Water treatment methods and water treatment agent compositions

Abstract

This invention provides a water treatment method that can inhibit the deterioration of a reverse osmosis membrane and the formation of slime while treating water containing at least one of a chlorine-based oxidant and a bromine-based oxidant or iodide ions. The water treatment method includes a reverse osmosis membrane treatment step of obtaining concentrated water and permeate water from the treated water using a reverse osmosis membrane. In this method, for the treated water containing at least one of a chlorine-based oxidant and a bromine-based oxidant, at least 1 mol of iodide ions are added relative to 1 mol of free chlorine and free bromine in the treated water; or, for the treated water containing iodide ions, at least one of a chlorine-based oxidant and a bromine-based oxidant is added in such a way that the free chlorine and free bromine contents are less than 1 mol relative to 1 mol of iodide ions in the treated water.

Description

Technical Field

[0001] The present invention relates to a water treatment method and a water treatment agent composition for use in the water treatment method. Background Technology

[0002] Reverse osmosis (RO) membrane treatment is used in many processes such as pure water production, wastewater recycling, and seawater desalination. Its application has increased, particularly in wastewater recycling, given the recent water shortages. RO membrane treatment typically involves pretreatment processes such as sand filtration and membrane filtration. Chlorine-based oxidants like hypochlorous acid are used to inhibit the formation of slime caused by microbial growth in these pretreatment processes. However, if hypochlorous acid or other chlorine-based oxidants flow into the RO membrane, it will significantly degrade membrane performance. Therefore, a reducing agent is added to the feed water to the RO membrane to decompose hypochlorous acid and thus inhibit membrane degradation.

[0003] However, if water with low slime-inhibiting effect is supplied to the reverse osmosis membrane by adding a reducing agent, microorganisms may proliferate on the membrane surface and produce biofouling, leading to failures such as reduced permeate flow and increased supply pressure.

[0004] Therefore, biofouling agents (sludge control agents) that are unlikely to cause membrane degradation are added to the feed water of reverse osmosis membranes containing reducing agents, such as chloramine, chlorammoniasulfonic acid and other stabilizing chlorine compounds, and stabilizing hypobromoic acid compositions containing bromine oxidants such as bromine and sulfamic acid.

[0005] However, if the amount of reducing agent added is excessive, the bactericide will be reduced and consumed by the reducing agent, resulting in an increase in the amount added and the cost of the drug. If the amount of reducing agent added is too small, there is a problem of membrane deterioration caused by residual hypochlorous acid, etc.

[0006] For example, Patent Document 1 describes the following: In reverse osmosis membrane treatment, before treating water containing sodium hypochlorite with reverse osmosis membrane treatment, sodium bisulfite as a reducing agent is added, and then potassium iodide is added to generate iodine, thereby inhibiting microbial contamination.

[0007] However, there is no clear record of the required amount of potassium iodide for hypochlorous acid and the like. If the amount of potassium iodide added is too small, there is a possibility of deterioration of the reverse osmosis membrane due to the unreduced hypochlorous acid and the like. If the amount of potassium iodide added is excessive, the cost of the reagents will increase.

[0008] On the other hand, iodide aqueous solutions, which are made by dissolving iodide salts in water, are used in the cleaning process during the manufacture of polarizers, as etching solutions, as reagents for quantitative analysis of various substances, as reducing agents for residual chlorine in treated water such as tap water, and as bactericides in water treatment.

[0009] As is well known, iodine in aqueous iodide solutions is oxidized upon contact with air, thus releasing iodine. Iodine is sublimable and corrosive, therefore the containers for storing aqueous iodide solutions must be made of expensive materials. Furthermore, iodine has oxidizing power; therefore, when using aqueous iodide solutions to reduce residual chlorine in treated water, the original reducing power is sometimes not achieved. Thus, it is necessary to suppress the release of iodine from aqueous iodide solutions.

[0010] Patent document 2 describes the following: In an example, a 1N sodium hydroxide aqueous solution containing 5% by weight of potassium iodide remained colorless at room temperature for one week.

[0011] In addition, Patent Document 3 describes the following: by adding an alkaline compound to a liquid with a concentration of less than 10% by mass of at least one of elemental iodine, iodine-containing compounds, iodide ions, and iodine-containing ions to adjust the pH, it is possible to suppress the emission of iodine into the air during evaporation and concentration.

[0012] However, neither Patent Document 2 nor Patent Document 3 describes a method for suppressing iodine release when the iodide salt content in the aqueous solution is low and the iodide solution is at a high concentration. When the iodide salt content in the aqueous solution is low, for example in water treatment applications, large quantities of the aqueous solution are sometimes required, raising concerns about increased transportation, storage, and manufacturing costs due to the increased usage. Therefore, there is a need for a water treatment agent composition that provides a high-concentration iodide aqueous solution to suppress iodine release.

[0013] Prior art literature

[0014] Patent documents

[0015] Patent Document 1: JP Japanese Patent Application Publication No. 56-033009

[0016] Patent Document 2: JP 2010-271141

[0017] Patent Document 3: JP 2006-232662 Summary of the Invention

[0018] (The problem the invention aims to solve)

[0019] The object of the present invention is to provide a water treatment method and a water treatment agent composition for the water treatment method, which can suppress the deterioration of the reverse osmosis membrane and the formation of slime in the reverse osmosis membrane treatment of water containing at least one of a chlorine-based oxidant and a bromine-based oxidant or iodide ions.

[0020] In addition, the present invention aims to provide a water treatment composition containing a high concentration of iodide salt and in which the release of iodine is inhibited.

[0021] (Technical solution used to solve the problem)

[0022] This invention provides a water treatment method, comprising: a reverse osmosis membrane treatment step, wherein concentrated water and permeate are obtained by using a reverse osmosis membrane on water to be treated. In the water treatment method, for the water to be treated containing at least one of a chlorine-based oxidant and a bromine-based oxidant, at least 1 mol of iodide ions are added relative to 1 mol of free chlorine and free bromine in the water to be treated; or, for the water to be treated containing iodide ions, at least one of a chlorine-based oxidant and a bromine-based oxidant is added relative to 1 mol of iodide ions in the water to make the free chlorine and free bromine content less than 1 mol.

[0023] Based on the water treatment method, preferably, when the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the water to be treated after the addition of the iodide ions × the time of addition of the iodide ions (h), is 1.25 (mg / L·h) or less.

[0024] Based on the aforementioned water treatment method, preferably, when at least one of the chlorine-based oxidant and the bromine-based oxidant is added to the treated water containing the iodide ions, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water after the addition of at least one of the chlorine-based oxidant and the bromine-based oxidant × the addition time (h) of at least one of the chlorine-based oxidant and the bromine-based oxidant, is 1.25 (mg / L·h) or less.

