Method for producing membrane element

Abstract

Provided is a method for producing a membrane element that includes a blocking rate adjustment step of adjusting a blocking rate by passing a chemical solution through a primary side channel of a membrane element. When the chemical solution is passed in the blocking rate adjustment step, a pressure difference added to both membrane surfaces of the membrane element is set to be smaller than an osmotic pressure of the chemical solution.

Description

Membrane element manufacturing method

[0001] The present invention relates to a method for producing a membrane element.

[0002] Industrial wastewater discharged from factories contains a mixture of various organic substances, but has traditionally been incinerated as is. Incineration consumes a large amount of energy, and improvements are needed from the perspectives of reducing environmental impact (environmental protection) and building a recycling-oriented society (sustainable society). Therefore, methods have been proposed to reduce energy consumption by subjecting industrial wastewater to various membrane processes to concentrate and reduce its volume before incinerating it.

[0003] As such membrane treatment, reverse osmosis (RO), which has been widely used in seawater desalination and the like, is an advantageous method. Reverse osmosis is a technology that applies pressure to the concentrated solution side separated by a semipermeable membrane, such as a reverse osmosis membrane (RO membrane) or a nanofiltration membrane (NF membrane), and selectively allows a solvent to permeate through the semipermeable membrane by increasing the operating pressure difference between the treated liquid side (primary side) and the permeation side (secondary side) to be greater than the osmotic pressure difference between the concentrated solution side and the dilute solution side. Membrane treatment using reverse osmosis typically uses a membrane element containing a semipermeable membrane. However, due to the high rejection of the membrane (membrane element), the use conditions of the membrane element are somewhat limited, resulting in a narrow range of application. For example, the membrane element used has an upper limit on the operable operating pressure difference, which limits the concentration of the solution to which reverse osmosis can be applied. Furthermore, as the treatment progresses, the treated water becomes more concentrated, and if the gradually increasing osmotic pressure exceeds the maximum operating pressure of the membrane element, membrane treatment becomes impossible. Furthermore, membrane clogging is likely to occur when the water being treated is concentrated. Therefore, in order to apply reverse osmosis to the treatment of concentrating industrial wastewater containing high concentrations of mixtures, or to the treatment of concentrating industrial wastewater containing low concentrations of mixtures to high concentrations, improvements to the semipermeable membrane and membrane element are required.

[0004] The above-mentioned operating conditions and application range can be alleviated or broadened by using a semipermeable membrane or membrane element with a low rejection. For example, by setting the semipermeable membrane rejection low and using a membrane element equipped with a semipermeable membrane that preferentially permeates water and slightly permeates solutes such as mixtures, the pressure difference between the primary and secondary pressures (transmembrane pressure difference) is reduced, thereby easing the operating conditions and broadening the application range. Furthermore, clogging of the semipermeable membrane can be suppressed. By using such a membrane element, even highly concentrated treated water can be concentrated, and even low-concentration treated water can be concentrated to a high concentration by single-stage or multi-stage membrane treatment. It is important that the semipermeable membranes and membrane elements used in such methods have a low rejection adjusted and controlled to keep the gradually increasing osmotic pressure of the treated liquid within the maximum operating pressure of the membrane element. Several methods for reducing the rejection of semipermeable membranes have been proposed. For example, Patent Document 1 describes a method in which an aqueous solution of chlorine-based inorganic compounds is passed through an RO membrane to chemically damage the RO membrane.

[0005] Patent No. 5037175

[0006] Here, it is relatively easy to adjust and control the rejection of a semipermeable membrane to a predetermined value for a semipermeable membrane or a flat membrane before it is incorporated into a membrane element. However, because a semipermeable membrane incorporated into a membrane element is tightly packed within a casing (housing), it is difficult to uniformly chemically damage the semipermeable membrane in the thickness and plane directions, making it difficult to adjust and control the rejection to the desired value. For example, one method for adjusting the rejection of a membrane element is to pass an aqueous solution containing a very low concentration of chlorine-based compounds through the membrane element to cause permeation through the semipermeable membrane and chemically damage the semipermeable membrane. However, this method requires a long period of time for passing the aqueous solution containing chlorine-based compounds (time of contact with the membrane), resulting in poor operational efficiency. Furthermore, there is no correlation between the flow time of the aqueous solution containing chlorine-based compounds and the amount of decrease in rejection. As described below, once a certain flow time is exceeded, the rejection rapidly and significantly decreases, making it impossible to adjust the rejection to the desired value. This problem also exists in the method described in Patent Document 1. Specifically, in this method, "2.4 ml of 0.167 mg / l sodium hypochlorite aqueous solution is ３ In this case, a long time of 90 days is required for the flow of the solution at a flow rate of 1000 / h. Furthermore, there is no correlation between the time of flow and the amount of decrease in permeability, and it is not possible to control the amount of decrease in rejection to achieve the desired rejection.

[0007] An object of the present invention is to provide a method for producing a membrane element that can produce a membrane element having a predetermined rejection rate in a short period of time by controlling chemical damage to the membrane caused by a chemical solution.

