WO2024071005A1 - 多孔質膜および造水方法 - Google Patents

多孔質膜および造水方法 Download PDF

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Publication number
WO2024071005A1
WO2024071005A1 PCT/JP2023/034640 JP2023034640W WO2024071005A1 WO 2024071005 A1 WO2024071005 A1 WO 2024071005A1 JP 2023034640 W JP2023034640 W JP 2023034640W WO 2024071005 A1 WO2024071005 A1 WO 2024071005A1
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Prior art keywords
porous membrane
membrane
water
reverse osmosis
porous
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PCT/JP2023/034640
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English (en)
French (fr)
Japanese (ja)
Inventor
高志 橘
俊 志村
正行 花川
茂久 花田
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Toray Industries Inc
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Toray Industries Inc
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Priority to EP23872230.0A priority Critical patent/EP4596088A4/en
Priority to KR1020257008472A priority patent/KR20250078440A/ko
Priority to AU2023353924A priority patent/AU2023353924A1/en
Priority to CN202380062540.XA priority patent/CN119816358A/zh
Priority to JP2023568518A priority patent/JP7697531B2/ja
Publication of WO2024071005A1 publication Critical patent/WO2024071005A1/ja
Anticipated expiration legal-status Critical
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/02—Reverse osmosis; Hyperfiltration ; Nanofiltration
    • B01D61/025—Reverse osmosis; Hyperfiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/14—Ultrafiltration; Microfiltration
    • B01D61/145—Ultrafiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
    • B01D61/58—Multistep processes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D69/00—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor
    • B01D69/02—Semi-permeable membranes for separation processes or apparatus characterised by their form, structure or properties; Manufacturing processes specially adapted therefor characterised by their properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/06—Organic material
    • B01D71/30—Polyalkenyl halides
    • B01D71/32—Polyalkenyl halides containing fluorine atoms
    • B01D71/34—Polyvinylidene fluoride
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/441—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by reverse osmosis
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
    • C02F1/444—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by ultrafiltration or microfiltration
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/02—Details relating to pores or porosity of the membranes
    • B01D2325/0283—Pore size
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/20—Specific permeability or cut-off range
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/52—Crystallinity
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2103/00—Nature of the water, waste water, sewage or sludge to be treated
    • C02F2103/08—Seawater, e.g. for desalination
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A20/00—Water conservation; Efficient water supply; Efficient water use
    • Y02A20/124—Water desalination
    • Y02A20/131—Reverse-osmosis

Definitions

  • the present invention relates to porous membranes for use in fields such as water treatment, pharmaceutical manufacturing, food industry, and fermentation.
  • porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in a variety of fields, including water treatment (such as water purification or wastewater treatment), medical fields (such as blood purification), and the food industry.
  • water treatment such as water purification or wastewater treatment
  • medical fields such as blood purification
  • food industry Recently, there has been a demand for highly fouling-resistant porous membranes that can efficiently filter easily fouling raw filtrates that have been considered difficult to filter in the past. Easily fouling raw filtrates are characterized by a higher than normal amount of fouling substances contained in the raw filtrate, and these fouling substances often contain organic matter. Common examples of organic matter include proteins, polysaccharides, and humic acid.
  • the porous membrane is clogged with the contaminants contained in the raw filtrate, resulting in a problem of reduced filtration flux through the porous membrane.
  • the pores in the porous membrane are likely to become clogged due to the interaction between the porous membrane and the organic matter, which causes the organic matter to be adsorbed onto the porous membrane.
  • the porous membrane becomes clogged and the number of pores decreases, high pressure is required for filtration, and the contaminants are pressed against the porous membrane surface with high pressure, making adsorption more likely.
  • one effective method for eliminating the adsorption of organic matter and recovering the filtration flux is to use chemicals, for example oxidizing agents such as sodium hypochlorite or alkalis such as sodium hydroxide, to decompose and remove the organic matter that has accumulated in the porous membrane.
  • chemicals for example oxidizing agents such as sodium hypochlorite or alkalis such as sodium hydroxide
  • oxidizing agents such as sodium hypochlorite or alkalis such as sodium hydroxide
  • Patent Documents 1 and 2 disclose a method for increasing the proportion of stable ⁇ -structure crystals in polyvinylidene fluoride resin (hereinafter also referred to as "PVDF resin"). It is disclosed that an increase in the amount of stable crystals improves chemical resistance and increases cleaning strength.
  • PVDF resin polyvinylidene fluoride resin
  • the ⁇ -type structure crystals in the porous membranes of polyvinylidene fluoride resin in Patent Documents 1 and 2 are TGTG' (T: Trans, G: Gauche) structures. Since ⁇ -type structure crystals are twisted structures, the dipole moments are arranged in alternating directions and have no polarity. Therefore, they have a weak interaction with polar water molecules, and conversely, they have a strong interaction with organic matter due to hydrophobic interactions. In other words, porous membranes made of polyvinylidene fluoride resin with ⁇ -type structure crystals are prone to adsorb organic matter and are prone to blockage. Therefore, they need to be washed with a large amount of chemicals.
  • TGTG' T: Trans, G: Gauche
  • Patent Documents 1 and 2 improve the chemical resistance of the porous membrane, but the chemicals are expensive and may have a negative effect on module components equipped with the porous membrane and on processes other than the porous membrane. For example, the adhesive that bonds the porous membrane may deteriorate, resulting in a shortened lifespan of the porous membrane module, or when another separation membrane with relatively low chemical resistance is used downstream of the porous membrane, this may lead to deterioration of the downstream separation membrane.
  • Patent Documents 2 and 3 disclose porous membranes that contain not only ⁇ -type structure crystals but also a large amount of ⁇ -type structure crystals, which are relatively metastable phases.
  • ⁇ -type structure crystals have a TTTT planar zigzag structure, dipole moments are arranged in the same direction, and the membrane surface has a large number of electron donor components and is considered to have polarity.
  • Patent Document 2 discloses a membrane in which the proportion of infrared absorbance derived from ⁇ -type structure crystals among the infrared absorbance derived from ⁇ -type structure crystals and ⁇ -type structure crystals is 32.5%.
  • a membrane in which the ratio (H ⁇ /H ⁇ ) of ⁇ -type structure crystals (H ⁇ ) to ⁇ -type structure crystals (H ⁇ ) is 0.38.
  • Patent Document 3 also discloses a membrane in which the abundance ratio of ⁇ -crystal structure to ⁇ -crystal structure is 0.11. Therefore, these membranes have a strong interaction with water molecules that also have polarity, and conversely, organic matter is difficult to adsorb.
  • the method for producing a porous membrane in Patent Document 2 tends to form a porous membrane with a relatively fast phase separation rate and large pore size.
  • the large pore size resulted in an insufficient number of pores.
  • dirt components easily enter the porous membrane and cause clogging, and the desired filtration rate cannot be obtained.
  • the membrane-forming stock solution contains polyvinylidene fluoride resin as a polymer and a mixture of multiple good solvents as a solvent, the self-diffusion coefficient of the polymer is not taken into consideration, and the good solvent does not contain the hydrogen-bonding solvent described below.
