Zero discharge treatment method and device for PVA fiber production wastewater

Abstract

The present application relates to a kind of PVA fiber production wastewater treatment method and device.Wastewater is after entering multiple medium filter by homogeneous adjustment pH, reduce the content of suspended solids in wastewater.Multiple medium water production uses ultrafiltration system to reduce the content of suspended solids and colloid.Ultrafiltration water production enters the first reverse osmosis membrane concentration, the first reverse osmosis membrane concentrated water is used to remove hard filter to remove hard, the water production of remove hard filter is used to further concentrate by second reverse osmosis membrane system.Second reverse osmosis membrane concentrated water is after ion concentration deployment system, concentrated water is treated by crystallization, most of crystallization mother liquor is backflowed to ion concentration deployment system, ion concentration deployment water production is further concentrated by third reverse osmosis membrane, third reverse osmosis membrane concentrated water is treated by power plant ash spray, the water production of three reverse osmosis units is further reduced by senior oxidation after reuse.

Description

Technical Field

[0001] This invention relates to a zero-discharge treatment method and apparatus for wastewater, belonging to the field of water treatment technology, specifically to a "zero-discharge" recycling process and apparatus for PVA fiber production wastewater. Background Technology

[0002] PVA fiber production generates a large amount of pollutants. In addition, the production process requires the addition of the additive sodium sulfate, so PVA fiber production wastewater is characterized by high salt content and high organic pollutant content.

[0003] How to handle high-salt, high-organic wastewater is a difficult and challenging aspect of chemical wastewater treatment. Wastewater from PVA fiber production has a salt content as high as 3000-7000 mg / L and an organic matter content as high as 100-500 mg / L, and also contains substances such as PVA. Direct discharge of this wastewater into a biological wastewater treatment plant is problematic because the large volume, high salt content, and high organic matter concentration cause high load and poor operation of the biological system due to the high organic matter and salt concentration. Direct discharge of this wastewater without treatment will impact the aquatic ecosystem, potentially leading to soil salinization; furthermore, the large volume of wastewater directly discharged violates the principle of sustainable water resource utilization.

[0004] By adopting appropriate methods to reuse salt and water from wastewater and subject organic matter to biochemical treatment, a new opportunity can be provided for the treatment of PVA fiber production wastewater. Simultaneously, this approach can utilize wastewater and waste salts as resources, turning wastewater into valuable resources, significantly reducing enterprise operating costs, and minimizing or eliminating the environmental impact of industrial production.

[0005] In conclusion, exploring suitable wastewater treatment methods is of great significance for the sustainable development of PVA fiber production and environmental protection. Summary of the Invention

[0006] This invention provides a "zero-discharge" treatment method and apparatus for PVA production wastewater, primarily applied to wastewater treatment in PVA fiber production. After treatment using physicochemical, membrane integration, concentration and crystallization, and advanced oxidation technologies, the wastewater and waste salts are utilized as resources, reducing wastewater discharge and achieving green production, thereby protecting the environment.

[0007] A zero-discharge treatment method for PVA fiber production wastewater includes the following steps:

[0008] Step 1 is the pretreatment step for wastewater;

[0009] Step 2: Treat the wastewater obtained in Step 1 using an ultrafiltration membrane;

[0010] Step 3: The permeate from the ultrafiltration membrane is filtered using a first reverse osmosis membrane to obtain a first concentrate and a first filtrate.

[0011] Step 4: The first concentrate is concentrated using a second reverse osmosis membrane to obtain a second concentrate and a second filtrate.

[0012] Step 5: The second concentrate is filtered through a nanofiltration membrane to obtain nanofiltration permeate and nanofiltration concentrate, thereby separating monovalent ions from divalent ions.

[0013] Step 6: Crystallize the nanofiltration concentrate to obtain sodium sulfate, and then concentrate the nanofiltration permeate using a third reverse osmosis membrane.

[0014] The permeate obtained from the first, second, or third reverse osmosis membrane is reused after advanced oxidation treatment. During the operation of the first and second reverse osmosis membranes, the water recovery rate is 60-75%, and the operating pressure range is 1-4 MPa. During the operation of the third reverse osmosis membrane, the membrane type can be disc-type high-pressure reverse osmosis or high-pressure spiral wound reverse osmosis, and the system operating pressure is 6-12 MPa.

[0015] The COD range of PVA fiber production wastewater is 100-500 mg / L, the total hardness range is 50-200 mg / L, the turbidity range is 10-30 NTU, the sodium sulfate range is 3000-5000 ppm, the sodium chloride range is 100-500 ppm, and the pH range is 4-9.

[0016] The pretreatment includes one of the following: homogenization, multi-media filtration, and pH adjustment.

[0017] The multi-media filter is a filter in which one or more of the following materials are used as filter media: manganese sand, magnetite, ceramsite, quartz sand, activated carbon, or fly ash; the particle size of the media gradually increases from top to bottom, with the uppermost layer having a particle size of 0.4 to 0.6 mm, the middle layer having a particle size of 0.6 to 1.6 mm, and the lowermost layer having a particle size of 2 to 4 mm.

