Low-temperature wet oxidation treatment of chemical wastewater zero discharge method

Abstract

This invention belongs to the field of chemical wastewater treatment technology, and discloses a low-temperature wet oxidation method for zero-discharge treatment of chemical wastewater. It is suitable for high-salt, recalcitrant concentrated wastewater generated from reverse osmosis concentration, with a COD of 500-2000 mg / L, containing benzene ring or heterocyclic recalcitrant organic compounds, a B / C ratio ≤ 0.2, and a TDS ≥ 30000 mg / L. The method first pre-treats the wastewater to remove suspended solids and large particulate impurities. Through solid-liquid separation, the suspended solids in the influent of the catalytic wet oxidation unit are controlled to be no higher than 5 mg / L. Catalytic oxidation is performed at 150-180 degrees Celsius and 0.5-2.2 MPa, achieving a COD removal rate of no less than 85%. The wastewater is then concentrated by membrane evaporation and crystallization, with the permeate and condensate recycled. The crystallized salt is utilized as a resource, achieving zero discharge. This method effectively reduces the energy consumption and equipment corrosion risk of wet oxidation reactions, improves the removal efficiency of recalcitrant organic compounds, avoids catalyst deactivation and membrane fouling, allows for the resource recovery of crystallized salt, has low operating costs, and is suitable for zero-discharge projects for various types of recalcitrant organic chemical wastewater.

Description

Technical Field

[0001] This invention belongs to the field of chemical wastewater treatment technology, and in particular to a method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation. Background Technology

[0002] In typical zero-discharge processes, after front-end biological treatment and reverse osmosis concentration, the resulting RO concentrate typically has a COD of 500-2000 mg / L and a TDS exceeding 30000 mg / L. The residual organic matter in this high-salt, low-COD concentrate is mostly benzene rings and heterocyclic compounds that are difficult to degrade. Conventional Fenton oxidation and ozone catalytic oxidation become drastically inefficient in high-salt environments, with COD removal rates of only 10%-20%. This fails to meet the influent requirements of subsequent nanofiltration and evaporation crystallization systems, becoming a key bottleneck for the stable operation of zero-discharge systems.

[0003] The current mainstream treatment process in the industry mainly adopts a combined route of "pretreatment + biological treatment + membrane concentration + evaporation crystallization". This process relies on microbial degradation to reduce organic matter, and then uses membrane separation and evaporation to separate and recover water and salt. It is widely used in the treatment of easily biodegradable wastewater and has the advantages of mature technology and low operating cost. However, for chemical wastewater with low biodegradability and containing recalcitrant organic matter, this process has obvious limitations: microorganisms have difficulty decomposing benzene rings and heterocyclic organic compounds, the COD of the effluent is difficult to meet the standards, and the residual recalcitrant organic matter will cause membrane element fouling and excessive organic matter in the crystallized salt, making resource recovery impossible and requiring hazardous waste disposal, which greatly increases the disposal cost.

[0004] Another mainstream pretreatment route is the high-temperature wet oxidation process, which uses a high temperature of over 200℃ and a high pressure of over 3MPa to oxidize and decompose recalcitrant organic matter, effectively improving the biodegradability of wastewater. However, it has extremely high energy consumption and prominent equipment corrosion problems in a high-temperature, high-humidity, and acidic environment. The equipment investment and operating costs far exceed the industry's acceptable range, making it difficult to promote on a large scale.

[0005] In addition, existing processes generally suffer from insufficient removal of suspended solids in the pretreatment stage, which easily leads to catalyst pore blockage and deactivation in subsequent catalytic units and excessively rapid flux decline in membrane systems. Furthermore, the low recovery rate in the membrane concentration stage significantly increases the processing load on subsequent evaporation systems, further raising operating costs. Therefore, there is an urgent need for a technology that can systematically address these shortcomings and achieve low-cost, highly stable zero-discharge technology for recalcitrant organic chemical wastewater. Summary of the Invention

[0006] To address the shortcomings of existing methods for treating recalcitrant organic chemical wastewater, such as high energy consumption, easy catalyst deactivation, rapid membrane fouling, low resource utilization rate of crystallized salts, and high operating costs for zero-discharge treatment, this invention provides a low-temperature wet oxidation method for zero-discharge treatment of chemical wastewater.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for zero-discharge treatment of chemical wastewater using low-temperature wet oxidation, wherein the chemical wastewater is high-salt, recalcitrant concentrated wastewater produced by reverse osmosis concentration, with a COD of 500~2000 mg / L, containing benzene ring or heterocyclic recalcitrant organic matter, a B / C ratio ≤ 0.2, and a TDS ≥ 30000 mg / L; the method includes the following steps: S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, resulting in pretreated effluent; S1a. The pretreated effluent is then subjected to solid-liquid separation to ensure that the suspended solids content in the influent to the catalytic wet oxidation reactor does not exceed 5 mg / L. S2. The pretreated effluent is fed into a catalytic wet oxidation reactor. After adding oxidant, an oxidation reaction is carried out under the action of a supported transition metal catalyst. The reaction temperature is controlled at 150℃~180℃, the reaction pressure at 0.5MPa~2.2MPa, and the hydraulic retention time at 30min~90min. Most of the recalcitrant organic pollutants in the wastewater are oxidized and decomposed into carbon dioxide and water to obtain oxidized effluent. After filtration, the suspended solids content of the oxidized effluent is not higher than 5mg / L, and the COD removal rate of the oxidized effluent is not less than 85%. S3. After filtration, the oxidized effluent is directly sent to the membrane concentration system for concentration and separation to obtain permeate and concentrate that meet the requirements for production reuse. S4. The concentrated liquid is sent to the evaporation crystallization system for solid-liquid separation. The separated crystallized salt is recycled as a resource, and the condensate generated by evaporation is reused in the production system to achieve zero discharge of chemical wastewater.