  • the self-diffusion coefficient of the polymer is not taken into consideration either, and the content of the polymer in the membrane-forming stock solution is as high as 30 wt%, so that the number of pores in the porous membrane produced is small, and the pure water permeability is likely to be low.
  • a porous membrane requires high pressure for filtration, and dirt components are pressed against the membrane surface with high pressure, which promotes adsorption and clogging of the porous membrane, and sufficient permeation liquid cannot be obtained when filtering a stock solution containing a large amount of dirt components.
  • the problem that the present invention aims to solve is to obtain a porous membrane that can maintain high filtrate permeability even for a filtrate stock solution that is easily contaminated as described above, which has been considered difficult to filter because the porous membrane is significantly contaminated.
  • porous membranes containing a large amount of ⁇ -structure crystals with emphasis on chemical resistance were common.
  • Patent Document 3 since the polymer concentration in the membrane-forming stock solution is high, the phase separation rate is relatively slow, and it is easy to form a porous membrane containing a large amount of ⁇ -structure crystals, which are a metastable phase, while the number of pores in the porous membrane is small and the pure water permeability is likely to be low.
  • Patent Document 2 even when the polymer concentration in the membrane-forming stock solution is low, it is easy to form a porous membrane containing a large amount of ⁇ -structure crystals by using a mixed solvent, but the phase separation rate is not sufficiently slow, so a porous membrane with a large pore size is likely to be formed. In other words, it was difficult to form a porous membrane that combines the three elements of being difficult to adsorb by containing a large amount of ⁇ -structure crystals, increasing the number of surface pores, ensuring a flow path, showing high pure water permeability, and having a relatively small surface pore size.
  • the present invention aims to provide a porous membrane that has a high content of ⁇ -structure crystals and excellent fouling resistance, and further has high pure water permeability, which reduces adsorption even to easily fouling filtrate raw solutions, and has a relatively small surface pore size that prevents fouling components from penetrating the porous membrane, ensuring a high filtration flux for a long period of time.
  • the present invention provides a porous membrane having the following configuration.
  • a membrane filtration device comprising the porous membrane according to any one of (1) to (6).
  • a freshwater production method including a pretreatment step of pretreating water to obtain reverse osmosis membrane feed water, and a reverse osmosis membrane treatment step of filtering the reverse osmosis membrane feed water through a reverse osmosis membrane to obtain permeate water, the water to be treated having a biopolymer concentration of 100 ⁇ g C/L or more, and the reverse osmosis membrane feed water having a biopolymer concentration of 75 ⁇ g C/L or less through pretreatment is supplied to the reverse osmosis membrane,
  • the pretreatment step includes an ultrafiltration membrane treatment section equipped with an ultrafiltration membrane, and the ultrafiltration membrane is the porous membrane according to any one of (1) to (6).
  • the present invention provides a porous membrane that has a high proportion of ⁇ -structure crystals to which organic matter does not easily adhere, a small surface pore size, and a sufficiently large number of pores. This makes it possible to provide a porous membrane that has high water permeability and excellent stain resistance, and can ensure high filtrate permeability for a long period of time even for filtrate raw solutions that are easily soiled.
  • FIG. 1 is a schematic diagram showing the filtration state of the porous membrane of the present invention.
  • FIG. 2 is a schematic diagram showing the filtration state using a conventional porous membrane.
  • FIG. 3 is a schematic diagram showing the filtration state using a conventional porous membrane.
  • FIG. 4 is a schematic diagram of the process for forming the porous membrane of the present invention.
  • FIG. 5 is an electron microscope photograph showing a surface (part) of a porous membrane according to one embodiment of the present invention.
  • FIG. 1 is a schematic diagram showing an example of a fresh water production method of the present invention.
  • a filtrate stock solution that is easily soiled is characterized by having a higher than normal amount of contaminants, and these contaminants often contain useful substances that are the subject of removal. Examples of organic matter that generally contain large amounts of proteins, polysaccharides, humic acid, etc.
  • the dirt related to "fouling resistance" in this invention refers to the clogging of the porous membrane with organic matter contained in the raw filtrate during the filtration process. As the clogging of the porous membrane with organic matter progresses, the filtration resistance increases and the amount of liquid that can pass through the porous membrane decreases.
  • porous membranes having ⁇ -type crystal structure are prone to adsorb organic matter, and to solve this problem, a method of decomposing and removing the accumulated organic matter using a large amount of chemicals is used as an effective means.
  • such chemicals not only decompose the accumulated organic matter, but also decompose the polymers that make up the membrane, and may also have a negative effect on module components equipped with the porous membrane and on processes other than the porous membrane. Therefore, a porous membrane having ⁇ -type crystal structure with excellent stain resistance is promising, but there has been no porous membrane that has ⁇ -type crystal structure and is highly permeable.
  • the present invention relates to a membrane having ⁇ -type crystal structure, which further has high pure water permeability, thereby reducing adsorption even for the filtrate raw solution that is easily stained, and ensuring a high filtration flux.
  • the porous membrane of the present invention with improved stain resistance will be described below.
  • the porous membrane according to the embodiment of the present invention is a porous membrane containing polyvinylidene fluoride resin, has surface A and surface B, and in the IR spectrum of surface A measured by ATR (attenuated total reflection) method, the ratio ( H ⁇ / H ⁇ ratio) of ⁇ -type structure crystal ( H ⁇ ) to ⁇ -type structure crystal ( H ⁇ ) in the crystal part of polyvinylidene fluoride resin is 0 or more and 0.5 or less, and the pure water permeability is 0.25m3 / m2 /h/50kPa or more and 1.2m3 / m2 /h/50kPa or less. Note that the H ⁇ / H ⁇ ratio of 0 indicates that ⁇ -type structure crystal ( H ⁇ ) is 100%.
  • the ratio (H ⁇ /H ⁇ ) of ⁇ -type structure crystals (H ⁇ ) and ⁇ -type structure crystals (H ⁇ ) in the crystal part of polyvinylidene fluoride resin is required to be 0 or more and 0.50 or less.
  • three structures are known as the crystal structure of polyvinylidene fluoride resin: ⁇ -type, ⁇ -type, and ⁇ -type, which exists in a very small amount.
  • we focused on ⁇ -type and ⁇ -type which are generally known to be formed more than ⁇ -type.
  • ⁇ -type structure crystals have a TTTT planar zigzag structure, dipole moments are arranged in the same direction, there are many electron donating components on the film surface, and they are considered to have polarity. Therefore, they have a strong interaction with water molecules, which also have polarity, and conversely, organic matter is difficult to adsorb.
  • H ⁇ /H ⁇ is 0 or more and 0.50 or less, thereby exhibiting excellent dirt resistance against organic matter.
  • the ratio of ⁇ structure crystals (H ⁇ ) to ⁇ structure crystals (H ⁇ ) in the crystalline portion of the polyvinylidene fluoride resin (H ⁇ /H ⁇ ratio) can be calculated by the following formula (1), and is preferably 0.25 to 0.40, and more preferably 0.30 to 0.40.
  • Ratio of ⁇ -type/ ⁇ -type structure crystals H ⁇ /H ⁇ Formula (1).
