Zero-discharge treatment method and device for PVA fiber production wastewater

Abstract

The invention relates to a PVA fiber production wastewater treatment method and device. The wastewater enters a multi-medium filter after being homogenized to adjust the pH, so that the content of suspended solids in the wastewater is reduced. And the content of suspended matters and colloids in multi-medium produced water is reduced by adopting an ultrafiltration system. Ultrafiltration produced water enters a first reverse osmosis membrane to be concentrated, first reverse osmosis membrane concentrated water is subjected to hardness removal through a hardness removal filter, and produced water of the hardness removal filter is further concentrated through a second reverse osmosis membrane system. After second reverse osmosis membrane concentrated water passes through an ion concentration allocation system, crystallization treatment is conducted on the concentrated water, most crystallization mother liquor flows back to the ion concentration allocation system, ion concentration allocation produced water is further concentrated through a third reverse osmosis membrane, and power plant ash spraying treatment is conducted on third reverse osmosis membrane concentrated water; and the produced water of the three reverse osmosis units is recycled after COD is further reduced through advanced oxidation.

Description

Technical Field

[0001] The invention relates to a zero-discharge treatment method and device for wastewater, belonging to the technical field of water treatment, and specifically to a "zero-discharge" recovery process and device for PVA fiber production wastewater. Background Art

[0002] PVA fiber production produces a large amount of pollutants. At the same time, the auxiliary agent sodium sulfate needs to be added during the PVA fiber production process. Therefore, the PVA fiber production wastewater has the characteristics of high salt content and high organic pollutant content.

[0003] How to solve the problem of high-salt and high-organic wastewater is the difficulty and pain point of chemical wastewater treatment. The salt content of PVA fiber production wastewater is as high as 3000-7000mg / L, the organic matter is as high as 100-500mg / L, and the wastewater contains PVA and other substances. The wastewater is directly discharged into the biochemical sewage treatment plant. Because of the large amount of water, high salt content, and high concentration of organic matter, the microorganisms have high organic matter concentration and high salt content, resulting in high load on the biochemical system and poor operation effect. If this wastewater is discharged directly without treatment, it will affect the ecological environment of the water body, and in severe cases, it can cause salinization of the land; secondly, the large amount of wastewater and direct discharge do not conform to the principle of sustainable use of water resources.

[0004] By adopting appropriate methods, the salt and water in the wastewater can be reused, and the organic matter can be sent to biochemical treatment, which will provide a new opportunity for the treatment of PVA fiber production wastewater. At the same time, this method can make resource utilization of wastewater and waste salt, turning wastewater into treasure, greatly reducing the operating costs of enterprises, and at the same time can reduce or avoid the impact of industrial production on the environment.

[0005] In summary, exploring appropriate wastewater treatment methods is of great significance to the sustainable development of PVA fiber production and environmental protection. Summary of the invention

[0006] The present invention provides a "zero discharge" treatment method and device for PVA production wastewater, which is mainly used for wastewater treatment in PVA fiber production. After the wastewater is treated by physical and chemical, membrane integration technology, concentration crystallization technology, and advanced oxidation technology, the resource utilization of wastewater and waste salt is realized, the discharge of wastewater is reduced, green production is realized, and the environment is protected.

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

[0008] Step 1, a step of pre-treating wastewater;

[0009] Step 2, treating the wastewater obtained in step 1 with an ultrafiltration membrane;

[0010] Step 3, filtering the produced water of the ultrafiltration membrane with a first reverse osmosis membrane to obtain a first concentrated liquid and a first filtrate;

[0011] Step 4, the first concentrated liquid is concentrated by a second reverse osmosis membrane to obtain a second concentrated liquid and a second filtrate;

[0012] Step 5, filtering the second concentrated liquid through a nanofiltration membrane to obtain a nanofiltration permeate and a nanofiltration concentrated liquid, so as to separate the monovalent ions from the divalent ions;

[0013] Step 6, the nanofiltration concentrated liquid is crystallized to obtain sodium sulfate, and the nanofiltration permeate is concentrated by a third reverse osmosis membrane.

[0014] The produced water obtained by the first reverse osmosis membrane, the second reverse osmosis membrane or the third reverse osmosis membrane is reused after advanced oxidation treatment; during the operation of the first reverse osmosis membrane and the second reverse osmosis membrane, the water recovery rate is 60-75%, and the operating pressure ranges from 1 to 4Mpa; during the operation of the third reverse osmosis membrane, the membrane form can be a disc-type high-pressure reverse osmosis or a high-pressure coil reverse osmosis, and the operating pressure of the system is 6-12MPa.

[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 comprises: one of homogenization, multi-media filtration and pH adjustment.

[0017] The multi-media filter is a filter with one or a combination of manganese sand, magnetite, ceramsite, quartz sand, activated carbon or fly ash as filter material; the particle size of the medium particles gradually increases from top to bottom, the particle size of the top layer is 0.4-0.6mm, the particle size of the middle layer is 0.6-1.6mm, and the particle size of the bottom layer is 2-4mm.