[0025] Based on the water treatment method, preferably, when adding the iodide ions to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane is set to 15 seconds or more.

[0026] This invention provides a water treatment method, comprising: a membrane filtration treatment step, wherein the water to be treated is subjected to membrane filtration treatment using a separation membrane; and a reverse osmosis membrane treatment step, wherein the membrane-filtered water obtained from the membrane filtration treatment step is subjected to a reverse osmosis membrane to obtain concentrated water and permeate water. In the water treatment method, for the water to be treated containing at least one of a chlorine-based oxidant and a bromine-based oxidant in the membrane filtration treatment step, at least 1 mol of iodide ions is added relative to 1 mol of free chlorine and free bromine in the water to be treated; or, for the water to be treated containing iodide ions in the membrane filtration treatment step, at least one of a chlorine-based oxidant and a bromine-based oxidant is added relative to 1 mol of iodide ions in the water to be treated in such a way that the free chlorine and free bromine contents are less than 1 mol.

[0027] Based on the water treatment method, preferably, when the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the water to be treated after the addition of the iodide ions × the time of addition of the iodide ions (h), is 1.25 (mg / L·h) or less.

[0028] Based on the aforementioned water treatment method, preferably, when at least one of the chlorine-based oxidant and the bromine-based oxidant is added to the treated water containing the iodide ions, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water after the addition of at least one of the chlorine-based oxidant and the bromine-based oxidant × the addition time (h) of at least one of the chlorine-based oxidant and the bromine-based oxidant, is 1.25 (mg / L·h) or less.

[0029] Based on the water treatment method, preferably, when adding the iodide ions to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane is set to 15 seconds or more.

[0030] Based on the water treatment method, preferably, when the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the process from the point of addition of the iodide ions to the reverse osmosis membrane treatment step is carried out in a closed system.

[0031] Based on the water treatment method, preferably, when adding the iodide ions to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the amount of iodide ions added is controlled so that the redox potential of the water to be treated after adding the iodide ions is below 550mV.

[0032] The present invention provides a water treatment agent composition for use in the water treatment method, comprising water and iodide salt.

[0033] In addition to the water treatment agent composition, preferably, the water treatment agent composition also contains iodine.

[0034] Based on the water treatment agent composition, preferably, the content of the iodide salt in the water treatment agent composition is 20% by mass or more, calculated as iodide ions, and the pH of the water treatment agent composition is a value calculated by the following formula (1) or more.

[0035] pH = 5.24 × log(iodide ion content (mass%)) - 8.27 (1)

[0036] In addition to the water treatment agent composition, preferably, the water treatment agent composition also contains an alkali agent.

[0037] Based on the water treatment agent composition, preferably, the alkaline agent is a hydroxide.

[0038] Based on the water treatment agent composition, preferably, the TOC of the water treatment agent composition is less than 10 mg / L.

[0039] (Invention Effects)

[0040] The present invention provides a water treatment method and a water treatment agent composition for the water treatment method, which can inhibit the deterioration of the reverse osmosis membrane and the formation of slime in the reverse osmosis membrane treatment of water containing at least one of a chlorine-based oxidant and a bromine-based oxidant or iodide ions.

[0041] The present invention provides a water treatment composition containing a high concentration of iodide salt, wherein the release of iodine is inhibited. Attached Figure Description

[0042] Figure 1 This is a schematic diagram illustrating an example of a water treatment apparatus used to implement the water treatment method according to an embodiment of the present invention.

[0043] Figure 2 This is a schematic diagram illustrating another example of a water treatment apparatus used to implement the water treatment method according to an embodiment of the present invention.

[0044] Figure 3 It is a graph representing a standard curve created by varying the concentration of hypochlorous acid to keep the ammonium ion concentration constant.

[0045] Figure 4This is a graph showing the relationship between the iodide ion content (mass%) in the water treatment agent compositions of Examples 5-8, Comparative Examples 3-6, and Reference Example 4 and the pH at which free iodine was not detected.

[0046] Figure 5 This is a graph showing the change in redox potential (mV) in Examples 9 and 10 relative to the elapsed time (seconds). Detailed Implementation

[0047] The following describes embodiments of the present invention. This embodiment is one example of implementing the present invention, and the present invention is not limited to this embodiment.

[0048] A schematic example of a water treatment apparatus for implementing the water treatment method according to the embodiments of the present invention is as follows: Figure 1 As shown.

[0049] The water treatment apparatus 1 includes a reverse osmosis membrane unit 14 as a reverse osmosis membrane treatment unit. This unit uses a reverse osmosis membrane to obtain concentrated water and permeate water for treated water containing at least one of chlorine-based oxidants and bromine-based oxidants, or for treated water containing iodide ions. The water treatment apparatus 1 may also include a treated water tank 10 for storing the treated water. The water treatment apparatus 1 may also include a membrane filtration unit 12 upstream of the reverse osmosis membrane unit 14 as a membrane filtration treatment unit for membrane filtration of treated water containing at least one of chlorine-based oxidants and bromine-based oxidants, or for treated water containing iodide ions, using a separation membrane.

[0050] exist Figure 1 In the water treatment apparatus 1, a treated water pipe 18 is connected to the treated water inlet of the treated water tank 10. The treated water outlet of the treated water tank 10 is connected to the treated water inlet of the membrane filtration device 12 via a treated water pipe 20. The membrane filtration treated water outlet of the membrane filtration device 12 is connected to the membrane filtration treated water inlet of the reverse osmosis membrane device 14 via a pump 16 and a membrane filtration treated water pipe 22. A permeate water pipe 24 is connected to the permeate outlet of the reverse osmosis membrane device 14, and a concentrate water pipe 26 is connected to the concentrate outlet. An addition pipe 28 may also be connected as an iodide ion addition device for adding iodide ions to at least one of the reagent inlet of the treated water tank 10, the treated water pipe 20, and the membrane filtration treated water pipe 22, or as an oxidant addition unit for adding at least one of a chlorine-based oxidant and a bromine-based oxidant. Figure 2 As shown, at least one of the treated water piping 18, the treated water piping 20, and the membrane filtration treated water piping 22 can be connected to a reducing agent addition piping 30 as a reducing agent addition unit.

[0051] The operation of the water treatment method and water treatment apparatus 1 in this embodiment will be explained.