  • the porous membrane of the present invention is characterized by a large amount of ⁇ -type structure crystals (H ⁇ ) in the crystal part of the polyvinylidene fluoride resin.
  • H ⁇ ⁇ -type structure crystals
  • the porous membrane of the present invention can generate a large number of fine holes even if the concentration of the PVDF polymer is low by controlling the phase separation rate of the polymer and the solvent. In the phase separation process of the polymer and the solvent, a relatively slow phase separation rate can be used to generate a small pore size and a large number of holes.
  • the self-diffusion coefficient of the polymer can be used as an indicator of the phase separation rate, and by optimizing the polymer concentration, the solvent and temperature of the coagulation bath, it is possible to obtain a porous membrane that has a large number of pores and a small surface pore size while having a large number of pores and a large amount of ⁇ -structure crystals and excellent stain resistance.
  • the porous membrane of the present invention is mainly composed of a crystalline polymer resin containing polyvinylidene fluoride, and within 50 ⁇ m from the surface A of the porous membrane, the polymer resin constituting the porous membrane has crystals, so the crystallinity is not limited, but it is more preferable that the crystallinity of PVDF is 30% or more.
  • the crystallinity of 30% or more when the turbidity collides with the porous membrane, it is easy to suppress the deformation of the pores, and the turbidity is easy to bounce off the membrane surface, which is preferable.
  • the crystallinity is more preferably 35% or more, and even more preferably 40% or more. However, if it is 80% or more, the flexibility of the porous membrane is lost, and it is easy to break during operation such as cross-flow operation, so it is preferable that it is 80% or less.
  • a crystallinity of 30% or more By having a small ratio of H ⁇ /H ⁇ and a crystallinity of 30% or more, it is easy to obtain the effect of strong interaction with polar water molecules and difficult adsorption of organic matter.
  • DSC differential scanning calorimeter
  • the slices within 50 ⁇ m from the outer surface of the porous membrane used for measuring the crystallinity are taken with a commercially available freezing microtome.
  • the porous membrane In the microtome, the porous membrane is moved a certain distance, and then the blade is brought into contact with the porous membrane to cut it.
  • the blade is set in a direction parallel to the surface of the porous membrane.
  • the porous membrane is cut once by moving the blade close to it at intervals of 5 ⁇ m. Then, the moving distance is set to 40 ⁇ m, and cutting is performed once more, so that a thickness of 40 ⁇ m to 45 ⁇ m can be taken from surface A.
  • the pure water permeability is more preferably 0.30 m 3 /m 2 /h/50 kPa or more and 1.2 m 3 /m 2 /h/50 kPa or less, and even more preferably 0.40 m 3 /m 2 /h/50 kPa or more and 1.2 m 3 /m 2 /h/50 kPa or less.
  • FIG. 1 is a conceptual schematic diagram showing a part of the cross section of a porous membrane.
  • the solid lines drawn on the surface portion 102 show a schematic mesh structure.
  • the filtration direction FL is the direction from the surface portion 102 toward the inside 103.
  • the organic matter in the filtrate stock solution often contains coarse dirt components 201 and fine dirt components 202.
  • the porous membrane is a ⁇ -type structure crystal, which has high polarity and suppresses the adsorption of the coarse dirt components 201 and the fine dirt components 202.
  • the porous membrane 101 includes the surface portion 102 and the inside 103.
  • the surface portion 102 has a denser structure than the interior portion 103, which makes it difficult for dirt components to penetrate into the porous membrane, and therefore is preferable because it tends to exhibit excellent dirt resistance. If the micropores in the surface portion 102 are dense, the coarse dirt components 201 cannot penetrate into the porous membrane and do not clog the surface layer 102 of the porous membrane, making the porous membrane less susceptible to dirt, which is preferable.
  • FIG. 2 shows a schematic diagram of a filtrate concentrate that is difficult to filter, when it is filtered using a porous membrane with large pores containing many ⁇ -structure crystals, as has been known up until now.
  • the porous membrane has low polarity due to the large amount of ⁇ -structure crystals, and it is easy for coarse organic dirt components 201 and fine dirt components 202 to be adsorbed to the porous membrane due to hydrophobic-hydrophobic interactions.
  • the pore size is large, the coarse dirt components 201 easily penetrate into the interior of the porous membrane and cause clogging, and because the number of pores is small, the filtrate permeability is low.
  • Figure 3 shows a schematic diagram of a filtrate stock solution that is difficult to filter, filtered through a porous membrane that has a relatively high content of ⁇ -structure crystals but low pure water permeability.
  • porous membranes often have a small number of pores and low pure water permeability.
  • the porous membrane has a high content of ⁇ -structure crystals, it adsorbs relatively little coarse dirt components 201 and fine dirt components 202.
  • the pure water permeability is low, high pressure is required for the filtrate stock solution containing fine dirt components 202 to pass through the porous membrane. Therefore, the fine dirt components 202 are strongly pressed against the pores in the porous membrane, and are gradually adsorbed into the porous membrane. This tends to reduce the filtrate permeability.
  • the porous membrane of the present invention further has a dextran removal rate of 45% or more and 80% or less of weight-average molecular weight of 40,000 Da, so that the coarse dirt substances and removal target substances in the filtrate liquid are prevented from penetrating into the membrane, and it is easy to show excellent dirt resistance.
  • the dextran removal rate is more preferably 50% or more and 80% or less.
  • the dextran removal rate can be calculated by the following formula (2) by filtering the dextran aqueous solution prepared so that the commercially available dextran with weight-average molecular weight of 40,000 Da is 1000 ppm and 25°C, with a cross-flow linear velocity of 1.0 m/sec and a transmembrane pressure difference of 10 kPa through the porous membrane.
  • T ⁇ (Refractive index of original solution) - (Refractive index of permeated solution) ⁇ / (Refractive index of original solution) x 100 ...
  • the cross-flow linear velocity is the flow rate of the raw liquid permeated in a direction perpendicular to the filtration direction divided by the cross-sectional area of the flow path
  • the transmembrane pressure is the difference in pressure between the raw liquid permeated and the permeated liquid permeated across the porous membrane.
  • the average surface pore diameter [nm] of the surface part of the porous membrane is 5.0 nm or more and 12 nm or less, since this prevents coarse dirt components in the filtrate and the objects to be removed from penetrating into the porous membrane, and tends to show high dirt resistance. It is even more preferable that the average surface pore diameter [nm] of the surface part is 5.0 nm or more and 8.0 nm or less.
  • the surface pore diameter is the diameter of the pores present in the surface when the surface of the porous membrane is observed.
  • the standard deviation of the surface pore diameter [nm] of the porous membrane is preferably 1.0 nm or more and 5.0 nm or less, which allows the porous membrane to be uniformly loaded with fouling substances and tends to exhibit excellent fouling resistance.
  • the standard deviation of the surface pore diameter is preferably 1.0 nm or more and 5.0 nm or less, which allows the porous membrane to be uniformly loaded with fouling substances and tends to exhibit excellent fouling resistance. It is more preferable that the standard deviation of the surface pore diameter is 1.0 nm or more and 4.0 nm or less, and even more preferable that the standard deviation of the surface pore diameter is 1.0 nm or more and 3.0 nm or less.
