Process for treating lignin purification wastewater

Abstract

A process method for treating lignin purification wastewater, comprising a pretreatment section, a deep treatment section and a regeneration section; the pretreatment section comprises a hardness removal adjusting unit, a tubular microfiltration unit, a bipolar membrane electrodialysis unit and a sludge concentration, dewatering and drying unit; the deep treatment section comprises a first-stage anaerobic unit, a second-stage anaerobic unit, a Fenton oxidation unit, a nanofiltration unit and a neutralization unit in sequence; the regeneration section comprises a single membrane electrodialysis unit, a regeneration unit and a methane storage unit. The bipolar membrane electrodialysis of the present application adopts a modified anion exchange membrane, most of the organic matters are intercepted in the dilute liquid, and most of the salt is converted into sulfuric acid and sodium hydroxide. The lignin purification wastewater adopts the process method of the present application, the wastewater realizes recycling and zero discharge, the waste gas tank is stored and can be used as fuel, the salt is converted into sulfuric acid and sodium hydroxide products, and the water, gas and solid are overall resourced.

Description

Technical Field

[0001] This invention relates to a method for treating lignin purification wastewater, belonging to the field of wastewater treatment technology. Background Technology

[0002] Bioenergy is a renewable resource. For fuel ethanol, corn and wheat are usually used as fermentation raw materials abroad. However, given my country's food security issues, it is not suitable to use grain as a raw material, so alternatives must be found.

[0003] Lignocellulose is a non-grain raw material with abundant resources, mainly derived from agricultural waste (such as wheat straw and stalks), industrial waste (such as fiber residue and sawdust from pulp and paper mills), and municipal waste (such as waste paper and packaging paper). Typically, lignocellulose undergoes processes such as acidification, enzymatic hydrolysis, and fermentation to obtain the main products, sugars or ethanol. The remaining mother liquor also contains a large amount of lignin. To obtain the byproduct high-purity lignin (which can be used as a reinforcing agent, mineral powder binder, concrete water-reducing agent, scale inhibitor, corrosion inhibitor, and coal-water slurry dispersant), the mother liquor requires further treatment, including solid-liquid separation, alkali dissolution, and acid precipitation. The wastewater generated during this process is the lignin purification wastewater.

[0004] The treatment of lignin purification wastewater has become a major challenge in the development of this technology. The main reason is that lignin purification wastewater is high in COD and sulfate, with a COD concentration greater than 2000 mg / L and a sulfate concentration greater than 5000 mg / L. The salt content mainly comes from sodium hydroxide added in the alkali dissolution unit and sulfuric acid added in the acidification and acid precipitation units. If the wastewater has a high organic content, anaerobic treatment and methane recovery are usually used. However, high-sulfate organic wastewater contains a large amount of sulfate, which, under anaerobic conditions, will be reduced to sulfur by sulfate-reducing bacteria (SRB). 2- These ions are highly biotoxic and have a significant inhibitory effect on microbial communities, severely impacting the removal of organic matter. Therefore, this method generally requires the sulfate concentration in the anaerobic reactor to be less than 2000 mg / L, as illustrated in patent CN103771670A. However, this method is not suitable for high-sulfate wastewater. Similarly, aerobic processes also face problems such as excessively high salt concentrations leading to a limited variety of salt-tolerant bacterial species and low treatment efficiency. Furthermore, aerobic processes also suffer from aeration dead zones and localized anaerobic processes. When the sulfate concentration is too high, it can also result in high sulfide concentrations in the water. In addition, aerobic processes also face the challenges of large footprint, high energy consumption, and high carbon emissions.

[0005] Anaerobic biological treatment helps hydrolyze large organic molecules into smaller ones and improves the biodegradability of wastewater. However, if the wastewater contains recalcitrant organic matter, anaerobic biological methods struggle to achieve discharge standards. Therefore, advanced oxidation processes are typically added at the end of biological treatment. Common advanced oxidation processes include ozone catalytic oxidation, electrocatalytic oxidation, photocatalysis, and Fenton oxidation. Electrocatalytic oxidation and photocatalysis are currently limited by high treatment costs, making industrial application difficult. The biggest problem with ozone catalytic oxidation is its low ozone utilization rate and the pollution it causes. Ozone pollution is now another complex pollution problem facing the world after acid rain and PM2.5. It is foreseeable that the application of ozone catalytic oxidation technology will be restricted as national regulations on ozone pollutants become more stringent. The biggest problem with Fenton oxidation is iron ion loss and iron sludge. This is because the Fenton reaction system is acidic, and ferrous ions act as catalysts for hydroxyl radical production. To meet discharge standards, the pH of the effluent needs to be adjusted to neutral by adding alkali, which generates a large amount of iron sludge. Proper treatment of this iron sludge has become a bottleneck in the development of Fenton oxidation technology. Summary of the Invention

[0006] To address the above shortcomings, this invention provides a process for treating lignin purification wastewater. Through this process, the wastewater is deeply treated and meets reuse standards. Most of the salts are utilized as resources in the form of sulfuric acid and sodium hydroxide, and most of the organic matter is utilized as resources in the form of methane.

