A method for treating high-salinity wastewater

Abstract

The application provides a high-salt wastewater treatment method, and belongs to the technical field of industrial wastewater treatment. The high-salt wastewater treatment method comprises the following steps: S1, sequentially pretreating and ultrafiltering wastewater to obtain ultrafiltration water; S2, performing desalination treatment on the ultrafiltration water through a nanofiltration membrane to obtain a nanofiltration permeate and a nanofiltration concentrate; S3, performing evaporation crystallization treatment on the nanofiltration permeate to obtain sodium chloride crystals and evaporation condensate; S4, sequentially performing chemical softening, electrocatalytic oxidation and biochemical treatment on the nanofiltration concentrate to obtain purified concentrate; and S5, combining the purified concentrate of step S4 with the evaporation condensate of step S3, and discharging after deep treatment. The application can stably realize efficient treatment of high-salt, high-hardness and high-organic-matter wastewater such as salted pepper water, and has environmental and economic benefits.

Description

Technical Field

[0001] This application relates to the field of industrial wastewater treatment technology, and in particular to a method for treating high-salinity wastewater. Background Technology

[0002] The chili processing industry generates large amounts of saline wastewater, commonly known as salted chili water, during salting and pickling processes. This type of wastewater is characterized by high salinity, high hardness, and high organic matter content: total dissolved solids (TDS) typically reach 40,000-60,000 mg / L, calcium and magnesium ion concentrations are 400-800 mg / L and 100-300 mg / L, respectively, and chemical oxygen demand (COD) can reach 8,000-15,000 mg / L. Furthermore, the wastewater contains capsaicin, oils, suspended debris, and other special components, making its treatment far more difficult than that of general industrial saline wastewater.

[0003] Currently, the main process for treating this type of high-salinity wastewater is "pretreatment + evaporation and crystallization". Pretreatment removes grease and suspended solids using methods such as dissolved air flotation. Then, the wastewater is directly fed into an evaporator for evaporation, concentration, and crystallization, producing mixed salts. The condensate is discharged after simple treatment, while the crystallized mixed salts are disposed of as hazardous waste by an external contractor.

[0004] However, in actual operation, the aforementioned traditional processes suffer from several drawbacks. High concentrations of calcium and magnesium ions in the raw water cause rapid formation of scale, primarily composed of calcium carbonate and calcium sulfate, on the heat exchange surfaces upon direct entry into the evaporation system. This leads to a sharp decline in heat exchange efficiency, requiring frequent evaporator shutdowns for cleaning, typically with a cleaning cycle of only a few weeks, severely impacting equipment capacity and lifespan. Furthermore, the precipitated solids are mixed salts with low purity, containing large amounts of organic matter and calcium and magnesium impurities, resulting in high treatment costs and wasted sodium chloride resources. In addition, capsaicin and other organic compounds in chili processing wastewater exhibit volatility and are difficult to biodegrade. During evaporation, some volatile organic compounds enter the condensate with the steam, causing COD levels in the condensate to exceed standards. This necessitates additional advanced oxidation or biological treatment units, increasing process complexity and investment. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a method for treating high-salinity wastewater.

[0006] Specifically, the first aspect of this application provides a method for treating high-salinity wastewater, including the following steps: S1: The wastewater is pretreated and ultrafiltered sequentially to obtain ultrafiltration permeate; S2: The ultrafiltration permeate is passed through a nanofiltration membrane for salt separation to obtain nanofiltration permeate and nanofiltration concentrate, wherein the nanofiltration membrane has a rejection rate of more than 90% for divalent ions and a rejection rate of less than 15% for monovalent chlorides; S3: The nanofiltration permeate is evaporated and crystallized to obtain sodium chloride crystals and evaporated condensate; S4: The nanofiltration concentrate is subjected to chemical softening, electrocatalytic oxidation and biochemical treatment in sequence to obtain purified concentrate; S5: Combine the purified concentrate from step S4 with the evaporated condensate from step S3, and discharge it after further treatment to meet the standards.

[0007] Further, the pretreatment in step S1 includes dissolved air flotation, adjusting the pH of the flotation influent to 7.0-8.0, and adding 200-400 mg / L of polyaluminum chloride and 2-5 mg / L of anionic polyacrylamide; The ultrafiltration uses an ultrafiltration membrane with a molecular weight cutoff of 100-200 kDa, an operating pressure of 0.1-0.3 MPa, and a product water SDI of less than 3.

