Composite semipermeable membrane, spiral membrane element, water treatment system, and water treatment method

Abstract

The composite semi-permeable membrane 12 of the present invention comprises a porous support membrane 12a and a surface layer 12b supported by the porous support membrane 12a. The elastic modulus of the membrane surface, calculated by force curve measurement using AFM in water, is 250 MPa or more and 500 MPa or less. The spiral membrane element 20 of the present invention comprises the composite semi-permeable membrane 12 of the present invention. The water treatment system 100 of the present invention comprises the spiral membrane element 20 of the present invention.

Description

Composite semi-permeable membranes, spiral membrane elements, water treatment systems and water treatment methods This invention relates to a composite semi-permeable membrane, a spiral membrane element, a water treatment system, and a water treatment method. In recent years, ZLD (Zero Liquid Discharge System) has attracted much attention. "ZLD" refers to the technological concept of reducing liquid waste discharged into rivers, oceans, and other natural environments to zero. In a ZLD system, membrane separation technology is used to concentrate wastewater. Then, solid waste is generated by evaporating the concentrated wastewater. The energy consumption in the final stage of solid waste generation is very high. Using membrane separation technology can significantly reduce the amount of wastewater generated in the final stage and suppress energy consumption. As described in Patent Document 1, a typical ZLD system comprises a plurality of reverse osmosis membrane units connected in series. Wastewater is concentrated in stages by the plurality of reverse osmosis membrane units. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent Application Publication No. 2020-44457 [The problem the invention aims to solve] In water treatment systems, it is recommended to replace the membrane when the salt rejection rate falls below the specified level. A longer membrane lifespan generally results in lower water treatment costs, which is preferable. Membrane lifespan is affected by various factors, including the purpose of the water treatment system, the composition of the raw water, the temperature of the raw water, and the operating pressure. The inventors have observed that in certain water treatment systems, such as the ZLD system, the membrane lifespan is shorter than predicted. Furthermore, they discovered that this is due to frequent and repeated system operation and shutdown. For example, in a well-known seawater desalination system, seawater is supplied to the system at a fixed flow rate, and fresh water is produced at the same fixed flow rate. On the other hand, in specific water treatment systems such as the ZLD system, the flow rate of wastewater (raw water) is sometimes not fixed. One reason for this variable wastewater flow rate is that it depends on the plant's operating conditions. Although buffer tanks are installed to stabilize the wastewater flow rate, the ZLD system automatically shuts down when the water level in the buffer tank falls below a specified level. Frequent switching between system operation and shutdown will subject the composite semipermeable membrane to water pressure or depressurization. As a result, the membrane will be damaged, and its impermeability will rapidly decrease. For example, using a high-density weave material in the permeable side flow path can suppress damage to the membrane. However, using a high-density weave material in the permeable side flow path will reduce the permeation flux. If the operating pressure is increased to compensate for the reduced permeation flux, the power consumption of the pump will increase. The object of this invention is to provide a composite semi-permeable membrane suitable for systems that frequently switch on and off, such as the ZLD system. Furthermore, this invention provides a spiral membrane element using the composite semi-permeable membrane, a water treatment system using the spiral membrane element, and a water treatment method using the spiral membrane element. [Technical Means for Solving the Problem] The present invention provides a composite semipermeable membrane comprising: a porous support membrane and a surface layer supported by the porous support membrane, wherein the elastic modulus of the membrane surface, as determined by force curve measurement using AFM (Atomic Force Microscopy) in water, is 250 MPa or more and 500 MPa or less. In another embodiment, the present invention provides a spiral membrane element having the composite semipermeable membrane of the present invention described above. In another embodiment, the present invention provides a water treatment system comprising the spiral membrane element of the present invention described above. In another embodiment, the present invention provides a water treatment method comprising: concentrating wastewater using a low-pressure RO membrane module; further concentrating the wastewater concentrated by the low-pressure RO membrane module using a medium-pressure RO membrane module; and further concentrating the wastewater concentrated by the medium-pressure RO membrane module using a high-pressure RO membrane module; wherein the medium-pressure RO membrane module includes the spiral membrane element of the present invention, the supply pressure of the low-pressure RO membrane module is lower than the supply pressure of the medium-pressure RO membrane module, the supply pressure of the high-pressure RO membrane module is higher than the supply pressure of the medium-pressure RO membrane module, and the supply pressure of the medium-pressure RO membrane module is 2.0 MPa or more and 4.0 MPa or less. [Effects of the Invention] According to the present invention, a composite semipermeable membrane suitable for systems that are frequently turned on and off, such as the ZLD system, can be provided. Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the following embodiments. (Embodiment 1) Figure 1 shows a spiral membrane element 20 according to Embodiment 1 of the present invention. The spiral membrane element 20 includes a water collection pipe 21 and a stacked body 22. The stacked body 22 is disposed around the water collection pipe 21. A raw water flow path and a permeate flow path are formed inside the stacked body 22. The water collection pipe 21 passes through the center of the stacked body 22. Raw water is supplied from one end face of the stacked body 22 into the interior of the spiral membrane element 20, flowing parallel to the length of the water collection pipe 21 in the raw water flow path. Within the spiral membrane element 20, the raw water is filtered to generate concentrate and permeate. The permeate is guided to the outside through the water collection pipe 21. The concentrate is discharged from the other end face of the stacked body 22 to the outside of the spiral membrane element 20. Examples of raw water that should be treated (filtered) by the spiral membrane element 