[0008] Preferably, the solid-liquid separation process is sand filtration or multi-media filtration.

[0009] Preferably, step S1 employs a coagulation and sedimentation process, using a mixture of polyaluminum chloride and polyacrylamide as the coagulant. The dosage of polyaluminum chloride is 50 mg / L to 200 mg / L, and the dosage of polyacrylamide is 1 mg / L to 5 mg / L. During the coagulation process, the mixture is first rapidly stirred at 200 rpm to 300 rpm for 1 min to 3 min, then slowly stirred at 50 rpm to 80 rpm for 10 min to 20 min. After settling for 30 min to 60 min, the supernatant is taken as the pretreated effluent, and the suspended solids content in the pretreated effluent is not higher than 20 mg / L.

[0010] Preferably, the oxidant added in step S2 is at least one of hydrogen peroxide and ozone, and the amount of oxidant added is 1.1 to 1.5 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent to the catalytic wet oxidation reaction is adjusted to 4 to 9. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 10% to 20%. Compressed air is continuously introduced during the reaction, and the air-to-water volume ratio is 5:1 to 20:1. The catalytic wet oxidation reactor is made of titanium or duplex stainless steel, and the inner wall is lined with a 0.5 mm to 2 mm thick polytetrafluoroethylene integral liner. The catalyst filling rate in the reactor is 20% to 60%.

[0011] Preferably, the supported transition metal catalyst support in step S2 is at least one of alumina, molecular sieve, and activated carbon, and the supported active component is a binary combination of iron and copper oxides with a molar ratio of 1:0.2 to 1:1, and the total loading of the active component is 1% to 8% of the support mass.

[0012] Preferably, the supported transition metal catalyst is prepared by an equal-volume impregnation method. Before loading the support, it is soaked in 10% dilute nitric acid for 2-4 hours, rinsed until neutral, and then dried. The impregnation time is 12-24 hours. After impregnation, it is dried at 105-120°C for 4-8 hours and then calcined at 400-600°C for 3-6 hours to obtain the finished product.

[0013] Preferably, the membrane concentration system in step S3 adopts a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the influent, 3 mg / L to 10 mg / L of organophosphorus scale inhibitor and 2 mg / L to 5 mg / L of isothiazolinone bactericide are added. The membrane concentration system adopts a partial concentrate reflux process with a reflux ratio of 15% to 25%. The overall concentration ratio of the system is 8 to 15 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 24 to 48 hours of operation, and the chemical cleaning is triggered when the transmembrane pressure difference rises to 1.3 to 1.5 times the initial value.

[0014] Preferably, the evaporation and crystallization in step S4 adopts the MVR evaporation process. Before the concentrate enters the evaporation system, its total calcium and magnesium hardness is not higher than 50 mg / L. The high-salt mother liquor generated during the evaporation process is refluxed to the inlet of the catalytic wet oxidation reactor, and the reflux ratio is 5%~15%.

[0015] Preferably, the water quality of the product water from the membrane concentration system and the condensate generated by evaporation and crystallization both meet the water replenishment requirements for chemical production and are fully reused. The nanofiltration membrane in the membrane concentration system separates the wastewater into concentrated water containing sodium sulfate and fresh water containing sodium chloride. Sodium sulfate is recovered by evaporation and crystallization on the nanofiltration concentrated water side, and sodium chloride is recovered by evaporation and crystallization on the nanofiltration fresh water side after reverse osmosis concentration. The crystallized salts all meet the industrial salt reuse standards.

[0016] The present invention has the following beneficial effects: This invention employs a medium-low temperature catalytic wet oxidation process, coupled with a supported iron-copper oxide binary catalyst. It eliminates the need for the stringent high-temperature and high-pressure reaction conditions required by existing high-temperature wet oxidation processes, significantly reducing reaction energy consumption and equipment corrosion risks. It can efficiently decompose benzene rings and heterocyclic recalcitrant organic compounds in wastewater, maintaining a high COD removal rate. It fundamentally avoids the problems of insufficient degradation efficiency of recalcitrant organic compounds and substandard effluent quality in traditional biochemical processes, providing stable and qualified feed water for subsequent membrane concentration and evaporation crystallization processes.