[0018] The hardness filter can be one of the following: ion exchange resin column, electro-deionization system, magnetic water processor, salt crystallization water softener, electronic descaling device, or chemical dosing precipitation filtration device.

[0019] The hardness removal filter is a chemical dosing precipitation filtration device and an ion exchange resin column. The added reagents include, but are not limited to, sodium carbonate, sodium hydroxide, calcium hydroxide, and lime. The reagent dosage exceeds the theoretical value by 0-200 mg / L, and the wastewater pH after reagent addition is not lower than 11.3. During the filtration process, two filtration methods are used: cross-flow filtration and dead-end filtration. The operating pressure is 0.01-1 MPa. Furthermore, the permeate from the hardness removal filter needs to be adjusted to a pH between 6 and 9. The acids added for pH adjustment include, but are not limited to, hydrochloric acid and sulfuric acid. The ion exchange resin used is a strong acid cation exchange resin. The wastewater flow rate within the resin column is 2-5 BV / h, and the permeate from the hardness removal filter is less than 5 mg / L.

[0020] The membrane configuration in the ultrafiltration process includes one of hollow fiber ultrafiltration membrane, tubular ultrafiltration membrane, and plate ultrafiltration membrane, and the material is selected from one of PVDF, PTFE, and PES.

[0021] During operation, the ultrafiltration membrane recovers its flux through periodic backwashing. The flux recovery rate is predicted by theoretical calculation. If the calculated recovery rate is high, the backwashing time can be appropriately reduced or the backwashing cycle can be extended. If the calculated recovery rate is low, the backwashing time can be appropriately increased or the backwashing cycle can be shortened.

[0022] The theoretically calculated predicted flux recovery rate R 修正 It is obtained by calculation using the following formula:

[0023] R 修正 =R×L, where L is the correction factor;

[0024] R%=α×T / (f β +γ)+ζ, where α, β, γ, and ζ are equation coefficients; T is the backwash time (s), and f is the backwash cycle (min);

[0025] If A≥10, L=1.15; if A≤5, L=0.70; for other values, L=1; A=(R1-R2) / R2, where R1 refers to turbidity removal rate and R2 refers to COD removal rate.

[0026] In step 5, the nanofiltration membrane temperature shall not exceed 42°C and the pressure shall not exceed 8 MPa; the recovery rate of the ion concentration adjustment system shall be between 70-95%, and the sodium sulfate content of the ion concentration concentrate shall not be less than 12 wt%.

[0027] Advanced oxidation mainly refers to ozone catalytic oxidation, which uses a specially formulated catalyst that can increase ozone utilization efficiency by 30%-50%. The COD content of the produced water from the advanced oxidation system is less than 5 mg / L, and the produced water is reused.

[0028] A zero-discharge treatment device for PVA fiber production wastewater includes:

[0029] Pretreatment equipment is used to pretreat wastewater;

[0030] Ultrafiltration membranes are used to treat the permeate from pretreatment units using ultrafiltration.

[0031] The first reverse osmosis membrane is used to filter ultrafiltration permeate.

[0032] The second reverse osmosis membrane is used to filter the concentrate from the first reverse osmosis membrane.

[0033] Nanofiltration membranes are used to filter the concentrate from the second reverse osmosis membrane to obtain nanofiltration permeate and nanofiltration concentrate, thereby separating monovalent ions from divalent ions.

[0034] A crystallizer is used to crystallize nanofiltration concentrate to obtain sodium sulfate;

[0035] The third reverse osmosis membrane is used to concentrate the nanofiltration permeate.

[0036] The pretreatment device is used to homogenize, filter through multiple media, and adjust the pH of wastewater.

[0037] It also includes a hardness removal filter, used to remove hardness from the concentrate of the first reverse osmosis membrane and supply the permeate to the second reverse osmosis membrane for treatment; the hardness removal filter is one of the following: ion exchange resin column, electro-deionization system, magnetic water quality processor, salt crystallization water softener, electronic descaling device, and chemical dosing precipitation filtration device.

[0038] Beneficial effects

[0039] 1. Membrane concentration significantly reduces wastewater discharge, making the "zero" discharge process for PVA fiber production wastewater economically feasible. 2. Reverse osmosis membrane treatment of PVA fiber production wastewater yields recycled water with advantages such as high water quality and stable process. It can be reused as production water as needed. 3. High-pressure nanofiltration membranes are used for salt separation, resulting in a concentrated solution with a sodium sulfate content of over 12%. Compared to methods such as electrodialysis, this significantly reduces energy consumption and investment. 4. The sodium sulfate obtained from the "zero discharge" PVA fiber production wastewater provided by this invention has a purity of over 97%, enabling the resource utilization of sodium sulfate. 5. This invention uses the concentrated reverse osmosis solution for ash spraying, which greatly reduces energy consumption and investment compared to conventional evaporation crystallization methods, while also avoiding the disposal of waste salts. Attached Figure Description

[0040] Figure 1 This is the process diagram of this patent.

