Water treatment method and water treatment agent composition

Abstract

To provide a water treatment method capable of suppressing degradation of reverse osmosis membranes and slime formation in reverse osmosis membrane treatment of treated water containing at least one of chlorine and bromine oxidants or iodide ions.SOLUTION: A water treatment method includes a reverse osmosis membrane treatment process to obtain concentrated water and permeate water from treated water by using a reverse osmosis membrane, in which at least 1 mol of iodide ions is added to the treated water containing at least one of chlorine oxidant and bromine oxidant for each 1 mol of free chlorine and free bromine in the treated water, or in which at least one of chlorine oxidant and bromine oxidant is added to the treated water containing iodide ions so that the amounts of free chlorine and free bromine become 1 mol or less relative to 1 mol of iodide ions in the treated water.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water treatment method and a water treatment agent composition used in the water treatment method. [Background technology]

[0002] Reverse osmosis membrane treatment, which uses reverse osmosis membranes, is used in numerous processes such as pure water production, wastewater recovery, and seawater desalination. In recent years, due to water shortages, its application in wastewater recovery has been increasing in particular. Reverse osmosis membrane treatment is generally applied after pretreatment processes such as sand filtration or membrane filtration. Chlorine-based oxidizing agents such as hypochlorous acid are used to suppress slime formation due to microbial growth in the pretreatment process. However, when chlorine-based oxidizing agents such as hypochlorous acid flow into the reverse osmosis membrane, the membrane performance deteriorates significantly. Therefore, reducing agents are added to the water supplied to the reverse osmosis membrane to decompose hypochlorous acid and suppress the deterioration of the reverse osmosis membrane.

[0003] However, if water with a low slime formation inhibitory effect due to the addition of a reducing agent is supplied to the reverse osmosis membrane, microorganisms may proliferate on the membrane surface, causing biofouling and leading to problems such as a decrease in permeate volume and an increase in supply pressure.

[0004] Therefore, to suppress biofouling, a bactericide (slime control agent) that is less likely to cause membrane degradation of the reverse osmosis membrane is added to the supply water of the reverse osmosis membrane to which a reducing agent has been added. This bactericide includes stabilized chlorine compounds such as chloramine and chlorosulfamic acid, as well as bromine-based oxidizing agents such as bromine and stabilized hypobromous acid compositions containing sulfamic acid.

[0005] However, if the amount of reducing agent added is excessive, the disinfectant is reduced and consumed by the reducing agent, leading to an increase in the amount added and the cost of the chemicals. Conversely, if the amount of reducing agent added is insufficient, film deterioration occurs due to residual hypochlorous acid, etc.

[0006] For example, Patent Document 1 describes that in reverse osmosis membrane treatment, sodium metabisulfite is added as a reducing agent to the water to be treated to which sodium hypochlorite has been added before the reverse osmosis membrane treatment, and potassium iodide is further added to generate iodine, thereby suppressing contamination by microorganisms.

[0007] However, there is no clear indication of the amount of potassium iodide required for hypochlorous acid, etc. If the amount of potassium iodide added is insufficient, the reverse osmosis membrane may deteriorate due to the unreduced hypochlorous acid, etc. If the amount of potassium iodide added is excessive, the cost of chemicals will increase. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 56-033009 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The object of the present invention is to provide a water treatment method that can suppress deterioration of the reverse osmosis membrane and suppress slime formation in reverse osmosis membrane treatment of water to be treated containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent or iodide ions, and a water treatment agent composition used in the water treatment method. [Means for solving the problem]

[0010] The present invention includes a reverse osmosis membrane treatment step in which a reverse osmosis membrane is used to obtain concentrated water and permeate from water to be treated. Hypochlorous acid, hypobromous acid, and their salts To the water to be treated, which contains at least one of the following, 1 mole or more of iodide ions are added per mole of free chlorine and free bromine in the water to be treated. Furthermore, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which the iodide ions are added × the time of iodide ion addition (h), is 1.25 (mg / L·h) or less. or, to the water to be treated which contains iodide ions, add the amount of iodide ions in the water to be treated which is 1 mol. Hypochlorous acid, hypobromous acid, and their saltsAdd at least one of the following so that the amount of free chlorine and free bromine is 1 mol or less. Furthermore, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which at least one of the hypochlorous acid, hypobromous acid, and their salts has been added, multiplied by the addition time (h) of at least one of the hypochlorous acid, hypobromous acid, and their salts, is 1.25 (mg / L·h) or less. This is a water treatment method.

