Method and device for cleaning separation membrane or concentration membrane

Abstract

The purpose of the present invention is to provide a method for cleaning a membrane that makes it possible to remove deposits adhering to a membrane surface, restore the permeability of the membrane, and maintain the filtering and concentration performance of the membrane, and that does not cause deterioration in the strength of a member constituting a membrane module. This method for cleaning a separation membrane or a concentration membrane is characterized by comprising a cleaning step for bringing a separation membrane or a concentration membrane into contact with a reduction cleaning liquid at a pH of not less than 7.0.

Description

Method and apparatus for cleaning separation or concentration membranes

[0001] The present invention relates to a method and apparatus for cleaning a separation membrane or a concentration membrane.

[0002] When membranes are used for separation or concentration, deposits contained in the liquid to be separated or concentrated, or deposits generated by reactions on or inside the membrane during separation or concentration, accumulate on the membrane surface or inside, reducing permeability. Therefore, by cleaning the membrane surface or inside periodically or when permeability decreases, separation or concentration can be performed while maintaining permeability. Cleaning is the process of removing deposits from the membrane, but it is essential that it does not damage the membrane or other components. Membrane cleaning methods are sometimes classified based on the size of the deposits removed; namely, methods that separate the deposits as clumps, methods that dissolve the deposits, and aggressive cleaning methods that break down the deposits by their molecular structure. For methods of separating clumps of deposits, surfactants function as this mechanism, providing relatively mild conditions that do not cause degradation to the membrane material. For methods of dissolving deposits, various acids, alkalis, oxidizing agents, reducing agents, and EDTA function as this mechanism, increasing water solubility and improving cleaning effectiveness. The method of breaking down the fouling is to use a strong alkali such as caustic soda or an oxidizing agent such as sodium hypochlorite, but these are harsh conditions that can cause the strength of the membrane material to deteriorate. While membrane deposits can be caused by a single substance, there are many cases where multiple substances are attached to the membrane. For example, when the deposits are inorganic or organic, acid cleaning and alkali cleaning can be combined.

[0003] As an example of chemical cleaning of membranes, there is known a method for treating chemical cleaning wastewater, in which a first step is performed using a mixed solution of sodium hydroxide and sodium hypochlorite as the cleaning chemical, and a second step is performed using a reducing acid solution such as a sodium bisulfite solution, a sodium hydrogen sulfite solution, a sodium sulfite solution, or a sodium thiosulfate solution as the cleaning chemical (Patent Document 1).

[0004] Furthermore, a method is known in which a filter is installed in the piping when cleaning a membrane module to prevent contaminants that have peeled off from the membrane from re-adhering (Patent Document 2). In this method, the chemical cleaning is not particularly limited, and the method describes a combination of acid cleaning using an acid such as hydrochloric acid, sulfuric acid, nitric acid, citric acid, or oxalic acid with alkaline cleaning using an alkali such as caustic soda or sodium hypochlorite, or the use of hydrogen peroxide, a surfactant, or the like.

[0005] Furthermore, a chemical cleaning method for membrane modules has been proposed in which the cleaning water is washed with purified water following chemical cleaning, in which the purified cleaning water discharged from the membrane module is treated in a recovery membrane module and reused (Patent Document 3). This document does not specifically limit the chemical solution, and describes aqueous solutions of inorganic acids such as hydrochloric acid, nitric acid, sulfuric acid, and hydrofluoric acid; aqueous solutions of organic acids such as citric acid and oxalic acid; aqueous solutions of hypochlorite, oxidizing agents and reducing agents such as hydrogen peroxide and bisulfites; and aqueous solutions of detergents and surfactants. Non-Patent Document 1, a review paper on membranes, provides a comprehensive description of various membrane types and the cleaning chemicals used for cleaning them.

[0006] Japanese Patent Application Laid-Open No. 2005-193119 Japanese Patent Application Laid-Open No. 2006-281022 Japanese Patent Application Laid-Open No. 2005-246361

[0007] Ram Chandra Bogati, “Membrane Fouling and Its Control in Drinking Water Membrane Filtration Process”, 2014

[0008] The methods described in Patent Documents 1 to 3 are unable to sufficiently remove deposits firmly attached to the membrane surface, and may not restore membrane permeability. Furthermore, the sodium hypochlorite described in the above patent documents also has the problem of reducing the strength of the components constituting the membrane module, which may cause problems such as membrane breakage during long-term use of the membrane module. Furthermore, the sodium hypochlorite described in Patent Documents 1 to 3 and Non-Patent Document 1 also has the problem of reducing the strength of the components constituting the membrane module, which may cause problems such as membrane breakage during long-term use of the membrane module. The present inventors have discovered that an effective means for removing deposits from the membrane is to use a cleaning solution that is alkaline and reducing, which can dissolve the deposits and also does not cause oxidative degradation of the material used as the membrane material, eliminating the concern of deterioration during long-term use, and have arrived at the present invention.

[0009] The present invention aims to provide a membrane cleaning method that can remove deposits attached to the membrane surface, restore the membrane permeability, maintain the filtration and concentration performance of the membrane, and prevent deterioration of the strength of the components that make up the membrane module.

[0010] That is, the present invention is as follows. [1] A method for cleaning a separation membrane or a concentration membrane, comprising a cleaning step of contacting a separation membrane or a concentration membrane with a reduction cleaning solution at a pH of 7.0 or higher. [2] The method for cleaning a separation membrane or a concentration membrane according to [1], wherein the reduction cleaning solution has an oxidation-reduction potential of -300 mV or lower. [3] The cleaning method according to [1] or [2], wherein the pH of the reduction cleaning solution is 7.0 or higher and 14.0 or lower. [4] The cleaning method according to any one of [1] to [3], wherein the oxidation-reduction potential of the reduction cleaning solution is -1200 mV or higher and -500 mV or lower. [5] The cleaning method according to any one of [1] to [4], wherein the reduction cleaning solution contains a reducing agent and an alkaline agent, the reducing agent is at least one selected from the group consisting of sodium hydrosulfite, thiourea dioxide, borohydride, and sodium formaldehyde sulfoxylate, and the alkaline agent is at least one selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and sodium bicarbonate. [6] The cleaning method according to any one of [1] to [5], wherein the cleaning step is carried out at a temperature of 1 to 100° C. [7] The cleaning method according to any one of [1] to [6], wherein the reduction cleaning solution further contains a surfactant. [8] The cleaning method according to any one of [1] to [7], wherein the separation membrane or the concentration membrane is a hollow fiber membrane or a flat membrane, the separation membrane or the concentration membrane is housed in a case, a module in which the separation membrane or the concentration membrane is housed in the case is a case-type module, and the cleaning step is a step of filling the case with the reduction cleaning solution and bringing the separation membrane or the concentration membrane into contact with the reduction cleaning solution. [9] The cleaning method according to any one of [1] to [8], wherein the separation membrane or the concentration membrane is a hollow fiber membrane or a flat membrane, the separation membrane or the concentration membrane is housed in a case, a module in which the separation membrane or the concentration membrane is housed in the case is a case-type module, and the cleaning step is a step of permeating the reduction cleaning solution through the surface and interior of the separation membrane or the concentration membrane and bringing the separation membrane or the concentration membrane into contact with the reduction cleaning solution.

[10] The cleaning method according to [8], wherein the case-type module is an external pressure filtration type.

[11] The cleaning method according to [9], wherein the case-type module is an external pressure filtration type.

[12] The cleaning method according to any one of [1] to

[11] , wherein the separation membrane or the concentration membrane is a membrane for separation or concentration in a wastewater treatment process, a water purification process, or a seawater desalination process.

[13] The cleaning method according to any one of [1] to

[12] , wherein the separation membrane or the concentration membrane is a UF membrane, MF membrane, RO membrane, FO membrane, or NF membrane.

[14] The cleaning method according to any one of [1] to

[13] , wherein the separation membrane or the concentration membrane is a membrane containing, as a constituent, at least one resin selected from the group consisting of PVDF, PE, PSF, PAN, PP, PTFE, ETFE, PA, and PES.

[15] The cleaning method according to any one of [1] to

[14] , wherein the cleaning step is a step of contacting at least one selected from the group consisting of the inner surface, the outer surface, and the inside of the separation membrane or the concentration membrane with the reduction cleaning solution.

[16] The method for cleaning the separation membrane or the concentration membrane according to any one of [1] to

[15] , wherein the separation membrane or the concentration membrane is a membrane used for separating or concentrating a solution after aerobic biological treatment.

[17] A cleaning method comprising the step of contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher to clean the separation membrane or the concentration membrane when any of the following conditions is met during operation of an apparatus equipped with the separation membrane or the concentration membrane: a predetermined operating pressure is reached, a predetermined increase in operating pressure is reached, a predetermined operating time is reached, the state of the liquid to be separated or concentrated changes to a predetermined state, or the permeability of the separation membrane or the concentration membrane decreases to a predetermined level.

[18] The cleaning method according to

[17] , comprising a step of contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher to clean the separation membrane or the concentration membrane when any of the following cases is met: when the predetermined operating pressure is reached, when the predetermined increase in operating pressure is reached, when the predetermined operating time is reached, when the liquid changes to the predetermined state of the separation or concentration target liquid, or when the permeability of the separation membrane or the concentration membrane is reduced to the predetermined value; and when any of the following cases is met: when a predetermined second operating pressure is reached, when the predetermined increase in operating pressure is reached, when the predetermined second operating time is reached, when the liquid changes to the predetermined state of the separation or concentration target liquid, or when the permeability of the separation membrane or the concentration membrane is reduced to the predetermined value.

[19] A cleaning device used in the cleaning method according to any of [1] to

[18] , having a part that brings the separation membrane or the concentration membrane into contact with the reducing cleaning solution.

[20] An apparatus equipped with a separation membrane or a concentration membrane, which cleans the separation membrane or the concentration membrane by contacting it with a reducing cleaning solution at a pH of 7.0 or higher when any of the following conditions is met: when a predetermined operating pressure is reached, when a predetermined increase in operating pressure is reached, when a predetermined operating time is reached, when the state of the liquid to be separated or concentrated changes to a predetermined state, or when the permeability of the separation membrane or the concentration membrane decreases to a predetermined level.

[21] The apparatus according to

[20] , wherein the separation membrane or the concentration membrane is cleaned by contacting it with a reducing cleaning solution at a pH of 7.0 or higher when any of the following cases is met: when the predetermined operating pressure is reached, when the predetermined increase in operating pressure is reached, when the predetermined operating time is reached, when the liquid changes to the predetermined state of the liquid to be separated or concentrated, or when the permeability of the separation membrane or concentration membrane is reduced; and when any of the following cases is met: when a predetermined second operating pressure is reached, when a predetermined increase in second operating pressure is reached, when the predetermined second operating time is reached, when the liquid changes to the predetermined state of the liquid to be separated or concentrated, or when the permeability of the separation membrane or concentration membrane is reduced.

[0011] Since the membrane cleaning method of the present invention has the above-mentioned configuration, it can remove deposits attached to the membrane surface, restore the membrane permeability, maintain the filtration and concentration performance of the membrane, and clean the membrane without causing deterioration in the strength of the components that make up the membrane module.

