Method for improving performance of composite separation membrane

Abstract

The invention discloses a post-treatment method for improving the performance of a composite separation membrane and relates to the technical field of membrane separation. The method comprises the following steps: on one hand, active amino groups of hydroxyethyl urea are combined with free carboxyl groups in the composite reverse osmosis membrane to generate amido bonds, so that the charge repulsion effect and the steric hindrance effect are enhanced, the permeation of ions is inhibited, and the salt rejection rate of the membrane is improved; on the other hand, hydrogen bonds can be formed between free alcoholic hydroxyl groups of hydroxyethyl urea and water molecules, so that more water permeation channels are provided, and the water production flux of the membrane is further increased.

Description

Technical Field

[0001] The invention relates to the technical field of membrane separation, and in particular to a method for improving the performance of a composite separation membrane. Background Art

[0002] Water resources are an indispensable natural resource in human life and production today. However, with the rapid development of society, the contradiction between water supply and demand has become increasingly prominent. How to effectively improve the utilization rate of water resources has become a top priority. Membrane separation, as an emerging technology, has both efficient separation performance and low energy consumption. It has been widely used in seawater desalination, water reuse, sewage treatment and other fields.

[0003] Reverse Osmosis (RO) is a new type of membrane separation technology driven by pressure difference, that is, it promotes water molecules to pass through the semipermeable membrane under pressure conditions higher than the osmotic pressure of the raw water, and intercepts inorganic ions, organic matter, colloids, microorganisms, pyrogens and other substances to obtain pure water resources. It has the characteristics of good separation effect, no phase change, simple and efficient. Reverse osmosis membrane is the core of reverse osmosis technology. Its main performance parameters include water flux and desalination rate. The higher the former value, the higher the membrane water output per unit area per unit time, and the higher the latter value, the less salt permeation, that is, the better the salt water separation effect.

[0004] Nanofiltration membranes are similar in structure to reverse osmosis membranes, that is, they generally include a non-woven fabric support layer, a polysulfone intermediate layer, and a polyamide separation layer, so they are collectively referred to as composite separation membranes. The polyamide layer controls the separation performance of the composite membrane, which is generally generated by polymerization of active amine monomers and active acyl chloride monomers at the water-oil boundary. Since both have extremely high activity, the reaction is usually completed in a very short time, which increases the difficulty of improving the performance of the composite membrane under the premise of a selected formulation process system.

[0005] The post-treatment process of the membrane refers to the process of using physical or chemical methods to optimize the performance of the membrane that has undergone interfacial polymerization and has achieved stable performance. Typical examples include: 1. DuPont uses tannic acid and polyvinyl methyl ether to treat damaged polyamide membranes to restore desalination performance; 2. Hebei University of Technology soaked commercial nanofiltration membranes in a mixed solution of sodium hydroxide and alcohol to increase the flux of the membrane.

[0006] However, these processes all have certain defects. The former needs to be under pressure to be effective, that is, it must be processed in the form of a membrane assembly, which is a cumbersome process; although the latter can increase the flux of the membrane, it will also have a significant impact on the retention performance of the membrane. In particular, currently commercial reverse osmosis membranes and nanofiltration membranes generally use polyester fibers as non-woven fabric support layers. Excessive sodium hydroxide concentrations will cause the non-woven fabric to hydrolyze, thereby losing mechanical properties and not being commercially feasible. Therefore, developing a composite separation membrane post-treatment method that simultaneously improves water flux and retention and meets the requirements of large-scale preparation is a technical problem that the industry urgently needs to solve. Summary of the invention

[0007] In view of the above problems, the present invention provides a method for improving the performance of a composite separation membrane, which can improve water flux, interception performance and COD interception rate and realize large-scale preparation.

[0008] The technical solution of the present invention is: A method for improving the performance of a composite separation membrane comprises the following steps: Step 1, preparing a hydroxyethyl urea aqueous solution; Step 2: membrane pretreatment; Step 3, soaking the membrane / membrane element in the above hydroxyethyl urea aqueous solution; the soaking temperature is 10-90° C., and the soaking time is 1-20 min; Step 4: Remove water stains on the surface of the membrane / membrane element (e.g., drain or dry). The drain or dry temperature is 20-90°C and the time is 0.5-15 minutes.

[0009] Specifically, the hydroxyethyl urea aqueous solution includes: 1-20wt% of hydroxyethyl urea, and the rest is pure water.

