Repair process, performance test method and performance test system for PVDF hollow fiber ultrafiltration membrane filaments

Abstract

The invention relates to a repair process, a performance testing method and a performance testing system for PVDF hollow fiber ultrafiltration membrane filaments, and the repair process comprises the following steps: firstly, sequentially cleaning waste membrane filaments by using a sodium hydroxide solution and a citric acid solution to remove surface impurities and dirt; pVDF powder, PEG, DMAc and PVP are mixed according to a certain proportion and stirred and dissolved at the temperature of 70 DEG C, and repairing liquid is formed; soaking the pretreated membrane filaments in the repair liquid for 1 hour, and then cleaning the membrane filaments with clear water and drying the membrane filaments. And finally, reassembling the repaired membrane filaments into the membrane assembly. The pure water flux of the scrapped membrane is recovered, the pollution resistance of the membrane is improved, and the recovery and production cost of membrane production of enterprises is reduced.

Description

Technical Field

[0001] The invention relates to a repair process, a performance testing method and a performance testing system for PVDF hollow fiber ultrafiltration membrane yarns, and belongs to the field of water treatment ultrafiltration membrane repair. Background Art

[0002] In recent years, with the advancement of industrialization, water pollution has intensified, triggering a series of water resource crises. As an advanced separation technology, membrane separation, particularly ultrafiltration membrane separation, has shown tremendous potential in wastewater purification. Ultrafiltration membranes effectively remove contaminants such as macromolecular organic matter, colloids, and bacteria from water through a sieving effect, achieving deep purification. Ultrafiltration membranes offer advantages such as low energy consumption, simple operation, and no phase transition. Consequently, they have been widely used in numerous fields, including energy, chemical engineering, and the environment, becoming a crucial technical support for the sustainable use of water resources.

[0003] Among numerous ultrafiltration membrane materials, polyvinylidene fluoride (PVDF) hollow fiber membranes stand out for their exceptional performance. They possess excellent mechanical strength, maintaining a stable structure and resisting breakage even in complex operating environments. They also exhibit excellent thermal stability, operating over a wide temperature range. They also possess exceptional chemical stability, exhibiting strong tolerance to acids, alkalis, and organic solvents, making them resistant to chemical attack. Furthermore, their excellent machinability allows them to be manufactured into membrane products of various shapes and specifications to meet the needs of diverse application scenarios. These advantages have led to widespread and mature use of PVDF hollow fiber membranes in industry.

[0004] However, in the actual water treatment process, PVDF membranes face the thorny problem of membrane fouling. Organic pollutants, colloids and other impurities in the water are easily adsorbed on the membrane surface, leading to membrane fouling and a decrease in membrane separation performance. Although physical and chemical cleaning methods can slow the rate of membrane fouling, the accumulation of difficult-to-clean pollutants on the surface and the damage to the membrane fibers during the cleaning process will eventually lead to the membrane being completely contaminated and reaching the end of its service life, requiring timely replacement. Since PVDF membrane fibers are difficult to degrade, the general treatment method is to landfill or incinerate them as solid waste. Landfilling will occupy a large amount of land resources, and the membrane fibers are difficult to degrade, which may cause long-term pollution to the soil and groundwater; incineration will produce harmful gases, polluting the atmospheric environment, and also causing a huge waste of resources.

[0005] To avoid these traditional treatment methods, some researchers have attempted to dissolve waste membrane filaments in solvents and reuse them as raw materials for secondary membrane formation. However, this method is costly, requires large amounts of solvent, and involves complex recycling processes. Therefore, a direct repair process for membrane filaments is needed to effectively increase their reuse rate, extend the membrane's lifespan, and reduce production costs. Summary of the Invention

[0006] The present invention provides a PVDF hollow fiber ultrafiltration membrane repair process, a performance testing method and a performance testing system, which can restore the pure water flux of scrapped membranes, improve the membrane's anti-pollution performance, and reduce the recovery and production costs of enterprise membrane production.