[0012] FIG. 1 is a conceptual diagram of maintenance cleaning and recovery cleaning. FIG. 2 is a diagram of EDX analysis results of membranes used in Examples 1 to 3 and Comparative Examples 1 to 5. FIG. 3 is a diagram of EDX analysis results of Comparative Example 2. FIG. 4 is a diagram of EDX analysis results of Comparative Example 3. FIG. 5 is a diagram of EDX analysis results of Example 2. FIG. 6 is a diagram of EDX analysis results of Example 3. FIG. 7 is a diagram explaining the wastewater treatment process in Examples 10 to 11 and Comparative Examples 9 to 10. FIG. 8 is a diagram explaining the hot water bath in Example 10 etc. FIG. 9 is a diagram explaining the circulation of the cleaning liquid in Example 11 etc.

[0013] Hereinafter, an embodiment for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present embodiment is an example for explaining the present invention, and the present invention is not limited to only the embodiment. In other words, the present invention can be modified in various ways without departing from the gist of the present invention.

[0014] {Method for Cleaning Separation Membrane or Concentration Membrane} The method for cleaning a membrane of this embodiment includes a cleaning step of contacting a separation membrane or a concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher.

[0015] [Separation Membrane and Concentration Membrane] The separation membrane and concentration membrane are not particularly limited as long as they are membranes used for separation or concentration. However, separation membranes that perform separation or concentration using pressure are prone to deposits on the membrane surface, necessitating continuous operation through repeated filtration and cleaning, and therefore require more effective cleaning. Examples of the separation membrane or concentration membrane include ultrafiltration membranes (UF membranes), microfiltration membranes (MF membranes), nanofiltration membranes (NF membranes), and reverse osmosis membranes (RO membranes), which are effectively used in the cleaning method of this embodiment. OARO membranes (osmosis-assisted reverse osmosis membranes) are concentration systems that utilize not only filtration pressure but also osmotic pressure differences by flowing at least a portion of the concentrated liquid to the secondary side. However, they are similar in that pressure is applied to the membrane surface, and like filtration membranes, deposits are prone to deposits and require cleaning, and are effectively used in the cleaning method of this embodiment. Forward osmosis membranes (FO membranes) are membranes used for separation and concentration, in which only solvents such as water move by flowing a draw solution with high osmotic pressure through the secondary side of the membrane. Since no pressure is applied to the membrane, pressure-induced deposition is minimal, but precipitation due to concentration polarization on the membrane surface is possible, and this is effectively used in the cleaning method of this embodiment. Membrane distillation membranes (MD membranes) allow only vapor with a high saturated vapor pressure to move from the liquid flowing on the primary side of the membrane to the secondary side, and do not require pressure. Therefore, deposition of deposits on the primary membrane surface is unlikely, but with long-term use, scale deposition and other deposits may occur, making cleaning necessary, and this is effectively used in the cleaning method of this embodiment. Many RO membranes, NF membranes, OARO membranes, and FO membranes use a thin layer of aromatic polyamide membrane, known as a composite semipermeable membrane, formed on the surface of a base membrane such as a UF membrane, to achieve separation. However, aromatic polyamides are prone to degradation due to oxidation, and sodium hypochlorite, a typical cleaning method that uses oxidation, cannot be used. In this respect, the reducing cleaning solution used in the cleaning method of this embodiment does not degrade aromatic polyamides and can be an effective cleaning method. These composite semipermeable membranes are capable of separating or concentrating at the molecular level, and are therefore used in wastewater treatment and seawater desalination to obtain highly pure water. A common pretreatment process for composite semipermeable membranes is to install a membrane with a larger pore size to reduce fouling of the composite semipermeable membrane.After cleaning the pretreatment membrane, trace amounts of cleaning solution components remain on the pretreatment membrane. When operation is resumed, trace amounts of cleaning solution may reach the composite semipermeable membrane, causing oxidative degradation, which can be a problem. However, the reduction cleaning solution described above is a cleaning method for the pretreatment membrane of a composite semipermeable membrane, and there is no risk of indirectly damaging the composite semipermeable membrane. Therefore, the pretreatment membrane of a composite semipermeable membrane is a suitable membrane for this cleaning solution. The separation membrane or the concentration membrane is preferably a UF membrane, MF membrane, RO membrane, FO membrane, or NF membrane. The ultrafiltration membrane may be a membrane with a pore size of 0.001 to 0.01 μm (e.g., a membrane capable of blocking particles of 2 nm to 0.1 μm). The molecular weight cutoff of the ultrafiltration membrane may be 1,000 to 150,000. A microfiltration membrane refers to a membrane with a pore size of several tens of nanometers to 10 μm, capable of blocking particles larger than 0.1 μm. Furthermore, the nanofiltration membrane may be a membrane with a pore size of 0.0005 μm or less, and refers to a membrane that can block particles of 1 nm or less. The molecular weight cutoff refers to a value measured by a method that determines the rejection rate when a solution containing a standard substance (dextran, insulin, etc.) with a known molecular weight is separated through the membrane. RO membranes, NF membranes, and FO membranes have nano-level separation sizes. While RO membranes can block all electrolyte ions, NF membranes can block only divalent inorganic ions. While RO membranes and NF membranes have a membrane structure that uses pressure as a driving force to obtain permeate water, FO membranes are membranes that use the osmotic pressure difference between two liquids via an aromatic polyamide layer to allow water to pass through, and have a membrane structure that does not require pressure resistance.

[0016] The separation membrane and the concentration membrane may be either an organic membrane or an inorganic membrane. Examples of the organic membrane include a membrane using a resin (for example, a membrane in which the mass ratio of the resin to 100 mass% of the membrane is 90 mass% or more (preferably 95 mass% or more)). Examples of the resin include a fluorine-based resin containing at least one selected from the group consisting of vinylidene fluoride resin (PVDF), chlorotrifluoroethylene resin (PCTFE), tetrafluoroethylene resin (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-monochlorotrifluoroethylene copolymer (ECTFE), hexafluoropropylene resin (HFP), and mixtures of these resins; polysulfone (PSF), polyethersulfone (PES), sulfonated polyethersulfone (S-PES), sulfonated polysulfone (S-PSF), and these resins. Examples of suitable resins include polysulfone resins containing at least one selected from the group consisting of polyethylene (PE), polypropylene (PP), polybutene (PB), polymethylpentene resin (PMP), cycloolefin resin (COP), and mixtures thereof; polyacrylonitrile resins containing at least one selected from the group consisting of acrylonitrile resins such as polyacrylonitrile (PAN), acrylonitrile copolymer resins, and mixtures thereof; polyamide resins (PA); cellulose; cellulose triacetate; and the like. The resins may be used alone or in combination. The resin preferably contains at least one selected from the group consisting of PVDF, PE, PSF, PAN, PP, PTFE, ETFE, PA, and PES, and more preferably contains only one selected from the group consisting of PVDF, PE, PSF, PAN, PP, PTFE, ETFE, PA, and PES. The organic membrane may also have a composite membrane structure in which a membrane layer is formed on a reinforcing material such as nonwoven fabric, braided cord, etc. Examples of the inorganic membrane include ceramic membranes made from metal oxides such as alumina, mullite, titania, and zirconia.

[0017] From the viewpoints of membrane strength, pore size uniformity, and ease of handling, the separation membrane and the concentration membrane are preferably organic membranes containing at least one resin selected from the group consisting of PVDF, PE, PSF, PAN, PP, PTFE, ETFE, PA, and PES.

[0018] The shapes of the separation membrane and the concentration membrane include hollow fiber membrane, tubular, flat membrane, etc., and hollow fiber membrane or flat membrane is preferred. The hollow fiber membrane may have an outer diameter of 0.5 mm to 5 mm, more preferably 0.7 mm to 3 mm, and even more preferably 1.0 mm to 2.0 mm. Hollow fiber membranes can maximize the membrane area per installation area, making filtration systems and concentration systems compact, and are a shape with many advantages in terms of ease of use and reduced equipment costs. In the case of inorganic membranes, shapes such as tubular, monolithic, and plate types are used.

[0019] The separation membrane and the concentration membrane may be submerged membranes that are used by immersing them without being housed in a case, or may be housed in a case. When housed in a case, the module in which the separation membrane or the concentration membrane is housed in the case may be a case-type module. Among these, a case-type module that can be operated under high pressure and thereby achieve a high flux is preferred. The case-type module is preferably a case-type module in which hollow fiber membranes or flat membranes are housed in a case. For example, it may be a module consisting of a hollow fiber membrane bundle in which multiple hollow fiber membranes are bundled and a tubular housing in which the hollow fiber membrane bundle is housed. Examples of case-type modules include hollow fiber membrane types, types filled with tubular membranes with larger diameters, spiral types in which flat membranes are wound, folded pleated types, and disk types in which multiple membranes are stacked. When filtration is performed using the case-type module using hollow fiber membranes, it may be a dead-end filtration type or an external pressure type. From the perspective of the membrane area to be filtered, an external pressure type module in which filtration is performed on the outer surface of the membrane is preferred.

[0020] Membrane cleaning includes maintenance cleaning and recovery cleaning, as shown in Figure 1. Maintenance cleaning typically involves contacting the membrane with a low-concentration cleaning solution for a short period of time, such as at regular intervals, to delay the increase in operating pressure. When membrane fouling is minor, increasing the number of cleanings with a low-concentration cleaning solution can suppress membrane fouling, even with a small amount of recovery per cleaning. Recovery cleaning is performed when permeability decreases, such as when the operating pressure reaches the operating pressure. Recovery cleaning is performed because the degree of fouling is high, so the concentration of the cleaning solution must be high. Recovery is facilitated by cleaning with a higher pH and lower redox potential. Both maintenance cleaning and recovery cleaning can be performed manually or automatically, and the above-mentioned reducing cleaning solution can be used for both cleaning. In particular, when maintenance cleaning or recovery cleaning is performed automatically, cleaning with a reducing cleaning solution is performed automatically when one of the following conditions is met: a predetermined operating pressure, a predetermined increase in operating pressure, a predetermined operating period, a predetermined change in the state of the raw water, a predetermined change in the state of the liquid to be separated or concentrated, or a predetermined decrease in the permeability of the separation or concentration membrane. Recovery cleaning is performed automatically when any of the following conditions occur: when a predetermined second operating pressure different from these conditions is reached, when a predetermined increase in the second operating pressure is reached, when a predetermined second operating period is reached, when the state of the liquid to be separated or concentrated changes to a predetermined second state, or when the permeability of the separation membrane or concentration membrane decreases to a predetermined second state.

[0021] The separation membrane and the concentration membrane are not particularly limited as long as they are membranes used for separation or concentration, but are preferably membranes having contaminants attached to their surface. The separation membrane and the concentration membrane are preferably membranes used for separation or concentration in a wastewater treatment process, a water purification process, or a seawater desalination process, with waste liquid attached to their surface. In particular, in wastewater treatment processes, membranes used for separation or concentration of solutions after biological treatment (preferably aerobic biological treatment or aerobic biological treatment using activated sludge) often have oxidized deposits on their surface, making them more suitable for cleaning with the reduction cleaning solution. The separation membrane and the concentration membrane are preferably membranes used for separation or concentration of solutions after aerobic biological treatment. Here, a membrane used for separation or concentration of solutions after aerobic biological treatment may be, for example, a membrane with biopolymers attached, which are generated by biological treatment such as aerobic biological treatment. The inventors have found that biopolymers generated by biological treatment are particularly easy to clean with the above-mentioned reduction cleaning solution, possibly because they are often attached by being physically pressed against the membrane rather than chemically bonded to it.