[0052] exist Figure 1 In the water treatment apparatus 1, treated water containing at least one of a chlorine-based oxidant and a bromine-based oxidant, or treated water containing iodide ions, is stored in the treated water tank 10 as needed via the treated water piping 18, and then supplied to the membrane filtration apparatus 12 via the treated water piping 20. In the membrane filtration apparatus 12, the treated water is subjected to membrane filtration treatment using a separation membrane (membrane filtration process). The membrane-filtered water obtained in the membrane filtration process is supplied to the reverse osmosis membrane apparatus 14 via the membrane-filtered water piping 22. In the reverse osmosis membrane apparatus 14, the membrane-filtered water is subjected to reverse osmosis membrane treatment to obtain concentrated water and permeate using a reverse osmosis membrane (reverse osmosis membrane treatment process). The permeate is discharged via the permeate piping 24, and the concentrated water is discharged via the concentrated water piping 26.

[0053] In the absence of a membrane filtration device 12 in the water treatment device 1, the treated water containing at least one of a chlorine-based oxidant and a bromine-based oxidant or the treated water containing iodide ions is fed to the reverse osmosis membrane device 14, in which the treated water is subjected to reverse osmosis membrane treatment (reverse osmosis membrane treatment process) to obtain concentrated water and permeate water using a reverse osmosis membrane.

[0054] In the water treatment method and water treatment apparatus 1 of this embodiment, the water treatment apparatus 1 includes a membrane filtration device 12. During the membrane filtration treatment process, for the water to be treated in the membrane filtration treatment process containing at least one of a chlorine-based oxidant and a bromine-based oxidant, 1 mol or more of iodide ions are added via the addition pipe 28 relative to 1 mol of free chlorine and free bromine in the water to be treated (iodide ion addition process). If the water treatment apparatus 1 does not include a membrane filtration device 12, for the water to be treated in the reverse osmosis membrane treatment process containing at least one of a chlorine-based oxidant and a bromine-based oxidant, 1 mol or more of iodide ions are added via the addition pipe 28 relative to 1 mol of free chlorine and free bromine in the water to be treated (iodide ion addition process). It should be noted that in this specification, "at least one of a chlorine-based oxidant and a bromine-based oxidant" is sometimes simply referred to as "chlorine-based oxidant, etc."

[0055] Alternatively, if the water treatment apparatus 1 is equipped with a membrane filtration device 12, during the membrane filtration treatment process, for the water to be treated in the membrane filtration treatment process containing iodide ions, at least one of a chlorine-based oxidant and a bromine-based oxidant is added through the addition pipe 28 in a manner that reduces the free chlorine and free bromine content to 1 mol or less relative to the amount of iodide ions in the water to be treated (oxidant addition process). If the water treatment apparatus 1 is not equipped with a membrane filtration device 12, for the water to be treated in the reverse osmosis membrane treatment process containing iodide ions, at least one of a chlorine-based oxidant and a bromine-based oxidant is added through the addition pipe 28 in a manner that reduces the free chlorine and free bromine content to 1 mol or less relative to the amount of iodide ions in the water to be treated (oxidant addition process).

[0056] By adding iodide ions to the water being treated using a reverse osmosis membrane containing chlorine-based oxidants, the chlorine-based oxidants, which can potentially degrade the reverse osmosis membrane, can be reduced. Through the chlorine-based oxidants, the iodide ions are oxidized into iodine, which has bactericidal properties, changing its form into a slime inhibitor that causes almost no degradation of the reverse osmosis membrane and suppresses slime formation. Therefore, by adding iodide ions to the water being treated containing chlorine-based oxidants, the degradation of the reverse osmosis membrane and slime formation can be inhibited.

[0057] Furthermore, by adding chlorine-based oxidants to the water being treated using a reverse osmosis membrane containing iodide ions, the iodide ions are oxidized into iodine, which has bactericidal properties, thus changing its form into a sludge inhibitor that causes almost no degradation of the reverse osmosis membrane and inhibits sludge formation. Chlorine-based oxidants that could potentially degrade the reverse osmosis membrane are reduced by the iodide ions. Therefore, by adding chlorine-based oxidants to the water being treated containing iodide ions, it is possible to inhibit the degradation of the reverse osmosis membrane and suppress sludge formation.

[0058] The location for adding iodide ions to treated water containing chlorinated oxidants, or the location for adding chlorinated oxidants, to treated water containing iodide ions, can be any one of the following: the treated water tank 10, the treated water piping 20 before the membrane filtration device 12, or the membrane filtration treated water piping 22 after the membrane filtration device 12. From the viewpoint of suppressing the deterioration of the membrane filtration device 12 caused by chlorinated oxidants, the location for adding iodide ions or chlorinated oxidants is preferably the treated water in the membrane filtration treatment process, i.e., the treated water tank 10 or the treated water piping 20 before the membrane filtration device 12.

[0059] As chlorine-based and bromine-based oxidants, any oxidant with a redox potential (ORP) higher than that of iodine is acceptable. Even a stabilized hypobromic acid composition containing bromine-based oxidants such as bound chlorine and bromine and aminosulfonic acid compounds can achieve the same effect. However, considering factors such as reaction rate, oxidants that are detected as free chlorine are preferred. Examples of oxidants that are detected as free chlorine include hypochlorous acid, hypobromic acid, or their salts.

[0060] The concentration of at least one of the chlorine-based oxidants and bromine-based oxidants in the treated water containing chlorine-based oxidants is, for example, in the range of 0.05 to 10 mg / L.

[0061] The concentration of iodide ions in the treated water containing iodide ions is, for example, in the range of 0.01 to 40 mg / L.

[0062] In this specification, the oxidizing power of the oxidant is expressed as total chlorine or free chlorine based on the DPD method. In this specification, "total chlorine" refers to the concentration determined by spectrophotometry using N,N-diethyl-p-phenylenediamine sulfate (DPD) as described in "JIS K 0120:2013, 33. Residual Chlorine". For example, 2.5 mL of 0.2 mol / L potassium dihydrogen phosphate solution is taken into a 50 mL colorimetric tube, and 0.5 g of DPD dilution powder (a solution prepared by crushing 1.0 g of N,N-diethyl-p-phenylenediamine sulfate and mixing it with 24 g of sodium sulfate) and 0.5 g of potassium iodide are added. A suitable amount of sample is added, water is added to the mark and dissolved, and the mixture is allowed to stand for approximately 3 minutes. The pink color is quantified by measuring the absorbance at a wavelength near 510 nm (or 555 nm) to determine the pink color. Furthermore, in this specification, "free chlorine" refers to the oxidizing power of the oxidant determined by the above-described "total chlorine" determination method without the addition of potassium iodide.

[0063] DPD is oxidized by oxidizing agents, such as chlorine, bromine, iodine, hydrogen peroxide, and ozone, which can be used as analytes. Chlorine quantified as total chlorine is in all oxidizing forms, including hypochlorous acid, hypochlorite ions, chlorine, chloramine, dichloramine, and other bound chlorine. Similarly, all oxidizing forms of bromine and iodine can also be measured. Free chlorine quantified is in forms that can be measured without potassium iodide in the above-mentioned total chlorine determination methods, such as hypochlorous acid, hypobromous acid, chlorine, bromine, and iodine.