[0041] Figure 2It is a comparison between the predicted and actual values ​​of the backwash flux recovery rate.

[0042] Figure 3 It is a comparison between the predicted and actual values ​​of the corrected backwash flux recovery rate. Detailed Implementation

[0043] The wastewater to be treated in this invention originates from the PVA fiber production industry. PVA fiber production wastewater mainly contains PVA, additives, and waste fibers, resulting in a large amount of PVA-containing wastewater. Due to its high salt and organic matter content, this wastewater is difficult to treat using simple physicochemical or biochemical processes. Originally, this wastewater was directly discharged into the factory's wastewater treatment plant. However, the high salt and organic matter content significantly impacted the wastewater treatment system, causing it to malfunction and become unstable. Furthermore, the wastewater's salt content reaches 3000-7000 mg / L. Direct discharge into the factory's wastewater treatment plant would affect the normal operation of the factory's biochemical system. However, if left untreated and directly discharged into water bodies, it would severely impact the stability of the aquatic ecosystem. Secondly, this wastewater may cause soil salinization. Finally, direct discharge of the wastewater not only wastes resources and violates the principles of sustainable development, but also increases the burden on the company's production.

[0044] Wastewater containing COD, hardness, turbidity, sodium sulfate, and sodium chloride. In one embodiment, the COD range of the effluent is 100-500 mg / L, the total hardness range is 50-100 mg / L, the turbidity range is 10-30 NTU, the sodium sulfate range is 3000-6000 ppm, the sodium chloride range is 100-500 ppm, and the pH range is 4-9.

[0045] In this invention, the sodium sulfate in the wastewater needs to be recycled and reused. Therefore, the cations other than sodium ions are impurity ions, such as calcium and magnesium ions.

[0046] The main process of this patent is as follows:

[0047] Step 1: Homogenize and adjust the pH of the wastewater.

[0048] Step 2 involves using a multi-media filter to remove suspended solids from the wastewater treated in Step 1. Commonly used multi-media filters include: quartz sand filters, activated carbon filters, anthracite filters, magnetite filters, manganese sand filters, quartz sand-ceramic filters, anthracite-quartz sand-magnetite filters, activated carbon-quartz sand-magnetite filters, and activated carbon-quartz sand filters, etc., primarily used to remove larger suspended particles. In some preferred embodiments, media filtration refers to using one or more of manganese sand, magnetite, ceramsite, quartz sand, activated carbon, or fly ash as the filter medium; the particle size of the medium gradually increases from top to bottom, with the uppermost layer having a particle size of 0.4–0.6 mm, the middle layer having a particle size of 0.6–1.6 mm, and the lowermost layer having a particle size of 2–4 mm.

[0049] Step 3 involves further removing suspended solids and colloids from the wastewater treated in Step 2 using ultrafiltration. The ultrafiltration membrane used here can have a molecular weight cutoff of 1000-200000. The material of these ultrafiltration membranes is not particularly limited, as long as it achieves the objective of removing the aforementioned water-soluble polymers and colloidal components. Examples include organic materials such as cellulose, cellulose esters, polysulfone, polyethersulfone, polyvinyl chloride, allyl chloride, polyolefins, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, and polytetrafluoroethylene; metals such as stainless steel; and inorganic materials such as ceramics. The material of the ultrafiltration membrane can be appropriately selected considering the properties of the hydrolysate or operating costs. From the perspective of ease of operation, organic materials are preferred, particularly polyvinyl chloride, polypropylene, polyvinylidene fluoride, polysulfone, and polyethersulfone. For the porous membrane constituting the ceramic separation membrane, a suitable selection from existing known ceramic materials can be made. For example, oxide materials such as alumina, zirconium oxide, magnesium oxide, silicon oxide, titanium oxide, cerium oxide, yttrium oxide, and barium titanate can be used; composite oxide materials such as cordierite, andalusite, forsterite, block talc, silica-alumina-oxygen-nitrogen ceramics, zircon, and ferrites can be used; nitride materials such as silicon nitride and aluminum nitride can be used; carbide materials such as silicon carbide can be used; hydroxide materials such as hydroxyapatite can be used; elemental materials such as carbon and silicon can be used; or inorganic composite materials containing two or more of these materials can be used. Natural minerals (clay, clay minerals, ceramic slag, silica sand, ceramic stone, feldspar, white sand) or blast furnace slag, fly ash, etc. can also be used. Preferably, one or more of alumina, zirconium dioxide, titanium oxide, magnesium oxide, and silicon oxide can be selected, and more preferably, ceramic powder composed mainly of alumina, zirconium dioxide, or titanium oxide can be used. Here, "as the main body" means that 50% or more (preferably 75% or more, more preferably 80% to 100% by mass) of the total ceramic powder is alumina or silicon dioxide.