[0022] Examples of the contaminants adhering to the membrane surface (e.g., contaminants adhering to the membrane surface during the wastewater treatment process, water purification process, or seawater desalination process) include suspended organic and inorganic substances and dissolved organic polymers contained in river water, lake water, underground water, raw industrial water, sewage, secondary sewage treatment water, industrial wastewater, domestic wastewater, seawater, and dyeing wastewater, as well as precipitates formed by the precipitation of dissolved components on the membrane surface. Other examples include precipitates contained in wastewater treatment in industrial fields such as the concentration of valuable materials in foods and beverages, the chemical industry, and petroleum refining. Membrane contaminants (fouling substances) include suspended fine particles contained in wastewater, polymerized organic substances and oils, inorganic substances precipitated on the membrane surface, and composites of these. Depending on the contaminant, cleaning with a mixture of sodium hypochlorite and sodium hydroxide, or inorganic or organic acids such as hydrochloric acid or citric acid, which are typical cleaning agents for conventional membranes, may not be sufficient to restore the membrane surface to its original state. The inventors' investigations have revealed that a high cleaning effect can be achieved and the membrane's inherent permeability restored when a reducing cleaning solution exhibits reducing properties on the alkaline side. The alkaline side is believed to dissolve organic contaminants that were originally attached, and the reducing action of the reducing agent is believed to further enhance the dissolution effect by changing the molecular structure of the attached contaminants. The cleaning action of the reducing cleaning solution restores the membrane's permeability before and after cleaning, and elemental analysis of the membrane surface has shown that oxygen atoms observed before cleaning are no longer observed after cleaning. In particular, aerobic biological treatment (e.g., aerobic treatment using activated sludge) is often used in wastewater treatment processes. In aerobic biological treatment, the wastewater is exposed to an oxidation-reduction potential of +50 mV to +150 mV for several hours. This oxidizes contaminants such as organic matter and suspended particles in the wastewater, making them susceptible to decomposition by the reducing action of the reducing cleaning solution. Therefore, the reducing cleaning solution has a significant cleaning effect.

[0023] [Reduction Cleaning Solution] The reduction cleaning solution may be a cleaning solution that donates electrons and removes oxygen from contaminants attached to a separation membrane or a concentration membrane. The reduction cleaning solution preferably contains a reducing agent and an alkaline agent, and more preferably further contains a surfactant. The reduction cleaning solution may also contain a solvent, additives, enzymes, etc., in addition to the above components. The reduction cleaning solution may be a liquid composition consisting of only a reducing agent and an alkaline agent, a liquid composition consisting of only a reducing agent, an alkaline agent, and a solvent, or a liquid composition consisting of only a reducing agent, an alkaline agent, a surfactant, and a solvent. The reduction cleaning solution of this embodiment is preferably a cleaning solution for a separation membrane or a concentration membrane that contains at least one reducing agent selected from the group consisting of sodium hydrosulfite, thiourea dioxide, borohydride, and sodium sulfoxylate formaldehyde, and at least one alkaline agent selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and sodium bicarbonate, and has a pH of 7.0 or higher.

[0024] (Reducing Agent) Examples of the reducing agent include hydrosulfite, thiourea dioxide, sodium borohydride, sodium sulfoxylate aldehyde, etc. Among these, at least one selected from the group consisting of sodium hydrosulfite, thiourea dioxide, borohydride, and sodium sulfoxylate formaldehyde is preferred, and hydrosulfite and thiourea dioxide are more preferred in terms of cleaning effect and time required for cleaning.

[0025] The mass ratio of the reducing agent relative to 100 mass% of the reduction cleaning solution is preferably 0.02 mass% or more, more preferably 0.05 to 10 mass%, and even more preferably 0.1 to 5 mass%, from the viewpoint of preventing deterioration of the strength of the membrane or module.

[0026] (Alkaline Agent) The alkaline agent is preferably one that can maintain the pH of the reduction cleaning solution at 7.0 or higher during the cleaning step, and more preferably one that can maintain the pH of the reduction cleaning solution at 7.0 or higher. The alkaline agent may be a mixture or a single compound, and examples thereof include sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, sodium bicarbonate, and lithium hydroxide. Among these, at least one selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and sodium bicarbonate is preferred, and sodium hydroxide, sodium carbonate, potassium hydroxide, and mixtures thereof are more preferred from the viewpoints of easier removal of contaminants attached to the membrane surface and better recovery of water permeability after cleaning.

[0027] The mass ratio of the alkaline agent relative to 100 mass% of the reduction cleaning solution is preferably 0.02 mass% or more, more preferably 0.05 to 10 mass%, and even more preferably 0.1 to 5 mass%, from the viewpoint of preventing deterioration of the strength of the membrane or module.

[0028] (Surfactant) As the surfactant, any of nonionic surfactants, anionic surfactants, cationic surfactants, and amphoteric surfactants may be used. Among them, nonionic surfactants and anionic surfactants are preferred from the viewpoint that when combined with the reducing agent and the alkaline agent, contaminants on the membrane surface are more easily removed and the water permeability after cleaning is more significantly restored. In addition to the cleaning effect, low foaming and rinsing properties that allow the surfactant to be washed away with a small amount of cleaning are also important when selecting a surfactant.

[0029] The mass ratio of the surfactant to 100 mass% of the reduction cleaning solution is preferably 0.01 to 5.0 mass%, more preferably 0.02 to 4.0 mass%, and even more preferably 0.05 to 3.0 mass%, from the viewpoints of the cleaning effect and the tendency of the reduction cleaning solution to foam due to the surfactant.

[0030] (Solvent) The solvent is preferably water from the viewpoints of the solubility of the reducing agent and alkaline agent, the permeability into the membrane when used as a reduction cleaning solution, and the solubility of the removed deposits.

[0031] (Additives) The reduction cleaning solution may contain various known additives, such as emulsifiers, dispersants, chelating agents, and stabilizers.

[0032] (pH) In the washing step, the pH of the reduction washing solution when contacting the separation membrane or the concentration membrane is 7.0 or higher, and is preferably 7.0 to 14.0, more preferably 9.0 to 14.0, and even more preferably 10.0 to 14.0, from the viewpoints of easier removal of contaminants adhering to the membrane surface, better recovery of water permeability after washing, and stronger reducing power of the reducing agent. The pH of the reduction washing solution is preferably 7.0 or higher, more preferably 7.0 to 14.0, even more preferably 9.0 to 14.0, and particularly preferably 10.0 to 14.0, from the viewpoints of easier removal of contaminants adhering to the membrane surface, better recovery of water permeability after washing, and stronger reducing power of the reducing agent. The pH can be measured using various pH meters commonly available on the market.

[0033] (Oxidation-Reduction Potential) An indicator of the reducing power of a reduction cleaning solution is the oxidation-reduction potential, which can be measured using various measurement methods, including an ORP meter. From the viewpoint of cleaning effect, the oxidation-reduction potential of the reduction cleaning solution is preferably −300 mV or less, more preferably less than −300 mV, and even more preferably less than −500 mV. It may also be −1200 mV or more. In particular, in the cleaning step, it is preferable that the pH of the reduction cleaning solution when brought into contact with the separation membrane or the concentration membrane is 7.0 or more (more preferably 7.0 to 14.0, even more preferably 10.0 to 14.0), and that the oxidation-reduction potential of the reduction cleaning solution is −300 mV or less (more preferably −1200 mV or more but less than −300 mV, even more preferably −1200 mV or more but less than −500 mV).

[0034] (Temperature) The above-mentioned cleaning step is preferably carried out at a temperature of 1 to 100°C, more preferably 5°C or higher, and even more preferably 10°C or higher, from the viewpoint of easier removal of contaminants adhering to the membrane surface and more excellent recovery of water permeability after cleaning. Furthermore, in order to prevent damage due to thermal expansion or deformation of the members constituting the membrane module, the temperature is more preferably 80°C or lower, even more preferably 60°C or lower, and particularly preferably 40°C or lower.

[0035] (Time) In the cleaning step, the time for contacting the separation membrane or the concentration membrane with the reduction cleaning solution may be 1 minute to 7 days, preferably 10 minutes to 3 days, and more preferably 20 minutes to 1 day. In the cleaning step, it is preferable to continuously contact the separation membrane or the concentration membrane with the reduction cleaning solution. A long contact time allows for a cleaning effect to be obtained even with a low concentration of the reduction cleaning solution. Generally, the cleaning effect can be calculated as the cleaning solution concentration multiplied by the cleaning time. For example, in the case of a cleaning solution with a 10-fold lower concentration, an immersion time 10 times longer can be considered to achieve the same level of cleaning effect. The appropriate time range is 1 to 100 times longer, with a more likely range being 1 to 50 times longer. The concentration range is 1 to 1 / 100, with a more likely range being 1 to 1 / 50. However, a long cleaning time shortens the separation or concentration time, and operational efficiency must also be taken into consideration when performing cleaning. The concentration primarily refers to the concentration of the reducing agent and alkaline agent.

[0036] In the washing step, the ratio of the inner and outer areas of the separation membrane or the concentration membrane to the mass of the reduction washing solution is 50 cm from the viewpoint of washing efficiency. ２ / g or less, and more preferably 30 cm ２ / g or less, more preferably 20 cm ２ / g or less. After cleaning with the above-mentioned reducing cleaning solution, cleaning with another cleaning solution may be performed. In particular, when the deposits on the membrane surface contain a large amount of inorganic components, cleaning with the above-mentioned reducing cleaning solution followed by cleaning with an acid may further restore the membrane permeability. Usable acids include inorganic acids such as hydrochloric acid, nitric acid, and sulfuric acid, and organic acids such as citric acid, oxalic acid, and acetic acid.

[0037] The above-mentioned cleaning step is preferably a step of contacting at least one selected from the group consisting of one surface of the membrane (e.g., the inner surface or the outer surface of the membrane) and the inside of the membrane with a reduction cleaning solution, and more preferably a step of contacting one surface of the membrane, both surfaces of the membrane, or both surfaces of the membrane and the inside of the membrane with a reduction cleaning solution.

[0038] In the above-mentioned cleaning step, the membrane may be immersed in the reduction cleaning solution, the membrane may be soaked with the reduction cleaning solution, the reduction cleaning solution may be circulated and flowed over the surface and inside of the membrane, or these may be combined.

[0039] In the above-mentioned washing step, when the membrane to be washed used for separation or concentration is a cartridge-type module, the membrane may be removed from the case and washed alone, or the membrane removed from the case may be placed in another case and washed. Also, when the membrane to be washed used for separation or concentration is not placed in a case, the membrane may be placed in a case and washed.