[0008] Furthermore, the nanofiltration membrane described in step S2 is a porous polyamide composite nanofiltration membrane with an operating pressure of 1.5-2.5 MPa.

[0009] Furthermore, a scale inhibitor is added to the nanofiltration inlet side, wherein the scale inhibitor is a polyaspartic acid or phosphonic acid-based carboxylic acid copolymer, and the dosage is 1-5 mg / L.

[0010] Furthermore, the evaporation and crystallization temperature in step S3 is 60-80℃, and trisodium phosphate is added to the evaporation feed as a scale inhibitor at a dosage of 0.1-0.5 mg / L; During the evaporation and crystallization process, the mother liquor is periodically discharged and incorporated into the nanofiltration concentrate of step S4 for further treatment.

[0011] Further, the chemical softening in step S4 specifically involves: adding sodium hydroxide to the nanofiltration concentrate to adjust the pH to 10.5-11.0, adding sodium carbonate to precipitate calcium and magnesium ions, and then adding a coagulant to perform solid-liquid separation to obtain softened effluent with a hardness of less than 80 mg / L based on calcium carbonate.

[0012] Furthermore, in step S4, the current density of the electrocatalytic oxidation is 10-30 mA / cm², and the hydraulic retention time is 40-90 min; the effluent from the electrocatalytic oxidation is dechlorinated and then enters the biochemical treatment.

[0013] Furthermore, the biochemical treatment in step S4 employs a salt-tolerant moving bed biofilm reactor with a filling rate of 30%-50% and a packing material specific surface area greater than 500 m². 2 / m 3 Inoculate with halophilic bacteria, with a hydraulic retention time of 24-48 h and dissolved oxygen of 2-4 mg / L.

[0014] Furthermore, the purified concentrate and the evaporated condensate described in step S5 are combined and then subjected to advanced treatment, including activated carbon adsorption. The final effluent COD is no greater than 50 mg / L and suspended solids are no greater than 10 mg / L.

[0015] Furthermore, it also includes a sludge treatment step: collecting the scum and sludge generated from pretreatment, the sludge generated from chemical softening, and the residual sludge generated from biochemical treatment, and then mechanically dewatering them. The filtrate generated from dewatering is returned to the pretreatment, and the dewatered sludge cake is transported off-site for disposal.

[0016] Furthermore, the wastewater is salt chili water produced during the chili processing, with a total dissolved solids of 40,000-60,000 mg / L, a calcium ion concentration of 400-800 mg / L, and a chemical oxygen demand of 8,000-15,000 mg / L.

[0017] The present invention has the following beneficial effects: This invention utilizes nanofiltration to separate most of the monovalent sodium chloride from the raw water through the membrane into the permeate, while divalent scale-forming ions and most organic matter are retained in the concentrate. This significantly reduces the calcium and magnesium hardness entering the evaporation and crystallization system, alleviating scale buildup in the evaporator at its source and extending the cleaning cycle. Furthermore, the high purity of sodium chloride in the nanofiltration permeate allows the sodium chloride crystals obtained from evaporation and crystallization to meet industrial salt standards, enabling recycling and avoiding the high costs associated with disposing of mixed salts as hazardous waste. Simultaneously, for the calcium and magnesium ions and organic matter retained in the nanofiltration concentrate, this method first removes calcium and magnesium hardness through chemical softening, then breaks down recalcitrant organic matter such as capsaicin through electrocatalytic oxidation, followed by further COD reduction through halophilic bacteria biochemical treatment. The treated effluent is then combined with the nanofiltration permeate for evaporation, preventing long-term accumulation of organic matter and reducing the problem of volatile organic compounds entering the condensate and causing excessive condensate levels. The overall process can reliably and efficiently treat high-salt, high-hardness, and high-organic-content wastewater such as salt-chili water, offering both environmental and economic benefits. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0019] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0020] An embodiment of the first aspect of this application provides a method for treating high-salinity wastewater, comprising the following steps: S1: The wastewater is pretreated and ultrafiltered sequentially to obtain ultrafiltration permeate; S2: The ultrafiltration permeate is passed through a nanofiltration membrane for salt separation to obtain nanofiltration permeate and nanofiltration concentrate, wherein the nanofiltration membrane has a rejection rate of more than 90% for divalent ions and a rejection rate of less than 15% for monovalent chlorides; S3: The nanofiltration permeate is evaporated and crystallized to obtain sodium chloride crystals and evaporated condensate; S4: The nanofiltration concentrate is subjected to chemical softening, electrocatalytic oxidation and biochemical treatment in sequence to obtain purified concentrate; S5: Combine the purified concentrate from step S4 with the evaporated condensate from step S3, and discharge it after further treatment to meet the standards.

