A hydrophilic ultrafiltration membrane for oil-water separation and a preparation method thereof

Abstract

The present invention provides a hydrophilic ultrafiltration membrane for oil-water separation and a preparation method thereof, wherein a blended membrane is prepared by blending polymethyl methacrylate with a membrane-forming polymer, and then ethylene glycol is grafted onto the membrane surface by an ester exchange reaction to prepare a hydrophilic ultrafiltration membrane, which exhibits good anti-pollution ability in separation fields such as oily wastewater treatment and material concentration. The preparation method of the hydrophilic ultrafiltration membrane for oil-water separation of the present invention has a simple preparation process, mild conditions, is easy to scale up industrially, and has good industrial application value.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer separation membranes, and in particular relates to a hydrophilic ultrafiltration membrane for oil-water separation and a preparation method thereof. Background Art

[0002] Oil-water mixtures come from a wide range of sources and have serious impacts on the ecological environment and production processes. They need to be efficiently separated from environmental protection, resource recycling, and safe production. Traditional oil-water separation technologies include flotation, flocculation, adsorption, electrical dehydration, and chemical demulsification. These technologies usually consume a lot of energy, add chemical agents, and produce a lot of sludge. Ultrafiltration membrane separation technology is a physical method that does not require external agents and has a simple treatment process. It can remove difficult-to-treat emulsified oil and dissolved oil, and is an ideal oil-water separation technology.

[0003] At present, organic ultrafiltration membranes are mostly made of hydrophobic materials such as polysulfone, polyethersulfone and polyvinylidene fluoride, which makes the ultrafiltration membrane hydrophobic. In practical applications, the filtration resistance increases continuously due to membrane contamination, the membrane filtration flux is seriously attenuated, and the cleaning is frequent and costly. In the oil-water separation process, membrane contamination is particularly obvious. Studies have shown that hydrophilic modification of separation membranes is the key to improving the anti-pollution ability of separation membranes. The main hydrophilic modification methods currently include blending modification technology and surface modification technology.

[0004] Blending modification is the process of adding one or more substances to the original membrane material and mixing them to form a macroscopic uniform material. In addition to retaining the original material's inherent properties, it can also overcome the defects of the original material. These modified materials structurally include hydrophilic segments and hydrophobic segments. During the gelation process, the hydrophilic segments will automatically accumulate on the membrane surface due to their high hydrophilicity, while the hydrophobic segments will entangle with the membrane material and be fixed inside the membrane.

[0005] Surface modification technology refers to improving the hydrophilicity and flux of ultrafiltration membranes through surface reactions, and improving the membrane's anti-pollution performance. The main methods for membrane surface modification include: adsorption, surface coating, surface chemical reaction, low-temperature plasma modification, irradiation modification, photografting modification, etc. The adsorption method and surface coating method are relatively simple to operate. The modifier is physically coated on the surface of the ultrafiltration membrane so that the modifier is adsorbed on the membrane surface. However, as the operation time increases, the modifier will gradually fall off the surface of the ultrafiltration membrane, and a permanent modification effect cannot be obtained. Surface chemical reaction can fix the modified monomer and graft the modified substance to the surface of ultrafiltration membrane. The modified substance is connected to the polymer through covalent bond and has good stability. However, the chemical modification process is complicated and requires pretreatment, which limits the scope of modification. Low-temperature plasma surface modification is a membrane modification method that has developed rapidly in recent years. It is relatively simple to operate and is not easy to pollute the air. However, the modification equipment is relatively complex and the modification mechanism is unclear, making it difficult to carry out targeted modification. Ultraviolet radiation graft polymerization is a commonly used ultrafiltration membrane surface modification method. Ultraviolet light is not easily absorbed by polymer membrane, but can be absorbed by photoinitiator to initiate reaction, which can achieve the purpose of surface modification without affecting the material itself. However, photoinitiator and catalyst need to be added, and the grafting rate is difficult to control. Interfacial cross-linking polymerization is a relatively new ultrafiltration membrane surface modification technology. It first adsorbs the polymer used for surface hydrophilic modification on the surface of ultrafiltration membrane, and then cross-links the hydrophilic modified polymer through cross-linking agent to form a network structure to improve the stability of the modified polymer on the membrane surface. This method is simple to operate, has low requirements on equipment, and the hydrophilic modified polymer has good stability on the membrane surface and has a wide range of applications.