[0040] When the membrane is a case-type module, from the viewpoint of cleaning efficiency, it is preferable to fill the case with the reduction cleaning solution and bring the separation membrane or the concentration membrane into contact with the reduction cleaning solution, or to permeate the reduction cleaning solution through the surface and interior of the separation membrane or the concentration membrane to bring the separation membrane or the concentration membrane into contact with the reduction cleaning solution. Examples of methods for filling the case with the reduction cleaning solution include filling the case with the reduction cleaning solution and leaving it to stand or introducing air into the case and stirring it with air bubbles. In this case, it is preferable to control the temperature, pH, and redox potential within the preferred ranges. Examples of methods for permeating the reduction cleaning solution through the surface and interior of the separation membrane or the concentration membrane include permeating the reduction cleaning solution through the exterior or interior of a hollow fiber membrane in a case-type module. For example, if pressure is not applied, the reduction cleaning solution flows along the membrane surface, so it is possible to clean only the membrane surface without cleaning the interior of the membrane. Alternatively, if pressure is applied, the reduction cleaning solution can permeate the interior of the membrane (i.e., flow inside the membrane) and clean the interior of the membrane as well.

[0041] When the case-type module is an internal pressure filtration module, the reduction cleaning solution may be permeated from the inside to the outside of the hollow fiber membrane or from the outside to the inside of the hollow fiber membrane, but from the viewpoint of cleaning efficiency, it is preferable to permeate from the outside to the inside of the hollow fiber membrane. Also, in the case of an external pressure filtration module, the reduction cleaning solution may be permeated from the inside to the outside of the hollow fiber membrane or from the outside to the inside of the hollow fiber membrane, but from the viewpoint of cleaning efficiency, it is preferable to permeate from the inside to the outside of the hollow fiber membrane. When flowing the chemical solution on the surface and inside of the membrane, it is possible to increase the flow rate up to the same maximum pressure as in normal filtration and concentration operation.

[0042] The washing step can be terminated by stopping contact of the separation membrane or the concentration membrane with the reduced washing solution.

[0043] The cleaning method of this embodiment can wash away contaminants adhering to the membrane surface or inside the membrane, maintaining the membrane's permeability. Even in the concentration of valuable resources, the membrane can become contaminated by organic and inorganic substances contained in the liquid to be concentrated, reducing its permeability and its separation and concentration function. Therefore, cleaning can be performed to restore the membrane. Unlike cleaning fibers or pulp, the cleaning method of this embodiment can remove contaminants adhering to the membrane without exposing it to high temperatures. Although the details are unclear, it is believed that contaminants adhering to the membrane are easily decomposed by reducing agents. Furthermore, by using a surfactant in combination, contaminants adhering to the membrane can be washed away extremely efficiently. This method is particularly effective in cleaning contaminants containing oxygen atoms.

[0044] In the cleaning method of this embodiment, for example, a membrane having contaminants attached thereto may be brought into contact with a reducing cleaning solution to clean the membrane. Specifically, an appropriate amount (for example, the ratio of the internal and external areas of the separation membrane or concentration membrane to the mass of the reducing cleaning solution) of the reducing cleaning solution is brought into contact with the membrane having contaminants attached thereto, and circulating cleaning is performed at the temperature described above for the time described above. The water permeability (for example, the water permeability retention rate described in the Examples below) before and after cleaning is compared and reported to the membrane provider. The cleaning wastewater may be treated as industrial wastewater, or may be neutralized with hydrochloric acid or the like and returned to the upstream of the wastewater treatment facility and discharged in small amounts.

[0045] The cleaning method of this embodiment preferably includes a step of cleaning the separation membrane or the concentration membrane by contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher when any of the following conditions is met during operation of an apparatus equipped with a separation membrane or a concentration membrane: when a predetermined operating pressure is reached, when a predetermined increase in operating pressure is reached, when a predetermined operating time is reached, when a predetermined state of the liquid to be separated or concentrated is changed, or when the permeability of the separation membrane or the concentration membrane is reduced to a predetermined level. Examples of the step of cleaning the separation membrane or the concentration membrane by contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher include the steps described above. In this specification, any of the following conditions may be referred to as the "first condition": when a predetermined operating pressure is reached, when a predetermined increase in operating pressure is reached, when a predetermined operating time is reached, when a predetermined state of the liquid to be separated or concentrated is changed, or when the permeability of the separation membrane or the concentration membrane is reduced to a predetermined level. The first condition may be a plurality of conditions, and cleaning may be performed when one of the plurality of conditions is satisfied. The first condition may be, for example, a condition for performing the maintenance cleaning.

[0046] The predetermined operating pressure can be appropriately determined depending on the device and the concentration membrane or separation membrane used, and may be, for example, 100 kPa. The predetermined operating pressure may be 1.5 times or 2.0 times the operating pressure when an unused separation membrane or concentration membrane is installed and cleaning is started.

[0047] When the predetermined increase in operating pressure is reached, the increase in operating pressure can be appropriately determined depending on the device and the concentration or separation membrane used, and may be, for example, 100 kPa. The increase in operating pressure may be the difference between the operating pressure at the time when an unused separation or concentration membrane is installed and cleaning is started and a pressure value 1.5 times the operating pressure, or may be the difference between the operating pressure and a pressure value 2.0 times the operating pressure.

[0048] The predetermined operation time can be appropriately determined depending on the device used and the concentration membrane or separation membrane used, and may be, for example, 48 hours after the start of use or the previous cleaning.

[0049] The predetermined state of the liquid to be separated or concentrated can be determined appropriately depending on the device and the concentration or separation membrane used. For example, the turbidity of the liquid to be separated or concentrated can be measured periodically or continuously, and when the turbidity reaches a certain level or higher, cleaning can be performed to prevent clogging of the membrane due to turbidity. The turbidity above a certain level may be, for example, 100 NTU or higher. The turbidity can be measured by passing the target water through a turbidity meter. In the cleaning method of this embodiment, contaminants may be substances that adhere to the separation or concentration membrane.

[0050] The permeability of a predetermined separation membrane or concentration membrane can be calculated from the relationship between the pressure applied between the membranes and the amount of permeated water by the following formula (1): Permeation performance (LMH / 100 kPa) = {total amount of water permeated by the module (L / min)) × 60 (min) × temperature correction of the permeated water temperature to the volume at 25°C} / membrane area of ​​the module (m ２) / filtration pressure (kPa) × 100 ... Formula (1) This permeation performance value may be, for example, 50 LMH / 100 kPa, or it may be ¼ of the initial permeation performance. Here, the total amount of water permeated by the module refers to the total amount of water permeating all of the hollow fiber membranes provided in the module. For example, when no module is used, it may be the total amount of water permeating the separation membrane or concentration membrane. Furthermore, the membrane area of ​​the module refers to the total surface area of ​​all of the hollow fiber membranes provided in the module, and for example, in the case of an external pressure type module, it may be the total external surface area of ​​all of the hollow fiber membranes. When no module is used, it may be the total surface area of ​​the separation membrane or concentration membrane.

[0051] The cleaning method of the present embodiment preferably includes a step of contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher to clean the separation membrane or the concentration membrane when any of the following cases is met: when the predetermined operating pressure is reached, when the predetermined increase in operating pressure is reached, when the predetermined operating time is reached, when the liquid changes to the predetermined state of the separation or concentration target liquid, or when the permeability of the separation membrane or concentration membrane is reduced; and when any of the following cases is met: when a predetermined second operating pressure is reached, when the predetermined increase in second operating pressure is reached, when the predetermined second operating time is reached, when the liquid changes to the predetermined state of the second separation or concentration target liquid, or when the permeability of the separation membrane or concentration membrane is reduced to the predetermined In this specification, any of the following cases may be referred to as a "second condition": when a predetermined second operating pressure is reached, when a predetermined increase in the second operating pressure is reached, when a predetermined second operating time is reached, when the state of the liquid to be separated or concentrated is changed to a predetermined second state, or when the permeability of the separation membrane or concentration membrane is reduced to a predetermined second state. The second condition may be a plurality of conditions, and cleaning may be performed when one of the plurality of conditions is satisfied. The first condition and the second condition are preferably the same parameter. For example, the first condition is preferably the operating pressure, and the second condition is preferably also the operating pressure. The second condition may be, for example, a condition for performing the recovery cleaning.

[0052] The second operating pressure may be the same as or higher than the predetermined operating pressure. The second operating pressure can be appropriately determined depending on the apparatus used and the concentration membrane or separation membrane used, and may be, for example, 150 kPa. The second operating pressure may be 1.6 times or 2.1 times the operating pressure when an unused separation membrane or concentration membrane is installed and cleaning is started. When the first condition and the second condition are both operating pressures, the second operating pressure is preferably higher than the first operating pressure, more preferably 20 kPa or more higher than the first operating pressure, and even more preferably 50 kPa or more higher than the first operating pressure.

[0053] The second operating pressure increase may be the same as or higher than the predetermined operating pressure increase. The second operating pressure increase can be appropriately determined depending on the device used and the concentration membrane or separation membrane used, and may be, for example, 150 kPa. The increase may be the difference between the operating pressure when an unused separation membrane or concentration membrane is installed and cleaning is started and a pressure value 1.6 times the operating pressure, or may be the difference between the operating pressure and a pressure value 2.1 times the operating pressure. When the first condition and the second condition are both operating pressure increase amounts, the second operating pressure increase amount is preferably higher than the first operating pressure increase amount, more preferably 20 kPa or more higher than the first operating pressure increase amount, and even more preferably 50 kPa or more higher than the first operating pressure increase amount.

[0054] The second operating time can be appropriately determined depending on the apparatus used and the concentration membrane or separation membrane used, and may be, for example, 240 hours. When the first condition and the second condition are both operating times, the second operating time is preferably longer than the first operating time, more preferably 5 hours or more longer than the first operating time, and even more preferably 10 hours or more longer than the first operating time.

[0055] The state of the second separation or concentration target liquid can be determined appropriately depending on the device and concentration membrane or separation membrane used. For example, the turbidity of the target liquid can be measured periodically or continuously, and when the turbidity reaches a certain level, cleaning can be performed to prevent membrane blockage due to turbidity. The turbidity may be, for example, 150 NTU or more. The turbidity in the state of the second separation or concentration target liquid is preferably higher than the turbidity in the predetermined state of the separation or concentration target liquid. The turbidity can be measured by flowing the target liquid through a turbidity meter. In the cleaning method of this embodiment, contaminants may refer to substances that adhere to the separation or concentration membrane. When both the first condition and the second condition result in a change in the state of the separation or concentration target liquid, the second turbidity is preferably higher than the first turbidity, more preferably 20 NTU or more higher than the first turbidity, and even more preferably 50 NTU or more higher than the first turbidity.

[0056] The second permeability is usually set lower than the permeability of the first separation membrane or concentration membrane. The second permeability may be 30 LMH / 100 kPa. When both the first condition and the second condition are permeable, the second permeability is preferably lower than the first permeability, more preferably 10 LMH / 100 kPa or more lower than the first permeability, and even more preferably 30 LMH / 100 kPa or more lower than the first permeability.

[0057] {Washing Apparatus} The washing apparatus of this embodiment has at least a portion that brings the separation membrane or the concentration membrane into contact with the reduction washing solution. The washing apparatus of this embodiment is preferably an apparatus used in the above-described washing method of this embodiment.