  • the standard deviation of the surface pore diameter can be determined by binarizing and analyzing an SEM image of the surface of the porous membrane as described above.
  • the number of surface pores observed on the surface of the porous membrane is 200/ ⁇ m 2 to 2000/ ⁇ m 2 , which is preferable because it is possible to disperse the contaminants in the raw filtrate in the porous membrane, and it is easy to perform stably filtering highly turbid water for a long period of time. Even if the contaminants in the raw filtrate block some of the surface pores as the filtration progresses, if the number of surface pores is large, it is easy to ensure a sufficient number of flow paths through which the raw filtrate passes through the porous membrane, which is preferable. If the number of surface pores is 200/ ⁇ m 2 or more, it is easy to ensure a sufficient number of flow paths until cleaning such as back pressure cleaning is performed, even if the raw filtrate is easily dirty.
  • the number of surface pores of the porous membrane is 290/ ⁇ m 2 to 1500/ ⁇ m 2 , and particularly preferably 350/ ⁇ m 2 to 1000/ ⁇ m 2.
  • the number of surface pores of the porous membrane is the number of pores present in the surface when the surface of the porous membrane is observed.
  • X is 30/ ⁇ m 2 /nm or more and 100/ ⁇ m 2 /nm or less, which prevents the dirt components in the filtrate from penetrating into the pores and ensures a sufficient number of flow paths through which the permeate permeates the porous membrane, making it easier to exhibit permeability, which is preferable.
  • X is more preferably 50/ ⁇ m 2 /nm or more and 100/ ⁇ m 2 /nm or less, and more preferably 60/ ⁇ m 2 /nm or more and 100/ ⁇ m 2 /nm or less.
  • the number of surface pores of the porous membrane can be obtained by binarizing an image obtained by SEM observing the surface of the porous membrane in the same manner as when the surface pore size was obtained, and analyzing it with Analyze Particles. The number of surface pores is divided by the area of the analyzed image to obtain the number of pores per unit area. As with the pore size, an image containing 1000 or more pores is analyzed and calculated.
  • the porous membrane in the present application preferably has a nanomesh structure on its surface.
  • the nanomesh structure is a structure in which the average cross-sectional pore diameter [nm] in the cross section of the outermost surface portion is 1 nm to 99 nm in the outermost surface portion from the surface to a thickness of 2 ⁇ m, and the number of cross-sectional pores is 100/ ⁇ m 2 to 1000/ ⁇ m 2.
  • the cross-sectional pore refers to a hole observed in the cross section of the outermost surface portion.
  • the average cross-sectional pore diameter [nm] in the cross section of the outermost surface portion is as fine as 1 nm to 99 nm, it is easy to capture coarse dirt components and objects to be removed in the filtrate at the outermost surface portion of the porous membrane and prevent them from penetrating into the porous membrane. Furthermore, since the number of cross-sectional pores is as large as 100/ ⁇ m 2 to 1000/ ⁇ m 2 , the pores are formed vertically, horizontally, and in all directions, the flow paths are easily connected in a straight line, and fine dirt components are less likely to clog when passing through the porous membrane. In addition, even if some of the pores become clogged, a flow path for the filtrate to pass through the porous membrane can be secured, and therefore the porous membrane is preferred as it tends to exhibit excellent resistance to fouling.
  • the porous membrane of the present invention can be obtained by a production method including: step (A) of dissolving a polymer in a solvent to obtain a polymer solution; and then step (B) of solidifying the polymer solution in a non-solvent to form a porous membrane; in the polymer solution obtained in step (A), the self-diffusion coefficient [m 2 /sec] of the dissolved polymer calculated by all-atom molecular dynamics calculation is 0.8 ⁇ 10 -11 m 2 /sec to 1.6 ⁇ 10 -11 m 2 /sec; the non-solvent used in step (B) contains 90 to 100% by weight of water; and the temperature of the non-solvent is 6°C to 45°C.
  • the type of polymer used in step (A) is not particularly limited, but specific examples include polysulfone resins, polyethersulfone resins, polyvinylidene fluoride resins, nylon, cellulose esters such as cellulose acetate or cellulose acetate propionate, fatty acid vinyl esters, polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, acrylic acid esters or methacrylic acid esters such as ethylene oxide, propylene oxide or polymethyl methacrylate, or copolymers thereof.
  • the polyvinylidene fluoride resin refers to a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride.
  • the copolymer of vinylidene fluoride refers to a polymer having a vinylidene fluoride residue structure.
  • the polymer having a vinylidene fluoride residue structure is typically a copolymer of a vinylidene fluoride monomer and other fluorine-based monomers.
  • fluorine-based monomers examples include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.
  • vinylidene fluoride copolymer examples include ethylene and the like other than the above-mentioned fluorine-based monomers may be copolymerized to an extent that does not impair the effects of the present invention.
  • the polyvinylidene fluoride resin comprises 50% by weight or more, and particularly preferably 60% by weight or more.
  • the weight-average molecular weight of the polymer is preferably 50 to 1,000,000 Da, since this makes it easier to control the self-diffusion coefficient (described below) to a relatively slow range within an appropriate range.
  • multiple types of polymers may be used in combination.
  • the solvent preferably contains a good solvent.
  • the term “good solvent” refers to a solvent capable of dissolving 5% by weight or more of a polymer even in a low temperature range of 60° C. or less.
  • the good solvent include N-methyl-2-pyrrolidone (hereinafter, "NMP"), 2-pyrrolidone (hereinafter, “2P”), ⁇ -caprolactam (hereinafter, “ ⁇ -CL”), dimethylsulfoxide (hereinafter, “DMSO”), dimethylacetamide (hereinafter, “DMAc”), dimethylformamide (hereinafter, “DMF”), methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea, trimethyl phosphate, or a mixture thereof.
  • NMP N-methyl-2-pyrrolidone
  • 2P 2-pyrrolidone
  • ⁇ -CL ⁇ -caprolactam
  • DMSO dimethylsulfoxide
  • the good solvent is contained in the solvent in an amount of 40% by weight or more, and particularly preferably in an amount of 60% by weight or more.
  • the "non-solvent" in step (B) refers to a solvent that does not dissolve or swell the polymer even when the non-solvent is heated to a high temperature up to the boiling point.
  • non-solvent examples include aliphatic hydrocarbons such as water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, and low molecular weight polyethylene glycol, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, and other chlorinated organic liquids or mixed solvents thereof.
  • aliphatic hydrocarbons such as water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichloroethylene, ethylene glycol, diethylene glycol, triethylene
  • the concentration (wt%) of the polymer in the polymer solution is preferably equal to or higher than the entanglement concentration in order to control the self-diffusion coefficient within an appropriate range, and more specifically, is preferably 10 to 40 wt%, more preferably 12 to 30 wt%, and particularly preferably 15 to 25 wt%.
  • the porous membrane formation process in step (B), in which the polymer solution is solidified in a non-solvent to form a porous membrane is a process for forming a porous membrane by non-solvent-induced phase separation.