[0017] This invention incorporates a dedicated solid-liquid separation process before the catalytic wet oxidation unit, strictly controlling the suspended solids content in the feed water. Simultaneously, the catalyst support undergoes pre-acid treatment to enhance the bonding strength between the active components and the support, effectively preventing catalyst pore blockage and active component dissolution and deactivation, thus significantly extending the catalyst's lifespan. The membrane concentration system employs a parameter-optimized concentrate recirculation process, coupled with scale inhibitor and bactericide addition and a graded flushing mechanism triggered by transmembrane pressure difference. This ensures a high system recovery rate while significantly reducing membrane element fouling rates, decreasing the frequency of chemical cleaning, and lowering membrane system maintenance costs and the processing load of subsequent evaporation processes.

[0018] This invention employs nanofiltration salt separation technology to achieve the fractional crystallization and recovery of sodium sulfate and sodium chloride. Combined with inert substance accumulation control measures during mother liquor reflux, it effectively reduces the organic matter content of the crystallized salt. Both types of crystallized salt obtained can meet industrial reuse standards, avoiding the additional costs associated with excessive organic matter in crystallized salt from traditional processes, which necessitate hazardous waste disposal. This process is specifically designed for the treatment of high-salt, recalcitrant concentrated wastewater generated by reverse osmosis concentration and can be widely applied to zero-discharge wastewater projects in various industries such as fine chemicals, coal chemicals, and pharmaceutical intermediate production, possessing extremely high value for promotion and application. Attached Figure Description

[0019] Figure 1 This is a flowchart of a low-temperature wet oxidation method for zero-discharge treatment of chemical wastewater proposed in this invention. Figure 2 This is a bar chart comparing COD removal rates under different reaction conditions proposed in this invention. Figure 3 This is a dual Y-axis line graph showing the continuous operation performance of the catalyst under different pH conditions proposed in this invention; Figure 4 This is a diagram showing the combined operating performance of the membrane concentration system proposed in this invention under different reflux ratios. Detailed Implementation

[0020] The following will refer to the appendices in the embodiments of the present invention. Figure 1-4The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1

[0022] The wastewater treated in this embodiment is concentrated water produced by the reverse osmosis system of a chemical plant. The COD is 500 mg / L, the TDS is 32000 mg / L, it contains benzene ring recalcitrant organic matter, the B / C ratio is 0.18, and the chloride ion concentration is 5500 mg / L. The catalytic wet oxidation reactor is made of titanium.

[0023] S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, yielding pretreated effluent. The coagulation sedimentation process uses a mixture of polyaluminum chloride and polyacrylamide, with a polyaluminum chloride dosage of 100 mg / L and a polyacrylamide dosage of 2 mg / L. The mixture is first rapidly stirred at 250 rpm for 2 minutes, then slowly stirred at 60 rpm for 15 minutes. After settling for 45 minutes, the supernatant is collected as the pretreated effluent.

[0024] S1a. The pretreated effluent undergoes precision filtration for solid-liquid separation to ensure that the suspended solids content in the influent entering the catalytic wet oxidation reactor does not exceed 5 mg / L. The measured suspended solids content in the influent was 3.8 mg / L.

[0025] S2. The filtered pretreated effluent is fed into a catalytic wet oxidation reactor. Hydrogen peroxide is used as the oxidant, and the dosage is 1.1 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent is adjusted to 4. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 10%. Compressed air is continuously introduced during the reaction, with an air-to-water volume ratio of 5:1. The inner wall of the reactor is lined with a 0.5 mm thick polytetrafluoroethylene integral liner. The reactor is filled with a supported transition metal catalyst with a filling rate of 20%. The catalyst uses alumina as a carrier, and the supported active components are a binary combination of iron and copper oxides with a molar ratio of 1:0.2. The total loading of the active components is 3% of the carrier mass. The catalyst is prepared by an equal volume impregnation method. Before loading, the carrier is soaked in 10% dilute nitric acid for 2 hours, rinsed until neutral, and then dried. The impregnation treatment time is 12 hours. After impregnation, it is dried at 105℃ for 4 hours and then calcined at 400℃ for 3 hours to obtain the finished product. The reaction temperature was controlled at 150℃, the reaction pressure at 0.5MPa, the hydraulic retention time at 90min, and the catalyst dosage at 2g / L. After precision filtration, the suspended solids content of the oxidized effluent was 4.5mg / L, and the COD removal rate was measured to be 86.7%.

[0026] After 200 hours of continuous operation, the COD removal rate remained at 86.2%, with iron leaching at 0.02 mg / L and copper leaching at 0.01 mg / L. No significant dissolution of the active components was observed. Pretreatment with dilute nitric acid on the support effectively improved the binding force of the active components, thus addressing the issue of catalyst deactivation. The catalyst was regenerated after 1000 hours of operation. First, it was backwashed with pure water to remove surface suspended solids, then soaked in 0.5% dilute nitric acid for 2 hours. After rinsing until neutral, it was dried at 120℃ for 4 hours and calcined at 450℃ for 2 hours. After regeneration, the COD removal rate recovered to 98.3% of the initial value, allowing for long-term cyclical use.

