Operating method of reverse osmosis membrane unit

Abstract

A method for operating a reverse osmosis membrane device is provided, comprising passing treated water containing scale components into a reverse osmosis membrane device having a reverse osmosis membrane element to obtain treated water. The reverse osmosis membrane element includes a reverse osmosis membrane and a flow path material. The dense layer of the reverse osmosis membrane contains carbon nanotubes and cellulose nanofibers, and the flow path material contains carbon nanotubes. The method for operating the reverse osmosis membrane device is characterized by adding a scale dispersant to the treated water.

Description

Technical Field

[0001] The present invention relates to an operating method of a reverse osmosis membrane device for treating water containing scale components by using the reverse osmosis membrane device. Prior Art

[0002] Reverse osmosis membrane systems are widely used for desalination of seawater and brackish water, production of industrial water and ultrapure water, and wastewater recovery. If a reverse osmosis membrane system is left operating continuously, scale such as calcium salts and silica will adhere to the reverse osmosis membrane.

[0003] One method for inhibiting the formation of calcium and silica scale on reverse osmosis membranes is operating under acidic conditions. Operating the system at a feed water pH of around 5 slows the formation of calcium and silica scale, stabilizing membrane performance. However, this method requires the addition of an acid to the feed water, and an alkali to neutralize the pH of the concentrate or permeate, increasing chemical costs. Furthermore, acidic conditions can degrade the reverse osmosis membrane's barrier performance.

[0004] Patent Documents 1 and 2 describe methods for suppressing scale deposition on reverse osmosis membranes by adjusting the Langelier index of the feed water to a value that is less prone to scale formation and performing regular flushing. However, the methods of Patent Documents 1 and 2 require the operation of changing the Langelier index of the feed water.

[0005] Patent Document 3 describes a reverse osmosis membrane containing carbon nanotubes and cellulose nanofibers in its dense layer. Patent Document 4 describes a flow path material containing carbon nanotubes. [Prior art literature] [Patent Document]

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-160179 Patent Document 2: Japanese Patent Application Laid-Open No. 2016-179442 Patent Document 3: Japanese Patent No. 6999958 Patent Document 4: International Publication No. 2019 / 131917 Summary of the Invention

[0007] [Problems to be solved by the invention] An object of the present invention is to provide a method for operating a reverse osmosis membrane device that can suppress scale adhesion to a reverse osmosis membrane (hereinafter sometimes referred to as an RO membrane) and stabilize membrane performance. [Methods for solving the problem]

[0008] The present inventors have studied methods for inhibiting the formation and adhesion of calcium fluoride and silica scale to RO membranes. They have discovered that by passing treated water supplemented with a dispersant through a reverse osmosis membrane device comprising a membrane element employing an RO membrane containing carbon nanotubes and cellulose nanofibers, and a flow path material containing carbon nanotubes, RO membrane fouling can be significantly reduced. Furthermore, they have discovered that RO membrane fouling can be further reduced by flushing the concentrated side with treated water or other water without pressurization.

[0009] The present invention is based on the above findings, and the gist of the present invention is as follows.

[0010] [1] A method for operating a reverse osmosis membrane device, wherein treated water containing scale components is passed through a reverse osmosis membrane device having a reverse osmosis membrane element to obtain treated water. The reverse osmosis membrane element includes a reverse osmosis membrane and a flow path material. The dense layer of the reverse osmosis membrane contains carbon nanotubes and cellulose nanofibers. The flow path material contains carbon nanotubes, and the operating method of the reverse osmosis membrane device is characterized in that: A scale dispersant is added to the treated water.

[0011] [2] The method for operating the reverse osmosis membrane device as described in [1] is characterized in that flushing is performed regularly.

[0012] [3] The method for operating a reverse osmosis membrane device as described in [1] or [2], wherein the treated water contains at least one of calcium fluoride, calcium carbonate and silicon dioxide as the scale component.

[0013] [4] The method for operating a reverse osmosis membrane device as described in any one of [1] to [3], characterized in that the scale dispersant has a phosphate group or a sulfonic acid group.

[0014] [5] The method for operating a reverse osmosis membrane device as described in [4] is characterized in that the scale dispersant is 2-phosphonobutane-1,2,4-tricarboxylic acid.

[0015] [6] The method for operating a reverse osmosis membrane device as described in any one of [1] to [5] is characterized in that the amount of scale dispersant added is 0.5 mg / L to 20 mg / L. [Effects of the Invention]

[0016] Based on the present invention, the adhesion of scale to the RO membrane can be suppressed, and the performance stabilization of the reverse osmosis membrane device can be achieved.