[0064] Furthermore, "total chlorine" can be converted to "total iodine". Specifically, the conversion is based on the molecular weight of chlorine and the molecular weight of iodine. That is, "total chlorine" × (126.9 / 35.45) ≈ "total chlorine" × 3.58 = "total iodine". "Free chlorine" can also be converted to "free iodine".

[0065] When iodide ions are added to hypochlorous acid, if a sufficient amount of iodide ions is added relative to hypochlorous acid, all the oxidizing power is due to iodine. However, if a insufficient amount of iodide ions is added relative to hypochlorous acid, a portion of the oxidizing power is caused by hypochlorous acid, and a portion by iodine. However, in the DPD method described above, both hypochlorous acid and iodine are detected as free chlorine, making it difficult to confirm whether sufficient iodide ions have been added to hypochlorous acid. Insufficient iodide ion addition raises concerns about residual free chlorine from hypochlorous acid leading to reverse osmosis membrane degradation. Therefore, it is necessary to selectively measure the free chlorine from hypochlorous acid in the hypochlorous acid-iodine mixture.

[0066] The inventors of this invention conducted in-depth research and found that the principle of the indophenol blue method used for the determination of ammonia nitrogen can be applied. Iodine is known to have low reactivity with amine compounds and can be suitably used without hindering the reaction based on this method.

[0067] [Determination of hypochlorous acid solutions containing potassium iodide using the indophenol blue method]

[0068] The method described in "JIS K 0102:2013 42.2 Indophenol Blue Spectrophotometry" aims to determine ammonium ions by creating a calibration curve by varying the amount of ammonium ions while keeping the concentration of hypochlorous acid constant. In contrast, in this determination, the purpose is to determine the concentration of hypochlorous acid. Therefore, the concentration of hypochlorous acid was first varied to keep the ammonium ion concentration constant, thereby verifying whether a standard curve could be created.

[0069] Therefore, a standard curve was created by adding ammonium ions to water to a concentration of 10 mg / L to change the hypochlorous acid content. The standard curve was created following the steps shown below, and the result R... 2 The value is 0.999. The measurement results are as follows: Figure 3 As shown. Additionally, the same method was used to convert hypochlorous acid to iodine and an attempt was made to determine the color, but no color was developed. This method reaffirms that color development occurs only with free chlorine derived from hypochlorous acid.

[0070] 1. Using sodium hypochlorite as free chlorine, add it to a 50mL graduated cylinder at a concentration of 0mg / L to 1.2mg / L, and add water to a final volume of 25mL.

[0071] 2. Add ammonium chloride solution to make the ammonium ion concentration 10 mg / L, and add water to 40 mL for mixing.

[0072] 3. Add 10 mL of sodium phenolate solution as specified in JIS K 0102 and mix.

[0073] 4. Keep the liquid temperature at 20-25℃ and let it stand for about 30 minutes.

[0074] 5. A portion of the solution was measured using absorbance around 630 nm.

[0075] Next, hypochlorous acid was added at a free chlorine concentration of 5 mg / L, and analysis was conducted on the hypochlorous acid in mixed solutions after potassium iodide was added at a concentration of iodide ions relative to free chlorine of 0.1 mol to 10 mol. The free chlorine concentration of the solution was measured before and after the addition of potassium iodide using a HACH DR3900 spectrophotometer, and almost no change in free chlorine concentration was observed. The determination of the free chlorine concentration derived from hypochlorous acid in the mixed solution was performed according to the following procedure. The results are shown in Table 1.

[0076] 1. Take sodium hypochlorite into a 50mL graduated cylinder, indicating that the free chlorine content is 5mg / L.

[0077] 2. Add potassium iodide solution in a concentration of 0.1–10 mol relative to free chlorine, and add water to make 25 mL.

[0078] 3. Add ammonium chloride solution to contain 10 mg / L of ammonium ions, and add water to 40 mL and mix.

[0079] 4. Add 10 mL of sodium phenolate solution as specified in JIS K 0102 and mix.

[0080] 5. Keep the liquid temperature at 20-25℃ and let it stand for about 30 minutes.

[0081] 6. Measure a portion of the solution using absorbance around 630 nm to determine the concentration of free chlorine originating from hypochlorous acid in the mixture.

[0082] 7. The concentration of free chlorine derived from iodine is determined by subtracting the concentration of free chlorine derived from hypochlorous acid from the concentration of free chlorine in the mixture.

[0083] [Table 1]

[0084]

[0085] In a mixture with 0.1 mol of KI relative to free chlorine, the concentration of free chlorine derived from hypochlorous acid is 3.9 mg / L, and the concentration of free chlorine derived from iodine, converted to free iodine at 1.1 mg / L, is also 3.9 mg / L. In a mixture with 0.5 mol of KI relative to free chlorine, the concentration of free chlorine derived from hypochlorous acid is 0.6 mg / L, and the concentration of free chlorine derived from iodine, converted to free iodine at 4.4 mg / L, is 15.6 mg / L. In a mixture with 1–10 mol of KI relative to free chlorine, all hypochlorous acid is reduced, and the concentration of free chlorine derived from hypochlorous acid is not quantified (quantification limit: 0.02 mg / L), while the concentration of free chlorine derived from iodine, converted to free iodine at 5.0 mg / L, is 17.9 mg / L.

[0086] In Patent Document 1, when iodine is used as an additive, a concentration of about 5 to 15 ppm is preferred. Furthermore, in the examples, an aqueous solution of potassium iodide is added in an amount sufficient to produce 15 ppm of iodine relative to the solution containing hypochlorous acid. However, as can be clearly seen from Table 1 above, the amount of potassium iodide added to produce 15 ppm (15 mg / L) of iodine is [I － With a free chlorine concentration of 0.5, the addition of potassium iodide becomes insufficient, and the free chlorine derived from hypochlorous acid may remain at 0.6 mg / L, thus degrading the reverse osmosis membrane.

[0087] In the iodide ion addition step, the free iodine concentration (CT) value, expressed as the concentration of free iodine generated in the treated water (mg / L) × the addition time (h) of iodide ions, obtained by adding 1 mol or more of iodide ions relative to 1 mol of free chlorine and free bromine in the treated water containing chlorine-based oxidants, is preferably 1.25 (mg / L·h) or less, more preferably 1.0 (mg / L·h) or less. In the oxidant addition step, the free iodine concentration (mg / L) generated in the treated water, expressed as the concentration of free iodide ions, obtained by adding at least one of a chlorine-based oxidant and a bromine-based oxidant in a manner where the free chlorine and free bromine content is 1 mol or less, is preferably 1.25 (mg / L·h) or less, more preferably 1.0 (mg / L·h) or less, obtained by adding at least one of a chlorine-based oxidant and a bromine-based oxidant in a manner where the free chlorine and free bromine content are 1 mol or less. If the free iodine CT value exceeds 1.25, it can sometimes lead to a deterioration in the quality of the water permeated through the reverse osmosis membrane.

