Separation membrane cleaning method and separation membrane cleaning agent used therein

Abstract

To provide a separation membrane cleaning method and a separation membrane cleaning agent used therefor, which do not cause membrane deterioration and highly effectively remove organic substances and biofouling.SOLUTION: A method for cleaning a separation membrane comprises the steps of: (1) cleaning the separation membrane with a separation membrane cleaning agent (X) containing at least one selected from a group consisting of a chelating agent, an organic acid and an inorganic acid; and (2) cleaning, after the step (1), the separation membrane with a separation membrane cleaning agent (Y) containing at least one chelating agent (C) selected from a group consisting of an oxidizing agent (A), a heavy metal compound (B) and a phosphonic acid-based chelating agent, an aminocarboxylic acid-based chelating agent and citric acid. The separation membrane cleaning agent contains at least the one chelating agent (C) selected from the group consisting of the oxidizing agent (A), the heavy metal compound (B) and the phosphonic acid-based chelating agent, the aminocarboxylic acid-based chelating agent and the citric acid.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a separation membrane cleaning method and a cleaning agent for a separation membrane used therein. [Background technology]

[0002] Separation membranes such as reverse osmosis (RO) membranes, ultrafiltration (UF) membranes, and microfiltration (MF) membranes are often used to separate components dissolved in water and purify the water. In such a separation process, suspended solids in the raw water and solids larger than the pore size of the membrane are blocked by the membrane, forming concentration polarization and a cake layer, while at the same time blocking the pores and increasing the filtration resistance, resulting in membrane fouling, and the transmembrane pressure rises as low flow rate operation continues. When a certain transmembrane pressure is reached, it becomes necessary to wash the separation membrane with a detergent to restore the original membrane filtration flux.

[0003] As cleaning agents for separation membranes, Patent Documents 1, 2, and 3 disclose oxidizing agents that utilize the Fenton reaction and are suitable for cleaning porous membranes made of polyvinylidene fluoride, known as microfiltration membranes (MF membranes). They utilize the Fenton reaction to remove organic matter and biofouling. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5421768 [Patent Document 2] Patent No. 5124815 [Patent Document 3] Patent No. 4384741 DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0005] However, the above-mentioned oxidizing agent not only removes organic matter and biofouling, but also has the potential to deteriorate the membrane itself, so that the scope and conditions of application are limited. In addition, although there are many types of separation membranes with different purposes and pore sizes, when a reverse osmosis membrane made of a polyamide material is cleaned with an oxidizing agent utilizing the Fenton reaction as described in Patent Documents 1, 2, and 3, the membrane is severely deteriorated and cannot be used.

[0006] An object of the present invention is to provide a separation membrane cleaning method which does not cause membrane deterioration and is highly effective in removing organic matter and biofouling, and a separation membrane cleaning agent used therein. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems and have found that an excellent cleaning effect can be obtained without membrane degradation by first cleaning and removing metal compounds adhering to the surface of a separation membrane, and then cleaning the separation membrane with a specific cleaning agent containing a Fenton reaction system oxidizing agent.

[0008] The present invention relates to As a chelating agent Ethidron Acid Depending on the contained separation membrane cleaning agent (X), Polyamide reverse osmosis membrane and after step (1), an oxidizing agent (A), a heavy metal compound (B), and Etidronic acid Consists of By using a cleaning agent (Y) for separation membranes containing a chelating agent (C), Polyamide reverse osmosis membrane The separation membrane cleaning method further comprises a step (2) of cleaning the separation membrane.

[0009] The oxidizing agent (A) is preferably at least one selected from the group consisting of hydrogen peroxide, percarbonate, peracetic acid, and salts thereof. The heavy metal compound (B) is preferably at least one selected from the group consisting of chlorides, sulfides and oxides of Fe, Mn, Co, Ni, Ti and Cu.

[0011] The present invention also provides a method for producing a semiconductor device comprising the steps of: A separation membrane cleaning agent for cleaning a polyamide-based reverse osmosis membrane, comprising:Oxidizing agents (A), heavy metal compounds (B) and Etidronic acid Consists of Contains chelating agent (C) pH is 3-5 It is also a cleaning agent for separation membranes.

