Process method for zero discharge of acrylonitrile flue gas desulfurization wastewater

Abstract

The application discloses a process method for zero discharge treatment of acrylonitrile flue gas desulfurization wastewater, which comprises a pretreatment section, a main reaction section and a deep treatment section; the pretreatment section comprises a molybdenum removal unit, a hardness removal and adjustment unit, a tubular microfiltration unit and a sludge concentration, dehydration and drying unit; the main reaction section comprises a bipolar membrane electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, an alkali absorption unit and a methane storage unit; the deep treatment section comprises a Fenton oxidation unit, a nanofiltration unit and a neutralization unit in sequence; the bipolar membrane electrodialysis unit adopts a modified anion exchange membrane, realizes separation of organic matters and salt and concentration of the salt, most of the organic matters are intercepted in the desalination liquid, and most of the salt is converted into sulfuric acid and sodium hydroxide; according to the process method, the wastewater is recycled and utilized, the waste gas tank is stored and can be used as fuel, the salt is converted into sulfuric acid and sodium hydroxide products, and water, gas and solid are integrally resourced.

Description

Technical Field

[0001] This invention relates to a method for treating acrylonitrile flue gas desulfurization wastewater, belonging to the field of wastewater treatment technology. Background Technology

[0002] Acrylonitrile is an important chemical raw material for the three major synthetic materials (fibers, rubber, and plastics), and it has a wide range of applications in the organic synthesis industry and people's economic life. The acrylonitrile production process generally adopts the propylene ammoxidation method. During the production process, the quench tower and the light component removal tower discharge a large amount of wastewater. The wastewater mainly contains pollutants such as acrylonitrile, succinic acid, methyl acrylate, and cyanide. The wastewater is highly toxic and the pollutants are difficult to treat. The simplest treatment method is concentration and incineration. That is, the wastewater is first concentrated by multi-effect evaporation and then incinerated. The flue gas produced by incineration will generate wastewater during the desulfurization and denitrification process. This wastewater is called acrylonitrile flue gas desulfurization wastewater.

[0003] Unlike the dehydration of flue gas from thermal power plants and catalytic cracking, acrylonitrile flue gas dehydration results in higher COD concentrations and higher molybdenum ion concentrations among heavy metal ions, but lower concentrations of other heavy metal ions. This is because thermal power plants use coal as fuel, and catalytic cracking uses oil adhering to the catalyst; both contain a significant amount of impurities, leading to higher heavy metal content in the flue gas dehydration. In contrast, acrylonitrile incineration boilers use wastewater and co-firing gas (dry gas) as fuel. The wastewater is generated from the propylene ammoxidation process, has a high organic content but generally low heavy metal ion concentrations. Furthermore, the dry gas itself contains no heavy metals. Therefore, the heavy metal ion concentrations in acrylonitrile flue gas dehydration are generally low. The high molybdenum ion concentration is due to the use of molybdenum-based catalysts (CN200510023601.5, CN201610327805.6) in the propylene ammoxidation process. During production, catalyst-loaded metal loss is inevitable, resulting in higher molybdenum content in the wastewater.

[0004] The "Emission Standard of Pollutants for Inorganic Chemical Industry" (GB31573-2015) stipulates that the water pollutant emission limit for molybdenum (Mo) ions must not exceed 0.5 mg / L. This is because excessive molybdenum intake can harm human health, disrupt energy metabolism, and cause a series of health problems. Furthermore, excessively high molybdenum levels can also affect the activity of sludge microorganisms. Therefore, the treatment of acrylonitrile flue gas desulfurization wastewater not only needs to address COD and salt levels but also the issue of excessively high molybdenum levels. Simultaneously, the treatment process must balance energy consumption, resource recovery, and zero emissions, making it a challenging overall process. Summary of the Invention

[0005] To address the above challenges, this invention provides a process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater, which achieves advanced treatment of the wastewater and meets reuse standards. Most of the salts are utilized as resources in the form of sulfuric acid and sodium hydroxide, most of the organic matter is utilized as resources in the form of methane, and the heavy metal molybdenum is efficiently removed without the need for additional reagents.

[0006] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater, comprising a pretreatment section, a main reaction section, and a deep treatment section;

[0008] The pretreatment section includes a molybdenum removal unit, a hardening and conditioning unit, a tubular microfiltration unit, and a sludge thickening, dewatering, and drying unit. Wastewater first enters the molybdenum removal unit to remove molybdenum, and the effluent enters the hardening and conditioning unit to remove hardness. After being filtered by the tubular microfiltration unit, it enters the bipolar membrane electrodialysis unit in the main reaction section. The chemical sludge generated by the molybdenum removal unit, the hardening and conditioning unit, and the tubular microfiltration unit enters the sludge thickening, dewatering, and drying unit. The water removed is returned to the inlet of the pretreatment section, and the dried sludge is sent out.

[0009] The main reaction section includes a bipolar membrane electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, an alkaline solution absorption unit, and a methane storage unit. The acid and alkaline solutions produced by the bipolar membrane electrodialysis are collected and reused. The desalinated solution enters the primary anaerobic unit, where it mainly undergoes organic acidification and sulfate reduction reactions, producing hydrogen sulfide gas. It then enters the secondary anaerobic unit, where it mainly undergoes methanogenesis. The generated methane is used as stripping gas and pumped back to the primary anaerobic unit. Most of the hydrogen sulfide produced in the primary anaerobic unit is carried out of the wastewater system and then enters the alkaline solution absorption unit. The purified gas produced by the alkaline solution absorption unit is recovered to a methane storage tank for later use.

[0010] The advanced treatment section includes, in sequence, a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit; the Fenton oxidation unit is the secondary anaerobic unit of the main reaction section; the effluent enters the nanofiltration unit, the concentrate from the nanofiltration unit is returned to the molybdenum removal unit, and after mixing, molybdenum can be removed efficiently; the permeate from the nanofiltration unit enters the neutralization unit, and after adding alkali to adjust it to neutral, the effluent meets the reuse standard.

[0011] The bipolar membrane electrodialysis is composed of anion exchange membrane, cation exchange membrane, and bipolar membrane, wherein the anion exchange membrane is a modified anion exchange membrane, the cation exchange membrane is a general-purpose cation exchange membrane, and the bipolar membrane is a general-purpose bipolar membrane.