[0021] In this embodiment, the wastewater mentioned in step S1 is salt chili water produced during the chili processing, with a total dissolved solids of 40,000-60,000 mg / L, a calcium ion concentration of 400-800 mg / L, and a chemical oxygen demand of 8,000-15,000 mg / L.

[0022] The wastewater stays in the equalization tank for 12 hours. The equalization tank is equipped with one underwater mixer with a power of 1.5 kW to balance the water quality and quantity.

[0023] The pretreatment includes dissolved air flotation (DAF). The influent to the FAF is treated with a 10% hydrochloric acid or 10% sodium hydroxide solution, controlled by an online pH meter to maintain the pH at 7.0-8.0. Flocculant 200-400 mg / L of polyaluminum chloride and coagulant aid anionic polyacrylamide (PAM, molecular weight 12 million) 2-5 mg / L are added. The dissolved air tank pressure is 0.5 MPa, and the dissolved air-water reflux ratio is 25%. The upward flow velocity in the contact zone is 20 mm / s, and the surface loading in the separation zone is 6 m³. 3 / (m 2•h), retention time 18 min. Control effluent oil content ≤10 mg / L, SS ≤50 mg / L. Scum and bottom heavy sludge are discharged into the sludge system; the treated bulk liquid is pumped into the ultrafiltration unit.

[0024] The ultrafiltration device uses a hydrophilic PVDF hollow fiber membrane with a molecular weight cutoff of 100-200 kDa and a single membrane area of ​​50 m². 2 The permeate has an SDI of less than 3 and a turbidity of less than 0.5 NTU, and then enters the nanofiltration system.

[0025] In this embodiment, the nanofiltration system in step S2 uses a porous polyamide composite nanofiltration membrane (NF270), which has a NaCl rejection rate of <10% and a Ca rejection rate of <10%. 2+ Mg 2+ Retention rate > 95%. Single membrane area 37m² 2 Operating pressure: 1.5-2.5 MPa.

[0026] A scale inhibitor, specifically polyaspartic acid or phosphonic acid-based carboxylic acid copolymer, is added to the nanofiltration feed water side at a dosage of 1-5 mg / L. Preferably, the scale inhibitor is polyaspartic acid (PASP, molecular weight 3000), prepared at a concentration of 5%, and added continuously to the outlet of the security filter to prevent slight supersaturation on the concentrate side. The operating pressure is 1.6-2.0 MPa, and the temperature is 25-30℃. The system recovery rate is 75%, i.e., permeate permeate yield is 0.625 m³. 3 / h (15 m) 3 / d), concentrate 0.208 m 3 / h (5m) 3 / d).

[0027] NF permeate (15 m 3 / d): TDS 48,000 mg / L, Na + 19200 mg / L, Ca 2+ 20 mg / L, Mg 2+ 8 mg / L, COD 160 mg / L. Pumped directly into the evaporator feed tank; NF concentrate (5 m 3 d): TDS 56,000 mg / L, Ca 2+ 2320 mg / L, Mg 2+ 768 mg / L, COD 38,400 mg / L. Discharged into a concentrate tank (the tank, after cleaning, is specifically for temporary storage of the concentrate; effective volume ≥ 8 m³). 3 (Assume stirring).