[0058] The part where the separation membrane or the concentration membrane is brought into contact with the reduced cleaning solution may be a part in the device where the separation membrane or the concentration membrane is provided, such as the case-type module described above.

[0059] The cleaning device of this embodiment may further include a tank for storing the reduction cleaning solution, a pump for adjusting the flow rate of the reduction cleaning solution, a pressure gauge, a flow meter, a heater for adjusting the temperature of the reduction cleaning solution, a data logger for recording the state, a limit element for preventing abnormal operating conditions, a turbidity meter, etc.

[0060] The cleaning device of this embodiment may be a device equipped with a separation membrane or a concentration membrane. For example, the cleaning device of this embodiment may be the same device as the device that separates or concentrates the liquid to be separated or concentrated, but the liquid flowing through the device may be changed from the liquid to be separated or concentrated to the reduction cleaning solution. In this case, multiple flow paths may be provided within the device, and the flow path used when separating or concentrating the liquid to be separated or concentrated may be different from the flow path used during the cleaning process. The cleaning device is preferably a device that cleans the separation membrane or the concentration membrane by contacting it with a reduction cleaning solution at a pH of 7.0 or higher when any of the following conditions is met: when a predetermined operating pressure is reached, when a predetermined increase in operating pressure is reached, when a predetermined operating time is reached, when the state of the liquid to be separated or concentrated is changed to a predetermined state, or when the permeability of the separation membrane or concentration membrane is reduced to a predetermined level. Furthermore, the cleaning device is preferably an apparatus that cleans the separation membrane or the concentration membrane by contacting the separation membrane or the concentration membrane with a reducing cleaning solution at a pH of 7.0 or higher when any of the following conditions is met: when the predetermined operating pressure is reached, when the predetermined operating pressure increase range is reached, when the predetermined operating time is reached, when the state of the separation or concentration target liquid is changed to the predetermined state, or when the permeability of the separation membrane or the concentration membrane is decreased to the predetermined state, and when any of the following conditions is met: when a predetermined second operating pressure is reached, when a predetermined second operating pressure increase range is reached, when the predetermined second operating time is reached, when the state of the separation or concentration target liquid is changed to the predetermined state, or when the permeability of the separation or concentration membrane is decreased to the predetermined state. The cleaning device of this embodiment may include a tank for storing the reducing cleaning solution, a pressure gauge for measuring the operating pressure, a timer for measuring the operating time, an analyzer for analyzing the state (e.g., concentration) of the concentration target liquid, and an analyzer for analyzing permeability.

[0061] {Powder Mixture} The powder mixture of this embodiment contains at least one reducing agent selected from the group consisting of sodium hydrosulfite, thiourea dioxide, borohydride, and sodium formaldehyde sulfoxylate, and at least one alkaline agent selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and sodium bicarbonate. The powder mixture may be a mixture consisting solely of the reducing agent and the alkaline agent, or may further contain other components. Examples of the reducing agent include, and preferably are, the same reducing agents as those described in the method for cleaning a separation membrane or a concentration membrane of this embodiment. Examples of the alkaline agent include, and preferably are, the same alkaline agents as those described in the method for cleaning a separation membrane or a concentration membrane of this embodiment. The mass ratio of the reducing agent to 100% by mass of the powder mixture is preferably 10 to 90% by mass, more preferably 20 to 80% by mass. The mass ratio of the alkaline agent to 100% by mass of the powder mixture is preferably 10 to 90% by mass, more preferably 20 to 80% by mass. The ratio of the total mass of the reducing agent and the alkaline agent to 100% by mass of the powder mixture is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more. 1 g of the powder mixture is mixed with 100 mL of water, stirred at room temperature for 3 minutes, and the pH at 25°C after the powder has completely disappeared is preferably 7.0 or more, more preferably 9.0 to 14.0, and even more preferably 10.0 to 14.0. The powder mixture is preferably a raw material for a reduction cleaning solution for molecular membranes or concentrated membranes. Here, the raw material for a reduction cleaning agent means that this powder, when dissolved in water at a concentration of 1% or more, becomes a reduction cleaning solution with a pH of 7 or more. The powder mixture can be used to produce the reduction cleaning solution in the method for cleaning a separation membrane or a concentrated membrane of the present embodiment described above.

[0062] The method for producing the reduction cleaning solution of this embodiment is preferably a method for producing the reduction cleaning solution having a pH of 7.0 or higher from the powder mixture of this embodiment described above to be brought into contact with the separation membrane or the concentration membrane. The separation membrane or the concentration membrane may be the same as the separation membrane or the concentration membrane in the method for cleaning a separation membrane or a concentration membrane of this embodiment described above, and the same is preferred. The reduction cleaning solution obtained by the method for producing the reduction cleaning solution of this embodiment can be used as the reduction cleaning solution in the method for cleaning a separation membrane or a concentration membrane of this embodiment described above.

[0063] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0064] Examples 1 to 3, Comparative Examples 1 to 6: Wastewater discharged from a dyeing factory was subjected to coagulation sedimentation, activated carbon adsorption, and biological treatment (i.e., treatments including anaerobic biological treatment and aerobic biological treatment). The treated solution was filtered using an MF membrane in a membrane module (UNA-620A manufactured by Asahi Kasei Corporation, spinning method: thermal induction type, membrane material: PVDF, pore size: 0.1 μm, membrane area: 50 m). ２A cleaning recovery test was conducted on a UNA-620A that had been used for approximately one year in a system in which water was treated using a reverse osmosis (RO) membrane after passing through a filtration system. One UNA-620A was removed from the treatment system, the membrane module was disassembled, and a hollow fiber membrane was cut into a length of approximately 30 cm. The sample was collected from the center of a 2-m length, inside a bundle of 6,600 membranes. The membrane was kept moist throughout the series of operations to prevent drying. Note that biopolymers derived from aerobic biological treatment were attached to the UNA-620A removed from the treatment system. Here, biopolymers can be identified using various techniques, but in the present invention, sugars and proteins of 104 Da or greater measured using a liquid chromatography-organic carbon detection (LC-OCD) system were defined as biopolymers. For the measurement method, see "Japan Society of Civil Engineers, Collection of Papers G (Environment), Vol. 72, No. 7 III_535-III_541, 2016" and "Kimura, K., Tanaka, K., Watanabe, Y.: Confirmation of the correlation between membrane fouling in microfiltration and biopolymer concentration in various Japanese surface water. Water Science and Technology: Water Supply, 15(2), 288-293., The hollow fiber membranes were cut to a length of 12 cm, and with one end sealed, pure water was pumped into the inside of the hollow fiber membrane from the other end at a pressure of 100 kPa. After the inside of the hollow fiber membrane was replaced with pure water, the weight of the pure water that flowed out to the outside of the membrane in 1 minute was measured to determine the water permeation rate. The water temperature was also measured at this time. The water permeation rate is defined by the following formula: Flux (LMH) = {(total amount of water flowing out of each hollow fiber membrane (L / min)) x 60 (min) x volume corrected for temperature at 25°C} / internal area of ​​membrane (m ２) ...Equation (2) Meanwhile, the water permeation rate of the membrane before product shipment is measured for each lot and known, and the water permeation retention (%) is defined by the following equation. The higher the water permeation retention, the more the membrane is judged to have its inherent water permeability. Water permeation retention (%) = (flux of membrane in that state) / (flux of unused membrane before shipment) x 100 ...Equation (3) In the present invention, the water permeation rate was measured by selecting 10 hollow fiber membranes, placing them in a test tube with an internal volume of approximately 35 ml, immersing them in a chemical solution for a predetermined period of time, and evaluating the average water permeation rate of the 10 membranes. The water permeation retention calculated from the average water permeation rate of 10 unwashed membranes cut out from the membrane module was a fairly low 4.3%.

[0065] Example 1 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, and 96% by mass of water. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (21-22°C) for 8 hours. The pH of this reduction cleaning solution at room temperature was measured using a pH meter (HM-20P, manufactured by DKK-TOA Corporation) to find it was 12.9, and the oxidation-reduction potential was measured using an ORP meter (RM-30P, manufactured by DKK-TOA Corporation) to find it was -795 mV. After 8 hours of immersion, the membrane was removed from the test tube, and the water permeation rate and water permeation retention were similarly measured, revealing that they had recovered to 33.8%. The following examples and comparative examples were measured using the pH meter and ORP meter described above. The pH meter was appropriately configured, and the ORP meter was periodically checked for normality by measuring the electromotive force using a standard solution and a reference electrode.

[0066] (Comparative Example 1) 50 g of a cleaning solution was prepared by mixing 0.5% by mass of sodium hypochlorite, 2% by mass of caustic soda, and 97.5% by mass of water. The cleaning solution was placed in a test tube, and 10 membranes cut out from the module were immersed in the solution and allowed to stand at room temperature of 21 to 22 ° C. for 8 hours. The pH of this cleaning solution at room temperature was 13.3, and the oxidation-reduction potential was +310 mV. After 8 hours of immersion, the membrane was removed from the test tube and the water permeation rate was measured in the same manner. The water permeation retention was measured, and the water permeation retention was found to be 7.8%, indicating that the cleaning effect was lower than in Example 1.

[0067] (Example 2) 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, and 96% by mass of water. The pH of the reduction cleaning solution at 40 ° C was 13.0, and the oxidation-reduction potential was -800 mV. The reduction cleaning solution was placed in a test tube, and 10 membranes cut out from the module were immersed therein. The test tube was placed in a thermostatic bath at 40 ° C. After confirming that the temperature inside the test tube had also reached 40 ° C, the membrane was removed from the test tube 8 hours later, and the water permeation rate was measured in the same manner. The water permeation retention was measured, resulting in a water permeation retention of 64.1%. It was confirmed that the water permeation retention was further improved by setting the temperature at 40 ° C. compared to Example 1.

[0068] (Comparative Example 2) 50 g of a cleaning solution was prepared by mixing 0.5% by mass of sodium hypochlorite, 2% by mass of caustic soda, and 97.5% by mass of water. The pH of the cleaning solution at 40 ° C. was 13.3, and the oxidation-reduction potential was +320 mV. The cleaning solution was placed in a test tube, and 10 membranes cut out from the module were immersed therein. The test tube was placed in a thermostatic bath at 40 ° C. After confirming that the temperature inside the test tube had also reached 40 ° C., the membrane was removed from the test tube 8 hours later, and the water permeation rate was measured in the same manner. The water permeation retention was measured, resulting in a water permeation retention of 12.7%. Although the immersion temperature was the same as in Example 2 at 40 ° C., no significant improvement in water permeation retention was observed.

[0069] (Example 3) 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, 2% by mass of Quinceorp 5G (anionic surfactant manufactured by Kotani Chemical), and 94% by mass of water. The pH of the reduction cleaning solution at 40 ° C was 12.8, and the redox potential was -765 mV. The reduction cleaning solution was placed in a test tube, and 10 membranes cut out from the module were immersed therein. The test tube was placed in a thermostatic bath at 40 ° C. After confirming that the temperature inside the test tube had reached 40 ° C, the membrane was removed from the test tube 8 hours later and the water permeation rate was measured in the same manner. The water permeation retention was measured, resulting in a water permeation retention of 74.0%. It was confirmed that the addition of a surfactant further improved the water permeation retention compared to Example 2.