[0012] The oxidizing agent (A) is preferably at least one selected from the group consisting of hydrogen peroxide, percarbonate, peracetic acid, and salts thereof. The heavy metal compound (B) is preferably at least one selected from the group consisting of chlorides, sulfides and oxides of Fe, Mn, Co, Ni, Ti and Cu. Effect of the Invention

[0013] By using the separation membrane cleaning method of the present invention to clean a separation membrane after filtration, it is possible to perform cleaning that is highly effective in removing organic matter, biofouling, and the like, without causing membrane deterioration. Furthermore, by using the cleaning agent of the present invention to clean the membrane, no membrane deterioration occurs, and an excellent cleaning effect can be obtained that is highly effective in removing organic matter and biofouling. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is an explanatory diagram showing an outline of a cleaning device used in the examples. [Diagram 2] FIG. 2 is an explanatory diagram showing the inside of a container of a cleaning device used in the examples. [Diagram 3] FIG. 1 is an explanatory diagram showing an outline of another cleaning device used in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will be described in detail below.

[0016] The method for cleaning a separation membrane of the present invention includes a step (1) of cleaning the separation membrane with a cleaning agent for separation membranes (X) containing at least one selected from the group consisting of a chelating agent, an organic acid, and an inorganic acid, and, after the step (1), a step (2) of cleaning the separation membrane with a cleaning agent for separation membranes (Y) containing the above-mentioned specific component. First, the separation membrane is washed with the above-mentioned separation membrane cleaning agent (X) to wash and remove heavy metal compounds adhering to the separation membrane surface (step (1)), and then the membrane surface is washed with the above-mentioned specific separation membrane cleaning agent (Y) (step (2)), whereby the Fenton reaction on the membrane surface can be appropriately suppressed, and thus, it becomes possible to effectively remove organic pollutants such as organic matter and biofouling adhering to the separation membrane while suppressing membrane deterioration.

[0017] The cleaning agent (X) for separation membranes used in step (1) is capable of cleaning and removing metal compounds attached to the surface of the separation membrane, and contains at least one selected from the group consisting of a chelating agent, an organic acid, and an inorganic acid that exerts this function. Furthermore, a basic chelating agent can also be preferably used in the present invention. Although there are many chelating agents that are organic acids, such compounds can be particularly preferably used.

[0018] As the "at least one selected from the group consisting of chelating agents, organic acids, and inorganic acids", for example, phosphonic acid chelating agents, aminocarboxylic acid chelating agents, sodium gluconate, tartaric acid, malic acid, citric acid, and other hydroxycarboxylic acids or salts thereof having a chelating ability are used. As the hydroxycarboxylic acid or salt thereof, it is preferable to use one having a chelating ability.

[0019] Examples of the phosphonic acid chelating agent include various phosphonic acids such as phosphonic acid, aminotris(methylenephosphonic acid), 1-hydroxyethane-1,1-diphosphonic acid (etidronic acid), ethylenediaminetetrakis(methylenephosphonic acid), hexamethylenediaminetetrakis(methylenephosphonic acid), diethylenetetraminepentakis(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid, and salts of these various phosphonic acids.

[0020] Examples of the aminocarboxylic acid chelating agent include various aminocarboxylic acids such as nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraminehexaacetic acid (TTHA) and salts of these various aminocarboxylic acids.

[0021] Alternatively, an organic acid having no chelating ability, such as acetic acid, or a salt thereof may be used. Furthermore, examples of inorganic acids that may be used include hydrochloric acid, sulfuric acid, and nitric acid.

[0022] Among these, in terms of metal oxide solubility, it is preferable to use a phosphonic acid-based chelating agent such as phosphonic acid, aminotris(methylenephosphonic acid), or 1-hydroxyethane-1,1-diphosphonic acid.

[0023] The amount of the above components is not particularly limited as long as the metal compounds can be removed, but is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, and is preferably 20% by mass or less, more preferably 10% by mass or less in the cleaning solution (X). By setting the amount within the above range, it is possible to remove the metal compounds without deteriorating the film.