  • the polymer solution comes into contact with the non-solvent, the polymer cannot be completely dissolved in the solvent, and phase separation occurs into a phase containing a large amount of polymer and a phase containing a large amount of solvent, and each phase coarsens as it merges with the surrounding identical phases.
  • FIG 4 is a schematic diagram showing the porous membrane formation process in step (B).
  • Phase separation progresses in the polymer solution in the order of Figure 4(a) to (f), and the areas with high polymer concentration (polymer-rich phase 301) become coarse.
  • the exchange of solvent and non-solvent progresses, and when the concentration of the non-solvent increases beyond a certain level, the polymer solidifies and the structure of the porous membrane is fixed.
  • the solvent-rich phase 302 becomes the pores of the porous membrane.
  • the inventors have found that in the production of the porous membrane of the present invention, by setting the self-diffusion coefficient of the polymer to a relatively low range of 0.8 ⁇ 10 ⁇ 11 m 2 /sec to 1.6 ⁇ 10 ⁇ 11 m 2 /sec, the progress of phase separation and coarsening can be suppressed, and the phase 301 containing a large amount of polymer and the phase 302 containing a large amount of solvent can be solidified in a state in which they are fine and exist in large quantities.
  • the self-diffusion coefficient is 0.8 ⁇ 10 ⁇ 11 m 2 /sec or more, the polymer and the solvent have a sufficient diffusion coefficient for phase separation, and pores are easily formed.
  • the self-diffusion coefficient is preferably 0.8 ⁇ 10 ⁇ 11 m 2 /sec to 1.4 ⁇ 10 ⁇ 11 m 2 /sec, and more preferably 0.8 ⁇ 10 ⁇ 11 m 2 /sec to 1.1 ⁇ 10 ⁇ 11 m 2 /sec.
  • the self-diffusion coefficient can be determined by all-atom molecular dynamics calculations. All-atom molecular dynamics calculations are a method for determining the trajectory of each atom by solving the equation of motion of a molecular ensemble system for all of the constituent atoms one by one. First, a polymer solution system is created to have the polymer concentration (weight %) that will actually be used. At this time, one model polymer chain is modeled to have a molecular weight of 800-6000 and 1/200-1/5 of the weight-average molecular weight of the polymer that will actually be used. Potential parameters used in molecular dynamics calculations include those of DREIDING [S. L. Mayo, B. D. Olafson, W. A. Goddard III, J. Phys. Chem.
  • a constant pressure and temperature ensemble is constructed by controlling the temperature to 25°C using the Nose-Hoover method [Hoover, W. G. Phys. Rev. A, 31, 1695 (1985)] and the pressure to 1 bar using the Andersen method [H. C. Andersen, J. Chem. Phys. 72, 2384 (1980)].
  • the short-range Lennard-Johns interaction is handled by applying a switch function from 1.0 nm and cutting off at 1.2 nm, and the long-range electrostatic interaction is calculated using the Particle Mesh Ewald method.
  • the unit cell length is adjusted to obtain the average density, and an additional calculation of 11 ns is performed in a constant temperature ensemble.
  • the mean square displacement (MSD) of each atom in the polymer is obtained, and the self-diffusion coefficient of the polymer is calculated from the following formula (3).
  • MSD mean square displacement
  • the self-diffusion coefficient of the polymer in the polymer solution is determined by taking the weighted average of the self-diffusion coefficients of each polymer based on the weight percentage of the polymer.
  • the non-solvent used for solidification contains 90 to 100% by weight of water, so that solidification is fast and the self-diffusion coefficient in the polymer solution is likely to affect the speed of phase separation and coarsening.
  • the temperature of the non-solvent is 6°C to 45°C, so that solidification is fast and the self-diffusion coefficient in the polymer solution is likely to affect the speed of phase separation and coarsening.
  • the temperature of the non-solvent is preferably 35°C to 40°C.
  • the solvent contains a hydrogen-bonding solvent having a molecular weight of 500 Da or less and hydrogen-bonding donor and hydrogen-bonding acceptor properties, which is preferable because it is easy to control the self-diffusion coefficient of the polymer to an appropriate range.
  • hydrogen-bonding donor properties means having positively polarized hydrogen atoms, specifically referring to having a hydroxyl group (OH group), a carboxyl group (COOH group), an amino group (NH group), etc.
  • Having hydrogen-bonding acceptor properties means having a lone electron pair, specifically referring to having a carbonyl group, an alkoxy group, a cyano group, etc.
  • Hydrogen bond donor and hydrogen bond acceptor solvents are not particularly limited, but specific examples include 2P, ⁇ -CL, 1,3-dimethylurea, N-methylacetamide, hydantoin, 2-imidazolidinone, DL-pyroglutamic acid, etc.
  • the polymer to be dissolved contains a polymer having hydrogen bond donor properties and/or hydrogen bond acceptor properties, since this makes it easier to control the self-diffusion coefficient of the polymer within an appropriate range.
  • the polymer has hydrogen bond donor properties and/or hydrogen bond acceptor properties, and interacts with the solvent having hydrogen bond donor properties and hydrogen bond acceptor properties through hydrogen bonds, making it easier to control the self-diffusion coefficient of the polymer to a relatively slow, appropriate range.
  • the polymer having hydrogen bond donor properties and/or hydrogen bond acceptor properties accounts for 10% to 50% by weight of the porous membrane, from the viewpoint of controlling the self-diffusion coefficient of the polymer within an appropriate range.
  • the hydrogen bonds between the polymer and the solvent are in an appropriate range, and the self-diffusion coefficient of the polymer is relatively slow and easy to control to an appropriate range.
  • the value is more preferably 2.0 to 9.0, and particularly preferably 5.4 to 8.0.
  • the hydrogen bonds between the polymer and the solvent are in an appropriate range, and the self-diffusion coefficient of the polymer is relatively slow and easy to control to an appropriate range.
  • the value is more preferably 1.5 to 2.3, and particularly preferably 1.8 to 2.3.
  • the porous membrane of the present invention may further include other layers.
  • the porous membrane of the present invention is disposed on the surface.
  • the other layer is not particularly limited as long as it is a component that can be overlapped with the porous membrane to form a layer, but it is preferable that the other layer is a support.
  • the "support” refers to a structure that has a higher breaking strength than the porous membrane for physically reinforcing the porous membrane.
  • the breaking strength (breaking strength per unit area) of the support is preferably 3 MPa or more, more preferably 10 MPa or more.
  • the breaking strength of the support is preferably 300 gf or more, more preferably 800 gf or more.
  • Liquids can be filtered by a membrane filtration device using the porous membrane of the present invention obtained as described above.
  • membrane filtration devices include, but are not limited to, a raw water tank, a booster pump, a module consisting of several thousand to several tens of thousands of the porous hollow fiber membrane of the present invention, a filtered water tank, and a backwash pump.
  • methods for filtering liquids include, but are not limited to, using the membrane filtration device to operate raw liquid such as industrial wastewater at an operating pressure of 10 kPa to 1 MPa to remove organic matter contained in the raw water.
  • Water production systems using reverse osmosis membranes are used in many industrial and water treatment fields, including seawater desalination, and have been proven to be superior in terms of separation performance and energy efficiency compared to other separation methods.