20 include wastewater discharged from factories, etc. Examples of wastewater include coking wastewater, coal chemical wastewater, and associated wastewater. Coking wastewater, coal chemical wastewater, and associated wastewater may contain aromatic compounds at concentrations ranging from several ppm to several hundred ppm. Aromatic compounds are known to cause chemical degradation of the composite semipermeable membrane. Therefore, wastewater containing aromatic compounds is difficult to treat. However, the composite semipermeable membrane of this embodiment can be used for a long time even with wastewater containing aromatic compounds. However, the raw water is not limited to wastewater. The laminate 22 includes a composite semi-permeable membrane 12, a raw water side flow path 13, and a permeable side flow path 14. The end face of the composite semi-permeable membrane 12 constitutes the end face of the laminate 22. In detail, the laminate 22 includes a plurality of composite semi-permeable membranes 12, a plurality of raw water side flow path paths 13, and a plurality of permeable side flow path paths 14. The raw water sideflow material 13 can be a component with a mesh structure made of resin materials such as polyester, polyethylene, and polypropylene. The through-flow material 14 can be a woven fabric made of resin materials such as polyester, polyethylene, and polypropylene. The through-flow material 14 is typically a Tricot woven fabric. A plurality of composite semi-permeable membranes 12 overlap each other, are sealed at three sides in a bag-like structure, and are wound around a water collection pipe 21. A raw water side flow path material 13 is disposed between the composite semi-permeable membranes 12, located outside the bag-like structure. The raw water side flow path material 13 ensures a space for raw water flow between the composite semi-permeable membranes 12. A permeable side flow path material 14 is disposed between the composite semi-permeable membranes 12, located inside the bag-like structure. The permeable side flow path material 14 ensures a space for permeable water flow between the composite semi-permeable membranes 12. A membrane leaf 11 is composed of a pair of composite semi-permeable membranes 12 and a permeable side flow path material 14. The open end of the membrane leaf 11 is connected to the water collection pipe 21, so that the permeable water flow path is connected to the water collection pipe 21. The water collection pipe 21 serves to collect the permeate water passing through each composite semipermeable membrane 12 and guide it to the outside of the spiral membrane element 20. A plurality of through holes 21h are provided along the length of the water collection pipe 21 at predetermined intervals. Permeate water flows into the water collection pipe 21 through these through holes 21h. Figure 2 is a cross-sectional view of the composite semi-permeable membrane 12 used in the spiral membrane element 20 shown in Figure 1. The composite semi-permeable membrane 12 includes a porous support membrane 12a, a surface layer 12b, and a coating layer 12c. The porous support membrane 12a, the surface layer 12b, and the coating layer 12c are laminated in this order. The surface layer 12b and the coating layer 12c are supported by the porous support membrane 12a. The surface layer 12b is disposed on the porous support membrane 12a. The coating layer 12c is disposed on the surface layer 12b. The coating layer 12c covers the surface layer 12b. Specifically, the coating layer 12c is in direct contact with the surface layer 12b. Figure 3 is a cross-sectional view illustrating the mechanism of damage to the composite semi-permeable membrane. When raw water is introduced into the spiral membrane element and water pressure F is applied to the composite semi-permeable membrane 121, a portion of the composite semi-permeable membrane 121 sinks into the groove 14m of the permeable side flow path material 14. The remaining portion of the composite semi-permeable membrane 121 is pressed against the permeable side flow path material 14, locally applying the load indicated by the dashed arrow. The groove 14m is, for example, the portion between the warp coils. When the introduction of raw water is stopped, the water pressure F is released from the composite semi-permeable membrane 121. The load indicated by the dashed arrow is also almost not applied. When the introduction of raw water begins again, water pressure F is applied to the composite semi-permeable membrane 121 again. When water pressure F is repeatedly applied and released, defects occur in the composite semi-permeable membrane 121 centered on the portion repeatedly pressed against the permeable side flow path material 14. Defects in the composite semi-permeable membrane 121 tend to occur in the thinner surface layer. As a result, the salt barrier efficiency of the composite semipermeable membrane 121 decreased rapidly. It is assumed that when the composite semipermeable membrane 121 has sufficient rigidity to resist water pressure F, the composite semipermeable membrane 121 is not easily damaged by the opening / closing of water pressure F. However, contrary to expectations, having appropriate flexibility helps to suppress damage caused by the opening / closing of water pressure F. That is, the elastic modulus of the membrane surface 12p of the composite semipermeable membrane 12 in this embodiment is between 250 MPa and 500 MPa. The elastic modulus of the membrane surface 12p is calculated by measuring the force curve using AFM (Atomic Force Microscope) in water. When the elastic modulus of the membrane surface 12p is within the above range, the composite semipermeable membrane 12 is not easily damaged even if the water pressure is repeatedly turned on and off. As a result, the lifespan of the spiral membrane element 20 is extended. The method for determining the force curve of AFM in water is as described in the Examples section. In the composite semipermeable membrane 12, the coating 12c contains a polymer. Using the coating 12c, the elastic modulus of the membrane surface 12p can be easily adjusted. The type of polymer contained in coating 12c differs from the type of polymer contained in surface layer 12b. Surface layer 12b is, for example, made of polyamide. The polymer contained in coating 12c includes, for example, at least one selected from the group consisting of polyvinyl alcohol, betaine polymers, and polyazoline. These polymers are hydrophilic (possessing hydrophilic groups). Using these polymers, the elastic modulus of the membrane surface 12p can be easily adjusted. The thickness of coating 12c is not particularly limited. The thickness of coating 12c is, for example, 10–1000 nm. The thickness of coating 12c is determined by measuring the thickness of coating 12c at any plurality of points (e.g., 10 points) on the cross-section of the composite semi-permeable membrane 12. The average of the obtained measurements can be considered as the thickness of coating 12c. Thickness measurement can be performed using SEM or TEM. The presence