[0013] In the water treatment method, Hypochlorous acid, hypobromous acid, and their salts When adding the iodide ions to the water to be treated, which contains at least one of the above, it is preferable to set the time from the addition of the iodide ions until they reach the reverse osmosis membrane to 15 seconds or more.

[0014] The present invention includes a membrane filtration step of performing membrane filtration on water to be treated using a separation membrane, and a reverse osmosis membrane treatment step of obtaining concentrated water and permeate from the membrane filtration water obtained in the membrane filtration step using a reverse osmosis membrane, wherein in the membrane filtration step Hypochlorous acid, hypobromous acid, and their salts To the water to be treated, which contains at least one of the following, 1 mole or more of iodide ions are added per mole of free chlorine and free bromine in the water to be treated. Furthermore, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which the iodide ions are added × the time of iodide ion addition (h), is 1.25 (mg / L·h) or less. or, in the treated water containing iodide ions in the membrane filtration process, add a certain amount of iodide ions to the treated water per 1 mol. Hypochlorous acid, hypobromous acid, and their salts Add at least one of the following so that the amount of free chlorine and free bromine is 1 mol or less. Furthermore, the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which at least one of the hypochlorous acid, hypobromous acid, and their salts has been added, multiplied by the addition time (h) of at least one of the hypochlorous acid, hypobromous acid, and their salts, is 1.25 (mg / L·h) or less. This is a water treatment method.

[0017] In the water treatment method, Hypochlorous acid, hypobromous acid, and their salts When adding the iodide ions to the water to be treated, which contains at least one of the above, it is preferable to set the time from the addition of the iodide ions until they reach the reverse osmosis membrane to 15 seconds or more.

[0018] In the water treatment method, Hypochlorous acid, hypobromous acid, and their salts When adding the iodide ions to the water to be treated, which contains at least one of the above, it is preferable to carry out the process from the point where the iodide ions are added to the reverse osmosis membrane treatment step in a closed system.

[0019] In the water treatment method, Hypochlorous acid, hypobromous acid, and their saltsWhen adding the iodide ion to the treated water containing at least one of them, it is preferable to control the addition amount of the iodide ion so that the oxidation-reduction potential of the treated water after adding the iodide ion becomes 550 mV or less.

[0020] The present invention is a water treatment agent composition used in the water treatment method and containing water and an iodide salt.

[0021] In the water treatment agent composition, it is preferable to further contain iodine.

Effect of the Invention

[0022] According to the present invention, in the reverse osmosis membrane treatment of treated water containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent or iodide ions, it is possible to provide a water treatment method capable of suppressing the deterioration of the reverse osmosis membrane and suppressing slime formation, and a water treatment agent composition used in the water treatment method.

Brief Description of the Drawings

[0023] [Figure 1] It is a schematic configuration diagram showing an example of a water treatment apparatus for carrying out the water treatment method according to an embodiment of the present invention. [Figure 2] It is a schematic configuration diagram showing another example of a water treatment apparatus for carrying out the water treatment method according to an embodiment of the present invention. [Figure 3] It is a graph showing a calibration curve created by changing the concentration of hypochlorous acid to make the ammonium ion constant. [Figure 4] It is a graph showing the change in oxidation-reduction potential (mV) with respect to the elapsed time (seconds) in Examples 5 and 6.

Modes for Carrying Out the Invention

[0024] Embodiments of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

[0025] Figure 1 shows a schematic example of a water treatment apparatus for carrying out a water treatment method according to an embodiment of the present invention.

[0026] The water treatment device 1 includes a reverse osmosis membrane device 14 as a reverse osmosis membrane treatment means for obtaining concentrated water and permeate water from water to be treated that contains at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent or water to be treated that contains iodide ions using a reverse osmosis membrane. The water treatment device 1 may also include a water tank 10 for storing water to be treated. The water treatment device 1 may also include a membrane filtration device 12 upstream of the reverse osmosis membrane device 14 as a membrane filtration treatment means for performing membrane filtration treatment on water to be treated that contains at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent or water to be treated that contains iodide ions using a separation membrane.