[0027] S3. The effluent from oxidation is filtered and then directly fed into the membrane concentration system, which employs a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the effluent is fed in, 3 mg / L of organophosphorus scale inhibitor and 2 mg / L of isothiazolinone bactericide are added. The membrane concentration system uses a partial concentrate reflux process with a reflux ratio of 15%. The measured membrane flux decay rate is 1.1% / h, the overall system concentration ratio is 8 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 24 hours. When the transmembrane pressure difference rises to 1.3 times the initial value, it is washed with 0.1% sodium hydroxide solution for alkaline washing and 0.2% citric acid solution for acid washing in sequence. The flux can be restored to more than 97% of the initial value.

[0028] S4. The concentrated liquid is fed into the MVR evaporation crystallization system. Its total calcium and magnesium hardness is no higher than 50 mg / L, requiring no additional softening agent. The high-salt mother liquor generated during evaporation is recycled to the inlet of the catalytic wet oxidation reactor at a reflux ratio of 5%. After 20 days of continuous operation, the chloride ion concentration of the mother liquor is monitored at 172,000 mg / L, and the TDS is 310,000 mg / L, requiring no external discharge. After nanofiltration and salt separation, sodium sulfate crystals are obtained by evaporation on the concentrated water side, and sodium chloride crystals are obtained by evaporation after reverse osmosis concentration on the desalinated water side. The organic matter content of the crystal salts is 0.08% and 0.05%, respectively, both meeting the industrial salt reuse standards and allowing for resource recovery. The condensate generated by evaporation and the permeate from the membrane concentration system both meet the water replenishment requirements for chemical production and are fully reused, achieving zero discharge of wastewater.

[0029] Example 2

[0030] The wastewater treated in this embodiment is concentrated water produced by the reverse osmosis system of a coal chemical enterprise. The COD is 2000 mg / L, the TDS is 48000 mg / L, it contains heterocyclic recalcitrant organic matter, the B / C ratio is 0.12, and the chloride ion concentration is 12000 mg / L. The catalytic wet oxidation reactor is made of titanium.

[0031] S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, resulting in pretreated effluent. The coagulation sedimentation process uses a mixture of polyaluminum chloride and polyacrylamide, with a polyaluminum chloride dosage of 150 mg / L and a polyacrylamide dosage of 4 mg / L. The mixture is first rapidly stirred at 280 rpm for 3 minutes, then slowly stirred at 70 rpm for 15 minutes. After settling for 40 minutes, the supernatant is collected as the pretreated effluent.

[0032] S1a. The pretreated effluent undergoes precision filtration for solid-liquid separation to ensure that the suspended solids content in the influent entering the catalytic wet oxidation reactor does not exceed 5 mg / L. The measured suspended solids content in the influent was 3.6 mg / L.

[0033] S2. The filtered pretreated effluent is fed into a catalytic wet oxidation reactor. Ozone is used as the oxidant, and the dosage is 1.5 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent is adjusted to 6. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 20%. Compressed air is continuously introduced during the reaction, with an air-to-water volume ratio of 20:1. The inner wall of the reactor is lined with a 2mm thick polytetrafluoroethylene integral liner. The reactor is filled with a supported transition metal catalyst with a filling rate of 60%. The catalyst uses molecular sieve as a carrier, and the supported active components are a binary combination of iron and copper oxides in a molar ratio of 1:1. The total loading of the active components is 8% of the carrier mass. The catalyst is prepared by an equal-volume impregnation method. Before loading, the carrier is soaked in 10% dilute nitric acid for 4 hours, rinsed until neutral, and dried. The impregnation treatment time is 24 hours. After impregnation, it is dried at 120℃ for 8 hours and then calcined at 600℃ for 6 hours to obtain the finished product. The reaction temperature was controlled at 180℃, the reaction pressure at 2.2MPa, the hydraulic retention time at 90min, and the catalyst dosage at 2g / L. After precision filtration, the suspended solids content of the oxidized effluent was 4mg / L, the COD removal rate was 92.3%, and the biodegradability B / C ratio was 0.51.

[0034] After 200 hours of continuous operation, the COD removal rate remained at 91.8%, the iron leaching amount was 0.03 mg / L, and the copper leaching amount was 0.02 mg / L, indicating stable activity.

[0035] S3. The effluent from oxidation is filtered and then directly fed into the membrane concentration system, which employs a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the effluent is fed in, 10 mg / L of organophosphorus scale inhibitor and 5 mg / L of isothiazolinone bactericide are added. The membrane concentration system uses a partial concentrate reflux process with a reflux ratio of 25%. The measured membrane flux decay rate is 1.8% / h, the overall system concentration ratio is 15 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 48 hours. When the transmembrane pressure difference rises to 1.5 times the initial value, it is washed with 0.1% sodium hydroxide solution for alkaline washing and 0.2% citric acid solution for acid washing in sequence. The flux can be restored to more than 96% of the initial value.

