Method for inhibiting biofouling on reverse osmosis membrane

Abstract

To provide a method for inhibiting biofouling on a reverse osmosis membrane by inhibiting biofilm formation on the reverse osmosis membrane, and a method for water treatment using a reverse osmosis membrane while inhibiting biofilm formation on the reverse osmosis membrane.SOLUTION: A method for inhibiting biofouling on a reverse osmosis membrane comprises: adding, in a flow path directly upstream of the reverse osmosis membrane, an alkaline solution containing a biofouling inhibitor, such as a halogen-based oxidizing agent or combined halogen, to water to be treated flowing through the flow path; and subsequently adding a solution containing an isothiazoline compound, such as 5-chloro-2-methyl-4-isothiazolin-3-one.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for suppressing biofouling in a reverse osmosis membrane and a method for treating water using a reverse osmosis membrane, and more particularly to a method for suppressing biofouling in a reverse osmosis membrane by suppressing the formation of a biofilm on the reverse osmosis membrane, and a method for treating water using a reverse osmosis membrane while suppressing the formation of a biofilm on the reverse osmosis membrane. [Background technology]

[0002] Reverse osmosis membranes are used in liquid concentration, desalination, pure water production, and other water treatments. For example, industrial water, city water, well water, river water, lake water, seawater, brackish water, industrial wastewater, etc. are used as raw water for water treatment devices. Known membranes used for reverse osmosis include cellulose acetate membranes, polyether membranes, polyamide membranes, etc.

[0003] Incidentally, when treating water in which conditions for microbial proliferation are favorable, the microorganisms may proliferate in the reverse osmosis membrane, forming a biofilm and causing blockage (biofouling). Biofouling inhibitors may be used to prevent biofouling. Examples of biofouling inhibitors include chlorine-based oxidizing agents. Also, bactericides may be used to reduce the number of microorganisms. Examples of bactericides include isothiazolinone compounds.

[0004] Various techniques have been proposed that involve the combined use of biofouling inhibitors and bactericides in water treatment.

[0005] For example, Patent Document 1 discloses a method for suppressing the reduction in the concentration of an isothiazolinone compound, which comprises supplying an isothiazolinone compound at a concentration of 0.2 mg / L or more and less than 1 mg / L to an aqueous system containing a combined chlorine agent consisting of a chlorine stabilizer and a chlorine-based oxidizing agent at a Cl2-equivalent concentration of 0.3 mg / L or more and 6 mg / L or less. According to Patent Document 1, in an actual aqueous system, the added isothiazolinone compound is consumed rapidly, and the remaining concentration is often low. Patent Document 1 states that the combined chlorine agent is, for example, monochlorosulfamic acid, which is obtained by the reaction of a chlorine stabilizer with a chlorine-based oxidizing agent.

[0006] Patent Document 2 discloses a method for inhibiting microorganisms in an aqueous system, which comprises adding one or more agents selected from a combination of a chlorine-based oxidizing agent and sulfamic acid or a salt thereof, or chlorosulfamic acid or a salt thereof to the aqueous system of a papermaking facility, and, after adding the agents, adding one or more slime control agents selected from 2,2-dibromo-2-nitroethanol, 2-bromo-2-nitro-1,3-propanediol, 2,2-dibromo-3-nitrilopropionamide, dichloroglyoxime, and 5-chloro-2-methyl-4-isothiazolin-3-one. The acid dissociation constant Ka of chlorosulfamic acid is 1.01×10 -1 and is strongly acidic. A 5% aqueous solution of ammonium sulfamate has a pH of 4.7 to 5.7 at 25°C.