[0006] Since the separation layer of the ultrafiltration membrane is thin, it is easily damaged by harsh environments, resulting in a decrease in the performance of the separation membrane. Therefore, the ideal separation membrane modification technology should have the characteristics of mild reaction conditions, simple equipment, and easy process scale-up. Compared with other modification technologies, surface chemical grafting has better stability, but chemical grafting on the surface of the separation membrane requires specific chemical sites. Traditional separation membrane materials have strong chemical stability and are not easy to undergo chemical grafting reactions. They need to be activated to produce active sites. However, the activation process of the separation membrane often causes the film-forming polymer chain to break, affecting the membrane pore structure and mechanical properties of the separation membrane. Therefore, if active groups can be introduced into the separation membrane through blending technology to provide active sites for chemical grafting, and can be carried out under mild reaction conditions for easy industrial scale-up, it will be an ideal separation membrane surface modification technology. Summary of the invention

[0007] In view of this, in order to solve the above problems, the present invention proposes a method for preparing a hydrophilic ultrafiltration membrane for oil-water separation, which has mild reaction conditions, simple process, is green and environmentally friendly, and is easy to scale up industrially. By blending polymethyl methacrylate with a film-forming polymer and then grafting ethylene glycol to the membrane surface through an ester exchange reaction, a hydrophilic ultrafiltration membrane with good hydraulic stability, strong anti-pollution ability, high water flux, good hydrophilicity, and good separation effect in the process of oily wastewater treatment is prepared.

[0008] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0009] A method for preparing a hydrophilic ultrafiltration membrane for oil-water separation comprises the following steps:

[0010] S1, a film-forming polymer is mixed with polymethyl methacrylate to prepare a blend film;

[0011] S2. Ethylene glycol is grafted onto the surface of the blended membrane through an ester exchange reaction to obtain a target product, a hydrophilic ultrafiltration membrane.

[0012] Furthermore, the S1 specifically comprises: dissolving the film-forming polymer, polymethyl methacrylate and additives in a solvent, mixing them evenly to obtain a film-forming solution, and preparing a blended film by an immersion sedimentation method after vacuum degassing;

[0013] The S2 specifically comprises: immersing the blended membrane in an ethylene glycol solution, grafting ethylene glycol onto the surface of the blended membrane through an ester exchange reaction, and obtaining a hydrophilic ultrafiltration membrane.

[0014] Further, the film-forming polymer is polysulfone, polyethersulfone, polyvinylidene fluoride or polyvinyl chloride;

[0015] Furthermore, the solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone or dimethyl sulfoxide.

[0016] Furthermore, the additive is a water-soluble additive, a water-insoluble additive, an inorganic salt compound or a combination thereof; the water-soluble additive is one or more of ethylene glycol, propylene glycol, glycerol, triethylene glycol, polyethylene glycol (200, 400 and 600), polyvinyl pyrrolidone, polyvinyl butyral, and polyvinyl acetate; the water-insoluble additive is one or more of propylene carbonate, γ-butyrolactone, diacetone alcohol, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, propylene glycol monoethyl ether acetate, and dipropylene glycol monoethyl ether acetate; the inorganic salt compound is one or more of lithium chloride, lithium nitrate, and calcium nitrate.

[0017] Furthermore, the sum of the mass of the film-forming polymer and the polymethyl methacrylate accounts for 5% to 30% of the total mass of the film-forming solution, the additive accounts for 0.5% to 20% of the total mass of the film-forming solution, and the remainder is the solvent; wherein the mass ratio of the film-forming polymer to the polymethyl methacrylate is 0.05:1 to 20:1.