[0050] Step 4 involves treating the wastewater after ultrafiltration in Step 3 using reverse osmosis to achieve desalination and COD separation. The permeate from reverse osmosis requires further treatment. The reverse osmosis membrane can retain salt ions and organic matter in the wastewater, producing a concentrated solution while simultaneously obtaining filtered permeate. The reverse osmosis membrane used in this invention typically consists of a reverse osmosis membrane element, a pressure vessel, and a pressurization pump. The treated liquid supplied to the reverse osmosis membrane device is usually pretreated by adding chemical agents such as bactericides, coagulants, reducing agents, and pH adjusters, and undergoing processes such as coagulation, sedimentation, sand filtration, activated carbon filtration, precision filtration, ultrafiltration, and permeation through a safety filter before being supplied to the device. Water treatment agents that can be used include scale inhibitors and bactericides. Based on water quality analysis results and calculations after on-site water sampling, it is recommended to select scale inhibitors with excellent scale inhibition and dispersion performance for iron, calcium, and magnesium ions, effectively controlling CaCO3, CaSO4, and SrSO4 scaling, exhibiting excellent overall scale inhibition effect, and demonstrating excellent dispersion and scale inhibition effects on metal oxides. Bactericides are used to disinfect reverse osmosis systems after a period of normal operation, as these systems may become contaminated with microorganisms due to various reasons. Non-oxidizing bactericides need to be added to the reverse osmosis system in a timely manner to ensure its normal operation. Here, a reverse osmosis membrane is a semi-permeable membrane that allows some components of a liquid, such as solvents, to pass through while preventing other components from passing through. Materials used for reverse osmosis membranes generally include polymers such as cellulose acetate polymers, polyamides, polyesters, polyimides, and vinyl polymers. Furthermore, in terms of structure, there are asymmetric membranes with a dense layer on at least one side of the membrane and micropores with gradually increasing pore size from this dense layer to the membrane interior or the surface of the other side, and composite membranes with a very thin active layer formed of other materials on the dense layer of the asymmetric membrane. Among these, the forms of reverse osmosis membranes include hollow fiber membranes and flat membranes. Typically, hollow fiber and flat membranes are preferred to have a membrane thickness of 10 μm to 1 mm, and the outer diameter of the hollow fiber is 50 μm to 4 mm. Furthermore, as a flat membrane, an asymmetric membrane is preferred, and as a composite membrane, a membrane supported by a substrate such as fabric, woven fabric, or nonwoven fabric is preferred. However, the method of the present invention can be used regardless of the material, membrane structure, or form of the reverse osmosis membrane, and is effective in any case. Representative reverse osmosis membranes include, for example, cellulose acetate or polyamide asymmetric membranes, and composite membranes having polyamide or polyurea active layers. A reverse osmosis membrane module is a material shaped for the practical use of the aforementioned reverse osmosis membrane. When the reverse osmosis membrane is in the form of a flat membrane, it can be used in a spiral, tubular, or plate-and-frame assembly. In the case of hollow fiber membranes, it can be used in an assembly based on bundles. The present invention is applicable regardless of the configuration of these reverse osmosis membrane modules.

[0051] Step 5 involves removing hardness from the reverse osmosis concentrate treated in Step 4 using a hardness filter to ensure the wastewater hardness is less than 5 mg / L. This hardness filter can be one of the following: an ion exchange resin column, an electro-deionization system, a magnetic water processor, a salt crystallization water softener, an electronic descaling device, or a chemical dosing precipitation filtration device. In the following embodiments, an ion exchange resin column is preferred for removing calcium and magnesium ions from the wastewater. Alternatively, a chemical dosing precipitation device, such as sodium carbonate or sodium hydroxide, can also be used for hardness removal.

[0052] Step 6: The reverse osmosis concentrate from step 5 (hardness removal) is further concentrated using reverse osmosis. The concentrated solution then enters an ion concentration adjustment device. This ion concentration adjustment device can be a nanofiltration membrane, capable of separating divalent (polyvalent) salts from monovalent salts in the permeate, allowing the monovalent salts to permeate as permeate. The nanofiltration membrane used in this invention is defined as a "pressure-driven membrane that blocks particles smaller than 2 nm and dissolved macromolecules." Its materials can be broadly classified into organic and inorganic nanofiltration membranes. Organic nanofiltration membranes primarily use organic materials, such as cellulose acetate (CA), sulfonated polysulfone (SPS), sulfonated polyethersulfone, polyvinyl alcohol (PVA), and polyamide (PA). Inorganic nanofiltration membranes use inorganic materials, such as alumina (Al₂O₃), titanium dioxide (TiO₂), and zirconium oxide (ZrO₂). Their structures can be hollow fiber, spiral wound, tubular, or flat sheet nanofiltration membranes. Each structure has its own characteristics. For example, hollow fiber and spiral wound membrane modules have high packing density and low cost, while plate and frame and tubular membrane modules are easy to clean and resistant to fouling. Nanofiltration membranes operate within the range between ultrafiltration and reverse osmosis, with a salt rejection rate between 20% and 98%, and a lower removal rate for soluble monovalent ions than for hypervalent ions. The operating pressure of nanofiltration membranes is typically 5-30 bar. Water treatment agents that can be used include, but are not limited to, those used to enhance the chlorine resistance, acid and alkali resistance, and backwashability of nanofiltration membranes, as well as agents for controlling membrane fouling. These agents help improve the lifespan of nanofiltration membranes and the effectiveness and quality of system operation.

