Process method for treating lignin purification wastewater

Abstract

A process method for treating lignin purification wastewater comprises a pretreatment section, a reaction section and a tail gas treatment section, the pretreatment section comprises an adjusting hardness removal unit, a tubular microfiltration unit, a bipolar membrane electrodialysis unit and a sludge thickening, dewatering and drying unit; the reaction section sequentially comprises a first-stage anaerobic unit, a second-stage anaerobic unit, an ozone catalytic oxidation unit and a BAF unit; the tail gas treatment section comprises an alkali liquor absorption unit and a methane storage unit. The modified anion exchange membrane is used for electrodialysis, so that most organic matters in the wastewater are intercepted in the desalinized solution, and most salt is converted into sulfuric acid and sodium hydroxide. In the process method, recycling and zero emission of wastewater are realized, waste gas is stored in a tank and can be used as fuel, salt is converted into sulfuric acid and sodium hydroxide products, and integral recycling of water, gas and solid is realized.

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] Petroleum is a non-renewable resource. Bioenergy is a renewable resource. For fuel ethanol, corn and wheat are commonly used as fermentation raw materials abroad, but grains are not suitable as raw materials in my country, so alternatives must be sought.

[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 COD concentrations exceeding 2000 mg / L and sulfate concentrations exceeding 5000 mg / L. The salt content primarily originates from sodium hydroxide added in the alkali dissolution unit and sulfuric acid added in the acidification and acid precipitation units. While anaerobic treatment and methane recovery are typically used when the wastewater has a high organic content, 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. Summary of the Invention

[0005] To address the above shortcomings, this invention provides a process for treating lignin purification wastewater, achieving zero wastewater discharge, converting salts into sulfuric acid and sodium hydroxide for reuse, and converting most organic matter into methane for resource recovery.

[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 treating lignin purification wastewater, including a pretreatment section, a reaction section, and a tail gas treatment section;

[0008] 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 reaction 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.

[0009] The reaction section sequentially includes a primary anaerobic unit, a secondary anaerobic unit, an ozone catalytic oxidation unit, and a BAF unit, with BAF effluent reused. The bipolar membrane electrodialysis unit produces desalinated water which enters the primary anaerobic unit, primarily for organic acidification and sulfate reduction, generating hydrogen sulfide gas. This then enters the secondary anaerobic unit, mainly for methanogenesis. The generated methane is pumped back to the primary anaerobic unit as stripping gas, carrying most of the hydrogen sulfide generated in the primary anaerobic unit out of the wastewater system and into the alkaline absorption unit of the tail gas treatment section. The secondary anaerobic unit... The effluent from the oxygen unit enters the BAF unit, and the BAF effluent is reused. The tail gas treatment section includes an alkali absorption unit and a methane storage unit. The alkali absorption unit purifies the methane and hydrogen sulfide mixture produced by the primary anaerobic unit to obtain high-purity methane, which is then stored in the methane storage unit for later use. After the alkali absorption unit has been running for a period of time, the lean alkali solution (low hydrogen sulfide content) becomes a rich alkali solution (high hydrogen sulfide content). At this time, the rich alkali solution is returned to the bipolar membrane electrodialysis regeneration section in the reaction section, and new alkali solution is simultaneously added to the alkali absorption unit.

[0010] The bipolar membrane electrodialysis unit 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.

[0011] The modified anion exchange membrane is prepared by the following steps:

[0012] Step a: Add polyvinyl alcohol to water to prepare a polyvinyl alcohol aqueous solution, then add positively charged amine compounds and β-cyclodextrin to the above solution respectively, mix and react to obtain a casting solution;

[0013] Step b: Cast the casting solution obtained in step a onto a horizontal panel, dry it, and obtain the base film;

[0014] Step c: Immerse the base film obtained in step b in a crosslinking solution composed of crosslinking agent, sodium sulfate, sulfuric acid, and water;

[0015] Step d: Wash the cross-linked membrane, alkalize it in an alkaline solution, and then soak it in water to obtain a polyvinyl alcohol anion exchange membrane;

[0016] Step e: Dissolve the polyanionic modifier and sodium chloride in Tris-HCl buffer solution, and adjust the pH to 8-9 with hydrochloric acid to obtain the modified solution;

[0017] Step f: Place the anion exchange membrane obtained in step d in the middle of the DC electrodeposition apparatus to form two compartments. Put the modified solution prepared in step e into the compartment on the cathode side and put water into the compartment on the anode side to carry out the electrodeposition reaction and obtain the modified anion exchange membrane.