[0012] The modified anion exchange membrane is prepared as follows:

[0013] Step a: Add polyvinyl alcohol to water to prepare a polyvinyl alcohol aqueous solution. Then add positively charged amine compounds, hydrophilic modifiers, and crosslinking agents to the above solution respectively. Add inorganic acid to adjust the pH. After mixing and stirring, the casting solution is obtained.

[0014] Step b: Cast the casting solution obtained in step a onto a horizontal panel, dry it, and obtain the base film;

[0015] Step c: After washing the base membrane obtained in step b, place it in an alkaline solution for alkalization, and then soak it in water to obtain a polyvinyl alcohol anion exchange membrane;

[0016] Step d: Dissolve dopamine in Tris-HCl buffer, adjust the pH with hydrochloric acid to obtain dopamine Tris buffer solution;

[0017] Step e: Place the anion exchange membrane obtained in step c in a dopamine Tris buffer solution and add copper sulfate. Stir the reaction while maintaining air circulation. After the reaction is complete, the modified anion exchange membrane is obtained.

[0018] Furthermore, the positively charged amine compound is selected from one or more of 2,3-epoxypropyltrimethylammonium chloride, polyethyleneimine, polyepoxychloropropaneamine, and quaternized chitosan, preferably 2,3-epoxypropyltrimethylammonium chloride; the mass ratio of the positively charged amine compound to polyvinyl alcohol is 0.1:1 to 0.6:1.

[0019] Furthermore, the hydrophilic modifier is selected from one or more of α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin, and the mass ratio of the hydrophilic modifier to polyvinyl alcohol is 0.05:1 to 0.5:1.

[0020] Furthermore, the crosslinking agent is selected from aldehyde or acid crosslinking agents, preferably glutaraldehyde, and the mass ratio of the crosslinking agent to polyvinyl alcohol is 0.01:1 to 0.1:1.

[0021] Furthermore, in step a, the inorganic acid is sulfuric acid, hydrochloric acid, or nitric acid, and the pH of the solution is adjusted to 4-6.

[0022] Furthermore, in step a, the polyvinyl alcohol aqueous solution has a mass fraction of 5% to 15%, and the reaction time with the positively charged amine compound, β-cyclodextrin, and crosslinking agent is 4 to 16 hours.

[0023] Furthermore, the polyvinyl alcohol aqueous solution is prepared by stirring at 60–90°C to dissolve the polyvinyl alcohol in water.

[0024] Furthermore, the drying process described in step b involves first drying at room temperature for 2–6 hours, and then continuing to dry under vacuum at 40–80°C for 4–10 hours.

[0025] Furthermore, the washing described in step c involves washing with water until the solution is neutral.

[0026] Furthermore, the alkalization is performed using a 0.5–3 mol / L sodium hydroxide aqueous solution for 12–24 hours.

[0027] Furthermore, the soaking time in step c is 12 to 24 hours.

[0028] Furthermore, the concentration of the Tris-HCl buffer in step d is 10–50 mmol / L.

[0029] Furthermore, the dopamine concentration in step d is 0.4–4 g / L, and the hydrochloric acid is used to adjust the pH range to 8–9.

[0030] Furthermore, in step e, the concentration of copper sulfate is 1–20 mmol / L, and the stirring reaction time is 1–12 h. The copper sulfate can induce rapid polymerization of dopamine, forming a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane, thereby modifying the membrane surface.

[0031] Furthermore, the modified anion exchange membrane is then stored in a sodium chloride solution. The mass concentration of the sodium chloride solution is 5–20 g / L.

[0032] Those skilled in the art should understand that most organic pollutants present in natural water bodies or wastewater, such as surfactants, humic acids, and proteins, are negatively charged. Dopamine has strong self-polymerization and adsorption capabilities and can increase the negative charge density on the membrane surface. Therefore, after modification with polydopamine, the anion exchange membrane becomes negatively charged, exhibiting electrostatic repulsion against negatively charged organic matter in the water, thus inhibiting organic matter contamination of the anion exchange membrane. Those skilled in the art should also understand that while the negative surface charge prevents organic matter contamination through electrostatic interaction, it also affects the migration rate of inorganic anions. The greater the ion charge, the greater the impact; therefore, sulfate ions are more affected than chloride ions. Adding hydrophilic modifiers such as β-cyclodextrin to the anion exchange membrane can impart strong hydrophilicity and alter the migration rates between anions. Specifically, the migration rate of less hydrated anions, such as bromide and nitrate ions, relative to chloride ions decreases, while the migration rate of more hydrated anions, such as sulfate ions, relative to chloride ions increases. On the other hand, increased membrane hydrophilicity reduces van der Waals forces between the membrane and organic solutes, decreasing attractiveness. Simultaneously, hydrogen bonding between the hydrophilic membrane and water molecules forms a hydration layer, which further hinders the adsorption of pollutants on the membrane surface. Therefore, the anion exchange membrane of this invention, after being modified with both a hydrophilic modifier and polydopamine, exhibits significantly increased antifouling ability and enhanced selective permeability to sulfate ions.

[0033] Furthermore, the molybdenum removal agent in the molybdenum removal unit is ferric ions from nanofiltration concentrate, with the pH controlled at 3-5, preferably 3.5-4.5, and the iron-molybdenum molar ratio controlled at 2:1-9:1. Those skilled in the art should understand that molybdenum in acrylonitrile flue gas desulfurization wastewater exists in the form of molybdate. Conventional molybdenum sulfide precipitation methods are ineffective. This invention uses an iron salt precipitation method, which can form a series of positively charged hydrolysis products with water molecules. These hydrolysis products then interact with negatively charged molybdate ions through electrostatic adsorption, surface complexation, co-precipitation, etc., thereby reducing the number of molybdenum ions.

[0034] Furthermore, the hardening agent used in the hardening unit is preferably a combination of sodium hydroxide, sodium carbonate, and PAM. Sodium carbonate is added at 1 to 3 times the mass concentration of calcium ions, the amount of sodium hydroxide added is greater than the mass concentration of magnesium ions, and the amount of PAM added is 1 to 20 mg / L. Sodium hydroxide also acts as a pH adjuster, and the pH value should be greater than 8 after addition.

[0035] Furthermore, the tubular microfiltration unit is mainly used to filter suspended solids, colloids, etc., to prevent clogging of the subsequent bipolar membrane electrodialysis membrane.