[0024] The pH of the cleaning agent for separation membranes (X) is preferably 2 to 10. Within this range, metal compounds can be removed without deteriorating the membrane.

[0025] The detergent (X) may contain various alkali metal hydroxides such as sodium hydroxide and potassium hydroxide for the purpose of adjusting the pH level.

[0026] In the present invention, the separation membrane cleaning agent (Y) used in step (2) also has a characteristic feature. The separation membrane cleaning agent (Y) contains an oxidizing agent (A), a heavy metal compound (B) and a chelating agent (C).

[0027] As described above, the cleaning agent for separation membranes (Y) of the present invention contains an oxidizing agent (A) and a heavy metal compound (B). Specifically, for example, a ferrous compound (Fe 2+ When HO 2 ) and hydrogen peroxide are added, hydroxyl radicals (HO 3 ) are generated according to the reaction shown below, and these hydroxyl radicals oxidize and decompose the organic pollutants on the membrane surface. H2O2+Fe 2+ →HO +OH - +Fe 3+

[0028] The Fenton reaction has a fast reaction rate and is difficult to control, making it difficult to suppress membrane deterioration. Therefore, in order to suppress the generation of the hydroxyl radical (HO·) and control the reaction rate, at least one chelating agent (C) selected from the group consisting of phosphonic acid chelating agents, aminocarboxylic acid chelating agents, and citric acid is used in combination with the oxidizing agent (A) and heavy metal compound (B), making it possible to achieve a balance between suppressing membrane deterioration and cleaning the membrane surface.

[0029] The oxidizing agent (A) used in the present invention is preferably at least one selected from the group consisting of hydrogen peroxide, percarbonate, peracetic acid, and salts thereof, etc. Among them, it is more preferable to use hydrogen peroxide.

[0030] In the cleaning agent for separation membranes (Y) of the present invention, the blending amount of the oxidizing agent (A) in the cleaning agent for separation membranes (Y) is preferably 0.01 mass% or more, more preferably 0.5 mass% or more, and preferably 10 mass% or less, more preferably 5 mass% or less, and even more preferably 3 mass% or less. By setting the amount within the above range, it is possible to remove the metal compounds without deteriorating the film.

[0031] The heavy metal compound (B) used in the present invention is preferably at least one selected from the group consisting of chlorides, sulfides, oxides, etc. containing Fe, Mn, Co, Ni, Ti, and Cu elements. From the viewpoints of solubility in water, cost, and ease of filtration and separation by adding an alkali such as sodium hydroxide to the cleaning solution after use, the heavy metal compound (B) used in the present invention is more preferably ferrous chloride.

[0032] In the cleaning agent for separation membranes (Y) of the present invention, the amount of the heavy metal compound (B) in the cleaning agent for separation membranes (Y) is preferably 0.0001% by mass or less, more preferably 0.002% by mass or less, and even more preferably 0.003% by mass or more. Also, the amount is preferably 0.05% by mass or less, more preferably 0.01% by mass or less, and even more preferably 0.005% by mass or less. By setting the amount within the above range, it is possible to remove the metal compounds without deteriorating the film.

[0033] The chelating agent (C) used in the cleaning agent (Y) for separation membranes is at least one chelating agent (C) selected from the group consisting of phosphonic acid chelating agents, aminocarboxylic acid chelating agents, and citric acid.

[0034] The purpose of using the chelating agent (C) is to suppress the generation of hydroxyl radicals (HO·) and control the reaction rate. To achieve this, the chelating agent (C) is used to chelating heavy metal compounds (B), such as Fe, present in the separation membrane cleaning agent (Y). 2+ As a chelating agent acting on the iron ions, it is preferable to use a phosphonic acid chelating agent, an aminocarboxylic acid chelating agent, or citric acid.