[0017] The mechanism that can suppress scale adhesion in the present invention is presumably as follows. (1) Carbon nanotubes or cellulose nanofibers are not hydrophilic materials and will cause a side-slip effect on the treated water. (2) Scale components are concentrated by the RO membrane and form scale nuclei near the reverse osmosis membrane surface. However, due to the sliding effect, the scale nuclei are less likely to remain on the reverse osmosis membrane surface. This can inhibit scale from adhering to the reverse osmosis membrane. (3) By adding a dispersant, the formation of scale nuclei can be suppressed, thereby reducing membrane fouling caused by scale. (4) By flushing, the scale nuclei remaining on the membrane surface can be discharged. Simple diagram description

[0018] FIG1 is an explanatory diagram of the flow in the embodiment. FIG. 2 is a graph showing the results of the example. Implementation Method

[0019] The water to be treated in the present invention contains scale components such as calcium fluoride, calcium carbonate, and silicon dioxide in an amount of 1 mg / L to 200 mg / L, particularly about 10 mg / L to 100 mg / L.

[0020] As the reverse osmosis membrane of the reverse osmosis membrane device used in the method for operating the reverse osmosis membrane device of the present invention, the reverse osmosis membrane described in Patent Document 3 is suitable.

[0021] Specifically, a suitable reverse osmosis membrane is the semipermeable composite membrane described in Patent Document 3, which comprises a semipermeable membrane formed by coating a porous support with a mixture of a first aqueous dispersion containing carbon nanotubes and a second aqueous dispersion containing cellulose nanofibers. The semipermeable composite membrane comprises cross-linked polyamide, carbon nanotubes, and cellulose nanofibers. The thickness of the semipermeable membrane in the semipermeable composite membrane is preferably approximately 10 nm to 200 nm.

[0022] Carbon nanotubes preferably have an average diameter (fiber diameter) of 5 nm to 30 nm. Cellulose nanofibers preferably have an average fiber diameter of 3 nm to 200 nm. The aspect ratio (fiber length / fiber diameter) of the cellulose nanofibers is preferably 10 to 1000.

[0023] The polyamide is preferably an aromatic polyamide.

[0024] The proportion of carbon nanotubes in the total amount of polyamide, carbon nanotubes and cellulose nanofibers is preferably 1% to 30% by mass, and the proportion of cellulose nanofibers is preferably 1% to 23.6% by mass.

[0025] As the material of the porous support, polysulfone, polyethersulfone, cellulose acetate, polyvinyl chloride, polyacrylonitrile, polyphenylene sulfide, polyphenylene sulfidesulfone, etc. can be used.

[0026] A suitable method for producing a semipermeable composite membrane is the following method, comprising: obtaining a mixed solution containing carbon nanotubes, cellulose nanofibers, and an amine component, preferably an aromatic amine component; and contacting the mixed solution with a porous support, and then using a crosslinking agent to cause the amine component in the mixed solution attached to the porous support to undergo a crosslinking reaction, thereby obtaining the semipermeable composite membrane. The step of obtaining the mixed solution comprises mixing a first aqueous dispersion containing the carbon nanotubes with a second aqueous dispersion containing the cellulose nanofibers. The aromatic amine is preferably at least one aromatic polyfunctional amine selected from the group consisting of m-phenylenediamine, p-phenylenediamine, 1,3,5-triaminobenzene, 1,2,4-triaminobenzene, 3,5-diaminobenzoic acid, 2,4-diaminotoluene, 2,4-diaminoanisole, amidol, xylylenediamine, N-methyl-m-phenylenediamine, and N-methyl-p-phenylenediamine. These may be used alone or in combination of two or more. As a crosslinking agent, an organic solvent solution containing an acyl chloride component such as trimesoyl chloride, terephthaloyl chloride, isophthaloyl chloride, or biphenyldimethyl chloride can be used. In addition to the crosslinking agent, an alkaline catalyst is also included to capture hydrochloric acid by-produced during interfacial polymerization. Examples of such alkaline catalysts include triethylamine and pyridine. In the step of obtaining a semipermeable composite membrane, the mixed solution obtained in the step of obtaining a mixed solution is applied to a porous support to form a membrane. A solution containing a crosslinking agent is then applied to the membrane to induce a polycondensation reaction and crosslinking. After heating and drying, the membrane is washed with distilled water to form a semipermeable membrane. When the mixed solution is applied to a porous support, it is preferably applied evenly using a bar coater or the like.