[0036] Furthermore, the bipolar membrane electrodialysis unit has a treatment time of 0.2–3 hours and a current density of 1–150 mA / cm². 2 The alkalinity of the alkaline solution produced by the bipolar membrane electrodialysis unit is 0.1–1.2 mol / L, and the acidity of the acid solution produced is 0.1–1.5 mol / L. The alkalinity and acidity of the solution are determined by neutralization titration, using phenolphthalein and methyl orange as indicators, respectively.

[0037] Furthermore, the dissolved oxygen in the primary anaerobic unit is controlled below 0.2 mg / L, the wastewater retention time is 1–12 h, the temperature is 25–35 °C, the pH is controlled at 5–6 by adding acid, and the bacterial strains are organic acidifying bacteria and sulfate-reducing bacteria.

[0038] Furthermore, the dissolved oxygen in the secondary anaerobic unit is controlled below 0.1 mg / L, the wastewater retention time is 2–24 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens.

[0039] Furthermore, the stripping gas from the primary anaerobic unit is methane produced by the secondary anaerobic unit, and the resulting methane and hydrogen sulfide mixture enters the alkaline absorption unit after stripping.

[0040] Furthermore, the initial alkali used in the alkali absorption unit is sodium hydroxide, which ensures the purity of the alkali solution produced by bipolar membrane electrodialysis. After the alkali absorption unit has been running for a period of time, the alkali-rich solution becomes alkali-poor solution. At this time, the alkali-poor solution is returned to the reaction section for bipolar membrane electrodialysis regeneration, and new alkali solution is replenished to the alkali absorption unit. The new alkali solution comes from the alkali solution prepared by bipolar membrane electrodialysis. After the hydrogen sulfide is absorbed by the alkali solution, it becomes sodium hydrosulfide or sodium sulfide. During the bipolar membrane electrodialysis regeneration process, it will be oxidized to sulfate by the action of the byproduct chlorine gas, and finally converted into sulfuric acid.

[0041] Furthermore, the amount of hydrogen peroxide in the Fenton oxidation unit is added at a mass ratio of hydrogen peroxide to wastewater COD of 1 to 10:1, the amount of iron added is added at a mass ratio of iron to hydrogen peroxide of 0.2 to 1:1, and the pH is 3 to 5; the iron is ferrous divalent, and the ferrous salt is selected from at least one of ferrous chloride, ferrous nitrate, and ferrous sulfate.

[0042] Furthermore, the nanofiltration unit has a water production rate of 60% to 95% and has a good retention effect on divalent and high-valence ions; specifically, after the effluent from the Fenton oxidation unit is treated by the nanofiltration unit, iron ions, sulfate ions, etc. are retained and enter the concentrate side, while sodium ions, chloride ions, etc. pass through the nanofiltration membrane and enter the product water side.

[0043] Furthermore, the neutralization unit adjusts the pH to 6-9 by adding alkaline solution to meet the requirements for water reuse or discharge. The alkaline solution used comes from the alkaline solution produced by dual-mode electrodialysis.

[0044] Compared with the prior art, the present invention has the following advantages:

[0045] (1) The modified anion exchange membrane and improved bipolar membrane electrodialysis of the present invention efficiently achieve the separation of organic matter and salt and the concentration of salt, and greatly increase the anti-fouling ability. After the improved bipolar membrane electrodialysis treatment, most of the organic matter is retained in the desalination solution, and most of the salt is converted into sulfuric acid and sodium hydroxide. The sulfuric acid can be reused in the Fenton oxidation unit and the molybdenum removal unit, and the sodium hydroxide can be used to adjust the pH of the hardening unit, the neutralization unit, the alkaline absorption unit, etc. In particular, it solves the problem of high cost caused by the large amount of acid and alkali used in the Fenton oxidation system.

[0046] (2) The two-stage anaerobic process of this invention efficiently removes COD and sulfate. Specifically, sulfate is converted into hydrogen sulfide in the first-stage anaerobic unit, and organic matter is converted into methane in the second-stage anaerobic unit and used as stripping gas to remove hydrogen sulfide generated in the first-stage anaerobic unit, preventing microbial poisoning of the sludge in the first-stage anaerobic unit. The COD and salt content of the effluent from both stages of anaerobic treatment are low, thereby reducing the cost of subsequent advanced oxidation treatment.

[0047] (3) The present invention uses Fenton oxidation and nanofiltration technology to achieve deep treatment and compliant reuse of acrylonitrile fume dewatering wastewater. Furthermore, the trivalent iron produced by Fenton oxidation is utilized as a resource by combining it with the nanofiltration unit and the molybdenum removal unit, i.e., it is used as a molybdenum removal agent, thereby saving agent consumption.

[0048] (4) The process method of this invention is used to recycle and reuse acrylonitrile flue gas desulfurization wastewater, store the waste gas in tanks and use it as fuel, and convert salt into sulfuric acid and sodium hydroxide products, thus realizing the overall resource utilization of water, gas and solids. In addition, compared with the technical route of producing sodium sulfate and sodium chloride products by concentration, evaporation and crystallization, the technical route of producing sulfuric acid and sodium hydroxide products by bipolar membrane electrodialysis has lower operating costs and higher product benefits.

[0049] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0050] Figure 1 The effect of molybdenum on the degradation of glucose by sludge microorganisms in Example 1;

[0051] Figure 2 Flowchart of acrylonitrile flue gas desulfurization wastewater treatment in Example 2. Detailed Implementation

[0052] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0053] This invention first investigated the effect of molybdenum on the degradation of glucose by sludge microorganisms:

[0054] A pure glucose solution with a COD concentration of 800 mg / L and a salt (Na2SO4) concentration of 1% was prepared. K2MoO4 was added to form different concentration gradients, and nutrients such as N and P were added. A salt-tolerant bacterial agent was added, and the MLSS was 1 g / L. The mixture was reacted in a constant temperature shaker at 30℃ and 150 r / min for 48 h. The COD values ​​before and after the reaction were measured.