[0018] The hardness removal filter can be one of an ion exchange resin column, an electric deionization system, a magnetic water quality processor, a salt crystallization water softener, an electronic descaling device, and a chemical dosing precipitation filtration device.

[0019] The hardness removal filter is a chemical dosing precipitation filtration device and an ion exchange resin column, and the added reagents include but are not limited to sodium carbonate, sodium hydroxide, calcium hydroxide, lime, etc., the amount of reagent added exceeds the theoretical value of 0-200 mg / L, and the pH of the wastewater is not lower than 11.3 after the addition of the reagents. During the filtration process, the filtration methods include cross-flow filtration and dead-end filtration, and the operating pressure is 0.01-1Mpa; and the water produced by the hardness removal filter needs to be adjusted to a pH between 6 and 9, and the acid added for adjusting the pH includes but is not limited to hydrochloric acid, sulfuric acid, etc.; the resin used in the ion exchange resin is a strong acid cationic resin, the flow rate of the wastewater in the resin column is 2-5BV / h, and the water produced by the hardness removal filter is less than 5mg / L.

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

[0021] During the operation of the ultrafiltration membrane, the membrane flux is restored through periodic backwashing, and the flux recovery rate is predicted by theoretical calculation. If the calculated recovery rate is high, the backwashing time during operation 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 theoretical calculations described above predict the flux recovery rate R 修正 It is calculated by the following formula:

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

[0024] R%=α×T / (f β +γ)+ζ, α, β, γ, ζ are equation coefficients; T is the backwash time, s, 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=(R 1 -R 2 ) / R2,R 1 Refers to the turbidity removal rate, R 2 Refers to the COD removal rate.

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

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

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

[0029] A pretreatment device for pre-treating wastewater;

[0030] Ultrafiltration membrane, used for ultrafiltration treatment of produced water from the pretreatment device;

[0031] The first reverse osmosis membrane is used for filtering the ultrafiltration water;

[0032] The second reverse osmosis membrane is used to filter the concentrated liquid of the first reverse osmosis membrane;

[0033] The nanofiltration membrane is used to filter the concentrated water from the second reverse osmosis membrane to obtain nanofiltration permeate and nanofiltration concentrate, so as to separate monovalent ions from divalent ions.

[0034] A crystallizer, used for crystallizing the nanofiltration concentrate to obtain sodium sulfate;

[0035] The third reverse osmosis membrane is used for concentrating the nanofiltration permeate.

[0036] The pretreatment device is used to perform homogenization, multi-media filtration and pH adjustment operations on wastewater.

[0037] It also includes a hardness removal filter for removing hardness from the concentrated water of the first reverse osmosis membrane and supplying the produced water to the second reverse osmosis membrane for treatment; the hardness removal filter is one of an ion exchange resin column, an electrodeionization system, a magnetic water quality processor, a salt crystallization softener, an electronic descaling device, and a chemical dosing precipitation filtration device.

[0038] Beneficial Effects

[0039] 1. The membrane concentration is adopted to greatly reduce the wastewater discharge, making the "zero" discharge process of PVA fiber production wastewater economically feasible; 2. The PVA fiber production wastewater is treated by reverse osmosis membrane, and the recycled water obtained has the advantages of good water quality and stable process. It can be reused as production water as needed; 3. The high-pressure nanofiltration membrane is used for salt separation treatment, and the sodium sulfate content of the concentrated liquid is more than 12%. Compared with the use of electrodialysis and other methods, it greatly reduces energy consumption and investment; 4. The purity of sodium sulfate obtained by the "zero discharge" of PVA fiber production wastewater provided by the present invention is more than 97%, which can realize the resource utilization of sodium sulfate; 5. The concentrated solution of reverse osmosis is used for ash spraying treatment, which greatly reduces energy consumption and investment compared with the conventional evaporation crystallization, and avoids the disposal of waste salt. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0042] Figure 3 It is the comparison between the predicted value and the actual value of the corrected backwash flux recovery rate. DETAILED DESCRIPTION

[0043] The wastewater to be treated by the present invention comes from the PVA fiber production industry. The PVA fiber production wastewater mainly contains PVA, additives, waste silk, etc., thereby generating a large amount of wastewater containing PVA. The wastewater is difficult to be treated using simple physical and chemical, biochemical and other processes due to its high salt content and high organic content. This wastewater was originally directly discharged into the factory sewage treatment plant, but because the wastewater has high salt content and high organic matter, it has a significant impact on the sewage treatment system, resulting in the sewage treatment system being unable to operate normally and stably. However, the salt content of this wastewater reaches 3000-7000 mg / L, and it is directly discharged into the factory sewage treatment plant, which will affect the normal operation of the factory biochemical system. However, if it is not treated and directly discharged into the water body, it will seriously affect the stability of the water body ecosystem; secondly, this wastewater may cause soil salinization; finally, if the wastewater is directly discharged, it will not only cause a waste of resources, but also not conform to the principle of sustainable development, and increase the burden on the production of the enterprise.