[0088] In the iodide ion addition or oxidant addition process, the method of adding iodide ions or chloride-based oxidants to the treated water can be either continuous addition or intermittent addition, with periods of addition between periods of no addition. From the perspective of chemical costs, intermittent addition is preferred.

[0089] In the iodide ion addition process or the oxidant addition process, it is preferred to add intermittently for more than 10 seconds and less than 3 hours during the addition period, and for more than 5 seconds and less than 48 hours during the no-addition period.

[0090] Alternatively, it is preferable to add a reducing agent during this additive-free period. For example... Figure 2 As shown, the reducing agent is added to the treated water in the membrane filtration process or the treated water (membrane-filtered treated water) in the reverse osmosis membrane treatment process via the reducing agent addition pipe 30 (reducing agent addition process). During periods without addition, the reducing agent can be added to the treated water at any of the following locations: treated water pipe 18, treated water pipe 20 before the membrane filtration unit 12, or membrane-filtered treated water pipe 22 after the membrane filtration unit 12. Preferably, the reducing agent is added before the addition of iodide ions or chloride-based oxidants.

[0091] In the absence of a reducing agent during the non-addition period, deterioration of the reverse osmosis membrane can sometimes occur. Examples of reducing agents include sulfites such as sodium sulfite, bisulfites such as sodium bisulfite, thiosulfates such as sodium thiosulfate, hydrazine, hydroxylamine, and hydrogen sulfide. From a safety perspective, sulfites, bisulfites, and thiosulfates are preferred, with thiosulfates being more preferred.

[0092] Sulfites and bisulfites, as shown in formulas (1) and (2) below, react with free iodine in equimolar amounts with the reducing agent. In contrast, thiosulfates, as shown in formula (3) below, react with free iodine in 1 / 2 molar amounts with the reducing agent. In the case where a reducing agent is added during the period without addition and remains, the reducing agent reduces the free iodine generated during the addition period. However, by using thiosulfates, the amount of reduction can be suppressed compared to sulfites and bisulfites.

[0093] I₂ + SO₃ 2－ +H₂O→H₂SO₄+2I － Equation (1)

[0094] I₂ + HSO₃ － +H₂O→2I － +3H + +SO4 2－ Equation (2)

[0095] I₂ + 2S₂O₃ 2－ →2I － +S4O6 2－ Equation (3)

[0096] When adding iodide ions to the water to be treated, which contains at least one of a chlorine-based oxidant and a bromine-based oxidant, it is preferable to set the time from the addition of iodide ions to their arrival at the reverse osmosis membrane to be 15 seconds or more, more preferably 20 seconds or more. If the time from the addition of iodide ions to their arrival at the reverse osmosis membrane is less than 15 seconds, there is a possibility that the chlorine-based oxidant or the like may not be sufficiently reduced by the iodide ions, leading to deterioration of the reverse osmosis membrane.

[0097] Preferably, the process from the point where iodide ions are added to the reverse osmosis membrane unit 14 where the reverse osmosis membrane treatment process is performed is carried out in a closed system. When there is atmospheric exposure or aeration, the amount of free iodine may sometimes decrease. Therefore, the addition of iodide ions is preferably done through a pipeline into the piping.

[0098] The amount of iodide ions added is preferably controlled such that, after adding 1 mol or more of iodide ions relative to 1 mol of free chlorine and free bromine in the treated water containing chlorine-based oxidants, the oxidation-reduction potential (ORP) of the treated water treated by the reverse osmosis membrane is 550 mV or less. This ORP is preferably 540 mV or less, more preferably 520 mV or less. If the ORP exceeds 550 mV, it can sometimes lead to deterioration of the reverse osmosis membrane. The amount of iodide ions added to the treated water can be easily managed by the ORP. For example, when adding iodide ions as part of a water treatment agent composition containing water and iodide salt, even if the free chlorine content in the treated water is unstable, the amount of iodide ions added can be managed by the ORP.

[0099] As the iodide ions added to the water being treated, only 1.0 mol or more of iodide ions need to be added relative to 1 mol of free chlorine and free bromine. The iodide ions can be added as solid iodide salts such as sodium iodide or potassium iodide, or as an aqueous solution pre-dissolved with sodium iodide or potassium iodide, or by dissolving iodine in an aqueous solution containing free iodide. From a treatment perspective, adding as an aqueous solution is preferred; from a storage perspective, adding as an aqueous solution without free iodine is more preferred.

[0100] <Water Treatment Agent Composition>

[0101] The water treatment agent composition used in the water treatment method of this embodiment is a composition containing water and iodide salt. If the content of chlorine-based oxidants or the like in the water to be treated is low (e.g., 0.1 mg / L or less), the amount of iodine generated by oxidation by the iodide salt is less. In such cases, the water treatment agent composition may also contain iodine.

[0102] The water treatment agent composition used in the water treatment method of this embodiment contains water and iodide salt, the content of iodide salt in the water treatment agent composition is 20% by mass or more based on iodide ions, and the pH of the water treatment agent composition is preferably a value calculated by the following formula (1) or more.

[0103] pH = 5.24 × log(iodide ion content (mass%)) - 8.27 (1)

[0104] The inventors of this invention have discovered that in a water treatment agent composition containing water and an iodide aqueous solution, by setting the pH to a value calculated by the above formula (1) or higher, the release of iodine is suppressed even at a high concentration of iodide salt content of 20% by mass or higher, based on iodide ions.

[0105] If the pH of the water treatment agent composition is less than the value calculated by the above formula (1), iodine will be released during storage, raising concerns about leakage into the storage container and a decrease in the original reducing power. The pH of the water treatment agent composition is preferably the value calculated by the above formula (1) plus 0.35 or more.

[0106] The iodide salt contained in the water treatment agent composition is an inorganic salt of iodine, such as sodium iodide, potassium iodide, lithium iodide, copper iodide, zinc iodide, etc. From the perspective of cost, sodium iodide or potassium iodide is preferred. The water treatment agent composition may contain one type of iodide salt or two or more types.

[0107] As for water, there are no special restrictions; examples include tap water and purified water.

[0108] The iodide salt content in the water treatment agent composition is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. If the iodide salt content is less than 20% by mass, there are concerns about increased costs associated with the transportation, storage, and addition of the drug due to increased usage. The upper limit for the iodide salt content is, for example, 56% by mass or less.