[0018] Furthermore, in the process of preparing the membrane-making solution, the reaction temperature is 5 to 120° C. and the reaction time is 1 to 50 h. In the process of preparing the blended membrane by immersion sedimentation method, the membrane is replaced with deionized water for 24 h after formation.

[0019] Furthermore, the ethylene glycol solution contains water and a catalyst; the catalyst accounts for 0.0001% to 5% of the total mass of the ethylene glycol solution, and the water accounts for 0.0001% to 5% of the total mass of the ethylene glycol solution.

[0020] Furthermore, the catalyst is an alkaline inorganic salt, specifically one or more of sodium hydroxide, potassium hydroxide or calcium hydroxide.

[0021] Furthermore, the time of the transesterification reaction is 0.5 to 50 hours, and the temperature of the transesterification reaction is 5 to 80°C.

[0022] A hydrophilic ultrafiltration membrane for oil-water separation, wherein the hydrophilic ultrafiltration membrane is an asymmetric structure of a finger-shaped pore support.

[0023] Furthermore, the hydrophilic ultrafiltration membrane is a flat membrane or a hollow fiber membrane.

[0024] Compared with the prior art, the hydrophilic ultrafiltration membrane for oil-water separation and the preparation method thereof described in the present invention have the following advantages:

[0025] (1) The method for preparing the hydrophilic ultrafiltration membrane for oil-water separation of the present invention has mild reaction conditions and a simple process. Active groups are introduced into the separation membrane by blending technology, providing active sites for chemical grafting. The reaction conditions of the transesterification reaction are mild, which is convenient for industrial scale-up. It is an ideal separation membrane surface modification technology with good industrial application value.

[0026] (2) The hydrophilic ultrafiltration membrane for oil-water separation described in the present invention has good hydraulic stability, strong anti-pollution ability, high water flux, and good hydrophilicity. The blended membrane is prepared by mixing a film-forming polymer with polymethyl methacrylate, and ethylene glycol is grafted onto the surface of the blended membrane through an ester exchange reaction, thereby improving the hydrophilicity and anti-pollution ability of the membrane. It has good application value in the fields of oily wastewater treatment, material concentration, and municipal wastewater treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1This is a cross-sectional photograph of the hollow fiber hydrophilic ultrafiltration membrane described in the embodiment of the present invention;

[0028] Figure 2 This is a cross-sectional photo of the flat hydrophilic ultrafiltration membrane described in the embodiment of the present invention. DETAILED DESCRIPTION

[0029] Unless otherwise defined, the technical terms used in the following examples have the same meanings as those generally understood by those skilled in the art to which the present invention belongs. The test reagents used in the following examples, unless otherwise specified, are all conventional biochemical reagents; the experimental methods, unless otherwise specified, are all conventional methods.

[0030] The present invention will be described in detail below with reference to the embodiments.

[0031] Example 1

[0032] preparation:

[0033] 32g polysulfone, 20g polymethyl methacrylate, 8g polyethylene glycol (molecular weight 400) were added to 240g dimethylacetamide and stirred at 70°C for 24 hours to completely dissolve to obtain a membrane solution. After vacuum degassing, a hollow fiber membrane was prepared by immersion sedimentation method. The specific spinning conditions were as follows:

[0034] The spinning temperature was 80°C, the dry spinning distance was 70 mm, the core liquid was deionized water, and the core liquid flow rate was 0.8 ml min -1 , the gel bath was water, the water bath temperature was 50°C, and the blended membrane was obtained after rinsing with deionized water.

[0035] 5 g of deionized water and 5 g of sodium hydroxide were added to 200 g of ethylene glycol to obtain an ethylene glycol solution, and the blended membrane was immersed in the ethylene glycol solution and reacted at 60° C. for 20 h to obtain the target product, a hollow fiber hydrophilic ultrafiltration membrane.

[0036] Performance Test:

[0037] The pure water flux of the hollow fiber hydrophilic ultrafiltration membrane prepared in Example 1 is 150 L·m -2 ·h -1 , the bovine serum albumin rejection rate was 75.5% and the water contact angle was 34°.