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

[0045] In the present invention, the sodium sulfate in the wastewater needs to be recovered and reused, so other cations except the sodium ion are impurity ions, such as calcium, magnesium and other cations.

[0046] The main process of this patent is:

[0047] Step 1: homogenize and adjust the pH value of the wastewater;

[0048] In the second step, the wastewater treated in the first step is subjected to a multi-media filter to remove suspended solids in the wastewater; commonly used multi-media filters may include: quartz sand filter, activated carbon filter, anthracite filter, magnetite filter, manganese sand filter, quartz sand-ceramic filter, anthracite-quartz sand-magnetite filter, activated carbon-quartz sand-magnetite filter, activated carbon-quartz sand filter, etc.; they are mainly used to remove larger suspended solid particles. In some preferred embodiments, medium filtration refers to the use of one or a combination of manganese sand, magnetite, ceramsite, quartz sand, activated carbon or fly ash as the filter medium; the particle size of the medium particles gradually increases from top to bottom, with the particle size of the top layer being 0.4-0.6 mm, the particle size of the middle layer being 0.6-1.6 mm, and the particle size of the bottom layer being 2-4 mm.

[0049] In the third step, the wastewater treated in the second step is further subjected to ultrafiltration to remove suspended solids and colloids in the wastewater; the ultrafiltration membrane used here can be a membrane with a molecular weight cutoff of 1000 to 200000. The material of these ultrafiltration membranes is not particularly limited as long as it can achieve the purpose of the present invention of removing the above-mentioned water-soluble polymers and colloid components. It can be cited as organic materials such as cellulose, cellulose ester, polysulfone, polyethersulfone, polyvinyl chloride, propylene chloride, polyolefin, polyvinyl alcohol, polymethyl methacrylate, polyvinylidene fluoride, polytetrafluoroethylene, or metals such as stainless steel, or inorganic materials such as ceramics. The material of the ultrafiltration membrane can be appropriately selected in consideration of the properties of the hydrolyzate or the running cost. From the perspective of ease of operation, organic materials are preferred, preferably polyvinyl chloride, polypropylene, polyvinylidene fluoride, polysulfone, and polyethersulfone. The material of the porous membrane constituting the ceramic separation membrane can be appropriately selected from existing known ceramic materials. For example, oxide materials such as aluminum oxide, zirconium oxide, magnesium oxide, silicon oxide, titanium oxide, cerium oxide, yttrium oxide, and barium titanate can be used; composite oxide materials such as cordierite, mullite, forsterite, steatite, silicon aluminum oxide ceramics, zircon, and ferrite; nitride materials such as silicon nitride and aluminum nitride; carbide materials such as silicon carbide; hydroxide materials such as hydroxyapatite; elemental materials such as carbon and silicon; or inorganic composite materials containing two or more of them. Natural minerals (clay, clay minerals, pottery slag, silica sand, pottery stone, feldspar, white sand) or blast furnace slag, fly ash, etc. can also be used. Among them, it is preferred to select one or more of aluminum oxide, zirconium dioxide, titanium oxide, magnesium oxide, and silicon oxide, and more preferably a ceramic powder composed mainly of aluminum oxide, zirconium dioxide or titanium oxide. The term "mainly" used herein means that 50% by mass or more (preferably 75% by mass or more, more preferably 80% to 100% by mass) of the entire ceramic powder is alumina or silica.