of coating 12c can be confirmed by scanning electron microscopy or transmission electron microscopy. The compositional analysis of the polymer contained in coating 12c can be performed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), or time-of-flight secondary ion mass spectrometry (TOF-SIMS). Various factors determine the elastic modulus of the membrane surface 12p. Specifically, these include: the material of the surface layer 12b, the thickness of the surface layer 12b, the surface roughness (Ra) of the surface layer 12b, the material of the coating 12c, the thickness of the coating 12c, and the temperature at which the surface layer 12b is formed. After the surface layer 12b is formed, the elastic modulus of the membrane surface 12p is increased by heat treatment. Heat treatment can be performed, for example, by exposing the membrane to warm water at approximately 50°C. Furthermore, if the material of the coating 12c is softer or has high hydrophilicity, the elastic modulus of the membrane surface 12p tends to decrease. If the thickness of the coating 12c increases, the elastic modulus of the membrane surface 12p tends to decrease. However, if the thickness of the coating 12c is excessively increased, the transmission flux (m) will decrease. 3 / m 2 / day) decreases. In the spiral membrane element 20, the permeable side flow path material 14, which is a Tricot woven fabric, has a weave density of 31-60 warp loops and 34-52 weft loops. The composite semi-permeable membrane 12 is in contact with the permeable side flow path material 14. In this embodiment, the weave density of the permeable side flow path material 14 is adjusted in conjunction with the composite semi-permeable membrane 12. The effect of adjusting the elastic modulus of the membrane surface of the composite semi-permeable membrane 12 and the effect of adjusting the weave density of the permeable side flow path material 14 complement each other, ensuring the required permeation flux and further improving the durability against water pressure opening / closing. Furthermore, a warp loop refers to the longitudinal loop of a knitted fabric. A weft loop refers to the transverse loop of a knitted fabric. The density of a knitted fabric (knitting density) is expressed by the number of warp loops per inch (warp loop density) and the number of weft loops per inch (weft loop density). Knitting density can also be expressed as (warp loop density) × (weft loop density). The degree of damage to the composite semipermeable membrane 12 can be determined by the decrease in salt rejection rate after the on / off test. Salt rejection rate is defined, for example, by the NaCl rejection rate. Specifically, a NaCl aqueous solution of a specified concentration is supplied to the spiral membrane element 20 at a specified supply pressure, and the NaCl rejection rate is measured. Then, the supply of the NaCl aqueous solution is stopped. Subsequently, the supply of the NaCl aqueous solution is restarted. This cycle is repeated n times (e.g., 7000 times). The supply pressure is, for example, 2.0 MPa to 4.0 MPa. The degree of damage to the composite semipermeable membrane 12 can be determined by the difference between the NaCl rejection rate of the first test and the NaCl rejection rate of the nth test. The NaCl rejection rate can be determined according to Japanese Industrial Standard JIS K 3805 (1990). Specifically, an aqueous NaCl solution is passed through a composite semi-permeable membrane of a specified size at a specified supply pressure. After a 30-minute preparation phase (first time), the conductivity of the permeate and supply water is measured using a conductivity meter. Based on the results and the calibration curve (concentration-conductivity), the NaCl rejection rate can be calculated using the following formula. Alternatively, ion chromatography can be used to determine the concentration instead of conductivity measurement. • NaCl rejection rate (%) = (1 - (NaCl concentration in permeate water / NaCl concentration in supply water)) × 100 The composite semi-permeable membrane 12 is typically a reverse osmosis membrane (RO membrane). However, the composite semi-permeable membrane 12 can also be a nanofiltration membrane (NF membrane). In this specification, "NF membrane" refers to a composite semi-permeable membrane with a NaCl rejection rate of 5% or higher but less than 93% when filtering a NaCl aqueous solution with a concentration of 2000 mg / L under conditions of supply pressure 1.55 MPa and 25°C. "RO membrane" refers to a composite semi-permeable membrane with a NaCl rejection rate of 93% or higher when filtering a NaCl aqueous solution with a concentration of 2000 mg / L under conditions of supply pressure 1.55 MPa and 25°C. The composite semipermeable membrane 12 can be manufactured by the following method. First, a porous support membrane 12a is prepared as the support. The porous support membrane 12a is not particularly limited as long as it is a membrane on which a surface layer 12b can be formed. The porous support membrane 12a is used to form an ultrafiltration membrane with a microporous layer having an average pore size of 0.01 to 0.4 μm on a nonwoven fabric. Examples of materials forming the microporous layer include polyurethane, polyarylene ether urethane, polyimide, polyvinylidene fluoride, and polyether ether imide. From the viewpoint of chemical stability, mechanical stability, and thermal stability, polyurethane or polyarylene ether urethane can be used. Alternatively, a self-supporting porous support membrane made of a thermosetting resin such as epoxy resin with the aforementioned average pore size can also be used. The thickness of the porous support membrane 12a is not particularly limited; for example, it can be in the range of 10 to 200 μm, or in the range of 20 to 75 μm. In this specification, "average pore diameter" refers to the value calculated using the following method: First, the surface or cross-section of the film or layer is observed using an electron microscope (e.g., a scanning electron microscope), and the diameters of a plurality of observed pores (e.g., any 10 pores) are measured. The average value of the measured pore diameters is defined as the "average pore diameter." "Pore diameter" refers to the major axis of the pore; more specifically, it refers to the diameter of the smallest circle that can enclose the pore. Next, a first solution containing the raw material for the surface layer 12b is brought into contact with a porous support membrane 12a. The first solution is typically an aqueous solution containing a polyfunctional amine that serves as the raw material for the surface layer 12b (hereinafter referred to as an "amine aqueous solution"). By bringing the amine aqueous solution into contact with the porous support membrane 12a, an amine-containing layer is formed on the surface of the porous support membrane 12a. In addition to water, the amine aqueous solution may also contain a polar solvent other than water, such as an alcohol. Alternatively, a polar solvent other than water, such as an alcohol, may