[0027] In the water treatment apparatus 1 shown in Figure 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 and the treated water inlet of the membrane filtration apparatus 12 are connected by a treated water pipe 20. The membrane filtration treated water outlet of the membrane filtration apparatus 12 and the membrane filtration treated water inlet of the reverse osmosis membrane apparatus 14 are connected via a pump 16 to a membrane filtration treated water pipe 22. A permeate pipe 24 is connected to the permeate outlet of the reverse osmosis membrane apparatus 14, and a concentrated water pipe 26 is connected to the concentrated water outlet. An additive pipe 28 may be connected to at least one of the chemical inlet of the treated water tank 10, the treated water pipe 20, and the membrane filtration treated water pipe 22, as an iodide ion adding means for adding iodide ions or an oxidizing agent adding means for adding at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent. As shown in Figure 2, a reducing agent addition pipe 30 may be connected to at least one of the treated water pipe 18, treated water pipe 20, and membrane filtration treated water pipe 22 as a reducing agent addition means for adding a reducing agent.

[0028] The operation of the water treatment method and water treatment apparatus 1 according to this embodiment will be described.

[0029] In the water treatment apparatus 1 shown in Figure 1, water to be treated containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent, or water to be treated containing iodide ions, is stored in the water tank 10 as needed through the water to be treated pipe 18, and then sent to the membrane filtration apparatus 12 through the water to be treated pipe 20. In the membrane filtration apparatus 12, the water to be treated is subjected to membrane filtration using a separation membrane (membrane filtration process). The membrane-filtered water obtained in the membrane filtration process is sent to the reverse osmosis membrane apparatus 14 through the membrane-filtered water pipe 22. In the reverse osmosis membrane apparatus 14, the membrane-filtered water is subjected to reverse osmosis membrane treatment using a reverse osmosis membrane to obtain concentrated water and permeate (reverse osmosis membrane treatment process). The permeate is discharged through the permeate pipe 24, and the concentrated water is discharged through the concentrated water pipe 26.

[0030] If the water treatment device 1 does not have a membrane filtration device 12, the water to be treated containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent, or the water to be treated containing iodide ions, is sent to the reverse osmosis membrane device 14, where the water to be treated is subjected to reverse osmosis membrane treatment using a reverse osmosis membrane to obtain concentrated water and permeate (reverse osmosis membrane treatment step).

[0031] In the water treatment method and water treatment apparatus 1 according to this embodiment, if the water treatment apparatus 1 is equipped with a membrane filtration apparatus 12 and a membrane filtration treatment process is performed, 1 mole or more of iodide ions are added to the water to be treated in the membrane filtration treatment process through an addition pipe 28 for every 1 mole of free chlorine and free bromine in the water to be treated (iodide ion addition process). If the water treatment apparatus 1 is not equipped with a membrane filtration apparatus 12, 1 mole or more of iodide ions are added to the water to be treated in the reverse osmosis membrane treatment process through an addition pipe 28 for every 1 mole of free chlorine and free bromine in the water to be treated (iodide ion addition process). Note that in this specification, "at least one of chlorine-based oxidizing agents and bromine-based oxidizing agents" may be simply referred to as "chlorine-based oxidizing agents, etc."

[0032] Alternatively, if the water treatment device 1 is equipped with a membrane filtration device 12 and a membrane filtration treatment process is performed, at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent is added to the water to be treated in the membrane filtration treatment process, which contains iodide ions, through the addition pipe 28, so that the amount of free chlorine and free bromine is 1 mol or less per 1 mol of iodide ions in the water to be treated (oxidizing agent addition process). If the water treatment device 1 is not equipped with a membrane filtration device 12, at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent is added to the water to be treated in the reverse osmosis membrane treatment process, which contains iodide ions, through the addition pipe 28, so that the amount of free chlorine and free bromine is 1 mol or less per 1 mol of iodide ions in the water to be treated (oxidizing agent addition process).

[0033] By adding iodide ions to the water being treated in reverse osmosis membrane treatment that contains chlorine-based oxidizing agents, etc., it is possible to reduce the chlorine-based oxidizing agents, etc., which may cause deterioration of the reverse osmosis membrane. The iodide ions are oxidized by the chlorine-based oxidizing agents, etc., to iodine, which has bactericidal properties, and change into a slime inhibitor that suppresses slime formation and hardly deteriorates the reverse osmosis membrane. Therefore, by adding iodide ions to the water being treated that contains chlorine-based oxidizing agents, etc., it is possible to suppress both the deterioration of the reverse osmosis membrane and slime formation.