[0053] Step 7: The concentrate from the ion concentration adjustment device in step 6 is fed into a crystallization device for crystallization purification. The resulting crystalline salt is reused; a small amount of the crystallization mother liquor is discharged, while most is recycled back to the nanofiltration influent. Since the concentrate mainly contains divalent salts, sodium sulfate can be obtained after crystallization.

[0054] Step 8: The permeate from the ion concentration adjustment device in step 6 is concentrated using reverse osmosis 3. The concentrated water from reverse osmosis is discharged into the power plant for ash spraying treatment. The permeate from the adjustment device mainly contains sodium chloride. After concentration, filtrate water is obtained.

[0055] Step 9: After collecting the permeate from the three reverse osmosis stages, the wastewater is treated with advanced oxidation to achieve resource utilization.

[0056] In some typical implementations, the above technical solutions may be:

[0057] The "zero discharge" treatment method for PVA fiber production wastewater includes the following steps:

[0058] Step 1: Homogenize and adjust the pH of the wastewater.

[0059] Step 2: Use a multi-media filter to remove suspended solids from the wastewater after Step 1 treatment;

[0060] Step 3: Use ultrafiltration to further remove suspended solids from the wastewater after step 2.

[0061] Step 4: The wash water from multi-media and ultrafiltration is treated and then enters the homogenization tank.

[0062] Step 5: The wastewater treated in step 3 is treated with reverse osmosis 1 to achieve wastewater desalination.

[0063] Step 6: Remove hardness from the concentrate of reverse osmosis 1 using a hardness removal filter;

[0064] Step 7: The low-hardness reverse osmosis concentrate obtained in step 6 is further concentrated by reverse osmosis 2.

[0065] Step 8: The reverse osmosis concentrate obtained in step 7 is processed using an ion concentration adjustment device.

[0066] Step 9: The ion-modified permeate obtained in step 8 is further concentrated using reverse osmosis 3. The concentrated water from reverse osmosis 3 is used for ash spraying treatment in the power plant.

[0067] Step 10: The permeate from the three reverse osmosis systems is further reduced in COD using advanced oxidation before being reused in the system.

[0068] Step 11: The ion-mixed concentrate obtained in step 7 is crystallized using crystallization technology. The mother liquor of the crystallization system is returned to the ion-mixing system, and a small amount of concentrate can be discharged from the crystallization system.

[0069] The wastewater refers to PVA-containing wastewater generated during the PVA fiber production process. After being homogenized and pH adjusted, the wastewater enters the subsequent processing stages.

[0070] The wastewater contains COD (mainly PVA), hardness, suspended solids, sodium sulfate, and a small amount of sodium chloride.

[0071] The wastewater has a COD range of 100-500 mg / L, a total hardness range of 50-200 mg / L, a turbidity range of 10-30 NTU, a sodium sulfate range of 3000-5000 ppm, a sodium chloride range of 100-500 ppm, and a pH range of 4-9.

[0072] The second step of media filtration refers to using one or more of the following as the filter medium: manganese sand, magnetite, ceramsite, quartz sand, activated carbon, or fly ash. The particle size of the medium gradually increases from top to bottom, with the uppermost layer having a particle size of 0.4 to 0.6 mm, the middle layer having a particle size of 0.6 to 1.6 mm, and the lowermost layer having a particle size of 2 to 4 mm.

[0073] The ultrafiltration process described in step 3 uses, but is not limited to, hollow fiber ultrafiltration membranes, tubular ultrafiltration membranes, and plate ultrafiltration membranes, and the materials are not limited to PVDF, PTFE, and PES.

[0074] During operation, it may also include optimization control steps for the ultrafiltration process, specifically referring to:

[0075] The treatment method used in step 4 is air flotation and coagulation sedimentation. The treated wastewater is returned to the homogenization tank, and the scum or sediment is further treated.

[0076] The reverse osmosis filtration process in step 5 has a water recovery rate of 60-75% and an operating pressure range of 1-4 MPa. The membrane element material used can be polyethersulfone or cellulose acetate.

[0077] In step 6, chemicals need to be added to the hard filter. These chemicals include, but are not limited to, sodium carbonate, sodium hydroxide, calcium hydroxide, and lime.

[0078] In step 6, the dosage of the chemical hardening agent exceeds the theoretical value by 0-200 mg / L, and the pH of the wastewater is not lower than 11.3 after the agent is added.