[0008] The present inventors investigated and considered the progress and extent of chemical damage to the semipermeable membrane caused by the chemical solution, based on the premise that the use of a chemical solution containing a relatively high concentration of a damaging component can cause chemical damage to the semipermeable membrane in a short period of time. As a result, they found that the time during which the chemical solution passes through the semipermeable membrane (contact time) drops sharply after a certain point, and as a result, they were unable to find a specific relationship between the time of passage and the rejection rate (degree of chemical damage) that holds throughout the entire time of passage. Therefore, instead of the method commonly used as a treatment method for membrane modules in which the chemical solution is passed through the semipermeable membrane (a method in which the chemical solution is passed through the primary flow path and the water in the chemical solution is passed through and circulated in the secondary flow path), they came up with the idea of ​​applying a method in which the chemical solution is passed through the primary flow path, but water is not passed through or circulated in the secondary flow path, leaving an air layer. Further investigations based on this idea have revealed that, by setting the pressure difference applied to both membrane surfaces of the membrane module to be smaller than the osmotic pressure of the chemical solution to be used, it is possible to uniformly chemically damage the semipermeable membrane incorporated in the housing in both the thickness and planar directions, even when using a membrane module. Furthermore, it has been discovered that a linear relationship exists between the product of the concentration of the damaging component in the chemical solution and the time the chemical solution is passed through (contact time with the chemical solution) (the exposure intensity of the damaging component to the semipermeable membrane), and the amount of chemical damage to the semipermeable membrane (rejection rate) over the entire range of fluctuations in the exposure intensity. Furthermore, it has been discovered that by determining the exposure intensity based on this linear relationship, a predetermined rejection rate can be achieved and set in a short period of time. The present invention was completed based on these findings.

[0009] That is, the objects of the present invention have been achieved by the following means. <1> A method for manufacturing a membrane element, including a rejection adjustment step of passing a chemical solution through a primary flow path of a membrane element to adjust the rejection, wherein when passing the chemical solution in the rejection adjustment step, the pressure difference applied to both membrane surfaces of the membrane element is set to be smaller than the osmotic pressure of the chemical solution. <2> A method for manufacturing a membrane element according to <1>, wherein the membrane element is a reverse osmosis membrane element or a nanofiltration membrane element. <3> A method for manufacturing a membrane element according to <1> or <2>, wherein the chemical solution is an aqueous solution containing hypochlorite. <4> A method for manufacturing a membrane element according to <3>, wherein the available chlorine concentration of the hypochlorite is 2 to 10 mass %.

[0010] The present invention provides a method for producing a membrane element that can control chemical damage to the membrane caused by a chemical solution, and as a result, can produce a membrane element having a predetermined rejection rate in a short period of time. The above and other features and advantages of the present invention will become more apparent from the following description.

[0011] Fig. 1 is a schematic diagram illustrating one embodiment of a system capable of carrying out the method for producing a membrane element of the present invention. Fig. 2 is a graph showing the relationship between the rejection rate and the product of the concentration of a damaging component in a chemical solution and the contact time of the chemical solution in Examples, Comparative Examples, and Reference Examples.

[0012] In the present invention and this specification, a semipermeable membrane that has a reduced rejection (performance) and allows water to pass preferentially while allowing solutes to pass through is referred to as a "loose membrane" or "deteriorated membrane." The rejection is not particularly limited and can be, for example, 90% or less, preferably 20 to 80%. On the other hand, a semipermeable membrane with a high rejection (a membrane whose rejection has not been reduced) is sometimes referred to as a "high-performance membrane." The rejection is not particularly limited, but is typically 95% or more. Unless otherwise specified, the rejection of a semipermeable membrane also includes the rejection of a membrane element. In the present invention and this specification, the reverse osmosis method encompasses both the RO method and the NF (nanofiltration) method. Therefore, unless otherwise specified, the terms RO membrane, RO membrane element, and RO membrane module encompass RO membrane and NF membrane, RO membrane element and NF membrane element, RO membrane module, and NF membrane module, respectively. In addition, in the present invention and this specification, a "membrane element" refers to a component equipped with a semipermeable membrane, such as a component constructed by integrating a semipermeable membrane, a support, and a flow path material, and a "membrane module" refers to an assembly in which one or more membrane elements are housed and integrated in a pressure vessel, which serves as a basic unit constituting a plant. Here, a membrane element or a membrane module having a loose membrane is referred to as a "loose membrane element" or a "loose membrane module," respectively. In the present invention and this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0013] [Method for manufacturing a membrane element] The method for manufacturing a membrane element of the present invention (hereinafter sometimes referred to as the "manufacturing method of the present invention") includes the following rejection adjustment step. <Rejection adjustment step> A step of adjusting the rejection of a membrane element by making the pressure difference applied to both membrane surfaces of a membrane element smaller than the osmotic pressure of the chemical solution and passing the chemical solution through the primary flow path of the membrane element As described above, the manufacturing method of the present invention is a method for adjusting and reducing the rejection of a semipermeable membrane (membrane element) by chemically damaging the semipermeable membrane by flowing the chemical solution (evenly) through the primary flow path while suppressing water permeation into the secondary flow path by setting the pressure difference applied to both membrane surfaces of a membrane element to be smaller than the osmotic pressure of the chemical solution.