[0007] The technical solution adopted by the present invention to solve its technical problem is:

[0008] A repair process for PVDF hollow fiber ultrafiltration membrane yarn specifically comprises the following steps:

[0009] Step S1, pretreatment, removing the waste membrane fibers from the membrane assembly, preparing a sodium hydroxide solution with a pH value of 11-12, immersing the waste membrane fibers in the sodium hydroxide solution for cleaning, further preparing a citric acid solution with a pH value of 1.5-2, washing the waste membrane fibers after washing with the sodium hydroxide solution in the citric acid solution, and finally washing with clean water;

[0010] Step S2, weighing polyvinylidene fluoride powder and polyethylene glycol powder, dissolving the two in N,N-dimethylacetamide solvent, adding polyvinyl pyrrolidone to the solution, and stirring the solution under a set constant temperature condition to form a membrane repair solution;

[0011] Step S3, soaking the waste membrane fibers cleaned in step S1 in the membrane fiber repair solution prepared in step S2, taking them out after soaking for a preset time, washing them with clean water to remove residual agents on the surface, and drying them in an oven to complete the repair;

[0012] Step S4: assemble the repaired membrane filaments into the membrane shell and perform flux testing to ensure the quality of the membrane filaments;

[0013] Furthermore, the waste membrane filaments removed from the membrane assembly are membranes with a service life of 3-6 years and a flux lower than 80% of a preset value;

[0014] Furthermore, in step S1, the sodium hydroxide solution and the citric acid solution are both prepared at room temperature, wherein the mass concentration of sodium hydroxide in the sodium hydroxide solution is 0.01%-0.05%, the alkaline washing time lasts for 0.5-4 hours, the mass concentration of citric acid in the citric acid solution is 2%-20%, the acid washing time also lasts for 0.5-4 hours, and the time for washing with clean water lasts for 1 hour;

[0015] Furthermore, in step S2, the prepared membrane repair solution comprises, by mass percentage, 10%-15% polyvinylidene fluoride, 6%-10% polyethylene glycol, 70% N,N-dimethylacetamide, and 6%-10% polyvinylpyrrolidone;

[0016] Furthermore, in step S2, the constant temperature is set to 70°C and the stirring time is ≥ 6h;

[0017] Furthermore, in step S3, the temperature of the membrane repair solution is 10°C-40°C, and the number of reuses is 5-10 times; the waste membrane is placed vertically in the membrane repair solution and soaked for 1 hour;

[0018] Furthermore, in step S3, the repaired membrane fibers are rinsed with clean water for 30 minutes and then placed in an oven at 40° C. for drying for 6 hours.

[0019] A performance testing method for a membrane assembly, wherein the pure water flux, rejection rate, pure water flux recovery rate, tensile strength, and hydrophilicity of the membrane assembly are tested for the membrane assembly repaired by the repair process;

[0020] Among them, the test method of pure water flux is to test the pure water flux at room temperature and under a pressure of 0.1Mpa;

[0021] The retention rate test method is to use polyethylene glycol as the solute to prepare a polyethylene glycol solution with a mass concentration of 200 mg / L. The prepared polyethylene glycol solution is filtered through a membrane module at a pressure of 0.1 MPa. After stabilization for 30 minutes, the filtrate and the original solution are collected and the organic carbon content of the filtrate and the original solution is measured using a total organic carbon analyzer.

[0022] The test method for the pure water flux recovery rate is to use pure water to rinse the polyethylene glycol organic matter remaining on the membrane assembly. The washing process lasts for 20 minutes and then the pure water flux is tested.

[0023] The tensile strength test method is to prepare the membrane assembly into a standard sample, and use a universal testing machine to measure the tensile strength of the waste membrane filaments and the repaired membrane filaments. Each sample is tested three times and the average value is taken.

[0024] The hydrophilicity test method is to add water drops on the membrane and use a contact angle meter to measure the water contact angle of the membrane. Each sample is measured at three locations and the average value is taken.