[0007] Patent Document 3 teaches that hypochlorite ions and sulfamic acid react as shown in the following formula to form N-monochlorosulfamate ions or N,N-dichlorosulfamate ions, thereby stabilizing the active ingredient of a chlorine-based oxidizing agent. ClO - + H2NSO3H → HClNSO3 - + H2O 2ClO - + H2NSO3H + H + → Cl2NSO3 - + 2H2O In other words, when sulfamic acid as a chlorine stabilizer is combined with hypochlorite as a chlorine-based oxidizing agent, the hypochlorite ion disappears and the reaction product contained in the agent is monochlorosulfamate ion or N,N-dichlorosulfamate ion. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2017-88534 A [Patent Document 2] JP 2017-128598 A [Patent Document 3] JP 2015-44765 A Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a method for suppressing biofouling in a reverse osmosis membrane by suppressing the formation of a biofilm on the reverse osmosis membrane, and a method for treating water using a reverse osmosis membrane while suppressing the formation of a biofilm on the reverse osmosis membrane. [Means for solving the problem]

[0010] As a result of investigations conducted in order to achieve the above object, the present invention including the following embodiments has been completed.

[0011] [1] In the flow path immediately upstream of the reverse osmosis membrane, The method includes adding an alkaline liquid containing a biofouling inhibitor to the water to be treated flowing through the flow path, and then adding a liquid containing an isothiazolinone compound. A method for inhibiting biofouling in reverse osmosis membranes.

[0012] [2] The method according to [1], wherein the biofouling inhibitor is at least one selected from the group consisting of halogen-based oxidizing agents and bound halogens. [3] The method according to [1] or [2], wherein the alkaline solution containing the biofouling inhibitor has a pH of 12 or higher.

[0013] [4] The water to be treated flowing through the flow path is Add an alkaline solution containing a biofouling inhibitor, Immediately downstream, a liquid containing an isothiazolinone compound is added, and performing reverse osmosis membrane treatment immediately downstream thereof. Water treatment methods.

[0014] [5] The water treatment method according to [4], wherein the biofouling inhibitor is at least one selected from the group consisting of halogen-based oxidizing agents and bound halogens. [6] The water treatment method according to [4] or [5], wherein the alkaline solution containing the biofouling inhibitor has a pH of 12 or higher. Effect of the Invention

[0015] The amount of alkaline solution, which is one of the water treatment agents, added is usually set so that the desired pH is achieved when it is mixed uniformly with the water to be treated. However, when the alkaline solution is added to the flow path through which the water to be treated flows, a region showing a local high pH appears at the addition location. Isothiazoline compounds, such as 5-chloro-2-methyl-4-isothiazolin-3-one, are easily decomposed under high pH. The isothiazoline compounds present in the local region showing a high pH that appears due to the addition of the alkaline solution decompose. As a result, the amount of isothiazoline compounds reaching the downstream area is reduced, and the desired bactericidal effect may not be achieved.

[0016] In contrast, when the biofouling suppression method of the present invention is used, the weight loss due to the decomposition of the isothiazolinone compound is extremely small, and the formation of a biofilm on the reverse osmosis membrane is effectively suppressed, thereby suppressing fouling in the reverse osmosis membrane. According to the water treatment method of the present invention, the water to be treated can be treated while effectively suppressing the formation of a biofilm on the reverse osmosis membrane. The method of the present invention can extend the usable life of the reverse osmosis membrane and reduce the frequency of replacement or cleaning of the reverse osmosis membrane. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] The water treatment method of the present invention includes adding an alkaline liquid containing a biofouling inhibitor to the water to be treated flowing through a flow path, adding a liquid containing an isothiazolinone compound immediately downstream of the alkaline liquid, and then performing reverse osmosis membrane treatment immediately downstream of the alkaline liquid.

[0018] Furthermore, the method for suppressing biofouling in a reverse osmosis membrane of the present invention includes adding an alkaline liquid containing a biofouling inhibitor to the water to be treated flowing through a flow path immediately upstream of the reverse osmosis membrane, and then adding a liquid containing an isothiazolinone compound.

[0019] The water to be treated that is the subject of the present invention is water to be treated by reverse osmosis membrane treatment. Examples of the raw water to be treated include industrial water, city water, well water, river water, lake water, seawater, brackish water, industrial wastewater, etc. It is preferable that the raw water to be treated is pretreated by pressure filtration, gravity filtration, coagulation sedimentation treatment, pressure flotation filtration, osmotic membrane treatment, membrane pretreatment, etc. The water to be treated flows through a flow path immediately upstream of the reverse osmosis membrane. Examples of the flow path include waterways such as channels and culverts; pipes; and manholes. The water to be treated that is the subject of the present invention is not particularly limited, but has a pH of preferably 10 or less, more preferably 9.5 or less, and even more preferably 9 or less. The lower limit of the pH of the water to be treated is not particularly limited as long as it is within the range in which reverse osmosis membrane treatment can be applied, and is, for example, about 2.