[0018] 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.

[0019] Furthermore, the positively charged amine compound is 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.

[0020] Furthermore, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.05:1 to 0.4:1.

[0021] 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 and β-cyclodextrin is 2–10 h.

[0022] Furthermore, the polyvinyl alcohol aqueous solution is prepared by stirring at 60–90°C to dissolve the polyvinyl alcohol in water.

[0023] 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.

[0024] Furthermore, the crosslinking agent is selected from aldehyde or acid crosslinking agents, preferably glutaraldehyde.

[0025] Furthermore, the crosslinking solution contains, by weight, 2%–8% of the crosslinking agent, 4%–15% of sodium sulfate, and 1%–4% of sulfuric acid, with the remainder being water.

[0026] Furthermore, in step c, the base membrane is immersed in the crosslinking solution for 1 to 6 hours; crosslinking is formed in the membrane through an acetal reaction.

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

[0028] Furthermore, the alkalization is performed using a 0.5–3 mol / L sodium hydroxide aqueous solution for 12–24 hours.

[0029] Furthermore, the soaking time in step d is 12–24 hours.

[0030] Furthermore, the polyanionic modifier is one or more of sodium poly4-styrene sulfonate, sodium p-styrene sulfonate, sodium polyethylene sulfonate, and sodium polypropylene sulfonate, preferably sodium poly4-styrene sulfonate.

[0031] Furthermore, in step e, the mass concentration of the polyanionic modifier in the modified solution is 1–10 g / L, and the mass concentration of sodium chloride is 3–30 g / L.

[0032] Furthermore, the concentration of the Tris–HCl buffer in step e is 10–50 mmol / L.

[0033] Furthermore, the electrodeposition reaction time in step f is 0.2–2 h, and the current density is 1–50 mA / cm². 2 A polyanion modifier was deposited on the surface of the anion exchange membrane using an electrodeposition method to modify the membrane surface.

[0034] Furthermore, after obtaining the modified anion exchange membrane, it is stored in a sodium chloride solution with a mass concentration of 5–20 g / L.

[0035] 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. The anion exchange membrane of this invention, after modification with a polyanion modifier, has a negatively charged surface, exhibiting electrostatic repulsion against negatively charged organic matter in the water, effectively inhibiting organic matter contamination of the anion exchange membrane. While the negative surface charge prevents organic matter contamination through electrostatic action, it also affects the migration rate of inorganic anions; the higher 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 sulfate ions, with higher hydration levels, 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 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 a polyanion modifier and β-cyclodextrin, exhibits significantly increased antifouling ability and markedly enhanced selective permeability to sulfate ions.

[0036] 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.

[0037] Furthermore, the tubular microfiltration unit is mainly used to filter suspended solids, colloids, etc., to prevent subsequent electrodialysis membrane clogging.

[0038] 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.

[0039] Furthermore, the dissolved oxygen in the primary anaerobic unit is controlled below 0.2 mg / L, the wastewater retention time is 1–8 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.

[0040] Furthermore, the dissolved oxygen in the secondary anaerobic unit is controlled below 0.1 mg / L, the wastewater retention time is 2–36 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens.

[0041] 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 is absorbed by the alkaline solution after stripping.

[0042] Furthermore, in the ozone catalytic oxidation unit, ozone is added at a mass concentration ratio of ozone to wastewater COD of 0.1 to 2:1, the reaction time is 10 to 120 minutes, and the catalyst is a conventional supported metal catalyst.

