A method for treating high-salinity high-organic high-hardness wastewater

Abstract

The application provides a high-salt high-organic high-hardness wastewater treatment method, which comprises the following steps: S101, preheating wastewater and then conveying the wastewater to a thin film evaporator to obtain steam and concentrated liquid through evaporation; S201, condensing the steam to obtain condensed water and conveying the condensed water to a rectifying tower to obtain tower kettle liquid and organic-containing steam; S202, discharging the concentrated liquid and collecting the concentrated liquid; S301, condensing the organic-containing steam to obtain an organic solution and conveying the organic solution back to the rectifying tower for continuous rectification and / or discharging and collecting; S401, conveying the tower kettle liquid to a biochemical tank, removing part of organic matters, and then performing sludge settlement treatment to obtain biochemical produced water; S501, conveying the biochemical produced water to a coagulation sedimentation tank, adding a coagulant, and then performing sedimentation to obtain sludge and sedimentation produced water, discharging the sludge, and dewatering the sludge; S601, conveying the sedimentation produced water to a heterogeneous oxidation device, and then performing oxidation, and finally filtering the sedimentation produced water through an immersed ultrafiltration membrane to obtain ultrafiltration produced water; and S701, filtering the ultrafiltration produced water through a reverse osmosis membrane, recycling reverse osmosis water, and returning reverse osmosis concentrated water to S101 to mix with the wastewater and then perform the treatment process again.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of wastewater treatment, in particular to a high-salt high-organic high-hardness wastewater treatment method. BACKGROUND

[0002] The wastewater discharged in the production process of pharmaceutical, printing and dyeing, fine chemical and other industries has the characteristics of high salt, high organic matter and high hardness, usually containing more than 100000 mg / L of salt, more than 15000 mg / L of COD concentration, and more than 1500 mg / L of total hardness (calculated as CaCO3). If the traditional biochemical method is used for treatment, the high salt content in the water will inhibit the growth of microorganisms, resulting in the loss of biological activity of microorganisms, and the purpose of treating wastewater cannot be achieved. If the multi-effect evaporation crystallization or MVR evaporation crystallization process is used for treatment, on the one hand, the high organic matter concentration in the water will cause the salt crystallization of the evaporator to be blocked, and at the same time will have a great impact on the solid-liquid separation of the crystal slurry; on the other hand, the high hardness in the water will cause the evaporator to be seriously scaled and cleaned frequently, and the process system cannot be normally and stably operated.

[0003] At present, the commonly used treatment method for this type of wastewater is evaporation pond treatment, which has the advantages of relying on natural airing effect, evaporating water in wastewater, and sending the produced crystalline salt to a dry salt pool for landfill treatment, and has the characteristics of low project investment, simple operation, low operation and maintenance cost. However, the biggest disadvantage of evaporation pond treatment is that it occupies a large area and is strongly dependent on natural climate, and at the same time, it will emit a large amount of odor, forming secondary air pollution and threatening human health. In recent years, with the strengthening of national environmental protection policy, the evaporation pond treatment technology for concentrated salt water has gradually withdrawn from the stage of water treatment industry. Therefore, it has important application value and practical significance to study a new type of high-salt, high-organic, high-hardness wastewater treatment process technology.

[0004] Therefore, the present application is proposed. SUMMARY

[0005] The purpose of the present application is to provide a high-salt high-organic high-hardness wastewater treatment method for treating wastewater with the characteristics of high salt, high organic matter and high hardness.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] The present application provides a high-salt high-organic high-hardness wastewater treatment method, comprising the following steps:

[0008] S101. The wastewater is preheated and then sent to a thin film evaporator, and after distribution and forced film formation in the thin film evaporator, steam and concentrated liquid are obtained by evaporation;

[0009] S201. The steam obtained in S101 is sent to a condenser to condense into condensed water, and the condensed water is sent to a rectifying tower for rectification to obtain tower bottom liquid and steam containing organic substances;

[0010] S202. The concentrated liquid obtained in S101 is discharged.

[0011] S301. The steam containing organic substances obtained in S201 is condensed to obtain an organic solution, which is sent back to the rectifying tower for continuous rectification and / or discharged for collection.