[0010] Specifically, the pretreatment of the membrane in step 2 includes: a. When preparing membranes on a large scale: Including the synthesis and drying process of the polyamide separation layer; b. When the performance of commercial diaphragms / membrane elements is improved: Including pure water immersion treatment; C. Performance recovery of failed membrane elements: Including acid and alkali cleaning and pure water cleaning.

[0011] Specifically, the synthesis and drying process of the polyamide separation layer specifically includes: a1, a polyamide separation layer is synthesized on the surface of the base membrane by interfacial polymerization process to form a primary composite separation membrane sheet MA01; a2, MA01 undergoes a heating and cleaning process to form a pre-treated membrane MA02; The pretreated MA02 is subjected to a hydroxyethyl urea aqueous solution to generate MA03; MA03 is drained or dried to complete the modification to generate the finished product MA04.

[0012] Specifically, the base membrane in a1 includes an ultrafiltration membrane in the middle layer and a non-woven fabric in the bottom layer. The ultrafiltration membrane is mainly composed of polysulfone or polyethersulfone and is prepared by a phase transition method.

[0013] Specifically, the interfacial polymerization in a1 refers to a polymerization reaction in which a mixed aqueous solution of active amine monomers and an oily solution of active acyl chloride monomers are in surface contact to form a polyamide network structure.

[0014] The mixed aqueous phase solution of active amine monomers includes: 0.5-5.0% of m-phenylenediamine or piperazine, 0.05-0.3% of sodium hydroxide, and the rest is pure water; The oil phase solution of the active acyl chloride monomer includes: 0.05-0.3% of trimesoyl chloride, and the rest is one of n-hexane, n-heptane and IsoparG.

[0015] Specifically, the heating process in a2 refers to a process in which MA01 passes through a hot air environment to volatilize the residual oil phase solvent on its surface and increase the internal cross-linking degree of the polyamide separation layer to improve the performance of the membrane. The hot air temperature is controlled at 40-90°C, and the time for MA01 to pass through the hot air environment is 1-10 minutes.

[0016] Specifically, the cleaning process in a2 includes, but is not limited to, acid cleaning, alkali cleaning, oxidation-reduction treatment and hot water treatment to clean the residue inside the membrane MA01.

[0017] Specifically, when the performance of commercial membranes / membrane elements is improved, it includes: Take the existing commercial diaphragm and go through the unwinding process and the cleaning water tank pretreatment process in sequence to form the diaphragm MB01; Or, take the existing commercial membrane element and immerse it in the cleaning water tank to form the membrane element MC01; The obtained MB01 or MC01 is subjected to the hydroxyethyl urea aqueous solution in step 3 to generate MB02 or MC02; MB02 or MC02 is drained or dried to complete the modification and generate the finished product MA03 or MC03.

[0018] Specifically, the cleaning water tank is mainly low-molecular alcohol aqueous solution; The low molecular alcohols include one or more solutions of methanol, ethanol, n-propanol, isopropanol, and n-butanol.

[0019] Specifically, when the performance of the failed membrane element is restored, it includes: According to the pollution type of the failed membrane element, acid washing, alkali washing and water washing are performed respectively to form membrane element MD01; The obtained MD01 is soaked in the hydroxyethyl urea aqueous solution in step three to generate MD02; MD02 is drained or dried to complete the modification to generate the finished product MD03.

[0020] Beneficial effects of the present invention: This case adopts a simple process, that is, immersing the composite separation membrane in an aqueous solution containing hydroxyethyl urea. This method: on the one hand, the active amine groups of hydroxyethyl urea combine with the free carboxyl groups in the composite reverse osmosis membrane to form amide bonds, which strengthens the charge repulsion effect and steric hindrance effect, inhibits the penetration of ions, and improves the salt retention rate of the membrane ( Figure 1 On the other hand, the free alcohol hydroxyl groups of hydroxyethyl urea can form hydrogen bonds with water molecules, providing more water permeation channels, thereby increasing the water flux of the membrane ( Figure 2 ). This method is suitable for large-scale preparation of high-performance composite separation membrane sheets, improvement of the performance of existing commercial membrane sheets, improvement of the performance of existing commercial membrane elements, and performance recovery of failed membrane elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a microscopic structure diagram of the polyamide layer before and after modification according to the present invention; Figure 2 It is the reaction structural formula of the hydroxyethyl urea and polyamide of the present invention. DETAILED DESCRIPTION

[0022] This method is applicable to the large-scale preparation of high-performance composite separation membrane sheets, improvement of the performance of existing commercial membrane sheets, improvement of the performance of existing commercial membrane elements, and performance recovery of failed membrane elements. The following describes the technical solutions for different application scenarios.