[0007] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0008] This invention provides a process for treating lignin purification wastewater, including a pretreatment section, a deep treatment section, and a regeneration section;

[0009] The pretreatment section includes a hardening and conditioning unit, a tubular microfiltration unit, a bipolar membrane electrodialysis unit, and a sludge thickening, dewatering, and drying unit. Wastewater first undergoes hardening and conditioning in the hardening and conditioning unit, then passes through the tubular microfiltration unit for filtration before entering the bipolar membrane electrodialysis unit. The acid and alkali solutions produced by the bipolar membrane electrodialysis unit are collected and reused, while the desalinated solution enters the primary anaerobic unit of the deep treatment section. The chemical sludge produced by the hardening and conditioning unit and the tubular microfiltration unit 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.

[0010] The advanced treatment section sequentially includes a primary anaerobic unit, a secondary anaerobic unit, a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit. The effluent from the neutralization unit is reused. The desalinated liquid from the bipolar membrane electrodialysis unit enters the primary anaerobic unit, where organic acidification and sulfate reduction are mainly carried out, generating hydrogen sulfide gas. It then enters the secondary anaerobic unit, where methanogenesis is mainly carried out. The generated methane is used as stripping gas and pumped to the primary anaerobic unit. Most of the hydrogen sulfide generated in the primary anaerobic unit is carried out of the wastewater system and enters the regeneration unit of the regeneration section. The effluent from the secondary anaerobic unit is sequentially treated by the Fenton oxidation unit and the nanofiltration unit. The effluent from the nanofiltration unit is divided into nanofiltration concentrate and nanofiltration permeate. The nanofiltration permeate enters the neutralization tank, and the nanofiltration concentrate enters the single-membrane electrodialysis unit of the regeneration section.

[0011] The regeneration section includes a single-membrane electrodialysis unit, a regeneration unit, and a methane storage unit. The effluent from the single-membrane electrodialysis unit is divided into mother liquor and anion exchange liquid. The anion exchange liquid is returned to the inlet of the bipolar membrane electrodialysis unit in the pretreatment section, while the mother liquor enters the regeneration unit and reacts with the methane and hydrogen sulfide mixture generated by the first-stage anaerobic process in the deep treatment section. The reacted water is returned to the Fenton oxidation unit in the deep treatment section, and the gas is recovered to the methane storage unit for later use.

[0012] 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.

[0013] The modified anion exchange membrane is prepared by the following steps:

[0014] Step a: Add polyvinyl alcohol to water to prepare a polyvinyl alcohol aqueous solution. Then add positively charged amine compounds, β-cyclodextrin, and crosslinking agents to the above solution respectively. Add inorganic acid to adjust the pH. After mixing and stirring, the casting solution is obtained.

[0015] Step b: Cast the casting solution obtained in step a onto a horizontal panel, dry it, and obtain the base film;

[0016] 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;

[0017] Step d: Dissolve dopamine in Tris-HCl buffer, adjust the pH with hydrochloric acid to obtain dopamine Tris buffer solution;

[0018] 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.

[0019] Furthermore, the lignin purification wastewater described in this invention is wastewater generated during the treatment of lignocellulose through processes such as acidification, enzymatic hydrolysis, and fermentation (to obtain the main products sugars and ethanol), and the lignin separation and purification of the remaining mother liquor. This purification process includes, but is not limited to, solid-liquid separation, alkali dissolution, and acid precipitation of the mother liquor. The COD concentration of the lignin purification wastewater is greater than 2000 mg / L, and the sulfate concentration is greater than 5000 mg / L. The main organic components of the wastewater are easily biodegradable sugars and ethanol, and it also includes a small amount of less biodegradable lignin.

[0020] 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.

[0021] Furthermore, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.05:1 to 0.4:1.

[0022] 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.

[0023] Furthermore, the inorganic acid mentioned in step a is sulfuric acid, hydrochloric acid, or nitric acid, and the pH of the solution is adjusted to 4-6.