[0014] Generally, membrane or membrane module treatment is performed by permeating a chemical solution through a semipermeable membrane, as described in Patent Document 1. That is, the pressure difference (transmembrane pressure) applied to both membrane surfaces is set to be equal to or greater than the osmotic pressure of the chemical solution. In contrast, in the production method of the present invention, when the chemical solution is passed through the primary flow path of the membrane element, the transmembrane pressure is set to be smaller than the osmotic pressure of the chemical solution. This prevents water in the chemical solution from passing through the semipermeable membrane and into the secondary flow path (the secondary flow path is an air layer at this time). This allows for highly and uniform control of chemical damage (oxidation) of the semipermeable membrane by the chemical solution in both the thickness direction and the planar direction, even for semipermeable membranes tightly integrated within a casing. As a result, the rejection rate of the semipermeable membrane can be set to a predetermined value in a short time. Although the reasons for this are not clear in detail, in conventional treatment methods, the permeation rate of damaging components in the chemical solution is slower than that of water. Therefore, for damaging components that permeate with water, a concentration distribution (concentration gradient) occurs at least in the thickness direction of the semipermeable membrane, which makes the degree of concentration distribution and the rate of progression more likely to change. As a result, it is thought that chemical damage in at least the thickness direction of the semipermeable membrane is difficult to control and changes rapidly. In contrast, in the rejection adjustment process of the present invention, since water is not circulated (permeated) through the semipermeable membrane, the permeation rate of the damaging component is constant in the thickness direction of the semipermeable membrane due to diffusion control, and the occurrence of a concentration distribution of the damaging component at least in the thickness direction of the semipermeable membrane can be suppressed. As a result, chemical damage can be caused to progress uniformly in the thickness direction and planar direction of the semipermeable membrane (highly controlled rejection), and a linear relationship is established between the exposure intensity of the damaging component to the semipermeable membrane and the rejection over the entire range of fluctuations in the exposure intensity. It is thought that based on this linear relationship, a predetermined rejection can be achieved and set in a short period of time.

[0015] In a membrane module, there are generally two types of rejection (removal rate): apparent rejection and true rejection. In the present invention, the "rejection rate" refers to the apparent rejection rate Robs [-] defined by the following formula (1): Formula (1): Robs = 1 - (Cp / Cb) In formula (1), Cb is the concentration of the liquid to be treated [mol / m ３ ], and Cp is the permeate concentration [mol / m３ In the present invention, the rejection of the membrane element or membrane (membrane incorporated in the membrane element) to be treated is not particularly limited, but typically exhibits a high rejection, preferably 95% or more. On the other hand, the rejection of the membrane element or membrane produced by the production method of the present invention is not particularly limited and can be set to an appropriate rejection. For example, the rejection of the loose membrane mentioned above can be mentioned. In this way, the production method of the membrane element of the present invention can convert a semipermeable membrane into a loose membrane by reducing the rejection, and therefore can also be called a production method of a loose membrane element.

[0016] [Membrane Element] The membrane element used in the present invention is not particularly limited as long as it is a component formed by integrating a membrane, a support, and a flow path agent, and a general membrane element can be used. Examples of membrane elements include membrane elements equipped with RO membranes or NF membranes, such as reverse osmosis membrane elements and nanofiltration membrane elements. In the rejection adjustment step of the present invention, the rejection of the semipermeable membrane incorporated in the membrane element can be highly controlled, so the structure of the membrane element used is not particularly limited, and examples include spiral membrane elements and hollow fiber membrane elements. In particular, spiral membrane elements, in which the membrane is housed in a spirally wound state in a casing and therefore rejection adjustment is more difficult, are preferably used.

[0017] Examples of semipermeable membranes incorporated into membrane elements include semipermeable membranes used in reverse osmosis, such as RO membranes and NF membranes. The materials constituting these semipermeable membranes are not particularly limited, and examples include cellulose acetate, aromatic polyamide, polyvinyl alcohol, and polysulfone, with aromatic polyamide being preferred. The above materials may be either non-crosslinked or crosslinked. The degree of crosslinking in the crosslinked material is not particularly limited, and may be fully crosslinked or partially crosslinked.

[0018] The membrane element may be manufactured as appropriate, or a commercially available product may be used.

[0019] [Chemical Solution] The chemical solution used in the production method of the present invention is typically a mixture of a damaging component and water, and is preferably an aqueous solution containing the damaging component. The damaging agent can be appropriately determined depending on the material of the semipermeable membrane to be treated, the desired rejection, and other factors, and examples thereof include inorganic halogen-based oxidizing agents, oxygen-based oxidizing agents, and organic compound oxidizing agents. Preferred inorganic halogen-based oxidizing agents are inorganic oxidizing agents containing chlorine atoms, such as chlorate, hypochlorite, chlorite, chlorine dioxide, chloramine, and N-chloroisocyanurate. The cations constituting the various salts are not particularly limited, and examples thereof include cations of elements belonging to Group 1 or Group 2 of the periodic table, ammonium cations, and organic cations. An example of an organic compound oxidizing agent is dimethyl sulfoxide. Specific examples of the chemical solution include an aqueous solution containing hypochlorite, an aqueous dimethylformamide solution, and an aqueous dimethyl sulfoxide solution.

