template in an iron ion solution for adsorption; the doping with biochar method is to mix and grind the biochar material with the composite template; the constructing tertiary pore structure method is to use silica nanospheres as templates to construct multi-level channels on the composite template; In step S2, the preparation method of the chitosan-nanocellulose composite template includes: S210, adding chitosan powder to the acetic acid solution, stirring until completely dissolved, to obtain a chitosan solution; S220, dispersing the nanocellulose in water to obtain a nanocellulose suspension; S230, mixing and stirring the chitosan solution and the nanocellulose suspension to form a mixed solution; S240, adding glutaraldehyde cross-linking agent to the mixed solution to initiate a cross-linking reaction; S250, drying the cross-linked mixed solution under vacuum to obtain a chitosan-nanocellulose composite template.

2. The process according to claim 1, characterized in that Preprocessing in S1 includes: S11, adding calcium hydroxide solution to the coal gasification wastewater, adjusting the pH and stirring; S12, coagulating and flocculating the wastewater treated in S11, and filtering the wastewater through an ultrafiltration membrane; S13, subjecting the wastewater treated by S12 to sedimentation separation, and subjecting the separated supernatant to gas stripping.

3. The process according to claim 1, characterized in that The method of preloading iron ions is further as follows: S211, dissolving ferric chloride in water to obtain an iron ion solution; S221, immersing the chitosan-nanocellulose composite template in an iron ion solution and stirring to allow the iron ions to be fully adsorbed onto the surface of the material; S231, washing the immersed composite template with water to remove unabsorbed iron ions; S241, drying the washed composite template under vacuum to obtain an iron ion-loaded chitosan-nanocellulose composite template.

4. The process according to claim 1, characterized in that The method of doping biochar is further: S221, pyrolyzing rice husk under a nitrogen atmosphere to obtain rice husk biochar, grinding and sieving to obtain rice husk biochar nanosheets; S222, the chitosan-nanocellulose composite template is mixed with the rice husk biochar nanosheets, and fully ground to make the biochar evenly dispersed in the composite template.

5. The process according to claim 1, characterized in that The method of constructing the tertiary pore structure is further as follows: S231, preparing silica nanospheres by sol-gel method; S232, mixing the chitosan-nanocellulose composite template and the silica nanospheres uniformly; S233, drying the mixture in a vacuum to fix the silica nanospheres on the composite template; S234, soaking the cured composite material in a sodium hydroxide solution to remove the silica nanospheres, thereby obtaining a composite template having a tertiary pore structure.

6. The process according to claim 1, characterized in that Step S3 includes: S31, the wastewater containing precipitate in S2 is subjected to gradient stirring; in the first stage, stirring is performed at 120-180 rpm for 15-25 min; in the second stage, stirring is performed at 40-60 rpm for 8-12 min; S32, subjecting the wastewater treated in S31 to sedimentation separation, and dehydrating the sediment.

7. The process according to claim 1, characterized in that Step S4 includes: S41, processing the precipitate in S3 to prepare a biofilm carrier, and filling the biofilm carrier into an anaerobic and aerobic two-stage biochemical reactor; S42, introducing the supernatant in S3 into an anaerobic reactor for anaerobic biological treatment; S43, introducing the effluent from the anaerobic reactor in S42 into the aerobic reactor for aerobic biological treatment.

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