[0025] Furthermore, the calculation formula for pure water flux is:

[0026]

[0027] In the formula, J0 is the pure water flux of the membrane, the unit is L / (m 2 ·h), V is the volume of filtrate passing through in time t, in L, A is the area of the membrane, in m 2 , t is the test time, unit is h;

[0028] The calculation formula for the retention rate is:

[0029]

[0030] In the formula, R is the retention rate of the membrane, the unit is %, C is the mass concentration of the filtrate, the unit is mg / L, C0 is the mass concentration of the original solution, the unit is mg / L;

[0031] The calculation formula for pure water flux recovery rate is:

[0032]

[0033] In the formula, FRR is the pure water flux recovery rate of the membrane, the unit is %, and J is the pure water recovery flux of the membrane, the unit is L / (m 2 ·h), J0 is the pure water flux of the membrane, the unit is L / (m 2 h);

[0034] A performance testing system, used in the performance testing method, comprises a pure water tank, a mechanical pump, and an outer shell. The outer shell is a hollow cylindrical structure. An annular clamp is installed in the inner cavity of the outer shell to fix the fiber membrane repaired by the repair process. End caps are installed at both ends of the outer shell to seal the outer shell.

[0035] The pure water tank is connected to the inlet of the mechanical pump through a pipeline, and the outlet of the mechanical pump is connected to the water inlet of the outer shell through a pipeline; a pressure gauge is installed on the pipeline between the outlet of the mechanical pump and the water inlet of the outer shell, and a pressure sensor is set in the pressure gauge;

[0036] A first rotor flowmeter is installed at the inlet of the outer shell, and a second rotor flowmeter and a third rotor flowmeter are respectively connected to the produced water outlet and the concentrated water outlet of the outer shell through pipelines.

[0037] Through the above technical solution, compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. The repair process of PVDF hollow fiber ultrafiltration membrane provided by the present invention and the design of chemical cleaning pretreatment process are designed because long-term use leads to irreversible contaminants on the membrane surface and inside that cannot be removed by simple cleaning. This design removes a large amount of contaminants on the membrane surface, facilitating subsequent repair of the membrane structure.

[0039] 2. The repair process for PVDF hollow fiber ultrafiltration membrane provided by the present invention uses a hydrophilic repair fluid that can promote the regeneration and repair of the pore structure on the membrane surface, improve the hydrophilicity and anti-fouling properties of the membrane surface; and the repair fluid can be reused multiple times, thereby improving utilization efficiency;

[0040] 3. The repair process for PVDF hollow fiber ultrafiltration membrane filaments provided by the present invention repairs the membrane filaments, promotes the recycling of resources, reduces the production and recycling costs of membranes for enterprises, and avoids environmental problems caused by solid waste disposal;

[0041] 4. The performance testing method provided by the present invention has a clear operation process, clear parameters for each step, and relatively low technical requirements for operators, ensuring the eligibility of the repaired membrane filaments. It is easy to promote and apply in enterprise production practices and has high practical value.

[0042] 5. The performance testing system provided by the present invention can accurately control the test pressure, adapt to the testing of membrane modules of different specifications, and improve the accuracy of test data and the versatility of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will be further described below with reference to the accompanying drawings and examples.

[0044] Figure 1 It is a schematic flow chart of the repair process provided by the present invention;

[0045] Figure 2 Schematic diagram of the structure of the membrane assembly in the embodiment provided by the present invention;

[0046] Figure 3 The waste membrane filaments removed from the membrane assembly of the embodiment provided by the present invention;

[0047] Figure 4 It is a schematic diagram of a performance testing system according to a preferred embodiment of the present invention.

[0048] In the figure: 1 is a pure water tank, 2 is a mechanical pump, 3 is a pressure gauge, 4 is a first rotor flowmeter, 5 is a membrane assembly, 6 is a second rotor flowmeter, and 7 is a third rotor flowmeter. DETAILED DESCRIPTION

[0049] The present invention will now be described in further detail with reference to the accompanying drawings. It should be understood that the terms "first," "second," and the like do not denote the importance of components and, therefore, should not be construed as limiting the present invention. The specific dimensions used in this embodiment are merely illustrative of the technical solution and do not limit the scope of protection of the present invention.

[0050] As explained in the background technology, since PVDF membrane filaments are difficult to degrade, the general treatment method is to landfill or incinerate them as solid waste, which will cause environmental pollution and waste of resources. To avoid using this method, the waste membrane filaments can be re-dissolved in a solvent and used as raw materials for secondary membrane formation. However, this method is expensive and the recycling process is complicated. Therefore, it is necessary to develop a process technology for directly repairing membrane filaments to effectively increase the reuse rate of membrane filaments, extend the service life of the membrane, and reduce the production cost of enterprises.

