Porous membrane and water production method

Abstract

With porous membranes in the related art, if the porous membrane is used with a filtration undiluted solution that is easily contaminated, membrane closure occurred due to turbidity separated by filtration and, as a result, membrane clogging occurred, and permeation performance readily deteriorated. The purpose of the present invention is to provide a porous membrane that has high permeability and excellent stain resistance, and is capable of maintaining high filtrate permeability with respect to a filtration undiluted solution that is easily contaminated.　A porous membrane according to the present invention comprises polyvinylidene fluoride resin, and, given surface A as one surface thereof and surface B as the other surface thereof, has a ratio (Hα / Hβ) of an α-type structural crystal (Hα) to a β-type structural crystal (Hβ) in a crystalline portion of the polyvinylidene fluoride resin measured by an attenuated total reflection (ATR) method at the surface A from 0 to 0.5, a pure water permeability of 0.25 to 1.2 m3 / m2 / h / 50 kPa, and an average value of a surface pore diameter of a surface pore of the surface A of the porous membrane from 5.0 to 12.0 nm.

Description

Technical Field

[0001] The present invention relates to a porous membrane used in the fields of water treatment, pharmaceutical manufacturing, food industry, fermentation, and the like.

Background Art

[0002] In recent years, porous membranes such as microfiltration membranes and ultrafiltration membranes have been used in various fields such as the water treatment field for purifying water or treating wastewater, the medical field such as blood purification, and the food industry field. Recently, there has been a demand for a highly fouling-resistant porous membrane that can efficiently filter even a filtration stock solution that has been difficult to filter and is prone to fouling. A filtration stock solution that is prone to fouling is characterized in that the amount of contaminants contained in the filtration stock solution is larger than normal, and the contaminants often contain organic substances. Examples of organic substances generally include proteins, polysaccharides, humic acids, and the like.

[0003] However, when filtering the above-described fouling-prone filtration stock solution, there is a problem that the porous membrane is clogged by the large amount of contaminants contained in the filtration stock solution, and as a result, the filtration flux of the porous membrane decreases. In particular, in the case of a fouling-prone filtration stock solution containing a large amount of organic substances, the interaction between the porous membrane and the organic substances causes the organic substances to adsorb to the porous membrane, etc., which is the starting point, and the pores of the porous membrane are easily clogged. Furthermore, when the porous membrane is clogged and the number of pores decreases, a high pressure is required during filtration, and the fouling components are pressed against the porous membrane surface at a high pressure, so that adsorption is more likely to occur. In order to suppress the decrease in the filtration flux, it is important to reduce the adsorption of organic substances to the porous membrane.

[0004] Here, as one method for eliminating the adsorption of organic substances and restoring the filtration flux, it is effectively used as a means to decompose and remove organic substances and the like accumulated on the porous membrane using chemicals such as oxidizing agents like sodium hypochlorite and alkalis like sodium hydroxide. However, such chemicals can not only decompose the accumulated organic substances but also decompose the polymer constituting the membrane. Therefore, if chemical cleaning is continuously performed, the membrane pores become coarsened, etc., making it difficult to maintain the separation performance and mechanical strength of the membrane over a long period.

[0005] On the other hand, Patent Documents 1 and 2 disclose methods for increasing the proportion of α-type structural crystals, which are stable crystals, in polyvinylidene fluoride resin (hereinafter also referred to as "PVDF resin"). It is disclosed that having more stable crystals improves chemical resistance and allows for an increase in cleaning intensity.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the α-type structural crystals in the porous membranes of polyvinylidene fluoride resins in Patent Documents 1 and 2 have a TGTG’ (T: Trans, G: Gauche) structure. Since the α-type structural crystals have a twisted structure, the dipole moments are arranged in alternating directions and have no polarity. Therefore, the interaction with polar water molecules is weak, and conversely, the interaction with organic substances is strong due to hydrophobic interaction. That is, a porous membrane made of polyvinylidene fluoride resin with α-type structural crystals is likely to adsorb organic substances and thus is likely to become blocked. Therefore, it is necessary to wash with a large amount of chemicals. Although Patent Documents 1 and 2 enhance the chemical resistance of the porous membrane, the chemicals are expensive and may also have an adverse effect on the module members equipped with the porous membrane and other processes. For example, the adhesive for bonding the porous membrane may deteriorate, resulting in a shortened lifespan of the porous membrane module. When using another separation membrane with relatively low chemical resistance after the porous membrane, it may lead to the deterioration of the subsequent separation membrane.

[0008] Patent Documents 2 and 3 disclose a porous membrane that contains not only α-type structural crystals but also a relatively metastable β-type structural crystals. The β-type structural crystals have a planar zigzag structure of TTTT, the dipole moments are arranged in the same direction, and there are many electron-donating components on the membrane surface, which is considered to have polarity. For example, Patent Document 2 discloses a membrane in which the ratio of the infrared absorbance derived from α-type structural crystals to the total infrared absorbance derived from α-type and β-type structural crystals is 32.5%. That is, a membrane is disclosed in which the ratio (H α ) of α-type structural crystals (H β ) to β-type structural crystals (H α / H β ) is 0.38. Patent Document 3 also discloses a membrane in which the abundance ratio of α-crystal structure to β-crystal structure is 0.11. Therefore, these membranes similarly have a strong interaction with polar water molecules, and conversely, organic substances are less likely to be adsorbed.

[0009] However, according to the findings of the present inventors, in the method for producing the porous membrane of Patent Document 2, the phase separation rate is relatively fast during film formation, and a porous membrane with a large pore diameter is likely to be formed. Since the pore diameter is large, the number of pores is not sufficient. In such a porous membrane, dirt components easily penetrate into the porous membrane and are likely to clog, and the desired filtration volume cannot be obtained. Although it is described in the casting solution that polyvinylidene fluoride resin is used as the polymer and a plurality of good solvents are mixed as the solvent, the self-diffusion coefficient of the polymer is not taken into account, and the hydrogen-bonding solvent described below is not included in the good solvent. Further, in the production method as described in Patent Document 3, the self-diffusion coefficient of the polymer is not taken into account in the same way. Moreover, the polymer content in the casting solution is as high as 30 wt%, the number of pores in the porous membrane to be formed is small, and the pure water permeability is likely to be low. Such a porous membrane requires a high pressure during filtration, and since the dirt components are pressed against the membrane surface at a high pressure, adsorption and blockage of the porous membrane are promoted, and sufficient permeate cannot be obtained in the filtration of a filtration stock solution with a large amount of dirt components.

[0010] The problem to be solved by the present invention is to obtain a porous membrane that can maintain high filtrate permeability even for a filtration stock solution that has been difficult to filter because significant fouling of the porous membrane occurs with respect to a filtration stock solution that is prone to fouling as described above. In the prior art, as in Patent Document 1, a porous membrane containing a large amount of α-type structure crystals that emphasizes chemical resistance has been common. As described above, in Patent Document 3, since the polymer concentration in the casting solution is high, the phase separation rate is likely to be relatively slow, and while it is easy to form a porous membrane containing a large amount of β-type structure crystals that are metastable phases, the number of pores in the porous membrane is small and the pure water permeability is likely to be low. Further, even when the polymer concentration in the casting solution is low as in Patent Document 2, while it is easy to form a porous membrane containing a large amount of β-type structure crystals by devising the use of a mixed solvent, since the phase separation rate is not sufficiently slow, a porous membrane with a large pore diameter is likely to be formed. That is, it has been difficult to form a porous membrane that has three elements: being difficult to adsorb by containing a large amount of β-type structure crystals, increasing the number of surface pores, securing a flow path, showing high pure water permeability, and having a relatively small surface pore diameter.

[0011] The present invention aims to provide a porous membrane in which a β-type structure crystal has excellent stain resistance, has high pure water permeability, reduces adsorption even for a filtration stock solution that is prone to fouling, and prevents fouling components from entering into the porous membrane due to a relatively small surface pore diameter, and can ensure a high filtration flux over a long period of time. [Means for Solving the Problems]

[0012] In order to solve the above problems, the present invention provides a porous membrane having the following configuration. (1) A porous membrane containing a polyvinylidene fluoride resin, with one surface being surface A and the other surface being surface B. The ratio (H α ) of the α-type structure crystal (H β ) to the β-type structure crystal (H α ) in the crystal part of the polyvinylidene fluoride resin measured by the ATR method (total reflection measurement method) on surface A is β , and the pure water permeability of the porous membrane is 0.25 m Not less than 0.25 and not more than 0.40 / m 3 / h / 50 kPa or more and 1.2 m 2 / m 3 / h / 50 kPa or less. The average value of the surface pore diameters of the surface pores on surface A of the porous membrane is 5.0 nm or more and 12.0 nm or less. The value X obtained by dividing the number [pieces / μm 2 of the surface pores on surface A of the porous membrane by the average value [nm] of the surface pore diameters of the surface pores is 30 pieces / μm 2 / nm or more and 100 pieces / μm 2 / nm or less. (2) The porous membrane according to (1), wherein the crystallinity of the vinylidene fluoride resin on surface A of the porous membrane is 30% or more. (3) The porous membrane according to (1) or (2), wherein the removal rate of 40,000 Da dextran of the porous membrane is 45% or more and 80% or less. (4) The porous membrane according to any one of (1) to (3), wherein the standard deviation of the surface pore diameters on surface A of the porous membrane is 1.0 nm or more and 5.0 nm or less. ​(5) The surface part of the porous membrane from the surface A to a thickness of 10 μm is the porous membrane according to any one of (1) to (4), which is denser than the inside. (6) A method for filtering a liquid using the porous membrane according to any one of (1) to (5). (7) A membrane filtration device comprising the porous membrane according to any one of (1) to (5). (8) In a water production method including a pretreatment step of pretreating raw water to obtain reverse osmosis membrane feed water and a reverse osmosis membrane treatment step of filtering the reverse osmosis membrane feed water with a reverse osmosis membrane to obtain permeated water, the raw water is raw water with a biopolymer concentration of 100 μgC / L or more, and the reverse osmosis membrane feed water with a biopolymer concentration of 75 μgC / L or less after pretreatment is supplied to the reverse osmosis membrane. The pretreatment step has 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 (5). It is a water production method. (9) The number of surface pores on the surface A of the porous membrane is 200 pores / μm 2 or more and 2000 pores / μm 2 or less. It is the water production method according to (8). (10) In the ultrafiltration membrane treatment of the pretreatment step, the raw water does not contain a flocculant. It is the water production method according to (8) or (9).