[0070] (Comparative Example 3) 50 g of a cleaning solution was prepared by mixing 0.5% by mass of sodium hypochlorite, 2% by mass of caustic soda, 2% by mass of Quince Soap 5G, and 95.5% by mass of water. The pH of the cleaning solution at 40 ° C was 13.2, and the oxidation-reduction potential was +280 mV. The solution was placed in a test tube, and 10 membranes cut out from the module were immersed in the solution. The test tube was placed in a thermostatic bath at 40 ° C. After confirming that the temperature inside the test tube had also reached 40 ° C, the membrane was removed from the test tube 8 hours later and the water permeation rate was measured in the same manner. The water permeation retention was measured, and the water permeation retention was 38.6%. Although the water permeation retention was recovered, the cleaning recovery effect was lower than in Example 3, and the effect was insufficient.

[0071] Comparative Example 4: 50 g of a cleaning solution was prepared by mixing 2.0% by mass of hydrosulfite and 98.0% by mass of water. The pH of the cleaning solution at 40°C was 5.53, and the oxidation-reduction potential was -624 mV. Ten membranes cut out from the module were immersed in the solution in a test tube, and the test tube was placed in a thermostatic bath at 40°C. After confirming that the temperature inside the test tube had also reached 40°C, the membranes were removed from the test tube 8 hours later and the water permeation rate was measured in the same manner. The water permeation retention was measured, which was 12.2%, indicating a low cleaning recovery effect.

[0072] Comparative Example 5: 50 g of a cleaning solution was prepared by mixing 2% by mass of caustic soda and 98% by mass of water. The pH of the cleaning solution at 40°C was 13.3, and the oxidation-reduction potential was -30 mV. The solution was placed in a test tube, and 10 membranes cut out from the module were immersed in the solution. The test tube was placed in a thermostatic bath at 40°C. After confirming that the temperature inside the test tube had also reached 40°C, the membranes were removed from the test tube 8 hours later, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured at 9.8%, indicating a low cleaning recovery effect and insufficient effectiveness.

[0073] (Comparative Example 6) 50 g of a cleaning solution was prepared by mixing 2% by mass of sodium sulfite, 2% by mass of caustic soda, 2% by mass of Quinthorpe 5G, and 94% by mass of water. The pH of the cleaning solution at 40°C was 13.3, and the redox potential was -125 mV. The solution was placed in a test tube, and 10 membranes cut out from the module were immersed in the solution. The test tube was placed in a thermostatic bath at 40°C. After confirming that the temperature inside the test tube had also reached 40°C, the membranes were removed from the test tube 8 hours later and the water permeation rate was measured in the same manner. The water permeation retention was measured, resulting in a water permeation retention of 12.9%, indicating a low cleaning recovery effect and insufficient effectiveness.

[0074]

[0075] After washing, each membrane was freeze-dried and then EDX was measured under the following conditions (using Hitachi Technologies SU8000, accelerating voltage 15 kV, collection time 60 seconds, measurement image width 150 μm). As shown in Figure 2, only C and F were detected in the unused PVDF membrane. This is -(CH ２ -CF ２ ) ｎ This is based on the structure of the -. Figures 3 and 4 show Comparative Examples 2 and 3, which had poor cleaning effectiveness. In addition to the C and F peaks of the membrane material, peaks for S, Cl, and Si are observed, as well as a peak for O. These are presumed to be elements resulting from deposits due to fouling. In Examples 2 and 3, as shown in Figures 5 and 6, the peaks for S, Cl, Si, and O have disappeared. In Examples 2 and 3, the water permeability retention rate recovered, but it was not 100% recovered, and some deposits are thought to remain on the membrane surface. The disappearance of the O peak is thought to be due to the O being removed from the deposits and decomposing them due to the reduction effect of the reducing cleaning solution. Thus, the reduction effect of the reducing cleaning solution is thought to change the properties of the deposits, making them easier to peel from the membrane. Furthermore, the cleaning effectiveness is thought to be further improved by heating the temperature in the range of 1 to 100°C and by adding a surfactant.

[0076] Examples 4 to 8, Comparative Example 7 Wastewater discharged from a liquid crystal factory was subjected to coagulation and sedimentation, followed by biological treatment (i.e., treatment including anaerobic biological treatment and aerobic biological treatment). The treated solution was passed through a membrane module (UNA-620A manufactured by Asahi Kasei Corporation) that filters the treated solution using an MF membrane, and then treated with a reverse osmosis membrane. A cleaning recovery test was conducted on the UNA-620A, which had been used for approximately 8 months in a system where one UNA-620A was removed from the treatment system, the membrane module was disassembled, and the hollow fiber membrane was cut out. The water permeation rate and water permeation retention were measured using the same procedures as in Example 1. The water permeation retention of the uncleaned membrane was 20.0%. Note that biopolymers derived from the aerobic biological treatment were attached to the UNA-620A removed from the treatment system.

[0077] Example 4: 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, 2% by mass of Quinthorpe 5G, and 94% by mass of water. The pH of the reduction cleaning solution at room temperature was 13.0, and the redox potential was -790 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (20-22°C) for 2 hours. After 2 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, and the membranes recovered to 101.2%. The fact that the water permeation retention rate exceeded 100% can be considered to be a variation when compared with the water permeation rate of unused membranes from the same lot. This indicates that cleaning recovery could be completed in a short time.

[0078] Example 5: 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, 2% by mass of Eriez K1248 (a nonionic surfactant manufactured by METEL), and 94% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.9, and the redox potential was -780 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (20-22°C) for two hours. After two hours, the membranes were removed from the test tube, and the water permeation rate and water permeation retention were measured in the same manner. The membranes recovered to 100.5%. This demonstrates that cleaning recovery can be completed in a short time, as in Example 4, if the surfactant is appropriately selected.

[0079] Example 6: 50 g of a reduction cleaning solution was prepared by mixing 0.1% by mass of sodium hydrosulfite, 0.1% by mass of caustic soda, 0.1% by mass of Quinthorpe 5G, and 99.7% by mass of water. The pH of the reduction cleaning solution at room temperature was 11.9, and the redox potential was -761 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (20-22°C) for 2 hours. After 2 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate recovered to 100.9%. This demonstrates that even if the concentration of the reduction cleaning solution is reduced, the cleaning effect of this wastewater remains unchanged.

[0080] Example 7: 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of thiourea dioxide, 2% by mass of caustic soda, 2% by mass of Eriez K1248, and 94% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.9, and the redox potential was -1012 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (20-22°C) for two hours. After two hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, and recovered to 101.5%. It was found that not only hydrosulfite but also thiourea dioxide, which has a reducing effect, had a high cleaning effect. Furthermore, when the membranes were inspected after the test, no degradation was observed.

[0081] Example 8: 50 g of a reduction cleaning solution was prepared by mixing 0.1% by mass of thiourea dioxide, 0.1% by mass of caustic soda, 0.1% by mass of Eriez K1248, and 99.7% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.1, and the redox potential was -936 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (20-22°C) for four hours. After four hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention was measured, and the membranes recovered to 99.7%.

[0082] (Comparative Example 7) 50 g of a cleaning solution was prepared by mixing 0.5% by mass of sodium hypochlorite, 2.0% by mass of caustic soda, and 97.5% by mass of water. The pH of the cleaning solution at room temperature was 13.3, and the oxidation-reduction potential was +300 mV. Ten membranes cut out from the module were immersed in the cleaning solution in a test tube and allowed to stand at room temperature of 20 to 22 ° C for 6 hours. Since the color of the membrane surface was such that deposits remained, the membrane was further rinsed with water and then immersed in a cleaning solution of 1% by mass of citric acid and 99% by mass of water for 2 hours. After a total of 8 hours, the membrane was removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, and it was found that the recovery rate had recovered to 39.5%, indicating that the cleaning recovery effect was insufficient.

[0083]

[0084] Example 9, Comparative Example 8 In a treatment process in which river water is pumped up, subjected to coagulation and sedimentation, and then filtered through a membrane to produce industrial water, a 2-m-long case-type module was operated for approximately one year, in which 6,000 polyethylene MF membranes with a pore size of 0.1 μm, an outer diameter of 1.2 mm, and an inner diameter of 0.6 mm were bundled and fixed at both ends. The membrane module was disassembled, and the hollow fiber membranes were cut out. The water permeation rate and water permeation retention were measured using the same procedures as in Example 1. The water permeation retention of the unwashed membrane was 48.6%.

[0085] Example 9 50 g of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, 2% by mass of Elise K1248, and 94% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.9, and the redox potential was -790 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (21-23 ° C) for 6 hours. After 6 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, and recovered to 90.2%. Furthermore, 50 g of a cleaning solution was prepared by mixing 1% by mass of citric acid and 99% by mass of water. Ten membranes that had been cleaned as described above were immersed in the cleaning solution in a test tube and allowed to stand at room temperature (21-23 ° C) for 2 hours. After thorough rinsing, the water permeation rate was measured, and the water permeation retention rate was 97.8%.

[0086] (Comparative Example 8) 50 g of a cleaning solution was prepared by mixing 0.5% by mass of sodium hypochlorite, 2% by mass of caustic soda, 2% by mass of Elise K1248, and 95.5% by mass of water. The pH of the cleaning solution at room temperature was 13.3, and the redox potential was +290 mV. Ten membranes cut from the module were immersed in the cleaning solution in a test tube and allowed to stand at room temperature (21-23 ° C) for 6 hours. After 6 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, and recovered to 69.3%. Furthermore, 50 g of a cleaning solution was prepared by mixing 1% by mass of citric acid and 99% by mass of water. The cleaning solution was placed in a test tube, and 10 of the membranes that had been cleaned as described above were immersed in the cleaning solution and allowed to stand at room temperature (21-23 ° C) for 2 hours. After thorough rinsing, the water permeation rate was measured, and the water permeation retention rate was 88.4%.

[0087]

[0088] Examples 10 and 11, Comparative Examples 9 and 10 Four MF membrane modules (UNA-620A manufactured by Asahi Kasei, indicated as MF membranes in FIG. 7) that had been used for eight months in the wastewater treatment process shown in FIG. 7 were each cleaned by the following method. Note that biopolymers derived from the aerobic biological treatment were attached to the removed MF membranes.

[0089] (Example 10) When pure water was filtered through the first membrane module by applying a transmembrane pressure difference of 20 kPa, the water permeability retention, which is the pure water permeability of the used membrane relative to the water permeability of an unused product before shipment, was 46.3%. The temperature when the pure water was filtered was 23.9 ° C, and the water permeability retention was calculated using a volume converted value at 25 ° C. As shown in Figure 8, the module was immersed in a temperature-adjustable hot water bath, and the temperature of the hot water bath was adjusted to 40 ° C. 50 kg of a reduction cleaning solution was prepared by mixing 2% by mass of sodium hydrosulfite, 2% by mass of caustic soda, 2% by mass of Quince Soap 5G, and 94% by mass of water. The pH of the reduction cleaning solution at 40 ° C was 12.9, and the redox potential was -795 mV. The solution was introduced into the case from the side tube of the membrane module and filled with water. It was immersed in this state for 8 hours. After that, the reduction cleaning solution was replaced with water and thoroughly rinsed, and then a transmembrane pressure of 20 kPa was applied to the membrane module to filter pure water. The water permeability retention, which is the water permeability capacity after cleaning relative to the water permeability capacity of an unused product before shipping, was 95.4%, and the water permeability retention was restored. The temperature when the pure water was filtered was 24.3°C, and the water permeability retention was calculated using a value converted to a volume at 25°C.