  • these water production systems have the problem that the transmembrane pressure increases rapidly due to the proliferation of microorganisms on the membrane surface or the adhesion of biological membranes (biofilms) to the membrane surface, i.e., biofouling, resulting in a decrease in the membrane's permeability and separation ability.
  • Japanese Patent No. 6561082 and Japanese Patent No. 6630689 focus on biopolymers contained as part of the organic matter in the water to be treated, and disclose a method of adjusting the biopolymers in the water to be treated so that they are below a predetermined threshold value, and then performing membrane separation.
  • the predetermined threshold value is any value within the range of 9 ⁇ g/L to 12 ⁇ g/L, or 9 ⁇ g/L to 17 ⁇ g/L.
  • Biopolymers are hydrophilic high molecular weight organic matter (polysaccharides, proteins, etc.) that has a molecular weight of approximately 10 to 20 kDa or more among organic matter.
  • the specified threshold disclosed in Patent No. 6,561,082 and Patent No. 6,630,689 was a very low concentration.
  • the biopolymer concentration is high in treated water such as general seawater and sewage wastewater treatment water, making it difficult to achieve the specified threshold disclosed in Patent No. 6,561,082 and Patent No. 6,630,689.
  • the intended supply target is a microfiltration membrane or ultrafiltration membrane rather than a reverse osmosis membrane, and since the fouling mechanism of the reverse osmosis membrane is different, it is difficult to apply the method as is.
  • the present invention suppresses the progression of fouling in a pretreatment process and reverse osmosis membrane for treated water with high biofouling potential when using a membrane to desalinate seawater, brackish water, etc., or when purifying sewage treatment water or industrial wastewater to obtain reused water.
  • the water production method of the present invention has any of the following configurations.
  • the water production method includes a pretreatment process for pretreating the water to be treated to obtain reverse osmosis membrane feed water, and a reverse osmosis membrane treatment process for filtering the reverse osmosis membrane feed water with a reverse osmosis membrane to obtain permeated water, in which the water to be treated, which has a biopolymer concentration of 100 ⁇ g C/L or more, is supplied to the reverse osmosis membrane with the reverse osmosis membrane feed water having a biopolymer concentration of 75 ⁇ g C/L or less by pretreatment, and the pretreatment process includes ultrafiltration membrane treatment, and the ultrafiltration membrane is the porous membrane described above.
  • the freshwater production method of the present invention is implemented in a freshwater production system in which water to be treated 401 is pretreated using an ultrafiltration membrane, then treated using a reverse osmosis membrane 402, and separated into permeate 403 and concentrated water 404.
  • water to be treated examples include seawater, brackish water, river water, lake water, groundwater, sewage, and secondary sewage treatment water. Since the water to be treated contains solid components such as turbidity, if it is directly filtered using a reverse osmosis membrane, the solid components will adhere to the membrane surface in large amounts, causing a sudden rise in differential pressure and making operation impossible. For this reason, the water to be treated 401 is first treated in a pretreatment process 405 to produce reverse osmosis membrane feed water 406, which is then supplied to the reverse osmosis membrane.
  • the reverse osmosis membrane feed water 406 which has a biopolymer concentration of 75 ⁇ g C/L or less by pretreatment, is supplied to the reverse osmosis membrane from the treated water 1 having a biopolymer concentration of 100 ⁇ g C/L or more. It is more preferable to supply the reverse osmosis membrane feed water 406, which has a biopolymer concentration of 50 ⁇ g C/L or less by pretreatment, to the reverse osmosis membrane from the treated water 401 having a biopolymer concentration of 100 ⁇ g C/L or more.
  • the biopolymer concentration of the treated water 401 having a biopolymer concentration of 100 ⁇ g C/L or more to less than 20 ⁇ g C/L by pretreatment so it is preferable to supply the reverse osmosis membrane feed water 406 to the reverse osmosis membrane with a biopolymer concentration of 20 ⁇ g C/L or more.
  • Biopolymers are hydrophilic organic polymers (polysaccharides, proteins, etc.) with a molecular weight of 10 to 20 kDa or more.
  • biopolymers see, for example, Huber, S. A., Balz, A., Abert, M., Pronk, W., 2011. Characterization of aquatic humic and non-humic matter with size-exclusion chromatography e organic carbon detection e organic carbon detection e organic nitrogen detection (LC-OCD-OND).
  • TOC can be measured by organic carbon exclusion chromatography (LC-OCD).
  • the LC-OCD method is an analytical method in which TOC components in a sample are fractionated by molecular weight and displayed as a chromatogram, and on the chromatogram, organic substances with larger molecular weights and hydrophilicity tend to have shorter retention times.
  • the measurement conditions for the LC-OCD method are a 250 mm x 20 mm TSK HW50S column, a flow rate of 1.1 mL/min, a sample injection amount of 1 mL, a UV wavelength of 254 nm, an acid injection amount to the OCD meter of 0.2 mL/min, a pH 6.85 phosphate buffer as the eluent, and a solution of 4 mL O-phosphoric acid (85%) and 0.5 g potassium peroxodisulfate added to 1 L of ultrapure water as the acidification solution.
  • an LC-OCD device manufactured by DOC-Labar
  • OCD meter wet total organic carbon meter
  • HPLC high performance liquid chromatography
  • the porous membrane of the present invention can be used as an ultrafiltration membrane. That is, the water production method of the present invention can use, as the ultrafiltration membrane, a porous membrane containing polyvinylidene fluoride resin, having a surface A and a surface B, in which, in an ATR (attenuated total reflection) method of surface A, the ratio (H ⁇ /H ⁇ ) of ⁇ -structure crystals (H ⁇ ) to ⁇ -structure crystals (H ⁇ ) in the crystal part of the polyvinylidene fluoride resin is 0 to 0.50, the pure water permeability is 0.25 m 3 /m 2 /h/50 kPa to 1.2 m 3 /m 2 /h/50 kPa, and the average surface pore size on surface A of the porous membrane is 5.0 nm to 12.0 nm.
  • a porous membrane with a removal rate of 40,000 Da dextran of 45% or more and 80% or less is preferable.
  • a dextran removal rate of 45% or more and 80% or less it is possible to prevent coarse contaminants and objects to be removed in the raw filtrate from penetrating into the membrane, and it is easy to show excellent resistance to dirt.
  • the biopolymer removal rate of the membrane is easy to be high, and the amount of flocculant added can be reduced or no flocculant can be used at all, making it possible to reduce the biopolymer concentration of the reverse osmosis membrane feed water to 75 ⁇ g C/L or less.
  • No flocculant is added to the water to be treated means that no flocculant is added to the water to be treated, that is, the water to be treated does not use a flocculant.
  • the type of flocculant is not limited as long as it flocculates biopolymers, and the concentration of the flocculant corresponds to 0% to 0.05%.
  • the dextran removal rate of the ultrafiltration membrane with a weight average molecular weight of 40,000 Da is 45% or more and 80% or less, and further 50% or more and 80% or less.
  • the reverse osmosis membrane 2 may be made of any material that can reduce the salt concentration so that the treated water can be used as drinking water, industrial water, municipal water, etc.