[0020] The concentration of the damaging component in the chemical solution is not particularly limited and can be appropriately determined taking into consideration, for example, the time difference depending on the location on the membrane, productivity, etc., as described below. The concentration of the damaging component in the chemical solution can be determined by conducting the preliminary experiment described below, in the same manner as the chemical solution flow time, preferably within the range described below. For example, when a sodium hypochlorite aqueous solution is used as the chemical solution, the sodium hypochlorite concentration can be 2 to 10 mass% in terms of effective chlorine concentration. Generally, the time from the start of flowing the chemical solution until contact with the solution and the time from the end of treatment until the chemical solution is discharged and cleaning is completed vary depending on the location on the membrane, and the actual treatment time varies depending on the location on the membrane. When the concentration of the damaging component in the chemical solution is high, the treatment time is shortened, resulting in a relatively large time difference depending on the location on the membrane. This causes uneven exposure intensity within the membrane, making it difficult to control the level of chemical damage. Furthermore, when the concentration of the damaging component in the chemical solution is low, it takes a long time to perform the rejection adjustment process (reducing the rejection to the desired value), resulting in low productivity. In consideration of these points, the treatment time in the production method of the present invention is preferably 30 minutes or more and less than 300 minutes, and more preferably 90 minutes or more and less than 180 minutes.

[0021] The temperature and pH of the chemical solution are not particularly limited and can be determined appropriately. For example, the temperature can be set to 0 to 45°C, but is preferably set to 20 to 30°C in consideration of the degree of chemical damage, workability, etc.

[0022] [Rejection Adjustment Step] The production method of the present invention (rejection adjustment step) can be carried out by appropriately selecting an apparatus configuration (system) that can pass the chemical solution through the primary flow path (concentration flow path) of the membrane element according to the method and conditions described below. One embodiment of a system for carrying out the production method of the present invention is shown in FIG. 1 . This system 1 includes a membrane element 2 to be treated, a chemical tank 3 that contains the chemical solution, and a wash water tank 4 that contains wash water. The chemical tank 3 has a transfer path 5 that connects the inlet of the chemical tank 3 to the primary flow path of the membrane element 2, and a transfer path 6 that connects the outlet of the primary flow path of the membrane element 2 to the chemical tank 3. The transfer path 5 has a pump 3A and a pump P1 in this order from the upstream side (chemical tank 3 side), and the transfer path 6 has a pump P2 and a valve in this order from the upstream side (membrane element 2 side). The transfer path 5 and the transfer path 6 form a circulation path including the chemical tank 3 and the membrane element 2, and the chemical stored in the chemical tank 3 is transferred to the membrane element 2 via the transfer path 5, flows through the primary flow path, and then is discharged from the membrane element 2 and returns to the chemical tank 3 via the transfer path 6. On the other hand, the wash water tank 4 has a transfer path 7 connecting the wash water tank 4 and the transfer path 5, and a transfer path 8 connecting the transfer path 6 and the wash water tank 4. The transfer path 7 has a pump 4A. The transfer path 7 (including the transfer path 5) and the transfer path 8 (including the transfer path 6) form a circulation path including the wash water tank 4 and the membrane element 2, and the wash water stored in the wash water tank 4 is transferred to the membrane element 2 via the transfer path 7 and the transfer path 5, flows through the primary flow path, and then is discharged from the membrane element 2 and returns to the wash water tank 4 via the transfer path 6 and the transfer path 8. A transfer path 9 equipped with a valve is connected to the outlet of the secondary flow path of the membrane element 2, and this transfer path 9 is connected to the transfer path 6. Using such a system 1, the production method of the present invention (rejection adjustment step) can be carried out.

[0023] As described above, the rejection adjustment step is a step of adjusting the rejection by passing a chemical solution through the primary flow path of the membrane element, and the condition for this step is to set the pressure difference (transmembrane pressure difference) applied to both membrane surfaces of the membrane element to be smaller than the osmotic pressure of the chemical solution. In this way, by passing the chemical solution only through the primary flow path without allowing the water in the chemical solution to pass through the semipermeable membrane, excessive or non-uniform chemical damage (deterioration) of the semipermeable membrane can be suppressed, and chemical damage can be controlled to a high degree or evenly.

[0024] The rejection adjustment step can be performed by any method, as long as it is performed by passing the chemical solution through the membrane element under pressure lower than the osmotic pressure. Typically, a pump is used to pass the chemical solution through the membrane element. Unlike separation and concentration, the rejection adjustment step does not require a pressure equal to or greater than the osmotic pressure. Therefore, the pump used to pass the chemical solution is not particularly limited, and a general-purpose pump can be used. For example, a centrifugal pump or a diaphragm pump can be used. However, since the chemical solution near the semipermeable membrane needs to be renewed during contact with the semipermeable membrane (liquid contact), for example, a pump capable of passing the chemical solution at a flow rate of 10 to 50 L / min for a 4-inch diameter membrane element is preferred, and a pump capable of passing the chemical solution at a flow rate of 15 to 25 L / min is more preferred. When the diameter of the membrane element is other than 4 inches, the flow rate for the pump can also be determined by converting the flow rate for the 4-inch diameter in proportion to the cross-sectional area.