[0050] In the fourth step, the wastewater after ultrafiltration in the third step is treated by reverse osmosis to achieve the purpose of desalination of wastewater and separation of COD. The water produced by reverse osmosis needs to be further treated; the reverse osmosis membrane can intercept salt ions and organic matter in the wastewater as a concentrate, and at the same time obtain filtered water. The reverse osmosis membrane used in the present invention is usually composed of a reverse osmosis membrane element, a pressure-resistant container, a pressure pump, etc. The treated liquid supplied to the reverse osmosis membrane device is usually supplied to the device after adding chemical agents such as bactericides, coagulants, reducing agents, pH regulators, and other pre-treatments such as coagulation, precipitation, sand filtration, activated carbon filtration, precision filtration, ultrafiltration, and passing through safety filters. The water treatment agents that can be used include scale inhibitors and bactericides. Based on the water quality analysis results and calculation analysis after on-site water sampling, it is recommended to use scale inhibitors with excellent scale inhibition and dispersion properties for iron, calcium, and magnesium ions to effectively control CaCO 3 、CaSO 4 、SrSO 4 Scaling, comprehensive anti-scaling effect is very good, and it has excellent dispersed anti-scaling effect on metal oxides. Bactericides are used in reverse osmosis systems. After a period of normal operation, due to various reasons, they will be contaminated by microorganisms. It is necessary to add non-oxidizing bactericides to the reverse osmosis system in time for sterilization to ensure the normal operation of the reverse osmosis system. Here, the so-called reverse osmosis membrane is a semi-permeable membrane that allows part of the components in the liquid, such as solvents, to pass through, but does not allow other components to pass through. As the material of the reverse osmosis membrane, generally used are polymer materials such as cellulose acetate polymers, polyamides, polyesters, polyimides, vinyl polymers, etc. In addition, as its structure, there are asymmetric membranes with a dense layer on at least one side of the membrane, micropores with gradually increasing pore sizes from the dense layer to the inside of the membrane or the surface of the other side, and composite membranes with a very thin active layer formed by other materials on the dense layer of the asymmetric membrane. Among them, as the form of reverse osmosis membrane, there are hollow fibers, flat membranes, etc. Usually, the preferred membrane thickness of hollow fibers and flat membranes is 10 μm to 1 mm, and the outer diameter of hollow fibers is 50 μm to 4 mm. In addition, as a flat membrane, an asymmetric membrane is preferred, and as a composite membrane, a membrane supported by a substrate such as a fabric, a woven fabric, or a non-woven fabric is preferred. However, the method of the present invention can be utilized independently of the material, membrane structure, or form of the reverse osmosis membrane, and is effective for any case. As representative reverse osmosis membranes, for example, cellulose acetate or polyamide asymmetric membranes, composite membranes with polyamide or polyurea active layers, and the like can be cited. The so-called reverse osmosis membrane assembly is a material that is shaped for the actual use of the above-mentioned reverse osmosis membrane. In the case where the reverse osmosis membrane is in the form of a flat membrane, it can be woven into a spiral, tubular, or plate and frame assembly for use. In addition, in the case of hollow fibers, it can be woven into an assembly on a bundle basis for use. The present invention can be applied independently of the structural form of these reverse osmosis membrane assemblies.

[0051] In step 5, the hardness of the concentrated reverse osmosis water treated in step 4 is removed by a hardness removal filter to ensure that the hardness of the wastewater is less than 5 mg / L; the hardness removal filter here can be one of an ion exchange resin column, an electrodeionization system, a magnetic water quality processor, a salt crystallization softener, an electronic descaling device, and a chemical dosing precipitation filtration device. In the following embodiments, an ion exchange resin column is preferably used to remove calcium and magnesium ions in the wastewater, and a dosing precipitation device can also be preferably used to remove hardness, such as sodium carbonate and sodium hydroxide.

[0052] In step 6, the reverse osmosis concentrated water treated with hardness removal in step 5 is further concentrated by reverse osmosis, and the concentrated solution of reverse osmosis enters the ion concentration adjustment device; the ion concentration adjustment device here may refer to a nanofiltration membrane, which can separate the divalent (multivalent) salts from the monovalent salts in the produced water, and allow the monovalent salts to pass through as produced water. The nanofiltration membrane used in the present invention is defined as a "pressure-driven membrane that blocks particles smaller than 2nm and dissolved macromolecules." Its material can be divided into two categories: organic nanofiltration membrane and inorganic nanofiltration membrane. Organic nanofiltration membranes are mainly made of organic matter. Common organic materials include cellulose acetate (CA), sulfonated polysulfone (SPS), sulfonated polyether sulfone, polyvinyl alcohol (PVA), polyamide (PA), etc. Inorganic nanofiltration membranes are made of inorganic matter. Common inorganic nanofiltration membrane materials include alumina (Al 2 O 3 ), titanium oxide (TiO 2 ), zirconium oxide (ZrO 2 ) and so on. Its structure can be hollow fiber, roll, tubular or flat nanofiltration membrane. Each structure has its own characteristics, for example, hollow fiber and roll membrane components have high filling density and low cost, while plate-frame and tubular membrane components are easy to clean and resistant to pollution. The operating range of nanofiltration membrane is between ultrafiltration and reverse osmosis, and its ability to intercept dissolved salts is between 20%-98%, and the removal rate of soluble monovalent ions is lower than that of high-valent ions. The operating pressure of nanofiltration membrane is generally 5-30bar. The water treatment agents that can be used include but are not limited to drugs used to enhance the chlorine resistance, acid and alkali resistance and backwashability of nanofiltration membranes, as well as agents used to control membrane fouling. These agents help to improve the service life of nanofiltration membranes and the effect and quality of system operation.

[0053] In step 7, the concentrated liquid from the ion concentration adjustment device in step 6 enters the crystallization device for crystallization purification, and the obtained crystallized salt is reused; a small amount of the crystallization mother liquor is discharged, and most of it is refluxed to the nanofiltration inlet water. Since the concentrated liquid mainly contains divalent salts, sodium sulfate can be obtained after crystallization.

[0054] In step 8, the produced water of the ion concentration adjustment device in step 6 is concentrated by reverse osmosis 3, and the concentrated water of reverse osmosis is discharged into the power plant for ash spraying treatment; the produced water of the adjustment device mainly contains sodium chloride, and after concentration, filtrate water is obtained.