[0023] The separation membrane of the composite separation membrane in this case includes a reverse osmosis membrane and a nanofiltration membrane, and the reverse osmosis membrane includes a household reverse osmosis membrane, an industrial brackish water reverse osmosis membrane, a seawater desalination reverse osmosis membrane, and the like.

[0024] Scenario 1: Large-scale preparation of high-performance composite separation membrane sheets; Step 1: synthesizing a polyamide separation layer on the surface of the base membrane by an interfacial polymerization process to form a primary composite separation membrane sheet MA01; Further, the base membrane refers to a composite material with polysulfone or polyethersulfone as the main body, prepared by a phase transition method, an ultrafiltration membrane with a surface pore size of 10-100 nm as the middle layer and a non-woven fabric as the bottom layer; Furthermore, the interfacial polymerization refers to a polymerization reaction in which a mixed aqueous solution of active amine monomers and an oily solution of active acyl chloride monomers are in surface contact to generate a polyamide network structure; Step 2, MA01 undergoes a heating and cleaning process to form a pre-treated membrane MA02; Furthermore, the heating process refers to a process in which MA01 passes through a hot air environment to volatilize the residual oil phase solvent on its surface and increase the internal cross-linking degree of the polyamide separation layer to improve the performance of the membrane; Furthermore, the hot air temperature is controlled at 40-90°C, and the time for MA01 to pass through the hot air environment is 1-10min; Furthermore, the cleaning process refers to the process in which the MA01 enters a multi-channel water tank after the heating process to clean the residue inside it; Furthermore, the cleaning process includes, but is not limited to, acid cleaning, alkali cleaning, oxidation-reduction treatment, hot water treatment and other processes; Step 3, MA02 is subjected to hydroxyethyl urea solution to generate MA03; Furthermore, the hydroxyethyl urea solution refers to an aqueous solution in which the effective monomer is hydroxyethyl urea, the concentration of hydroxyethyl urea is 1-20wt%, the soaking temperature is 10-90°C, and the soaking time is 1-20min; Step 4: MA03 is drained or dried to complete the modification and generate the finished product MA04 Furthermore, the draining or drying temperature is 20-90°C and the time is 0.5-15min. Scenario 2: Improvement of existing commercial diaphragm performance Step 1: Take the existing commercial membrane and process it through the unwinding process and water tank cleaning process to form the membrane MB01; Furthermore, the cleaning water tank is mainly low-molecular alcohol aqueous solution, and its main purpose is to wash away the moisturizer remaining in the membrane; Furthermore, the low molecular alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, etc., with a content of 10-50wt%, and a treatment time of 1min-24h; Step 2, MB01 is subjected to hydroxyethyl urea solution to generate MB02; Furthermore, the hydroxyethyl urea solution refers to an aqueous solution in which the effective monomer is hydroxyethyl urea, the concentration of hydroxyethyl urea is 1-20wt%, the soaking temperature is 10-90°C, and the soaking time is 1-20min; Step 3: MB02 is drained or dried to complete the modification and generate the finished product MA03; Furthermore, the draining or drying temperature is 20-90°C and the time is 0.5-15min. Scenario 3: Improvement of the performance of existing commercial membrane elements; Step 1: Take the existing commercial membrane element and immerse it in a cleaning water tank to form a membrane element MC01; Furthermore, the cleaning water tank is mainly low-molecular alcohol aqueous solution, and its main purpose is to wash away the moisturizer remaining in the membrane; Furthermore, the low molecular alcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, etc., with a content of 10-50wt%, and a treatment time of 1min-24h; Step 2, taking out MC01 and immersing it in a hydroxyethyl urea solution water tank for immersion treatment to generate MC02; Furthermore, the hydroxyethyl urea solution refers to an aqueous solution in which the effective monomer is hydroxyethyl urea, the concentration of hydroxyethyl urea is 1-20wt%, the soaking temperature is 10-90°C, and the soaking time is 1-20min; Step 3: MC02 is drained or dried to complete the modification and generate the finished product MC03 Furthermore, the draining or drying temperature is 20-90°C and the time is 0.5-15min. Scenario 4: Performance recovery of failed membrane elements; Step 1: According to the pollution type of the failed membrane element, acid washing, alkali washing and water washing are performed respectively to form membrane element MD01; Furthermore, the failed membrane element refers to a membrane element whose performance parameters in the water treatment system are lower than the customer's usage requirements and whose performance cannot be restored by a simple cleaning process; Furthermore, the acid cleaning refers to cleaning the membrane element with an acidic aqueous solution online or offline, the main purpose of which is to remove scaling pollutants on the membrane surface; Furthermore, the alkaline cleaning refers to cleaning the membrane element with an alkaline aqueous solution online or offline, the main purpose of which is to remove organic matter or biological contaminants on the membrane surface; Furthermore, the water washing refers to the online or offline pure water washing of the membrane element, the main purpose of which is to remove the acid and alkali solutions remaining on the surface during acid washing or alkali washing; Step 2, taking out MD01 and immersing it in a hydroxyethyl urea solution water tank for immersion treatment to generate MD02; Furthermore, the hydroxyethyl urea solution refers to an aqueous solution in which the effective monomer is hydroxyethyl urea, the concentration of hydroxyethyl urea is 1-20wt%, the soaking temperature is 10-90°C, and the soaking time is 1-20min; Step 3: MD02 is drained or dried to complete the modification and generate the finished product MD03; Furthermore, the draining or drying temperature is 20-90°C and the time is 0.5-15min. The embodiments of the present invention are described in detail below. The embodiments are exemplary and are only used to explain the present invention, but should not be construed as limiting the present invention.