[0025] The conditions for the rejection adjustment step are not particularly limited, as long as the pressure difference (transmembrane pressure difference) applied to both membrane surfaces of the membrane element is set to be smaller than the osmotic pressure of the chemical solution. The primary-side pressure (pressure applied to the chemical solution), secondary-side pressure, and pressure difference (synonymous with the pressure difference between the primary-side pressure and the secondary-side pressure) are all appropriately determined taking into consideration the osmotic pressure of the chemical solution, the maximum operating pressure, and the like. For example, the primary-side pressure can be 0.02 to 0.50 MPa, and the secondary-side pressure can be 0.01 to 0.49 MPa. In the present invention, the pressure difference can be 0.01 to 0.20 MPa, preferably 0.01 to 0.10 MPa, in order to allow the chemical solution to flow evenly through the primary side of the membrane while preventing it from penetrating to the secondary side. Furthermore, the pressure difference between the pressure difference and the osmotic pressure of the drug solution is not particularly limited, but the lower limit of the pressure difference can be, for example, 0.005 MPa or more, and preferably 0.01 MPa or more, in terms of high or uniform control of chemical damage, pressure controllability, etc.

[0026] The flow rate of the chemical solution is not particularly limited and can be determined as appropriate, and for example, for a membrane element with a diameter of 4 inches, it can be set to 10 to 50 L / min, and preferably 15 to 25 L / min. When the membrane element has a diameter other than 4 inches, the flow rate for a diameter of 4 inches can also be determined by converting it in proportion to the cross-sectional area.

[0027] In the rejection adjustment process, the rejection of the semipermeable membrane can be reduced by increasing the concentration of the damaging component in the chemical solution and by increasing the contact time (flow time) of the chemical solution with the semipermeable membrane. Furthermore, the rejection can be reduced linearly (straight-line) with respect to the product of the concentration of the damaging component and the contact time (exposure intensity of the damaging component to the semipermeable membrane). Therefore, the contact time between the chemical solution and the semipermeable membrane can be appropriately determined taking into account the concentration of the damaging component in the chemical solution, the rejection, etc., and is set to the contact time required to achieve the desired rejection. The contact time required to achieve the desired rejection can be determined, for example, by the following method 1 and / or method 2. The contact time thus determined in advance is used as the contact time in the rejection adjustment process using the membrane element. Method 1: The membrane element is disassembled in advance to remove the flat membrane to be treated, and a preliminary experiment is conducted in which the flat membrane is immersed in a chemical solution of a predetermined concentration to directly determine the contact time required to achieve the desired rejection. Method 2: In the same manner as in Method 1, the above preliminary experiment is carried out to determine the linear relationship between the rejection rate and the product of the concentration of the damaging component and the contact time, and the contact time is determined from this linear relationship. Note that in Method 2, the concentration of the damaging component in the chemical solution can also be determined instead of or in addition to the contact time.

[0028] In the present invention, the product of the concentration (mass%) of the damaging component and the contact time (min) is not particularly limited. For example, for a sodium hypochlorite aqueous solution, it can be 10 to 1,000 (mass% min), preferably 30 to 500 (mass% min), and more preferably 100 to 300 (mass% min). Furthermore, the contact time in the rejection adjustment process using a membrane element is determined as described above and can be significantly shortened compared to conventional rejection reduction conditions, such as the 90 days described in Patent Document 1, specifically, as described above. The contact time is measured from the point (0 seconds) when the chemical solution begins to pass through the membrane element. The linear relationship described above cannot be uniquely determined due to variations in the concentration of the damaging component, membrane size, and chemical flow rate, and is therefore usually determined by preliminary experiments such as those described above.

[0029] The secondary flow path of the membrane element is normally filled with air and does not allow water to pass through, but in the present invention, it may be filled in advance with a liquid that does not contain damaging components, such as water or an inert gas. By performing the rejection adjustment step in this manner, the rejection of the membrane can be set (reduced) to a predetermined value, preferably within the above range, in a short period of time.

[0030] In the production method of the present invention, the rejection of the semipermeable membrane can be set to a predetermined value by, for example, determining the liquid passage time that will result in the predetermined rejection and the appropriate concentration of the damaging component by the above-mentioned methods 1 and 2, and then carrying out the rejection adjustment step. However, the predetermined rejection can be easily adjusted by carrying out a production method (a preferred embodiment of the production method of the present invention) that applies method 2, which determines in advance the relationship between the rejection and the product of the concentration of the damaging component and the liquid passage time. That is, a preferred embodiment of the production method of the present invention is a production method for a membrane element that includes a rejection adjustment step of adjusting the rejection of the membrane element by passing a chemical solution through the primary flow path of the membrane element while making the pressure difference applied to both membrane surfaces of the membrane element smaller than the osmotic pressure of the chemical solution, and the rejection adjustment step includes the following substeps 1 to 3. Sub-step 1: A sub-step of determining the relationship (linear relationship) between the product of the concentration and the liquid-passing time of the damaging component contained in the chemical solution and the rejection rate, by changing at least one of the concentration and the liquid-passing time of the damaging component contained in the chemical solution when the pressure difference applied to both membrane surfaces of the membrane element is made smaller than the osmotic pressure of the chemical solution and the chemical solution is passed through the primary flow path of the membrane element. Sub-step 2: A sub-step of determining the product of the concentration and the liquid-passing time for a predetermined rejection rate from the relationship obtained in sub-step 1. Sub-step 3: A sub-step of setting the concentration and / or the liquid-passing time to satisfy the product obtained in sub-step 2, and passing the chemical solution through the primary flow path of the membrane element by making the pressure difference applied to both membrane surfaces of the membrane element smaller than the osmotic pressure of the chemical solution.