[0055] In step 9, the water produced by the three stages of reverse osmosis is collected and oxidized using advanced oxidation to achieve resource utilization of wastewater.

[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 value of the wastewater;

[0059] Step 2, using a multi-media filter to remove suspended solids from the wastewater treated in step 1;

[0060] Step 3, using ultrafiltration to further remove suspended matter from the wastewater treated in step 2;

[0061] Step 4: The wash water from the multi-media and ultrafiltration enters the homogenization tank after being treated;

[0062] Step 5, the wastewater treated in step 3 is treated by reverse osmosis 1 to achieve wastewater desalination;

[0063] Step 6, using a hardness removal filter to remove hardness from the concentrated water of the reverse osmosis 1;

[0064] Step 7, further concentrating the low-hardness reverse osmosis 1 concentrated water obtained in step 6 through reverse osmosis 2;

[0065] Step 8, treating the reverse osmosis concentrated water obtained in step 7 using an ion concentration adjustment device;

[0066] Step 9, the ion blended water obtained in step 8 is further concentrated by reverse osmosis 3, and the concentrated water of reverse osmosis 3 is used for ash injection treatment in the power plant;

[0067] Step 10: The produced water from the three reverse osmosis processes is further subjected to advanced oxidation to reduce COD and then reused in the system.

[0068] Step 11, the ion preparation concentrated water obtained in step 7 is crystallized using crystallization technology, the mother liquor of the crystallization system is refluxed to the ion preparation system, and a small amount of concentrated water can be discharged from the crystallization system.

[0069] The wastewater refers to the wastewater containing PVA generated in the production process of PVA fibers. The wastewater enters the subsequent process after being treated by homogenization and pH adjustment.

[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 medium filtration refers to using one or a combination of manganese sand, magnetite, ceramsite, quartz sand, activated carbon or fly ash as the filtration medium; the particle size of the medium particles gradually increases from top to bottom, the particle size of the top layer is 0.4-0.6mm, the particle size of the middle layer is 0.6-1.6mm, and the particle size of the bottom layer is 2-4mm;

[0073] The ultrafiltration treatment in step 3 is carried out using, but not limited to, hollow fiber ultrafiltration membrane, tubular ultrafiltration membrane, plate ultrafiltration membrane, etc., and the material is not limited to PVDF, PTFE, PES, etc.;

[0074] During the operation, the optimization control steps of the ultrafiltration operation process may also be included, specifically referring to:

[0075] The treatment method adopted in the fourth step is flotation and coagulation sedimentation, and the treated wastewater is returned to the homogenization tank, and the scum or sediment is further treated;

[0076] The reverse osmosis filtration treatment 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, it is necessary to add reagents to the hard filter, and the reagents include but are not limited to sodium carbonate, sodium hydroxide, calcium hydroxide, lime, etc.

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

[0079] The filtering methods adopted by the hardness removal filter in step 6 include cross-flow filtration and dead-end filtration, and the operating pressure is 0.01-1MPa.

[0080] In step 6, the pH of the water produced by the hardness removal 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, sulfuric acid, etc.

[0081] In step 6, the water produced by the hardness removal filter enters the ion exchange resin. The resin used is a strong acid cationic resin. The flow rate of the wastewater in the resin column is 2-5BV / h, and the water produced by the hardness removal filter is less than 5mg / L.

[0082] The ion exchange resin regeneration method in step 6 can adopt salt regeneration or acid-base regeneration.

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

[0084] The treatment adopted in step 8 is ion concentration adjustment, the membrane temperature adopted in ion adjustment is not more than 42°C, and the pressure is not more than 8MPa; the recovery rate of the ion concentration adjustment system is between 70-95%, and the sodium sulfate content of the ion adjustment concentrate is not less than 12wt%;

[0085] The treatment adopted in step 9 is reverse osmosis treatment. The membrane used can be disc-type high-pressure reverse osmosis or high-pressure spiral reverse osmosis. The operating pressure of the system is 6-12MPa. The concentrated liquid is used for ash spraying in power plants.

[0086] The advanced oxidation used in step 10 mainly refers to ozone catalytic oxidation. The catalyst used is a special catalyst, which can increase the ozone utilization efficiency by 30%-50%. The COD content of the produced water of the advanced oxidation system is lower than 5 mg / L, and the produced water is reused;

[0087] The crystallization technology used in step 11 can be evaporative crystallization or frozen crystallization. The purity of sodium sulfate obtained by the system is between 97% and 99%, the whiteness is not less than 82, the sodium sulfate yield is not less than 97%, and the crystallized salt is reused.