[0051] Figure 1 As shown, the repair process of PVDF hollow fiber ultrafiltration membrane provided by the present application specifically includes the following steps:

[0052] Step S1, pretreatment: Remove used membrane fibers from the membrane assembly that are 3-6 years old and have a flux less than 80% of the preset value. Prepare sodium hydroxide solution and citric acid solution at room temperature. This process is simple to operate and has low energy consumption. The pH value of the prepared sodium hydroxide solution is 11-12, and the mass concentration of sodium hydroxide in the sodium hydroxide solution is 0.01%-0.05%. The pH value of the prepared citric acid solution is 1.5-2, and the mass concentration of citric acid in the citric acid solution is 2%-20%. The sodium hydroxide solution and citric acid solution are prepared in a 500L plastic dispensing barrel with a drain valve to facilitate solution preparation, storage, and discharge. The plastic material also prevents chemical reactions between the solution and the metal barrel.

[0053] The disassembled waste membrane fibers are cleaned in sequence with sodium hydroxide solution and citric acid solution, and finally with clean water. Alkaline washing removes oil and other impurities on the surface of the membrane fibers, and acid washing further removes metal oxides and other dirt on the surface of the membrane fibers. In order to ensure that the impurities and dirt on the surface of the waste membrane fibers are completely removed, the alkaline washing and acid washing time are both lasted for 0.5-4 hours, and the water washing is used for 1 hour.

[0054] In step S2, a certain amount of polyvinylidene fluoride (PVDF) powder and polyethylene glycol (PEG) powder are weighed and dissolved in N,N-dimethylacetamide (DMAc) solvent, and a certain amount of polyvinyl pyrrolidone (PVP) is added to the solution to form a membrane repair solution.

[0055] Regarding the role of each component in the membrane filament repair solution, the solvent DMAc dissolves the organic pollutants on the membrane filament surface that have not been removed in step S1 during the repair process, further cleaning the membrane filament; PVDF repairs the membrane pore structure on the surface and restores the filtration performance of the membrane; PVP, as a porogen, is conducive to the formation of new membrane pores on the surface and maintains the separation performance of the membrane; PEG, as a hydrophilic agent, optimizes the hydrophilicity and hydrophobicity of the membrane surface and improves the membrane flux.

[0056] To achieve the best possible repair effect, this application provides a precise formulation for a membrane-filament repair solution. The components, measured by mass percentage, include 10%-15% polyvinylidene fluoride, 6%-10% polyethylene glycol, 70% N,N-dimethylacetamide, and 6%-10% polyvinylpyrrolidone. Stirring at a constant temperature of 70°C for at least six hours ensures complete dissolution of the powders and agents, forming a stable repair solution system and ensuring the stability and effectiveness of the repair solution.

[0057] Step S3, vertically place the waste membrane filaments cleaned in step S1 into a membrane filament repair liquid with a temperature of 10℃-40℃, soak for 1 hour and then take it out, so that the repair liquid can evenly penetrate into all parts of the membrane filament, especially the defective area, to ensure the consistency of the repair effect. The temperature of the repair liquid is controlled at 10℃-40℃. Within this temperature range, the fluidity of the repair liquid and the activity of each component are more suitable, which is conducive to the repair process. Use clean water to wash for 30 minutes to remove residual agents on the surface, place it in a 40℃ oven and continue to dry for 6 hours. This can not only ensure that the membrane filament structure is repaired and solidified, but also will not damage the membrane filament due to excessive temperature, and finally complete the repair.

[0058] The membrane repair fluid here can be reused, and can generally be reused 5-10 times, which reduces the repair cost and improves resource utilization.

[0059] In step S4, the repaired membrane filaments are assembled into the membrane housing and subjected to flux testing to ensure the membrane filaments are qualified. This is to reassemble the wet membrane assembly to avoid membrane filament damage or poor sealing problems that may occur during dry membrane assembly.