[0012] S401. The tower bottom liquid obtained in S201 is sent to a biochemical tank for biochemical treatment, and after removing part of the organic substances by the biochemical treatment, sludge settlement treatment is performed to obtain biochemical product water.

[0013] S501. The biochemical product water obtained in S401 is sent to a coagulation sedimentation tank, a coagulant is added and sedimentation is performed to obtain sludge and sedimentation product water, and the sludge is discharged and dewatered.

[0014] S601. The sedimentation product water obtained in S501 is sent to a heterogeneous oxidation device, and the sedimentation product water is oxidized by ozone in the heterogeneous oxidation device, and after the oxidation is completed, it is filtered by an ultrafiltration membrane to obtain ultrafiltration product water.

[0015] S701. The ultrafiltration product water obtained in S601 is filtered by a reverse osmosis membrane, the reverse osmosis product water is reused, and the reverse osmosis concentrated water is sent to S101 to be mixed with wastewater for treatment again.

[0016] In the process, the high-salinity, high-organic-substance and high-hardness wastewater is distributed on the heating surface of the thin film evaporator to form a liquid film, and after heating, the water evaporates and the remaining wastewater becomes concentrated with a large amount of crystalline salt particles (i.e. concentrated liquid). The concentrated liquid with a large amount of crystalline salt particles is discharged into the discharge tank for collection for subsequent drying or other treatment.

[0017] After being treated by the thin film evaporator, the condenser and the rectifying tower, the high-salinity, high-organic-substance and high-hardness wastewater can be converted into low-salinity, low-organic-substance and low-hardness wastewater for subsequent biochemical treatment.

[0018] Preferably or alternatively, the temperature of the wastewater preheating in step S101 is 60-65°C.

[0019] Preferably or alternatively, in step S201, the condensed water is rectified in the rectifying tower at a temperature of 60-100°C, the operating pressure of the rectifying tower is -85.0-0.0 KPa, and the separation efficiency of the rectifying tower is ≥ 90%.

[0020] The organic matters in the condensed water can be further removed by the rectification process, the treatment load of the subsequent biochemical treatment is reduced, the construction investment is reduced, and the operation cost is reduced.

[0021] Preferably or alternatively, in step S401, the biochemical tank is an anaerobic-anoxic-aerobic tank.

[0022] Preferably or alternatively, the residence time of the tower kettle liquid in the biochemical tank is 11-18h.

[0023] Preferably or alternatively, in step S501, the coagulation sedimentation tank is a high-density sedimentation tank or a mechanical clarifier.

[0024] Preferably or alternatively, in step S501, the turbidity of the sedimentation water of the coagulation sedimentation tank is ≤5NTU.

[0025] Preferably or alternatively, in step S601, the oxidation time is 40-60min.

[0026] The sedimentation water is subjected to ozone oxidation under the action of the heterogeneous catalyst in the heterogeneous oxidation device, and most of the organic matters in the water are oxidized into water and carbon dioxide.

[0027] Preferably or alternatively, in step S601, the ultrafiltration membrane is an immersed ultrafiltration membrane, and the pore size of the ultrafiltration membrane is 20-100nm.

[0028] Since the reverse osmosis membrane has certain requirements on water quality, in the method, the ultrafiltration membrane is used for filtration before the reverse osmosis membrane treatment, so as to further reduce the turbidity of the water and meet the water quality requirements of the reverse osmosis membrane.

[0029] Preferably or alternatively, in step S701, the water recovery rate of the reverse osmosis membrane is ≥75%.

[0030] After the action of the reverse osmosis membrane, the water quality of the produced water has reached the water quality requirements of the recycled water, and can be directly recycled to the production process of the enterprise.