  • Examples include membranes made of cellulose acetate and polyamide materials. Of these, those made of polyamide materials are particularly effective in the method of the present invention.
  • Polyamide membranes have low resistance to chlorine, which is most commonly used as a disinfectant to prevent the growth of biofilms, and even small concentrations of chlorine cause significant membrane deterioration, making it difficult to prevent biofouling. Therefore, the effects of implementing the present invention are evident.
  • the rate of decrease in permeability of the reverse osmosis membrane is preferably 0.20 (%/hr) or less, and more preferably 0.18 (%/hr) or less.
  • Ratio of ⁇ type/ ⁇ type structure crystals H ⁇ /H ⁇ Equation (1).
  • T ⁇ (refractive index of undiluted solution) - (refractive index of permeated solution) ⁇ / (refractive index of undiluted solution) x 100 ... formula (2).
  • the hollow fiber membrane was cut once by moving the blade close to it at 5 ⁇ m intervals. After that, the moving distance was set to 40 ⁇ m, and cutting was performed once more to collect a hollow fiber membrane section having a thickness of 40 to 45 ⁇ m from the surface.
  • the slices were set in a DSC (SEIKO DSC6200) and the temperature was raised from room temperature to 300°C at 5°C/min, the endothermic peak of each polymer resin was regarded as the heat of fusion, and the heat was divided by the heat of complete crystalline fusion of the polymer resin to calculate as a percentage.
  • the endothermic peak observed in the range of 100 to 190°C was regarded as the heat of fusion of the polyvinylidene fluoride resin, and the heat was divided by 104.6 J/g, which is the heat of complete crystalline fusion of the polyvinylidene fluoride resin, to calculate as a percentage.
  • (vi) Self-diffusion coefficient of polymer The self-diffusion coefficient was determined by all-atom molecular dynamics calculation. A polymer solution system was prepared so that the polymer concentration (wt%) would be the concentration actually used. At this time, one model polymer chain was modeled so that the molecular weight was 800 to 6000 and 1/200 to 1/5 of the weight average molecular weight of the polymer actually used. GAFF2 was used as the potential parameter used in the molecular dynamics calculation. A constant pressure and temperature ensemble was constructed by controlling the temperature to 25°C and the pressure to 1 bar.
  • the short-range Lennard-Johns interaction was handled by applying a switch function from 1.0 nm and cutting off at 1.2 nm, and the long-range electrostatic interaction was calculated by the Particle Mesh Ewald method.
  • the unit cell length was adjusted to obtain the average density, and an additional calculation of 11 ns was performed in the constant temperature ensemble.
  • the mean square displacement (MSD) of each atom of the polymer was determined using a 10 ns trajectory, and the self-diffusion coefficient was calculated from the following formula (3).
  • the pore sizes of 1000 or more pores are averaged.
  • the number of cross-sectional pores is divided by the area of the analyzed region to obtain the number of pores per unit area.
  • an image containing 1000 or more pores is analyzed and calculated.
  • the standard deviation of each cross-sectional pore size was calculated.
  • the total perimeter of the cross-sectional pore size was calculated by binarizing the TEM image of the cross-section of the porous membrane as described above, selecting Perim in Analyze Particles, and dividing the total by the area of the observed region.
  • a 250 mm ⁇ 20 mm TSK HW50S column was used, the flow rate was 1.1 mL/min, the sample injection amount was 1 mL, the UV wavelength was 254 nm, the acid injection amount to the OCD meter was 0.2 mL/min, pH 6.85 phosphate buffer was used as the eluent, and 4 mL O-phosphoric acid (85%) and 0.5 g potassium peroxodisulfate were added to 1 L of ultrapure water as the acidification solution.
  • (x) Permeability Reduction Rate of Reverse Osmosis Membrane The reverse osmosis membrane was operated at a pressure of 5.5 MPa and a recovery rate of 37%. The recovery rate is calculated by the flow rate of reverse osmosis membrane permeate water/(flow rate of reverse osmosis membrane permeate water+flow rate of reverse osmosis membrane concentrated water) ⁇ 100.
  • the permeability reduction rate of the reverse osmosis membrane was calculated based on the formula (4) using the flow rate of the permeate water 1 hour after the start of operation of the reverse osmosis membrane and the flow rate 50 hours after the start of operation.
  • the amount of permeate water (g) for 5 minutes was used as each flow rate.
  • Permeability decrease rate of reverse osmosis membrane [%/hr] (permeated water amount [g] after 51 hours from the start of operation)/(permeated water amount [g] after 1 hour from the start of operation)/50 ⁇ 100 (Equation (4)).
  • Example 1 The porous membrane used was a hollow fiber porous membrane containing a support obtained by the following manufacturing method. 38% by mass of PVDF (Kureha Corporation; KF1300, weight average molecular weight 350,000 Da) and 62% by mass of ⁇ -butyrolactone were mixed and dissolved at 160°C to prepare a support membrane stock solution. This support membrane stock solution was discharged from a double-tube nozzle while accompanying an 85% by mass ⁇ -butyrolactone aqueous solution as a hollow portion forming liquid.
  • the discharged support membrane stock solution was solidified in a cooling bath containing an 85% by mass ⁇ -butyrolactone aqueous solution at a temperature of 20°C installed 30 mm below the nozzle, to produce a hollow fiber support having a spherical structure.
  • PVDF1 Arkema; Kynar (registered trademark) 710, weight average molecular weight 180,000 Da
  • cellulose diacetate Eastman; CA-398-3
  • cellulose triacetate Eastman; CA-436-80S
  • NMP 68.7% by mass of NMP
  • 12.1% by mass of 2P were mixed and stirred at 120°C for 4 hours to prepare a polymer solution with the composition ratio shown in Table 1.
  • the polymer solution was uniformly applied to the outer surface of the hollow fiber support at 10 m/min (thickness 50 ⁇ m).
  • the support to which the polymer solution was applied was immersed in a coagulation bath of distilled water at 35° C. for 10 seconds to coagulate, forming a porous membrane having a three-dimensional mesh structure.
  • the evaluation results of the obtained porous membrane are shown in Table 1.
  • the surface portion of the porous membrane from the surface to a thickness of 10 ⁇ m was denser than the inside.
  • An image of the cross section of the outermost surface portion of the porous membrane observed with a TEM is shown in FIG. 5.
  • the porous membrane had a nanomesh structure based on the cross-sectional pore size and the number of cross-sectional pores.
  • the H ⁇ /H ⁇ of the obtained porous membrane was 0.37
  • the pure water permeability was 0.43 m 3 /m 2 /h/50 kPa
  • the dextran removal rate was 55%, which were good.
  • the crystallinity of the outer surface resin was high at 49%.
  • the value X obtained by dividing the number of surface pores by the average surface pore diameter was 66
  • the average surface pore diameter was 7.2 nm
  • the standard deviation was 2.5.
  • Example 2 A porous membrane was obtained in the same manner as in Example 1, except that the NMP in the polymer solution was changed to DMSO and the distilled water in the coagulation bath was set to 40°C.
  • the evaluation results of the porous membrane are shown in Table 1.