[0109] When the water treatment agent composition contains iodine, the iodine content is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 10% by mass or more. When the iodine content is less than 3% by mass, the bactericidal effect is sometimes insufficient. The upper limit of the iodine content is, for example, 30% by mass or less.

[0110] The water treatment agent composition may further contain an alkaline agent. As an alkaline agent, any agent capable of increasing the pH of the solution is acceptable; examples include hydroxides such as sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonium hydroxide, and tetramethylammonium hydroxide; carbonates such as sodium carbonate and potassium carbonate; and bicarbonates such as sodium bicarbonate and potassium bicarbonate. From the perspectives of safety and formulation cost, hydroxides such as sodium hydroxide, potassium hydroxide, and calcium hydroxide are preferred, and sodium hydroxide or potassium hydroxide is more preferred.

[0111] Furthermore, from the viewpoint of storage stability, the alkali agent is preferably present in the composition at 0.01% by mass or more, more preferably at 0.1% by mass or more. The upper limit of the alkali agent content is, for example, less than 10% by mass.

[0112] The iodide salt content in the water treatment agent composition, calculated as iodide ions, is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. If the iodide salt content, calculated as iodide ions, is less than 20% by mass, there is concern that the increased dosage may lead to higher pharmaceutical costs. The upper limit for the iodide salt content, for example, is 42% by mass or less, calculated as iodide ions.

[0113] The total iodine in the water treatment agent composition can be determined by the DPD (N,N-diethyl-p-phenylenediamine) method described above.

[0114] The water treatment agent composition of this embodiment preferably does not contain organic matter. "Does not contain organic matter" means that the TOC of the water treatment agent composition is 100 mg / L or less, preferably 10 mg / L or less.

[0115] The water treatment agent composition of this embodiment is suitable for use as a water treatment agent for reducing residual chlorine in treated water and for sterilizing treated water. The iodide ions contained in the water treatment agent composition can effectively reduce residual chlorine and render it harmless. The water treatment agent composition of this embodiment is particularly suitable for use as a sterilization adjuvant in reverse osmosis membrane treatment.

[0116] In water treatment systems, oxidants such as hypochlorous acid and hypobromic acid are often added to inhibit sterilization and slime formation. However, it is known that these oxidants, intended for sterilization and slime inhibition, are not consumed and remain in the treated water, potentially causing adverse effects on downstream water treatment systems by flowing into them. For example, the inflow of residual chlorine can cause corrosion in cooling towers, deterioration of reverse osmosis membranes in reverse osmosis systems, and significant oxidative degradation in resin towers and electrodeionization (EDI) systems.

[0117] The water treatment agent composition of this embodiment can suppress the effects on such downstream water treatment devices, and can suppress corrosion in cooling towers, performance degradation of reverse osmosis membranes in reverse osmosis membrane devices, and oxidative degradation in resin towers and electro-deionization (EDI) systems.

[0118] When using the above-described water treatment agent composition for water treatment, for example, the water treatment agent composition can be added to the water to be treated, which contains at least one of a chlorine-based oxidant and a bromine-based oxidant. Additionally, in a water treatment method that treats water containing at least one of a chlorine-based oxidant and a bromine-based oxidant using a reverse osmosis membrane, the above-described water treatment agent composition can be added to the water to be treated.

[0119] By adding iodide salts to water treated by reverse osmosis membrane treatment containing at least one of chlorine-based and bromine-based oxidants, the chlorine-based oxidants, which may cause deterioration of the reverse osmosis membrane, can be reduced. Through the chlorine-based oxidants, iodide ions are oxidized to iodine, which has bactericidal properties, changing its form into a sludge inhibitor that causes almost no deterioration of the reverse osmosis membrane and inhibits sludge formation. Therefore, by adding iodide ions to water containing chlorine-based oxidants, deterioration of the reverse osmosis membrane and sludge formation can be inhibited.

[0120] Example

[0121] The present invention will be described in more detail below with examples and comparative examples, but the present invention is not limited to the following examples.

[0122] <Example 1, Comparative Example 1>

[0123] Under the following test conditions, hypochlorous acid was added to the feed water (treated water) of the reverse osmosis membrane unit in the form of free chlorine to reach 5 mg / L. Potassium iodide was added after the high-pressure RO pump at concentrations of 0.5 mol, 1.0 mol, and 2.0 mol of iodide ions relative to free chlorine. Free chlorine was measured using a HACH DR3900 spectrophotometer. The results are shown in Table 2.

[0124] (Experimental conditions)

[0125] • Test apparatus: Reverse osmosis membrane element test apparatus

[0126] • Supply pressure: 0.2~0.35MPa

[0127] Water supply: Sagami raw well water (dechlorinated, bacteria count 2×10⁻⁶) 3 CFU / mL

[0128] Water temperature: 16~19℃

[0129] pH: 7.3–7.7

[0130] Hypochlorous acid concentration: Added to the water being treated at a concentration of 5 mg / L (based on free chlorine).

[0131] Potassium iodide: 99.8% potassium iodide manufactured by GODO SHIGEN CO.,LTD.

[0132] • Reverse osmosis membrane: Nitto Denko Corporation, 4-inch reverse osmosis membrane element (ESPA2)

[0133] • Water flow rate: 500L / h concentrate water, 125L / h permeate water

[0134] [Table 2]

[0135]

[0136] In Comparative Example 1, the reverse osmosis membrane was operated with potassium iodide aqueous solution added relative to free chlorine to make the iodide ion concentration 0.5 mol. After 1000 hours of operation, the EC rejection rate (based on conductivity) decreased from 98% to 90%, and the ionic silica rejection rate decreased from 98% to 85%. The residual oxidizing power from hypochlorous acid, which was not reduced by iodide ions, caused oxidative degradation of the reverse osmosis membrane. In Examples 1 and 2, potassium iodide aqueous solution was added with iodide ions at concentrations of 1.0 mol and 2.0 mol relative to free chlorine, respectively. After 1000 hours of operation, the EC rejection rate and ionic silica rejection rate showed almost no change, indicating almost no degradation of the reverse osmosis membrane. Furthermore, under both conditions, there was almost no increase in the flow pressure differential, resulting in sufficient slime suppression.

[0137] <Example 2, Reference Example 1>

[0138] use Figure 1 The water treatment apparatus shown was used to treat water under the following test conditions, with the free iodine CT value (mg / L·h), expressed as (free iodine in the treated water (mg·L)) × (time of iodide ion addition (h)), varying as shown in Table 3. The results are shown in Table 3.