[0031] Advantages

[0032] The high-salt high-organic high-hardness wastewater treatment method provided by the application solves the problem of the toxic effect of high-salt high-organic high-hardness wastewater on traditional biochemical sludge microorganisms, thereby causing difficulty in wastewater treatment, realizes the treatment and recycling of high-salt high-organic high-hardness wastewater, and overcomes a series of problems such as salt crystallization obstruction, crystallization slurry solid-liquid separation difficulty, evaporator scaling, and system instability encountered in the treatment of high-salt high-organic high-hardness wastewater by traditional multi-effect evaporation crystallization or MVR evaporation crystallization; the method is not limited by conditions such as the salt content, total hardness, and organic matter concentration of wastewater, has a wide application range, and can be implemented stably for a long time. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 A schematic diagram of a wastewater treatment system used in Example 2. DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the accompanying drawings and preferred experimental examples, but the scope of protection of the present application is not limited to the following specific examples.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. The professional terms used herein are only for the purpose of describing specific examples and are not intended to limit the scope of protection of the present application.

[0036] Example 1

[0037] The present application provides a high-salt high-organic high-hardness wastewater treatment method.

[0038] The wastewater treatment method is performed according to the following steps:

[0039] The high-salt high-organic high-hardness wastewater is preheated and then sent to a thin film evaporator, evaporated after distribution and forced film formation, to obtain steam and concentrated liquid. Most of the salt in the wastewater remains in the concentrated liquid, and the organic matter is separated together with the steam. The concentrated liquid is collected after the high-salt concentrated liquid is discharged for subsequent treatment. As a feasible treatment method, the concentrated liquid is further concentrated and then subjected to centrifugal solid-liquid separation treatment. The separated salt solid is packaged and then subjected to harmless treatment. The centrifugal mother liquor is dried, and the dried salt solid is also packaged and then subjected to harmless treatment.

[0040] The steam is condensed by a condensing device to obtain condensed water, and the condensed water is sent to a rectifying column for rectification to obtain column still liquid and steam containing organic matter. The steam containing organic matter contains a large amount of volatile organic matter, so the steam containing organic matter is condensed again, and the organic solution obtained by condensation is selected to be refluxed for rectification again or discharged and collected for centralized treatment.

[0041] The tower kettle liquid produced after rectification contains a small amount of difficult-to-vaporize organic components, and therefore the tower kettle liquid is transported to a biochemical tank composed of an anaerobic-anoxic-aerobic tank to remove part of the organic matter through denitrification and nitrification, and after sludge settlement treatment, biochemical product water is obtained.

[0042] The biochemical product water is transported to a coagulation sedimentation tank and a coagulant is added to make the solid suspensions in it flocculate and precipitate, so as to further reduce the turbidity and further remove a small amount of organic matter. After precipitation, the sludge and the precipitation product water are separated, the sludge is discharged and subjected to dewatering and drying treatment.

[0043] The precipitation product water is transported to a heterogeneous oxidation device and ozone is introduced to remove the remaining organic matter by oxidation. After oxidation, ultrafiltration membrane filtration is performed to further reduce the turbidity of the water, so that the ultrafiltration product water meets the water quality requirements of the reverse osmosis process.

[0044] The ultrafiltration product water is filtered by a reverse osmosis membrane. The reverse osmosis product water can be reused to the process because it meets the water quality requirements of the reuse water, and the reverse osmosis concentrated water is returned to the beginning of the process and mixed with the wastewater to be treated again.

[0045] Example Two

[0046] This embodiment further illustrates the treatment method of Example One in combination with specific equipment and systems. The wastewater treatment system and method provided by the embodiment of the present application comprises a pretreatment unit, a biochemical unit and a deep treatment unit arranged in sequence. Figure 1 As shown in the figure, the wastewater treatment system is connected to the wastewater discharge outlet of the boundary area and is composed of a pretreatment unit, a biochemical unit and a deep treatment unit arranged in sequence.

[0047] The pretreatment unit comprises a thin film evaporator, a first condenser, a second condenser and a rectification device.

[0048] The high-salinity, high-organic-matter and high-hardness wastewater produced by the boundary area is pumped and transported to the heat exchanger of the rectification tower kettle of the rectification device for preheating after being metered by a flow meter. After preheating, the wastewater is transported to the thin film evaporator.

[0049] The wastewater is distributed and forced to form a film in the thin film evaporator, and the steam generated by evaporation is condensed into condensed water by the first condenser, which is transported to the rectification tower for rectification to remove organic matter. The remaining concentrated liquid in the thin film evaporator is transported to the discharge tank for subsequent treatment.