[0043] Furthermore, the influent salt concentration of the BAF unit is less than 3000 mg / L, common biological strains are used, dissolved oxygen is controlled above 2 mg / L, and the wastewater retention time is 2 to 16 hours.

[0044] Furthermore, the alkali used in the alkali absorption unit is sodium hydroxide, which can ensure the purity of the alkali solution produced 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 into sulfate by the action of the byproduct chlorine gas, and finally transformed into sulfuric acid.

[0045] Furthermore, the methane purity in the methane storage unit is greater than 98%.

[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 have achieved efficient separation of organic matter and salt in lignin purification wastewater and have strong anti-pollution ability. After treatment by improved bipolar membrane electrodialysis, most of the organic matter is retained in the desalination liquid and most of the salt is converted into sulfuric acid and sodium hydroxide. The sulfuric acid can be recycled to the acidification and acid precipitation unit in the lignin purification process, and the sodium hydroxide can be recycled to the alkali dissolution unit and the alkali absorption unit in the tail gas treatment section of the lignin purification process, thus realizing the regeneration, recycling and utilization of acid and alkali.

[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, which is then 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 effluent from both stages of anaerobic treatment has low COD and salt content, thereby reducing the cost of ozone catalytic oxidation treatment and facilitating the selection of common composite microbial communities in the BAF unit, thus improving its treatment efficiency.

[0049] (3) The exhaust gas treatment unit of the present invention is simple and efficient. Thanks to the ability of bipolar membrane electrodialysis to achieve acid-base regeneration, the present invention adopts the simplest alkaline absorption method to efficiently remove acidic gas and produce high-purity methane. Sulfur is finally converted into sulfuric acid, and sodium hydroxide is used for regeneration.

[0050] (4) As a typical example of high sulfate organic wastewater, the process method of the present invention is used for lignin purification wastewater. The wastewater is recycled and zero-discharged, the waste gas is stored in tanks and can be used as fuel, and the salt is converted into sulfuric acid and sodium hydroxide products, realizing the overall resource utilization of water, gas and solids.

[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;

[0053] Figure 2 Schematic diagram of anion exchange membrane electrodeposition modification in Example 1. Detailed Implementation

[0054] 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.

[0055] Example 1

[0056] 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 solution 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 pumped back to the primary anaerobic digester as stripping gas, carrying most of the hydrogen sulfide produced in the primary anaerobic digester out of the wastewater system before entering the alkaline solution absorption unit. The effluent from the secondary anaerobic digester is reused after ozone catalytic oxidation and BAF treatment. The alkaline solution absorption unit purifies the methane and hydrogen sulfide mixture to obtain high-purity methane, which is stored in a methane storage tank for later use. After the alkaline solution absorption unit has been running for a period of time, the lean alkaline solution becomes rich alkaline solution. At this point, the rich alkaline solution is returned to the bipolar membrane electrodialysis unit for regeneration, while new alkaline solution is simultaneously added to the alkaline solution absorption unit.

[0057] The process method of this invention is used to treat lignin purification wastewater.

[0058] The wastewater from a lignin purification process has the following characteristics: COD 2820 mg / L, sulfate 8500 mg / L, chloride concentration 500 mg / L, calcium ion concentration 100 mg / L, magnesium ion concentration 80 mg / L, total salt content 13600 mg / L, and pH 3.5.

[0059] The bipolar membrane electrodialysis provided in this embodiment is a modified electrodialysis method. 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).

[0060] The modified anion exchange membrane described above was prepared by the following method:

[0061] Step a: Add polyvinyl alcohol to deionized water to prepare a 10% aqueous solution. Stir at 80°C to dissolve it. Then add glycidyltrimethylammonium chloride and β-cyclodextrin to the water. The mass ratio of polyvinyl alcohol to glycidyltrimethylammonium chloride is 1:0.5, and the mass ratio of polyvinyl alcohol to β-cyclodextrin is 1:0.2. React for 6 hours to obtain a viscous liquid, which is the casting solution.