[0024] Furthermore, in step a, 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.

[0025] Furthermore, the polyvinyl alcohol aqueous solution is prepared by stirring at 60–90°C to dissolve the polyvinyl alcohol in water.

[0026] 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.

[0027] Furthermore, the washing described in step c involves washing with water until the solution is neutral.

[0028] Furthermore, the alkalization in step c uses a 0.5–3 mol / L sodium hydroxide aqueous solution, and the alkalization time is 12–24 h.

[0029] Furthermore, the soaking time in step c is 12 to 24 hours.

[0030] Furthermore, the concentration of the Tris–HCl buffer in step d is 10–50 mmol / L.

[0031] Furthermore, in step d, the dopamine concentration is 0.4–4 g / L, and the hydrochloric acid is used to adjust the pH range to 8–9.

[0032] 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.

[0033] 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.

[0034] Those skilled in the art should understand that the organic matter in lignin purification wastewater mainly consists of lignin, sugars, and a small amount of organic acids. Lignin and sugars are electrically neutral, while organic acids 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 has a negatively charged surface, exhibiting electrostatic repulsion against negatively charged organic matter in the water, thus inhibiting organic matter fouling of the anion exchange membrane. Those skilled in the art should also understand that while the negative surface charge prevents organic matter fouling 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 β-cyclodextrin to the anion exchange membrane imparts strong hydrophilicity and alters the migration rates between anions. Anions with lower hydration levels, such as bromide and nitrate ions, show a decreased migration rate relative to chloride ions, while anions with higher hydration levels, such as sulfate ions, show an increased migration rate relative to chloride ions. On the other hand, increased membrane hydrophilicity reduces van der Waals forces between the membrane and organic solutes, decreasing attraction. 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 β-cyclodextrin and polydopamine, exhibits significantly increased antifouling ability and enhanced selective permeability to sulfate and sulfate ions.

[0035] 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.

[0036] Furthermore, the tubular microfiltration unit is mainly used to filter suspended solids, colloids, etc., to prevent clogging of the subsequent bipolar membrane electrodialysis membrane.

[0037] 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.

[0038] Furthermore, the dissolved oxygen in the primary anaerobic unit is controlled below 0.2 mg / L, the wastewater retention time is 1–16 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.

[0039] Furthermore, the dissolved oxygen in the secondary anaerobic unit is controlled below 0.1 mg / L, the wastewater retention time is 2–48 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens.

[0040] Furthermore, the stripping gas from the primary anaerobic unit is methane produced by the secondary anaerobic unit, and the methane and hydrogen sulfide mixture produced after stripping enters the regeneration unit.

[0041] Furthermore, the amount of hydrogen peroxide added in the Fenton oxidation unit is 1 to 12:1 by mass ratio of hydrogen peroxide to COD of wastewater, and the amount of iron added is 0.2 to 1.5:1 by mass ratio of iron to hydrogen peroxide, with a pH of 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 Fenton oxidation unit effluent is treated by nanofiltration, 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 by adding alkaline solution to a level of 6-9 to meet the requirements for water reuse or discharge.

[0044] Furthermore, the single-membrane electrodialysis unit uses a common anion exchange membrane. Under the action of electrode drive and the anion exchange membrane, sulfate ions in the nanofiltration concentrate permeate through the anion membrane into the concentrate side, including Fe. 2+ Fe 3+ The cations and most organic matter remain in the mother liquor; the single-membrane electrodialysis unit has a treatment time of 0.1–2 h and a current density of 1–50 mA / cm². 2 .

[0045] Furthermore, the regeneration unit introduces a mixture of methane and hydrogen sulfide to process Fe. 3+ In the reduction and regeneration process, methane does not participate in the reaction and is insoluble in water; therefore, the regeneration unit also purifies methane gas. Furthermore, by controlling the reaction between hydrogen sulfide and Fe... 3+ By controlling the concentration of ions and utilizing the slight solubility of hydrogen sulfide in water, methane with a purity greater than 95% can be obtained and stored in a methane storage unit.

[0046] Compared with the prior art, the present invention has the following advantages:

[0047] (1) The modified anion exchange membrane and improved bipolar membrane electrodialysis of the present invention efficiently separate organic matter and salt in lignin purification wastewater and have strong anti-pollution ability. After treatment by the improved bipolar membrane electrodialysis, 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 acidification, acid precipitation and Fenton oxidation units in the lignin purification process, and the sodium hydroxide can be reused in the alkali dissolution unit in the lignin purification process and used as pH adjustment in the neutralization tank. This realizes the regeneration, recycling and utilization of acid and alkali, and in particular solves the problem of high cost caused by the large amount of acid and alkali used in the Fenton oxidation system.