[0050] As a feasible subsequent treatment scheme, the concentrated liquid can be further concentrated and subjected to centrifugal solid-liquid separation treatment. The separated salt solid is packaged for harmless treatment, and the centrifugal mother liquor is dried. The dried salt solid is also packaged for harmless treatment.

[0051] The condensate water exchanges heat with the tower plate in the middle of the rectifying tower to separate the volatile organic matter and water, and the volatile organic matter moves upward in the rectifying tower, is collected at the top of the tower, and is condensed into an organic solution rich in volatile organic matter components by the second condenser. The organic solution is pumped back to the rectifying tower to be distilled again after being mixed with the condensate water, or a small amount of the organic solution is cooled and then transported to a storage tank for centralized treatment.

[0052] The remaining water and non-volatile organic matter in the rectifying tower move downward and are collected at the bottom of the rectifying tower. After being cooled by the heat exchanger (synchronously preheating the wastewater) at the tower bottom, the water and non-volatile organic matter are transported to the biochemical unit.

[0053] The biochemical unit comprises a biochemical tank, a secondary sedimentation tank, and a coagulation sedimentation tank arranged in sequence.

[0054] In this embodiment, the biochemical tank is an anaerobic-anoxic-aerobic tank. The wastewater first enters the anaerobic tank, then enters the anoxic tank for denitrification, and then enters the aerobic tank for nitrification to reduce the content of organic matter. After the treatment is completed, the wastewater is transported to the secondary sedimentation tank for sludge settlement, and the produced water is transported to the coagulation sedimentation tank.

[0055] In the coagulation sedimentation tank, a coagulant is added to flocculate and settle the solid suspension, reduce turbidity, and also remove a small amount of organic matter. After sedimentation, the produced water of the coagulation sedimentation tank is transported to the advanced treatment unit, and the sludge is discharged, collected, and concentrated for dewatering treatment.

[0056] The advanced treatment unit comprises a heterogeneous oxidation device, an immersed ultrafiltration membrane tank, and a reverse osmosis device arranged in sequence.

[0057] In the heterogeneous oxidation device, the produced water of the coagulation sedimentation tank is oxidized by ozone to decompose the organic matter into water and carbon dioxide, further reducing the concentration of organic matter in the water. The produced water of the heterogeneous oxidation device is transported to the immersed ultrafiltration membrane tank, filtered by the immersed ultrafiltration membrane, and the turbidity of the water is reduced to meet the water inlet conditions of the reverse osmosis device.

[0058] The produced water of the immersed ultrafiltration membrane tank is transported to the reverse osmosis device, filtered by the reverse osmosis membrane, so that the water quality of the reverse osmosis produced water meets the standard, and the reverse osmosis produced water can be reused for production of the enterprise, while the reverse osmosis concentrated water is returned to the wastewater inlet of the boundary area for treatment again.

[0059] Example Three

[0060] The following further details of Example Two are described in combination with specific process parameters.

[0061] In this embodiment, the wastewater generated in the boundary area is high-salt, high-organic, and high-hardness wastewater with a salt content ≥100,000 mg / L, a COD concentration ≥15,000 mg / L, a total hardness (calculated as CaCO3) ≥1,500 mg / L, and a pH of 6-9.

[0062] The wastewater from the boundary zone is pumped, metered by a flow meter, and then preheated in a heat exchanger in the tower bottom of the rectification tower of the rectification device, and then introduced into the thin film evaporator after preheating to 60-65℃.

[0063] The wastewater is distributed and forced to form a film in the thin film evaporator, and the steam generated by evaporation is condensed into condensed water by a first condenser and then transported into the rectification tower for rectification to remove volatile organic matter. The concentrated liquid remaining after evaporation is discharged and collected, and after further concentration, it is subjected to centrifugal solid-liquid separation treatment. The separated salt solid is packaged and then subjected to harmless treatment, and the centrifugal mother liquor is dried. The salt solid after drying is also packaged and then subjected to harmless treatment.

[0064] In this embodiment, the operating temperature of the rectification tower is 60-100℃, the operating pressure (gauge pressure) is -85.0-0.0kPa, and the separation efficiency of the rectification tower is ≥90%.