[0031] In the above-mentioned sub-step 1, it is preferable to carry out multiple types, preferably three or more types, of rejection adjustment steps in which at least one of the concentration of the damage-inflicting component and the liquid-passing time is changed, in order to more accurately determine the above relationship. In this case, the condition to be changed may be only the concentration, only the liquid-passing time, or both the concentration and the liquid-passing time. The conditions other than the concentration and the liquid-passing time in the rejection adjustment step in sub-step 1 are the same as the conditions in the rejection adjustment step in the production method of the present invention.

[0032] In the sub-process 2, after a plurality of types of rejection adjustment processes are performed in the sub-process 1, for example, as shown in FIG. 2 , for each rejection adjustment process, (concentration × liquid-flow time, rejection) is plotted as (X, Y) on a Cartesian coordinate system to obtain an approximation line showing the relationship between the product of the concentration and the liquid-flow time and the rejection.

[0033] In the above-mentioned substep 3, the product of the concentration and the liquid-flow time resulting in a predetermined rejection is determined from the approximation line obtained in substep 2, and the concentration and / or the liquid-flow time in the rejection adjustment step of substep 3 is determined. Thereafter, the determined concentration and / or the liquid-flow time are set, and substep 3 is carried out in the same manner as the rejection adjustment step in the production method of the present invention. The conditions in the rejection adjustment step of substep 3 other than the concentration and the liquid-flow time are the same as the conditions in the rejection adjustment step in the production method of the present invention.

[0034] [Cleaning Step] In the present invention, as described above, after the rejection adjustment step or substep 3 in the production method of the present invention is performed (after a predetermined liquid flow time has been reached), it is preferable to discharge the chemical solution from the membrane element and then perform a cleaning step in which a cleaning solution is passed through. In this way, by quickly discharging the chemical solution and stopping further progression of chemical damage, the rejection can be set to a predetermined value. The method for discharging the chemical solution from the membrane element is not particularly limited. For example, a gas may be passed through the membrane element, but passing cleaning water is preferred. The cleaning solution used in the cleaning step may be any solution that does not chemically damage the membrane, but is typically water, such as tap water, ion-exchanged water, purified water, or (ultra)pure water. The method and conditions for performing the cleaning step are not particularly limited as long as the cleaning solution can be passed through at least the primary side of the membrane element. For example, the same method as that for performing the rejection adjustment step can be applied to perform the cleaning step, and the conditions for performing the cleaning step can be determined appropriately. The cleaning step can be performed continuously in one go, or it is preferable to perform it multiple times with different cleaning solutions, as this reliably stops the progression of chemical damage.

[0035] [Other Steps] In the production method of the present invention (including preferred embodiments of the production method of the present invention), steps other than the rejection adjusting step (including substeps 1 to 3) and the washing step may be carried out. For example, a step of determining the contact time required to reach a desired rejection (preliminary experiment) may be included.

[0036] The production method of the present invention performs a simple rejection adjustment step in which the pressure difference applied to both membrane surfaces of the membrane element is set to be smaller than the osmotic pressure of the chemical solution and the chemical solution is passed through the primary flow path, thereby making it possible to highly control the rejection of the membrane (membrane element) and set the target rejection in a short period of time. Furthermore, commercially available membrane elements can be used, and membrane elements exhibiting the target rejection can be produced simply, quickly, and with good workability.

[0037] The manufacturing method of the present invention is a method for manufacturing a membrane element for treating a membrane element, but it can also be used for treating a membrane module in which a plurality of membrane elements are housed in a pressure vessel as an integrated unit, and the rejection of the plurality of membrane elements is adjusted at once to produce a membrane module in which a plurality of membrane elements set to a predetermined rejection are housed in a pressure vessel. That is, the manufacturing method of the membrane element of the present invention can also be applied to a membrane module, and in this case it can also be said to be a method for manufacturing a membrane module exhibiting a predetermined rejection.

[0038] In the production method of the present invention, the membrane element is the object of treatment, and therefore the membrane element itself before being incorporated into a plant can be the object of treatment alone, or the membrane element incorporated into a plant can be the object of treatment.

[0039] The membrane module produced by the production method of the present invention can be used as it is or together with other membrane elements, preferably housed in a pressure-resistant vessel as a membrane module, for treating various liquids to be treated. The liquid to be treated is not particularly limited, and examples thereof include various industrial wastewaters containing organic solvents and other organic compounds, seawater, and recycled liquids in which valuable materials are dissolved.