[0048] (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.

[0049] (3) Since the wastewater contains some organic matter that is difficult to biodegrade, the present invention uses Fenton advanced oxidation to further treat the two-stage anaerobic biological effluent. Then, the ferric iron produced by Fenton oxidation is intercepted by nanofiltration, separated by single membrane electrodialysis, and regenerated by the regeneration unit, and returned to the Fenton oxidation unit in the form of ferrous iron, so as to realize the recycling of iron catalyst and zero generation of iron sludge. Nanofiltration further desalinates the Fenton oxidation effluent, and the final nanofiltration product water has very low salt content and COD.

[0050] (4) As a typical example of high-sulfate organic wastewater, the process method of this invention for lignin purification wastewater achieves recycling and zero discharge, waste gas is stored in tanks and can be used as fuel, and salt is converted into sulfuric acid and sodium hydroxide products, realizing the overall resource utilization of water, gas, and solids. In addition, the technical route of producing sulfuric acid and sodium hydroxide products by bipolar membrane electrodialysis has lower operating costs and higher product returns compared with the technical route of producing sodium sulfate and sodium chloride products by concentration, evaporation, and crystallization.

[0051] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0052] Figure 1 Flowchart of the lignin purification wastewater treatment process in Example 1. Detailed Implementation

[0053] 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.

[0054] Example 1

[0055] The process flow diagram for treating lignin purification wastewater is as follows: Figure 1 As shown, the wastewater first undergoes hardness removal in a regulating and hardening tank, then passes through a tubular microfiltration system before entering a bipolar membrane electrodialysis unit. The acid and alkali solutions produced by the bipolar membrane electrodialysis are collected and reused, while the desalinated liquid enters the primary anaerobic digester. The chemical sludge produced in the regulating and hardening tank and tubular microfiltration unit enters a 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. The effluent from the primary anaerobic digester enters the secondary anaerobic digester. The methane gas produced in the secondary anaerobic digester is used as stripping gas and pumped back to the primary anaerobic digester, carrying away most of the hydrogen sulfide produced in the primary anaerobic digester from the wastewater system before entering the secondary anaerobic digester. The regeneration unit: the effluent from the secondary anaerobic digestion enters the Fenton oxidation unit, where hydrogen peroxide is added, and the iron catalyst comes from the regeneration unit; the effluent from the Fenton oxidation unit enters the nanofiltration unit, and the nanofiltration permeate enters the neutralization tank. After being adjusted to neutrality with alkali, the effluent is either reused or discharged, and the nanofiltration concentrate flows to the single-membrane electrodialysis unit; the effluent from the single-membrane electrodialysis unit is divided into mother liquor and anion exchange liquid. The anion exchange liquid is returned to the inlet of the bipolar membrane electrodialysis unit, and the mother liquor enters the regeneration unit to react with the methane and hydrogen sulfide mixture produced by the primary anaerobic digestion unit. The reacted water is returned to the Fenton oxidation unit, and the gas is recovered to the methane storage tank for later use.

[0056] The process method of this invention is used to treat lignin purification wastewater.

[0057] The wastewater from a lignin purification process has the following characteristics: COD 2650 mg / L, sulfate 8200 mg / L, chloride concentration 500 mg / L, calcium ion concentration 100 mg / L, magnesium ion concentration 80 mg / L, total salt content 13200 mg / L, and pH 3.5.

[0058] The bipolar membrane electrodialysis provided in this embodiment is a modified bipolar membrane electrodialysis. The anion exchange membrane is a modified anion exchange membrane with strong antifouling ability and high ion permeability. The cation exchange membrane is a general-purpose cation exchange membrane (Hefei Capgemini Polymer Co., Ltd., China, model CJ-MC-3). The bipolar membrane is a general-purpose bipolar membrane (imported). BP-1 membrane).

[0059] The modified anion exchange membrane described above was prepared by the following method:

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] The specific operating parameters for treating lignin purification wastewater are as follows:

[0067] Wastewater first enters the equalization and hardening tank, where sodium hydroxide (250 mg / L), sodium carbonate (180 mg / L), and PAM (6 mg / L) are added to raise 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 before entering a bipolar membrane electrodialysis unit. The chemical sludge produced in the equalization and hardening tank and the tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the equalization and hardening tank, while the dried sludge is sent off-site. The bipolar membrane electrodialysis treatment time is 0.7 hours, and the current density is 35 mA / cm³. 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.48 mol / L, the alkalinity of the alkaline solution was 0.4 mol / L, the salt content of the desalination solution was 2.7 g / L, the COD was 3046 mg / L, the water production rate (the percentage of desalination solution to the total influent) was 79%, the sulfate ion selective permeability was 82.1%, the chloride ion selective permeability was 87.8%, and the organic matter rejection rate was 90.8%.