[0027] The flow path material for the reverse osmosis membrane device used in the reverse osmosis membrane device operating method of the present invention is preferably a mesh-shaped water treatment flow path material comprising a molded article containing a polypropylene resin and carbon nanotubes as described in Patent Document 4, wherein the carbon nanotubes are blended in an amount of 5.3 to 18 parts by mass per 100 parts by mass of the polypropylene resin.

[0028] The carbon nanotubes used in this flow path material preferably have a structure similar to a rolled-up graphene sheet, with a diameter ranging from several nanometers to tens of nanometers and a length ranging from tens to thousands of times greater than the diameter. Carbon nanotubes are categorized into single-walled carbon nanotubes (CNTs), which consist of a substantially single graphene sheet, and multi-walled carbon nanotubes (MNTs), which consist of two or more graphene sheets. Both single-walled and multi-walled CNTs can be used.

[0029] The water treatment flow path material can be formed into a mesh shape using a predetermined mold. For example, the water treatment flow path material can be manufactured by the method described in the examples of Patent Document 4.

[0030] The flow path material of the example of Patent Document 4 includes a mesh-shaped molded product produced by molding a composition containing a polypropylene resin and carbon nanotubes.

[0031] As the scale dispersant used in the present invention, those having a phosphoric acid group or a sulfonic acid group are suitable, and particularly suitable are inorganic polyphosphoric acids such as sodium hexametaphosphate or sodium tripolyphosphate, and phosphonic acids such as hydroxyethylene diphosphonic acid or phosphonobutane tricarboxylic acid.

[0032] Furthermore, as the scale dispersant, a polymer-based scale dispersant having a sulfonic acid group and a carboxyl group is also suitable.

[0033] Examples of the polymer having a sulfonic acid group and a carboxyl group include copolymers of a monomer having a sulfonic acid group and a monomer having a carboxyl group, or terpolymers with other monomers copolymerizable with these monomers.

[0034] Examples of monomers having a sulfonic acid group include conjugated diene sulfonic acids such as 2-methyl-1,3-butadiene-1-sulfonic acid, unsaturated (meth)allyl ether monomers having a sulfonic acid group such as 3-(meth)allyloxy-2-hydroxypropanesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-hydroxy-3-acrylamidopropanesulfonic acid, styrenesulfonic acid, methallylsulfonic acid, vinylsulfonic acid, allylsulfonic acid, isopentylsulfonic acid, or salts thereof. Preferred examples include 3-allyloxy-2-hydroxy-1-propanesulfonic acid (HAPS) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). These monomers may be used alone or in combination of two or more.

[0035] Examples of the monomer having a carboxyl group include acrylic acid (AA), methacrylic acid, crotonic acid, methacrylic acid, vinylacetic acid, atropic acid, maleic acid, fumaric acid, itaconic acid, hydroxyethyl acrylate, or salts thereof, and preferably acrylic acid and methacrylic acid.

[0036] Examples of monomers copolymerizable with these monomers include olefins such as isobutylene, and amides such as N-tert-butyl acrylamide (N-tBAA) and N-vinylformamide.

[0037] The weight average molecular weight of the polymer scale dispersant is preferably 1,000 to 50,000.

[0038] In the present invention, the amount of the scale dispersant added to the treated water is preferably 0.5 mg / L to 20 mg / L, particularly about 1 mg / L to 10 mg / L.

[0039] In the present invention, by performing regular flushing, the membrane fouling of the reverse osmosis membrane can be further reduced.

[0040] Flushing refers to the process of opening the on-off valve on the concentrate water discharge piping while the feed pump is running continuously. This allows the RO feed water to flow to the primary side (concentrated water side) of the membrane with little to no permeation through the membrane, and then be discharged from the concentrate water discharge piping to the outside of the system. Since the RO feed water barely permeates the membrane, nearly all of the feed water flows through the primary membrane surface. This allows a larger volume of water to flow at a faster rate than during normal operation, when permeate water is being collected, effectively flushing away any dirt clogging the membrane surface.

[0041] The frequency of flushing is not particularly limited and can be appropriately determined according to the water quality of the treated water or the RO treatment conditions. It is usually preferably implemented at a rate of about 1 minute to 60 minutes at a rate of several hours to 10 days. [Example]

[0042] [Example 1, Comparative Example 1, Comparative Example 2] <Summary> The reverse osmosis membrane system shown in FIG1 was used. The following reverse osmosis membrane elements were used. The following water to be treated was passed through the reverse osmosis membrane system and the flux change over time was measured.