[0020] In the present invention, the biofouling inhibitor may be, for example, at least one selected from the group consisting of halogen-based oxidizing agents and bound halogens.

[0021] The halogen-based oxidizing agent can be at least one selected from the group consisting of perchloric acid, perchlorate, chloric acid, chlorate, chlorous acid, chlorite, hypochlorous acid, hypochlorite, perbromic acid, perbromate, bromic acid, bromate, bromite, bromite, hypobromite, and hypobromite.Of these, at least one selected from the group consisting of perchlorate, chlorate, chlorite, hypochlorite, perbromate, bromate, bromite, and hypobromite is preferred, and hypochlorite is more preferred.Perchlorate, chlorate, chlorite, hypochlorite, perbromate, bromate, bromite, and hypobromite can be sodium salt, potassium salt, calcium salt, magnesium salt, aluminum salt, etc., and preferably sodium salt.

[0022] The bound halogen may be at least one selected from the group consisting of monochloramine, dichloramine, trichloramine, monobromamine, dibromamine, and tribromamine. The bound halogen may be a compound obtained by reacting a halogen-based oxidizing agent such as hypochlorous acid with ammonia or an amine. A commercially available product may be used as the biofouling inhibitor. Examples of commercially available biofouling inhibitors include Kuriverter IK-110 (manufactured by Kurita Water Industries, bound chlorine-based biofouling inhibitor), Orpersion E2661 series (manufactured by Organo, bound bromine-based biofouling inhibitor), and Stabrex (manufactured by Nalco, bound bromine-based biofouling inhibitor).

[0023] The biofouling inhibitors as described above can be used alone or in combination of two or more.

[0024] The amount of the biofouling inhibitor to be added is preferably 0.03 mg or more and 6 mg or less, more preferably 0.2 mg or more and 3 mg or less, in terms of Cl2 or Br2 concentration, per 1 L of the water to be treated.

[0025] The alkaline liquid containing the biofouling inhibitor has a pH of preferably 10 or more, more preferably 12 or more, and even more preferably 13 or more. The alkaline liquid containing the biofouling inhibitor may contain a pH regulator. Examples of the pH regulator include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; and alkali metal carbonates such as sodium carbonate and potassium carbonate. The solvent that can be used in the alkaline liquid containing the biofouling inhibitor is preferably soft water or hard water, and more preferably soft water. The hardness of the soft water is preferably less than 120 mg / L, more preferably less than 100 mg / L, and even more preferably less than 60 mg / L. The hardness is a value defined by the following formula. Hardness (mg / L) = Calcium concentration (mg / L) x 2.5 + Magnesium concentration (mg / L) x 4.1

[0026] The concentration of the biofouling inhibitor in the alkaline liquid containing the biofouling inhibitor is preferably 0.5 to 16 mass %, more preferably 1 to 14 mass %. The pH of the water being treated in the area where the alkaline solution containing the biofouling inhibitor is added is non-uniform, and there may be areas with low pH and areas with high pH. As the water flows down the flow path, the pH of the water being treated becomes uniform. This is more noticeable as the amount of water being treated increases.

[0027] In order to more quickly uniformize the pH of the water to be treated to which an alkaline solution containing a biofouling inhibitor has been added, a mixing promotion means such as a static mixer, a line mixer, a flow path throttle mechanism such as an orifice or a venturi, a filter, a pump, a tank, etc., may be installed in the flow path through which the water to be treated flows. In addition, in order to more quickly uniformize the pH of the water to be treated to which an alkaline solution containing a biofouling inhibitor has been added, for example, in a junction structure consisting of a straight flow path for the water to be treated and an injection flow path for the alkaline solution containing a biofouling inhibitor connected to the straight flow path in a T-shape (at right angles), the outlet (or outlet nozzle) of the injection flow path is located in a fast flow area (for example, the center of the straight flow path) or the flow path around the straight flow path at the part where the outlet (or outlet nozzle) of the injection flow path is located is widened like a basin or buffer tank.