[0034] Furthermore, by adding a chlorine-based oxidizing agent to the reverse osmosis membrane treatment water containing iodide ions, the iodide ions are oxidized by the chlorine-based oxidizing agent to iodine, which has bactericidal properties, and transform into a slime inhibitor that suppresses slime formation without significantly degrading the reverse osmosis membrane. Chlorine-based oxidizing agents that may cause degradation of the reverse osmosis membrane are reduced by the iodide ions. Therefore, by adding a chlorine-based oxidizing agent to the treatment water containing iodide ions, it is possible to suppress both the degradation of the reverse osmosis membrane and slime formation.

[0035] The location where iodide ions are added to the treated water containing a chlorine-based oxidizing agent, or where a chlorine-based oxidizing agent is added to the treated water containing iodide ions, may be any 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 deterioration of the membrane filtration device 12 due to the chlorine-based oxidizing agent, it is preferable that the location where iodide ions or a chlorine-based oxidizing agent is added is the treated water in the membrane filtration process, i.e., the treated water tank 10, or the treated water piping 20 before the membrane filtration device 12.

[0036] Any oxidizing agent with a higher oxidation-reduction potential (ORP) than iodine can be used as the chlorine-based or bromine-based oxidizing agent. Similar effects can be obtained with combined chlorine or a stabilized hypobromous acid composition containing a bromine-based oxidizing agent such as bromine and a sulfamic acid compound. However, in terms of reaction speed, it is preferable to use an oxidizing agent that is detected as free chlorine. Typical examples of oxidizing agents that are detected as free chlorine include hypochlorous acid, hypobromous acid, or their salts.

[0037] The concentration of at least one of the chlorine-based oxidizing agent and the bromine-based oxidizing agent in the treated water containing the chlorine-based oxidizing agent is, for example, in the range of 0.05 to 10 mg / L.

[0038] 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.

[0039] In this specification, the oxidizing power of an oxidizing agent is expressed as total chlorine or free chlorine by the DPD method. In this specification, "total chlorine" refers to the concentration determined by the spectrophotometric method using N,N-diethyl-p-phenylenediammonium 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 placed in a 50 mL colorimetric tube, 0.5 g of DPD dilution powder (1.0 g of N,N-diethyl-p-phenylenediammonium sulfate pulverized and mixed with 24 g of sodium sulfate) is added, 0.5 g of potassium iodide is added, an appropriate amount of the sample is added, water is added to the mark to dissolve, and it is left to stand for about 3 minutes. The resulting pink to pinkish color is quantified by measuring the absorbance around a wavelength of 510 nm (or 555 nm). Furthermore, in this specification, "free chlorine" refers to the oxidizing power of the oxidizing agent determined by the "total chlorine" measurement method described above, without adding potassium iodide.

[0040] DPD is oxidized by oxidizing agents, such as chlorine, bromine, iodine, hydrogen peroxide, and ozone, which can be used as oxidizing agents. The forms of chlorine quantified as total chlorine include all forms with oxidizing power, such as hypochlorous acid, hypochlorite ions, and combined chlorine such as chlorine, chloramine, and dichloramine. Similarly, all forms of bromine and iodine with oxidizing power can be measured. Free chlorine is quantified in the above "total chlorine" measurement method without adding potassium iodide, and examples include hypochlorous acid, hypobromous acid, chlorine, bromine, and iodine.

[0041] Furthermore, "total chlorine" can be converted to "total iodine." Specifically, this conversion is based on the molecular weight of chlorine and iodine. That is, "total chlorine" × (126.9 / 35.45) ≈ "total chlorine" × 3.58 = "total iodine." "Free chlorine" can also be converted to "free iodine" in the same way.

[0042] When iodide ions are added to hypochlorous acid, if a sufficient amount of iodide ions is added relative to the hypochlorous acid, all the oxidizing power will be due to iodine. However, if a sufficient amount of iodide ions is not added relative to the hypochlorous acid, some of the oxidizing power will be due to hypochlorous acid and some will be due to iodine. However, in the above DPD method, both hypochlorous acid and iodine are detected as free chlorine, making it difficult to confirm whether a sufficient amount of iodide ions has been added relative to the hypochlorous acid. If the amount of iodide ions added is insufficient, there is a concern that free chlorine derived from hypochlorous acid will remain and cause deterioration of the reverse osmosis membrane. Therefore, it is necessary to selectively measure the free chlorine derived from hypochlorous acid in the mixture of hypochlorous acid and iodine.