[0043] <Water to be treated> The following chemicals were mixed and adjusted to pH 5.5 using hydrochloric acid and sodium hydroxide. Calcium chloride dihydrate (special grade, Fujifilm Wako Pure Chemical Industries, Ltd.): 294 mg / L Sodium fluoride (special grade, Fujifilm Wako Pure Chemical Industries, Ltd.): 24 mg / L 2-Phosphonobutane-1,2,4-tricarboxylic acid (PBTC, LANXESS): 2.5 mg / L

[0044] <Membrane Element of Example 1> A spiral reverse osmosis membrane element employs the RO membrane described in Example 1 of Patent Document 3, comprising carbon nanotubes and cellulose nanofibers in its dense layer and manufactured by the following RO membrane manufacturing method, and the flow path material comprising carbon nanotubes, manufactured by the following flow path material manufacturing method described in Patent Document 4. The element has the following structure. The membrane area is 1300 cm², the element diameter is 5 cm, and the element axial length is 30 cm.

[0045] 《Manufacturing Method of RO Membrane》 According to Example 1 of Patent Document 3, an RO membrane is manufactured by the following steps (1) to (5). (1) Step of obtaining a first aqueous dispersion: 2 g of multiwalled carbon nanotubes (FT9110, manufactured by Cnano, with an average diameter of 15 nm (the average diameter is the arithmetic average of measurements taken at 200 or more locations using a scanning electron microscope)) were manually stirred in 10 g of distilled water (first mixing step). The mixture was then placed on a triple roll with a 50 mm diameter (EXAKT M-50 I, manufactured by Nagase Screen Printing Laboratories, Ltd.) at a roll temperature of 25°C to 40°C and kneaded for 3 minutes to 10 minutes (second mixing step) to obtain a first aqueous dispersion. The roll gap was 0.001 mm to less than 0.01 mm, and the roll speed ratios were V1 = 1, V2 = 1.8, and V3 = 3.3, with roll speed V3 being a peripheral speed of 1.2 m / s.

[0046] (2) Step of obtaining a second aqueous dispersion: An aqueous dispersion of cellulose nanofibers (TEMPO-oxidized cellulose nanofibers) was diluted with water to prepare an aqueous dispersion of cellulose nanofibers having a concentration of 0.4% by mass. This dispersion was then stirred at 20,000 rpm for 30 seconds using a blender (MX1200XTX manufactured by Waring) to obtain a second aqueous dispersion containing cellulose nanofibers.

[0047] (3) Steps for obtaining a mixed solution (coating solution): The following substances were prepared: 105 g of an additive (an aqueous solution containing 10% by mass of triethylamine (TEA) and 20% by mass of camphorsulfonic acid (CSA)) and 0.45 g of sodium lauryl sulfate (SLS) were added to 100 g of distilled water, stirred and dissolved using a magnetic stirrer, 1.8 g of the previously obtained first aqueous dispersion containing 16.7% by mass of carbon nanotubes was added and stirred, and 10.5 g of m-phenylenediamine and 18 g of isopropyl alcohol (IPA) were added and stirred and dissolved; and a required amount of the second aqueous dispersion containing 0.4% by mass of cellulose nanofibers was aliquoted into 100 g of distilled water, stirred using a magnetic stirrer, 105 g of an additive (an aqueous solution containing 10% by mass of TEA and 20% by mass of CSA) and 0.45 g of SLS were added, stirred using a magnetic stirrer, and 10.5 g of m-phenylenediamine was added. The mixture was stirred with 18 g of isopropyl alcohol (IPA), adjusted to a total of 600 g with distilled water, and stirred using a magnetic stirrer to obtain a mixed solution (coating solution) containing 3.5% by mass of m-phenylenediamine, 3.5% by mass of TEA, 0.15% by mass of SLS, 7% by mass of CSA, 6% by mass of IPA, carbon nanotubes, and cellulose nanofibers.

[0048] The concentration of carbon nanotubes in the mixed solution was 0.05% by mass, and the concentration of cellulose nanofibers in the mixed solution was 0.003% by mass.