[0028] In this embodiment, step S3 uses a feed pump to deliver 15m 3The permeate from the NF reactor is fed into the evaporator. Trisodium phosphate is added to the evaporation feed as a scale inhibitor at a dosage of 0.1-0.5 mg / L; a polyether defoamer is added intermittently at 5 mg / L. The evaporation and crystallization temperature is 60-80℃. The solution is concentrated until NaCl is supersaturated, with a solids content of 25%. It is then discharged into a crystallization tank for 30 minutes to precipitate 0.5-1.0 mm crystals. The crystals are separated by centrifugation, with a filter cake moisture content of 3%. After air-jet drying, the industrial-grade NaCl yield is 0.73 t / d (based on a permeate salt content of 720 kg / d and a recovery rate of 95%), with a purity ≥95% and whiteness ≥70. This is sold as a byproduct. To prevent organic matter accumulation from affecting salt quality, 5% is discharged daily from the circulating mother liquor and combined with the NF concentrate in the nanofiltration concentrate tank of step S4 for further treatment. The evaporation condensate production is approximately 14.8 m³. 3 / d, TDS<80 mg / L, COD<40 mg / L. All water flows into the clear water tank by gravity.

[0029] In this embodiment, step S4 involves collecting the NF concentrate and evaporation mother liquor in a concentrate tank, and using a stirrer to equalize the water quality. The mixed liquor quality is: TDS ≈ 58,000 mg / L, Ca... 2+ ≈2,100 mg / L, COD≈35,000 mg / L.

[0030] The chemical softening process specifically involves: adding 30% NaOH solution to the nanofiltration concentrate to adjust the pH to 10.5-11.0, with the dosage controlled by an online pH meter; adding 20% ​​Na2CO3 solution to precipitate calcium and magnesium ions; and then adding anionic polyacrylamide (PAM) as a coagulant aid for solid-liquid separation to obtain softened effluent with a hardness of less than 80 mg / L (calculated as calcium carbonate). The sludge, consisting of calcium carbonate and magnesium hydroxide precipitates, is discharged into a wastewater tank.

[0031] In this embodiment, the electrocatalytic oxidation in step S4 uses a ruthenium-based oxide-coated titanium anode (manufactured by Shaanxi Youchuang Technology), a titanium plate as the cathode, an electrode spacing of 12 mm, and an effective electrode area of ​​10 m². 2 No external electrolyte is required; the wastewater itself has a high TDS (~58,000 mg / L) for conductivity. Sodium sulfite is added to the electrocatalytic effluent, with the dosage controlled according to the ORP (Original Rate of Return) to ensure ORP < 200 mV for residual chlorine removal. The current density for electrocatalytic oxidation is 10-30 mA / cm². 2 The tank voltage is approximately 5.5 V, and the hydraulic retention time is 40-90 min. The COD of the effluent is reduced from 30,000 mg / L to approximately 20,000 mg / L. The effluent from the electrocatalytic oxidation is dechlorinated and then enters the biochemical treatment process.

[0032] In this embodiment, step S4 employs an anoxic-aerobic (A / O) two-stage moving bed biofilm reactor (MBBR). The anoxic stage utilizes denitrification (if nitrate recirculation is present) or facultative hydrolysis to further improve biodegradability, while the aerobic stage is the primary degradation stage. Because the total nitrogen in the influent is not high (mainly supplemented with urea), stage A can be set as a facultative selective stage to inhibit filamentous bacteria and enhance shock resistance.

[0033] The suspended filler is made of high-density polyethylene (HDPE, molecular weight 200,000), modified with added hydrophilic groups (details of the modification are as follows). The modification method is as follows: under ice-water bath and stirring, concentrated sulfuric acid is slowly added to a saturated potassium dichromate solution to prepare a chromic acid cleaning solution (potassium dichromate: concentrated sulfuric acid: water ≈ 5g: 100mL: 10mL). The HDPE filler is immersed in the chromic acid cleaning solution and treated at 25~40℃ for 10~30 minutes. The filler is then removed, rinsed with plenty of tap water, and then cleaned three times with deionized water in an ultrasonic cleaner to thoroughly remove residual acid and chromium ions. Finally, it is dried in an oven at 60℃ for 4 hours to obtain the surface-modified filler. The core purpose of this modification is to transform the hydrophobic surface into a hydrophilic surface and introduce active sites to promote the initial attachment and rapid biofilm formation of halophilic microorganisms.