[0090] (Example 11) When pure water was filtered by applying a transmembrane pressure difference of 20 kPa to the second membrane module, the water permeation retention was 44.2%. The temperature when the pure water was filtered was 23.9°C, and the water permeation retention was calculated using a value converted to a volume at 25°C. As shown in Figure 9, 100 kg of a reduction cleaning solution containing 2 mass% sodium hydrosulfite, 2 mass% caustic soda, 2 mass% Quince Soap 5G, and 94 mass% water was prepared in a temperature-controllable tank. The pH of the reduction cleaning solution at 40°C was 13.0, and the oxidation-reduction potential was -800 mV. A heater was installed in the tank to maintain the reduction cleaning solution at 40°C. 0.5 m of the reduction cleaning solution was added to the membrane module. ３The solution was sent at a flow rate of 1000 ml / hr, and the reduction cleaning solution was continued to flow on the surface of the membrane and the inside and inner surfaces of the membrane by filtration for 8 hours. After that, the reduction cleaning solution was replaced with water and thoroughly rinsed. After applying a transmembrane pressure of 20 kPa to the membrane module and filtering pure water, the water permeation retention was 99.5%, and the water permeation retention was recovered. The temperature when the pure water was filtered was 24.6 ° C, and the water permeation retention was calculated using a volume-converted value at 25 ° C.

[0091] (Comparative Example 9) When the third membrane module was subjected to a transmembrane pressure difference of 20 kPa to filter pure water, the water retention rate was 47.6%. The temperature when the pure water was filtered was 23.9 ° C., and the water retention rate was calculated using a volumetric value converted to 25 ° C. As shown in FIG. 8, the module was immersed in a temperature-adjustable hot water bath, and the temperature of the hot water bath was adjusted to 40 ° C. 50 kg of cleaning solution was prepared by mixing 0.5 mass% sodium hypochlorite, 2 mass% caustic soda, 2 mass% Quince Soap 5G, and 95.5 mass% water. The pH of the cleaning solution at 40 ° C. was 13.3, and the redox potential was +310 mV. The solution was introduced into the case from the side tube of the membrane module and filled with water. It was immersed in this state for 8 hours. The cleaning solution was then replaced with water, and after thorough rinsing, a transmembrane pressure of 20 kPa was applied to the membrane module to filter pure water. The water retention rate was 69.4%, which was lower than that of Example 10, which differed only in the cleaning solution. The temperature when the pure water was filtered was 24.4°C, and the water retention rate was calculated using a value converted to a volume at 25°C.

[0092] (Comparative Example 10) When pure water was filtered by applying a transmembrane pressure difference of 20 kPa to the fourth membrane module, the water retention rate was 48.4%. The temperature when the pure water was filtered was 23.9°C, and the water retention rate was calculated using a volume converted value at 25°C. As shown in Figure 9, 100 kg of a cleaning solution containing 0.5 mass% sodium hypochlorite, 2 mass% caustic soda, 2 mass% Quince Soap 5G, and 95.5 mass% water was prepared in a temperature-controllable tank. The pH of the above cleaning solution at 40°C was 13.3, and the oxidation-reduction potential was +310 mV. A heater was installed in the tank to maintain the cleaning solution at 40°C. 0.5 m of cleaning solution was poured into the membrane module. ３The cleaning solution was sent at a rate of 1 / hr and continued to flow on the surface of the membrane and the inside and inner surfaces of the membrane by filtration for 8 hours. After that, the cleaning solution was replaced with water and thoroughly rinsed. After that, a transmembrane pressure of 20 kPa was applied to the membrane module to filter pure water, and the water permeability retention was 78.3%, which was lower than that of Example 11, which differed only in the cleaning solution. The temperature when the pure water was filtered was 24.8 ° C., and the water permeability retention was calculated using a volume-converted value at 25 ° C.

[0093]

[0094] Examples 12-13, Comparative Examples 11-12: Deterioration of membrane elongation due to cleaning solution was confirmed. PVDF membranes and PE membranes were immersed in a test tube with a capacity of 35 ml for 10 days, and the tensile strength and tensile elongation were measured using a tensile tester with a chuck distance of 5 cm and a pulling speed of 200 mm / min. The presence or absence of deterioration was determined by the retention rate after 10 and 20 days relative to the strength and elongation of the membrane before immersion. In actual membrane operation, filtration and cleaning are repeated, and 20 days of chemical immersion is a method for expressing the strength and elongation of the membrane when filtration and cleaning are performed over the expected lifespan of the membrane module.

[0095] (Example 12) Sodium hydrosulfite 2% by mass, caustic soda 2% by mass, Quinceorp 5G 2% by mass, and water 94% by mass were mixed. The pH at 40 ° C was 12.8, and the redox potential was -790 mV. A PVDF membrane was immersed in the reduction cleaning solution maintained at 40 ° C. After 10 days, the strength and elongation retention rates relative to the unused membrane were 101.1% and 103.8%, respectively, and after 20 days, the strength and elongation retention rates relative to the unused membrane were 102.1% and 108.8%, respectively, confirming that no deterioration had occurred.

[0096] (Example 13) Sodium hydrosulfite was mixed at a ratio of 2% by mass, caustic soda at 2% by mass, Quince Soap 5G at 2% by mass, and water at 94% by mass. The pH at 40 ° C. was 12.8, and the redox potential was -790 mV. A PE membrane was immersed in the reduction cleaning solution maintained at 40 ° C. After 10 days, the strength and elongation retention rates relative to the unused membrane were 99.2% and 102.5%, respectively, and after 20 days, the strength and elongation retention rates relative to the unused membrane were 102.5% and 96.8%, respectively, confirming that no degradation had occurred. PE membranes are manufactured from general-purpose resins, and although their manufacturing costs are low, they are known to have low chemical resistance. However, the fact that cleaning using this technology did not result in deterioration in both strength and elongation after 20 days suggests the possibility of long-term use.

[0097] (Comparative Example 11) Sodium hypochlorite 0.5% by mass, caustic soda 2% by mass, Quince Soap 5G 2% by mass, and water 95.5% by mass were mixed. The pH at room temperature was 13.0, and the redox potential was +310 mV. A PVDF membrane was immersed in the cleaning solution maintained at 20-23 ° C. After 10 days, the strength and elongation retention rates relative to the unused membrane were 95.0% and 84.0%, respectively, and after 20 days, the strength and elongation retention rates relative to the unused membrane were 97.2% and 87.1%, respectively. It was confirmed that deterioration had occurred compared to the reduction cleaning solution of Example 12.

[0098] (Comparative Example 12) Sodium hypochlorite 0.5% by mass, caustic soda 2% by mass, Quince Soap 5G 2% by mass, and water 95.5% by mass were mixed. The pH at room temperature was 13.0, and the redox potential was +310 mV. A PE membrane was immersed in the cleaning solution maintained at 20-23 ° C., and after 10 days, the strength and elongation retention rates relative to the unused membrane were 86.2% and 79.9%, respectively. After 20 days, the strength and elongation retention rates relative to the unused membrane were 85.6% and 66.9%, respectively. It was confirmed that deterioration had occurred compared to the reduction cleaning solution of Example 13.

[0099]

[0100] Thus, it was found that the cleaning solution using sodium hypochlorite caused more deterioration than the reduction cleaning solution, even though it was at 20 to 23°C, a temperature condition that is less susceptible to deterioration. It was confirmed that cleaning using the reduction cleaning solution not only has a high cleaning effect, but also does not deteriorate the mechanical properties of the membrane.

[0101] Example 14: To remove bacteria and proteins from the seasoning liquid (salt content 5%) of prosciutto, a pencil module SLP-1053 (polysulfone UF membrane, molecular weight cutoff 10,000, inner diameter 1.4 mm, 140 membranes, membrane area 0.12 m) manufactured by Asahi Kasei Corporation was used. ２ Filtration was carried out until the 29 L stock solution was concentrated 10 times. The initial flux was 13 L / m ２ / hr, but after 40 hours when the 10-fold concentration was completed, it was 4 L / m ２ / hr. The water permeability retention of the membrane after concentration had decreased to 5%. When this module was contacted for 2 hours with a reduction cleaning solution containing 2% by mass of hydrosulfite, 2% by mass of caustic soda, and 96% by mass of water, with a pH of 13.1 at 40°C, an ORP of -795 mV, and a temperature of 40°C, the water permeability retention was 84%, indicating sufficient recovery.

[0102] (Comparative Example 13) Concentration was carried out using the same raw water, the same membrane module, and the same conditions as in Example 14. The initial flux was 13 L / m ２ / hr, but after 40 hours when the 10-fold concentration was completed, it was 4 L / m ２ / hr. The water permeability retention of the membrane after concentration had decreased to 6%. When this module was contacted for 2 hours with a cleaning solution containing 0.5 wt% sodium hypochlorite, 2 wt% caustic soda, and 97.5 wt% water, with a pH of 13.2 at 40°C, an oxidation-reduction potential of +320 mV, and a temperature of 40°C, the water permeability retention was 61%, indicating insufficient recovery after cleaning.

[0103]

[0104] Example 15, Comparative Example 14 Using an RO element manufactured by Dupont Corporation (FilmTec TW30-1812-100HR) using an aromatic polyamide membrane, wastewater from a dyeing factory was treated biologically (i.e., treatment including anaerobic biological treatment and aerobic biological treatment), sand filtered, and adsorbed with activated carbon (TOC 10-20 mg / L, TDS 5000-8000 mg / L) as raw water. The operation was carried out at a constant operating pressure until the membrane permeate rate decreased to 40%. The element was then disassembled, and the membrane was cut out. Note that biopolymers derived from the aerobic biological treatment were attached to the membrane cut out from the RO element.

[0105] (Example 15) The disassembled contaminated membrane was thoroughly rinsed with pure water and then placed in a thin-layer flow flat membrane test cell C10-T (membrane area 60 cm) manufactured by Nitto Denko Corporation. ２ ) and the water permeability retention was measured. The measurement was performed at a liquid temperature of 25 ° C., 300 ppm salt water was pressurized at 100 kPa, and the weight of permeated water per hour was measured. The conductivity of the permeated water was also measured, and the salt rejection rate was also calculated. When the contaminated membrane was measured, the water permeability retention rate for the membrane after cleaning with respect to the unused membrane was 42%, and the salt rejection rate was 98.3%. Next, the cut membrane was immersed in a reduction cleaning solution (pH 11.8, acid reduction potential -740 mV) containing 0.1 mass% hydrosulfite, 0.1 mass% caustic soda, and 0.1 mass% Quince Soap 5G for 8 hours, and then similarly measured. The water permeability for the membrane after cleaning with respect to the unused membrane was 92%, and the salt rejection rate was 97.9%, and the rejection rate did not decrease and the water permeability was restored.