[0079] In step 6, the hard filter uses two filtration methods: cross-flow filtration and dead-end filtration, with an operating pressure of 0.01-1 MPa.

[0080] In step 6, the pH of the water produced by the hard filter needs to be adjusted to between 6 and 9. The acid added to adjust the pH includes, but is not limited to, hydrochloric acid and sulfuric acid.

[0081] In step 6, the permeate from the hard filter enters the ion exchange resin. The resin used is a strong acid cation exchange resin. The flow rate of the wastewater in the resin column is 2-5 BV / h, and the permeate from the hard filter is less than 5 mg / L.

[0082] In step 6, the ion exchange resin can be regenerated using either salt regeneration or acid-base regeneration.

[0083] Step 7 is reverse osmosis treatment, with a water recovery rate of 60%-75% and an operating pressure of 1-4 MPa. The membrane element used is the same as that in step 5.

[0084] Step 8 involves ion concentration adjustment. The membrane temperature used for ion adjustment shall not exceed 42°C and the pressure shall not exceed 8MPa. The recovery rate of the ion concentration adjustment system shall be between 70-95%, and the sodium sulfate content of the ion concentration concentrate shall not be less than 12wt%.

[0085] Step 9 uses reverse osmosis treatment. The membrane type used can be disc-type high-pressure reverse osmosis or high-pressure spiral wound reverse osmosis. The system operating pressure is 6-12 MPa. The concentrate is used for ash spraying treatment in power plants.

[0086] Step 10 mainly uses ozone catalytic oxidation, and the catalyst used is a specially made catalyst that can increase ozone utilization efficiency by 30%-50%. The COD content of the water produced by the advanced oxidation system is less than 5 mg / L, and the water is reused.

[0087] The crystallization technique used in step 11 can be either evaporation crystallization or freeze crystallization. The sodium sulfate obtained by the system has a purity of 97%-99%, a whiteness of not less than 82, and a sodium sulfate yield of not less than 97%. The crystallized salt is recycled.

[0088] Example 1

[0089] After homogenization, the wastewater quality is as follows: water volume is 120m³. 3The wastewater concentration was 4.2. The COD was 460 mg / L, total hardness was 85 mg / L, turbidity was 15 NTU, influent TDS was 4200 mg / L, sodium sulfate was 3900 mg / L, sodium chloride was 210 mg / L, and pH was 4.2. After adjusting the pH to 8 and using multi-media filtration, the turbidity in the wastewater was 0.5 NTU. The multi-media effluent was filtered through a tubular ceramic membrane at an operating pressure of 0.18 MPa, with a recovery rate of 95%, resulting in an effluent turbidity of 0.1 NTU and an effluent SDI of 3. The ultrafiltration effluent was concentrated using reverse osmosis 1 at an operating pressure of 2.5 MPa, with a recovery rate of 75%. During operation, the ultrafiltration membrane flux was restored through periodic backwashing. The TDS of the reverse osmosis 1 clarified solution was 120 mg / L, COD was 18 mg / L, and the TDS of the concentrated solution was 31040 mg / L. The reverse osmosis 1 concentrate is filtered through a hardening filter, producing water with a hardness of 0.3 mg / L. The hardening-reduced reverse osmosis 1 concentrate is further concentrated using reverse osmosis 2 at an operating pressure of 3.5 MPa, achieving a recovery rate of 60%. The concentrate has a TDS of 58,000 mg / L, while the reverse osmosis 2 clarified solution has a TDS of 150 mg / L and a COD of 21 mg / L. The reverse osmosis 2 concentrate is then desalted using a nanofiltration membrane at an operating pressure of 6.0 MPa. The concentrate reaches a sulfate concentration of 83,000 mg / L and a TDS of 124,300 mg / L, while the nanofiltration clarified solution has a TDS of 3,500 mg / L. The nanofiltration concentrate is then crystallized using freeze crystallization at -3°C. This yields 1.1 t / h of sodium sulfate with approximately 0.5 t of water; sodium sulfate has a purity of 98.5% and a whiteness of 84. The system produces approximately 5 m³ of sodium sulfate per hour. 3 The mother liquor from the crystallization process is recycled to the nanofiltration system. The nanofiltration clarified liquid is further concentrated and reduced in volume using reverse osmosis (RO) stage 3. The RO stage 3 permeate has a TDS of 210 mg / L and a COD of 210 mg / L. The RO stage 3 concentrate is then sent to the power plant for soot spraying. The clarified liquid produced by the three-stage RO process is further reduced in COD using ozone catalytic oxidation technology, ensuring that the recycled water system has a COD < 5 mg / L, a salt content not exceeding 280 mg / L, and a flow rate of 115 m³ / h. 3 / h.