[0028] Isothiazoline compounds are compounds having a five-membered heterocyclic ring containing a nitrogen atom and a sulfur atom. Isothiazoline compounds are substances that are believed to have antibacterial properties and to exhibit a biofilm adhesion inhibitory effect. Specific examples of the isothiazolin compound include 5-chloro-2-methyl-4-isothiazolin-3-one (abbreviation: Cl-MIT), 2-methyl-4-isothiazolin-3-one (abbreviation: MIT), 4,5-dichloro-2-methyl-4-isothiazolin-3-one, 2-ethyl-4-isothiazolin-3-one, 2-n-octyl-4-isothiazolin-3-one, 5-chloro-2-ethyl-4-isothiazolin-3-one, 5-chloro-2-t-octyl-4-isothiazolin-3-one, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one, 4,5-dichloro-2-cyclohexyl-4-isothiazolin-3-one, 1,2-benzoisothiazolin-3-one, etc. These isothiazolin compounds can be used alone or in combination of two or more. Of these, Cl-MIT and MIT are preferred, with Cl-MIT being more preferred.

[0029] The amount of the isothiazoline compound added is preferably 0.01 mg or more and 1 mg or less, more preferably 0.02 mg or more and 0.5 mg or less, per liter of the water to be treated.

[0030] The isothiazolinone compound may be added as a solid, but is preferably added in the form of a solution, dispersion, or suspension by adding it to a solvent. The solvent that can be used for the liquid containing the isothiazolinone compound is preferably soft water or hard water, more preferably soft water. The hardness of the soft water is preferably less than 120 mg / L, more preferably less than 100 mg / L, and even more preferably less than 60 mg / L.

[0031] The concentration of the isothiazolin compound in the liquid containing the isothiazolin compound is preferably 0.01 to 30% by mass, and more preferably 0.1 to 10% by mass. The liquid containing the isothiazolin compound may contain magnesium salts such as magnesium chloride and magnesium nitrate, and copper salts such as copper chloride. The liquid containing the isothiazolin compound has a pH of preferably 7 or less, more preferably 4 or less, and further preferably 2 or less. The lower limit of the pH of the isothiazolin compound solution is not particularly limited, and is, for example, 0.5.

[0032] The water to be treated in the area where the isothiazolinone compound is added has a non-uniform pH, and may have low and high pH areas. As the water flows down the flow path, the pH of the water to be treated becomes uniform.

[0033] In order to more quickly uniformize the pH of the water to be treated to which the isothiazolin compound has been added, a mixing promotion means such as a static mixer, a line mixer, a flow path throttling mechanism such as an orifice or a venturi, a filter, a pump, a tank, etc., may be installed in the flow path through which the water to be treated flows. In addition, in order to more quickly uniformize the pH of the water to be treated to which the isothiazolin compound has been added, for example, in a junction structure consisting of a straight flow path for the water to be treated and an injection flow path for a liquid containing the isothiazolin compound connected to the straight flow path in a T-shape (at right angles), the outlet (or outlet nozzle) of the injection flow path is positioned in a region in the straight flow path where the flow is fast (for example, the center of the straight flow path), or the flow path around the straight flow path in the part where the outlet (or outlet nozzle) of the injection flow path is located is expanded like a box or a buffer tank.

[0034] The uniform pH of the water to be treated after the addition of the isothiazolin compound is preferably 10 or less, more preferably 9.5 or less, and even more preferably 9 or less. Within this pH range, the isothiazolin compound is hardly decomposed and can fully exert its bactericidal effect. If the uniform pH of the water to be treated after the addition of the isothiazoline compound does not fall within this range, it is preferable to adjust the pH to fall within the above range by adding an acid or the like upstream of the addition position of the isothiazoline compound. The lower limit of the uniform pH of the water to be treated after the addition of the isothiazoline compound is not particularly limited as long as it is within the range in which reverse osmosis membrane treatment can be applied, and is, for example, about 2.