[0036] S4. The concentrated liquid is fed into the MVR evaporation crystallization system. Its total calcium and magnesium hardness is no higher than 50 mg / L, requiring no additional softening agent. The high-salt mother liquor generated during evaporation is recycled to the inlet of the catalytic wet oxidation reactor at a reflux ratio of 15%. After 25 days of continuous operation, the chloride ion concentration of the mother liquor is monitored at 181,500 mg / L, and the TDS is 330,000 mg / L, both below the threshold, requiring no external discharge. After nanofiltration and salt separation, sodium sulfate crystals are obtained by evaporation on the concentrated water side, and sodium chloride crystals are obtained by evaporation after reverse osmosis concentration on the desalinated water side. The organic matter content of the crystals is 0.05%, meeting the industrial salt reuse standards and allowing for resource recovery. The condensate generated by evaporation and the permeate from the membrane concentration system both meet the water replenishment requirements for chemical production and are fully reused, achieving zero discharge of wastewater.

[0037] Example 3

[0038] The wastewater treated in this embodiment is concentrated water produced by the reverse osmosis system of a pharmaceutical intermediate production enterprise. The COD is 1200 mg / L, the TDS is 38000 mg / L, it contains a mixture of benzene rings and heterocyclic recalcitrant organic matter, the B / C ratio is 0.15, the chloride ion concentration is 6500 mg / L, and the catalytic wet oxidation reactor is made of titanium.

[0039] S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, yielding pretreated effluent. The coagulation sedimentation process uses a mixture of polyaluminum chloride and polyacrylamide, with a polyaluminum chloride dosage of 80 mg / L and a polyacrylamide dosage of 3 mg / L. The mixture is first rapidly stirred at 220 rpm for 2 minutes, then slowly stirred at 55 rpm for 18 minutes. After settling for 50 minutes, the supernatant is collected as the pretreated effluent.

[0040] S1a. The pretreated effluent undergoes precision filtration for solid-liquid separation to ensure that the suspended solids content in the influent entering the catalytic wet oxidation reactor does not exceed 5 mg / L. The measured suspended solids content in the influent was 3.5 mg / L.

[0041] S2. The filtered pretreated effluent is fed into a catalytic wet oxidation reactor. Hydrogen peroxide is used as the oxidant, and the dosage is 1.3 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent is adjusted to 7. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 15%. Compressed air is continuously introduced during the reaction, with an air-to-water volume ratio of 12:1. The inner wall of the reactor is lined with a 1mm thick polytetrafluoroethylene integral liner. The reactor is filled with a supported transition metal catalyst with a filling rate of 40%. The catalyst uses activated carbon as a carrier, and the supported active components are a binary combination of iron and copper oxides with a molar ratio of 1:0.6. The total loading of the active components is 8% of the carrier mass. The catalyst is prepared by an equal volume impregnation method. Before loading, the carrier is soaked in 10% dilute nitric acid for 3 hours, rinsed until neutral, and dried. The impregnation treatment time is 18 hours. After impregnation, it is dried at 110℃ for 6 hours and then calcined at 500℃ for 4.5 hours to obtain the finished product. The reaction temperature was controlled at 165℃, the reaction pressure at 1.5MPa, the hydraulic retention time at 60min, and the catalyst dosage at 2g / L. After filtration, the suspended solids content of the oxidized effluent was 3.7mg / L, the COD removal rate was measured to be 89.1%, and the biodegradability B / C ratio was 0.47.

[0042] After 200 hours of continuous operation, the COD removal rate remained at 88.7%, the iron leaching amount was 0.015 mg / L, and the copper leaching amount was 0.01 mg / L, indicating stable activity.

[0043] S3. The effluent from oxidation is filtered and then directly fed into the membrane concentration system, which employs a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the effluent is fed in, 6 mg / L of organophosphorus scale inhibitor and 3.5 mg / L of isothiazolinone bactericide are added. The membrane concentration system uses a partial concentrate recirculation process with a recirculation ratio of 20%. The measured membrane flux decay rate is 1.4% / h, the overall system concentration ratio is 12 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 36 hours. When the transmembrane pressure difference rises to 1.4 times the initial value, it is washed with 0.1% sodium hydroxide solution for alkaline washing and 0.2% citric acid solution for acid washing in sequence. The flux can be restored to more than 96.5% of the initial value.

[0044] S4. The concentrated liquid is fed into the MVR evaporation crystallization system. Its total calcium and magnesium hardness is no higher than 50 mg / L, requiring no additional softening agent. The high-salt mother liquor generated during evaporation is recycled to the inlet of the catalytic wet oxidation reactor at a reflux ratio of 10%. After 30 days of continuous operation, the chloride ion concentration of the mother liquor is monitored at 155,000 mg / L, and the TDS is 320,000 mg / L, both below the threshold, requiring no external discharge. After nanofiltration and salt separation, sodium sulfate crystals are obtained by evaporation on the concentrated water side, and sodium chloride crystals are obtained by evaporation after reverse osmosis concentration on the desalinated water side. The organic matter content of the crystals is 0.06%, meeting the industrial salt reuse standards and allowing for resource recovery. The condensate generated by evaporation and the permeate from the membrane concentration system both meet the water replenishment requirements for chemical production and are fully reused, achieving zero discharge of wastewater.

[0045] Example 4

[0046] The wastewater treated in this embodiment is the reverse osmosis concentrate from a zero-discharge project in a chemical industrial park. The COD is 860 mg / L, the TDS is 52000 mg / L, it contains benzene ring recalcitrant organic matter, the B / C ratio is 0.08, and the chloride ion concentration is 18000 mg / L. The catalytic wet oxidation reactor is made of titanium.