be used instead of water. Polyfunctional amines are amines that have a plurality of reactive amino groups. Examples of polyfunctional amines include aromatic polyfunctional amines, aliphatic polyfunctional amines, and alicyclic polyfunctional amines. Examples of aromatic polyfunctional amines include: m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, 1,3,5-triaminobenzene, 1,2,4-triaminobenzene, 3,5-diaminobenzoic acid, 2,4-diaminotoluene, 2,6-diaminotoluene, N,N'-dimethylm-phenylenediamine, 2,4-diaminoanisole, amylin, phenylenediamine, etc. Examples of aliphatic polyfunctional amines include ethylenediamine, propylenediamine, tris(2-aminoethyl)amine, and N-phenyl-ethylenediamine. Examples of alicyclic polyfunctional amines include 1,3-cyclohexanediamine, 1,2-cyclohexanediamine, 1,4-cyclohexanediamine, piperazine, and piperazine derivatives. One of the polyfunctional amines selected from these polyfunctional amines may be used alone, or two or more may be used in combination. To facilitate the formation of an amine-containing layer and improve the performance of the surface 12b, polymers such as polyvinyl alcohol, polyvinylpyrrolidone, polyacrylic acid, sorbitol, and glycerol can be added to the amine aqueous solution. The concentration of the amine component in the aqueous amine solution can be in the range of 0.1% to 15% by mass, or in the range of 1% to 10% by mass. By appropriately adjusting the concentration of the amine component, defects such as pinholes in the surface layer 12b can be suppressed. Furthermore, a surface layer 12b with excellent salt barrier properties can be formed. Moreover, if the concentration of the amine component is appropriately adjusted, the thickness of the surface layer 12b can also be appropriately adjusted, thereby obtaining a composite semipermeable membrane 12 that can achieve sufficient permeability. There is no particular limitation on the method for contacting the amine aqueous solution with the porous support membrane 12a. Methods such as immersing the porous support membrane 12a in the amine aqueous solution, coating the porous support membrane 12a with the amine aqueous solution, or spraying the porous support membrane 12a with the amine aqueous solution can be appropriately employed. Furthermore, after the step of contacting the amine aqueous solution with the porous support membrane 12a, a step of removing excess amine aqueous solution from the porous support membrane 12a can also be performed. For example, by extending the amine-containing layer with a rubber roller, excess amine aqueous solution can be removed from the porous support membrane 12a. By removing excess amine aqueous solution, a surface layer 12b of appropriate thickness can be formed. Next, the second solution is brought into contact with the amine-containing layer. The second solution is a solution containing other raw materials for the surface layer 12b. Specifically, the second solution is a solution containing a polyfunctional amide brine (hereinafter referred to as "amide brine solution"), which is another raw material for the surface layer 12b. When the amide brine solution is brought into contact with the amine-containing layer, a polymerization reaction between the amine and the amide brine occurs at the interface between the amine-containing layer and the amide brine solution layer. This forms the surface layer 12b. So-called polyfunctional amide halides are amide halides with multiple reactive carbonyl groups. Examples of polyfunctional amide halides include aromatic polyfunctional amide halides, aliphatic polyfunctional amide halides, and alicyclic polyfunctional amide halides. Examples of aromatic polyfunctional chlorobenzene halides include: pyromellitic trimethylolpropionate, terephthalic acid chlorobenzene, isophthalic acid chlorobenzene, biphenyl dimethylolpropionate, naphthyl dimethylolpropionate, benzotrisulfonate, benzene disulfonate, chlorosulfonylurea benzodimethylolpropionate, etc. Examples of aliphatic polyfunctional amide halides include: propane dimethyl chloride, butane dimethyl chloride, pentane dimethyl chloride, propane trimethyl chloride, butane trimethyl chloride, pentane trimethyl chloride, pentane dimethyl chloride, and hexane dimethyl chloride. Examples of alicyclic polyfunctional chlorohydrins include: cyclopropane trimethylchlorohydrin, cyclobutane tetramethylchlorohydrin, cyclopentane trimethylchlorohydrin, cyclopentane tetramethylchlorohydrin, cyclohexane trimethylchlorohydrin, tetrahydrofuran tetramethylchlorohydrin, cyclopentane dimethylchlorohydrin, cyclobutane dimethylchlorohydrin, cyclohexane dimethylchlorohydrin, and tetrahydrofuran dimethylchlorohydrin. One of the polyfunctional amides selected from these polyfunctional amides may be used alone, or two or more may be used in combination. Aromatic polyfunctional amides may be used to obtain a surface layer 12b with excellent salt-barrier properties. Furthermore, polyfunctional amides with three or more components may be used as at least a part of the polyfunctional amide components to form a cross-linked structure. Organic solvents, especially nonpolar organic solvents, can be used as solvents for acetic acid halides. There are no particular limitations on the type of organic solvent, as long as it has low solubility in water, does not degrade the porous support membrane 12a, and can dissolve the polyfunctional acetic acid halides. Examples of suitable organic solvents include saturated hydrocarbons such as cyclohexane, heptane, octane, and nonane, and halogenated hydrocarbons such as 1,1,2-trichlorotrifluoroethane. Saturated hydrocarbons with boiling points below 300°C or below 200°C can also be used. The concentration of the acetic acid brine in the brine solution can be in the range of 0.01–5% by mass, or 0.05–3% by mass. By appropriately adjusting the concentration of the acetic acid brine, unreacted amine and acetic acid components can be reduced. Furthermore, defects such as pinholes on the surface layer 12b can be suppressed, thereby providing a composite semipermeable membrane 12 with excellent salt-barrier properties. Moreover, by appropriately adjusting the concentration of the acetic acid brine, the thickness of the surface layer 12b can also be appropriately adjusted, thereby providing a composite semipermeable membrane 12 capable of achieving sufficient permeability. There is no particular limitation on the method of contacting the amide brine solution with the amine-containing layer. The amine-containing layer and the porous support membrane 12a can be immersed together in the amide brine solution, or the amide brine solution can be coated onto the surface of the amine-containing layer. The contact time between the amine-containing layer and the amide brine solution is, for example, 10 seconds to 5 minutes or 30 seconds to 1 minute. Alternatively, after contacting the amine-containing layer with the amide brine solution, a step of removing excess amide brine solution from the amine-containing layer can be performed. Next, the surface layer 12b and the porous support membrane 12a are heated together to dry them. By heating the surface layer 12b, its mechanical strength and heat resistance can be improved. The heating temperature is, for example, 70–200°C or 80–130°C. The heating time is, for example, 30 seconds to 10 minutes or 40 seconds to 7 minutes. Alternatively, after the drying step is performed at room temperature, a further drying step can be performed using a dryer at an ambient temperature higher than room temperature. Furthermore, to facilitate the formation of the surface layer 12b or to improve the performance of the desired composite semi-permeable membrane 12, various additives can be added to the amine aqueous solution and / or acetic acid solution. Examples of such additives include: surfactants such as sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and sodium lauryl sulfate; alkaline compounds such as sodium hydroxide, trisodium phosphate, and triethylamine, which are effective in removing hydrogen halides generated by polymerization; acetic catalysts; and solubility parameters of 8–14 (cal / cm³). 