[0035] The location where the liquid containing the isothiazoline compound is added is immediately downstream of the location where the alkaline liquid containing the biofouling inhibitor is added, and preferably is the location where the pH of the treated water to which the alkaline liquid containing the biofouling inhibitor has been added becomes uniform or downstream of that location.

[0036] The location where the pH of the treated water to which an alkaline solution containing a biofouling inhibitor has been added becomes uniform can be determined by measurements in an actual plant, by measurements in a simulated plant, or by simulations using computational fluid dynamics, analytical mechanics, etc. In addition, the location where the pH of the water to be treated to which the alkaline liquid containing the biofouling inhibitor is added becomes uniform may be determined by empirical rule. For example, the location where the alkaline liquid containing the biofouling inhibitor is added is set to a location downstream of the location where the alkaline liquid containing the biofouling inhibitor is added, the location where the pH of the water to be treated becomes uniform may be determined by empirical rule. H Alternatively, a location three times as far away as the target area may be set as the location where the pH of the water to be treated to which the alkaline solution containing the biofouling inhibitor has been added becomes uniform. D H =4A / P A is the area of ​​the cross section perpendicular to the flow where the fluid is flowing, and P is the length of the part where the fluid is in contact with the flow path (wetted perimeter length).

[0037] In the present invention, a reducing agent may be added immediately downstream of the location where the liquid containing the isothiazoline compound is added and immediately upstream of the reverse osmosis membrane. Examples of the reducing agent include nitrous acid, nitrites, sulfurous acid, sulfites, bisulfites, thiosulfate, and thiosulfate. Examples of the nitrites, sulfites, bisulfites, and thiosulfates include sodium salts, potassium salts, calcium salts, magnesium salts, and aluminum salts, and preferably sodium salts. Of these, sodium hydrogen sulfite or sodium sulfite is preferably used. The reducing agent is preferably added in the form of an aqueous solution.

[0038] The amount of the reducing agent to be added is not particularly limited, but is preferably 1 g equivalent or more, more preferably 1.5 to 4 g equivalent, and even more preferably 2 to 3 g equivalent relative to 1 g equivalent of the halogen-based oxidizing agent contained in the water immediately before the reducing agent is added. Note that 1 g equivalent is the value obtained by dividing the mass of 1 mole by the valence.

[0039] The water to be treated in the area where the reducing agent is added may have a non-uniform pH, with some areas having a low pH and others having a high pH. As the water flows down the flow path, the pH of the water to be treated becomes uniform. In order to more quickly make the pH of the water to be treated uniform, measures similar to those described above may be taken.

[0040] Although the halogen-based oxidizing agent or the bonded halogen inhibits the proliferation of microorganisms, it may also corrode the reverse osmosis membrane. If the halogen-based oxidizing agent or the bonded halogen is reduced by a reducing agent, the effect of inhibiting the proliferation of microorganisms is lost, and fouling is likely to occur in the reverse osmosis membrane. Therefore, in the present invention, it is preferable to first add an isothiazolinone compound to the water to be treated that contains a biofouling inhibitor, and then add a reducing agent to reduce the halogen-based oxidizing agent or the bonded halogen. Since the content of the isothiazolinone compound is not significantly reduced by the addition of a reducing agent, the effect of inhibiting the proliferation of microorganisms is maintained even after the halogen-based oxidizing agent or the bonded halogen is consumed by the reduction reaction, and fouling can be inhibited in the reverse osmosis membrane.

[0041] The water to be treated after the above-mentioned chemical treatment is subjected to reverse osmosis membrane treatment. The reverse osmosis membrane used in the present invention is a semipermeable membrane used to separate low molecular weight substances or ions from water by using reverse osmosis. Reverse osmosis is a process in which a concentrated liquid and a permeated liquid are placed across a semipermeable membrane, and a pressure exceeding the osmotic pressure is applied to the concentrated liquid side to cause the water to permeate to the permeated liquid side. There are no particular limitations on the material used for the semipermeable membrane, and examples of the material include polyamide, polysulfone, polyimide, and cellulose.