[0139] (Experimental conditions)

[0140] • Test water: Sagami raw well water (dechlorinated, bacterial count 2×10⁻⁶) 3 CFU / mL

[0141] • Chemical: A water treatment agent composition containing free iodine (potassium iodide content: 20% by mass)

[0142] pH: 7.0

[0143] • Reverse osmosis membranes: Nitto Denko ES20, ESPA2, LFC3; Toray Industries TML10D

[0144] [Table 3]

[0145]

[0146] It is known that by keeping the CT value below 1.25, the concentration of free iodine in the permeate water can be suppressed, thereby inhibiting the deterioration of the permeate water quality. Furthermore, in Reference Example 1, the addition was performed under conditions equivalent to the free iodine CT value in Patent Document 1.

[0147] <Example 3, Reference Example 2>

[0148] A potassium iodide aqueous solution (potassium iodide content: 20% by mass) was added to pure water containing hypochlorous acid to achieve a total chlorine concentration of 0.5 mg / L. The solution was then stored in an iboy wide-mouth flask (AS ONE) under the following storage conditions with stirring. The residual percentage of total chlorine after a given time was calculated. The results are shown in Table 4.

[0149] Example 3: Close the lid and seal.

[0150] Refer to Example 2-1: Remove the cover to open the top.

[0151] Refer to Example 2-2: Remove the cover for air aeration.

[0152] [Table 4]

[0153]

[0154] In Example 3, even after 1140 minutes, 100% of the total chlorine remained. However, in Reference Example 2-1, the residual rate of total chlorine decreased after 10 minutes, reaching 0 after 1140 minutes. In Reference Example 2-2, the residual rate of total chlorine decreased after 10 minutes, similar to Reference Example 2-1, and was even lower than that of Reference Example 2-1. This demonstrates that in a closed system, there is almost no reduction in total chlorine while maintaining sufficient sterilization effectiveness.

[0155] <Example 4, Comparative Example 2, Reference Example 3>

[0156] Under the following experimental conditions, sodium hypochlorite was added to the test water to achieve a free chlorine concentration of 1 mg / L, and potassium iodide was added in amounts ranging from 0.1 to 10 mol relative to the free chlorine content. The oxidation-reduction potential (ORP) was measured using a portable ORP meter (TOA DKK, RM-30P type). The results are shown in Table 5. The free chlorine concentration showed almost no change before and after the addition of potassium iodide.

[0157] (Experimental conditions)

[0158] • Test water: Sagami raw well water (dechlorinated)

[0159] • pH: 7.0 (adjusted after adding sodium hypochlorite)

[0160] [Table 5]

[0161] <![CDATA[[I - [Free chlorine] ORP See Example 3 0 754 Comparative Example 2-1 0.1 708 Comparative Example 2-2 0.5 686 Example 4-1 1.0 546 Example 4-2 2.0 516 Example 4-3 3.0 507

[0162] In Reference Example 3, without the addition of potassium iodide, the ORP of the test water containing 1 mg / L of sodium hypochlorite as free chlorine was 754 mV. It can be seen that in Comparative Examples 2-1 and 2-2, when potassium iodide was added at a ratio of 0.1 mol to 0.5 mol of iodide ions relative to free chlorine, the resulting ORPs were as high as 708 mV and 686 mV, respectively, potentially leading to membrane degradation. In Examples 4-1, 4-2, and 4-3, when potassium iodide was added at a ratio of 1.0 mol, 2.0 mol, and 3.0 mol of iodide ions relative to free chlorine, the resulting ORPs were as low as 546 mV, 516 mV, and 507 mV, respectively, indicating a low likelihood of membrane degradation. Therefore, it is preferable to control the amount of iodide ions added so that the oxidation-reduction potential (ORP) of the treated water is below 550 mV.

[0163] As described above, according to the embodiments, in the reverse osmosis membrane treatment of water containing at least one of chlorine-based oxidants and bromine-based oxidants, the deterioration of the reverse osmosis membrane and the formation of slime can be suppressed.

[0164] <Examples 5-8, Comparative Examples 3-6, Reference Example 4>

[0165] Following the compositions shown in Tables 6-10, potassium iodide, pH adjuster (acid or alkali), and water were mixed, and storage stability tests were conducted. The effective components were determined after 7, 14, and 30 days of storage under light-proof, sealed conditions at room temperature (25±2℃) with their respective compositions and pH values. The amount of free iodine was determined using a HACH DR3900 spectrophotometer.

[0166] [Table 6]

[0167]

[0168] [Table 7]

[0169]

[0170] [Table 8]

[0171]

[0172] [Table 9]

[0173]

[0174] [Table 10]

[0175]

[0176] Water, potassium iodide, 0.35% hydrochloric acid, or 0.48% potassium hydroxide were mixed according to the proportions shown in Table 6 (Example 5, Comparative Example 3), resulting in a substantially homogeneous liquid. At pH 4.93, the solution immediately after preparation turned yellow, indicating the detection of 0.0004% by mass of free iodine. Under other conditions, the solution immediately after preparation was colorless and transparent, and no free iodine was detected (detection limit: <0.10 mg / 100 g). Storage tests were conducted using the obtained preparation, and the results showed that setting the pH to 11 or higher suppressed iodine formation after storage at 25°C for 30 days. The results are shown in Table 11. It should be noted that in Table 11, "〇" indicates that no free iodine was detected after storage at 25°C for 30 days, and "×" indicates that free iodine was detected after storage at 25°C for 30 days.

[0177] Water, potassium iodide, 0.35% hydrochloric acid, or 0.48% potassium hydroxide were mixed according to the proportions shown in Table 7 (Example 6, Comparative Example 4). The resulting liquid was substantially homogeneous, and the freshly prepared solution was colorless and transparent under all conditions, with no detectable free iodine. Storage tests were conducted using the obtained formulation, and the results showed that setting the pH to 11 or higher suppressed iodine formation after storage at 25°C for 30 days. The results are shown in Table 11.

[0178] Water, potassium iodide, 0.35% hydrochloric acid, or 0.48% potassium hydroxide were mixed according to the proportions shown in Table 8 (Example 7, Comparative Example 5), Table 9 (Example 8, Comparative Example 6), and Table 10 (Reference Example 4). The results yielded a generally homogeneous liquid. Under all conditions, the freshly prepared solution was colorless and transparent, and no free iodine was detected. Storage tests were conducted using the obtained formulation. The results showed that by maintaining pH values ​​of 10, 8, and 6 or higher, iodine formation was suppressed after storage at 25°C for 30 days. The results are shown in Table 11.

[0179] [Table 11]

[0180]

[0181] Figure 4The graph shows the relationship between the iodide ion content (mass%) in the composition and the pH at which no free iodine was detected (the lower limit of pH with an evaluation result of "0" in Table 11). From this result, the above equation (1) is derived as pH = 5.24 × log(iodide ion content (mass%)) - 8.27.