[0034] The C / N / P balance in the electrocatalytic effluent is unbalanced, necessitating the replenishment of nitrogen and phosphorus. Urea and potassium dihydrogen phosphate should be added according to a BOD5:N:P ratio of 100:5:1, adjusted based on the effluent conditions. Chemicals to be added: Urea, requiring 4.75 kg / day per day, prepared as a 10% solution and continuously added to the inlet of the anoxic tank using a metering pump; Potassium dihydrogen phosphate, requiring 1.92 kg / day per day, prepared as a 5% solution and added at the same location. A commercially available halophilic compound bacterial agent (manufactured by Airini (Shandong) Environmental Protection Technology Co., Ltd.) should be purchased. This compound bacterial group includes introduced halophilic archaea and salt-tolerant denitrifying bacteria. During the initial water injection, clean water is injected into the reaction tank, and the salinity is adjusted to 50,000-60,000 mg / L using industrial NaCl (matching the influent TDS). Urea and potassium dihydrogen phosphate are added according to the calculated dosage. Commercial bacterial agents are added to maintain the DO in the aerobic tank at 3-4 mg / L, the temperature at 28-30℃, and the retention time at 36 hours. Starting from the second day, the daily influent flow rate is increased in steps of 20%, 40%, 60%, 80%, and 100% of the design flow rate, maintaining each gradient for 2-3 days. The decision to increase the flow rate is based on whether the effluent COD drops to the target range and there is no acute inhibition. During this process, microorganisms naturally attach to the packing material to form a biofilm. A pale yellow, semi-transparent film is generally visible to the naked eye on days 7-10, and the film thickness reaches 0.5-1 mm on days 15-20, with a stable COD removal rate. Effluent parameters: COD ≤ 80 mg / L, NH3-N ≤ 5 mg / L, SS ≤ 30 mg / L. The effluent flows by gravity into the clear water pool.

[0035] The clear water tank receives two streams of water—evaporation condensate (COD 40 mg / L) and biologically treated effluent (COD 80 mg / L), resulting in a combined COD of approximately 51 mg / L. The clear water tank utilizes an activated carbon filter, with the addition of 10 mg / L of powdered activated carbon or a trace amount of hydrogen peroxide for advanced treatment, ensuring that the final effluent has a COD ≤ 50 mg / L and SS ≤ 10 mg / L. The treated effluent meets these standards and is stored in clear water tanks for metered discharge.

[0036] The sludge treatment system collects flotation scum and bottom sludge, chemically softened sludge, and residual sludge from biological treatment. A screw press is used for sludge pressing, with PAM added at 4 mg / L for conditioning, a pressing pressure of 0.8 MPa, and the sludge cake moisture content ≤58%, which is then transported off-site for disposal. All filtrate is recycled to the equalization tank.

[0037] Example The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0038] The following examples and comparative examples all use 20 m 3 The treatment target is salt and chili water; influent water quality: TDS 50000 mg / L, Ca... 2+ 600 mg / L, Mg 2+ 200 mg / L, COD 12000 mg / L, pH 5.8, capsaicin content approximately 8000 SHU.