[0062] 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.

[0063] Step c: The vacuum-dried membrane is further immersed in a mixed solution composed of glutaraldehyde, sodium sulfate, sulfuric acid, and deionized water, with the mass percentages of glutaraldehyde, sodium sulfate, and sulfuric acid being 6%:8%:2% and the remainder being deionized water. The membrane is soaked for 4 hours to form crosslinks in the membrane through an acetal reaction.

[0064] Step d: Wash the cross-linked membrane with deionized water until neutral, then alkalize it in a 1 mol / L sodium hydroxide aqueous solution for 16 h, and then soak it in deionized water for 16 h to obtain the anion exchange membrane to be modified.

[0065] Step e: Dissolve the polyanionic modifier and sodium chloride in 20 mmol / L Tris–HCl buffer, adjust the pH to 8.5 with hydrochloric acid, the mass concentration of the polyanionic modifier is 2 g / L, and the mass concentration of sodium chloride is 10 g / L; the polyanionic modifier is sodium poly4-styrene sulfonate.

[0066] Step f: Surface modification is performed using an electrodeposition method, such as... Figure 2As shown, the anion exchange membrane to be modified was placed in the middle of a DC electrodeposition apparatus, forming two compartments. The cathode-side compartment contained the modification solution, and the anode-side compartment contained deionized water. The electrodeposition time was 0.7 h, and the current density was 20 mA / cm². 2 ;

[0067] Step g: 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 lignin purification wastewater are as follows:

[0069] 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 electrodialysis treatment time is 0.6 hours, and the current density is 35 mA / cm³. 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.45 mol / L, the alkalinity of the alkaline solution was 0.38 mol / L, the salt content of the desalination solution was 3.1 g / L, the COD was 3211 mg / L, the water production rate (the percentage of desalination solution to the total influent) was 80%, the sulfate ion selective permeability was 81.6%, the chloride ion selective permeability was 87.5%, and the organic matter rejection rate was 91.1%.

[0070] The electrodialysis desalination solution enters the primary anaerobic stage, with dissolved oxygen controlled below 0.15 mg / L. The pH is maintained at 5.5 by adding sulfuric acid. The wastewater retention time is 6 hours, and the temperature is 30°C. The bacterial strains are organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD is reduced to 2755 mg / L, and the salt content is reduced to 1437 mg / L. In the secondary anaerobic stage, dissolved oxygen is controlled below 0.1 mg / L, and the pH is maintained at 8.1 by adding sodium hydroxide. The wastewater retention time is 14 hours, and the temperature is 35°C. The bacterial strain is methanogenic bacteria. The effluent COD is reduced to 2755 mg / L, and the salt content is reduced to 1437 mg / L. The OD was 368 mg / L and the salt content was 1366 mg / L. The effluent from the secondary anaerobic process entered ozone catalytic oxidation, where the ozone dosage was 300 mg / L, the reaction time was 40 minutes, and the catalyst was an iron catalyst supported on activated carbon composite material. The effluent COD was 125 mg / L and then entered the BAF. The BAF used ordinary composite biological bacteria, with dissolved oxygen controlled above 2 mg / L and a residence time of 6 hours. The effluent COD was as low as 48 mg / L and the salt content was 1385 mg / L, meeting the reuse requirements.

[0071] 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, thereby obtaining methane with a purity of 98.6%, 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 time, the alkali-lean solution is returned to the bipolar membrane electrodialysis regeneration stage in the reaction section, while new alkali solution is replenished to the alkali absorption unit. The new alkali solution comes from the alkali solution prepared by the bipolar membrane electrodialysis. 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 the bipolar membrane electrodialysis. The excess alkali and acid solutions are recycled to the acidification, acid precipitation, and alkali dissolution units in the lignin purification process.

[0072] 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 recovery.

[0073] Example 2

[0074] use Figure 1 The process shown is used to treat lignin purification wastewater.