[0068] The bipolar membrane electrodialysis desalination solution enters the primary anaerobic stage, where dissolved oxygen is controlled below 0.15 mg / L, pH is maintained at 5.5 by acidification, wastewater retention time is 5 h, temperature is 30℃, and the bacterial strains are organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD is reduced to 2820 mg / L, and the salt content is reduced to 1712 mg / L. In the secondary anaerobic stage, dissolved oxygen is controlled below 0.1 mg / L, pH is maintained at 8.1 by alkaliification, wastewater retention time is 18 h, temperature is 35℃, and the bacterial strains are methanogenic bacteria. The effluent COD is 265 mg / L, and the salt content is 1651 mg / L. The secondary anaerobic effluent, at a concentration of g / L, enters the Fenton oxidation unit. The Fenton oxidation unit adds hydrogen peroxide at a 4:1 ratio to wastewater COD, specifically 1060 mg / L. The iron-based catalyst comes from the regeneration unit's recycled water, resulting in an iron ion concentration of 1010 mg / L after mixing. The pH is adjusted to 3.5 by adding acid, ultimately reducing the COD from 265 mg / L to 51 mg / L. The effluent then enters nanofiltration. The nanofiltration permeability is 89%, with iron and sulfate ion rejection rates both exceeding 98%. The nanofiltration permeate enters a neutralization tank, where alkali is added to adjust the pH to 6.5. The final effluent has a COD of 55 mg / L and a salt content of 1422 mg / L, meeting the requirements for reclaimed water quality.

[0069] Nanofiltration concentrate is then fed into a single-membrane electrodialysis system. The treatment time is 0.4 hours, and the current density is 20 mA / cm³. 2The anion exchange membrane used is a product of Asahi Glass Corporation of Japan (SELEMION AMV), with a sulfate ion selective permeability of 71% and an iron ion rejection rate of over 98%. The anion solution is returned to the bipolar membrane electrodialysis feed water, and the mother liquor enters the regeneration unit. The total iron ion content of the regeneration unit feed water is less than 10% ferrous iron. After reacting with the hydrogen sulfide mixture, the total iron ion content of the regeneration unit effluent is greater than 95% ferrous iron. The effluent is recycled to the Fenton oxidation unit, and the insoluble gas from the regeneration unit is sent to the methane storage tank. The methane purity is 97.5%.

[0070] The alkali and acid used to adjust the pH in the above units are both derived from the alkali and acid solutions prepared by bipolar membrane electrodialysis.

[0071] As can be seen from this embodiment, the present invention can effectively treat lignin purification wastewater. The bipolar membrane electrodialysis with the modified anion exchange membrane as the core efficiently separates organic matter from salts, and finally achieves wastewater reuse and zero discharge. Salts are converted into sulfuric acid and sodium hydroxide for reuse, and most of the organic matter is converted into high-purity methane for resource utilization. In addition, the present invention also solves the problem of high cost caused by the large amount of acid and alkali used in the Fenton oxidation system, and realizes the recycling of iron catalyst.

[0072] Example 2

[0073] use Figure 1 The process shown is used to treat lignin purification wastewater.

[0074] The wastewater from a lignin purification process has the following characteristics: COD 4200 mg / L, sulfate 11500 mg / L, chloride concentration 1000 mg / L, calcium ion concentration 150 mg / L, magnesium ion concentration 80 mg / L, total salt content 19100 mg / L, and pH 3.3.

[0075] 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.

[0076] Wastewater first enters the equalization and hardening tank, where sodium hydroxide (260 mg / L), sodium carbonate (200 mg / L), and PAM (6 mg / L) are added to raise the solution pH to 8.1. 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 before entering a bipolar membrane electrodialysis unit. The chemical sludge produced in the equalization and hardening tank and the tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the equalization and hardening tank, while the dried sludge is sent off-site. The bipolar membrane electrodialysis treatment time is 0.8 hours, and the current density is 45 mA / cm³. 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.68 mol / L, the alkalinity of the alkaline solution was 0.54 mol / L, the salt content of the desalination solution was 3.5 g / L, the COD was 4914 mg / L, the water production rate (the percentage of desalination solution to the total influent) was 77%, the sulfate ion selective permeability was 81.5%, the chloride ion selective permeability was 88.9%, and the organic matter rejection rate was 90.1%.