[0039] Example 1 A method for treating high-salinity wastewater includes the following steps: S1: Raw water enters the equalization tank for 6 hours to achieve equilibrium, then is pumped into the dissolved air flotation (DAF) system. The DAF influent is passed through a 10% sodium hydroxide solution, with the pH maintained at 7.5 by an online pH meter interlock control. 250 mg / L of polyaluminum chloride flocculant and 2 mg / L of anionic polyacrylamide (PAM, molecular weight 12 million) coagulant aid are added. The dissolved air tank pressure is 0.5 MPa, and the dissolved air-water reflux ratio is 25%. The upward flow velocity in the contact zone is 20 mm / s, and the surface loading in the separation zone is 6 m³. 3 / (m 2•h), retention time 18 min. Control effluent oil content 8 mg / L, SS 35 mg / L. Scum and bottom heavy sludge are discharged into the sludge system; treated flotation effluent is pumped into the ultrafiltration unit; The effluent from the air flotation process enters a PVDF ultrafiltration membrane with a molecular weight cutoff of 150 kDa. The transmembrane pressure difference is 0.2 MPa, the recovery rate is 90%, and the SDI of the permeate is 2.5. S2: Ultrafiltration permeate enters the nanofiltration membrane (NF270 type), operating pressure 1.8 MPa, recovery rate 75%. Nanofiltration permeate 15 m³ / d (Ca 2+ 20 mg / L, COD 160 mg / L, NaCl permeability 93%), nanofiltration concentrate 5 m³ / d (Ca 2+ 2320 mg / L, COD 38400 mg / L); add 2 mg / L of polyaspartic acid scale inhibitor; S3: Use a feed pump to feed 15m 3 The permeate from the nanofiltration process (NF) is fed into the evaporator. Trisodium phosphate (0.3 mg / L) is added to the evaporation feed as a scale inhibitor; a polyether defoamer (5 mg / L) is added intermittently. The evaporation crystallization temperature is 68℃. The solution is concentrated until NaCl is supersaturated, with a solids content of 25%. It is then discharged into a crystallization tank for 30 minutes to allow crystals to precipitate. The crystals are separated using a centrifuge, with a filter cake moisture content of 3%. After further airflow drying, the industrial-grade NaCl yield is 0.72 t / d, with a purity of 96.2% and a whiteness of 73. 5% of the circulating mother liquor is discharged daily and combined with the concentrate in the nanofiltration concentrate tank of step S4 for further treatment. The evaporation condensate production is approximately 14.8 m³. 3 / d, TDS75 mg / L, COD 38 mg / L; S4: Mix 5 m³ / d of nanofiltration concentrate with 0.75 m³ / d of evaporated mother liquor (TDS ~300 g / L) to form 5.75 m³ / d, and then perform chemical softening, electrocatalytic oxidation, and biochemical treatment sequentially. For chemical softening, add 30% NaOH solution to adjust the pH to 10.8, add 38.5 kg / d of 20% Na₂CO₃ solution, and 1 mg / L of anionic polyacrylamide. The effluent after precipitation has a hardness (CaCO₃) of 72 mg / L, a COD of 30000 mg / L, and a B / C ratio of 0.09. Softened effluent enters the electrocatalytic reactor, using a ruthenium oxide-coated titanium anode (DSA electrode) and a titanium plate as the cathode. The current density is 20 mA / cm², the cell voltage is approximately 5.5 V, and the hydraulic retention time is 60 min. After dechlorination (sodium sulfite to ORP 180 mV), the COD decreases to 19800 mg / L, and the B / C ratio increases from 0.09 to 0.39. The effluent from the electrocatalytic reactor enters a two-stage moving bed biofilm reactor (MBBR) with a salt-tolerant anoxic-aerobic (A / O) configuration. Halophilic bacteria are inoculated and form a biofilm. The suspended packing material is high-density polyethylene (HDPE) with a filling rate of 40%. The aerobic tank maintains a DO concentration of 3 mg / L, a temperature of 28℃, and a hydraulic retention time of 36 h. The effluent COD is 76 mg / L, and NH3-N is 3.2 mg / L. S5: Combine the purified concentrate from step S4 with the evaporation condensate from step S3, add 10 mg / L of powdered activated carbon, contact for 30 min, and then filter. The resulting effluent from the clear water tank has a COD of 46 mg / L and SS of 8 mg / L, meeting discharge standards. The sludge is pressed using a screw press, with PAM added at a conditioning level of 4 mg / L, a pressing pressure of 0.8 MPa, and a sludge cake moisture content of 56%, and then transported for disposal.

[0040] Results: After 180 days of continuous operation, the evaporator heat transfer coefficient decreased by less than 5%, the salt production purity remained stable, the membrane flux was maintained at 92% of the initial value, and the total operating cost (including electricity and chemicals) was 28.6 yuan / ton of water.

[0041] Example 2 This embodiment is basically the same as Embodiment 1, except that in step S2, the nanofiltration recovery rate is increased to 80%, the nanofiltration permeate is 16 m³ / d, and the concentrate is 4 m³ / d. (Concentrate Ca) 2+ The concentration of NaOH was increased to 2850 mg / L, and the COD was 46500 mg / L. In step S4, the dosage of NaOH for chemical softening was increased to 48 kg / d, while other conditions remained unchanged.

[0042] Results: The final MBBR effluent COD was 82 mg / L, and the COD of the mixed clear water tank effluent was 49 mg / L. Salt production was 0.75 t / d with a purity of 95.1%. After 150 days of operation, the average flux of the nanofiltration membrane decreased by 10%, which is still acceptable.