[0043] As a result of diligent research by the inventors, it was found that the principle of the indophenol blue method, which is used for measuring ammonia nitrogen, can be applied. Iodine is known to have low reactivity with amine compounds and can be suitably used in this method without significantly inhibiting the reaction.

[0044] [Measurement of hypochlorous acid solution containing potassium iodide using the indophenol blue method] The method described in "JIS K 0102:2013, 42.2 Indophenol Blue Spectrophotometric Method" is intended for measuring ammonium ions, and a calibration curve is created by changing the amount of ammonium ions while keeping the concentration of hypochlorous acid constant. In contrast, since the purpose of this measurement is to measure the concentration of hypochlorous acid, we first verified whether it is possible to create a calibration curve by changing the concentration of hypochlorous acid while keeping the amount of ammonium ions constant.

[0045] Therefore, a calibration curve was created by adding ammonium ions to water to a concentration of 10 mg / L and varying the hypochlorous acid content. When the calibration curve was created using the procedure shown below, R 2The value was 0.999. The measurement results are shown in Figure 3. Furthermore, when attempting to measure using the same method but replacing hypochlorous acid with iodine, no color development occurred, confirming once again that only free chlorine derived from hypochlorous acid develops color in this method.

[0046] 1. Place sodium hypochlorite in a 50 mL graduated cylinder so that it contains 0 mg / L to 1.2 mg / L of free chlorine, and add water to make a total volume of 25 mL. 2. Add ammonium chloride solution to reach a concentration of 10 mg / L of ammonium ions, then add water up to 40 mL and mix. 3. Add 10 mL of sodium phenoxide solution specified in JIS K 0102 and mix. 4. Maintain the liquid temperature at 20-25°C and leave it for approximately 30 minutes. 5. Measure a portion of this solution by its absorbance at approximately 630 nm.

[0047] Next, hypochlorous acid was added to a mixed solution to achieve a free chlorine concentration of 5 mg / L, and potassium iodide was added to ensure that the iodide ion concentration was between 0.1 mol and 10 mol relative to the free chlorine concentration. The hypochlorous acid in the mixed solution was then analyzed. Using a HACH DR3900 spectrophotometer, the free chlorine concentration of the solution was measured before and after the addition of potassium iodide, and there was almost no change in the free chlorine concentration. The free chlorine concentration derived from hypochlorous acid in the mixed solution was analyzed using the following procedure. The results are shown in Table 1.

[0048] 1. Add sodium hypochlorite to a 50 mL graduated cylinder so that it contains 5 mg / L of free chlorine. 2. Add potassium iodide solution to the free chlorine to a concentration of 0.1 to 10 mol, then add water to make a total volume of 25 mL. 3. Add ammonium chloride solution to a total concentration of ammonium ions of 10 mg / L, then add water to a total of 40 mL and mix. 4. Add 10 mL of sodium phenoxide solution specified in JIS K 0102 and mix. 5. Maintain the liquid temperature at 20-25°C and leave it for approximately 30 minutes. 6. A portion of this solution is measured using absorbance at approximately 630 nm to determine the concentration of free chlorine derived from hypochlorous acid in the mixture. 7. Determine the free chlorine concentration derived from iodine by subtracting the free chlorine concentration derived from hypochlorous acid from the free chlorine concentration of the mixed solution.

[0049] [Table 1]

[0050] In a mixture with a KI of 0.1 mol relative to free chlorine, the free chlorine concentration from hypochlorous acid was 3.9 mg / L, and the free chlorine concentration from iodine was 1.1 mg / L, which converted to 3.9 mg / L of free iodine. In a mixture with a KI of 0.5 mol relative to free chlorine, the free chlorine concentration from hypochlorous acid was 0.6 mg / L, and the free chlorine concentration from iodine was 4.4 mg / L, which converted to 15.6 mg / L of free iodine. In a mixture with a KI of 1 to 10 mol relative to free chlorine, all hypochlorous acid was reduced, the free chlorine concentration from hypochlorous acid could not be quantified (limit of quantification: 0.02 mg / L), and the free chlorine concentration from iodine was 5.0 mg / L, which converted to 17.9 mg / L of free iodine.