[0077] The bipolar membrane electrodialysis desalination solution enters the primary anaerobic stage, where dissolved oxygen is controlled below 0.15 mg / L, pH is maintained at 5.4 by adding acid, wastewater retention time is 6 h, temperature is 30℃, and the bacterial strains are organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD is reduced to 4188 mg / L, and the salt content is reduced to 2415 mg / L. In the secondary anaerobic stage, dissolved oxygen is controlled below 0.1 mg / L, pH is maintained at 8.2 by adding alkali, wastewater retention time is 24 h, temperature is 35℃, and the bacterial strains are methanogenic bacteria. The effluent COD is 306 mg / L, and the salt content is 2288 mg / L. / L; the secondary anaerobic effluent enters the Fenton oxidation unit; the Fenton oxidation unit adds hydrogen peroxide at a mass ratio of 3.7:1 to wastewater COD, specifically 1132 mg / L. The iron-based catalyst comes from the regeneration unit's recycled water, and the iron ion concentration after mixing is 1150 mg / L. The pH is adjusted to 3.5 by adding acid, and the final COD is reduced from 306 mg / L to 55 mg / L. The effluent then enters the nanofiltration unit; the nanofiltration permeability is 88%, and the iron ion and sulfate ion rejection rates are both greater than 98%; the nanofiltration permeate enters the neutralization tank, and the pH is adjusted to 6.6 by adding alkali. The final effluent has a COD of 58 mg / L and a salt content of 1452 mg / L, meeting the requirements for reclaimed water quality.

[0078] Nanofiltration concentrate is then fed into a single-membrane electrodialysis system. The treatment time is 0.45 h, and the current density is 25 mA / cm³. 2The anion exchange membrane used is a product of Asahi Glass Corporation of Japan (SELEMION AMV), with a sulfate ion selective permeability of 73.6% and an iron ion rejection rate of over 98%. The anion solution is returned to the bipolar membrane electrodialysis feed water, and the mother liquor enters the regeneration unit. The total iron ion content of the regeneration unit feed water is less than 10% ferrous ions. After reacting with the hydrogen sulfide mixture, the total iron ion content of the regeneration unit effluent is greater than 95% ferrous ions. The effluent is recycled to the Fenton oxidation unit, and the insoluble gas from the regeneration unit is sent to the methane storage tank. The methane purity is 97.8%.

[0079] The alkali and acid used to adjust the pH in the above-mentioned unit are both derived from alkaline and acid solutions prepared by bipolar membrane electrodialysis. This embodiment demonstrates that the present invention can effectively treat lignin purification wastewater of different concentrations, achieving resource utilization of water, air, and solids in the treatment process.

[0080] Comparative Example 1

[0081] The treatment of lignin purification wastewater was the same as in Example 1, and the process route and implementation steps were also the same. The difference was that both the anion and cation exchange membranes and the bipolar membrane in the bipolar membrane electrodialysis were general-purpose membranes. The anion exchange membrane was a product of Asahi Glass Corporation of Japan (SELEMION AMV), the cation exchange membrane was a product of Hefei Kaijie Polymer Co., Ltd. of China (model CJ-MC-3), and the bipolar membrane was imported. BP-1 membrane.

[0082] Specifically, as in Example 1, the wastewater, after being treated by a conditioning and hardening tank and tubular microfiltration, enters a bipolar membrane electrodialysis system. The bipolar membrane electrodialysis treatment time is 0.7 hours, and the current density is 35 mA / cm². 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.36 mol / L, the alkalinity of the alkaline solution was 0.27 mol / L, the salt content of the desalinated solution was 5.5 g / L, the COD was 2781 mg / L, the permeate yield (percentage of desalinated solution to total influent) was 79%, the sulfate ion selective permeate rate was 65.3%, the chloride ion selective permeate rate was 76.5%, and the organic matter rejection rate was 82.9%. The bipolar membrane electrodialysis desalinated solution entered the primary anaerobic digester, with dissolved oxygen controlled at 0.15 mg / L. For effluent below L, the pH was controlled at 5.5 by adding acid, the wastewater retention time was extended to 7 hours, the temperature was 30℃, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD was reduced to 2052 mg / L, and the salt content was reduced to 3914 mg / L. For secondary anaerobic digestion, the dissolved oxygen was controlled below 0.1 mg / L, the pH was controlled at 8.1 by adding alkali, the wastewater retention time was extended to 24 hours, the temperature was 35℃, and the bacterial strains were methanogenic bacteria. The effluent COD was 933 mg / L, and the salt content was 3036 mg / L.