[0043] Example 3 This embodiment is basically the same as that of embodiment 1, except that the electrocatalytic current density in step S4 is reduced to 15 mA / cm² and the hydraulic residence time is extended to 75 min.

[0044] Results: The effluent COD decreased from 30,000 mg / L to 21,500 mg / L, with a B / C ratio of 0.32. The MBBR effluent COD was 85 mg / L, and the COD of the mixed clear water tank effluent was 50 mg / L. Energy consumption decreased by approximately 18%, but the biochemical load was slightly higher, requiring periodic addition of a small amount of carbon source (sodium acetate).

[0045] Example 4 This embodiment is basically the same as embodiment 1, except that the hydraulic retention time in the aerobic tank in step S4 is 30 hours.

[0046] Results: The COD of the effluent was 88 mg / L, and the COD of the effluent from the mixed clear water tank was 53 mg / L, which is slightly higher than 50 mg / L. The activated carbon dosage needs to be increased to 15 mg / L to meet the standard.

[0047] Comparative Example 1 Wastewater is directly fed into the evaporator (evaporation temperature 75℃), and 10 mg / L of trisodium phosphate scale inhibitor is added.

[0048] Results: By day 12, significant scaling had occurred on the heat exchange tubes, reducing the heat transfer coefficient by 25%, necessitating a shutdown for acid washing. The produced mixed salts had a purity of only 81.3% and a whiteness of 52, requiring outsourcing as hazardous waste at a cost of 4800 yuan / ton. The evaporation condensate had a COD of 120 mg / L, requiring additional biological treatment. The system could not operate continuously and stably, resulting in a total cost as high as 45 yuan / ton of water.

[0049] Comparative Example 2 This comparative example is basically the same as Example 1, except that the nanofiltration concentrate is not chemically softened, electrocatalytically oxidized and biochemically treated, but is directly mixed with the evaporated condensate and discharged.

[0050] Results: The mixed TDS reached 16000 mg / L and COD 9200 mg / L, far exceeding the emission limits and causing serious pollution. This indicates that the present invention achieves the separation of salt and most organic matter through nanofiltration. The salt produced by permeate evaporation ensures salt purity, and the concentrated liquid is treated separately to meet emission standards. This invention can solve the problems of easy scaling of evaporators, poor salt quality, and failure of concentrated liquid to meet emission standards in the treatment of high-salt organic wastewater. Compared with the treatment method of direct evaporation producing hazardous waste, it significantly reduces the overall treatment cost, can achieve long-term stable operation of the system, and the treated effluent meets the emission requirements, showing good prospects for engineering applications.

[0051] Comparative Example 3 This comparative example is basically the same as Example 1, except that the electrocatalytic oxidation step is omitted in step S4, and the softened water (COD 30000 mg / L, B / C 0.09) is directly fed into the salt-tolerant MBBR for biological treatment.

[0052] Results: After 14 days of operation, the effluent COD was still as high as 15200 mg / L, capsaicin was not degraded, microbial activity in the MBBR was inhibited, sludge settling was poor, and the effluent was turbid. Mixed discharge could not meet the standards.

[0053] Comparative Example 4 This comparative example is basically the same as Example 1, except that the chemical softening step is omitted in step S4, and the hardness (CaCO3) of the nanofiltration concentrate is about 5800 mg / L, and it is directly fed into the electrocatalytic reactor.

[0054] Results: After 24 hours of operation, white calcium scale deposited on the electrode surface, current efficiency decreased by 35%, and effluent COD removal rate dropped to 18%. Subsequently, due to the large influx of calcium ions, calcium salts precipitated on the packing surface of the MBBR, reducing biofilm formation and causing effluent COD to deteriorate to 2100 mg / L. This indicates that chemical softening is a necessary prerequisite for protecting the electrocatalytic electrode and biochemical processes.

[0055] Comparative Example 5 This comparative example is basically the same as Example 1, except that the two-stage moving bed biofilm reactor (MBBR) in step S4 is replaced with a conventional activated sludge process, and no halophilic bacteria are inoculated.