Effect of the Invention

[0013] According to the present invention, a porous membrane with a high proportion of β-type structure crystals to which organic substances hardly adhere, a small surface pore diameter, and a sufficient number of pores can be obtained. Thereby, it is possible to provide a porous membrane that has high water permeability and excellent fouling resistance and can ensure high filtrate permeability over a long period even for a filtration stock solution that is easily fouled.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the present invention is not limited thereto. In this specification, "mass" and "weight" are synonymous. Also, the filtration stock solution that is easily soiled is characterized in that the amount of dirt substances contained in the filtration stock solution is more than normal, and the dirt substances often contain useful substances to be removed and the like. Organic substances generally contain a large amount of, for example, proteins, polysaccharides, humic acids, etc.

[0016] The dirt related to the "anti-fouling property" of the present invention represents that the porous membrane is blocked by organic substances contained in the filtration stock solution in the filtration process. When the blockage of the porous membrane by organic substances progresses, the filtration resistance increases and the amount of liquid that can permeate through the porous membrane decreases.

[0017] Generally, a porous membrane having an α-type crystal structure easily adsorbs organic substances. To solve this problem, a method of decomposing and removing the accumulated organic substances and the like using a large amount of chemicals has been used as an effective means. However, such chemicals not only decompose the accumulated organic substances but also decompose the polymer constituting the membrane, and may further have an adverse effect on the module member equipped with the porous membrane and another process. Therefore, a porous membrane having a β-type crystal structure with excellent fouling resistance is promising, but there has been no porous membrane having a β-type crystal structure and high water permeability. The present invention relates to a membrane having a β-type crystal structure, and further has high pure water permeability, so that adsorption can be reduced even for a filtration stock solution that is easily fouled, and a high filtration flux can be ensured. The porous membrane with improved fouling resistance of the present invention will be described below.

[0018] <Regarding the porous membrane> The porous membrane according to an embodiment of the present invention is a porous membrane containing a polyvinylidene fluoride resin, having a surface A and a surface B, and in the IR spectrum measured by the ATR method (total reflection measurement method) of the surface A, in the crystal part of the polyvinylidene fluoride resin, the ratio (H α ) of the α-type crystal (H β ) and the β-type crystal (H α / H β ratio) is 0 or more and 0.5 or less, and the pure water permeability is 0.25 m 3 / m 2 / h / 50 kPa or more and 1.2 m 3 / m 2 / h / 50 kPa or less. Note that the ratio of H α / H β being 0 indicates that the β-type crystal (H β ) is 100%.

[0019] The ratio (H α ) of the α-type crystal (H β ) and the β-type crystal (H α / H β) needs to be 0 or more and 0.50 or less. Here, as the crystal structures of polyvinylidene fluoride resin, three structures, namely α-type, β-type, and γ-type with an extremely small amount of existence, are known. Among these, generally, attention was paid to the α-type and β-type, which are known to form more compared to the γ-type. The β-type structure crystal has a planar zigzag structure of TTTT, the dipole moments are arranged in the same direction, there are many electron-donating components on the film surface, and it is considered to have polarity. Therefore, the interaction with water molecules having the same polarity is strong, and conversely, it is difficult for organic substances to adsorb. H α / H β is 0 or more and 0.50 or less to exhibit excellent stain resistance against organic substances. In the crystal part of the polyvinylidene fluoride resin, the ratio of the α-type structure crystal (H α ) to the β-type structure crystal (H β ) (the ratio of H α / H β ) can be calculated by the following formula (1), more preferably 0.25 or more and 0.40 or less, and even more preferably 0.30 or more and 0.40 or less. H α / H β The smaller the ratio of, the more preferable the β-type structure crystal. Ratio of α-type / β-type structure crystal = H α / H β ···· Formula (1).

[0020] The porous membrane of the present invention is characterized in that there are many β-type structure crystals (H β ) in the crystal part of the polyvinylidene fluoride resin. In the prior art, the β-type structure crystal (H β) In order to obtain , it was necessary to increase the concentration of the PVDF polymer in the film-forming process. When the polymer concentration of the PVDF polymer is high, the resulting porous membrane has an increased polymer density, resulting in a low void fraction and a small number of pores. Although there have been attempts to reduce the PVDF polymer concentration in the film-forming process to obtain β-type structure crystals, since the concentration is low, large pores are likely to be formed, resulting in a sparse porous membrane with a small number of pores. In contrast, the porous membrane of the present invention can generate a large amount of fine pores even when 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, by setting a relatively slow phase separation rate, the pore diameter can be reduced and a large number of pores can be generated. The self-diffusion coefficient of the polymer can be used as an index of the phase separation rate, and by optimizing the polymer concentration, the solvent and temperature of the coagulation bath, a porous membrane with a large amount of β-type structure crystals, excellent antifouling properties, a small surface pore diameter, and a large number of pores can be obtained.

[0021] The porous membrane of the present invention is mainly composed of a crystalline polymer resin containing polyvinylidene fluoride, and within 50 μm from the surface A of the porous membrane, if the polymer resin constituting the porous membrane has crystals, the crystallinity is not limited, but the crystallinity of PVDF is more preferably 30% or more. When the crystallinity is 30% or more, it is easy to suppress the deformation of the pores when the turbidity collides with the porous membrane, and it is also easy for the turbidity to bounce back from the membrane surface, which is preferable. In order to better suppress the deformation of the pores and the bouncing back of the turbidity, the crystallinity is more preferably 35% or more, and even more preferably 40% or more. However, when it exceeds 80%, the flexibility of the porous membrane is lost, and it is likely to be damaged, for example, in operations such as cross-flow operation, so it is preferably 80% or less. H α / H βBy having a small ratio and a crystallinity of 30% or more, it is easy to obtain the effect that the interaction with polar water molecules is strong and it is difficult for organic substances to adsorb. When determining the crystallinity of the porous membrane, it can be calculated from the measurement results of a differential scanning calorimeter (hereinafter referred to as DSC). For the section within 50 μm from the outer surface of the porous membrane used for measuring the crystallinity, it is collected and used with a commercially available cryomicrotome. With the microtome, after moving the porous membrane at a certain moving distance, the blade can be brought into contact with the porous membrane for cutting. The blade is installed in a direction parallel to the surface of the porous membrane. First, move the distance closer to the blade at 5 μm intervals and cut the porous membrane once. Then, with the moving distance set to 40 μm, cut it one more time, and it is possible to collect a thickness of 40 μm to 45 μm from surface A.

[0022] The pure water permeability is 0.25 m 3 / m 2 / h / 50 kPa or more and 1.2 m 3 / m 2 / h / 50 kPa or less, it becomes possible to perform a filtration operation at a relatively low pressure even for a filtration stock solution that is prone to fouling. Since the fouling components are not pressed against the membrane surface at a high pressure, adsorption and blockage of the porous membrane are suppressed, and sufficient permeate can be obtained even in the filtration of a filtration stock solution with a large amount of fouling components. 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 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.

[0023] Fig. 1 shows a schematic diagram of filtering a stock solution with high filtration difficulty using the porous membrane of the present invention. Fig. 1 is a conceptual schematic diagram showing a part of the cross-section of the porous membrane. In Fig. 1, the solid line drawn on the surface portion 102 schematically shows a mesh structure. The filtration direction FL is the direction from the surface portion 102 to the interior 103. The organic matter in the stock solution often contains a coarse dirt component 201 and a fine dirt component 202. Since the porous membrane is a β-type structure crystal, it has a high polarity and suppresses the adsorption of the coarse dirt component 201 and the fine dirt component 202. Furthermore, because of its high pure water permeability, the fine dirt component 202 can be passed through the porous membrane at a relatively low pressure. At this time, since the fine dirt component 202 is not strongly pressed against the pores in the porous membrane, it is easy to suppress the adsorption and blockage of the fine dirt component 202 to the porous membrane, and a high filtrate permeability can be maintained. Also, the porous membrane 101 includes a surface portion 102 and an interior 103. Since the surface portion 102 has a denser structure than the interior 103, dirt components are less likely to penetrate into the interior of the porous membrane, so it is easy to exhibit excellent fouling resistance, which is preferable. When the fine pores of the surface portion 102 are dense, the coarse dirt component 201 cannot penetrate into the porous membrane and does not block the surface layer 102 of the porous membrane, and the porous membrane is not easily fouled, which is preferable.