[0051] Patent Document 1 states that when iodine is used as an additive, a concentration of approximately 5 to 15 ppm is preferable, and in the examples, it is stated that an aqueous solution of potassium iodide is added to a solution containing hypochlorous acid in an amount sufficient to generate 15 ppm of iodine. However, as is clear from Table 1 above, the amount of potassium iodide to be added to generate 15 ppm (15 mg / L) of iodine is [I - The ratio of free chlorine to sodium iodide (QI) is 0.5, indicating that the amount of potassium iodide added was insufficient, resulting in a residual free chlorine concentration of 0.6 mg / L derived from hypochlorous acid, which could potentially degrade the reverse osmosis membrane.

[0052] In the iodide ion addition step, the free iodine CT value, expressed as the free iodine concentration (mg / L) × the iodide ion addition time (h) obtained by adding 1 mole or more iodide ions to 1 mole of free chlorine and free bromine in the water to be treated containing a chlorine-based oxidizing agent, is preferably 1.25 (mg / L·h) or less, and more preferably 1.0 (mg / L·h) or less. In the oxidizing agent addition step, the free iodine CT value, expressed as the free iodine concentration (mg / L) × the addition time (h) of at least one of the chlorine-based oxidizing agent and the bromine-based oxidizing agent, obtained by adding at least one of the chlorine-based oxidizing agent and the bromine-based oxidizing agent to 1 mole of iodide ions in the water to be treated containing iodide ions, such that the amount of free chlorine and free bromine is 1 mole or less, is preferably 1.25 (mg / L·h) or less, and more preferably 1.0 (mg / L·h) or less. If the free iodine CT value exceeds 1.25, it can lead to a deterioration in the water quality of the permeate from the reverse osmosis membrane.

[0053] In the iodide ion addition step or the oxidizing agent addition step, the method of adding iodide ions or chlorine-based oxidizing agents to the water to be treated may be continuous addition, or intermittent addition, which involves adding iodide ions or chlorine-based oxidizing agents to the water to be treated and then adding them during periods when no iodide ions or chlorine-based oxidizing agents are added. Intermittent addition is preferred from the standpoint of chemical costs, etc.

[0054] In the iodide ion addition step or the oxidizing agent addition step, it is preferable to use intermittent addition, where the addition period is continuously 10 seconds or more and 3 hours or less, and the non-addition period is continuously 5 seconds or more and less than 48 hours.

[0055] Also, it is preferable to add a reducing agent during this additive-free period. As shown in FIG. 2, the reducing agent is added to the water to be treated in the membrane filtration treatment step or the water to be treated (membrane filtration treated water) in the reverse osmosis membrane treatment step through the reducing agent addition pipe 30 (reducing agent addition step). The location where the reducing agent is added to the water to be treated during the additive-free period may be any of the water to be treated pipe 18, the water to be treated pipe 20 before the membrane filtration device 12, and the membrane filtration treated water pipe 22 after the membrane filtration device 12. The location of adding the reducing agent is preferably before the location of adding iodide ions or chlorine-based oxidants, etc.

[0056] If the reducing agent is not added during the additive-free period, it may cause deterioration of the reverse osmosis membrane. Examples of the reducing agent include sulfites such as sodium sulfite, bisulfites such as sodium bisulfite, thiosulfates such as sodium thiosulfate, hydrazine, hydroxylamine, hydrogen sulfide, etc. Among these, sulfites, bisulfites, and thiosulfates are preferable from the viewpoints of safety, etc., and thiosulfates are more preferable.

[0057] As shown in the following formulas (1) and (2), sulfites and bisulfites react with free iodine in an equimolar amount with respect to the reducing agent, while thiosulfates react with free iodine in a 1 / 2 molar amount with respect to the reducing agent as shown in the following formula (3). If the reducing agent remains after being added during the additive-free period, it reduces the free iodine generated during the additive period, but by using thiosulfates, the reduction amount can be suppressed more than that of sulfites and bisulfites.