[0035] Examples of phosphonic acid chelating agents include various phosphonic acids such as phosphonic acid, aminotris(methylenephosphonic acid), 1-hydroxyethane-1,1-diphosphonic acid (etidronic acid), ethylenediaminetetrakis(methylenephosphonic acid), hexamethylenediaminetetrakis(methylenephosphonic acid), diethylenetetraminepentakis(methylenephosphonic acid), and 2-phosphonobutane-1,2,4-tricarboxylic acid, and salts of these various phosphonic acids.

[0036] Examples of aminocarboxylic acid chelating agents include various aminocarboxylic acids such as nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraminehexaacetic acid (TTHA) and salts of these various aminocarboxylic acids.

[0037] Furthermore, a non-acidic chelating agent may be used as long as it has the above-mentioned action.

[0038] As the chelating agent (C), it is preferable to use a phosphonic acid-based chelating agent such as phosphonic acid, aminotris(methylenephosphonic acid), 1-hydroxyethane-1,1-diphosphonic acid, etc., particularly from the viewpoint of resistance to oxidative decomposition.

[0039] The chelating agent having the above-mentioned action may be used alone or in combination of two or more kinds. The salt of the chelating agent may be a sodium salt, a potassium salt, a lithium salt, an ammonium salt, an amine salt, an alkanolamine salt, or the like.

[0040] In the cleaning agent for separation membranes (Y) of the present invention, the blending amount of the chelating agent used for the above purpose is preferably 0.0001 mass% or more, more preferably 0.003 mass% or more, and even more preferably 0.004 mass% or more in the cleaning agent for separation membranes (Y), and is preferably 0.05 mass% or less, and more preferably 0.01 mass% or less. If the amount of the chelating agent is small, the membrane tends to deteriorate, whereas if the amount is large, the membrane tends to be insufficiently washed.

[0041] Furthermore, the cleaning agent for separation membranes (Y) of the present invention can be used to form, for example, Fe by the Fenton reaction. 2+ From Fe 3+ The iron ions that have been oxidized to Fe 2+ In order to reduce the chelating agent (C) to 100% by weight and continue the reaction, it is preferable to use a hydroxycarboxylic acid such as gluconic acid or a salt thereof in combination with the chelating agent (C). Sodium gluconate is particularly preferable. Furthermore, by adding the above-mentioned hydroxycarboxylic acid and its salt, the heavy metals used are dissolved in the aqueous solution and can be washed away without adhering to the membrane surface.

[0042] The amount of the hydroxycarboxylic acid and its salt in the cleaning agent for separation membranes (Y) is preferably 0.001% by mass or more, more preferably 0.002% by mass or more, and even more preferably 0.003% by mass or more, and is preferably 2% by mass or less.

[0043] The chelating agent (C) may be the same as the component that can be used in the above-mentioned cleaning agent for separation membranes (X). However, the purpose and action of the chelating agent (C) are different from those of the cleaning agent (X). The amount of the chelating agent (C) is appropriately adjusted to allow the respective actions to be exerted.

[0044] The pH of the cleaning agent for separation membranes (Y) of the present invention is preferably 2-7, more preferably 3-5, further preferably 3-4, and most preferably 4. By adjusting the pH to the above range, it is possible to oxidatively decompose and remove organic pollutants without deteriorating the membrane.

[0045] The cleaning agent for separation membranes (Y) of the present invention can be produced by mixing the above-mentioned oxidizing agent (A), heavy metal compound (B) and chelating agent (C) in a solvent. The mixing can be performed by a known method, and may be performed simply by mixing. The solvent used in the present invention is preferably water from the viewpoint of the impact on the environment. In the production of the cleaning agent for separation membranes (Y) of the present invention, from the viewpoints of removing organic pollutants, preventing the heavy metal compounds in the cleaning solution from adhering to the membrane surface, and preventing the decomposition of the oxidizing agent (A) itself, it is preferable to add the oxidizing agent (A) in a state in which the heavy metal compound (B), preferably the iron ion which is a heavy metal ion, is completely dissolved.

[0046] The method of supplying the oxidizing agent (A) used in the cleaning agent for separation membranes (Y) of the present invention is not particularly limited. For example, when oxygen peroxide is used, in addition to supplying hydrogen peroxide as it is, an oxidizing agent that generates hydrogen peroxide in an aqueous solution such as sodium percarbonate or sodium perborate may be used.