[0024] Fig. 2 shows a schematic diagram of filtering a stock solution with high filtration difficulty using a porous membrane having many large pores with the α-type structure crystals known so far. Since the porous membrane has many α-type structure crystals, it has a low polarity, and the coarse dirt component 201 and the fine dirt component 202, which are organic substances, are likely to be adsorbed to the porous membrane by hydrophobic-hydrophobic interaction. Also, because the pore diameter is large, the coarse dirt component 201 easily penetrates to the inside of the porous membrane and is easily clogged. Furthermore, because the number of pores is small, the filtrate permeability is low.

[0025] Furthermore, while there are relatively many β-type structure crystals known so far, a schematic diagram of filtering a filtration stock solution with a high filtration difficulty using a porous membrane with low pure water permeability is shown in Fig. 3. Such a porous membrane often has a small number of pores and low pure water permeability. Since the porous membrane has many β-type structure crystals, the adsorption of coarse dirt components 201 and fine dirt components 202 is relatively small. However, because of the low pure water permeability, a high pressure is required for the filtration stock solution containing the fine dirt component 202 to pass through the porous membrane. Therefore, the fine dirt component 202 is strongly pressed against the pores in the porous membrane, and adsorption into the porous membrane gradually occurs. Therefore, the filtrate permeability tends to be low.

[0026] The porous membrane of the present invention further has a dextran removal rate of 45% or more and 80% or less for dextran with a weight average molecular weight of 40,000 Da, thereby preventing coarse dirt substances and substances to be removed in the filtration stock solution from entering the membrane and showing excellent fouling 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) using a dextran aqueous solution prepared so that commercially available dextran with a weight average molecular weight of 40,000 Da is 1000 ppm and the temperature is 25°C, filtering the porous membrane at a cross-flow linear velocity of 1.0 m / sec and a transmembrane differential pressure of 10 kPa. T ={(refractive index of the stock solution)-(refractive index of the permeate)} / (refractive index of the stock solution)×100 ····Formula (2) Here, the cross-flow linear velocity is a value obtained by dividing the flow rate in a direction perpendicular to the filtration direction of the filtration stock solution by the cross-sectional area of the flow path of the flow. The transmembrane differential pressure is the difference between the pressure on the filtration stock solution side and the pressure on the permeate side across the porous membrane.

[0027] When the average value of the surface pore diameter [nm] of the surface portion of the porous membrane is 5.0 nm or more and 12 nm or less, it is possible to prevent coarse dirt components and objects to be removed in the filtration stock solution from entering the porous membrane, and it is likely to exhibit high fouling resistance, which is preferable. More preferably, the average value of the surface pore diameter [nm] of the surface portion is 5.0 nm or more and 8.0 nm or less. The surface pore diameter is the diameter of the pores in the plane when observing the surface of the porous membrane. When determining the surface pore diameter of the porous membrane, an image obtained by SEM observing the surface of the porous membrane is binarized using the free software "ImageJ". When binarizing, after creating a background with 1 pixel using Subtract Background, select the condition: RenyiEntropy with Threshold (the binarization threshold). In the obtained binarized image, by selecting Area with Analyze Particles, the area of each pore is obtained, and the diameter calculated assuming each pore is a circle is taken as the surface pore diameter. When obtaining the average value of the surface pore diameter, the pore diameters of more than a thousand pores are averaged.

[0028] When the standard deviation of the surface pore diameter [nm] of the porous membrane is 1.0 nm or more and 5.0 nm or less, the load of dirt substances on the porous membrane can be made uniform, and it is likely to exhibit excellent fouling resistance, which is preferable. When the standard deviation is 1.0 nm or more and 5.0 nm or less, the load of dirt substances on the porous membrane can be made uniform, and it is likely to exhibit excellent fouling resistance, which is preferable. More preferably, the standard deviation of the surface pore diameter is 1.0 nm or more and 4.0 nm or less, and even more preferably 1.0 nm or more and 3.0 nm or less. The standard deviation of the surface pore diameter can be obtained by binarizing and analyzing the SEM image observing the surface of the porous membrane as described above.

[0029] The number of surface pores observed on the surface of the porous membrane is 200 pores / μm 2 ~2000 pores / μm 2By being such, it is possible to disperse the dirt components in the filtrate within the porous membrane, and it is preferable because it is easy to perform the filtration of high-turbidity water stably for a long time. Even if some of the surface pores are blocked by the dirt components in the filtrate as the filtration progresses, if the number of surface pores is large, it is easy to sufficiently secure the number of flow paths through which the filtrate passes through the porous membrane, which is preferable. If the number of surface pores is 200 pores / μm 2 or more, even for a filtrate that is prone to fouling, it is easy to sufficiently secure the flow paths until cleaning such as backwashing is performed. The number of surface pores of the porous membrane is 290 pores / μm 2 ~1500 pores / μm 2 is more preferable, and 350 pores / μm 2 ~1000 pores / μm 2 is particularly preferable. The number of surface pores of the porous membrane is the number of pores existing in the plane when observing the surface of the porous membrane. When determining the number of surface pores of the porous membrane, the image obtained by SEM observing the surface of the porous membrane is binarized using the free software "ImageJ". When binarizing, after creating the background as 1 pixel with Subtract Background, select the condition: RenyiEntropy with Threshold (the threshold of binarization). In the obtained binarized image, determine the number of pores in the observed range by Analyze Particles. When determining the number of surface pores per unit area [pores / μm 2 , observe more than a thousand pores and divide the number of those pores by the total area of the observed region. The value obtained by dividing the number of surface pores by the average value of the surface pore diameter: X is 30 pores / μm 2 / nm or more and 100 pores / μm 2 / nm or less, because it can prevent the dirt components in the filtrate from entering the pores and can sufficiently secure the number of flow paths through which the permeate passes through the porous membrane, it is easy to exhibit permeate properties, which is preferable. X is 50 pores / μm 2 / nm or more and 100 pores / μm 2 / nm or less is more preferable, and 60 pores / μm 2 / nm or more and 100 pores / μm 2It is more preferable that it is below / nm. The number of surface pores of the porous membrane can be obtained by binarizing the image obtained by SEM observing the surface of the porous membrane in the same manner as when obtaining the surface pore diameter, and analyzing it with Analyze Particles. By dividing the area of the analyzed image by the number of surface pores, the number of pores per unit area is obtained. Similar to the pore diameter, an image containing 1000 or more pores is analyzed and calculated.

[0030] In addition, the porous membrane in the present application preferably has a nano-network structure on its surface portion. The nano-network structure means that in the surface portion of the porous membrane, in the outermost surface portion from the surface to a thickness of 2 μm, the average value of the cross-sectional pore diameter [nm] in the cross-section of the outermost surface portion is 1 nm to 99 nm, and the number of cross-sectional pores is 100 pores / μm 2 ~1000 pores / μm 2 It is. By having a nano-network structure, the porous membrane is likely to exhibit excellent antifouling properties, which is preferable. The cross-sectional pores refer to the pores observed in the cross-section of the outermost surface portion. Since the average value of the 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 prevent coarse dirt components and objects to be removed in the filtration stock solution from being captured on the outermost surface portion of the porous membrane and entering the porous membrane. Furthermore, since the number of cross-sectional pores is 100 pores / μm 2 ~1000 pores / μm 2 is large, the pores are formed vertically, horizontally, in all directions, and the flow paths are likely to be linearly connected, and it is difficult for fine dirt components to be clogged when passing through the porous membrane. Also, even when some of the pores are clogged, a flow path for the filtration stock solution to pass through the porous membrane can be secured, so the porous membrane is likely to exhibit excellent antifouling properties, which is preferable.

[0031] <Regarding the manufacturing method of the porous membrane> The porous membrane of the present invention includes a step (A) of dissolving a polymer in a solvent to obtain a polymer solution, and then a porous membrane forming step (B) of coagulating 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] calculated by the all-atom molecular dynamics calculation of the dissolved polymer is 0.8×10 -11 m 2 / sec~1.6×10 -11 m2 It can be obtained by a production method in which 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, at / sec.

[0032] The type of polymer used in step (A) is not particularly limited. 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, polyvinyl pyrrolidone, polymers of acrylic acid esters or methacrylic acid esters such as ethylene oxide, propylene oxide or polymethyl methacrylate, or copolymers thereof.

[0033] In particular, in order to use the porous membrane for filtration for a long period of time, it is preferable to periodically wash the accumulated dirt components with chemicals, and it is particularly preferable to contain a polyvinylidene fluoride resin having excellent chemical resistance. The polyvinylidene fluoride resin refers to a homopolymer of vinylidene fluoride or a copolymer of vinylidene fluoride. Here, 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. Examples of such fluorine-based monomers include vinyl fluoride, tetrafluoroethylene, hexafluoropropylene or chlorotrifluoroethylene. In the above copolymer of vinylidene fluoride, ethylene or the like other than the above fluorine-based monomers may be copolymerized to such an extent that the effects of the present invention are not impaired.

[0034] The polyvinylidene fluoride resin preferably contains 50% by weight or more, and particularly preferably 60% by weight or more, based on the weight of the porous membrane being 100%. The weight average molecular weight of the polymer is preferably 5 to 1 million Da, as it is easy to control the self-diffusion coefficient described later within a relatively slow and suitable range. Also, a plurality of types of polymers may be mixed and used.