[0025] Embodiment 1: S1, respectively prepare the m-phenylenediamine aqueous phase solution, the trimesoyl chloride oil phase solution, the post-treatment cleaning solution, the 10wt% hydroxyethyl urea modified solution, etc., and sequentially place them into the interfacial polymerization section water tank and the post-treatment water tank of the composite separation membrane sheet large-scale coating equipment; S2, placing the polysulfone-based membrane into the unwinding device of the composite separation membrane sheet large-scale coating equipment, opening the equipment and sequentially undergoing the processes of interfacial polymerization, heat treatment, post-treatment cleaning, post-treatment modification, draining or drying, etc., to complete the modification, and the obtained membrane sheet is recorded as MA-A; S3, repeating the modification process of S2, but using pure water instead of hydroxyethyl urea solution, to prepare a membrane as a reference group, denoted as MA-B; S4. Take the MA-A and MA-B diaphragms and put them into the test bench for performance testing. The test conditions are 225psi operating pressure and 2000ppm NaCl solution. The results are as follows: serial number MA-A1 MA-A2 MA-B1 MA-B2 Salt rejection rate 99.49% 99.61% 99.30% 99.27% Water production flux 31.66LMH 31.66LMH 29.19LMH 28.93LMH Example 2 S1. Take 50% ethanol aqueous solution to soak and clean the commercial reverse osmosis membrane for 12 hours; S2, placing the cleaned membrane into a 10wt% hydroxyethyl urea solution for modification, the temperature of the modified solution is controlled at 30°C, and the modification time is 10min; S3, placing the modified membrane in the air for draining at an ambient temperature of 35°C for 15 minutes. The membrane after draining is recorded as MB-A, and the membrane without modification is recorded as MB-B; S4. Place the above diaphragm into the test bench for performance testing. The test conditions are 225 psi operating pressure and 2000 ppm NaCl solution. The results are as follows: serial number MB-A1 MB-A2 MB-B1 MB-B2 Salt rejection rate 99.24% 99.33% 99.02% 98.66% Water production flux 34.91LMH 34.91LMH 29.92LMH 16.62LMH The average desalination rate of the unmodified reverse osmosis membrane is 98.84%, and the average water production flux is 29.92LMH; the average desalination rate of the modified reverse osmosis membrane is 99.28%, and the average water production flux is 34.91LMH. The desalination rate has increased by about 0.42%, and the flux has increased by about 4.99LMH.