[0047] In addition, the cleaning agent (Y) for separation membranes of the present invention may contain a substance that promotes cleaning. Examples include surfactants. Specifically, examples include anionic surfactants such as soap, higher alcohol sulfate salts, alkylbenzenesulfonates, alkylnaphthalenesulfonates, and higher alcohol phosphate salts; cationic surfactants such as primary amine salts, secondary amine salts, tertiary amine salts, and quaternary ammonium salts; amphoteric surfactants such as amine oxides such as alkyldimethylamine oxides, alkyldimethylamino fatty acid betaines, and alkylcarboxymethylhydroxyethylimidazolium betaines; and nonionic surfactants such as polyoxyethylene alkylphenyl ethers, polyoxypropylene alkylphenyl ethers, polyoxyethylene alkyl ethers, polyoxyethylene alkyl esters, polyethylene glycol alkyl esters, ethylene oxide adducts of polypropylene glycols, and propylene oxide adducts of polypropylene glycols.

[0048] Moreover, various alkali metal hydroxides such as sodium hydroxide and potassium hydroxide may be added to the cleaning agent for separation membranes (Y) of the present invention for adjusting the pH.

[0049] The separation membrane cleaning method of the present invention includes the following: A method of supplying a cleaning liquid from the raw water side and filtering the entire amount for cleaning (full amount filtration cleaning), a method of circulating the cleaning liquid from the raw water side for cleaning, which may be either filtered (circulation filtration cleaning) or not filtered (circulation non-permeation cleaning). Other cleaning methods include a method of filling the separation membrane with cleaning liquid and immersing it for a predetermined time (immersion cleaning), a method of backflow cleaning with the cleaning liquid from the filtration side (backflow cleaning), a cleaning method in which the cleaning liquid backflowed from the filtration side is stored in the separation membrane and immersed for a predetermined time (backflow immersion cleaning), and the like, each of which may be used alone or in combination, or a cleaning method in which air bubbles are introduced into the raw water side simultaneously with each cleaning (simultaneous air cleaning), or a cleaning method in which air bubbles are introduced into the raw water side following each cleaning.

[0050] In the separation membrane cleaning method of the present invention, the method of contacting the cleaning agent with the separation membrane may or may not involve passing the cleaning solution in the cross-sectional direction of the membrane. When the cleaning solution is passed in the cross-sectional direction of the membrane, the passing direction may be from the raw water side membrane surface or from the filtration side membrane surface.

[0051] The above-mentioned cleaning methods may be appropriately adopted for the steps (1) and (2) of the cleaning method of the present invention. In the present invention, particularly in step (2), better results can be obtained by combining circulation washing with immersion washing.

[0052] The treatment time in step (1) is not particularly limited as long as it can remove inorganic components, and a treatment time of 1 hour or more will be sufficient to obtain the desired effect, preferably 18 hours or more. Furthermore, the treatment time in step (2) may be appropriately set so long as it is possible to remove organic components, and is preferably 1 hour or more, and more preferably 18 hours or more.

[0053] The types of separation membranes that can be cleaned by the cleaning method of the present invention include reverse osmosis membranes, NF membranes, ultrafiltration (UF) membranes, and microfiltration (MF) membranes. In particular, when cleaning polyamide-based reverse osmosis membranes, the effect of suppressing membrane deterioration is maximized.

[0054] In addition, the separation membrane may be subjected to alkaline washing before or after the above step (1), or after the step (2). Examples of the alkaline agent used for washing include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide. EXAMPLES

[0055] The present invention will be specifically described below with reference to examples and comparative examples, but these are not intended to limit the scope of the present invention.