[0060] Obviously, in order to ensure the accuracy and comparability of the test results, the performance test system for the repaired PVDF hollow fiber membrane is also very important. Preferably, Figure 4As shown, the performance testing system provided includes a pure water tank 1, an outer shell, and a mechanical pump 2. The outer shell is made of high-strength, corrosion-resistant materials such as stainless steel or high-strength engineering plastics. In actual operation, the outer shell wall thickness is designed based on the required test pressure to ensure sufficient pressure resistance. The inner wall of the outer shell is smooth to reduce water flow resistance and ensure uniform fluid distribution during the test. An annular clamp is installed in the inner cavity of the outer shell to secure the reassembled wetted membrane assembly. The pure water tank is used to store the pure water required for testing. The pure water tank is connected to the inlet of the mechanical pump via a pipeline, and the outlet of the mechanical pump is connected to the water inlet of the outer shell via a pipeline. A pressure gauge 3 is installed in the pipeline between the mechanical pump outlet and the water inlet of the outer shell. The pressure gauge is equipped with a pressure sensor. The pressure sensor ensures a pressure of 0.1 MPa in the performance testing device. A first rotameter 4 is installed at the inlet of the outer shell. The product water outlet and the concentrate water outlet of the outer shell are connected by pipelines to a second rotameter 6 and a third rotameter 7, respectively. The product water is discharged after passing through the second rotameter, and the concentrate water is discharged after passing through the third rotameter.

[0061] Start the mechanical pump to transport the water in the pure water tank to the wet membrane assembly. By adjusting the speed of the mechanical pump or the opening of the valve, combined with the readings of the pressure gauge and the first rotor flowmeter, adjust the water pressure and flow entering the wet membrane assembly to the set value. After a period of stable operation, record the readings of the rotor flowmeter at the water outlet (filtrate) and the concentrate outlet of the wet membrane assembly to prepare for subsequent test methods.

[0062] After the performance test system is set up, the test method of the repaired membrane of this application is continued. The test content includes testing the pure water flux, retention rate, pure water flux recovery rate, tensile strength and hydrophilicity of the membrane component 5.

[0063] The test method for pure water flux is to test the pure water flux at room temperature and under a pressure of 0.1Mpa;

[0064] The calculation formula for pure water flux is:

[0065]

[0066] In the formula, J0 is the pure water flux of the membrane, the unit is L / (m 2 ·h), V is the volume of filtrate passing through in time t, in L, A is the area of the membrane, in m 2 , t is the test time, the unit is h.

[0067] The retention rate test method is to use polyethylene glycol as the solute to prepare a polyethylene glycol solution with a mass concentration of 200 mg / L. The prepared polyethylene glycol solution is filtered under a pressure of 0.1 MPa. After stabilization for 30 minutes, the filtrate and the original solution are collected and the TOC of the filtrate and the original solution is measured using a total organic carbon analyzer.

[0068] The calculation formula for the retention rate is:

[0069]

[0070] In the formula, R is the retention rate of the membrane, the unit is %, C is the mass concentration of the filtrate, the unit is mg / L, and C0 is the mass concentration of the original solution, the unit is mg / L.

[0071] The tensile strength test method is to prepare the membrane assembly into a standard sample, and use a universal testing machine to measure the tensile strength of the waste membrane filaments and the repaired membrane filaments. Each sample is tested three times and the average value is taken.

[0072] The calculation formula for pure water flux recovery rate is:

[0073]

[0074] In the formula, FRR is the pure water flux recovery rate of the membrane, the unit is %, and J is the pure water recovery flux of the membrane, the unit is L / (m 2 ·h), J0 is the pure water flux of the membrane, the unit is L / (m 2 ·h).

[0075] The test method for tensile strength is to prepare the membrane into a standard specimen, and use a universal testing machine to measure the tensile strength of the waste membrane filaments and the repaired membrane filaments. Each sample is tested three times and the average value is taken.

[0076] The test method for hydrophilicity and hydrophobicity is to add water drops on the membrane fibers and use a contact angle meter to test the water contact angle of the membrane fibers. Three positions of each sample are measured and the average value is taken.

[0077] Next, this application provides relevant embodiments to verify the feasibility and effectiveness of the above solution.

[0078] The equipment used in the entire process includes: raw water tank, medicine dispensing box, mechanical pump, several valves, electric mixer, blast drying oven, homemade flux testing device, automatic contact angle tester, universal testing machine, total organic carbon (TOC) meter, etc.