[0055] like Figure 1As shown, without the addition of Mo, the COD removal rate of the glucose solution after 48 hours of biochemical reaction was 86%. However, after adding Mo at a concentration of 100 mg / L, the COD removal rate dropped to 82.2%, and the COD removal rate gradually decreased with increasing Mo concentration. This indicates that high concentrations of Mo ions inhibit the activity of sludge microorganisms and affect their ability to degrade organic matter. Since the reaction time was only 48 hours, it cannot be ruled out that under long-term operation conditions, high concentrations of Mo ions would further affect the activity of sludge microorganisms. Furthermore, considering that some emission standards, such as the "Emission Standard of Pollutants for Inorganic Chemical Industry" (GB31573-2015), require that the Mo ion concentration not exceed 0.5 mg / L, it is necessary to remove Mo before the acrylonitrile fume dewatering biochemical treatment.

[0056] Example 1

[0057] The process flow diagram for treating acrylonitrile flue gas desulfurization wastewater is as follows: Figure 2 As shown, the flue gas desulfurization wastewater sequentially enters the molybdenum removal tank, the equalization and hardening tank, and the tubular microfiltration. The effluent from the tubular microfiltration enters the bipolar membrane electrodialysis. The chemical sludge generated from the molybdenum removal tank, the equalization and hardening tank, and the tubular microfiltration enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off. The acid and alkali solutions generated from the bipolar membrane electrodialysis are collected and reused. The desalinated liquid sequentially enters the primary anaerobic digester, the secondary anaerobic digester, and the Fenton oxidation unit. The methane gas generated from the secondary anaerobic digester is used as stripping gas and pumped to the primary anaerobic digester, carrying most of the hydrogen sulfide generated from the primary anaerobic digester out of the wastewater system. After treatment by the alkali absorption unit, it is recovered to the methane storage tank. The Fenton oxidation effluent enters the nanofiltration unit. The nanofiltration concentrate is returned to the molybdenum removal tank. After mixing, molybdenum can be removed efficiently. The nanofiltration permeate enters the neutralization tank. After adding alkali to adjust to neutrality, the effluent meets the reuse standards.

[0058] The process method of this invention is used to treat acrylonitrile flue gas desulfurization wastewater.

[0059] The wastewater from the desulfurization of acrylonitrile flue gas has the following characteristics: COD 600 mg / L, sulfate 7200 mg / L, chloride concentration 2800 mg / L, calcium ion concentration 100 mg / L, magnesium ion concentration 50 mg / L, molybdenum ion concentration 60 mg / L, total salt content 15500 mg / L, and pH 5.

[0060] The bipolar membrane electrodialysis provided in this embodiment is a modified bipolar membrane electrodialysis, wherein the anion exchange membrane is a modified anion exchange membrane with strong anti-fouling ability and high ion permeability, and the cation exchange membrane is a general-purpose cation exchange membrane (Hefei Capgemini Polymer Co., Ltd., China, model CJ-MC-3).

[0061] The modified anion exchange membrane described above was prepared by the following method:

[0062] Step a: Add polyvinyl alcohol to deionized water to prepare a 10% aqueous solution. Stir at 80°C to dissolve it. Then add 2,3-epoxypropyltrimethylammonium chloride, β-cyclodextrin, and glutaraldehyde to the water. The mass ratio of 2,3-epoxypropyltrimethylammonium chloride to polyvinyl alcohol is 0.5:1, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.2:1, and the mass ratio of glutaraldehyde to polyvinyl alcohol is 0.04:1. Adjust the pH to 5 with hydrochloric acid and react for 6 hours to obtain a viscous liquid, which is the casting solution.

[0063] Step b: Cast the casting solution onto a horizontal glass plate and dry it at room temperature for 3 hours. Then, continue to dry the formed film under vacuum at 60°C for 6 hours to obtain the base film.

[0064] Step c: Wash the base membrane obtained in step b with deionized water until neutral, then place it in a 1 mol / L sodium hydroxide aqueous solution for alkalization for 16 h, and then soak it in deionized water for 16 h to obtain a polyvinyl alcohol anion exchange membrane.

[0065] Step d: Dissolve dopamine in 20 mmol / L Tris–HCl buffer, adjust the pH to 8.5 with hydrochloric acid to obtain a dopamine Tris buffer solution with a dopamine mass concentration of 0.6 g / L.

[0066] Step e: Place the polyvinyl alcohol anion exchange membrane to be modified in a dopamine Tris buffer solution and add 3 mmol / L copper sulfate. Stir and react for 4 hours. Keep the air circulating during the reaction. Copper sulfate can induce rapid polymerization of dopamine and form a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane. After the reaction is completed, the modified anion exchange membrane is obtained.

[0067] Step f: Remove the modified anion exchange membrane and place it in a sodium chloride solution with a mass concentration of 10 g / L for later use.

[0068] The specific operating parameters for treating acrylonitrile flue gas desulfurization wastewater are as follows:

[0069] Wastewater first enters the molybdenum removal tank, where it is mixed with nanofiltration concentrate and acid is added to adjust the pH to 4, resulting in a large amount of precipitate. After settling, the molybdenum ion concentration in the effluent decreases to below 1 mg / L, and then it enters the equalization and hardening tank. The equalization and hardening tank is prepared by adding chemicals at a concentration of 200 mg / L sodium hydroxide, 150 mg / L sodium carbonate, and 8 mg / L PAM, raising the solution pH to 8.2. After clarification, the calcium ion concentration in the supernatant decreases to below 10 mg / L, and the magnesium ion concentration decreases to below 5 mg / L. The effluent is then filtered through a tubular microfiltration system and enters a bipolar membrane electrodialysis system. The chemical sludge produced in the molybdenum removal tank, equalization and hardening tank, and tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off-site.

[0070] The effluent from the tubular microfiltration system enters a bipolar membrane electrodialysis system. The salt content of the effluent from the tubular microfiltration system is approximately 15.7 g / L, and the COD is 570 mg / L. The bipolar membrane electrodialysis treatment time is 0.6 h, and the current density is 30 mA / cm². 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.45 mol / L, the alkalinity of the alkaline solution was 0.37 mol / L, the salt content of the desalinated solution was approximately 3.6 g / L, the COD was 676 mg / L, the permeate rate (percentage of desalinated solution to total influent) was 80%, the sulfate ion selective permeate rate was 79.5%, the chloride ion selective permeate rate was 85.7%, and the organic matter rejection rate was 94.9%. The bipolar membrane electrodialysis desalinated solution entered the primary anaerobic digester, with dissolved oxygen controlled at 0.15 ppm. For effluent with dissolved oxygen levels below 0.1 mg / L, pH was controlled at 5.5 by adding acid, wastewater retention time was 5 hours, temperature was 30°C, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD was reduced to 527 mg / L, and the salt content was reduced to 2136 mg / L. For secondary anaerobic digestion, dissolved oxygen was controlled at 0.1 mg / L, pH was controlled at 8 by adding alkali, wastewater retention time was 8 hours, temperature was 35°C, and the bacterial strains were methanogenic bacteria. The effluent COD was 162 mg / L, and the salt content was 2090 mg / L.