[0083] 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. Under the same energy consumption, the concentrations of both acid and alkali solutions are significantly reduced, and the salt concentration of the desalinated solution is excessively high (5.5 g / L). Subsequent primary anaerobic treatment, with extended retention time, resulted in an effluent salt content as high as 3914 mg / L, especially with relatively high sulfate levels. Even with extended secondary anaerobic retention time, the effluent COD remained as high as 933 mg / L, significantly increasing the processing pressure on the Fenton oxidation unit. Changes in COD and salt content indicate a significant decrease in overall methane production during secondary anaerobic treatment, along with increased sulfate reduction reactions, which explains the decreased efficiency of secondary anaerobic treatment. Conversely, a significant increase in hydrogen sulfide production during primary anaerobic treatment resulted in relatively less methane in the stripped gas, leading to a decrease in hydrogen sulfide stripping efficiency and ultimately a continuous deterioration of the reaction environment during primary anaerobic treatment.

[0084] Comparative Example 2

[0085] The treatment of lignin purification wastewater 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 nanofiltration concentrate directly enters the regeneration unit without single-membrane electrodialysis.

[0086] Specifically, as in Example 1, the wastewater sequentially passes through a molybdenum removal tank, a hardening and equalization tank, tubular microfiltration, bipolar membrane electrodialysis, two-stage oxidation, Fenton oxidation, and nanofiltration before entering the regeneration unit. The COD of the effluent from the two-stage oxidation is 265 mg / L, and the salt content is 1651 mg / L. After Fenton oxidation, the COD decreases to 51 mg / L. Due to the addition of iron ions (1010 mg / L) and acid in the Fenton oxidation unit, the salt content increases to 3215 mg / L. The nanofiltration permeate rate is 89%, and the iron ion and sulfate ion rejection rates are both greater than 98%. The nanofiltration permeate enters the neutralization tank, where alkali is added to adjust the pH to 6.5. The final effluent has a COD of 1651 mg / L. The COD of the nanofiltration concentrate is 55 mg / L, and the salt content is 1356 mg / L, which meets the requirements for recycled water quality. The COD of the nanofiltration concentrate is 86 mg / L, and the salt content is 24138 mg / L, of which the iron ion concentration is 9165 mg / L and the sulfate concentration is 9422 mg / L. The nanofiltration concentrate flows to the regeneration unit, is regenerated by a hydrogen sulfide mixture, and then flows back to Fenton oxidation to complete the local circulation. Since there is no sulfate removal unit during the circulation process, the sulfate concentration of the nanofiltration concentrate rises to 18767 mg / L after the second circulation.

[0087] As can be seen from this comparative example, without single-membrane electrodialysis, there is no sulfate removal process in the local circulation process of Fenton oxidation, nanofiltration, and regeneration units, which leads to an increasing sulfate concentration in the wastewater, eventually resulting in salt precipitation and scaling, and ultimately system failure.

[0088] Comparative Example 3

[0089] The treatment of lignin purification wastewater 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 regeneration unit.

[0090] Specifically, as in Example 1, the wastewater was sequentially treated through a molybdenum removal tank, a hardening and equalization tank, tubular microfiltration, bipolar membrane electrodialysis, two-stage oxidation, Fenton oxidation, nanofiltration, and single-membrane electrodialysis. After the above unit treatments, the iron ion concentration of the mother liquor from the single-membrane electrodialysis was 9075 mg / L, the proportion of ferrous ions in the total iron ions was less than 10%, mainly ferric ions, the salt content was 16143 mg / L, and the pH was 3.1. Considering all factors, this mother liquor did not meet the discharge standards. If alkali was added for neutralization, a large amount of iron sludge would be generated, so how to dispose of it became a major problem. In addition, the Fenton oxidation system required the replenishment of fresh ferrous salts, and the methane and hydrogen sulfide mixture produced by the primary anaerobic digestion could not be properly treated. Calculations showed that the methane purity in the methane storage tank was as low as 78.4% at this point.

[0091] As can be seen from this comparative example, compared with Example 1, the purity of methane in the final methane storage tank is severely affected by the lack of a regeneration unit, and Fenton oxidation requires the continued addition of ferrous salt, which significantly increases the treatment cost. At the same time, the wastewater containing ferric iron generated after Fenton oxidation cannot be properly disposed of.