[0047] S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, yielding pretreated effluent. The coagulation sedimentation process uses a mixture of polyaluminum chloride and polyacrylamide, with a polyaluminum chloride dosage of 120 mg / L and a polyacrylamide dosage of 2.5 mg / L. The mixture is first rapidly stirred at 260 rpm for 2.5 min, then slowly stirred at 65 rpm for 12 min. After settling for 35 min, the supernatant is collected as the pretreated effluent.

[0048] S1a. The pretreated effluent undergoes precision filtration for solid-liquid separation to ensure that the suspended solids content in the influent entering the catalytic wet oxidation reactor does not exceed 5 mg / L. The measured suspended solids content in the influent was 3.2 mg / L.

[0049] S2. The filtered pretreated effluent is fed into a catalytic wet oxidation reactor. Hydrogen peroxide is used as the oxidant, and the dosage is 1.2 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent is adjusted to 5. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 12%. Compressed air is continuously introduced during the reaction, with an air-to-water volume ratio of 8:1. The inner wall of the reactor is lined with a 1mm thick polytetrafluoroethylene integral liner. The reactor is filled with a supported transition metal catalyst with a filling rate of 30%. The catalyst uses activated carbon as a carrier, and the supported active components are a binary combination of iron and copper oxides with a molar ratio of 1:0.3. The total loading of the active components is 2% of the carrier mass. The catalyst is prepared by an equal-volume impregnation method. Before loading, the carrier is soaked in 10% dilute nitric acid for 2.5 hours, rinsed until neutral, and dried. The impregnation treatment time is 16 hours. After impregnation, it is dried at 110℃ for 5 hours and then calcined at 450℃ for 4 hours to obtain the finished product. The reaction temperature was controlled at 150℃, the reaction pressure at 0.7MPa, the hydraulic retention time at 30min, and the catalyst dosage at 2g / L. After filtration, the suspended solids content of the oxidized effluent was 3.8mg / L, and the COD removal rate was measured to be 86.5%.

[0050] As a control, the same batch of RO concentrate was treated with conventional Fenton oxidation at a ferrous ion dosage of 4 mmol / L and an H2O2 / Fe ratio of [missing value]. 2+ At a molar ratio of 4:1 and pH 3.5, the COD removal rate was only 16.2% after 120 min of reaction. Using ozone catalytic oxidation with an ozone dosage of 200 mg / L and a reaction time of 60 min, the COD removal rate was only 12.8%. This method demonstrates significantly better performance under high-salt conditions than conventional advanced oxidation processes.

[0051] After the catalyst ran continuously for 200 hours, the COD removal rate remained at 86.1%, the iron leaching amount was 0.01 mg / L, the copper leaching amount was 0.008 mg / L, and there was no significant leaching of the active components.

[0052] S3. The effluent from oxidation is filtered and then directly fed into the membrane concentration system, which employs a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the effluent enters the system, 5 mg / L of organophosphorus scale inhibitor and 3 mg / L of isothiazolinone bactericide are added. The membrane concentration system uses a partial concentrate recirculation process with a recirculation ratio of 18%. The overall concentration ratio of the system is 10 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 30 hours. When the transmembrane pressure difference rises to 1.3 times the initial value, it is washed with 0.1% sodium hydroxide solution for alkaline washing and 0.2% citric acid solution for acid washing in sequence. The flux can be restored to more than 97% of the initial value.

[0053] S4. The concentrate is fed into the MVR evaporation crystallization system, with a total calcium and magnesium hardness not exceeding 50 mg / L. The high-salt mother liquor generated during evaporation is recycled to the inlet of the catalytic wet oxidation reactor at a reflux ratio of 8%. After 20 days of continuous operation, the chloride ion concentration of the mother liquor is monitored at 190,000 mg / L, and the TDS is 340,000 mg / L, both below the threshold, requiring no external discharge. After nanofiltration and salt separation, sodium sulfate crystals are obtained by evaporation on the concentrate side, and sodium chloride crystals are obtained by evaporation after reverse osmosis concentration on the desalination side, both meeting the industrial salt reuse standards. All condensate and permeate from the membrane concentration system are reused, achieving zero discharge with no external wastewater discharge.

[0054] Comparative Example This comparative example uses the existing mainstream process of "pretreatment + high-temperature wet oxidation + biochemical treatment + membrane concentration + evaporation crystallization," without a solid-liquid separation step before catalytic oxidation, without adding a catalyst, with a reaction temperature of 220℃ and a reaction pressure of 3.5MPa. The suspended solids content of the pretreated effluent is 25mg / L, which is directly fed into the oxidation reactor. The membrane concentration system has no concentrate reflux process. Testing showed that the COD removal rate of the oxidation effluent was 58.2%, the B / C ratio was 0.22, the membrane system recovery rate was 72%, the membrane flux decay rate was 3.8% / h, and the organic matter content of the crystalline salt was 0.85%, which does not meet industrial salt standards and requires hazardous waste disposal. The operating cost is significantly higher than that of the process of this invention. A blank control experiment was also set up. The COD removal rate was 51.3% at 150℃ without a catalyst, 58.2% at 180℃ without a catalyst, and 62.1% at 120℃ with 2g / L catalyst, all failing to meet the 85% removal requirement.