[0056] [Example 1] <Cleaning equipment> The cleaning effect and membrane deterioration suppression of the RO membrane were confirmed using a flat membrane test device shown in Figures 1 and 2. As shown in Figure 1, in this flat membrane test device, RO membrane feed water is supplied from a pipe 6 by a high-pressure pump 3 to a raw water chamber 1A below a flat membrane cell 2 in which an RO membrane is set in a sealed container 1. As shown in Figure 2, the sealed container 1 is composed of a lower case 1a on the raw water chamber 1A side and an upper case 1b on the permeate chamber 1B side, and a flat membrane cell 2 is fixed between the lower case 1a and the upper case 1b. The flat membrane cell 2 has an RO membrane 2B fixed thereto by an O-ring, and is supported by a spacer 2A from the raw water side and a porous support plate 2B from the permeate side. The RO membrane permeate is taken out from a pipe 7 through the permeate chamber 1B above the flat membrane cell 2. The concentrated water is taken out from a pipe 8. The pressure inside the sealed container 1 is adjusted by a pressure gauge 4 installed in the water supply pipe 6 and a pressure regulating valve 5 installed in the concentrated water extraction pipe 8. The RO membrane 2B of the flat membrane cell 2 was an aromatic polyamide RO membrane (TMG20-400 manufactured by Toray) that had been in operation for three years to treat water taken from a river.

[0057] <Cleaning solution> Separation membrane cleaning agent (X) A liquid prepared by adding 1% by mass of 1-hydroxyethane-1,1-diphosphonic acid (etidronic acid) to 100% by mass of ion-exchanged water and adjusting the pH to 6 with sodium hydroxide was used as a separation membrane cleaning agent (X).

[0058] Separation membrane cleaning agent (Y) A solution prepared by adding 1 mass% hydrogen peroxide, 0.004 mass% ferrous chloride tetrahydrate as a heavy metal compound (B), 0.009 mass% 1-hydroxyethane-1,1-diphosphonic acid (etidronic acid) as a chelating agent (C) and 0.04 mass% sodium gluconate to 100 mass% pure water and adjusting the pH to 4 with sodium hydroxide was used as a separation membrane cleaning agent (Y).

[0059] <Cleaning method> 1. Before cleaning, a 1500mg / L aqueous solution of sodium chloride was passed through the fouled membrane at 0.8MPa to measure the permeation flux and salt rejection. 2. After that, each cleaning agent was supplied and cleaning was performed. The cleaning operation was performed in the following order for both steps (1) and (2): 2 hours of circulation cleaning, 14 hours of immersion cleaning, and 2 hours of circulation cleaning. No operating pressure was applied during circulation cleaning. 3. After each washing step, a 1500 mg / L aqueous sodium chloride solution was passed through the column at 0.8 MPa to determine the permeation flux and salt rejection rate.

[0060] The cleaning ability and membrane deterioration were evaluated based on the following criteria. (1) Cleanability The recovery rate was calculated using the following formula, and the cleaning ability was evaluated according to the following criteria. The recovery rate also increases when membrane degradation occurs, so when membrane degradation does not occur and the recovery rate is high, the cleaning ability can be said to be good. Recovery rate (%) = (Permeation flux after washing - Permeation flux before washing) / Permeation flux before washing × 100 The permeation flux was calculated by measuring the volume of the permeated water that permeated the membrane in 5 minutes using a measuring cylinder. 〇…Recovery of 10% or more △…Recovery between 5% and 10% ×…Less than 5% recovery

[0061] (2) Membrane deterioration The rate of decrease in salt rejection was calculated using the following formula, and the membrane deterioration was evaluated according to the following criteria. Decrease rate of salt rejection (%) = (Salt rejection before washing - Salt rejection after washing) / Salt rejection before washing x 100 The salt rejection rate was calculated by the following formula. Desalination rate (%): 100-Permeate water conductivity / Feed water conductivity x 100 ○…Decrease of 1% or less △: Decrease of more than 1% and less than 5% ×…Decrease of more than 5%

[0062] (3) Overall Judgment ○: Both cleaning and membrane deterioration were ○ △: At least one of cleaning property or membrane deterioration property is △, and the other is not × ×: Washability or membrane deterioration was ×

[0063] The results are shown in Table 1.

[0064] [Comparative Example 1] Cleaning was carried out in the same manner as in Example 1, except that cleaning in step (1) was not carried out and only cleaning in step (2) was carried out. The results are shown in Table 1.