[0071] The Fenton oxidation unit adds hydrogen peroxide at a mass ratio of 3:1 to wastewater COD, specifically 486 mg / L. Ferrous chloride is added at a mass ratio of 0.6:1 to iron to hydrogen peroxide, translating to an iron molar concentration of 4.9 mmol / L. The pH is adjusted to 3.5 by adding sulfuric acid, ultimately reducing the COD from 162 mg / L to 44 mg / L. The effluent then enters a nanofiltration system; the nanofiltration permeability is 88%, with iron and molybdenum ion rejection rates both exceeding 98%. The nanofiltration concentrate enters a molybdenum removal tank, with a calculated iron-molybdenum molar ratio of 6.2. The nanofiltration permeate then enters a neutralization tank, where alkali is added to adjust the pH to 6.5. The final effluent has a COD of 45 mg / L, a salt content of 1345 mg / L, and a molybdenum ion concentration below 0.1 mg / L, meeting the requirements for reclaimed water quality.

[0072] Methane gas produced in the secondary anaerobic digestion stage is pumped to the primary anaerobic digestion stage as stripping gas. The methane and hydrogen sulfide composite gas produced in the primary anaerobic digestion stage enters the alkali absorption unit. The alkali absorption unit absorbs hydrogen sulfide and a small amount of carbon dioxide from the mixed gas, thus obtaining methane with a purity of 98.8%, which is then stored in a methane storage tank. After the alkali absorption unit has been running for a period of time, the alkali-rich solution becomes alkali-lean solution. At this point, the alkali-lean solution is returned to the bipolar membrane electrodialysis regeneration stage, while new alkali solution, which comes from the alkali solution prepared by bipolar membrane electrodialysis, is replenished to the alkali absorption unit. In addition, the alkali and acid used to adjust the pH in the above units are both from the alkali and acid solutions prepared by bipolar membrane electrodialysis.

[0073] As demonstrated in this embodiment, the present invention can effectively treat acrylonitrile flue gas desulfurization wastewater. The bipolar membrane electrodialysis, with a modified anion exchange membrane as its core, efficiently separates organic matter from salts and concentrates the salts. Most organic matter is retained in the desalination solution, and most salts are concentrated in the concentrate, which is beneficial for subsequent anaerobic treatment. The salts are ultimately converted into sulfuric acid and sodium hydroxide for reuse, while most organic matter is ultimately converted into methane. Due to the special characteristics of acrylonitrile flue gas desulfurization wastewater, some organic matter is difficult to treat biochemically. The present invention employs Fenton advanced oxidation to further treat the effluent from the two-stage anaerobic biological treatment. The trivalent iron produced by Fenton oxidation is retained by nanofiltration and returned to the molybdenum removal tank at the front end of the process, achieving efficient molybdenum removal and significantly reducing the consumption of molybdenum removal reagents. The acids and alkalis used in the molybdenum removal tank, the equalization and hardening tank, the Fenton oxidation, the two-stage anaerobic treatment, the alkaline absorption, and the neutralization tank all come from the bipolar membrane electrodialysis, achieving self-sufficiency and significantly reducing the cost of acid and alkali addition and the operating cost of Fenton oxidation.

[0074] Example 2

[0075] use Figure 2 The process shown is for treating acrylonitrile flue gas desulfurization wastewater.

[0076] The wastewater from the desulfurization of acrylonitrile flue gas has the following characteristics: COD 750 mg / L, sulfate 10500 mg / L, chloride concentration 4500 mg / L, calcium ion concentration 150 mg / L, magnesium ion concentration 100 mg / L, molybdenum ion concentration 100 mg / L, total salt content 23800 mg / L, and pH 5.

[0077] The process route and implementation steps for treating this wastewater in this embodiment are the same as in Embodiment 1. The bipolar membrane electrodialysis in the process route is the same as in Embodiment 1, using modified bipolar membrane electrodialysis. The anion exchange membrane is a modified anion exchange membrane, the cation exchange membrane is a general-purpose cation exchange membrane (Hefei Kaijie Polymer Co., Ltd., China, model CJ-MC-3), and the bipolar membrane is a general-purpose bipolar membrane (imported). (BP-1 membrane). In the preparation process of the modified anion exchange membrane, except that the positively charged amine compound in step a is polyethyleneimine, the mass ratio of polyethyleneimine to polyvinyl alcohol is 0.5:1, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.25:1, the mass ratio of glutaraldehyde to polyvinyl alcohol is 0.05:1, and the reaction time in step a is 8 hours, everything else is the same as in Example 1.

[0078] The specific operating parameters for treating acrylonitrile flue gas desulfurization wastewater are as follows:

[0079] Wastewater first enters the molybdenum removal tank, where it is mixed with nanofiltration concentrate and acid is added to adjust the pH to 4.2, resulting in a large amount of precipitate. After settling, the molybdenum ion concentration in the effluent decreases to below 1 mg / L, and then it enters the equalization and hardening tank. The equalization and hardening tank is prepared by adding chemicals at a concentration of 250 mg / L sodium hydroxide, 300 mg / L sodium carbonate, and 10 mg / L PAM, raising the solution pH to 8.4. After clarification, the calcium ion concentration in the supernatant decreases to below 10 mg / L, and the magnesium ion concentration decreases to below 5 mg / L. The effluent is then filtered through a tubular microfiltration system and enters a bipolar membrane electrodialysis system. The chemical sludge produced in the molybdenum removal tank, equalization and hardening tank, and tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off-site.