[0035] The solvent preferably contains a good solvent. Here, the "good solvent" refers to a solvent that can dissolve 5% by weight or more of the polymer even in a low-temperature range of 60°C or lower. Examples of the good solvent include N-methyl-2-pyrrolidone (hereinafter, "NMP"), 2-pyrrolidone (hereinafter, "2P"), ε-caprolactam (hereinafter, "ε-CL"), dimethyl sulfoxide (hereinafter, "DMSO"), dimethylacetamide (hereinafter, "DMAc"), dimethylformamide (hereinafter, "DMF"), methyl ethyl ketone, acetone, tetrahydrofuran, tetramethylurea, or trimethyl phosphate, or a mixed solvent thereof. It is more preferable that the good solvent accounts for 40% by weight or more of the solvent, and particularly preferably 60% by weight or more. By containing a large amount of the good solvent, the polymer chains expand in the polymer solution, and it is preferable because the self-diffusion coefficient described below can be easily controlled within a suitable range where it is relatively slow. Here, the "non-solvent" in step (B) refers to a solvent that does not dissolve or swell the polymer even when heated to the boiling point. Examples of the non-solvent include water, hexane, pentane, benzene, toluene, methanol, ethanol, carbon tetrachloride, o-dichlorobenzene, trichlorethylene, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butylene glycol, pentanediol, hexanediol, or a low-molecular-weight polyethylene glycol, such as aliphatic hydrocarbons, aromatic hydrocarbons, aliphatic polyhydric alcohols, aromatic polyhydric alcohols, chlorinated hydrocarbons, or other chlorinated organic liquids, or a mixed solvent thereof. In order to control the self-diffusion coefficient within a suitable range, the concentration (weight%) of the polymer in the polymer solution is preferably not less than the entanglement concentration. More specifically, 10 to 40% by weight is preferable, 12 to 30% by weight is more preferable, and 15 to 25% by weight is particularly preferable.

[0036] The porous membrane forming step of forming a porous membrane by coagulating the polymer solution of step (B) in a non-solvent is a step of forming a porous membrane by so-called non-solvent induced phase separation. When 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 rich in polymer and a phase rich in solvent. Each phase coarsens while merging with the same surrounding phase.

[0037] Figure 4 is a schematic diagram showing the formation process of the porous membrane of step (B). In the order from (a) to (f) of Figure 4, phase separation proceeds in the polymer solution, and the portion where the concentration of the polymer is high (phase 301 rich in polymer) coarsens. During this phase separation and / or coarsening process, the exchange of the solvent and the non-solvent proceeds. When the concentration of the non-solvent becomes higher than a certain level, the polymer solidifies and the structure of the porous membrane is fixed. At this time, the phase 302 rich in solvent becomes the pores of the porous membrane.

[0038] As a result of continued intensive studies, the inventors found that in the production of the porous membrane of the present invention, when the self-diffusion coefficient of the polymer is in 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 it is possible to solidify in a state where the phase 301 rich in polymer and the phase 302 rich in solvent each exist in a fine and large amount. When 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 are likely to form pores. When the diffusion of the polymer is so slow that phase separation is difficult, it is difficult to form pores. When the self-diffusion coefficient is 1.6×10 -11 m 2 / sec or less, the progress of excessive phase separation and coarsening can be suppressed, and the coalescence of pores can be suppressed, making it easy to form many fine pores. That is, by appropriately controlling the self-diffusion coefficient of the polymer in a relatively low range, a porous membrane having many fine pores can be formed. The self-diffusion coefficient is 0.8×10 -11 m 2 / sec to 1.4×10 -11 m 2It is more preferably / sec, and 0.8×10 -11 m 2 / sec to 1.1×10 -11 m 2 / sec is more preferable.

[0039] Examples of the method for determining the self-diffusion coefficient include the method determined by all-atom molecular dynamics calculation. All-atom molecular dynamics calculation is a method for obtaining the trajectory of each atom by solving the equations of motion of the molecular population system one by one for all constituent atoms. First, a polymer solution system is prepared so as to have the polymer concentration (weight %) actually used. At this time, one model polymer chain is modeled so that its molecular weight is 800 to 6000 and is 1 / 200 to 1 / 5 of the weight average molecular weight of the polymer actually used. As the potential parameters used in the molecular dynamics calculation, known parameters such as DREIDING [S.L. Mayo, B.D. Olafson, W.A. Goddard III, J. Phys. Chem. 94, 8897 (1990)], GAFF [J. Wang, R.M. Wolf, J.W. Caldwell, P.A. Kollman, D.A. Case, J. Comput. Chem. 25, 1157 (2004)], OPLS-AA [W.L. Jorgensen, D.S. Maxwell, Julian Tirado-Rives, J. Am. Chem. Soc. 118, 11225 (1996)], CHARMM [B.R. Brooks, R.E. Bruccoleri, B.D. Olafson, D.J. States, S. Swaminathan, M. Karplus, J. Comput. Chem. 4, 187 (1983)] can be used, but parameters that reproduce physical quantities such as density and aggregation energy, which are physical quantities representing the aggregation state of the solution system, are preferable.

[0040] As a result of the inventors' examination of the reproducibility of the aggregated state, it was found that it is particularly preferable to use GAFF or OPLS-AA. Also, a constant pressure-temperature ensemble is constructed by controlling the temperature to 25 °C by the Nose-Hoover method [Hoover, W.G. Phys. Rev. A, 31, 1695 (1985).] and the pressure to 1 bar by Andersen's method [H.C. Andersen, J. Chem. Phys. 72, 2384 (1980)]. At this time, for the short-range Lennard-Johns interaction, a switch function is applied from 1.0 nm and cutoff is performed at 1.2 nm, and the long-range electrostatic interaction is calculated by the Particle Mesh Ewald method. After performing molecular dynamics calculations until the density becomes constant in the constant pressure-temperature ensemble, the unit cell length is adjusted to the average density, and additional calculations of 11 ns are performed in the constant temperature ensemble. Using a 10-ns trajectory, the mean square displacement (MSD) of each atom of the polymer is obtained, and the self-diffusion coefficient of the polymer is calculated from the following formula (3). At this time, it is confirmed that the range of MSD and t used for the calculation of D is such that the value obtained by dividing log(MSD) by log(t) is in the range of 0.9 to 1.1. When a polymer solution is prepared by mixing multiple types of polymers, the self-diffusion coefficient of each polymer is the weighted average value based on the weight percentage of the polymer, which is the self-diffusion coefficient of the polymer in the polymer solution.

[0041] D = MSD / 6t ····· Formula (3) D: Self-diffusion coefficient t: Time In the production of the porous membrane of the present invention, since the non-solvent used for coagulation contains 90 to 100% by weight of water, coagulation is rapid, and the self-diffusion coefficient in the polymer solution is likely to affect the rates of phase separation and coarsening. Also, since the temperature of the non-solvent is 6°C to 45°C, coagulation is rapid, and the self-diffusion coefficient in the polymer solution is likely to affect the rates of phase separation and coarsening. The temperature of the non-solvent is preferably 35°C to 40°C. By setting the temperature of the non-solvent to 35°C to 40°C, coagulation becomes particularly rapid, and a large number of β-type structure crystals can be formed in the crystal form. That is, as described above, while suppressing the self-diffusion coefficient of the polymer in the polymer solution to a low range, and efficiently exchanging the solvent in the polymer solution and the non-solvent in the coagulation bath to accelerate coagulation more, it is possible to increase the proportion of β-type structure crystals and obtain a porous membrane that satisfies both a large number of surface pores.

[0042] Regarding the production of the porous membrane of the present invention, in the above-described step (A), the solvent preferably contains a hydrogen-bonding solvent having a molecular weight of 500 Da or less and having hydrogen-bond donor properties and hydrogen-bond acceptor properties, because it is easy to control the self-diffusion coefficient of the polymer within an appropriate range. Having hydrogen-bond donor properties means having a positively polarized hydrogen atom, and specifically refers to having a hydroxy group (OH group), a carboxyl group (COOH group), an amino group (NH group), etc. Having hydrogen-bond acceptor properties means having a lone pair of electrons, and specifically refers to having a carbonyl group, an alkoxy group, a cyano group, etc. Since the solvent has hydrogen-bond donor properties and hydrogen-bond acceptor properties, the interaction due to hydrogen bonding between the solvents is strong, suppressing the movement of the solvated polymer and making it easy to control the self-diffusion coefficient of the polymer to a relatively slow and suitable range. The solvents having hydrogen-bond donor properties and hydrogen-bond acceptor properties are not particularly limited, but specific examples include 2P, ε-CL, 1,3-dimethylurea, N-methylacetamide, hydantoin, 2-imidazolidinone, DL-pyroglutamic acid, and the like.

[0043] In the above step (A), it is preferable that the polymer to be dissolved includes a polymer having hydrogen bond donor property and / or hydrogen bond acceptor property, because it is easy to control the self-diffusion coefficient of the polymer within an appropriate range. Since the polymer has hydrogen bond donor property and / or hydrogen bond acceptor property, it interacts with a solvent having hydrogen bond donor property and hydrogen bond acceptor property by hydrogen bonding, and it is easy to control the self-diffusion coefficient of the polymer within a relatively slow and suitable range. From the viewpoint of controlling the self-diffusion coefficient of the polymer within an appropriate range, it is more preferable that the polymer having hydrogen bond donor property and / or hydrogen bond acceptor property is contained in the porous membrane in an amount of 10% by weight to 50% by weight.