[0040] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0041] Example 1: Production of a membrane element with reduced rejection. A TM810V membrane element (trade name, built-in crosslinked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used as a membrane element. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 2.0% by mass (assumed osmotic pressure of 1.2 MPa) was passed through the element for 19 minutes at a pressure difference of 0.06 MPa (primary pressure: 0.07 MPa, secondary pressure: 0.01 MPa) applied to both membrane surfaces (hereinafter referred to as membrane surface differential pressure) and a flow rate of 18 L / min. This was used for a rejection adjustment step. After passing the solution through the element (19 minutes after the start of the flow), the sodium hypochlorite aqueous solution was promptly drained from the membrane element, and the element was then washed three times with changing water (washing volume: 20 L of water per wash) to carry out a cleaning step. The rejection of the membrane element thus produced was measured using the following method. The results are shown in Table 1.

[0042] <Method of Measuring Rejection Rate> In Example 1, the apparent rejection rate of the element was calculated using the following formula, taking into account the increase in concentration within the element.

[0043] In equation (2), the initial feed flow rate is Qf [m ３ / s], initial salt concentration is Wf [wt%], final feed flow rate is Qr [m ３ / s], and the final salt concentration is Wr [wt%]. The initial salt concentration is not the concentration of the feed liquid, but the concentration after circulation including hold-up water. The final feed flow rate and final salt concentration were calculated by measuring the permeate flow rate and permeate salt concentration and calculating from the material balance on the feed side and permeate side.

[0044] Example 2 A TM810V membrane element (product name, built-in cross-linked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 5.0% by mass (assumed osmotic pressure: 1.2 MPa) was passed through the membrane element at a membrane surface differential pressure of 0.06 MPa (primary pressure: 0.07 MPa, secondary pressure: 0.01 MPa) and a flow rate of 18 L / min for 19 minutes to perform a rejection adjustment step. After passing the solution through the membrane element, the sodium hypochlorite aqueous solution was promptly drained from the membrane element. The membrane element was then washed three times with changing water (washing volume: 20 L / time) to perform a washing step. The rejection of the membrane element thus produced was measured in the same manner as in Example 1. The results are shown in Table 1.

[0045] Example 3 A TM810V membrane element (product name, built-in cross-linked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 2.3% by mass (assumed osmotic pressure: 1.2 MPa) was passed through the membrane element at a membrane surface differential pressure of 0.06 MPa (primary pressure: 0.07 MPa, secondary pressure: 0.01 MPa) and a flow rate of 18 L / min for 62 minutes to perform a rejection adjustment step. After passing the solution through the membrane element, the sodium hypochlorite aqueous solution was promptly drained from the membrane element. The membrane element was then washed three times with changing water (washing volume: 20 L / time) to perform a washing step. The rejection of the membrane element thus produced was measured in the same manner as in Example 1. The results are shown in Table 1.

[0046] Comparative Example 1 A TM810V membrane element (product name, built-in cross-linked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 1.9% by mass (assumed osmotic pressure: 1.2 MPa) was passed through the membrane element at a membrane surface differential pressure of 1.99 MPa (primary pressure: 2.00 MPa, secondary pressure: 0.01 MPa) and a flow rate of 10 L / min for 20 minutes to perform a rejection adjustment step. After passing the solution through the membrane element, the sodium hypochlorite aqueous solution was promptly drained from the membrane element. The membrane element was then washed three times with changing water (washing volume: 20 L / time) to perform a washing step. The rejection of the membrane element thus produced was measured in the same manner as in Example 1. The results are shown in Table 1.

[0047] Comparative Example 2 A TM810V membrane element (product name, built-in cross-linked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 1.9% by mass (assumed osmotic pressure: 1.2 MPa) was passed through the membrane element at a membrane surface differential pressure of 1.99 MPa (primary pressure: 2.00 MPa, secondary pressure: 0.01 MPa) and a flow rate of 10 L / min for 50 minutes to perform a rejection adjustment step. After passing the solution through the membrane element, the sodium hypochlorite aqueous solution was promptly drained from the membrane element. The membrane element was then washed three times with changing water (washing volume: 20 L / time) to perform a washing step. The rejection of the membrane element thus produced was measured in the same manner as in Example 1. The results are shown in Table 1.

[0048] Comparative Example 3 A TM810V membrane element (product name, built-in cross-linked fully aromatic polyamide RO membrane, 4-inch diameter, 99.8% rejection (salt rejection), manufactured by Toray Industries, Inc.) was used. A sodium hypochlorite aqueous solution with an effective chlorine concentration of 1.9% by mass (assumed osmotic pressure: 1.2 MPa) was passed through the membrane element at a membrane surface differential pressure of 1.99 MPa (primary pressure: 2.00 MPa, secondary pressure: 0.01 MPa) and a flow rate of 10 L / min for 73 minutes to perform a rejection adjustment step. After passing the solution through the membrane element, the sodium hypochlorite aqueous solution was promptly drained from the membrane element. The membrane element was then washed three times with changing water (washing volume: 20 L / time) to perform a washing step. The rejection of the membrane element thus produced was measured in the same manner as in Example 1. The results are shown in Table 1.