3 ) 1 / 2 Compounds, etc. By performing the above steps, a membrane with a porous support membrane 12a and a surface layer 12b can be obtained. The thickness of the surface layer 12b is not particularly limited, for example, it is 0.05 to 2 μm, or it can be 0.1 to 1 μm. Furthermore, this specification describes a method for directly forming a surface layer 12b on the surface of a porous support membrane 12a using interfacial polymerization. However, the surface layer 12b can also be formed on other supports besides the porous support membrane 12a, and the obtained surface layer 12b can be transferred to the porous support membrane 12a for integration. In other words, the surface layer 12b can also be transferred from other supports to the porous support membrane 12a. Subsequently, coating 12c is formed. Coating 12c can be formed by contacting an aqueous solution containing the polymer described above with the surface layer 12b to form a polymer-containing layer, followed by drying the polymer-containing layer. The method of contacting the aqueous solution with the surface layer 12b is not particularly limited. The surface layer 12b and the porous support membrane 12a can be immersed together in the aqueous solution, or the aqueous solution can be coated onto the surface of the surface layer 12b. The contact time between the surface layer 12b and the aqueous solution is, for example, 10 seconds to 5 minutes. Alternatively, after contacting the surface layer 12b with the aqueous solution, a step of removing excess aqueous solution from the surface layer 12b can be performed. In addition to water, the aqueous solution may also contain a polar solvent other than water, such as an alcohol. A polar solvent other than water, such as an alcohol, can also be used instead of water. Next, the polymer-containing layer is heated to dry it. By heating the polymer-containing layer, the mechanical strength and heat resistance of the coating 12c can be improved. The heating temperature is, for example, 80–150°C. The heating time is, for example, 10–300 seconds. Alternatively, after the drying step is performed at room temperature, a further drying step can be performed using a dryer at an ambient temperature higher than room temperature. By implementing the above steps, a composite semi-permeable membrane 12 with a porous support membrane 12a, a surface layer 12b, and a coating layer 12c can be obtained. (Embodiment 2) Figure 4 is a structural diagram of the water treatment system according to Embodiment 2. The water treatment system 100 includes a plurality of RO membrane modules. The plurality of RO membrane modules include a low-pressure RO membrane module 110, a medium-pressure RO membrane module 120, and a high-pressure RO membrane module 130. The low-pressure RO membrane module 110, the medium-pressure RO membrane module 120, and the high-pressure RO membrane module 130 are connected to each other in this order to filter wastewater. At least one of the following is selected from the group consisting of low-pressure RO membrane module 110, medium-pressure RO membrane module 120, and high-pressure RO membrane module 130: a spiral membrane element 20 as described in Embodiment 1. The spiral membrane element 20 exhibits excellent durability against water pressure switching on / off cycles. Therefore, even with repeated switching on / off cycles, the salt barrier performance of the module including the spiral membrane element 20 can be maintained for a long period in the water treatment system 100. In detail, the medium-pressure RO membrane module 120 includes the spiral membrane element 20 described in Embodiment 1. Because the supply pressure in the low-pressure RO membrane module 110 is relatively low, repeated opening and closing of the water pressure is unlikely to damage the composite semi-permeable membrane used in the low-pressure RO membrane module 110. A permeable side flow path material with a dense weave can be used in the high-pressure RO membrane module 130. It is difficult to achieve a balance between structure and supply pressure in the medium-pressure RO membrane module 120. Therefore, by using the spiral membrane element 20 of Embodiment 1 in the medium-pressure RO membrane module 120, the salt barrier performance of the medium-pressure RO membrane module 120 can be maintained for a long period. Spiral membrane elements can also be used in low-pressure RO membrane module 110 and high-pressure RO membrane module 130. The water treatment system 100 is, for example, a ZLD system. As mentioned above, there are situations where the operation and shutdown of the ZLD system are frequently switched. Therefore, the water treatment system 100 of this embodiment is particularly suitable for ZLD applications. A flow path 2a is connected to the inlet of the low-pressure RO membrane module 110. A flow path 2b is connected to the concentrate outlet of the low-pressure RO membrane module 110 and the inlet of the medium-pressure RO membrane module 120. A flow path 2c is connected to the concentrate outlet of the medium-pressure RO membrane module 120 and the inlet of the high-pressure RO membrane module 130. The low-pressure RO membrane module 110 has at least one low-pressure RO membrane element housed in a pressure-resistant container. The low-pressure RO membrane module 110 may have a plurality of low-pressure RO membrane elements or only one low-pressure RO membrane element. The medium-pressure RO membrane module 120 has at least one medium-pressure RO membrane element housed in a pressure-resistant container. The medium-pressure RO membrane module 120 may have a plurality of medium-pressure RO membrane elements or only one medium-pressure RO membrane element. The high-pressure RO membrane module 130 has at least one high-pressure RO membrane element housed in a pressure-resistant container. The high-pressure RO membrane module 130 may have a plurality of high-pressure RO membrane elements or only one high-pressure RO membrane element. The supply pressure of the low-pressure RO membrane module 