[0090] Example 2

[0091] Wastewater quality after homogenization: water volume 150m 3The initial concentration of the feed water was 4859 mg / L, COD was 550 mg / L, total hardness was 110 mg / L, turbidity was 20 NTU, TDS was 4859 mg / L, sodium sulfate was 4282 mg / L, sodium chloride was 178 mg / L, and pH range was 4.6. After adjusting the pH to 8.8, the turbidity after multi-media filtration was 0.7 NTU. Multi-media permeate was filtered using a PTFE tubular ultrafiltration membrane at an operating pressure of 0.2 MPa, with a recovery rate of 94%, resulting in an effluent turbidity of 0.1 NTU and an effluent SDI of 2.5. The ultrafiltration effluent was concentrated using reverse osmosis (RO) 1 at an operating pressure of 2.3 MPa, with a recovery rate of 60%. During operation, the ultrafiltration membrane flux was restored through periodic backwashing. The RO 1 clarified solution had a TDS of 80 mg / L, COD of 14 mg / L, and a concentrated solution TDS of 28040 mg / L. The reverse osmosis (RO) 1 concentrate is filtered using a hardening filter, producing water with a hardness of 0.3 mg / L. After hardening removal, the RO 1 concentrate is further concentrated using RO 2 at an operating pressure of 3.5 MPa, achieving a recovery rate of 75%. The concentrate has a TDS of 57200 mg / L, while the RO 2 clarified solution has a TDS of 136 mg / L and a COD of 21 mg / L. The RO 2 concentrate is then desalted using a nanofiltration membrane at an operating pressure of 6.5 MPa, resulting in a sulfate concentration of 81500 mg / L and a TDS of 122030 mg / L. The nanofiltration clarified solution has a TDS of 3300 mg / L. The nanofiltration concentrate is then crystallized using a freeze-crystallization process. The freeze-crystallization temperature is controlled at 0℃, yielding 0.8 t / h of sodium sulfate with approximately 0.3 t of water; sodium sulfate has a purity of 98.1% and a whiteness of 82. The system produces approximately 6 m³ of sodium sulfate per hour. 3 The mother liquor from the crystallization process is recycled to the nanofiltration system. The nanofiltration clarified liquid is further concentrated and reduced in volume using reverse osmosis (RO) stage 3. The RO stage 3 permeate has a TDS of 210 mg / L and a COD of 210 mg / L. The RO stage 3 concentrate is then sent to the power plant for soot spraying. The clarified liquid produced by the three-stage RO process is further reduced in COD using ozone catalytic oxidation technology, ensuring that the recycled water system has a COD < 5 mg / L, a salt content not exceeding 3000 mg / L, and a flow rate of 110 m³ / h. 3 / h.

[0092] Example 3

[0093] Establishment of optimized control procedures for ultrafiltration membrane operation:

[0094] During operation, ultrafiltration membranes can recover flux through backwashing. Backwashing effectively removes particulate filter cake deposited on the membrane surface, resulting in a high recovery rate, but it is less effective at removing organic contaminants from the membrane surface. Furthermore, backwashing cycles (f) can be set, such as every 15 / 25 / 30 minutes, and backwash times (T) can be set, such as 5 / 10 / 15 / 20 seconds. Longer backwash cycles lead to greater contaminant deposition on the membrane surface, affecting the backwash recovery rate. Shorter cycles, on the other hand, affect treatment efficiency and result in excessive backwash water consumption. Regarding backwash time, shorter times tend to lead to insufficient recovery, while longer times also cause issues with treatment efficiency and backwash water consumption. This patent first establishes an empirical formula for predicting the pure water flux recovery rate after backwashing, and conducts multiple preliminary operational tests under different raw water quality conditions.

[0095] The wastewater has a COD range of 220-410 mg / L, a total hardness range of 122-156 mg / L, a turbidity range of 12-24 NTU, a sodium sulfate range of 3520-3780 ppm, a sodium chloride range of 240-374 ppm, and a pH range of 5-7. After pre-filtration using a multi-media filter, filtration is performed on an ultrafiltration membrane with a molecular weight cutoff of 200,000. During operation, the membrane surface flow rate is 0.5 m / s, and the pressure is 0.2 MPa. When the flux drops to 25% of the original pure water flux, backwashing is initiated. The backwashing time T is 5-25 s, and the cycle frequency is 15-60 min. After backwashing, the pure water flux is measured, and the recovery rate (R, %) compared to the initial water flux before backwashing is calculated. After fitting an empirical formula, the formula for flux recovery rate versus backwashing time / frequency is obtained:

[0096] R%=19.5×T / (f 0.5 +3.13)+48.44

[0097] The relationship between theoretical and experimental values ​​is as follows: Figure 2As shown, the theoretical and experimental values ​​are generally in agreement, but significant differences remain at certain points. Analysis suggests that the different types of membrane fouling caused by PVA particles and small organic molecules lead to variations in the cleaning recovery rate. A higher concentration of PVA particles facilitates backwashing removal, while a higher concentration of small organic molecules reduces the backwashing rate. The concentration of PVA particles significantly affects the turbidity of the feed solution. However, because PVA particles are not easily oxidized in COD detection and are stable, they have little impact on the COD value. Small organic molecules primarily affect the COD value and have some influence on turbidity detection, but this influence is not significant. Therefore, flux calculations of the turbidity and COD values ​​in the feed and permeate can represent the degree of PVA particle and organic matter deposition on the membrane surface. In other words, the turbidity removal rate primarily reflects the degree of PVA particle deposition on the membrane surface, while the COD removal rate primarily reflects the degree of organic matter deposition. The following formula can be used to obtain a ratio measuring the amount of PVA particles and organic matter deposited on the membrane surface:

[0098] The ratio A = (R1 - R2) / R2, where R1 refers to the turbidity removal rate and R2 refers to the COD removal rate. The turbidity removal rate is obtained by an online turbidity meter, and the COD removal rate is obtained by an online COD analyzer.