[0049] [Reference Examples 1 to 6] In Examples 1 to 3 and Comparative Examples 1 to 3, the rejection adjustment step was carried out in the same manner as in each Example and Comparative Example, except that in place of the membrane element, a flat membrane removed from this membrane element was used, and the flat membrane was immersed in the chemical solution for the same time as the liquid passing time. Furthermore, the flat membrane was treated by carrying out the cleaning step in the same manner as in Examples 1 to 3 and Comparative Examples 1 to 3, except that the flat membrane removed from the chemical solution after the rejection adjustment step was immersion-washed three times in 1 L of water. The rejection of the obtained flat membrane was measured in the same manner as in Example 1. The results of each Reference Example are shown as "Flat membrane rejection" in the column for the Example or Comparative Example in which the conditions for the rejection adjustment step were the same.

[0050] Table 1 shows the measured rejection rates and the conditions of the rejection rate adjusting step in each Example and Comparative Example. In Table 1, "concentration" indicates the concentration of the aqueous sodium hypochlorite solution (effective chlorine concentration), and "liquid contact time" indicates the time during which the aqueous sodium hypochlorite solution was passed through (Examples and Comparative Examples) or the time of immersion in the aqueous sodium hypochlorite solution (Reference Example).

[0051] Figure 2 is a graph showing the relationship between concentration x contact time and rejection, where "concentration x contact time" and "rejection rate" in Table 1 are plotted as (X, Y) on a Cartesian coordinate system, and (concentration x contact time, rejection rate). The approximate dashed line in Figure 2 shows the relationship between the product of concentration and contact time and rejection rate in the Reference Example. The rejection rate adjustment steps in Examples 1 to 3 and the preparation of Figure 2 correspond to the above-mentioned method 2 for determining the contact time required to reach the target rejection rate, i.e., sub-step 1 in a preferred embodiment of the production method of the present invention. The approximate line showing the relationship between the product of concentration and contact time and rejection rate in Examples 1 to 3 is Y ≈ -0.004X + 0.9(R ２ = 0.9998).

[0052] The results in Table 1 and Figure 2 reveal the following. First, when a flat membrane is used, a linear relationship is established between the concentration of the chemical solution (sodium hypochlorite aqueous solution) x contact time (immersion time) and rejection in Reference Examples 1 to 3, which correspond to Examples 1 to 3, and Reference Examples 4 to 6, which correspond to Comparative Examples 1 to 3. In contrast, Comparative Examples 1 to 3, in which the differential pressure when passing the chemical solution (sodium hypochlorite aqueous solution) through the membrane element is set to a pressure equal to or greater than the (assumed) osmotic pressure of the chemical solution, are able to reduce the rejection of the RO membrane. However, in Comparative Example 3, in which the contact time was longer, the rejection dropped sharply and significantly compared to Comparative Example 2, and a linear relationship is not established between the concentration x contact time and rejection. Therefore, when a chemical solution is passed through a membrane element at a differential pressure equal to or greater than the osmotic pressure, it is not possible to adjust the rejection of the membrane element to a predetermined value based on the concentration × contact time, particularly when the concentration × contact time exceeds 95 (mass% min). In other words, it is not possible to manufacture a membrane element that exhibits a predetermined rejection based on the concentration × contact time. On the other hand, Examples 1 to 3, in which the differential pressure was set to a pressure lower than the (assumed) osmotic pressure of the chemical solution when passing the chemical through the membrane element, are able to reduce the rejection of the RO membrane. Moreover, even for membrane elements that are more difficult to treat uniformly than flat membranes, the linear relationship between the concentration × contact time and the rejection, as shown by the approximation line above, is established. Therefore, it is clear that by performing the rejection adjustment process using the concentration and / or contact time set based on the concentration × contact time, it is possible to adjust the rejection of the membrane element to a predetermined value, i.e., it is possible to manufacture a membrane element that exhibits a predetermined rejection.

[0053] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0054] This application claims priority based on Japanese Patent Application No. 2024-054170, filed on March 28, 2024, the contents of which are incorporated herein by reference as part of the present specification.

[0055] 1 System 2 Membrane element 3 Chemical tank 3A Pump 4 Washing water tank 4A Pump 5-9 Transfer paths P1, P2 Pump

Claims

1. A method for manufacturing a membrane element, which includes a rejection adjustment step of passing a chemical solution through a primary flow path of a membrane element to adjust the rejection, wherein when passing the chemical solution in the rejection adjustment step, the pressure difference applied to both membrane surfaces of the membrane element is set to be smaller than the osmotic pressure of the chemical solution.

2. The method for producing a membrane element according to claim 1, wherein the membrane element is a reverse osmosis membrane element or a nanofiltration membrane element.

3. A method for manufacturing a membrane element according to claim 1 or 2, wherein the chemical solution is an aqueous solution containing hypochlorite.

4. The method for producing a membrane element according to claim 3, wherein the available chlorine concentration of the hypochlorite is 2 to 10 mass %.

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