110 is, for example, 0.5 MPa or higher but less than 2.0 MPa. The supply pressure of the medium-pressure RO membrane module 120 is, for example, 2.0 MPa or higher but less than 4.0 MPa. The supply pressure of the high-pressure RO membrane module 130 is, for example, greater than 4.0 MPa but less than 8.0 MPa. That is, the supply pressure of the low-pressure RO membrane module 110 is lower than the supply pressure of the medium-pressure RO membrane module 120. The supply pressure of the medium-pressure RO membrane module 120 is lower than the supply pressure of the high-pressure RO membrane module 130. By appropriately adjusting the supply pressure of each RO membrane module and using the spiral membrane element 20 of Embodiment 1 in the medium-pressure RO membrane module 120, a water treatment system 100 with excellent durability in response to water pressure switching on / off can be constructed. The low-pressure RO membrane module 110, medium-pressure RO membrane module 120, and high-pressure RO membrane module 130 each have a permeate flow path material made of Tricot woven fabric. The weave density of the permeate flow path material used in the low-pressure RO membrane module 110 is lower than that used in the medium-pressure RO membrane module 120. The weave density of the permeate flow path material used in the medium-pressure RO membrane module 120 is lower than that used in the high-pressure RO membrane module 130. With this configuration, the permeate flux of each module is adequately ensured, and the composite semi-permeable membrane in each module is not easily damaged even if the water treatment system 100 is repeatedly operated and stopped. Wastewater that has undergone appropriate pretreatment is introduced into the low-pressure RO membrane module 110 through flow path 2a. The wastewater is concentrated in the low-pressure RO membrane module 110. Examples of pretreatment methods include wastewater treatment using sand filtration (such as silica sand) or wastewater treatment using UF (ultrafiltration membrane) or MF (microfiltration membrane). The wastewater concentrated in the low-pressure RO membrane module 110 is introduced into the medium-pressure RO membrane module 120 through flow path 2b. The medium-pressure RO membrane module 120 further concentrates the wastewater concentrated in the low-pressure RO membrane module 110. The wastewater concentrated in the medium-pressure RO membrane module 120 is introduced into the high-pressure RO membrane module 130 through flow path 2c. The high-pressure RO membrane module 130 further concentrates the wastewater concentrated in the medium-pressure RO membrane module 120. The permeate outlets of the low-pressure RO membrane module 110, the medium-pressure RO membrane module 120, and the high-pressure RO membrane module 130 are respectively connected to flow paths 4a, 4b, and 4c. The permeate is supplied to factories and other facilities for reuse through flow paths 4a, 4b, and 4c. The water treatment system 100 further includes an ultra-high pressure RO membrane module 140, an NF membrane module 150, and an ultra-high pressure RO membrane module 160. The ultra-high pressure RO membrane modules 140 and 160 operate at a supply pressure higher than that of the high pressure RO membrane module 130. A flow path 2d connects the concentrate outlet of the high pressure RO membrane module 130 to the inlet of the ultra-high pressure RO membrane module 140. A flow path 2e connects the concentrate outlet of the high pressure RO membrane module 130 to the inlet of the NF membrane module 150. A flow path 2f connects the concentrate outlet of the ultra-high pressure RO membrane module 140. A flow path 2g connects the concentrate outlet of the NF membrane module 150 to the inlet of the ultra-high pressure RO membrane module 160. A flow path 2h connects the permeate outlet of the NF membrane module 150 to the inlet of the ultra-high pressure RO membrane module 160. Wastewater concentrated by the high-pressure RO membrane module 130 is introduced into the ultra-high-pressure RO membrane module 140 through flow path 2d. The ultra-high-pressure RO membrane module 140 further concentrates the wastewater concentrated by the high-pressure RO membrane module 130. The wastewater concentrated by the high-pressure RO membrane module 130 is introduced into the NF membrane module 150 through flow path 2e. The NF membrane module selectively removes divalent ions from the wastewater concentrated by the high-pressure RO membrane module 130. The permeate from the NF membrane module 150 is introduced into the ultra-high-pressure RO membrane module 160 through flow path 2h. The ultra-high-pressure RO membrane module 160 further filters the permeate filtered by the NF membrane module 150. The permeate outlets of the ultra-high pressure RO membrane module 140 and 160 are connected to flow path 4d and flow path 4e, respectively. The permeate is supplied to factories and other facilities for reuse through flow paths 4d and 4e. The concentrated water is sent to electrolysis units, evaporators, and other facilities for post-treatment through flow paths 2f, 2i, and 2g. Pumps, valves, sensors, etc. are configured as needed in each flow path of the water treatment system 100. Furthermore, "supply pressure" refers to the pressure applied to the raw water near the module inlet. The area near the module inlet is, for example, the space between the upstream component and the flow path within the module. [Example] (Manufacturing Example 1: Manufacturing Example of Composite Semi-permeable Membrane) An amine aqueous solution was prepared by mixing 3.0% by mass of m-phenylenediamine, 0.15% by mass of sodium dodecyl sulfate, 2.15% by mass of triethylamine, 0.31% by mass of sodium hydroxide, 6% by mass of camphor sulfonic acid, and 1% by mass of isopropanol. The amine aqueous solution was coated onto a polyurethane porous support membrane formed on a polyester nonwoven fabric. Excess amine aqueous solution was then removed, thereby forming an amine-containing layer. On the other hand, a pyromellitic chloroform solution was prepared by dissolving 0.20% by mass of pyromellitic chloroform in a cycloalkane solvent (Exxsol D40, manufactured by ExxonMobil). The surface of the amine-containing layer was immersed in the pyromellitic chloroform solution for 7 seconds. Excess pyromellitic chloroform solution was then removed. This allowed an interfacial polymerization reaction to occur, forming a surface layer. The surface layer was air-dried for 20 seconds and then heated in a hot air dryer at 140°C for 3 minutes. Through these steps, a composite semi-permeable membrane is obtained having a non-woven fabric substrate, a polyurethane porous support membrane, and a polyamide surface layer in sequence. An aqueous solution of polyvinyl alcohol (PVA) containing 0.165% by mass (PVA) with a saponification degree of ≥99% and a viscosity of 62.0–72.0 MPa·s (25°C) for a 4% by mass solution was prepared. The surface of the composite semipermeable membrane was brought into contact with this PVA aqueous solution for 10 seconds. Subsequently, the