[0026] Example 3 S1. Take 50% ethanol aqueous solution to soak and clean the commercial nanofiltration membrane for 12 hours; S2, placing the cleaned membrane into a 10wt% hydroxyethyl urea solution for modification, the temperature of the modified solution is controlled at 30°C, and the modification time is 10min; S3, placing the modified membrane in the air for draining at an ambient temperature of 35°C for 15 minutes. The membrane after draining is recorded as MB1-A, and the membrane without modification is recorded as MB1-B; S4. Place the above diaphragm into the test bench for performance testing. The test conditions are 100 psi operating pressure and 2000 ppm MgSO4 solution. The results are as follows: serial number MB1-A1 MB1-A2 MB1-B1 MB1-B2 Salt rejection rate 99.49% 99.40% 99.25% 99.24% Water production flux 50.05LMH 44.04LMH 34.03LMH 44.04MH The unmodified nanofiltration membrane has an average magnesium sulfate rejection rate of 99.24% and an average water production flux of 39.03LMH; the modified nanofiltration membrane has an average magnesium sulfate rejection rate of 99.45% and an average water production flux of 47.04LMH. The desalination rate increased by about 0.21% and the flux increased by about 8.01LMH.

[0027] Example 4 S1. Take 50% ethanol aqueous solution to soak and clean the commercial reverse osmosis membrane element for 12 hours; S2. Place the cleaned membrane element into a water tank containing 10 wt% hydroxyethyl urea solution for modification. The temperature of the modified solution is controlled at 30°C and the modification time is 10 min. S3, take out the modified membrane element and put it into the air for draining, the ambient temperature is 30-35℃, the time is 60min, the membrane element after draining is recorded as MC-A, and the membrane element without modification is recorded as MC-B; S4. Place the above membrane element into the test device for performance testing. The test conditions are 225 psi operating pressure and 2000 ppm NaCl solution. The results are as follows: serial number MC-A1 MC-A2 MC-B1 MC-B2 Salt rejection rate 99.65% 99.61% 99.21% 99.43% Water production 40.09m³ / d 41.78m³ / d 37.88m³ / d 39.17m³ / d The unmodified reverse osmosis membrane element has an average desalination rate of 99.32% and an average water production of 38.52 m³ / d; the modified reverse osmosis membrane element has an average desalination rate of 99.63% and an average water production flux of 40.93 m³ / d. The desalination rate has increased by about 0.31% and the flux has increased by about 1.55 m³ / d.

[0028] Example 5 S1. Take the expired commercial reverse osmosis membrane element and put it into the cleaning equipment for acid washing, alkali washing and water washing respectively; S2. Place the cleaned membrane element into a water tank containing 10 wt% hydroxyethyl urea solution for modification. The temperature of the modified solution is controlled at 30°C and the modification time is 30 min. S3, take out the modified membrane element and put it into the air for draining, the ambient temperature is 30-35℃, the time is 60min, the membrane element after draining is recorded as MD-A, and the membrane element without modification is recorded as MD-B; S4. Place the above membrane element into the test device for performance testing. The test conditions are 225 psi operating pressure and 2000 ppm NaCl solution. The results are as follows: serial number MD-A1 MD-A2 MD-B1 MD-B2 Salt rejection rate 98.88% 97.72% 96.54% 95.97% Water production flux 27.38m³ / d 27.85m³ / d 22.19m³ / d 25.34m³ / d The average desalination rate of the unmodified reverse osmosis membrane element is 96.26%, and the average water production is 23.77m³ / d; the average desalination rate of the modified reverse osmosis membrane element is 98.30%, and the average water production flux is 27.62m³ / d. The desalination rate has increased by about 2.04%, and the flux has increased by about 3.85m³ / d.

[0029] Case Study Example 1 proves that the method described in this case can be used for large-scale preparation of high-performance composite separation reverse osmosis membrane sheets; Examples 2 and 3 demonstrate that the method described in this case can be used to improve the performance of commercial composite separation reverse osmosis membranes and nanofiltration membranes; Example 4 demonstrates that the method described in this case can be used to improve the performance of commercial composite separation reverse osmosis membrane elements; Example 5 demonstrates that the method described in this case can be used to restore the performance of damaged or failed reverse osmosis membrane elements.

[0030] In this case, the composite membrane separation membrane is immersed in an aqueous solution of hydroxyethyl urea to achieve the purpose of improving both water flux and interception performance. A typical effect is that the commercial reverse osmosis membrane treated by the method of the present invention has a sodium chloride interception rate increased from 98.84% to 99.28% under the same test conditions, and the water flux increased from 29.92LMH to 34.91LMH; the composite nanofiltration membrane has a magnesium sulfate interception rate increased from 99.24% to 99.45%, and the water flux increased from 39LMH to 47LMH.