[0099] After subtracting the small change in R2 caused by organic matter from R1, the numerator basically represents the amount caused by PVA particle deposition on the membrane surface, while the denominator represents the amount of organic matter deposited. Comparison revealed that a larger A value indicates more PVA particle deposition, leading to a higher backwash recovery rate, while a smaller A value indicates a larger amount of organic matter deposition and a smaller actual backwash recovery rate. Therefore, when predicting the backwash pre-recovery rate, a correction term is introduced into the above empirical formula, namely:

[0100] R 修正 =R×L, where L is the correction factor;

[0101] If A≥10, L=1.15; if A≤5, L=0.70; otherwise, L=1.

[0102] After correction of the predicted values, the comparison between the predicted values ​​and the experimental values ​​is as follows: Figure 3 As shown, the degree of deviation is significantly reduced, with the correlation coefficients between the predicted and experimental values ​​before and after correction being 0.755 and 0.849, respectively.

[0103] The above correction values ​​are estimated based on experience. They can also be further improved by performing data fitting regression after accumulating more data.

[0104] After determining the calculation equation for the backwash recovery rate, in actual operation, if the calculated recovery rate is high, the backwash time during operation can be appropriately reduced or the backwash cycle can be extended. If the calculated recovery rate is low, the backwash time can be appropriately increased or the backwash cycle can be shortened.

Claims

1. A zero discharge treatment method of PVA fiber production wastewater, characterized by, It comprises the following steps: Step 1, pretreatment of wastewater; Step 2, ultrafiltration membrane treatment of wastewater obtained in step 1; Step 3, first reverse osmosis membrane treatment of water produced by the ultrafiltration membrane to obtain first concentrated liquid and first filtrate; Step 4, second reverse osmosis membrane treatment of the first concentrated liquid to obtain second concentrated liquid and second filtrate; Step 5, nanofiltration membrane treatment of the second concentrated liquid to obtain nanofiltration permeate and nanofiltration concentrated liquid, so as to separate monovalent ions from divalent ions; Step 6, crystallization treatment of the nanofiltration concentrated liquid to obtain sodium sulfate, and third reverse osmosis membrane treatment of the nanofiltration permeate; The membrane configuration in the ultrafiltration membrane treatment comprises one of hollow fiber ultrafiltration membrane, tubular ultrafiltration membrane or plate ultrafiltration membrane, and the material is selected from one of PVDF, PTFE or PES; in the operation process of the ultrafiltration membrane, the membrane flux is recovered through periodic backwashing, and the recovery rate is predicted through theoretical calculation; if the calculated recovery rate is high, the backwashing time in the operation process is reduced or the backwashing cycle is prolonged, and if the calculated recovery rate is low, the backwashing time is increased or the backwashing cycle is shortened; The flux recovery rate is calculated by the following formula: Flux recovery rate R 修正 = R x L, where L is a correction factor; R% = a x T / (f β + y) + z, a, b, g, z are equation coefficients; T is backwash time, unit is s, f is backwash period, unit is min; If A≥10, L=1.15; if A≤5, L=0.70; if A is other values, L=1; A=(R1-R2) / R2, R1 refers to the turbidity removal rate, and R2 refers to the COD removal rate; The COD range in PVA fiber production wastewater is 100-500 mg / L, the total hardness range is 50-200 mg / L, the turbidity range is 10-30 NTU, the sodium sulfate concentration range is 3000-5000 ppm, the sodium chloride concentration range is 100-500 ppm, and the pH range is 4-9.

2. The PVA fiber production wastewater zero-emission treatment method according to claim 1, characterized by, The water produced by the first reverse osmosis membrane, the second reverse osmosis membrane or the third reverse osmosis membrane is reused after advanced oxidation treatment; in the operation process of the first reverse osmosis membrane and the second reverse osmosis membrane, the water recovery rate is 60-75%, and the operation pressure range is 1-4 MPa; in the operation of the third reverse osmosis membrane, the membrane form is disc-type high-pressure reverse osmosis or high-pressure spiral reverse osmosis, and the system operation pressure is 6-12 MPa.

3. The PVA fiber production wastewater zero-emission treatment method according to claim 1, characterized by, The pretreatment comprises one of homogenization, multi-medium filtration and pH adjustment.

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