[0088] Example 1

[0089] Water quality after homogenization: water volume is 120m 3 / h, COD is 460mg / L, total hardness is 85mg / L, turbidity is 15NTU, inlet TDS is 4200mg / L, sodium sulfate is 3900mg / L, sodium chloride is 210mg / L, pH is 4.2. After adjusting the pH of the wastewater to 8 and using multi-media filtration, the turbidity in the wastewater is 0.5NTU. The multi-media effluent is filtered through a tubular ceramic membrane, with an operating pressure of 0.18MPa, a recovery rate of 95%, an effluent turbidity of 0.1NTU, and an effluent SDI of 3. The ultrafiltration effluent is concentrated by reverse osmosis 1, with an operating pressure of 2.5MPa and a recovery rate of 75%. During the operation of the ultrafiltration membrane, the membrane flux is restored by periodic backwashing. The TDS of the clear liquid of reverse osmosis 1 is 120mg / L, the COD is 18mg / L, and the TDS of the concentrated liquid is 31040mg / L. The concentrated liquid of reverse osmosis 1 is filtered by a hardness removal filter, and the hardness of the water produced by the hardness removal filter is 0.3 mg / L. The concentrated liquid of reverse osmosis 1 after hardness removal is further concentrated by reverse osmosis 2, with an operating pressure of 3.5 MPa, a recovery rate of 60%, a TDS of 58,000 mg / L for the concentrated liquid, a TDS of 150 mg / L for the clear liquid of reverse osmosis 2, and a COD of 21 mg / L. The concentrated liquid of reverse osmosis 2 is separated by a nanofiltration membrane, with an operating pressure of 6.0 MPa, a sulfate ion in the concentrated liquid reaching 83,000 mg / L, a TDS of 124,300 mg / L, and a TDS of 3,500 mg / L for the clear liquid of nanofiltration. The concentrated liquid of nanofiltration is crystallized by freeze crystallization. The freeze crystallization temperature is controlled at -3°C, and the obtained Glauber's salt is 1.1 t / h, containing about 0.5 t of water; the purity of sodium sulfate is 98.5%, and the whiteness is 84. The system produces about 5m 3 / h crystallization mother liquor is returned to the nanofiltration system. The nanofiltration clear liquid is further concentrated and reduced by reverse osmosis 3. The TDS and COD of the water produced by reverse osmosis 3 are 210mg / L and 210mg / L respectively. The concentrated liquid of reverse osmosis 3 enters the power plant for ash spraying. The clear liquid produced by the three-stage reverse osmosis is further reduced in COD by ozone catalytic oxidation technology to ensure that the COD of the recycled water system is less than 5mg / L, the salt content does not exceed 280mg / L, and the water volume is 115m 3 / h.

[0090] Example 2

[0091] Wastewater quality after homogenization: water volume 150m 3 / h, COD is 550mg / L, total hardness is 110mg / L, turbidity is 20NTU, inlet TDS is 4859mg / L, sodium sulfate is 4282mg / L, sodium chloride is 178mg / L, and pH range is 4.6. Adjust pH to 8.8, and the turbidity is 0.7NTU after multi-media filtration. Multi-media produced water is filtered by PTFE tubular ultrafiltration membrane, the operating pressure is 0.2MPa, the recovery rate is 94%, the effluent turbidity is 0.1NTU, and the effluent SDI is 2.5. The ultrafiltration effluent is concentrated by reverse osmosis 1, the operating pressure is 2.3MPa, the recovery rate is 60%, and the ultrafiltration membrane is used to recover the membrane flux through periodic backwashing during operation. The TDS of the clear liquid of reverse osmosis 1 is 80mg / L, the COD is 14mg / L, and the TDS of the concentrated liquid is 28040mg / L. The concentrated liquid of reverse osmosis 1 is filtered by a hardness removal filter, and the hardness of the water produced by the hardness removal filter is 0.3 mg / L. After hardness removal, the concentrated liquid of reverse osmosis 1 is further concentrated by reverse osmosis 2, with an operating pressure of 3.5 MPa, a recovery rate of 75%, a TDS of 57200 mg / L for the concentrated liquid, a TDS of 136 mg / L for the clear liquid of reverse osmosis 2, and a COD of 21 mg / L. The concentrated liquid of reverse osmosis 2 is separated by a nanofiltration membrane, with an operating pressure of 6.5 MPa, a sulfate ion in the concentrated liquid reaching 81500 mg / L, a TDS of 122030 mg / L, and a TDS of 3300 mg / L for the clear liquid of nanofiltration. The concentrated liquid of nanofiltration is crystallized by freeze crystallization. The freeze crystallization temperature is controlled at 0°C, and the obtained sodium sulfate is 0.8 t / h, with a water content of about 0.3 t; the purity of sodium sulfate is 98.1%, and the whiteness is 82. The system produces about 6m 3 / h crystallization mother liquor is returned to the nanofiltration system. The nanofiltration clear liquid is further concentrated and reduced by reverse osmosis 3. The TDS and COD of the water produced by reverse osmosis 3 are 210mg / L and 210mg / L respectively. The concentrated liquid of reverse osmosis 3 enters the power plant for ash spraying. The clear liquid produced by the three-stage reverse osmosis is further reduced in COD by ozone catalytic oxidation technology to ensure that the COD of the recycled water system is less than 5mg / L, the salt content does not exceed 3000mg / L, and the water volume is 110m 3 / h.