[0055] Experimental data tables and explanations Table 1 Comparison of COD removal rates under different reaction conditions

[0056] The data in the table shows that after adding the iron-copper binary catalyst, the COD removal rate can reach more than 85% at 150℃, which is much higher than the group without catalyst at the same temperature and the group without catalyst at 180℃. The treatment requirements cannot be met at 120℃, which proves that the reaction temperature range of 150℃~180℃ set by the present invention is reasonable, and it has the advantages of both treatment efficiency and energy consumption, solving the problem of excessive energy consumption of traditional wet oxidation.

[0057] Table 2. Catalyst Continuous Operation Performance under Different pH Conditions

[0058] The data in the table show that the catalyst supported on the support pretreated with dilute nitric acid showed no significant decrease in activity after 200 hours of continuous operation within the pH range of 4 to 9. The amount of active component leaching was extremely low, far lower than that of the untreated support group. This proves that the support pretreatment measures can effectively improve the binding force between the active component and the support, effectively reduce the risk of active component leaching, and solve the defects of existing catalysts such as easy deactivation and short service life.

[0059] Table 3. Performance Comparison of Membrane Concentration Systems at Different Reflux Ratios

[0060] The data in the table shows that when the reflux ratio is in the range of 15% to 25%, the system recovery rate is stably maintained at around 87%, the membrane flux decay rate is less than 2% / h, and the cleaning cycle can reach more than 24 days. When the reflux ratio is less than 15%, the concentration ratio is insufficient, the recovery rate is only slightly improved, but the decay rate increases significantly. When the reflux ratio is higher than 25%, the membrane fouling rate increases sharply, and the cleaning cycle is shortened by nearly half. This proves that the reflux ratio range of 15% to 25% set in this invention is reasonable, which can take into account both the system recovery rate and the operational stability, and solves the defects of existing membrane systems such as low recovery rate, rapid fouling, and high operation and maintenance costs.

[0061] refer to Figure 2 This figure visually illustrates the impact of reaction temperature and catalyst addition on the decomposition efficiency of recalcitrant organic matter. As shown, without catalyst addition, even at a reaction temperature of 180℃, the COD removal rate is still less than 60%, failing to meet subsequent treatment requirements; with catalyst addition at 120℃, the removal rate is only 62.1%, also failing to meet the standard; however, when only a binary catalyst of iron-copper oxide is added within the 150℃~180℃ range, the COD removal rate can stably reach over 85%, far exceeding the treatment efficiency of traditional non-catalyst-based wet oxidation. This eliminates the need for high-temperature reaction conditions above 200℃, effectively reducing reaction energy consumption and equipment corrosion risks. It solves the shortcomings of existing wet oxidation processes, such as high energy consumption and low removal efficiency of recalcitrant organic matter, verifying the rationality of the reaction parameter settings in this invention.

[0062] refer to Figure 3 This figure illustrates the effect of support pretreatment on the catalyst's operational stability. As shown, the iron-copper oxide binary catalyst pretreated with dilute nitric acid maintained a COD removal rate above 86% after 200 hours of continuous operation within a wide pH range of 4-9, with total dissolution of active components all below 0.05 mg / L and no significant activity decay. In contrast, the untreated catalyst, after 200 hours of operation under the same conditions, showed a COD removal rate dropping to 72.3%, with active component dissolution exceeding 10 times that of the pretreated group, indicating significant deactivation. This figure verifies that support pretreatment effectively enhances the bonding force between the active components and the support, effectively extends the catalyst's lifespan, and addresses the shortcomings of existing catalysts such as easy dissolution and deactivation.

[0063] refer to Figure 4 This figure illustrates the impact of the concentrate reflux ratio on the operating performance of the membrane system. As shown, when the reflux ratio is below 15% or above 25%, the membrane flux decay rate increases to over 3.5% / h, resulting in extremely rapid membrane fouling and a significantly shortened cleaning cycle. Furthermore, when the reflux ratio is above 25%, the system recovery rate drops below 87%, failing to meet design requirements. Only within the reflux ratio range of 15% to 25% can the system recovery rate be stably maintained at around 87%, while the membrane flux decay rate remains below 2% / h, significantly reducing the risk of membrane fouling. This figure verifies the rationality of the reflux ratio parameter setting in this invention, effectively addressing the shortcomings of existing membrane concentration systems, such as low recovery rate, rapid fouling, and high operation and maintenance costs.