[0044] Furthermore, when the value obtained by dividing the number of moles of the hydrogen bond acceptor functional group contained in the solvent in the above step (A) by the number of moles of the hydrogen bond donor functional group contained in the polymer having hydrogen bond donor property and / or hydrogen bond acceptor property is 1.0 to 12, the hydrogen bond between the polymer and the solvent is within an appropriate range, and it is easy to control the self-diffusion coefficient of the polymer within a relatively slow and suitable range. It is more preferable that the value is 2.0 to 9.0, and particularly preferable that the value is 5.4 to 8.0.

[0045] When the value obtained by dividing the number of moles of the hydrogen bond donor functional group contained in the solvent in the above step (A) by the number of moles of the hydrogen bond acceptor functional group contained in the polymer having hydrogen bond donor property and / or hydrogen bond acceptor property is 0.5 to 5.0, the hydrogen bond between the polymer and the solvent is within an appropriate range, and it is easy to control the self-diffusion coefficient of the polymer within a relatively slow and suitable range. It is more preferable that the value is 1.5 to 2.3, and particularly preferable that the value is 1.8 to 2.3.

[0046] The porous membrane of the present invention may further include other layers. In that case, it is preferable that the porous membrane of the present invention is disposed on the surface portion. By disposing the porous membrane of the present invention on the surface portion, it is difficult for the components contained in the stock solution for filtration to penetrate into the inside of the porous membrane, and high permeation performance can be maintained over a long period of time. The above-mentioned other layer is not particularly limited as long as it is a component capable of overlapping with the porous membrane to form a layered structure, but it is preferable that the above-mentioned other layer is a support. Here, the "support" refers to a structure having a higher breaking strength than the porous membrane for physically reinforcing the porous membrane. In order to increase the breaking strength of the support, the breaking strength (breaking strength per unit area) of the support is preferably 3 MPa or more, and more preferably 10 MPa or more. When the composite membrane in which the other layer and the porous membrane of the present invention are arranged in layers is in the form of a hollow fiber, the breaking strength of the support is preferably 300 gf or more, and more preferably 800 gf or more.

[0047] Liquid can be filtered by the membrane filtration device using the porous membrane of the present invention obtained as described above. Examples of the membrane filtration device include, but are not limited to, those equipped with a raw water tank, a booster pump, a module composed of several thousand to several tens of thousands of the porous hollow fiber membranes of the present invention, a filtered water tank, a backwash pump, and the like. As a method for filtering a liquid, for example, a stock solution such as industrial wastewater can be operated at an operating pressure of 10 kPa to 1 MPa using the above membrane filtration device to remove organic substances and the like contained in the raw water, but it is not limited thereto.

[0048] The water production system using a reverse osmosis membrane has been applied in many industries and water treatment fields, starting from the desalination of seawater, and has been proven to be superior in terms of separation performance, energy efficiency, etc. compared to other separation methods. On the other hand, in this water production system, there is a problem that the membrane differential pressure rises rapidly due to the growth of microorganisms on the membrane surface or the attachment of a biofilm (biofouling) to the membrane surface, resulting in a decrease in the permeability and separability of the membrane.

[0049] When the membrane differential pressure increases or the permeability and separation performance of the membrane decrease due to biofouling, it is common to clean the membrane. Examples of cleaning methods include so-called flushing cleaning, where filtration is temporarily stopped and raw water or filtrate is supplied to the membrane surface for cleaning, and chemical cleaning using a cleaning agent. However, once fouling progresses, even if cleaning is performed, the membrane differential pressure, permeability, and separation performance do not fully recover, the frequency of cleaning gradually increases, and eventually, operation becomes impossible, requiring membrane replacement.

[0050] In Japanese Patent Publication Nos. 6561082 and 6630689, attention is paid to biopolymers contained as part of the organic matter in the water to be treated, and a method of adjusting the biopolymers in the water to be treated to be below a predetermined threshold value and performing membrane separation is disclosed. The predetermined threshold value is either a value in the range of 9 μg / L or more and 12 μg / L or less, or 9 μg / L or more and 17 μg / L or less. Biopolymers are hydrophilic polymer organic substances (such as polysaccharides and proteins) having a molecular weight of generally 10 to 20 kDa or more among organic substances.

[0051] However, as a result of measuring the biopolymer concentration in seawater during the inventors' study of the present invention, it was 122 to 218 μgC / L, and the predetermined threshold values disclosed in Japanese Patent Publication Nos. 6561082 and 6630689 were extremely low concentrations. In water to be treated such as general seawater and treated sewage, the biopolymer concentration is high as described above, and it is difficult to achieve the predetermined threshold values disclosed in Japanese Patent Publication Nos. 6561082 and 6630689. In addition, it is assumed that a microfiltration membrane or an ultrafiltration membrane is used as the supply target instead of a reverse osmosis membrane, and since it is different from the fouling mechanism of a reverse osmosis membrane, it is difficult to apply it as it is.

[0052] Therefore, when obtaining fresh water by desalinating seawater, brackish water, etc. using a membrane, or when obtaining recycled water by purifying treated wastewater, industrial wastewater, etc., the present invention suppresses the progress of fouling in the pretreatment process and the reverse osmosis membrane for the water to be treated with a high bioclogging potential. To solve the above problems, the water production method in the present invention comprises any of the following configurations. In a water production method including a pretreatment step of pretreating the water to be treated to obtain reverse osmosis membrane feed water and a reverse osmosis membrane treatment step of filtering the reverse osmosis membrane feed water with a reverse osmosis membrane to obtain permeate water, the water production method is such that the water to be treated with a biopolymer concentration of 100 μgC / L or more is supplied to the reverse osmosis membrane as the reverse osmosis membrane feed water with the biopolymer concentration reduced to 75 μgC / L or less by pretreatment, the pretreatment step has ultrafiltration membrane treatment, and the ultrafiltration membrane is the porous membrane described above.

[0053] The water production method of the present invention is implemented in a water production system in which the water to be treated 401 is pretreated with an ultrafiltration membrane and then treated with a reverse osmosis membrane 402 to be separated into permeate water 403 and concentrated water 404. Examples of the water to be treated include seawater, brackish water, river water, lake water, groundwater, sewage, secondary treated sewage, etc. Since the water to be treated contains solid components such as turbidity, if it is directly filtered with a reverse osmosis membrane, the amount of solid components adhering to the membrane surface increases, the differential pressure rises rapidly, and operation becomes impossible. Therefore, the water to be treated 401 is pretreated in the pretreatment step 405 in advance and then supplied to the reverse osmosis membrane as the reverse osmosis membrane feed water 406.

[0054] In the present invention, in order to suppress the progress of fouling of the reverse osmosis membrane, the treated water 1 with a biopolymer concentration of 100 μgC / L or more is supplied to the reverse osmosis membrane as the reverse osmosis membrane feed water 406 with a biopolymer concentration of 75 μgC / L or less by pretreatment. It is more preferable to supply the reverse osmosis membrane feed water 406 to the reverse osmosis membrane by setting the biopolymer concentration of the treated water 401 with a biopolymer concentration of 100 μgC / L or more to 50 μgC / L or less by pretreatment. On the other hand, a great deal of energy and cost are required to reduce the biopolymer concentration of the treated water 401 with a biopolymer concentration of 100 μgC / L or more to less than 20 μgC / L by pretreatment. It is preferable to supply the reverse osmosis membrane feed water 406 to the reverse osmosis membrane with the biopolymer concentration of 20 μgC / L or more.

[0055] Biopolymers are hydrophilic polymer organic substances (polysaccharides, proteins, etc.) with a molecular weight of generally 10 to 20 kDa or more among organic substances. As the definition and measurement method of biopolymers, for example, as described in Huber, S.A., Balz, A., Abert, M., Pronk, W., 2011. Characterisation 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). Water Research 45(2), 879-885, it can be measured by size-exclusion chromatography with organic carbon detection (LC-OCD). Here, the LC-OCD method is an analytical method that fractionates the TOC components in a sample by molecular weight and shows them as a chromatogram. On the chromatogram, organic substances with a larger molecular weight and higher hydrophilicity tend to have a shorter retention time. As the measurement conditions of the LC-OCD method, a 250 mm × 20 mm TSK HW50S column is used, the flow rate is set to 1.1 mL / min, the sample injection volume is set to 1 mL, the UV wavelength is set to 254 nm, the acid injection volume into the OCD meter is set to 0.2 mL / min, a pH 6.85 phosphate buffer is used as the eluent, and as the acidification solution, a solution prepared by adding 4 mL of O-phosphoric acid (85%) and 0.5 g of potassium peroxodisulfate to 1 L of ultrapure water can be adopted. As the measurement device used for the LC-OCD method, for example, an LC-OCD device (manufactured by DOC-Labar) obtained by connecting a wet total organic carbon meter (OCD meter) to high-performance liquid chromatography (HPLC) can be used.

[0056] The porous membrane of the present invention can be used as an ultrafiltration membrane. That is, the water production method of the present invention uses, as the ultrafiltration membrane, a porous membrane containing a polyvinylidene fluoride resin, having a surface A and a surface B, and in the ATR method (total reflection measurement method) of the surface A, the α-type structural crystal (H α ) and β-type structural crystal (Hβ ) ratio (H α / H β ) is 0 or more and 0.50 or less, and the pure water permeability is 0.25 m 3 / m 2 / h / 50 kPa or more and 1.2 m 3 / m 2 / h / 50 kPa or less, and a porous membrane having an average value of the surface pore diameter on the surface A of the porous membrane of 5.0 nm or more and 12.0 nm or less can be used.