[0038] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 120ppm, the suspended particle content was 60ppm, and the median particle size was 3 microns.

[0039] After filtration using the hollow fiber hydrophilic ultrafiltration membrane prepared in Example 1, the oil content in the permeate was less than 10 ppm, the content of suspended particles was 1 ppm, and the median particle size was less than 1 micron. The water flux of the membrane in oily wastewater was 30 L·m -2 ·h -1 , the flow recovery rate of the membrane was 85% after a simple flush.

[0040] Example 2

[0041] preparation:

[0042] 36g of polysulfone, 18g of polymethyl methacrylate, and 8g of ethylene glycol were added to 238g of dimethylacetamide and stirred at 80°C for 24 hours to completely dissolve to obtain a membrane-making solution. After vacuum degassing, a hollow fiber membrane was prepared by immersion sedimentation method. The specific spinning conditions were as follows:

[0043] The spinning temperature was 50°C, the dry spinning distance was 100 mm, the core liquid was deionized water, and the core liquid flow rate was 1.0 ml min -1 , the gel bath was water, the water bath temperature was 50°C, and the blended membrane was obtained after rinsing with deionized water.

[0044] 1 g of deionized water and 1 g of sodium hydroxide were added to 200 g of ethylene glycol to obtain an ethylene glycol solution. The blended membrane was immersed in the ethylene glycol solution and reacted at 60° C. for 24 hours to obtain a hollow fiber hydrophilic ultrafiltration membrane.

[0045] Performance Test:

[0046] The pure water flux of the hollow fiber hydrophilic ultrafiltration membrane prepared in Example 2 is 200 L·m -2 ·h -1 , the bovine serum albumin rejection rate was 55.5% and the water contact angle was 40°.

[0047] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 120ppm, the suspended particle content was 60ppm, and the median particle size was 3 microns.

[0048] After filtration using the hollow fiber hydrophilic ultrafiltration membrane prepared in Example 2, the oil content in the permeate was less than 16 ppm, the content of suspended particles was 2 ppm, and the median particle size was less than 1 micron. The water flux of the membrane in oily wastewater was 30 L·m -2 ·h -1 , the flow recovery rate of the membrane was 75% after a simple flush.

[0049] Example 3

[0050] 32g polysulfone, 18g polymethyl methacrylate, and 3g lithium nitrate were added to 247g dimethylformamide and stirred at 80°C for 24 hours to completely dissolve to obtain a film-making solution. After vacuum degassing, a flat film was scraped by immersion sedimentation method. The specific film scraping conditions are as follows:

[0051] The film-making liquid temperature was 50°C, the scraper height was 200 μm, the film-scraping speed was 0.1 m / s, the gel bath was water, the water bath temperature was 15°C, and the blended film was obtained after rinsing with deionized water.

[0052] 1 g of deionized water and 2 g of sodium hydroxide were added to 200 g of ethylene glycol to obtain an ethylene glycol solution. The blended membrane was immersed in the ethylene glycol solution and reacted at 60° C. for 24 h to obtain a flat hydrophilic ultrafiltration membrane.

[0053] Performance Test:

[0054] The pure water flux of the flat hydrophilic ultrafiltration membrane prepared in Example 3 is 280 L·m -2 ·h -1 , the bovine serum albumin retention rate was 35.5%, and the water contact angle was 48°.

[0055] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 100ppm, the suspended particle content was 50ppm, and the median particle size was 3 microns.

[0056] After filtration using the flat hydrophilic ultrafiltration membrane prepared in Example 3, the oil content in the permeate was less than 14 ppm, the content of suspended particles was 2 ppm, and the median particle size was less than 1 micron. The water flux of the membrane in oily wastewater was 35 L·m -2 ·h -1 , the flow recovery rate of the membrane after simple flushing was 73%.