[0075] The wastewater from a lignin purification process has the following characteristics: COD 4500 mg / L, sulfate 12000 mg / L, chloride concentration 1000 mg / L, calcium ion concentration 150 mg / L, magnesium ion concentration 80 mg / L, total salt content 19850 mg / L, and pH 3.3.

[0076] 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 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 0.55:1, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.3:1, and the reaction time in step a is 7h, everything else is the same as in Example 1.

[0077] 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 electrodialysis treatment time is 0.7 hours, and the current density is 45 mA / cm³. 2 After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.66 mol / L, the alkalinity of the alkaline solution was 0.52 mol / L, the salt content of the desalination solution was 4.2 g / L, the COD was 5203 mg / L, the water production rate (the percentage of desalination solution to the total influent) was 78%, the sulfate ion selective permeability was 80.8%, the chloride ion selective permeability was 88.7%, and the organic matter rejection rate was 90.2%.

[0078] The electrodialysis desalination solution enters the primary anaerobic stage, with dissolved oxygen controlled below 0.15 mg / L. The pH is maintained at 5.4 by adding sulfuric acid. The wastewater retention time is 8 hours, and the temperature is 30°C. The bacterial strains are organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD is reduced to 4486 mg / L, and the salt content is reduced to 1978 mg / L. In the secondary anaerobic stage, dissolved oxygen is controlled below 0.1 mg / L, and the pH is maintained at 8.2 by adding sodium hydroxide. The wastewater retention time is 18 hours, and the temperature is 35°C. The bacterial strains are methanogenic bacteria. The effluent COD is reduced to 4486 mg / L, and the salt content is reduced to 1978 mg / L. The OD was 431 mg / L and the salt content was 1835 mg / L. The effluent from the secondary anaerobic process entered ozone catalytic oxidation, where the ozone dosage was 350 mg / L, the reaction time was 40 minutes, and the catalyst was an iron catalyst supported on activated carbon composite material. The effluent COD was 154 mg / L and then entered the BAF. The BAF used ordinary composite biological bacteria, with dissolved oxygen controlled above 2 mg / L and a residence time of 7 hours. The effluent COD was as low as 54 mg / L and the salt content was 1844 mg / L, meeting the reuse requirements.

[0079] 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, thereby 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 time, the alkali-lean solution is returned to the bipolar membrane electrodialysis regeneration stage in the reaction section, while new alkali solution is replenished to the alkali absorption unit. The new alkali solution comes from the alkali solution prepared by the bipolar membrane electrodialysis. 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 the bipolar membrane electrodialysis. The excess alkali and acid solutions are recycled to the acidification, acid precipitation, and alkali dissolution units in the lignin purification process.

[0080] As can be seen from this embodiment, the present invention can effectively treat lignin purification wastewater of different concentrations, and the treatment process realizes the resource utilization of water, air and solids.

[0081] Comparative Example 1

[0082] The treatment of lignin purification wastewater was carried out in the same manner as in Example 1, with the same process route and implementation steps. The difference was that both the anion and cation exchange membranes in the bipolar membrane electrodialysis were general-purpose membranes. The anion exchange membrane used was a product of Asahi Glass Co., Ltd. (SELEMION AMV) from Japan, and the cation exchange membrane used was a product of Hefei Kaijie Polymer Co., Ltd. (model CJ-MC-3) from China. The bipolar membrane was imported. BP-1 membrane.