[0042] Reverse osmosis membranes are usually used in the form of membrane modules integrated with other components. Examples of membrane modules include casing-contained membrane modules and tank-immersed membrane modules. Casing-contained membrane modules are modules in which a reverse osmosis membrane, its support, flow path material, and other components are housed and integrated in a casing. Examples of casing-contained membrane modules include modules using flat membranes such as spiral-type modules, modules using tubular membranes such as capillary-type modules, and hollow-fiber-type modules.

[0043] The location where the reverse osmosis membrane treatment is carried out is immediately downstream of the location where the above-mentioned chemical treatment has been carried out, and is preferably a location where the pH of the water to be treated after the chemical treatment becomes uniform or downstream of that location.

[0044] The location where the pH of the treated water after chemical treatment becomes uniform can be determined in a similar manner to that used to determine the location where the pH of the treated water to which an alkaline solution containing a biofouling inhibitor has been added becomes uniform.

[0045] The present invention will now be described in more detail with reference to examples and comparative examples, although the following examples are not intended to limit the scope of the present invention.

[0046] [Experimental Example 1] An aqueous solution containing 5-chloro-2-methyl-4-isothiazolin-3-one was added to 100 mL of biologically treated water sampled from the biological treatment tank in an amount that would result in 0.005 mg of 5-chloro-2-methyl-4-isothiazolin-3-one, and the solution was stirred for 30 seconds. An aqueous solution (pH = 13.9) containing a biofouling inhibitor (Kuriverter IK-110 (Kurita Water Industries)) was added to the solution in an amount that would result in 100 mg of biofouling inhibitor, and the solution was allowed to stand for 3 minutes and then stirred for 10 minutes. The total ATP of the solution was measured using an ATP test kit (Aquasnap) manufactured by hygiena.

[0047] [Experimental Example 2] An aqueous solution (pH = 13.9) containing a biofouling inhibitor (Kuriverter IK-110 (Kurita Water Industries, Ltd.)) was added to 100 mL of biologically treated water sampled from the biological treatment tank in an amount that would result in 100 mg of biofouling inhibitor, and the solution was stirred for 30 seconds. An aqueous solution containing 5-chloro-2-methyl-4-isothiazolin-3-one was added to the solution in an amount that would result in 0.005 mg of 5-chloro-2-methyl-4-isothiazolin-3-one, and the solution was allowed to stand for 3 minutes, and then stirred for 10 minutes. The total ATP of the solution was measured using an ATP test kit (Aquasnap) manufactured by hygiena. The total ATP in Experimental Example 2 was 79, compared to the total ATP of 100 in Experimental Example 1, and the total ATP was reduced by 21%.

[0048] From the above results, it can be seen that the method of the present invention can effectively suppress the formation of a biofilm on a reverse osmosis membrane, thereby suppressing fouling in the reverse osmosis membrane.

Claims

1. In the flow path immediately upstream of the reverse osmosis membrane, The method includes adding an alkaline liquid containing a biofouling inhibitor to the water to be treated flowing through the flow path, and then adding a liquid containing an isothiazolinone compound. A method for inhibiting biofouling in reverse osmosis membranes.

2. 2. The method of claim 1, wherein the biofouling inhibitor is at least one selected from the group consisting of halogen-based oxidants and bound halogens.

3. The method according to claim 1 or 2, wherein the alkaline liquid containing the biofouling inhibitor has a pH of 12 or more.

4. The water to be treated flowing through the flow path is Add an alkaline solution containing a biofouling inhibitor, Immediately downstream, a liquid containing an isothiazolinone compound is added, and performing reverse osmosis membrane treatment immediately downstream thereof. Water treatment methods.

5. 5. The water treatment method according to claim 4, wherein the biofouling inhibitor is at least one selected from the group consisting of halogen-based oxidizing agents and bound halogens.

6. 6. The water treatment method according to claim 4, wherein the alkaline liquid containing the biofouling inhibitor has a pH of 12 or more.