[0080] The effluent from the tubular microfiltration system enters a bipolar membrane electrodialysis system. The salt content of the effluent from the tubular microfiltration system is approximately 24.2 g / L, and the COD is 755 mg / L. The bipolar membrane electrodialysis treatment time is 0.8 h, and the current density is 35 mA / cm³. 2 After treatment, the acidity of the acidic solution produced by bipolar membrane electrodialysis was 0.73 mol / L, the alkalinity of the alkaline solution was 0.57 mol / L, the salt content of the desalinated solution was approximately 4.6 g / L, the COD was 909 mg / L, the permeate yield (percentage of desalinated solution to total influent) was 78%, the sulfate ion selective permeate rate was 83.5%, the chloride ion selective permeate rate was 88.7%, and the organic matter rejection rate was 94.6%. The bipolar membrane electrodialysis desalinated solution entered the primary anaerobic digester, with dissolved oxygen controlled at 0.15. For effluent with dissolved oxygen levels below 0.1 mg / L, pH was controlled at 5.6 by adding acid, wastewater retention time was 8 hours, temperature was 30℃, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD was reduced to 718 mg / L, and the salt content was reduced to 2915 mg / L. For secondary anaerobic digestion, dissolved oxygen was controlled at 0.1 mg / L, pH was controlled at 8 by adding alkali, wastewater retention time was 9 hours, temperature was 35℃, and the bacterial strains were methanogenic bacteria. The effluent COD was 184 mg / L, and the salt content was 2810 mg / L.

[0081] The Fenton oxidation unit adds hydrogen peroxide at a mass ratio of 2.8:1 to wastewater COD, specifically 526 mg / L. Ferrous chloride is added at a mass ratio of 0.8:1 to iron, translating to an iron molar concentration of 7.5 mmol / L. The pH is adjusted to 3.6 by adding sulfuric acid, ultimately reducing the COD from 188 mg / L to 55 mg / L. The effluent then enters a nanofiltration system; the nanofiltration permeability is 87%, with iron and molybdenum ion rejection rates both exceeding 98%. The nanofiltration concentrate enters a molybdenum removal tank, with a calculated iron-molybdenum molar ratio of 5.5. The nanofiltration permeate then enters a neutralization tank, where sodium hydroxide is added to adjust the pH to 6.6. The final effluent has a COD of 57 mg / L, a salt content of 1433 mg / L, and a molybdenum ion concentration below 0.1 mg / L, meeting the requirements for reclaimed water quality.

[0082] Methane gas produced in the secondary anaerobic digestion stage is pumped to the primary anaerobic digestion stage as stripping gas. The methane and hydrogen sulfide composite gas produced in the primary anaerobic digestion stage enters the alkali absorption unit. The alkali absorption unit absorbs hydrogen sulfide and a small amount of carbon dioxide from the mixed gas, thus obtaining methane with a purity of 98.9%, which is then stored in a methane storage tank. After the alkali absorption unit has been running for a period of time, the alkali-rich solution becomes alkali-lean solution. At this point, the alkali-lean solution is returned to the bipolar membrane electrodialysis regeneration stage, while new alkali solution, which comes from the alkali solution prepared by bipolar membrane electrodialysis, is replenished to the alkali absorption unit. In addition, the alkali and acid used to adjust the pH in the above units all come from the alkali and acid solutions prepared by bipolar membrane electrodialysis.

[0083] As can be seen from this embodiment, the present invention can effectively treat acrylonitrile flue gas desulfurization wastewater of different concentrations, and the treatment process realizes the resource utilization of water, gas and solids.

[0084] Comparative Example 1

[0085] The treatment of acrylonitrile flue gas desulfurization wastewater quality is the same as in Example 1, and the process route and implementation steps are also the same as in Example 1. The difference is that the anion and cation exchange membranes and the bipolar membrane in the bipolar membrane electrodialysis are both general-purpose membranes. The anion exchange membrane is a product of Asahi Glass Corporation of Japan (SELEMION AMV), the cation exchange membrane is a product of Hefei Kaijie Polymer Co., Ltd. of China (model CJ-MC-3), and the bipolar membrane is imported. BP-1 membrane.

[0086] Specifically, consistent with Example 1, the wastewater, after passing through a molybdenum removal tank, a hardening and equalization tank, and tubular microfiltration, enters a bipolar membrane electrodialysis system. The salt content of the tubular microfiltration effluent is approximately 15.7 g / L, and the COD is 570 mg / L. The bipolar membrane electrodialysis treatment time is 0.6 h, and the current density is 30 mA / cm². 2After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.34 mol / L, the alkalinity of the alkaline solution was 0.25 mol / L, the salt content of the desalinated solution was approximately 6.1 g / L, the COD was 604 mg / L, the permeate rate (percentage of desalinated solution to total influent) was 80%, the sulfate ion selective permeate rate was 67.6%, the chloride ion selective permeate rate was 75.8%, and the organic matter rejection rate was 84.8%. The bipolar membrane electrodialysis desalinated solution entered the primary anaerobic digester, with dissolved oxygen controlled below 0.15 mg / L, pH controlled at 5.5 by adding acid, wastewater retention time of 8 hours, and temperature of 30°C. At ℃, the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria, and the effluent COD was reduced to 468 mg / L, and the salt content was reduced to 3852 mg / L. The dissolved oxygen in the secondary anaerobic unit was controlled below 0.1 mg / L, the pH was controlled at 8 by adding alkali, the wastewater retention time was 16 h, the temperature was 35℃, the bacterial strains were methanogenic bacteria, and the effluent COD was 338 mg / L. The COD removal rate of this unit was only 27.8%, while the COD removal rate of the secondary anaerobic unit in Example 1 was 69.3%. The salt content of the effluent in this unit was 3330 mg / L, while the salt content of the secondary anaerobic effluent in Example 1 was 2090 mg / L.

[0087] This comparative example shows that bipolar membrane electrodialysis using unmodified anion and cation exchange membranes has poor ability to separate organic matter and salts. Furthermore, the anion exchange membrane is easily fouled by organic matter, resulting in an excessively high salt concentration in the final desalinated solution (6.1 g / L), while the concentration of acids and alkalis produced decreases. Subsequent primary anaerobic treatment significantly increased the primary anaerobic retention time compared to Example 1, but the effluent salt content remained as high as 3852 mg / L. This also affected the biological activity of methanogenic bacteria in the secondary anaerobic environment. Even with the increased secondary anaerobic retention time, the final effluent COD and salt concentrations remained high, significantly increasing the pressure on subsequent Fenton oxidation and nanofiltration treatments, potentially leading to substandard effluent and failure to meet reclaimed water quality requirements.