  • the surface portion of the porous membrane up to a thickness of 10 ⁇ m from the surface was denser than the inside.
  • the H ⁇ / H ⁇ of the obtained porous membrane was 0.27, the pure water permeability was 0.29 m3 / m2 /h/50 kPa, and the dextran removal rate was 63%, which were good results.
  • Example 3 A porous membrane was obtained in the same manner as in Example 1, except that the NMP in the polymer solution was changed to DMF and the distilled water in the coagulation bath was set to 25° C.
  • the evaluation results of the porous membrane are shown in Table 1.
  • the surface portion of the porous membrane up to a thickness of 10 ⁇ m from the surface was denser than the inside.
  • the H ⁇ / H ⁇ of the obtained porous membrane was 0.24, the pure water permeability was 0.45 m3 / m2 /h/50 kPa, and the dextran removal rate was 47%, which were good results.
  • Example 1 In the membrane production of Example 1, the solvent in the polymer solution was all NMP, and a porous membrane was obtained in the same manner, except that the composition ratio was changed. The evaluation results of the porous membrane are shown in Table 2.
  • the porous membrane did not have a nanomesh structure.
  • the surface part of the porous membrane from the surface to a thickness of 10 ⁇ m was denser than the inside.
  • the H ⁇ /H ⁇ of the obtained porous membrane was 1.0, the pure water permeability was 0.41 m 3 /m 2 /h/50 kPa, and the dextran removal rate was 55%.
  • X was 13, the average surface pore size was 15 nm, and the standard deviation was 6.0.
  • the crystallinity of the outer surface resin was low at 11%.
  • the initial ratio (F2/F1) of the filtration flux after the permeation volume reached 400 L/m 2 (F2) to the initial ratio (F1) of the filtration flux immediately after the start of filtration was 0.38, and the filtration flux could not be maintained after long-term use.
  • Comparative Example 2 A porous membrane was obtained by the same membrane production as in Comparative Example 1 except that the coagulation bath temperature was 6°C. The evaluation results of the porous membrane are shown in Table 2. The surface portion of the porous membrane from the surface to a thickness of 10 ⁇ m was denser than the inside. The H ⁇ / H ⁇ of the obtained porous membrane was 0.10, the pure water permeability was 0.10 m3 / m2 /h/50 kPa, and the dextran removal rate was 70%, which did not satisfy the conditions. Furthermore, X was 10, the average surface pore size was 8.0 nm, and the standard deviation was 1.9.
  • Example 3 In the film formation of Example 1, PVDF1 in the polymer solution was replaced with PVDF2 (Solef9009 manufactured by Solvay) and the composition ratio was changed, and the film was formed in the same manner as in Comparative Example 1 to obtain a porous film.
  • the evaluation results of the porous film are shown in Table 2.
  • the porous film did not have a nanomesh structure.
  • the surface part of the porous film from the surface to a thickness of 10 ⁇ m was denser than the inside.
  • the H ⁇ /H ⁇ of the obtained porous film was 0.22, the pure water permeability was 0.21 m 3 /m 2 /h/50 kPa, and the dextran removal rate was 57%.
  • a porous membrane was obtained by forming a membrane in the same manner as in Example 1, except that the distilled water in the coagulation bath was set to 21 ° C. The evaluation results of the porous membrane are shown in Table 2.
  • the H ⁇ /H ⁇ of the obtained porous membrane was 0.63, the pure water permeability was 0.022 m 3 /m 2 /h / 50 kPa, and the dextran removal rate was 66%, which did not satisfy the conditions. Furthermore, X was 0.90, the average surface pore size was 15 nm, and the standard deviation was 8.5. (vii) When the long-term stability was evaluated by the filtration evaluation method for industrial wastewater, the amount of permeated liquid was extremely small, making it difficult to measure the filtration flux.
  • Example 4 In the flow shown in Figure 6, the permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 1 (biopolymer concentration 122 ⁇ g C/L) was used as the water to be treated, and the porous membrane of Example 1 was used for pretreatment. As a result, the biopolymer concentration of the reverse osmosis membrane feed water was 35 ⁇ g C/L.
  • This reverse osmosis membrane feed water was pressurized with a high-pressure pump and filtered through the reverse osmosis membrane to obtain permeate water.
  • the reverse osmosis membrane was a spiral reverse osmosis membrane with a polyamide membrane material, a salt rejection rate of 99.8%, and a membrane area of 37 m2 .
  • the initial permeability ratio of the ultrafiltration membrane was 0.71
  • the permeability decrease rate of the reverse osmosis membrane was 0.15%/hr.
  • Example 5 In the flow shown in Figure 6, permeated water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 2 (biopolymer concentration 218 ⁇ g C/L) was used as the water to be treated, and the porous membrane of Example 1 was used for pretreatment. As a result, the biopolymer concentration of the reverse osmosis membrane feed water was 47 ⁇ g C/L.
  • This reverse osmosis membrane feed water was pressurized with a high-pressure pump and filtered through the reverse osmosis membrane to obtain permeated water.
  • the reverse osmosis membrane was a spiral reverse osmosis membrane with a polyamide membrane material, a salt rejection rate of 99.8%, and a membrane area of 37 m2 .
  • the initial permeability ratio of the ultrafiltration membrane was 0.63
  • the permeability decrease rate of the reverse osmosis membrane was 0.18%/hr.
  • the permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 1 (biopolymer concentration 122 ⁇ g C/L) was used as the water to be treated, and the porous membrane of Comparative Example 1 was used for pretreatment. As a result, the biopolymer concentration of the reverse osmosis membrane feed water was 85 ⁇ g C/L.
  • This reverse osmosis membrane feed water was pressurized with a high-pressure pump and filtered through the reverse osmosis membrane to obtain permeate water.
  • the reverse osmosis membrane was a spiral reverse osmosis membrane with a polyamide membrane material, a salt rejection rate of 99.8%, and a membrane area of 37 m2 .
  • the initial permeability ratio of the ultrafiltration membrane was 0.53
  • the permeability decrease rate of the reverse osmosis membrane was 0.25%/hr, showing that the permeability decrease rate of the reverse osmosis membrane was large.
  • Table 3 The results are shown in Table 3.
  • the permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 2 (biopolymer concentration 218 ⁇ g C/L) was used as the water to be treated, and the porous membrane of Comparative Example 1 was used for pretreatment. As a result, the biopolymer concentration of the reverse osmosis membrane feed water was 139 ⁇ g C/L.
  • This reverse osmosis membrane feed water was pressurized with a high-pressure pump and filtered through the reverse osmosis membrane to obtain permeate water.
  • the reverse osmosis membrane was a spiral reverse osmosis membrane with a polyamide membrane material, a salt rejection rate of 99.8%, and a membrane area of 37 m2 .
  • the initial permeability ratio of the ultrafiltration membrane was 0.45
  • the permeability decrease rate of the reverse osmosis membrane was 0.45%/hr.
  • the biopolymer concentration in the treated water was very high
  • the permeability decrease of the ultrafiltration membrane was large, and the permeability decrease rate of the reverse osmosis membrane was also significantly large.

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