[0083] Specifically, as in Example 1, the wastewater, after being treated by a regulating and hardening tank and tubular microfiltration, enters electrodialysis. The electrodialysis treatment time is 0.6 hours, and the current density is 35 mA / cm². 2After treatment, the acidity of the acid solution produced by bipolar membrane electrodialysis was 0.33 mol / L, the alkalinity of the alkaline solution was 0.25 mol / L, the salt content of the desalinated solution was 5.8 g / L, the COD was 2933 mg / L, the permeate rate (percentage of desalinated solution to total influent) was 80%, the sulfate ion selective permeate rate was 63.4%, the chloride ion selective permeate rate was 73.3%, and the organic matter rejection rate was 83.2%. The electrodialysis desalinated solution entered the primary anaerobic digester, with dissolved oxygen controlled below 0.15 mg / L, and the pH controlled at 5.5 by adding sulfuric acid. Wastewater retention... The treatment time was 10 hours, the temperature was 30℃, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD was reduced to 1956 mg / L, and the salt content was reduced to 3145 mg / L. The dissolved oxygen in the secondary anaerobic unit was controlled below 0.1 mg / L, and the pH was controlled at 8.1 by adding sodium hydroxide. The wastewater retention time was 18 hours, the temperature was 35℃, and the bacterial strain was methanogenic bacteria. The effluent COD was 965 mg / L, and the salt content was 2438 mg / L. Due to the high COD concentration in the effluent, the ozone dosage in the catalytic oxidation unit needed to be significantly increased, which greatly increased the treatment cost.

[0084] This comparative example shows that electrodialysis using unmodified cation and anion exchange membranes has poor ability to separate organic matter and salts, and the anion exchange membrane is easily fouled by organic matter. Under the same energy consumption, the concentrations of both acid and alkali solutions produced are significantly reduced, and the salt concentration of the desalinated solution is excessively high (5.8 g / L). Subsequent primary anaerobic treatment, with extended retention time, resulted in an effluent salt content as high as 3145 mg / L, especially with relatively high sulfate levels. Even with extended secondary anaerobic retention time, the effluent COD remained as high as 965 mg / L, significantly increasing the processing pressure on the catalytic 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, compared to a relatively smaller amount of methane produced by stripping, leads to a decrease in hydrogen sulfide stripping efficiency during primary anaerobic treatment, ultimately resulting in a continuous deterioration of the reaction environment during primary anaerobic treatment.

[0085] Comparative Example 2

[0086] 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 two-stage anaerobic process is changed to a single-stage anaerobic process.

[0087] Specifically, consistent with Example 1, the wastewater sequentially passes through a molybdenum removal tank, a hardening and equalization tank, a tubular microfiltration system, and a bipolar membrane electrodialysis system before entering a single-stage anaerobic system. The influent to the single-stage anaerobic system comes from the effluent of the bipolar membrane electrodialysis desalination solution, with a salt content of 3100 mg / L and a COD of 3211 mg / L. The dissolved oxygen in the single-stage anaerobic system is controlled below 0.15 mg / L, the pH is adjusted to 7, the temperature is 33°C, and the bacterial strains are a mixture of organic acidifying bacteria, sulfate-reducing bacteria, and methanogenic bacteria. The wastewater retention time is extended to 22 hours. The methane and hydrogen sulfide mixed gas produced during the anaerobic reaction is pumped out and transferred to the alkaline absorption unit. The final effluent COD is reduced to 1845 mg / L, and the salt content is reduced to 2369 mg / L, which is significantly higher than the COD (368 mg / L) and salt content (1366 mg / L) of the two-stage anaerobic effluent in Example 1.

[0088] 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 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.