[0065] [Comparative Example 2] In the step (2), washing was carried out in the same manner as in Example 1, except that the chelating agent (C) was not used. The results are shown in Table 1.

[0066] [Table 1]

[0067] From the results in Table 1, in Example 1, the permeation flux increased after cleaning and the desalination performance was also good, so it can be said that membrane deterioration was suppressed and the membrane was sufficiently cleaned. On the other hand, when step (1) was not performed as in Comparative Example 1, the desalination performance was poor and membrane deterioration occurred. In addition, when the chelating agent (C) was not used as the cleaning agent (Y) in step (2) as in Comparative Example 2, the desalination performance was also poor and membrane deterioration occurred.

[0068] [Examples 2 to 17] Washing was carried out in the same manner as in Example 1, except that the contents of the respective components were set to the conditions shown in Tables 2 or 3. The results are shown in Tables 2 and 3.

[0069] [Table 2]

[0070] [Table 3]

[0071] From the results in Tables 2 and 3, good results were obtained in both permeation flux and desalination performance in each Example, membrane deterioration was suppressed, and the separation membrane was cleaned well.

[0072] [Examples 18 to 26] Cleaning was performed in the same manner as in Example 1, except that the cleaning conditions were as shown in Table 4. In Example 20, step (1) was simply circulation washing for 2 hours. In Examples 21 to 23, the time of immersion washing in step (1) was changed. In Example 24, step (2) was performed by only circulating cleaning for 1 hour. In Examples 25 and 26, the time of immersion cleaning in step (2) was changed. The results are also shown in Table 4.

[0073] [Table 4]

[0074] As can be seen from the results in Table 4, in each Example, good results were obtained in terms of both permeation flux and desalination performance, membrane deterioration was suppressed, and the separation membrane was cleaned well.

[0075] [Examples 27 to 34] Washing was carried out in the same manner as in Example 1, except that the components shown in Table 5 were used. The results are also shown in Table 5.

[0076] [Table 5]

[0077] As can be seen from the results in Table 5, in each Example, good results were obtained in terms of both permeation flux and desalination performance, membrane deterioration was suppressed, and the separation membrane was cleaned well.

[0078] [Example 35] <Cleaning equipment> The cleaning effect of the RO membrane was investigated using an 8-inch RO membrane test device shown in Figure 3. In this test device, water to be fed to the RO membrane is supplied from a tank 10 by a high-pressure pump 11 through a pipe 12 to an RO membrane element 13. The supplied liquid is returned to the tank 10 through a concentrated water pipe line 14 or a permeate water pipe line 15, and is used again as feed water. The RO membrane element 13 used was one that had been in operation for three years treating water taken from a river (TMG20-400, aromatic polyamide RO membrane, manufactured by Toray).

[0079] <Cleaning solution> Separation membrane cleaning agent (X) A liquid obtained by adding 1% by mass of etidronic acid to ion-exchanged water and adjusting the pH to 6 with sodium hydroxide was used as a separation membrane cleaning agent (X). Separation membrane cleaning agent (Y) A solution prepared by adding 1 mass% hydrogen peroxide, 0.004 mass% ferrous chloride tetrahydrate, 0.009 mass% etidronic acid, and 0.04 mass% sodium gluconate to pure water and adjusting the pH to 4 with sodium hydroxide was used as a separation membrane cleaning agent (Y).

[0080] <Cleaning method> 1. Before cleaning, a 1500mg / L aqueous solution of sodium chloride was passed through the fouled membrane at 0.8MPa to measure the permeation flux and salt rejection. 2. After that, in step (1), the separation membrane cleaning agent (X) was used for 2 hours of circulation cleaning, followed by 14 hours of immersion cleaning, and then 2 hours of circulation cleaning in that order. No pressure was applied during the circulation cleaning. 3. After that, the permeation flux and salt rejection after washing were determined in the same manner as in 1 above. 4. Next, the separation membrane cleaning agent (Y) was used for 2 hours of circulation cleaning, followed by 18 hours of immersion cleaning, followed by 2 hours of circulation cleaning. No pressure was applied during the circulation cleaning. 5. After that, the permeation flux and salt rejection after washing were determined in the same manner as in 1 above.