[0079] The external pressure type LWU-P1150 model ( Figure 2 The membrane fibers of PVDF ultrafiltration membrane (shown) Figure 3In order to repair and recycle the dismantled membrane filaments, the pretreatment process is as follows: first, the scrapped membrane filaments are removed from the membrane assembly, and the removed membrane filaments are cleaned with a 0.05% sodium hydroxide solution for 30 minutes, and then with a 2% citric acid solution, and finally with clean water for use.

[0080] Repair solution preparation process: PVDF powder, PEG, DMAc, and PVP are prepared in a mass ratio of 10:10:70:10. The volume of the prepared solution is 500 L and stirred at a constant temperature of 70°C for 6 hours until all the above reagents are dissolved to form a membrane filament repair solution.

[0081] Repair process: Soak the pretreated membrane fibers in the above-mentioned membrane fiber repair solution at 20°C to repair the membrane pore structure for 1 hour. Then take out the membrane fibers and wash them with clean water to remove the residual agent on the surface. Then place them in a 40°C oven to dry to ensure that the membrane fiber structure is repaired and solidified.

[0082] Membrane filament post-processing process: The repaired membrane filaments are reassembled into LWU-P1150 membrane modules to test their pure water flux, rejection rate test, pure water recovery, tensile strength and hydrophilicity.

[0083] After testing, the pure water flux of the waste membrane before repair was 24L / (m 2 ·h), the pure water flux of the repaired membrane is 30L / (m 2 h), an increase of 25%. The repaired membrane achieved a PEG rejection rate of 75.6%, a 50% increase compared to the old membrane. The repaired membrane flux recovery rate was 63.1%, and the tensile strength increased by 28% compared to the unrepaired old membrane. The contact angle of the repaired membrane was 35.8°, compared to 54.8° for the old membrane. The smaller contact angle indicates that the repaired membrane has become more hydrophilic.

[0084] In summary, the repair process for PVDF hollow fiber ultrafiltration membrane filaments provided in this application provides a new way for the disposal of PVDF hollow fiber ultrafiltration membrane filaments, prolongs the service life of the membrane, can directly repair the membrane filaments, promotes the recycling of resources, and avoids environmental problems caused by solid waste treatment. The membrane filament repair process technology is simple to operate and can be carried out at the project implementation site without the need to return to the factory for repair, reducing transportation costs. The repair liquid in the process can be reused many times and has high utilization efficiency. In addition, in addition to being applicable to PVDF hollow fiber membrane components, the process is also applicable to flat membranes or rolled membrane structures, and has strong applicability.

[0085] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art and, unless defined as such herein, will not be interpreted in an idealized or overly formal sense.

[0086] The meaning of "and / or" in this application means that both situations where each exists alone or both exist at the same time are included.

[0087] The term “connection” as used in this application may mean a direct connection between components or an indirect connection between components via other components.

[0088] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A repair process for PVDF hollow fiber ultrafiltration membrane yarn, characterized in that: The specific steps include: Step S1, pretreatment, removing the waste membrane fibers from the membrane assembly, preparing a sodium hydroxide solution with a pH value of 11-12, immersing the waste membrane fibers in the sodium hydroxide solution for cleaning, further preparing a citric acid solution with a pH value of 1.5-2, washing the waste membrane fibers after washing with the sodium hydroxide solution in the citric acid solution, and finally washing with clean water; Step S2, weighing polyvinylidene fluoride powder and polyethylene glycol powder, dissolving the two in N,N-dimethylacetamide solvent, adding polyvinyl pyrrolidone to the solution, and stirring the solution under a set constant temperature condition to form a membrane repair solution; Step S3, soaking the waste membrane fibers cleaned in step S1 in the membrane fiber repair solution prepared in step S2, taking them out after soaking for a preset time, washing them with clean water to remove residual agents on the surface, and drying them in an oven to complete the repair; In step S4, the repaired membrane filaments are assembled into the membrane shell and flux testing is performed to ensure the eligibility of the membrane filaments.

2. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: The waste membrane filaments removed from the membrane assembly are membranes with a service life of 3 to 6 years, and their flux is lower than 80% of the preset value.

3. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: In step S1, the sodium hydroxide solution and the citric acid solution are both prepared at room temperature, wherein the mass concentration of sodium hydroxide in the sodium hydroxide solution is 0.01%-0.05%, the alkaline washing time lasts for 0.5-4 hours, the mass concentration of citric acid in the citric acid solution is 2%-20%, the acid washing time also lasts for 0.5-4 hours, and the time for washing with clean water lasts for 1 hour.

4. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: In step S2, the prepared membrane repair solution has a mass percentage ratio of 10%-15% polyvinylidene fluoride, 6%-10% polyethylene glycol, 70% N,N-dimethylacetamide, and 6%-10% polyvinylpyrrolidone.

5. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: In step S2, the constant temperature is set to 70° C. and the stirring time is ≥ 6 h.

6. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: In step S3, the temperature of the membrane repair liquid is 10°C-40°C, and the number of reuses is 5-10 times; the waste membrane fibers are placed vertically in the membrane repair liquid and the soaking time is 1 hour.

7. The repair process for PVDF hollow fiber ultrafiltration membrane according to claim 1, characterized in that: In step S3, the repaired membrane fibers are rinsed with clean water for 30 minutes and then placed in an oven at 40° C. to dry for 6 hours.

8. A performance testing method for a membrane module, characterized in that: For the membrane assembly repaired by any one of the repair processes described in claims 1 to 7, the pure water flux, rejection rate, pure water flux recovery rate, tensile strength and hydrophilicity of the membrane assembly are tested; Among them, the test method of pure water flux is to test the pure water flux at room temperature and under a pressure of 0.1Mpa; The retention rate test method is to use polyethylene glycol as the solute to prepare a polyethylene glycol solution with a mass concentration of 200 mg / L. The prepared polyethylene glycol solution is filtered through a membrane module at a pressure of 0.1 MPa. After stabilization for 30 minutes, the filtrate and the original solution are collected and the organic carbon content of the filtrate and the original solution is measured using a total organic carbon analyzer. The test method for the pure water flux recovery rate is to use pure water to rinse the polyethylene glycol organic matter remaining on the membrane assembly. The washing process lasts for 20 minutes and then the pure water flux is tested. The tensile strength test method is to prepare the membrane assembly into a standard sample, and use a universal testing machine to measure the tensile strength of the waste membrane filaments and the repaired membrane filaments. Each sample is tested three times and the average value is taken. The test method for hydrophilicity and hydrophobicity is to add water drops on the membrane fibers and use a contact angle meter to measure the water contact angle of the membrane fibers. Three positions of each sample are measured and the average value is taken.

9. The performance testing method of a membrane module according to claim 8, characterized in that: The calculation formula for pure water flux is: In the formula, J0 is the pure water flux of the membrane, the unit is L / (m 2 ·h), V is the volume of filtrate passing through in time t, in L, A is the area of the membrane, in m 2 , t is the test time, unit is h; The calculation formula for the retention rate is: In the formula, R is the retention rate of the membrane, the unit is %, C is the mass concentration of the filtrate, the unit is mg / L, C0 is the mass concentration of the original solution, the unit is mg / L; The calculation formula for pure water flux recovery rate is: In the formula, FRR is the pure water flux recovery rate of the membrane, the unit is %, and J is the pure water recovery flux of the membrane, the unit is L / (m 2 ·h), J0 is the pure water flux of the membrane, the unit is L / (m 2 ·h).

10. A performance testing system, characterized in that: For use in the performance testing method described in claim 8, comprising a pure water tank, a mechanical pump, and a membrane housing, wherein the membrane housing is an outer shell having a hollow cylindrical structure, an annular clamp is installed in the inner cavity of the outer shell for fixing the membrane assembly repaired by the repair process described in any one of claims 1 to 7; end caps are installed at both ends of the outer shell to seal the outer shell; The pure water tank is connected to the inlet of the mechanical pump through a pipeline, and the outlet of the mechanical pump is connected to the water inlet of the outer shell through a pipeline; a pressure gauge is installed on the pipeline between the outlet of the mechanical pump and the water inlet of the outer shell, and a pressure sensor is set in the pressure gauge; A first rotor flowmeter is installed at the inlet of the outer shell, and a second rotor flowmeter and a third rotor flowmeter are respectively connected to the produced water outlet and the concentrated water outlet of the outer shell through pipelines.

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