[0049] (4) Preparation of porous support: A 30 cm long by 20 cm wide wet nonwoven fabric comprising a blend of polyester fibers having a single filament fineness of 0.5 dtex and polyester fibers having a single filament fineness of 1.5 dtex, having an air permeability of 0.7 cm³ / cm²-second and an average pore size of 7 μm or less, is fixed on a glass plate. A solution (20°C) containing a 15 wt% polysulfone concentration in a dimethylformamide (DMF) solvent is cast thereon to a total thickness of 210 μm to 215 μm, and the polysulfone support is immediately immersed in water to produce a porous polysulfone support.

[0050] (5) Steps to obtain RO membrane: The mixed solution (3) (coating solution) was applied to the surface of an 80 cm2 porous support using a bar coater (#6 wired bar) at a speed of 10 mm / s. After removing excess aqueous solution from the surface of the porous support using a rubber scraper, 4 ml of a room temperature IP solvent solution containing 0.18% by mass of trimesic acid chloride was applied to completely wet the membrane surface. To remove excess solution from the membrane, the membrane surface was held vertically to remove liquid. After that, the membrane was dried in a thermostatic bath at 120°C for 3 minutes and then immersed in distilled water for cleaning to produce an RO membrane.

[0051] 《Manufacturing method of flow path material》 According to Example 1 of Patent Document 4, a composition containing 18 parts by mass of carbon nanotubes per 100 parts by mass of polypropylene resin is molded using a predetermined mold to produce a mesh-shaped molded article. The mesh used has a mesh opening of approximately 4 mm in length and 4 mm in width, a wire diameter of approximately 0.4 mm, and a thickness of approximately 0.7 mm.

[0052] <Membrane Element of Comparative Example 1> The membrane element was the same as that of Example 1 except that the flow path material used in the membrane element of Comparative Example 2 was used as the flow path material.

[0053] <Membrane Element of Comparative Example 2> Commercially available RO mini element (TW30-2012-125, DOW).

[0054] <RO element for pre-concentration> Six ES202-inch RO elements (manufactured by Nitto Denko Corporation) were used.

[0055] <Experimental Procedure> Figure 1 shows an overview of the water flow test apparatus. Treated water containing calcium chloride, sodium fluoride, and PBTC was passed from treated water tank 1 to pre-concentration RO element 3 via pump 2, where it was concentrated fivefold. This concentrated water was then passed from concentration water tank 4 to RO mini-element 6 via pump 5, and the permeate flow rate (flux) was measured over time. The water temperature was 31°C to 34°C.

[0056] <Results> FIG2 shows the change over time in the flux ratio (a value obtained by dividing the measured flux by the initial flux).

[0057] As shown in FIG2 , it can be seen that the flux ratio of Comparative Example 2 is maintained higher than that of Comparative Example 1, but in Example 1, it can be maintained even higher for a long time.

[0058] While the present invention has been described in detail using specific aspects, it will be apparent to those skilled in the art that various modifications can be made without departing from the intent and scope of the present invention. This application is based on Japanese patent application No. 2022-124183 filed on August 3, 2022, the entire contents of which are incorporated herein by reference.

[0059] 1: Treated water tank 2: Pump 3: Pre-concentration RO element 4: Concentrated water tank 5: Pump 6:RO mini components

Claims

1. A method of operating a reverse osmosis membrane device, comprising introducing treated water containing scale components into a reverse osmosis membrane device having a reverse osmosis membrane element to obtain treated water, wherein the reverse osmosis membrane element includes a reverse osmosis membrane and a flow path material, the reverse osmosis membrane is a semi-permeable composite membrane having a dense layer on a porous support, the dense layer containing carbon nanotubes and cellulose nanofibers, the flow path material containing carbon nanotubes, and the method of operating the reverse osmosis membrane device is characterized in that a scale dispersant is added to the treated water.

2. The method of operating the reverse osmosis membrane device as described in claim 1, wherein, Rinse regularly.

3. The method of operating the reverse osmosis membrane device as described in claim 1, wherein, The treated water contains at least one of calcium fluoride, calcium carbonate, and silicon dioxide as a component of the scale.

4. The method of operating the reverse osmosis membrane device as described in claim 1, wherein, The scale dispersant has a phosphate group or a sulfonic acid group.

5. The method of operating the reverse osmosis membrane device as described in claim 1, wherein, The scale dispersant is 2-phosphatidylbutane-1,2,4-tricarboxylic acid.

6. A method of operating a reverse osmosis membrane apparatus as described in any one of claims 1 to 5, wherein, The amount of the scale dispersant added is 0.5 mg / L to 20 mg / L.

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