[0182] The solution in which free iodine is detected is pale yellow in appearance, becoming darker yellow as the concentration increases. Thus, the free iodine in the solution causes discoloration of the container, therefore it is preferable to store it under conditions where almost no iodine is generated. If the solution meets the iodide salt content and pH value of the examples, almost no iodine will be released, allowing for long-term stable storage.

[0183] Thus, in the examples, a water treatment agent composition containing a high concentration of iodide salt and in which the release of iodine is suppressed is obtained.

[0184] <Examples 9 and 10>

[0185] Sodium hypochlorite (free chlorine concentration of 1 mg / L) and potassium iodide (relative to free chlorine of 1.5 mol) were mixed in pure water, and the redox potential (ORP) was measured over time. In Example 9, potassium iodide was added to the sodium hypochlorite solution, and in Example 10, sodium hypochlorite was added to the potassium iodide solution. The ORP was measured using a portable ORP meter (TOA DKK, RM-30P model). The results are shown below. Figure 5 .

[0186] In Example 9, the redox potential before mixing was above 700 mV, but it decreased slowly after mixing, and stabilized at around 550 mV after more than 15 seconds. In Example 10, the redox potential before mixing was below 400 mV, but it increased slowly after mixing, and stabilized at around 550 mV after more than 15 seconds.

[0187] As described above, in either Example 9 or Example 10, the redox potential remained stable at around 550 mV. However, in Example 9, a high redox potential was maintained until approximately 15 seconds. Therefore, when hypochlorous acid is injected followed by iodide ions, it is preferable to set the injection point such that the time until the iodide ions reach the reverse osmosis membrane is 15 seconds or more. When sodium hypochlorite is injected following iodide ions, there are no particular limitations on the setting of the injection point.

[0188] (Label Explanation)

[0189] 1 Water treatment unit, 10 Treated water tank, 12 Membrane filtration unit, 14 Reverse osmosis membrane unit, 16 Pump, 18, 20 Treated water piping, 22 Membrane filtration treated water piping, 24 Permeate water piping, 26 Concentrated water piping, 28 Addition piping, 30 Reducing agent addition piping.

Claims

1. A water treatment method, characterized in that, include: The reverse osmosis membrane treatment process uses a reverse osmosis membrane to produce concentrated water and permeate water from the water being treated. In the water treatment method, For the treated water containing at least one of a chlorine-based oxidant and a bromine-based oxidant, if at least 1 mol of iodide ions are added relative to 1 mol of free chlorine and free bromine in the treated water, the free iodine CT value, expressed as the free iodine concentration generated in the treated water after the addition of the iodide ions multiplied by the addition time of the iodide ions, is less than 1.25 mg / L·h, where the unit of free iodine concentration is mg / L and the unit of addition time is hours (h). For the treated water containing iodide ions, at least one of a chlorine-based oxidant and a bromine-based oxidant is added relative to 1 mol of iodide ions in the treated water, in a manner that makes the free chlorine and free bromine content less than 1 mol. The free iodine CT value, expressed as the free iodine concentration generated in the treated water after the addition of at least one of the chlorine-based oxidant and the bromine-based oxidant, multiplied by the addition time of at least one of the chlorine-based oxidant and the bromine-based oxidant, is less than 1.25 mg / L·h, where the unit of free iodine concentration is mg / L and the unit of addition time is hours (h).

2. The water treatment method according to claim 1, characterized in that, When the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane is set to be 15 seconds or more.

3. The water treatment method according to claim 1 or 2, characterized in that, When the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the process from the point of addition of the iodide ions to the reverse osmosis membrane treatment step is carried out in a closed system.

4. The water treatment method according to claim 1 or 2, characterized in that, When adding the iodide ions to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the amount of iodide ions added is controlled so that the redox potential of the water to be treated after adding the iodide ions is below 550 mV.

5. A water treatment method, characterized in that, include: The membrane filtration process involves using a separation membrane to filter the water being treated. as well as The reverse osmosis membrane treatment process involves using a reverse osmosis membrane to obtain concentrated water and permeate water from the membrane-filtered water obtained in the membrane filtration process. In the water treatment method, For the treated water containing at least one of a chlorine-based oxidant and a bromine-based oxidant in the membrane filtration process, with 1 mol or more of iodide ions added relative to 1 mol of free chlorine and free bromine in the treated water, the free iodine CT value, expressed as the free iodine concentration generated in the treated water after the addition of the iodide ions multiplied by the iodide ion addition time, is below 1.25 mg / L·h. The unit of free iodine concentration is mg / L, and the unit of addition time is hours (h). For the treated water containing iodide ions in the membrane filtration process, at least one of a chlorine-based oxidant and a bromine-based oxidant is added relative to the amount of iodide ions in the treated water (1 mol) to make the amount of free chlorine and free bromine less than 1 mol. The free iodine CT value, expressed as the free iodine concentration generated in the treated water after the addition of at least one of the chlorine-based oxidant and the bromine-based oxidant, multiplied by the addition time of at least one of the chlorine-based oxidant and the bromine-based oxidant, is less than 1.25 mg / L·h. The unit of free iodine concentration is mg / L, and the unit of addition time is hours (h).

6. The water treatment method according to claim 5, characterized in that, When the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane is set to be 15 seconds or more.

7. The water treatment method according to claim 5 or 6, characterized in that, When the iodide ions are added to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the process from the point of addition of the iodide ions to the reverse osmosis membrane treatment step is carried out in a closed system.

8. The water treatment method according to claim 5 or 6, characterized in that, When adding the iodide ions to the water to be treated containing at least one of the chlorine-based oxidant and the bromine-based oxidant, the amount of iodide ions added is controlled so that the redox potential of the water to be treated after adding the iodide ions is below 550 mV.

9. A water treatment agent composition, characterized in that, The water treatment method according to any one of claims 1 to 8 The water treatment agent composition contains water and iodide salt.

10. The water treatment agent composition according to claim 9, characterized in that, The water treatment agent composition also contains iodine.

11. The water treatment agent composition according to claim 9 or 10, characterized in that, The iodide salt content in the water treatment agent composition is 20% by mass or more, calculated as iodide ions. The pH of the water treatment agent composition is greater than or equal to the value calculated by the following formula (1). pH = 5.24 × log (iodide ion content (mass%)) - 8.27 (1).

12. The water treatment agent composition according to claim 11, characterized in that, The water treatment agent composition also contains an alkali agent.

13. The water treatment agent composition according to claim 11, characterized in that, The TOC of the water treatment agent composition is below 10 mg / L.

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