[0088] Comparative Example 2

[0089] The water quality of the flue gas desulfurization wastewater treated is the same as in Example 1, and the process route and implementation steps are also basically the same as in Example 1. The difference is that the two-stage anaerobic process is changed to a single-stage anaerobic process.

[0090] Specifically, as in Example 1, the wastewater was sequentially treated by a molybdenum removal tank, a hardening and equalization tank, a tubular microfiltration system, and a bipolar membrane electrodialysis system. The bipolar membrane electrodialysis desalinated solution then entered a single-stage anaerobic digester. The influent salt content was 3.6 g / L, and the COD was 676 mg / L. The dissolved oxygen in the single-stage anaerobic digester was controlled below 0.15 mg / L, the pH was adjusted to 7, and the temperature was 33°C. The bacterial strains included organic acidifying bacteria, sulfate-reducing bacteria, and methanogenic bacteria. The wastewater retention time was extended to 16 hours. The methane and hydrogen sulfide mixture produced during the anaerobic reaction was pumped out and transferred to the alkaline absorption unit. The final effluent COD was reduced to 445 mg / L, and the salt content was reduced to 2876 mg / L, significantly higher than the COD (162 mg / L) and salt content (2090 mg / L) of the two-stage anaerobic digester in Example 1.

[0091] This comparative example shows that the treatment efficiency of single-stage anaerobic digestion is much lower than that of two-stage anaerobic digestion. This is because two-stage anaerobic digestion achieves the separation and efficient operation of the acid-sulfate reducing phase and the methanogenic phase through the adjustment of pH, temperature, bacterial species, and stripping gas, resulting in a significant decrease in the salt content and COD of the effluent. In contrast, with single-stage anaerobic digestion, the acid-sulfate reducing phase and the methanogenic phase interfere with each other, and the reaction pH and temperature are not optimal. In particular, the methanogenic bacteria have a weak tolerance to hydrogen sulfide, which ultimately leads to a decrease in COD and sulfate treatment efficiency. As this deteriorates over time, it eventually leads to the collapse of the reaction system.

[0092] Comparative Example 3

[0093] The water quality of the flue gas desulfurization wastewater treated is the same as in Example 1, and the process route and implementation steps are also basically the same as in Example 1. The difference is that there is no nanofiltration unit after the Fenton oxidation unit.

[0094] Specifically, consistent with Example 1, the wastewater, after passing through a molybdenum removal tank, a hardening and equalization tank, and tubular microfiltration, enters a bipolar membrane electrodialysis system. The bipolar membrane electrodialysis permeate rate (percentage of desalinated water to total influent) is 80%. The desalinated water from the bipolar membrane electrodialysis system undergoes two stages of anaerobic treatment before entering the Fenton oxidation unit. Assuming consistency with Example 1, hydrogen peroxide is added at a COD mass ratio of 3:1, specifically at a dosage of 486 mg / L. Ferrous chloride is added at a ferric to hydrogen peroxide mass ratio of 0.6:1, translating to an iron molar concentration of 4.9 mmol / L. The pH is adjusted to 3.5 by adding sulfuric acid. The COD of the Fenton oxidation effluent is 44 mg / L, and the salt content is 2980 mg / L. The effluent contains a large amount of ferric ions. In order to effectively utilize ferric ions for front-end molybdenum removal, the Fenton oxidation effluent needs to be partially recirculated. The recirculation percentage is set at 50%. The final water volume when water balance is reached is 166.7% of the initial water volume, and the iron-molybdenum molar ratio in the molybdenum removal tank decreases to 3.2, resulting in a decrease in molybdenum removal efficiency. The Fenton oxidation effluent enters the neutralization tank, and sodium hydroxide is added to adjust the pH to 6.5. In addition to the salt content rising to 3230 mg / L, a large amount of ferric hydroxide precipitate is also generated, which does not meet the requirements for salt content and suspended solids of reclaimed water.

[0095] In Example 1, after nanofiltration was used, the calculated water volume at which water balance was reached was 110.6% of the initial water volume, under the condition that the nanofiltration water production rate was 88%.

[0096] As can be seen from this comparative example, compared with Example 1, when the nanofiltration unit is not used, in order to effectively utilize the iron ions oxidized by Fenton, an effluent recirculation is required, which ultimately increases the overall treatment capacity by 50.7%. This results in a significant increase in energy consumption of each unit, a decrease in the molybdenum removal efficiency of the molybdenum removal tank, and ultimately, the effluent from the neutralization tank does not meet the standards.