[0064] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for zero-discharge treatment of chemical wastewater using low-temperature wet oxidation, wherein the chemical wastewater is high-salt, recalcitrant concentrated wastewater generated by reverse osmosis concentration, with a COD of 500~2000 mg / L, containing benzene ring or heterocyclic recalcitrant organic matter, a B / C ratio ≤ 0.2, and a TDS ≥ 30000 mg / L; characterized in that, Includes the following steps: S1. The chemical wastewater is pretreated by coagulation sedimentation, filtration, and ultrafiltration to remove suspended solids and large particulate impurities, resulting in pretreated effluent; S1a. The pretreated effluent is then subjected to solid-liquid separation to ensure that the suspended solids content in the influent to the catalytic wet oxidation reactor does not exceed 5 mg / L. S2. The pretreated effluent is fed into a catalytic wet oxidation reactor. After adding oxidant, an oxidation reaction is carried out under the action of a supported transition metal catalyst. The reaction temperature is controlled at 150℃~180℃, the reaction pressure at 0.5MPa~2.2MPa, and the hydraulic retention time at 30min~90min to obtain oxidized effluent. After filtration, the suspended solids content of the oxidized effluent is not higher than 5mg / L, and the COD removal rate of the oxidized effluent is not less than 85%. S3. After filtration, the oxidized effluent is directly sent to the membrane concentration system for concentration and separation to obtain permeate and concentrate that meet the requirements for production reuse. S4. The concentrated liquid is sent to the evaporation crystallization system for solid-liquid separation. The separated crystallized salt is recycled as a resource, and the condensate generated by evaporation is reused in the production system to achieve zero discharge of chemical wastewater.

2. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, The solid-liquid separation process is sand filtration or multi-media filtration.

3. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, Step S1 employs a coagulation and sedimentation process. The coagulant is a mixture of polyaluminum chloride and polyacrylamide. The dosage of polyaluminum chloride is 50 mg / L to 200 mg / L, and the dosage of polyacrylamide is 1 mg / L to 5 mg / L. During the coagulation process, the mixture is first rapidly stirred at 200 rpm to 300 rpm for 1 min to 3 min, then slowly stirred at 50 rpm to 80 rpm for 10 min to 20 min. After settling for 30 min to 60 min, the supernatant is taken as the pretreated effluent. The suspended solids content in the pretreated effluent is not higher than 20 mg / L.

4. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, The oxidant added in step S2 is at least one of hydrogen peroxide and ozone. The amount of oxidant added is 1.1 to 1.5 times the theoretical oxygen demand corresponding to the COD of the wastewater. The pH of the influent to the catalytic wet oxidation reaction is adjusted to 4 to 9. The reactor adopts an upflow water distribution structure with a bottom water distribution plate opening rate of 10% to 20%. Compressed air is continuously introduced during the reaction, and the air-to-water volume ratio is 5:1 to 20:

1. The catalytic wet oxidation reactor is made of titanium or duplex stainless steel, and the inner wall is lined with a 0.5 mm to 2 mm thick polytetrafluoroethylene integral liner. The catalyst filling rate in the reactor is 20% to 60%.

5. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, The supported transition metal catalyst support in step S2 is at least one of alumina, molecular sieve, and activated carbon. The supported active component is a binary combination of iron and copper oxides with a molar ratio of 1:0.2 to 1:1, and the total loading of the active component is 1% to 8% of the support mass.

6. A method for zero-discharge treatment of chemical wastewater using low-temperature wet oxidation according to claim 5, characterized in that, The supported transition metal catalyst was prepared by an equal-volume impregnation method. Before loading the support, it was soaked in 10% dilute nitric acid for 2-4 hours, rinsed until neutral, and then dried. The impregnation time was 12-24 hours. After impregnation, it was dried at 105-120℃ for 4-8 hours and then calcined at 400-600℃ for 3-6 hours to obtain the finished product.

7. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, The membrane concentration system in step S3 employs a series combination of ultrafiltration, nanofiltration, and reverse osmosis. Before the influent, 3 mg / L to 10 mg / L of organophosphorus scale inhibitor and 2 mg / L to 5 mg / L of isothiazolinone bactericide are added. The membrane concentration system uses a partial concentrate recirculation process with a recirculation ratio of 15% to 25%. The overall concentration ratio of the system is 8 to 15 times, and the recovery rate is not less than 87%. The system is hydraulically flushed every 24 to 48 hours of operation, with the transmembrane pressure difference rising to 1.3 to 1.5 times the initial value as the trigger condition for chemical cleaning.

8. The method for zero-discharge treatment of chemical wastewater by low-temperature wet oxidation according to claim 1, characterized in that, The evaporation and crystallization in step S4 adopts the MVR evaporation process. Before the concentrate enters the evaporation system, its total calcium and magnesium hardness is not higher than 50 mg / L. The high-salt mother liquor generated during the evaporation process is refluxed to the inlet of the catalytic wet oxidation reactor, with a reflux ratio of 5% to 15%.

9. A method for zero-discharge treatment of chemical wastewater using low-temperature wet oxidation according to claim 1, characterized in that, The water quality of the product water and the condensate generated by evaporation and crystallization in the membrane concentration system both meet the water replenishment requirements for chemical production and are fully reused. The nanofiltration membrane in the membrane concentration system separates the wastewater into concentrated water containing sodium sulfate and fresh water containing sodium chloride. Sodium sulfate is recovered by evaporation and crystallization on the nanofiltration concentrated water side, and sodium chloride is recovered by evaporation and crystallization on the nanofiltration fresh water side after reverse osmosis concentration. The crystallized salts all meet the industrial salt reuse standards.

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