[0057] Example 4

[0058] preparation:

[0059] 51g polyvinylidene fluoride, 18g polymethyl methacrylate, and 9g propylene glycol were added to 222g dimethylformamide and stirred at 70°C for 24 hours to completely dissolve to obtain a film-making solution. After vacuum degassing, a flat film was scraped by immersion sedimentation method. The specific film scraping conditions are as follows:

[0060] The film-making liquid temperature was 60°C, the scraper height was 150 μm, the film-scraping speed was 0.15 m / s, the gel bath was water, the water bath temperature was 15°C, and the blended film was obtained after rinsing with deionized water.

[0061] 1 g of deionized water and 2 g of sodium hydroxide were added to 200 g of ethylene glycol to obtain an ethylene glycol solution. The blended membrane was immersed in the ethylene glycol solution and reacted at 60° C. for 24 h to obtain a flat hydrophilic ultrafiltration membrane.

[0062] Performance Test:

[0063] The pure water flux of the flat hydrophilic ultrafiltration membrane prepared in Example 4 is 300 L·m -2 ·h -1 , the bovine serum albumin rejection rate was 45.5%, and the water contact angle was 45°.

[0064] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 90ppm, the suspended particle content was 50ppm, and the median particle size was 2 microns.

[0065] After filtration using the flat hydrophilic ultrafiltration membrane prepared in Example 4, the oil content in the permeate was less than 15 ppm, the content of suspended particles was 1.5 ppm, and the median particle size was less than 1 micron. The water flux of the membrane in oily wastewater was 32 L·m -2 ·h -1 , the flow recovery rate of the membrane was 70% after a simple flush.

[0066] Comparative Example 1

[0067] preparation:

[0068] 32g polysulfone and 20g polymethyl methacrylate were added to 248g dimethylacetamide and stirred at 70°C for 24 hours to completely dissolve to obtain a membrane-making solution. After vacuum degassing, a hollow fiber membrane was prepared by immersion sedimentation method. The specific spinning conditions were as follows:

[0069] The spinning temperature was 80°C, the dry spinning distance was 70 mm, the core liquid was deionized water, and the core liquid flow rate was 0.8 ml·min -1 , the gel bath is water, the water bath temperature is 50°C, and the cellulose hollow fiber membrane is obtained after rinsing with deionized water.

[0070] Performance Test:

[0071] The spun cellulose hollow fiber membrane was washed in flowing deionized water for 24 hours and then placed in a 50% glycerol aqueous solution for standby use. Scanning electron microscopy showed that the membrane had an asymmetric structure and the pure water flux of the membrane was 120 L·m -2 ·h -1 , the bovine serum albumin rejection rate was 42.5%, and the water contact angle was 78°.

[0072] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 120ppm, the suspended particle content was 60ppm, and the median particle size was 3 microns.

[0073] After filtration by cellulose hollow fiber membrane, the oil content in the permeate is less than 10ppm, the content of suspended particles is 1ppm, and the median particle size is less than 1 micron. The water flux of the membrane in oily wastewater is 10L·m -2 ·h -1 , the flow recovery rate of the membrane was 40% after a simple flush.

[0074] Comparative Example 2

[0075] preparation:

[0076] 36g polyvinylidene fluoride, 12-polymethyl methacrylate, 152g dimethylacetamide, stirred at 80°C for 24 hours to completely dissolve, to obtain a membrane-forming solution, after vacuum degassing, a flat membrane was scraped by immersion sedimentation method, and the specific membrane forming conditions are as follows:

[0077] The material liquid temperature is 60°C, the scraper height is 200 μm, the scraping speed is 0.15 m / s, the gel bath is water, the water bath temperature is 15°C, and the blended membrane is obtained after rinsing with deionized water.

[0078] Performance Test:

[0079] The spun flat membrane was washed in flowing deionized water for 24 hours and then placed in a 50% glycerol aqueous solution for standby use. Scanning electron microscopy showed that the membrane had an asymmetric structure and the pure water flux of the membrane was 120 L·m -2 ·h -1 , the bovine serum albumin retention rate is 65.5% and the water contact angle is 80°.