Claims

1. A process for treating lignin purification wastewater, comprising a pretreatment section, a reaction section, and a tail gas treatment 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 reaction 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 reaction section sequentially includes a primary anaerobic unit, a secondary anaerobic unit, an ozone catalytic oxidation unit, and a BAF unit, with the BAF effluent being reused. The bipolar membrane electrodialysis unit produces desalinated liquid which enters the primary anaerobic unit, primarily for organic acidification and sulfate reduction, generating hydrogen sulfide gas. This liquid then enters the secondary anaerobic unit, where methanogenesis occurs. The generated methane is pumped back to the primary anaerobic unit as stripping gas, carrying most of the hydrogen sulfide generated in the primary anaerobic unit out of the wastewater system and into the alkaline absorption unit of the tail gas treatment section. The effluent from the secondary anaerobic unit enters the BAF unit, where the BAF effluent is reused. The tail gas treatment section includes an alkaline solution absorption unit and a methane storage unit. The alkaline solution absorption unit purifies the methane and hydrogen sulfide mixture produced by the primary anaerobic unit to obtain high-purity methane, which is then stored in the methane storage unit for later use. After the alkaline solution absorption unit has been running for a period of time, the lean alkaline solution (low hydrogen sulfide content) becomes a rich alkaline solution (high hydrogen sulfide content). At this time, the rich alkaline solution is returned to the bipolar membrane electrodialysis regeneration section in the reaction section, while new alkaline solution is replenished to the alkaline solution absorption unit. in, The bipolar membrane electrodialysis unit 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 by the following steps: Step a: Add polyvinyl alcohol to water to prepare a polyvinyl alcohol aqueous solution, then add positively charged amine compounds and β-cyclodextrin to the above solution respectively, mix and react to obtain a casting solution; Step b: Cast the casting solution obtained in step a onto a horizontal panel, dry it, and obtain the base film; Step c: Immerse the base film obtained in step b in a crosslinking solution composed of crosslinking agent, sodium sulfate, sulfuric acid, and water; Step d: Wash the cross-linked membrane, alkalize it in an alkaline solution, and then soak it in water to obtain a polyvinyl alcohol anion exchange membrane; Step e: Dissolve the polyanionic modifier and sodium chloride in Tris-HCl buffer solution, and adjust the pH to 8-9 with hydrochloric acid to obtain the modified solution; Step f: Place the anion exchange membrane obtained in step d in the middle of the DC electrodeposition apparatus to form two compartments. Put the modified solution prepared in step e into the compartment on the cathode side and put water into the compartment on the anode side to carry out the electrodeposition reaction and obtain the modified anion exchange membrane.

2. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, in step a, the positively charged amine compound is one or more of 2,3-epoxypropyltrimethylammonium chloride, polyethyleneimine, polyepoxychloropropaneamine, and quaternized chitosan; the mass ratio of the positively charged amine compound to polyvinyl alcohol is 0.1:1 to 0.6:1; the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.05:1 to 0.4:1; the mass fraction of the polyvinyl alcohol aqueous solution is 5% to 15%; and the reaction time with the positively charged amine compound and β-cyclodextrin is 2 to 10 hours.

3. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the crosslinking agent in step c is selected from aldehyde or acid crosslinking agents. The crosslinking solution contains, by weight, 2%–8% crosslinking agent, 4%–15% sodium sulfate and 1%–4% sulfuric acid, with the remainder being water. The base membrane is immersed in the crosslinking solution for 1–6 hours.

4. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the alkalization in step d 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 polyanion modifier in step e is one or more of sodium poly(4-styrene sulfonate), sodium p-styrene sulfonate, sodium polyethylene sulfonate, and sodium polypropylene sulfonate; the mass concentration of the polyanion modifier in the modification solution is 1–10 g / L, the mass concentration of sodium chloride is 3–30 g / L, and the concentration of Tris–HCl buffer is 10–50 mmol / L.

6. The process method according to claim 1, characterized in that, In the preparation of the modified anion exchange membrane, the electrodeposition reaction time in step f is 0.2–2 h, and the current density is 1–50 mA / cm². 2 .

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 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.

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 1–8 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 2–36 h, the temperature is 30–40 °C, the pH is controlled at 7–9 by adding alkali, and the bacteria are methanogens.

11. The process method according to claim 1, characterized in that, In the ozone catalytic oxidation unit, ozone is added at a mass concentration ratio of ozone to wastewater COD of 0.1 to 2:1, the reaction time is 10 to 120 minutes, and the catalyst is a conventional supported metal catalyst.

12. The process method according to claim 1, characterized in that, The BAF unit has an influent salt concentration of less than 3000 mg / L, uses common biological strains, controls dissolved oxygen at more than 2 mg / L, and has a wastewater retention time of 2–16 h.

13. The process method according to claim 1, characterized in that, The alkali used in the alkali absorption unit is sodium hydroxide, which can ensure the purity of the alkali solution produced 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 into sulfate by the action of the byproduct chlorine gas, and finally transformed into sulfuric acid.

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