composite semipermeable membrane was air-dried for 30 seconds, and then heated in a hot air dryer at 20°C for 2 minutes. This yielded the composite semipermeable membrane of Manufacturing Example 1, which has a PVA coating. (The elastic modulus of the membrane surface was calculated by measuring the force curve of the AFM in water.) Figure 5 illustrates the method for measuring the force curve of the AFM in water. The elastic modulus of the composite semipermeable membrane of Manufacturing Example 1 was determined by the following method. First, the composite semipermeable membrane of Manufacturing Example 1 was cut into 2 cm × 2 cm pieces to obtain a test piece 101. Then, as shown in Figure 5, the test piece 101 was fixed on the glass plate 104 of the fixture using a liquid measurement fixture (Closed Fluid Cell), a fixing pin 102, and a clamping plate 103 manufactured by Asylum Technology. Subsequently, approximately 100 μL of ultrapure water 105 was added to the test piece 101. The test piece 101 is moved vertically, and a load is applied to the surface of the test piece 101 while a spherical probe 106 is pressed in. Then, the spherical probe 106 is pulled away from the test piece 101. The deflection or warping (displacement) of the cantilever 107 when the spherical probe 106 is pulled away from the test piece 101 is detected using a photodiode in the form of the displacement of laser light 108, and a force curve is measured. The force curve is converted into the load and the deformation of the membrane surface using the program provided with the device. The force curve is measured at any 5 points within a 5 μm × 5 μm measurement area. Furthermore, the elastic modulus is calculated by fitting the measurement to a Heltz model using the analytical software provided with the device. • Measuring Apparatus: MFP-3D-SA (manufactured by Asylum Technology) • Cantilever: Spring constant 40 N / m • Spherical Probe: Nanosensors, with a front-end radius of curvature of 0.4 μm, silicone (100), Poisson's ratio 0.17, and elastic modulus 150 GPa • Measuring Environment: Ultrapure water (28–30°C) • Indentation and Retraction Speeds: 1 Hz • Number of Measurements: 5 The elastic modulus E of the composite semipermeable membrane surface in Example 1 sample The value (MPa) was obtained by substituting the values ​​into the following Hertzian elastic contact theory formula and using the accompanying software. The obtained elastic modulus is 340 MPa. (Hertzian elastic contact theory formula) h=〔3 / 4[{(1-ν) probe 2 ) / E probe}+{(1-ν sample 2 ) / E sample}] 2 / 3 F 2 / 3 r -1 / 3 h: Deformation of the membrane surface (average value) ν probe The probe's Poisson's ratio is 0.17ν. sample The Poisson's ratio of the test piece was 0.33 (as a representative value for the resin, 0.33 (a fixed value) was used). probe The probe's elastic modulus is 150 GPa. sample Elastic modulus of the specimen (MPa) F: arbitrary r: radius of curvature of the probe tip (10 nm) (Manufacturing Example 2: Manufacturing Example of Composite Semipermeable Membrane) Except that a betaine polymer (manufactured by Osaka Organic Chemical Industry Co., Ltd., LAMBIC-1100W) was used instead of PVA, the composite semipermeable membrane of Manufacturing Example 2 was manufactured in the same way as Manufacturing Example 1. (Manufacturing Example 3: Manufacturing Example of Composite Semipermeable Membrane) Except that PEOX (poly(2-ethyl-2-azolin)) was used instead of PVA, the composite semipermeable membrane of Manufacturing Example 3 was manufactured in the same way as Manufacturing Example 1. (Manufacturing Example 4: Manufacturing Example of Composite Semipermeable Membrane) Except that the PVA coating was not applied, the composite semipermeable membrane of Manufacturing Example 4 was manufactured in the same way as Manufacturing Example 1. [Open / Close Test] The composite semi-permeable membrane and the permeable side flow material were placed in a pressure-resistant container, and the initial NaCl rejection rate was measured using the method described above. Subsequently, the steps of continuously supplying pressurized RO water (25°C) for 90 seconds and stopping the RO water supply were repeated 7000 times. After repeating 7000 times, the NaCl rejection rate was measured. (Samples 1-4) An opening / closing test was conducted using the composite semipermeable membrane obtained in Manufacturing Example 1, the permeable side flow path material shown in Table 1, and the supply pressure shown in Table 1. The results are shown in Table 1. The permeable side flow path material was deposited on the composite semipermeable membrane in such a way that the surface with the protrusions of the warp loops (rough surface) was in contact with the surface of the polyester nonwoven fabric of the composite semipermeable membrane. (Sample 5) An opening / closing test was conducted using the composite semipermeable membrane obtained in Manufacturing Example 2, the permeable side flow path material shown in Table 1, and the supply pressure shown in Table 1. The results are shown in Table 1. (Sample 6) An opening / closing test was conducted using the combination of the composite semipermeable membrane obtained in Manufacturing Example 3, the permeable side flow path material shown in Table 1, and the supply pressure shown in Table 1. The results are shown in Table 1. (Samples 7 and 8) An opening / closing test was conducted using the combination of the composite semipermeable membrane obtained in Manufacturing Example 4, the permeable side flow path material shown in Table 1, and the supply pressure shown in Table 1. The results are shown in Table 1. (Sample 9) An opening / closing test was conducted using the composite semipermeable membrane obtained in Manufacturing Example 1, the permeable side flow path material shown in Table 1, and the supply pressure shown in Table 1. The results are shown in Table 1. [Table 1] Based on the results of samples 1 and 7, it can be understood that the difference in salt rejection rate before and after the on / off test for the combination of samples 1 is smaller than the difference in salt rejection rate before and after the on / off test for the combination of samples 7. That is, the combination of samples 1 exhibits excellent durability against water pressure on / off cycles. The combination of samples 1 is suitable for systems that are frequently on / off, such as the ZLD system. Based on the results of samples 1 and 2, it can be understood that the decrease in salt rejection rate increases with increasing supply pressure. However, based on the results of samples 2 and 8, it can be understood that the difference in salt rejection rate before and after the on / off test for the sample 2 combination is smaller than the difference in salt rejection rate before and after the on / off test for the sample 8 combination. That is, the sample 2 combination exhibits excellent durability against water pressure on / off cycles. The sample 2 combination is suitable for systems that are frequently on / off, such as the ZLD system. Based on the results