[0057] As the ultrafiltration membrane, a porous membrane having a removal rate of 40 kDa dextran of 45% or more and 80% or less is preferable. When the dextran removal rate is 45% or more and 80% or less, it is possible to prevent coarse contaminants and objects to be removed in the filtration stock solution from entering the membrane, and it is easy to exhibit excellent fouling resistance. In addition, the bio-polymer removal rate of the membrane tends to be high, the amount of flocculant added can be reduced, or the bio-polymer concentration of the reverse osmosis membrane feed water can be reduced to 75 μgC / L or less without using any flocculant at all. That the treated water does not contain a flocculant means that no flocculant is added to the treated water, that is, the treated water does not use a flocculant. The flocculant is not limited to the type of compound as long as it can aggregate bio-polymers, and the concentration of the flocculant corresponds to 0% to 0.05%. In particular, it is particularly preferable that the removal rate of dextran having a weight average molecular weight of 40 kDa of the ultrafiltration membrane is 45% or more and 80% or less, and further 50% or more and 80% or less.

[0058] The reverse osmosis membrane 2 can be made of any material as long as it can reduce the salt concentration so that the treated water can be used for drinking water, industrial water, municipal water, etc. For example, those made of cellulose acetate-based or polyamide-based materials can be mentioned. Among these, those made of polyamide-based materials are particularly effective in the method of the present invention. Polyamide-based membranes have low resistance to chlorine, which is the most commonly used bactericide to prevent the growth of biofilms, and membrane deterioration occurs significantly even with a small concentration of chlorine, so it is difficult to prevent biofouling. Therefore, the effect of implementing the present invention appears significantly.

[0059] The lower the water permeability reduction rate of the reverse osmosis membrane, the longer the chemical cleaning interval of the reverse osmosis membrane and the lower the replacement frequency. In addition, when the reverse osmosis membrane is operated at a low flow rate, an increase in the operating differential pressure can be suppressed. As a result, the number of reverse osmosis membranes required for water production is reduced, which is preferable. The water permeability reduction rate of the reverse osmosis membrane is preferably 0.20 (% / hr) or less, and more preferably 0.18 (% / hr) or less.

Examples

[0060] Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto. First, the measurement method and evaluation method are shown below.

[0061] (i) Measurement of the ratio of α-type structure crystals and β-type structure crystals in the crystalline part of the PVDF resin Using an IRTracer-100 from Shimadzu Corporation, the IR spectrum of the membrane surface was measured by the ATR method (total reflection measurement method) with a resolution of 8 cm -1 . In the measurement area and the obtained spectrum, the peak height (H -1 ) of the signal of the α-type structure crystals appearing at the position of 763 cm α and the peak height (H -1 ) of the signal of the β-type structure crystals appearing at 840 cm β ) were used to calculate the ratio of α-type structure crystals and β-type structure crystals using the following formula.

[0062] Ratio of α-type / β-type structure crystals = H α / H β ······ Formula (1).

[0063] (ii) Pure water permeability A small module with a length of about 10 cm composed of about 1 to 10 hollow fiber membranes was fabricated. Distilled water was fed from surface A under the conditions of a temperature of 25 °C and a filtration differential pressure of 18.6 kPa, and the total amount was filtered. The value obtained by measuring the permeation water volume (m 3 ) for a certain period of time was calculated by converting it to per unit time (hr), per unit effective membrane area (m 2 ) and per 50 kPa.

[0064] (iii) Evaluation method for dextran removal rate with weight-average molecular weight of 40,000 Da Dextran (manufactured by Aldrich; weight-average molecular weight 40,000 Da) was mixed into 1000 ppm distilled water to prepare a dextran aqueous solution. The prepared dextran aqueous solution was supplied to a porous membrane at 25 °C with an intermembrane differential pressure of 10 kPa, and cross-flow filtration was performed at a cross-flow linear velocity of 1.0 m / sec, and the permeate was sampled. At the timing of sampling the permeate, the dextran aqueous solution (stock solution) supplied to the porous membrane was sampled. The refractive indices of the permeate and the stock solution were measured, and the removal rate: T was determined based on Equation (2). T = {(refractive index of stock solution) - (refractive index of permeate)} / (refractive index of stock solution) × 100 ··· Equation (2).

[0065] (iv) Measurement of crystallinity of polymer resin constituting the porous membrane Measurement of the crystallinity of the polymer resin of the porous membrane in the shape of a hollow fiber membrane is shown. For the measurement, a section within 50 μm from the outer surface of the hollow fiber membrane was used. The outer surface of the hollow fiber membrane was cut open with a commercially available cryomicrotome (manufactured by Leica; Jung CM3000) to collect samples. The hollow fiber membrane immersed in distilled water was frozen at -20 °C using a cryomicrotome (manufactured by Leica; Jung CM3000), and a blade was installed in a direction parallel to the surface of the hollow fiber membrane. First, the hollow fiber membrane was cut once with the blade approaching it at intervals of 5 μm in the moving distance. Then, with the moving distance set to 40 μm, it was cut one more time to collect a hollow fiber membrane section with a thickness of 40 - 45 μm from the surface. When the section was set in a DSC (manufactured by SEIKO: DSC6200) and heated 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 this heat quantity was divided by the heat of fusion of the completely crystalline polymer resin and calculated as a percentage. For example, for a polyvinylidene fluoride-based resin, the endothermic peak observed in the range of 100 - 190 °C was regarded as the heat of fusion of the polyvinylidene fluoride-based resin, and this heat quantity was divided by 104.6 J / g, which is the heat of fusion of the completely crystalline polyvinylidene fluoride-based resin, and calculated as a percentage.

[0066] (v) Regarding the surface structure (pore diameter, number, standard deviation) of the surface part After the porous membrane was vacuum-dried at 25°C overnight, it was observed at a magnification of 30,000 to 100,000 times using SEM (manufactured by Hitachi High-Technologies Corporation; S-5500). The image obtained by SEM observing the surface of the porous membrane was binarized using the free software "ImageJ". When binarizing, after creating the background with 1 pixel using Subtract Background, the condition: RenyiEntropy was selected with Threshold (the threshold of binarization). In the obtained binarized image, by selecting Area with Analyze Particles, the area of each pore was determined, and the diameter calculated assuming each pore as a circle was taken as the surface pore diameter. When obtaining the average value of the surface pore diameter, the pore diameters of 1000 or more pores were averaged. Similarly, the standard deviation of each surface pore diameter was obtained. The number of pores was divided by the area of the observed region to obtain the number of pores per unit area.

[0067] (vi) Self-diffusion coefficient of polymer The self-diffusion coefficient was determined by all-atom molecular dynamics calculations. A polymer solution system was prepared to have the polymer concentration (weight %) actually used. At this time, one model polymer chain was modeled such that the molecular weight was 800 to 6000 and the weight-average molecular weight of the polymer actually used was 1 / 200 to 1 / 5. GAFF2 was used for the potential parameters used in the molecular dynamics calculations. A constant pressure-temperature ensemble was constructed by controlling the temperature at 25 °C and the pressure at 1 bar. At this time, for the short-range Lennard-Johns interaction, a switching function was applied from 1.0 nm and cutoff was performed at 1.2 nm, and the long-range electrostatic interaction was calculated by the Particle Mesh Ewald method. After performing molecular dynamics calculations in the constant pressure-temperature ensemble until the density became constant, the unit cell length was adjusted to the average density, and additional calculations of 11 ns were performed in the constant temperature ensemble. Using a 10-ns trajectory, the mean squared displacement (MSD) of each atom of the polymer was obtained, and the self-diffusion coefficient was calculated from the following formula (3). At this time, it was confirmed that the range of MSD and t used for the calculation of D was such that the value obtained by dividing log(MSD) by log(t) was in the range of 0.9 to 1.1. When multiple types of polymers were mixed to form a polymer solution, the self-diffusion coefficient of each polymer was taken as the weighted average value based on the weight % of the polymer as the self-diffusion coefficient of the polymer in the polymer solution. D = MSD / 6t ····· Formula (3) D: Self-diffusion coefficient, t: Time.

[0068] (vii) Filtration evaluation method for chemical plant wastewater Wastewater from a chemical plant (TOC; 30 mg / L, turbidity; 11 NTU) was supplied to a porous membrane at 25 °C with a transmembrane pressure difference of 100 kPa and filtered in total, and the permeate volume was measured. When the permeate volume reached 28 L / m 2 and reverse filtration was carried out until the reverse filtration permeation volume became 3 L / m 2 at a transmembrane pressure difference of 150 kPa. Filtration and reverse filtration were repeated, and the filtration flux (F1) immediately after the start of filtration and the permeate volume per unit area of 400 L / m 2Ratio of the filtration flux immediately after the start of filtration (F2) to that at the start (F1) after the above steps: The initial ratio of the filtration flux was calculated. The closer the initial ratio of the filtration flux is to 1, the better the initial characteristics are maintained. A guideline for the porous membrane to be less likely to clog and enable good filtration even after long-term use is 50% or more.