[0065] The condensed water exchanges gas and liquid through the trays in the middle of the rectification tower, separating volatile organic matter and water. The volatile organic matter moves upward in the rectification tower, forming steam containing organic matter and collecting at the top of the tower, and is condensed into an organic solution rich in volatile organic matter components by a second condenser. The organic solution is pumped back to the rectification tower for rectification again, or a small amount of organic solution is cooled, discharged and collected, and then subjected to harmless treatment.

[0066] The remaining water and a small amount of non-volatile organic matter in the rectification tower move downward and collect at the bottom of the rectification tower, forming tower bottom liquid, which is cooled by the heat exchanger in the tower bottom of the rectification tower (simultaneously preheating the wastewater) and then transported to the biochemical unit.

[0067] At this time, the water quality introduced into the biochemical unit has a salt content of ≤1000mg / L, a COD concentration of ≤1500mg / L, a total hardness (calculated as CaCO3) of ≤50mg / L, and a pH of 6-9.

[0068] That is, the high-salt, high-organic, and high-hardness wastewater from the boundary zone is treated into low-salt, low-organic, and low-hardness wastewater through the foregoing treatment.

[0069] In this embodiment, the biochemical tank of the biochemical unit is an anaerobic-anoxic-aerobic tank. The wastewater first enters the anaerobic tank, then enters the anoxic tank for denitrification, and then enters the aerobic tank for nitrification to remove part of the organic matter. The total hydraulic retention time of the wastewater in the biochemical tank is 11-18h. After treatment, it is introduced into the secondary sedimentation tank for sludge settling, and the produced water is introduced into the coagulation sedimentation tank.

[0070] In this embodiment, the coagulation sedimentation tank is a high-density sedimentation tank or a mechanical clarifier, and the turbidity of the produced water treated by the coagulation sedimentation tank is ≤5NTU.

[0071] In the coagulation sedimentation tank, coagulant is added to make solid suspension flocculate and settle, and a small amount of organic matter is also removed.

[0072] The water produced by the coagulation sedimentation tank is transported to the advanced treatment unit, and the sludge produced by the sedimentation is discharged, collected and concentrated for dewatering treatment.

[0073] The water produced by the coagulation sedimentation tank is subjected to ozone oxidation in the heterogeneous oxidation device to decompose organic matter into water and carbon dioxide for further reducing the concentration of organic matter in the water, and the residence time for oxidation is 40-60 min. The water produced by the heterogeneous oxidation device is transported to the submerged ultrafiltration membrane tank.

[0074] In the embodiment, the membrane assembly of the submerged ultrafiltration membrane tank is a curtain membrane or a flat plate membrane, and the membrane filtration pore size is 20-100 nm. In this embodiment, the ultrafiltration process is used to reduce the turbidity in the water to meet the requirements of the subsequent reverse osmosis process.

[0075] The water produced by the submerged ultrafiltration membrane tank is transported to the reverse osmosis device, and is filtered by the reverse osmosis membrane in the reverse osmosis device to make the water quality of the reverse osmosis product meet the standard, and the water can be recycled for production of the enterprise, and the concentrated water of the reverse osmosis is transported back to the wastewater inlet of the boundary area for treatment again.

[0076] In this embodiment, the recovery rate of the water produced by the reverse osmosis device is greater than or equal to 75%. After treatment, the salt content of the water produced by the reverse osmosis device is ≤300 mg / L, the COD concentration is ≤50 mg / L, the total hardness (calculated as CaCO3) is ≤50 mg / L, and the pH is 6-9, which can meet the water standard of the enterprise.