Claims

1. A process for treating lignin purification wastewater, comprising a pretreatment section, a deep treatment section, and a regeneration section; The pretreatment section includes a hardening and conditioning unit, a tubular microfiltration unit, a bipolar membrane electrodialysis unit, and a sludge thickening, dewatering, and drying unit. Wastewater first undergoes hardening and conditioning in the hardening and conditioning unit, then passes through the tubular microfiltration unit for filtration before entering the bipolar membrane electrodialysis unit. The acid and alkali solutions produced by the bipolar membrane electrodialysis unit are collected and reused, while the desalinated solution enters the primary anaerobic unit of the deep treatment section. The chemical sludge produced by the hardening and conditioning unit and the tubular microfiltration unit 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. The advanced treatment section sequentially includes a primary anaerobic unit, a secondary anaerobic unit, a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit. The effluent from the neutralization unit is reused. The desalinated liquid from the bipolar membrane electrodialysis unit enters the primary anaerobic unit, where organic acidification and sulfate reduction are mainly carried out, generating hydrogen sulfide gas. It then enters the secondary anaerobic unit, where methanogenesis is mainly carried out. The generated methane is used as stripping gas and pumped to the primary anaerobic unit. Most of the hydrogen sulfide generated in the primary anaerobic unit is carried out of the wastewater system and enters the regeneration unit of the regeneration section. The effluent from the secondary anaerobic unit is sequentially treated by the Fenton oxidation unit and the nanofiltration unit. The effluent from the nanofiltration unit is divided into nanofiltration concentrate and nanofiltration permeate. The nanofiltration permeate enters the neutralization tank, and the nanofiltration concentrate enters the single-membrane electrodialysis unit of the regeneration section. The regeneration section includes a single-membrane electrodialysis unit, a regeneration unit, and a methane storage unit. The effluent from the single-membrane electrodialysis unit is divided into mother liquor and anion exchange liquid. The anion exchange liquid is returned to the inlet of the bipolar membrane electrodialysis unit in the pretreatment section, while the mother liquor enters the regeneration unit and reacts with the methane and hydrogen sulfide mixture generated by the first-stage anaerobic process in the deep treatment section. The reacted water is returned to the Fenton oxidation unit in the deep treatment section, and the gas is recovered to the methane storage unit for later use. in, The bipolar membrane electrodialysis system consists 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. The modified anion exchange membrane is prepared by the following steps: Step a: Polyvinyl alcohol is added to water to prepare a polyvinyl alcohol aqueous solution. Then, a positively charged amine compound, β-cyclodextrin, 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. 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 of the modified anion exchange membrane, in step a, 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 β-cyclodextrin to polyvinyl alcohol is 0.05:1 to 0.4:1; 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 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.

5. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the concentration of Tris–HCl buffer in step d is 10–50 mmol / L; the mass concentration of dopamine is 0.4–4 g / L; the pH range of hydrochloric acid is adjusted to 8–9; the concentration of copper sulfate 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 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.

7. 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 .

8. 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 1–16 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.

9. 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 2–48 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens.

10. The process method according to claim 1, characterized in that, The amount of hydrogen peroxide added in the Fenton oxidation unit is 1-12:1 by mass ratio of hydrogen peroxide to COD of wastewater, and the amount of iron added is 0.2-1.5:1 by mass ratio of iron to hydrogen peroxide, with a pH of 3-5. The iron is ferrous iron, and the ferrous salt is selected from at least one of ferrous chloride, ferrous nitrate, and ferrous sulfate.

11. The process method according to claim 1, characterized in that, The neutralization unit adjusts the pH to 6-9 by adding alkaline solution.

12. The process method according to claim 1, characterized in that, The single-membrane electrodialysis unit uses a common anion exchange membrane. Under the action of electrode drive and anion exchange membrane, sulfate ions in the nanofiltration concentrate permeate through the anion membrane into the concentrate side, including Fe. 2+ Fe 3+ The cations and most organic matter remain in the mother liquor; the single-membrane electrodialysis unit has a treatment time of 0.1–2 h and a current density of 1–50 mA / cm². 2 .

13. The process method according to claim 1, characterized in that, The regeneration unit introduces a mixture of methane and hydrogen sulfide to regenerate Fe. 3+ During the reduction and regeneration process, methane does not participate in the reaction and is insoluble in water, thus playing a purification role.

14. The process method according to claim 13, characterized in that, In the regeneration unit, the reaction of hydrogen sulfide and Fe is controlled. 3+ By controlling the concentration of ions and utilizing the slight solubility of hydrogen sulfide in water, methane with a purity greater than 95% can be obtained and stored in a methane storage unit.

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