[0031] The present invention provides a new treatment and modification method, which is suitable for the large-scale preparation of high-performance composite separation membrane sheets, the improvement of the performance of commercial composite separation membrane sheets / membrane elements, the recovery of the performance of damaged or failed separation membrane elements, etc. It has commercial feasibility and provides an important idea for the study of composite membrane structure and mechanism.

[0032] As described above, although the present invention has been described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself, and various changes in form and details may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for improving the performance of a composite separation membrane, characterized in that: The following steps are involved: Step 1, preparing a hydroxyethyl urea aqueous solution; The hydroxyethyl urea aqueous solution comprises 1-20wt% hydroxyethyl urea, and the remainder is pure water; Step 2: pretreatment of the membrane; Step 3, immersing the membrane sheet / membrane element in the above-mentioned hydroxyethyl urea aqueous solution; Step 4: Clean the water stains on the surface of the diaphragm / membrane element.

2. A method for improving the performance of a composite separation membrane according to claim 1, characterized in that: Step 2: membrane pretreatment includes: a. When preparing membranes on a large scale: Including the synthesis and drying process of the polyamide separation layer; b. When the performance of commercial diaphragms / membrane elements is improved: Including pure water immersion treatment; C. Performance recovery of failed membrane elements: Including acid and alkali cleaning and pure water cleaning.

3. A method for improving the performance of a composite separation membrane according to claim 2, characterized in that: The synthesis and drying process of the polyamide separation layer specifically includes: a1, a polyamide separation layer is synthesized on the surface of the base membrane by interfacial polymerization process to form a primary composite separation membrane sheet MA01; a2, MA01 undergoes a heating and cleaning process to form a pre-treated membrane MA02; The pretreated MA02 is subjected to a hydroxyethyl urea aqueous solution to generate MA03; MA03 is drained or dried to complete the modification to generate the finished product MA04.

4. A method for improving the performance of a composite separation membrane according to claim 3, characterized in that: The base membrane described in a1 includes an ultrafiltration membrane in the middle layer and a non-woven fabric in the bottom layer.

5. A method for improving the performance of a composite separation membrane according to claim 3, characterized in that: The interfacial polymerization in a1 refers to a polymerization reaction in which a mixed aqueous solution of active amine monomers and an oily solution of active acyl chloride monomers are in surface contact to form a polyamide network structure.

6. A method for improving the performance of a composite separation membrane according to claim 3, characterized in that: The heating process in a2 refers to the process in which MA01 passes through a hot air environment to volatilize the residual oil phase solvent on its surface and increase the internal cross-linking degree of the polyamide separation layer to improve the performance of the membrane.

7. A method for improving the performance of a composite separation membrane according to claim 3, characterized in that: The cleaning process in a2 includes, but is not limited to, acid cleaning, alkaline cleaning, redox treatment and hot water treatment to clean the internal residue of the membrane MA01.

8. A method for improving the performance of a composite separation membrane according to claim 2, characterized in that: When commercial membrane / element performance is improved, it specifically includes: Take the existing commercial diaphragm and go through the unwinding process and the cleaning water tank pretreatment process in sequence to form the diaphragm MB01; Or, take the existing commercial membrane element and immerse it in the cleaning water tank to form the membrane element MC01; The obtained MB01 or MC01 is subjected to the hydroxyethyl urea aqueous solution in step 3 to generate MB02 or MC02; MB02 or MC02 is drained or dried to complete the modification and generate the finished product MA03 or MC03.

9. A method for improving the performance of a composite separation membrane according to claim 8, characterized in that: The cleaning water tank is mainly low-molecular alcohol aqueous solution; The low molecular alcohols include one or more solutions of methanol, ethanol, n-propanol, isopropanol, and n-butanol.

10. A method for improving the performance of a composite separation membrane according to claim 2, characterized in that: When the performance of failed membrane elements is restored, it specifically includes: According to the pollution type of the failed membrane element, acid washing, alkali washing and water washing are performed respectively to form membrane element MD01; The obtained MD01 is soaked in the hydroxyethyl urea aqueous solution in step three to generate MD02; MD02 is drained or dried to complete the modification to generate the finished product MD03.

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