[0092] Example 3

[0093] Establishment of the operation optimization control steps of ultrafiltration membrane:

[0094] During the operation of the ultrafiltration membrane, the flux can be restored by backwashing. During backwashing, the particle filter cake deposited on the membrane surface has a good removal effect and a high degree of recovery, while the removal effect of organic pollutants on the membrane surface is low. In addition, during backwashing, the backwashing cycle (f) can be set, such as backwashing every 15min / 25min / 30min, etc., and the time (T) for each backwashing can also be set, such as 5s / 10s / 15s / 20s, etc. If the backwashing cycle is long, it is easy to cause more pollution deposition on the membrane surface, affecting the backwashing recovery rate. If the cycle is short, on the one hand, it affects the treatment efficiency, and on the other hand, it also leads to excessive consumption of backwashing water. As for the backwashing time, the shorter the time, the more likely it is to lead to insufficient recovery, and the longer the time, the more problems will arise in the treatment efficiency and flushing water consumption. This patent first establishes an empirical formula for predicting the pure water flux recovery rate after backwashing, and conducts multiple groups of preliminary operation tests under different raw water quality conditions.

[0095] The COD range of the wastewater is 220-410 mg / L, the total hardness range is 122-156 mg / L, the turbidity range is 12-24 NTU, the sodium sulfate range is 3520-3780 ppm, the sodium chloride range is 240-374 ppm, and the pH range is 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. The membrane surface flow rate during the operation 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 turned on. 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 when filtering before this backwashing is calculated. After fitting with the empirical formula, the formula for flux recovery rate and backwashing time / frequency is obtained:

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

[0097] The relationship between the theoretical value and the experimental value is as follows Figure 2As shown, it can be seen that the theoretical value and the experimental value are basically consistent, but there are still large differences at some points. After analysis, it is believed that the different types of membrane fouling caused by PVA particles and small organic molecules on the surface will lead to differences in cleaning recovery rates. If there are more PVA particles, it is easy to recoil and remove them, and if there are more small organic molecule pollutants, the recoil cleaning rate will decrease; the concentration of PVA particles will significantly affect the turbidity of the feed solution. At the same time, since PVA particles are not easily oxidized in COD detection and have stable properties, it is difficult for them to affect the COD value; while small organic molecules mainly affect the COD value, and have a certain effect on turbidity detection, but it is not very significant. Therefore, the flux calculation of the turbidity and COD values ​​in the feed and permeate can represent the degree of deposition of PVA particles and organic matter in the solution on the membrane surface, that is: the turbidity removal rate mainly reflects the degree of deposition of PVA particles on the membrane surface, while the COD removal rate mainly reflects the degree of deposition of organic matter on the membrane surface. A ratio of the amount of PVA particles and organic matter deposited on the membrane surface is obtained by the following formula:

[0098] Ratio A=(R 1 -R 2 ) / R2,R 1 Refers to the turbidity removal rate, R 2 Refers to COD removal rate. Turbidity removal rate is obtained by online turbidity meter detection, COD removal rate is obtained by online COD detector detection.

[0099] R 1 Small amount of changes caused by organic matter R 2 After deduction, the numerator can basically represent the amount caused by PVA particle deposition on the membrane surface, while the denominator represents the amount of organic matter deposition. After comparison, it is found that if the A value is too large, the PVA particles are deposited more, which will lead to a high backwash recovery rate, while if the A value is too small, the amount of organic matter deposition is large and the actual backwash recovery amount is small; therefore, when predicting the backwash pre-recovery rate, a correction term is introduced into the above calculation empirical formula, that is:

[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; for other values, L=1;

[0102] After the correction of the predicted value, the comparison between the predicted value and the experimental value is as follows: Figure 3 As shown, it can be seen that the degree of deviation is significantly reduced, and the correlation coefficients between the predicted value and the experimental value before and after correction are 0.755 and 0.849, respectively.

[0103] The above correction values ​​are estimated and set through experience, and can also be used to perform data fitting regression after accumulating more data to further improve their accuracy.

[0104] After determining the above calculation equation for the backwash recovery rate, during 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 for PVA fiber production wastewater, characterized in that: The steps include: Step 1, a step of pre-treating wastewater; Step 2, treating the wastewater obtained in step 1 with an ultrafiltration membrane; Step 3, filtering the produced water of the ultrafiltration membrane with a first reverse osmosis membrane to obtain a first concentrated liquid and a first filtrate; Step 4, the first concentrated liquid is concentrated by a second reverse osmosis membrane to obtain a second concentrated liquid and a second filtrate; Step 5, filtering the second concentrated liquid through a nanofiltration membrane to obtain a nanofiltration permeate and a nanofiltration concentrated liquid, so as to separate the monovalent ions from the divalent ions; Step 6, the nanofiltration concentrated liquid is crystallized to obtain sodium sulfate, and the nanofiltration permeate is concentrated by a third reverse osmosis membrane.

2. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: The produced water obtained by the first reverse osmosis membrane, the second reverse osmosis membrane or the third reverse osmosis membrane is reused after advanced oxidation treatment; during the operation of the first reverse osmosis membrane and the second reverse osmosis membrane, the water recovery rate is 60-75%, and the operating pressure ranges from 1 to 4Mpa; during the operation of the third reverse osmosis membrane, the membrane form can be a disc-type high-pressure reverse osmosis or a high-pressure coil reverse osmosis, and the operating pressure of the system is 6-12MPa.

3. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: 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.

4. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: The pretreatment comprises: one of homogenization, multi-media filtration, and pH adjustment; the multi-media filter is a filter using one or a combination of manganese sand, magnetite, ceramsite, quartz sand, activated carbon or fly ash as filter material; the particle size of the medium particles gradually increases from top to bottom, the particle size of the top layer is 0.4-0.6 mm, the particle size of the middle layer is 0.6-1.6 mm, and the particle size of the bottom layer is 2-4 mm.

5. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: The hardness removal filter can be one of an ion exchange resin column, an electric deionization system, a magnetic water quality processor, a salt crystallization water softener, an electronic descaling device, and a chemical dosing precipitation filtration device.

6. The zero-discharge treatment method for PVA fiber production wastewater according to claim 5, characterized in that: The hardness removal filter is a chemical dosing precipitation filter and an ion exchange resin column, and the added agent includes but is not limited to sodium carbonate, sodium hydroxide, calcium hydroxide, lime, etc., the dosage of the agent exceeds the theoretical value of 0-200 mg / L, and the pH of the wastewater is not less than 11.3 after the addition of the agent; During the filtration process, the filtration methods include cross-flow filtration and dead-end filtration, with an operating pressure of 0.01-1Mpa; and, the pH of the water produced by the hardness removal filter needs to be adjusted to between 6-9, and the acid added for adjusting the pH includes but is not limited to hydrochloric acid, sulfuric acid, etc.; the resin used in the ion exchange resin is a strong acid cationic resin, and the flow rate of the wastewater in the resin column is 2-5BV / h, and the water produced by the hardness removal filter is less than 5mg / L.

7. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: The membrane configuration in the ultrafiltration treatment includes one of a hollow fiber ultrafiltration membrane, a tubular ultrafiltration membrane, and a plate ultrafiltration membrane, and the material is selected from one of PVDF, PTFE, and PES. During the operation of the ultrafiltration membrane, the membrane flux is recovered by periodic backwashing, and the flux recovery rate is predicted by theoretical calculation. If the calculated recovery rate is high, the backwashing time during the operation 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; The theoretical calculations described above predict the flux recovery rate R 修正 It is calculated by the following formula: R 修正 =R×L, where L is the correction factor; R%=α×T / (f β +γ)+ζ, α, β, γ, ζ are equation coefficients; T is the backwash time, s, f is the backwash cycle, min; If A≥10, L=1.15; if A≤5, L=0.70; for other values, L=1; A=(R1-R2) / R2, R1 refers to the turbidity removal rate, and R2 refers to the COD removal rate.

8. The zero-discharge treatment method for PVA fiber production wastewater according to claim 1, characterized in that: In step 5, the temperature of the nanofiltration membrane does not exceed 42°C, and the pressure does not exceed 8MPa; the recovery rate of the ion concentration preparation system is between 70-95%, and the sodium sulfate content of the ion preparation concentrate is not less than 12wt%; advanced oxidation mainly refers to ozone catalytic oxidation, and the catalyst used is a special catalyst, which can increase the ozone utilization efficiency by 30%-50%, and the COD content of the produced water of the advanced oxidation system is less than 5mg / L, and the produced water is reused.

9. A zero-discharge treatment device for PVA fiber production wastewater, characterized in that: include: A pretreatment device for pre-treating wastewater; Ultrafiltration membrane, used for ultrafiltration treatment of produced water from the pretreatment device; The first reverse osmosis membrane is used for filtering the ultrafiltration water; The second reverse osmosis membrane is used to filter the concentrated liquid of the first reverse osmosis membrane; The nanofiltration membrane is used to filter the concentrated water from the second reverse osmosis membrane to obtain nanofiltration permeate and nanofiltration concentrate, so as to separate monovalent ions from divalent ions. A crystallizer, used for crystallizing the nanofiltration concentrate to obtain sodium sulfate; The third reverse osmosis membrane is used for concentrating the nanofiltration permeate.

10. The zero-discharge treatment device for PVA fiber production wastewater according to claim 9, characterized in that: The pretreatment device is used for homogenizing, multi-media filtering and pH adjustment of wastewater; it also includes a hardness removal filter for removing hardness from the concentrated water of the first reverse osmosis membrane and supplying the produced water to the second reverse osmosis membrane for treatment; the hardness removal filter is one of an ion exchange resin column, an electrodeionization system, a magnetic water quality processor, a salt crystallization softener, an electronic descaling device, and a chemical dosing precipitation filtration device.

Images