[0069] (viii) Cross-sectional structure of the outermost surface (presence or absence of a nano-network structure, pore diameter, number, standard deviation) The porous membrane embedded using a commercially available embedding agent for frozen tissue sections (manufactured by Tissue-Tek; O.C.T. Compound) was cut into 100-nm-thick sections perpendicular to the surface at -40 °C using a cryo-ultramicrotome (manufactured by Leica; FC7), and vacuum-dried at room temperature for 12 hours. The cross-section of the outermost surface of the porous membrane was observed with a TEM (manufactured by JEOL Ltd.; JEM-1400Plus) to obtain an image, which was binarized using the free software "ImageJ". When binarizing, the condition "Minimum" was selected for Threshold (the binarization threshold). In the obtained binarized image, the area of each pore was determined by selecting "Area" in Analyze Particles, and the calculated diameter assuming each pore as a circle was taken as the cross-sectional pore diameter.

[0070] When obtaining the average value of the cross-sectional pore diameters, the pore diameters of 1000 or more pores were averaged. Also, by dividing the number of cross-sectional pores by the area of the analyzed region, the number of pores per unit area was obtained. Similar to the pore diameter, an image containing 1000 or more pores was analyzed and calculated. Similarly, the standard deviation of each cross-sectional pore diameter was obtained. The total perimeter of the cross-sectional pore diameters was determined by binarizing the TEM image of the cross-section of the porous membrane as described above and selecting "Perim" in Analyze Particles to obtain the perimeter of each pore, and then dividing the sum by the area of the observed region. In the outermost surface of the porous membrane, within the surface to a thickness of 2 μm from the surface, when the average value of the cross-sectional pore diameters [nm] in the cross-section of the outermost surface is 1 nm to 99 nm, and the number of cross-sectional pores is 100 pores / μm 2 ~1000 pores / μm 2 has a network structure, it was determined to have a nano-network structure.

[0071] (ix) Bio-polymer concentration, bio-polymer removal rate The bio-polymer concentrations of the water to be treated and the reverse osmosis membrane feed water were measured by size-exclusion chromatography with organic carbon detection (LC-OCD) as described in Huber, S.A., Balz, A., Abert, M., Pronk, W., 2011. Characterisation 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). Water Research 45(2), 879-885. A 250 mm × 20 mm TSK HW50S column was used, the flow rate was set at 1.1 mL / min, the sample injection volume was 1 mL, the UV wavelength was 254 nm, the acid injection volume into the OCD meter was 0.2 mL / min, a pH 6.85 phosphate buffer was used as the eluent, and for the acidification solution, 4 mL of O-phosphoric acid (85%) and 0.5 g of potassium peroxydisulfate were added to 1 L of ultrapure water.

[0072] (x) Reverse osmosis membrane water permeability decline rate The reverse osmosis membrane was operated at a pressure of 5.5 MPa and a recovery rate of 37%. The recovery rate is calculated as the flow rate of the permeate water through the reverse osmosis membrane / (the flow rate of the permeate water through the reverse osmosis membrane + the flow rate of the concentrated water through the reverse osmosis membrane) × 100. Here, the reverse osmosis membrane water permeability decline rate was determined based on Equation (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. Here, for each flow rate, the amount of permeate water (g) over 5 minutes was used. Reverse osmosis membrane water permeability decline rate [% / hr] = (amount of permeate water [g] 51 hours after the start of operation) / (amount of permeate water [g] 1 hour after the start of operation) / 50 × 100 ··· Equation (4).

[0073] (xi) Ultrafiltration membrane initial water permeability ratio The water to be treated was supplied to the ultrafiltration membrane so that the transmembrane differential pressure was 100 kPa and filtered in full volume, and the permeate water volume was measured. The permeate water volume was 28 L / m2 When it reaches this state, reverse osmosis permeation is carried out until the reverse osmosis permeation water volume becomes 3 L / m² so that the transmembrane differential pressure becomes 150 kPa. Filtration and reverse osmosis are repeated, and the flow rate F1(g) of the permeated water immediately after the start of filtration through the ultrafiltration membrane and the permeated liquid volume per unit area are 400 L / m² 2 or more. Then, the flow rate F2(g) of the permeated water immediately after the start of filtration is measured. Here, each flow rate is calculated using the permeated water volume (g) for 5 minutes, and the ratio of the flow rates of the permeated water (F2 / F1) is calculated and used as the initial permeability ratio. 2

[0074] (Example 1) As the porous membrane, a hollow fiber porous membrane containing a support obtained by the following production method was used. 38% by mass of PVDF (manufactured by Kuraray Co., Ltd.; KF1300, weight average molecular weight 350,000 Da) and 62% by mass of γ-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-orifice die while accompanying an 85% by mass aqueous solution of γ-butyrolactone as a hollow part forming liquid. The discharged support membrane stock solution was solidified in a cooling bath containing an 85% by mass aqueous solution of γ-butyrolactone at a temperature of 20 °C installed 30 mm below the die to produce a hollow fiber-shaped support having a spherical structure.

[0075] 12% by mass of PVDF1 (manufactured by Arkema; Kynar® 710, weight average molecular weight 180,000 Da), 4.8% by mass of cellulose diacetate (manufactured by Eastman; CA-398-3), 2.4% by mass of cellulose triacetate (manufactured by Eastman; CA-436-80S), 68.7% by mass of NMP, and 12.1% by mass of 2P were mixed and stirred at 120 °C for 4 hours to prepare a polymer solution having the composition ratio shown in Table 1.

[0076] Next, a polymer solution was uniformly applied to the outer surface of the above hollow fiber support at a rate of 10 m / min (with a thickness of 50 μm). One second after the application of the polymer solution, the coated support was immersed in a coagulation bath of distilled water at 35°C for 10 seconds for coagulation to form a porous membrane having a three-dimensional network 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 μm was denser than the interior. An image of the cross-section of the outermost surface portion of the porous membrane observed by TEM is shown in Figure 5. It was determined to have a nano-network structure based on the cross-sectional pore diameter and the number of cross-sectional pores. The H α / H β was 0.37, the pure water permeability was 0.43 m 3 / m 2 / h / 50 kPa, and the dextran removal rate was 55%, which was good. Also, the crystallinity of the resin in the outer surface portion was as high as 49%. Furthermore, the value X obtained by dividing the number of surface pores by the average value of the surface pore diameters was 66, the average surface pore diameter was 7.2 nm, and the standard deviation was 2.5. (vii) In the filtration evaluation method for factory wastewater, when evaluating the long-term stability, the initial ratio (F2 / F1) of the filtration flux between 400 L / m 2 after (F2) and immediately after the start of filtration (F1) was 0.63, and the filtration flux could be maintained even after long-term use.

[0077] (Example 2) In the membrane formation of Example 1, except that NMP in the polymer solution was changed to DMSO and the distilled water in the coagulation bath was set to 40°C, membrane formation was carried out in the same manner to obtain a porous membrane. The evaluation results of the porous membrane are shown in Table 1. The surface portion of the porous membrane from the surface to a thickness of 10 μm was denser than the interior. The H α / H β was 0.27, the pure water permeability was 0.29 m 3 / m 2 / h / 50 kPa, and the dextran removal rate was 63%, which was good.

[0078] Furthermore, X was 78, the average surface pore diameter was 7.3 nm, and the standard deviation was 1.8. (vii) In the filtration evaluation method for factory wastewater, when evaluating the long-term stability, the permeate volume was 400 L / m 2The initial ratio (F2 / F1) of the filtration flux between after (F2) and immediately after the start of filtration (F1) was 0.55, and the filtration flux could be maintained even after long-term use.

[0079] ( Reference Example 1 ) In the film formation of Example 1, a porous membrane was obtained in the same manner except that 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 part of the porous membrane from the surface to a thickness of 10 μm was denser than the inside. The H α / H β was 0.24, the pure water permeability was 0.45 m 3 / m 2 / h / 50 kPa, and the dextran removal rate was 47%, which was good.

[0080] Furthermore, X was 48, the average surface pore diameter was 8.5 nm, and the standard deviation was 3.1. (vii) In the filtration evaluation method of industrial wastewater, when evaluating the long-term stability, the permeate volume was 400 L / m 2 The initial ratio (F2 / F1) of the filtration flux between after (F2) and immediately after the start of filtration (F1) was 0.50, and the filtration flux could be maintained even after long-term use.

[0081] (Comparative Example 1) In the film formation of Example 1, a porous membrane was obtained in the same manner except that all the solvents in the polymer solution were NMP and the composition ratio was changed. The evaluation results of the porous membrane are shown in Table 2. The porous membrane did not have a nano-network structure. The surface part of the porous membrane from the surface to a thickness of 10 μm was denser than the inside. The H α / H β was 1.0, the pure water permeability was 0.41 m 3 / m 2 / h / 50 kPa, and the dextran removal rate was 55%. Furthermore, X was 13, the average surface pore diameter was 15 nm, and the standard deviation was 6.0. Also, the crystallinity of the resin in the outer surface part was as low as 11%. (vii) In the filtration evaluation method of industrial wastewater, when evaluating the long-term stability, the permeate volume was 400 L / m 2The initial ratio (F2 / F1) of the filtration flux between after (F2) and immediately after the start of filtration (F1) was 0.38, and the filtration flux could not be maintained after long-term use.

[0082] (Comparative Example 2) In the film formation of Comparative Example 1, a porous membrane was obtained in the same manner except that the coagulation bath temperature was set to 6°C. The evaluation results of the porous membrane are shown in Table 2. The surface part of the porous membrane from the surface to a thickness of 10 μm was denser than the inside. The H α / H β was 0.10, the pure water permeability was 0.10 m 3 / m 2 / h / 50 kPa, the dextran removal rate was 70%, and the conditions were not satisfied. Furthermore, X was 10, the average surface pore diameter was 8.0 nm, and the standard deviation was 1.9. (vii) In the filtration evaluation method of industrial wastewater, when the long-term stability was evaluated, the permeate volume was 400 L / m 2 The initial ratio (F2 / F1) of the filtration flux between after (F2) and immediately after the start of filtration (F1) was 0.37, and the filtration flux could not be maintained after long-term use.