[0106] (Comparative Example 14) A cut-out membrane was immersed in a 0.1% by mass aqueous solution of caustic soda (pH 11.9, oxidation-reduction potential -28 mV) for 8 hours, and then similar measurements were performed. The water permeability of the washed membrane was 68% compared to that of an unused membrane, and the salt removal rate was 97.8%. Although the removal rate did not decrease, the recovery of water permeability was insufficient.

[0107] Examples 16-17, Comparative Example 15: The effectiveness of the cleaning solution used in maintenance cleaning was confirmed in a water treatment process in which secondary sewage effluent was used as raw water and filtered through an Asahi Kasei Corporation membrane module UNA620A, followed by RO treatment to obtain pure water and recycle the water. The operating conditions were a raw water temperature of 20-25°C, a filtration flux of 50 LMH for 29 minutes, and a backwash of 100 LMH for 1 minute in which the filtrate was sent through the membrane to the raw water side for cleaning. After 23 hours of filtration and backwashing, a 1-hour maintenance cleaning was performed in which the cleaning solution was passed from the filtration side of the membrane to the raw water side at a flow rate of 3 L / min for 20 minutes, followed by a further 40 minutes of static time.

[0108] (Example 16) When a reduction cleaning solution containing 0.1 mass % hydrosulfite and 400 mass ppm caustic soda, pH 7.4, and oxidation-reduction potential −685 mV was used as the cleaning solution and the system was operated for one month, the operating pressure did not increase suddenly from 40 kPa to 44 kPa, and the system could continue operation.

[0109] (Example 17) When a reduction cleaning solution containing 400 ppm by mass of hydrosulfite and 300 ppm by mass of caustic soda, having a pH of 10.7 and an oxidation-reduction potential of −670 mV was used as the reduction cleaning solution and operated for one month, the operating pressure did not increase suddenly from 40 kPa to 46 kPa, and the operation could be continued.

[0110] (Comparative Example 15) When a cleaning solution containing 600 ppm by mass of sodium hypochlorite, a pH of 12.0, and an oxidation-reduction potential of +505 mV was used as the cleaning solution and operated for one month, the operating pressure increased from 40 kPa to 110 kPa, and the effect as maintenance cleaning was low.

[0111] Examples 18-20, Comparative Example 16 Wastewater discharged from a semiconductor factory was subjected to biological treatment (i.e., treatment including anaerobic biological treatment and aerobic biological treatment), and the treated solution was passed through a membrane module (UNA-620A manufactured by Asahi Kasei Corporation) that filters the treated solution using an MF membrane, followed by water treatment using a reverse osmosis membrane. A cleaning recovery test was conducted on the UNA-620A, which had been used for approximately four months in a system. One UNA-620A was removed from the treatment system, the membrane module was disassembled, and the hollow fiber membrane was cut out. The water permeation rate and water permeation retention were measured using the same procedures as in Example 1. The water permeation retention of the uncleaned membrane was 47.3%. Note that biopolymers derived from the aerobic biological treatment were attached to the UNA-620A removed from the treatment system.

[0112] Example 18 50 g of a reduction cleaning solution was prepared by mixing 1% by mass of sodium hydrosulfite, 1% by mass of caustic soda, and 98% by mass of water. The pH of the reduction cleaning solution at room temperature was 13.0, and the redox potential was -821 mV. Ten membranes cut out from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (21-23°C) for 2 hours. After 4 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention was measured, and it had recovered to 97.6%.

[0113] Example 19 50 g of a reduction cleaning solution was prepared by mixing 0.1% by mass of sodium hydrosulfite, 0.1% by mass of caustic soda, and 99.8% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.0, and the redox potential was -789 mV. Ten membranes cut out from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (21-23°C) for 2 hours. After 4 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention was measured, and the membranes recovered to 91.8%.

[0114] Example 20: 50 g of a reduction cleaning solution was prepared by mixing 0.05% by mass of sodium hydrosulfite, 0.05% by mass of caustic soda, and 99.9% by mass of water. The pH of the reduction cleaning solution at room temperature was 11.8, and the redox potential was -771 mV. Ten membranes cut out from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (22-24°C) for 2 hours. After 4 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention was measured, and the membranes recovered to 88.6%.

[0115] (Comparative Example 16) 50 g of a reduction cleaning solution was prepared by mixing 0.1% by mass of sodium hypochlorite, 0.1% by mass of caustic soda, and 99.8% by mass of water. The pH of the reduction cleaning solution at room temperature was 12.2, and the oxidation-reduction potential was +421 mV. Ten membranes cut from the module were immersed in the reduction cleaning solution in a test tube and allowed to stand at room temperature (22-24°C) for 2 hours. After 4 hours, the membranes were removed from the test tube, and the water permeation rate was measured in the same manner. The water permeation retention rate was measured, but only recovered to 67.2%. From the above, although Examples 18 to 20 all had a cleaning effect, a higher pH and a lower ORP resulted in better cleaning recovery.

[0116]

[0117] Example 21, Comparative Example 17 Using a Nitto Denko RO element: CAP7-LD, raw water was wastewater (TOC 10-20 mg / L, TDS 1000-2000 mg / L) from a chemical plant that had been biologically treated (i.e., treated with anaerobic and aerobic biological treatment), sand filtered, and adsorbed with activated carbon. The operation was carried out at a constant operating pressure until the membrane permeate volume decreased to 75%. The element was then disassembled, and the membrane was excised. Biopolymers derived from the aerobic biological treatment were attached to the membrane removed from the RO element. The permeate volume was determined using a Nitto Denko Membrane Master C10-T. The permeate volume and salt rejection rate were calculated using the following formula according to the instructions provided with the C10-T: permeate volume = 0.24 × f × V / T, where f is the water temperature conversion coefficient, and V (mL) is the volume of water obtained by filtration in a given time T (minutes). The salt removal rate was calculated by the following formula: Salt removal rate = (raw water conductivity - permeated water conductivity) / raw water conductivity x 100.

[0118] (Example 21) The disassembled contaminated membrane was thoroughly rinsed with pure water and then placed in a thin-layer flow flat membrane test cell C10-T (membrane area 60 cm) manufactured by Nitto Denko Corporation. ２ ) and the water permeability retention was measured. The measurement was performed at a raw water temperature of 25 ° C., 500 ppm salt water pressurized at 700 kPa, and circulated at 0.8 L / min, and the weight of permeated water per hour was measured. The conductivity of the permeated water was also measured, and the salt rejection rate was also calculated. The contaminated membrane was cut out and the water permeability and salt rejection rate were measured under the same conditions as for an unwashed membrane. The water permeability retention rate for the membrane after washing with respect to the unused membrane was 78.0%, and the salt rejection rate was 98.0%. Next, the membrane was immersed in a reduction cleaning solution (pH 11.8, acid reduction potential -820 mV) containing 1 mass% hydrosulfite and 0.1 mass% caustic soda for 16 hours, and then rinsed with pure water. The water permeability and salt rejection rate were measured under the same conditions as for an unwashed membrane. The water temperature was 25 ° C. After washing the unused membrane, the water permeability through the membrane was 97.9%, the salt rejection rate was 97.8%, and the water permeability recovered by 19.9% ​​without any decrease in the rejection rate.

[0119] (Comparative Example 17) The contaminated membrane was cut out and the water permeability and filtrate conductivity were measured under the same conditions as in Example 21. The water permeability retention rate for the membrane after cleaning was 81.2% and the salt removal rate was 97.8%. Next, the membrane was immersed in a 0.1% by mass aqueous caustic soda solution (pH 11.9, oxidation-reduction potential -35 mV) for 16 hours, and the same measurements were taken. The water permeability for the membrane after cleaning was 82.1% and the salt removal rate was 97.7%. Although the removal rate did not decrease, the water permeability recovery was only 0.9%, and a sufficient cleaning recovery effect was not obtained.

[0120]

[0121] According to the membrane cleaning method of the present invention, deposits adhering to the membrane surface can be removed, the membrane permeability can be restored, and the filtration performance of the membrane can be maintained.

Claims

1. A washing step of contacting a separation membrane or a concentration membrane with a reduction washing solution at a pH of 7.0 or higher, The pH of the reduction cleaning solution is 7.0 or more and 14.0 or less, The oxidation-reduction potential of the reduction cleaning solution is −1200 mV or more and −300 mV or less. A method for cleaning a separation membrane or a concentration membrane, comprising:

2. 2. The cleaning method according to claim 1, wherein the oxidation-reduction potential of the reducing cleaning solution is −1200 mV or more and −500 mV or less.

3. the reducing cleaning solution contains a reducing agent and an alkaline agent, the reducing agent is at least one selected from the group consisting of sodium hydrosulfite, thiourea dioxide, borohydride, and sodium formaldehyde sulfoxylate; The alkaline agent is at least one selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, and sodium bicarbonate. The cleaning method according to claim 1 or 2.

4. The cleaning method according to claim 1 or 2, wherein the cleaning step is carried out at a temperature of 1 to 100°C.

5. The cleaning method according to claim 1 or 2, wherein the reducing cleaning solution further contains a surfactant.

6. The separation membrane or the concentration membrane is a hollow fiber membrane or a flat membrane, The separation membrane or the concentration membrane is housed in a case, a module in which the separation membrane or the concentration membrane is housed in a case is a case-type module, The cleaning step is a step of filling the case with the reduction cleaning solution and bringing the separation membrane or the concentration membrane into contact with the reduction cleaning solution. The cleaning method according to claim 1 or 2.

7. The separation membrane or the concentration membrane is a hollow fiber membrane or a flat membrane, The separation membrane or the concentration membrane is housed in a case, a module in which the separation membrane or the concentration membrane is housed in a case is a case-type module, The cleaning step is a step of contacting the separation membrane or the concentration membrane with the reduction cleaning solution by permeating the surface and the inside of the separation membrane or the concentration membrane. The cleaning method according to claim 1 or 2.

8. 7. The cleaning method according to claim 6, wherein the case-type module is of an external pressure filtration type.

9. 8. The cleaning method according to claim 7, wherein the case-type module is of an external pressure filtration type.

10. The cleaning method according to claim 1 or 2, wherein the separation membrane or the concentration membrane is a membrane for separation or concentration in a wastewater treatment process, a water purification process, or a seawater desalination process.

11. The cleaning method according to claim 1 or 2, wherein the separation membrane or the concentration membrane is a UF membrane, an MF membrane, an RO membrane, an FO membrane, or an NF membrane.

12. 3. The cleaning method according to claim 1, wherein the separation membrane or the concentration membrane is a membrane containing, as a constituent component, at least one resin selected from the group consisting of PVDF, PE, PSF, PAN, PP, PTFE, ETFE, PA, and PES.

13. 3. The cleaning method according to claim 1, wherein the cleaning step is a step of contacting at least one selected from the group consisting of an inner surface, an outer surface, and an inside of the separation membrane or the concentration membrane with the reduction cleaning solution.

14. 3. The cleaning method according to claim 1, wherein the separation membrane or the concentration membrane is a membrane that has been used for separating or concentrating a solution after aerobic biological treatment.

15. 3. A cleaning device used in the cleaning method according to claim 1, comprising a portion for bringing the separation membrane or the concentration membrane into contact with the reduction cleaning solution.