[0080] Oilfield produced water was used for oil-water separation test. Before the test, the oil content in the produced water was 120ppm, the suspended particle content was 60ppm, and the median particle size was 3 microns.

[0081] After the flat membrane filtration, the oil content in the permeate is less than 20ppm, the content of suspended particles is 1ppm, and the median particle size is less than 1 micron. The water flux of the membrane in oily wastewater is 10L·m -2 ·h -1 , after simple flushing, the flow recovery rate of the separation membrane was 30%.

[0082] From the performance test of the embodiment and the comparative example, it can be seen that the hollow fiber type and flat plate type hydrophilic ultrafiltration membranes described in the embodiment have strong anti-pollution ability and high water flux. After treating oily wastewater, after simple washing, the flow recovery rate of the membrane is much higher than that of the separation membrane prepared in the comparative example. The cross-sectional structure of the hollow fiber type and flat plate type hydrophilic ultrafiltration membrane prepared in the embodiment is shown in FIG. Figure 1 and Figure 2 .

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a hydrophilic ultrafiltration membrane for oil-water separation, characterized in that: The following steps are involved: S1, dissolving a film-forming polymer, polymethyl methacrylate and an additive in a solvent, mixing them evenly to obtain a film-forming solution, and preparing a blended film by an immersion sedimentation method after vacuum degassing; S2. Immersing the blended membrane in an ethylene glycol solution, grafting ethylene glycol onto the surface of the blended membrane through an ester exchange reaction, and obtaining a hydrophilic ultrafiltration membrane.

2. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: The film-forming polymer is polysulfone, polyethersulfone, polyvinylidene fluoride or polyvinyl chloride; the solvent is dimethylacetamide, dimethylformamide, N-methylpyrrolidone or dimethyl sulfoxide.

3. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: The additive is a water-soluble additive, a water-insoluble additive, an inorganic salt compound or a combination thereof; the water-soluble additive is one or more of ethylene glycol, propylene glycol, glycerol, triethylene glycol, polyethylene glycol, polyvinyl pyrrolidone, polyvinyl butyral, and polyvinyl acetate; the water-insoluble additive is one or more of propylene carbonate, γ-butyrolactone, diacetone alcohol, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, propylene glycol monoethyl ether acetate, and dipropylene glycol monoethyl ether acetate; the inorganic salt compound is one or more of lithium chloride, lithium nitrate, and calcium nitrate.

4. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: The sum of the masses of the film-forming polymer and the polymethyl methacrylate accounts for 5% to 30% of the total mass of the film-forming solution, the additive accounts for 0.5% to 20% of the total mass of the film-forming solution, and the remainder is the solvent; wherein the mass ratio of the film-forming polymer to the polymethyl methacrylate is 0.05:1 to 20:

1.

5. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: In the process of preparing the membrane-making solution, the reaction temperature is 5 to 120° C. and the reaction time is 1 to 50 hours. In the process of preparing the blended membrane by the immersion sedimentation method, the membrane is replaced with deionized water for 24 hours after formation.

6. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: The ethylene glycol solution contains water and a catalyst; the catalyst accounts for 0.0001% to 5% of the total mass of the ethylene glycol solution, and the water accounts for 0.0001% to 5% of the total mass of the ethylene glycol solution.

7. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 6, characterized in that: The catalyst is one or more of sodium hydroxide, potassium hydroxide or calcium hydroxide.

8. The method for preparing a hydrophilic ultrafiltration membrane for oil-water separation according to claim 1, characterized in that: The time of the transesterification reaction is 0.5 to 50 hours, and the temperature of the transesterification reaction is 5 to 80°C.

9. A hydrophilic ultrafiltration membrane for oil-water separation prepared by the method according to any one of claims 1 to 8, characterized in that: The hydrophilic ultrafiltration membrane is an asymmetric structure of a finger-shaped pore support.

10. The hydrophilic ultrafiltration membrane for oil-water separation according to claim 9, characterized in that: The hydrophilic ultrafiltration membrane is a flat membrane or a hollow fiber membrane.

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