[0083] (Comparative Example 3) In the film formation of Example 1, a porous membrane was obtained in the same manner as in Comparative Example 1 except that PVDF1 in the polymer solution was changed to PVDF2 (manufactured by Solvay: Solef9009) and the composition ratio was changed. The evaluation results of the porous membrane are shown in Table 2. The porous membrane did not have a nano-network structure. The surface part of the porous membrane from the surface to a thickness of 10 μm was denser than the inside. The H α / H β was 0.22, the pure water permeability was 0.21 m 3 / m 2 / h / 50 kPa, and the dextran removal rate was 57%. Furthermore, X was 7.6, the average surface pore diameter was 10 nm, and the standard deviation was 4.1, and the conditions were not satisfied. (vii) In the filtration evaluation method of industrial wastewater, when the long-term stability was evaluated, the permeate volume was 400 L / m 2 The initial ratio (F2 / F1) of the filtration flux between after (F2) and immediately after the start of filtration (F1) was 0.40, and the filtration flux could not be maintained after long-term use.

[0084] (Comparative Example 4) A polymer solution was prepared by mixing 20% by mass of PVDF3 (manufactured by Kureha Corporation; KF Polymer #110, weight average molecular weight 280,000 Da) and 80% by mass of a mixed solvent (DMAc:GBL = 89:11), and stirring at 120 °C for 4 hours. Other than setting the distilled water in the coagulation bath to 21 °C, a porous membrane was obtained in the same manner as in Example 1. The evaluation results of the porous membrane are shown in Table 2. The H α / H β was 0.63, the pure water permeability was 0.022 m 3 / m 2 / h / 50 kPa, the dextran removal rate was 66%, and the conditions were not satisfied. Furthermore, X was 0.90, the average surface pore diameter was 15 nm, and the standard deviation was 8.5. (vii) In the filtration evaluation method of industrial wastewater, when evaluating the long-term stability, the permeate volume was extremely small, and it was difficult to measure the filtration flux.

[0085] (Example 4) Using the flow shown in Fig. 6, permeated water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 1 (biopolymer concentration 122 μgC / L) was used as the water to be treated, and as a result of using the porous membrane of Example 1 for pretreatment, the biopolymer concentration of the reverse osmosis membrane feed water was 35 μgC / L. This reverse osmosis membrane feed water was pressurized with a high-pressure pump and filtered through a reverse osmosis membrane to obtain permeated water. The reverse osmosis membrane was a spiral-type reverse osmosis membrane with a membrane material of polyamide, a salt rejection rate of 99.8%, and a membrane area of 37 m 2 .

[0086] Here, the initial value ratio of the water permeability of the ultrafiltration membrane was 0.71, the water permeability reduction rate of the reverse osmosis membrane was 0.15% / hr, and despite the high biopolymer concentration in the water to be treated, the filtration of the ultrafiltration membrane was stable without using a flocculant, and furthermore, the filtration of the reverse osmosis membrane was also stable. The results are shown in Table 3.

[0087] (Example 5) With the flow shown in Fig. 6, permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 2 (biopolymer concentration: 218 μgC / L) was used as the water to be treated, and as a result of using the porous membrane of Example 1 for pretreatment, the biopolymer concentration of the reverse osmosis membrane feed water was 47 μgC / L. This reverse osmosis membrane feed water was pressurized with a high-pressure pump and then filtered through the reverse osmosis membrane to obtain permeate water. The reverse osmosis membrane was a spiral-type reverse osmosis membrane with a membrane material of polyamide, a salt rejection rate of 99.8%, and a membrane area of 37 m 2 .

[0088] Here, the initial water permeability ratio of the ultrafiltration membrane was 0.63, and the water permeability decline rate of the reverse osmosis membrane was 0.18% / hr. Despite the very high biopolymer concentration in the water to be treated, the filtration of the ultrafiltration membrane was stable and the filtration of the reverse osmosis membrane was also stable without using a flocculant. The results are shown in Table 3.

[0089] (Comparative Example 5) With the flow shown in Fig. 6, permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 1 (biopolymer concentration: 122 μgC / L) was used as the water to be treated, and as a result of using the porous membrane of Comparative Example 1 for pretreatment, the biopolymer concentration of the reverse osmosis membrane feed water was 85 μgC / L. This reverse osmosis membrane feed water was pressurized with a high-pressure pump and then filtered through the reverse osmosis membrane to obtain permeate water. The reverse osmosis membrane was a spiral-type reverse osmosis membrane with a membrane material of polyamide, a salt rejection rate of 99.8%, and a membrane area of 37 m 2 .

[0090] Here, the initial water permeability ratio of the ultrafiltration membrane was 0.53, and the water permeability decline rate of the reverse osmosis membrane was 0.25% / hr. The decline rate of the permeability of the reverse osmosis membrane was large. The results are shown in Table 3.

[0091] (Comparative Example 6) Using the flow shown in FIG. 6, permeate water of the reverse osmosis membrane was produced from the water to be treated. That is, seawater 2 (biopolymer concentration: 218 μgC / L) was used as the water to be treated, and as a result of using the porous membrane of Comparative Example 1 for pretreatment, the biopolymer concentration of the reverse osmosis membrane feed water was 139 μgC / L. This reverse osmosis membrane feed water was filtered through a reverse osmosis membrane by pressurizing it with a high-pressure pump to obtain permeate water. The reverse osmosis membrane was a spiral-type reverse osmosis membrane with a membrane material of polyamide, a salt rejection rate of 99.8%, and a membrane area of 37 m 2 .

[0092] Here, the initial water permeability ratio of the ultrafiltration membrane was 0.45, the water permeability reduction rate of the reverse osmosis membrane was 0.45% / hr, and perhaps because the biopolymer concentration in the water to be treated was very high, the decrease in the water permeability of the ultrafiltration membrane was large, and the decrease rate of the permeability of the reverse osmosis membrane was extremely large. The results are shown in Table 3.

[0093]

Table 1

[0094]

Table 2

[0095]

Table 3

Explanation of Symbols

[0096] 101 Porous membrane 102 Surface part 103 Inside 201 Coarse dirt component 202 Fine dirt component 300 Polymer solution 301 Phase rich in polymer 302 Phase rich in solvent FL Filtration direction 401 Water to be treated 402 Reverse osmosis membrane 403 Permeate water 404 Concentrated water 405 Pretreatment process 406 Reverse osmosis membrane feed water 407 High-pressure pump 408 Liquid transfer pump 409 Supply pump 410 Cleaning agent 411 Bactericide

Claims

1. A porous membrane containing a polyvinylidene fluoride resin, with one surface being surface A and the other surface being surface B, and the ratio of the α-type structure crystals (H α ) to the β-type structure crystals (H β ) in the crystalline part of the polyvinylidene fluoride resin, measured by the ATR method (attenuated total reflection measurement method) on surface A, (H α / H β ratio) is 0.25 or more and 0.40 or less, and the pure water permeability of the porous membrane is 0.25 m ３ / m ２ / h / 50 kPa or more and 1.2 m ３ / m ２ / h / 50 kPa or less, the average value of the surface pore diameters of the surface pores on the surface A of the porous membrane is 5.0 or more and 12.0 nm or less, and the number of the surface pores [pieces / μm ２ divided by the average value of the surface pore diameters of the surface pores [nm]: X is 30 pieces / μm ２ / nm or more and 100 pieces / μm ２ / nm or less, and the porous membrane is as described above.

2. The porous membrane according to claim 1, wherein the crystallinity of the polyvinylidene fluoride resin on the surface A of the porous membrane is 30% or more.

3. The porous membrane according to claim 1 or 2, wherein the removal rate of 40,000 Da dextran of the porous membrane is 45% or more and 80% or less.

4. The porous membrane according to claim 1 or 2, wherein the standard deviation of the surface pore diameter on the surface A of the porous membrane is 1.0 nm or more and 5.0 nm or less.

5. The porous membrane according to claim 1 or 2, wherein the surface portion from the surface A of the porous membrane to a thickness of 10 μm is denser than the inside.

6. A method for filtering a liquid using the porous membrane according to claim 1 or 2.

7. A membrane filtration device comprising the porous membrane according to claim 1 or 2.

8. In a water production method including a pretreatment step of pretreating raw water to obtain reverse osmosis membrane feed water, and a reverse osmosis membrane treatment step of filtering the reverse osmosis membrane feed water with a reverse osmosis membrane to obtain permeate water, The raw water is raw water with a biopolymer concentration of 100 μg C / L or more, and the reverse osmosis membrane feed water with a biopolymer concentration of 75 μg C / L or less is obtained by the pretreatment step and supplied to the reverse osmosis membrane treatment step. The pretreatment step has an ultrafiltration membrane treatment section equipped with an ultrafiltration membrane. The water production method, wherein the ultrafiltration membrane is the porous membrane according to claim 1 or 2.

9. The number of surface pores on the surface A of the porous membrane is 200 pores / μm ２ or more and 2000 pores / μm ２ The water production method according to claim 8, wherein the number is 2000 pores / μm or less

10. The water production method according to claim 8, wherein in the ultrafiltration membrane treatment of the pretreatment step, the raw water does not contain a flocculant.

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