Claims

1. A process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater, comprising a pretreatment section, a main reaction section, and a deep treatment section; The pretreatment section includes a molybdenum removal unit, a hardening and conditioning unit, a tubular microfiltration unit, and a sludge thickening, dewatering, and drying unit. Wastewater first enters the molybdenum removal unit to remove molybdenum, and the effluent enters the hardening and conditioning unit to remove hardness. After being filtered by the tubular microfiltration unit, it enters the bipolar membrane electrodialysis unit in the main reaction section. The chemical sludge generated by the molybdenum removal unit, the hardening and conditioning unit, and the tubular microfiltration unit enters the sludge thickening, dewatering, and drying unit. The water removed is returned to the inlet of the pretreatment section, and the dried sludge is sent out. The main reaction section includes a bipolar membrane electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, an alkaline solution absorption unit, and a methane storage unit. The acid and alkaline solutions produced by the bipolar membrane electrodialysis are collected and reused. The desalinated solution enters the primary anaerobic unit for organic acidification and sulfate reduction reactions, producing hydrogen sulfide gas. It then enters the secondary anaerobic unit for methanogenesis. The generated methane is used as stripping gas and pumped back to the primary anaerobic unit. Most of the hydrogen sulfide produced in the primary anaerobic unit is carried out of the wastewater system and then enters the alkaline solution absorption unit. The purified gas produced by the alkaline solution absorption unit is recovered to a methane storage tank for later use. The advanced treatment section includes, in sequence, a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit; the Fenton oxidation unit is the secondary anaerobic unit of the main reaction section; the effluent enters the nanofiltration unit, the concentrate from the nanofiltration unit is returned to the molybdenum removal unit, and after mixing, molybdenum can be removed efficiently; the permeate from the nanofiltration unit enters the neutralization unit, and after adding alkali to adjust it to neutral, the effluent meets the reuse standard. in, The bipolar membrane electrodialysis system consists of an anion exchange membrane, a cation exchange membrane, and a bipolar membrane, wherein the anion exchange membrane is a modified anion exchange membrane, the cation exchange membrane is a general-purpose cation exchange membrane, and the bipolar membrane is a general-purpose bipolar membrane. The modified anion exchange membrane is prepared as follows: Step a: Polyvinyl alcohol is added to water to prepare a polyvinyl alcohol aqueous solution. Then, a positively charged amine compound, a hydrophilic modifier, and a crosslinking agent are added to the above solution respectively. An inorganic acid is added to adjust the pH. After mixing and stirring, a casting solution is obtained. The positively charged amine compound is selected from one or more of 2,3-epoxypropyltrimethylammonium chloride, polyethyleneimine, polyepoxychloropropaneamine, and quaternized chitosan. The hydrophilic modifier is selected from one or more of α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin. Step b: Cast the casting solution obtained in step a onto a horizontal panel, dry it, and obtain the base film; Step c: After washing the base membrane obtained in step b, place it in an alkaline solution for alkalization, and then soak it in water to obtain a polyvinyl alcohol anion exchange membrane; Step d: Dissolve dopamine in Tris-HCl buffer, adjust the pH with hydrochloric acid to obtain dopamine Tris buffer solution; Step e: Place the anion exchange membrane obtained in step c in a dopamine Tris buffer solution and add copper sulfate. Stir the reaction while maintaining air circulation. After the reaction is complete, the modified anion exchange membrane is obtained.

2. The process method according to claim 1, characterized in that, In the preparation process of the modified anion exchange membrane, the mass ratio of the positively charged amine compound to polyvinyl alcohol is 0.1:1 to 0.6:1; the mass ratio of the hydrophilic modifier to polyvinyl alcohol is 0.05:1 to 0.5:1; the crosslinking agent is selected from aldehyde or acid crosslinking agents, and the mass ratio of the crosslinking agent to polyvinyl alcohol is 0.01:1 to 0.1:

1.

3. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the inorganic acid in step a is sulfuric acid, hydrochloric acid or nitric acid, and the pH of the solution is adjusted to 4-6; the mass fraction of the polyvinyl alcohol aqueous solution is 5%-15%, and the reaction time with the positively charged amine compound, β-cyclodextrin and crosslinking agent is 4-16 h.

4. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the drying in step b is to first dry at room temperature for 2 to 6 hours, and then continue drying under vacuum at 40 to 80°C for 4 to 10 hours.

5. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the alkalization in step c uses a 0.5–3 mol / L sodium hydroxide aqueous solution for 12–24 h; the soaking time is 12–24 h; the dopamine concentration in step d is 0.4–4 g / L, and the pH range is adjusted to 8–9 with hydrochloric acid; the copper sulfate concentration in step e is 1–20 mmol / L, and the stirring reaction time is 1–12 h.

6. The process method according to claim 1, characterized in that, The molybdenum removal agent in the molybdenum removal unit is ferric ions from nanofiltration concentrate, with the pH controlled at 3-5 and the iron-molybdenum molar ratio controlled at 2:1-9:

1.

7. The process method according to claim 1, characterized in that, The hardening agent in the hardening unit is a combination of sodium hydroxide, sodium carbonate, and PAM. Sodium carbonate is added at 1 to 3 times the mass concentration of calcium ions, the amount of sodium hydroxide added is greater than the mass concentration of magnesium ions, and the amount of PAM added is 1 to 20 mg / L. Sodium hydroxide also acts as a pH adjuster, and the pH value should be greater than 8 after addition.

8. The process method according to claim 1, characterized in that, The bipolar membrane electrodialysis unit has a treatment time of 0.2–3 hours and a current density of 1–150 mA / cm². 2 .

9. The process method according to claim 1, characterized in that, The dissolved oxygen in the primary anaerobic unit is controlled below 0.2 mg / L, the wastewater retention time is 1–12 h, the temperature is 25–35 °C, the pH is controlled at 5–6 by adding acid, and the bacterial strains are organic acidifying bacteria and sulfate reducing bacteria.

10. The process method according to claim 1, characterized in that, The dissolved oxygen in the secondary anaerobic unit is controlled below 0.1 mg / L, the wastewater retention time is 2–24 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens; the stripping gas in the primary anaerobic unit is the methane produced by the secondary anaerobic unit, and the methane and hydrogen sulfide mixture produced after stripping enters the alkaline absorption unit.

11. The process method according to claim 1, characterized in that, The initial alkali used in the alkali absorption unit is sodium hydroxide, which ensures the purity of the alkali solution produced by bipolar membrane electrodialysis. After the alkali absorption unit has been running for a period of time, the alkali-rich solution becomes alkali-poor solution. At this time, the alkali-poor solution is returned to the reaction section for bipolar membrane electrodialysis regeneration, and new alkali solution is replenished to the alkali absorption unit. The new alkali solution comes from the alkali solution prepared by bipolar membrane electrodialysis. The hydrogen sulfide is absorbed by the alkali solution and becomes sodium hydrosulfide or sodium sulfide. During the bipolar membrane electrodialysis regeneration process, it will be oxidized to sulfate by the action of the byproduct chlorine gas, and finally converted into sulfuric acid.

12. The process method according to claim 1, characterized in that, The amount of hydrogen peroxide in the Fenton oxidation unit is added at a mass ratio of hydrogen peroxide to wastewater COD of 1 to 10:1, and the amount of iron added is added at a mass ratio of iron to hydrogen peroxide of 0.2 to 1:1, with a pH of 3 to 5; the iron is ferrous iron, and the ferrous salt is selected from at least one of ferrous chloride, ferrous nitrate, and ferrous sulfate.

13. The process method according to claim 1, characterized in that, The nanofiltration unit has a water production rate of 60% to 95%. After the Fenton oxidation unit effluent is treated by the nanofiltration unit, iron ions and sulfate ions are retained and enter the concentrate side, while sodium ions and chloride ions pass through the nanofiltration membrane and enter the product water side. The neutralization unit adjusts the pH to 6 to 9 by adding alkali solution.

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