[0077] The industrial wastewater treatment method provided by the present application solves the problem of the toxic effect of high-salt, high-organic and high-hardness wastewater on traditional biochemical sludge microorganisms, thereby causing difficulty in wastewater treatment, realizes the treatment and recycling of high-salt, high-organic and high-hardness wastewater, and overcomes a series of problems such as salt crystallization obstruction, difficulty in solid-liquid separation of crystal slurry, evaporation device scaling and system instability encountered in the treatment of high-salt, high-organic and high-hardness wastewater by traditional multi-effect evaporation crystallization or MVR evaporation crystallization. The method is not limited by conditions such as the salt content, total hardness and organic matter concentration of the wastewater, has a wide application range, and can be implemented stably for a long time.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent substitutions for some technical features. These modifications or substitutions do not change the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A high-salinity high-organic high-hardness wastewater treatment method, characterized by, The method comprises the following steps: S101. High-salinity, high-organic-matter and high-hardness wastewater with a salt content of ≥100000 mg / L, a COD concentration of ≥15000 mg / L, a total hardness of ≥1500 mg / L in terms of CaCO3, and a pH of 6-9 is pumped into a heat exchanger at the bottom of a rectifying tower of a rectifying device for preheating to 60-65°C, and then is transported to a thin film evaporator, evaporated to obtain steam and concentrated liquid after distribution and forced film formation in the thin film evaporator; S201. The steam obtained in S101 is condensed to obtain condensed water, and the condensed water is transported to the rectifying tower for rectification to obtain tower bottom liquid and organic matter-containing steam; S202. The concentrated liquid obtained in S101 is discharged and collected, and after further concentration, is subjected to centrifugal solid-liquid separation treatment, the separated salt solid is packaged and subjected to harmless treatment, and the centrifugal mother liquor is dried, and the dried salt solid is also packaged and subjected to harmless treatment; S301. The organic matter-containing steam obtained in S201 is condensed to obtain an organic solution, and the organic solution is transported back to the rectifying tower for continuous rectification and / or discharged and collected; S401. The tower bottom liquid obtained in S201 is cooled by the heat exchanger at the bottom of the rectifying tower as described in S101, and then is transported to a biochemical pool for biochemical treatment, and the water quality of the water introduced into the biochemical pool is a salt content of ≤1000 mg / L, a COD concentration of ≤1500 mg / L, a total hardness of ≤50 mg / L in terms of CaCO3, and a pH of 6-9; after removal of part of the organic matter by the biochemical treatment, the treated water is transported to a secondary sedimentation tank for sludge settlement treatment to obtain biochemical product water; The biochemical pool comprises, in sequence, an anaerobic tank, an anoxic tank and an aerobic tank; S501. The biochemical product water obtained in S401 is transported to a coagulation sedimentation tank, a coagulant is added and sedimentation is performed to obtain sludge and sedimentation product water, and the sludge is discharged and dewatered; S601. The sedimentation product water obtained in S501 is transported to a heterogeneous oxidation device, and the sedimentation product water is subjected to oxidation by introducing ozone into the heterogeneous oxidation device, and after the oxidation is completed, is filtered by an ultrafiltration membrane to obtain ultrafiltration product water; S701. The ultrafiltration product water obtained in S601 is filtered by a reverse osmosis membrane, the reverse osmosis product water is reused, the reverse osmosis water product has a salt content of ≤300 mg / L, a COD concentration of ≤50 mg / L, a total hardness of ≤50 mg / L in terms of CaCO3, and a pH of 6-9; and the reverse osmosis concentrated water is returned to S101 to be mixed with the wastewater for treatment again.

2. The wastewater treatment method according to claim 1, characterized by, In step S201, the condensate is rectified in the rectifying tower at a temperature of 60-100°C, the operation pressure of the rectifying tower is -85.0-0.0 KPa, and the separation efficiency of the rectifying tower is ≥90%.

3. The wastewater treatment method according to claim 1, characterized by, The residence time of the tower bottom liquid in the biochemical pool is 11-18 h.

4. The wastewater treatment method according to claim 1, characterized by, In step S501, the coagulation sedimentation tank is a high-density sedimentation tank or a mechanical clarifier.

5. The wastewater treatment method according to claim 4, characterized by, In step S501, the turbidity of the sedimentation product water of the coagulation sedimentation tank is ≤5 NTU.

6. The wastewater treatment method according to claim 1, characterized by, In step S601, the oxidation time is 40-60 min.

7. The wastewater treatment method according to claim 1, characterized by, In step S601, the ultrafiltration membrane is an immersed ultrafiltration membrane, and the pore size of the ultrafiltration membrane is 20-100 nm.

8. The wastewater treatment method according to claim 1, characterized by, In step S701, the water recovery rate of the reverse osmosis membrane is ≥75%.

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