of Specimens 3 and 4, it is understood that increasing the weave density of the permeable sideflow material improves the durability against water pressure during on / off cycles. In particular, as shown in Specimens 4, even under relatively high supply pressure (4.0 MPa), the use of a permeable sideflow material with an appropriate weave density significantly suppressed the decrease in salt rejection caused by the on / off test. This demonstrates a synergistic effect between the composite semi-permeable membrane and the permeable sideflow material. The combination of Specimens 3 and 4 is suitable for systems that undergo frequent on / off cycles, such as the ZLD system. The results of samples 5 and 6 suggest that good results can be obtained even if the coating material is changed. In sample 9, due to the excessively high supply pressure, the salt rejection rate decreased significantly before and after the on / off test. [Observation of Membrane Surface After Open / Close Test] After the open / close test, the membrane surfaces of the composite semipermeable membranes of samples 1, 3, and 7 were observed. Specifically, a staining solution with a concentration of 160 mg / L was prepared using a dye (Tokyo Chemical Industries, Ltd., Basic Violet 1). The staining solution was introduced into the composite semipermeable membrane under pressure of 1.5 MPa for 10 minutes. After the staining solution was introduced, the composite semipermeable membrane was washed with RO water, dried at room temperature, and then the membrane surface was observed at 50x magnification using a digital microscope (Keyence Corporation, VHX6000). The results are shown in Figures 6A, 6B, and 6C. Figure 6A is an optical microscope image of the composite semi-permeable membrane of sample 1 after staining. Figure 6B is an optical microscope image of the composite semi-permeable membrane of sample 3 after staining. Figure 6C is an optical microscope image of the composite semi-permeable membrane of sample 7 after staining. In Figures 6A, 6B, and 6C, the darker areas show the areas where defective dye has penetrated. The brighter areas show the areas where less dye has penetrated and there are fewer defects. As shown in Figure 6A, the composite semi-permeable membrane of sample 1 exhibits fewer defects. A few defects are generated parallel to the warp direction. As shown in Figure 6B, the composite semi-permeable membrane of sample 3 shows almost no defects. As shown in Figure 6C, the composite semi-permeable membrane of sample 7 exhibits significant defects along the warp direction. [Industrial Applicability] This invention is useful for wastewater treatment systems such as the ZLD system. 2a, 2b, 2c, 2d, 2e, 2f, 2g, 2h, 2i: Flow path 4a, 4b, 4c, 4d, 4e: Flow path 11: Membrane leaf 12, 121: Composite semi-permeable membrane 12a: Porous support membrane 12b: Surface layer 12c: Coating 12p: Membrane surface 13: Raw water side flow path material 14: Permeable side flow path material 14m: Tank 20: Spiral membrane element 21: Water collection pipe 21h: Through-hole 22: Laminated body 100: Water treatment system 101: Sample 102: Fixing pin 103: Pressure plate 104: Glass plate 105: Ultrapure water 106: Spherical probe 107: Cantilever 108: Laser beam 110: Low-pressure RO membrane module 120: Medium-pressure RO membrane module 130: High-pressure RO membrane module 140, 160: Ultra-high-pressure RO membrane module 150: NF membrane module F: Water pressure Figure 1 is a perspective view of the spiral membrane element of Embodiment 1. Figure 2 is a cross-sectional view of the composite semipermeable membrane used in the spiral membrane element shown in Figure 1. Figure 3 is a cross-sectional view illustrating the mechanism of damage to the composite semipermeable membrane. Figure 4 is a structural diagram of the water treatment system of Embodiment 2. Figure 5 is a diagram illustrating the method of measuring the force curve of AFM in water. Figure 6A is an optical microscope image of the composite semipermeable membrane of Sample 1 after staining. Figure 6B is an optical microscope image of the composite semipermeable membrane of Sample 3 after staining. Figure 6C is an optical microscope image of the composite semipermeable membrane of Sample 7 after staining. 11: Membranous leaf 12: Composite semi-permeable membrane 13: Raw water side flow road materials 14: Through sideflow road materials 20: Spiral membrane element 21: Water collection pipe 21h: Through hole 22: Laminated body

Claims

1. A composite semipermeable membrane comprising: a porous support membrane, a surface layer supported by the porous support membrane, and a coating covering the surface layer, wherein the surface layer is made of polyamide, the coating contains a polymer, the polymer comprising at least one selected from the group consisting of polyvinyl alcohol, betaine polymers, and polyazoline, wherein the elastic modulus of the membrane surface, calculated by force curve determination using AFM in water, is 250 MPa or more and 500 MPa or less, and the composite semipermeable membrane is suitable for ZLD systems.

2. A spiral membrane element comprising a composite semipermeable membrane as claimed in claim 1.

3. The spiral membrane element of claim 2, further comprising a permeable side flow path material, wherein the permeable side flow path material comprises a Tricot woven fabric having a weave density of 31 to 60 warp loops and 34 to 52 weft loops, wherein the composite semipermeable membrane is in contact with the permeable side flow path material.

4. A water treatment system comprising a spiral membrane element as claimed in claim 2, and wherein the system is a ZLD system.

5. The water treatment system of claim 4, comprising: a low-pressure RO membrane module, a medium-pressure RO membrane module for further concentrating the wastewater concentrated by the low-pressure RO membrane module, and a high-pressure RO membrane module for further concentrating the wastewater concentrated by the medium-pressure RO membrane module, wherein the medium-pressure RO membrane module includes the spiral membrane element of claim 2.

6. A water treatment method, comprising: Wastewater is concentrated using a low-pressure RO membrane module; The wastewater concentrated by the low-pressure RO membrane module is further concentrated using a medium-pressure RO membrane module; and the wastewater concentrated by the medium-pressure RO membrane module is further concentrated using a high-pressure RO membrane module. The above water treatment method is applicable to the ZLD system. The medium-pressure RO membrane module includes a spiral membrane element as claimed in claim 2. The supply pressure of the low-pressure RO membrane module is lower than the supply pressure of the medium-pressure RO membrane module. The supply pressure of the medium-pressure RO membrane module is lower than the supply pressure of the high-pressure RO membrane module, and the supply pressure of the medium-pressure RO membrane module is 2.0 MPa or more and 4.0 MPa or less.

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