[0081] The cleaning ability and membrane deterioration were evaluated based on the following criteria. The results are shown in Table 6.

[0082] (1) Cleanability The recovery rate was calculated using the following formula, and the cleaning ability was evaluated according to the following criteria. The recovery rate also increases when membrane degradation occurs, so when membrane degradation does not occur and the recovery rate is high, the cleaning ability can be said to be good. Recovery rate (%) = (Permeation flux after washing - Permeation flux before washing) / Permeation flux before washing × 100 〇…Recovery of 5% or more ×…Less than 5% recovery The permeate flux was read from a flow meter in the permeate line.

[0083] (2) Membrane deterioration The rate of decrease in salt rejection was calculated using the following formula, and the membrane deterioration was evaluated according to the following criteria. Decrease rate of salt rejection (%) = (Salt rejection before washing - Salt rejection after washing) / Salt rejection before washing x 100 The salt rejection rate was calculated by the following formula. Desalination rate (%): 100-Permeate water conductivity / Feed water conductivity x 100 ○…Decrease of 1% or less ×…Decrease of more than 1%

[0084] (3) Overall Judgment ○: Both cleaning and membrane deterioration were ○ ×: Washability or membrane deterioration was ×

[0085] [Example 36] Cleaning was carried out in the same manner as in Example 35, except that in step (1), only circulation cleaning was carried out for 1 hour. The results are shown in Table 6.

[0086] [Comparative Example 3] Cleaning was carried out in the same manner as in Example 1, except that cleaning in step (1) was not carried out and only cleaning in step (2) was carried out. The results are shown in Table 6.

[0087] [Table 6]

[0088] From the results in Table 6, it was found that in Examples 35 and 36, the permeation flux increased after cleaning and the desalination performance was also good, so that the membrane deterioration was suppressed and the membrane was sufficiently cleaned. When step (1) was not performed as in Comparative Example 3, the desalination performance was deteriorated and the membrane deterioration occurred. [Explanation of symbols]

[0089] 1: Container 2: Flat membrane cell 2A: Porous support 2B:RO membrane 2C: Spacer 3: High pressure pump 4: Pressure gauge 5: Pressure regulating valve 6,7,8:Pipe lines 9: O-ring 10: Tank 11: High pressure pump 12:Pipe line 13: RO membrane element 14: Concentrated water piping line 15: Permeate piping line

Claims

1. A process (1) for cleaning a polyamide reverse osmosis membrane with a separation membrane cleaning agent (X) containing etidronic acid as a chelating agent; After the step (1), a step (2) of cleaning the polyamide reverse osmosis membrane with a cleaning agent for separation membranes (Y) containing an oxidizing agent (A), a heavy metal compound (B), and a chelating agent (C) consisting of etidronic acid. The method for cleaning a separation membrane comprising the steps of:

2. 2. The method for cleaning a separation membrane according to claim 1, wherein the oxidizing agent (A) is at least one selected from the group consisting of hydrogen peroxide, percarbonate, peracetic acid, and salts thereof.

3. 3. The method for cleaning a separation membrane according to claim 1, wherein the heavy metal compound (B) is at least one selected from the group consisting of chlorides, sulfides and oxides of Fe, Mn, Co, Ni, Ti and Cu.

4. A separation membrane cleaning agent for cleaning polyamide-based reverse osmosis membranes, comprising an oxidizing agent (A), a heavy metal compound (B) and a chelating agent (C) consisting of etidronic acid, and having a pH of 3 to 5.

5. The cleaning agent for separation membranes according to claim 4, wherein the oxidizing agent (A) is at least one selected from the group consisting of hydrogen peroxide, percarbonate, peracetic acid, and salts thereof.

6. 6. The cleaning agent for a separation membrane according to claim 4 or 5, wherein the heavy metal compound (B) is at least one selected from the group consisting of chlorides, sulfides and oxides of Fe, Mn, Co, Ni, Ti and Cu.

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