[0056] Results: The sludge rapidly deactivated and expanded, and the system collapsed after 3 days. The COD of the effluent was no different from that of the influent. This indicates that inoculation with a suitable halophilic compound bacterial agent is the core prerequisite for ensuring the stable degradation of pollutants by the biochemical system under high salinity conditions. Ordinary activated sludge cannot maintain its activity in this high salinity environment, let alone effectively degrade recalcitrant organic matter. It is necessary to use specially acclimatized and cultured salt-tolerant bacteria to ensure stable and compliant biochemical treatment results.

[0057] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for treating high-salinity wastewater, characterized in that, Includes the following steps: S1: The wastewater is pretreated and ultrafiltered sequentially to obtain ultrafiltration permeate; S2: The ultrafiltration permeate is passed through a nanofiltration membrane for salt separation to obtain nanofiltration permeate and nanofiltration concentrate; S3: The nanofiltration permeate is evaporated and crystallized to obtain sodium chloride crystals and evaporated condensate; S4: The nanofiltration concentrate is subjected to chemical softening, electrocatalytic oxidation and biochemical treatment in sequence to obtain purified concentrate; S5: Combine the purified concentrate from step S4 with the evaporated condensate from step S3 for further processing.

2. The method for treating high-salinity wastewater according to claim 1, characterized in that, The pretreatment in step S1 includes dissolved air flotation, adjusting the pH of the flotation influent to 7.0-8.0, and adding 200-400 mg / L of polyaluminum chloride and 2-5 mg / L of anionic polyacrylamide; The ultrafiltration uses an ultrafiltration membrane with a molecular weight cutoff of 100kDa-200kDa, an operating pressure of 0.1-0.3 MPa, and a product water SDI of less than 3.

3. The method for treating high-salinity wastewater according to claim 1, characterized in that, The nanofiltration membrane mentioned in step S2 is a porous polyamide composite nanofiltration membrane with an operating pressure of 1.5-2.5 MPa; And / or, a scale inhibitor is added to the nanofiltration inlet side, wherein the scale inhibitor is a polyaspartic acid or phosphonic acid-based carboxylic acid copolymer, and the dosage is 1-5 mg / L.

4. The method for treating high-salinity wastewater according to claim 1, characterized in that, The evaporation and crystallization temperature in step S3 is 60-80℃. Trisodium phosphate is added to the evaporation feed as a scale inhibitor at a dosage of 0.1-0.5 mg / L.

5. The method for treating high-salinity wastewater according to claim 1, characterized in that, The chemical softening in step S4 specifically involves adding sodium hydroxide to the nanofiltration concentrate to adjust the pH to 10.5-11.0, adding sodium carbonate, and then adding a coagulant to perform solid-liquid separation, thereby obtaining softened effluent with a hardness of less than 80 mg / L based on calcium carbonate.

6. The method for treating high-salinity wastewater according to claim 1, characterized in that, The current density of the electrocatalytic oxidation in step S4 is 10-30 mA / cm², and the hydraulic retention time is 40-90 min; the effluent from the electrocatalytic oxidation is dechlorinated and then enters the biochemical treatment.

7. The method for treating high-salinity wastewater according to claim 1, characterized in that, The biochemical treatment described in step S4 involves inoculating halophilic bacteria with a hydraulic retention time of 24-48 h.

8. The method for treating high-salinity wastewater according to claim 1, characterized in that, The purified concentrate and evaporated condensate described in step S5 are combined and then subjected to advanced treatment, including activated carbon adsorption. The final effluent COD is no more than 50 mg / L and suspended solids are no more than 10 mg / L.

9. The method for treating high-salinity wastewater according to any one of claims 1-7, characterized in that, It also includes sludge treatment steps: collecting the scum and sludge generated from pretreatment, the sludge generated from chemical softening, and the residual sludge generated from biochemical treatment, and then mechanically dewatering them. The filtrate generated from dewatering is returned to the pretreatment, and the dewatered sludge cake is transported off-site for disposal.

10. The method for treating high-salinity wastewater according to any one of claims 1-7, characterized in that, The wastewater is salt chili water produced during chili processing, with a total dissolved solids of 40,000-60,000 mg / L, a calcium ion concentration of 400-800 mg / L, and a chemical oxygen demand of 8,000-15,000 mg / L.