[0058] I2+SO3 2- +H2O→H2SO4+2I - Formula (1) I2+HSO3 - +H2O→2I - +3H + +SO4 2- Formula (2) I2+2S2O3 2- →2I - +S4O6 2- Formula (3)

[0059] When adding iodide ions to treated water containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent, it is preferable to set the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane to 15 seconds or more, and more preferably to set it to 20 seconds or more. If the time from the addition of the iodide ions to their arrival at the reverse osmosis membrane is less than 15 seconds, the chlorine-based oxidizing agent and the like may not be sufficiently reduced by the iodide ions, which may degrade the reverse osmosis membrane.

[0060] It is preferable to carry out the process from the point where iodide ions are added to the reverse osmosis membrane apparatus 14 that performs the reverse osmosis membrane treatment in a closed system. If there is exposure to the atmosphere or aeration, the amount of free iodine may decrease. From this point of view, it is preferable to add iodide ions in a line by adding them to the piping.

[0061] It is preferable to control the amount of iodide ions added to reverse osmosis membrane treatment so that the oxidation-reduction potential (ORP) of the treated water is 550 mV or less after adding 1 mole or more iodide ions per 1 mole of free chlorine and free bromine in the treated water containing a chlorine-based oxidizing agent, etc. This oxidation-reduction potential is preferably 540 mV or less, and more preferably 520 mV or less. If this oxidation-reduction potential exceeds 550 mV, it may lead to deterioration of the reverse osmosis membrane. The amount of iodide ions added to the treated water can be easily controlled by the oxidation-reduction potential. For example, when adding iodide ions as a water treatment agent composition containing water and an iodide salt, the amount of iodide ions added to the treated water can be controlled by the oxidation-reduction potential even if the amount of free chlorine in the treated water is not stable.

[0062] The amount of iodide ions to be added to the treated water should be at least 1.0 mol per 1 mol of free chlorine and free bromine. The iodide ions may be added as solid iodide salts such as sodium iodide or potassium iodide, or as an aqueous solution in which iodide salts such as sodium iodide or potassium iodide have been dissolved beforehand, or as an aqueous solution containing free iodine, prepared by dissolving iodine in an aqueous solution containing iodide salts such as sodium iodide or potassium iodide. From the viewpoint of handling, it is preferable to add it as an aqueous solution, and from the viewpoint of storage, it is more preferable to add it as an aqueous solution that does not contain free iodine.

[0063] <Water treatment agent composition> The water treatment agent composition used in the water treatment method according to this embodiment is a composition containing water and an iodide salt. If the amount of chlorine-based oxidizing agents, etc., contained in the water to be treated is small (for example, 0.1 mg / L or less), the amount of iodine produced by oxidation by the iodide salt will be small. In such cases, the water treatment agent composition may further contain iodine.

[0064] The iodide salt contained in the water treatment agent composition is an inorganic salt of iodine, and examples include sodium iodide, potassium iodide, lithium iodide, copper iodide, zinc iodide, etc., with sodium iodide or potassium iodide being preferred from the viewpoint of cost and other factors. The water treatment agent composition may contain one type of iodide salt or two or more types.

[0065] There are no particular restrictions on the type of water used, but tap water, purified water, etc. are acceptable.

[0066] 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 is a concern that the cost of using the chemicals will increase due to the increased amount used, including transportation, storage, and addition of the chemicals. The upper limit of the iodide salt content is, for example, 56% by mass or less.

[0067] When a 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. If the iodine content is less than 3% by mass, the bactericidal effect may be insufficient. The upper limit for the iodine content is, for example, 30% by mass or less.

[0068] The water treatment agent composition may further contain an alkaline agent. The alkaline agent should be able to raise the pH of the solution, and 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. Of these, hydroxides such as sodium hydroxide, potassium hydroxide, and calcium hydroxide are preferred from the viewpoint of safety and manufacturing cost, and sodium hydroxide or potassium hydroxide are more preferred.

[0069] Furthermore, from the viewpoint of storage stability and other factors, it is preferable that the composition contains 0.01% by mass or more of the alkaline agent, and more preferably 0.1% by mass or more. The upper limit of the alkaline agent content is, for example, less than 10% by mass. [Examples]

[0070] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0071] <Example 1, Comparative Example 1> Under the following test conditions, hypochlorous acid was added to the feedwater (water to be treated) of the reverse osmosis membrane system to a concentration of 5 mg / L as free chlorine. Potassium iodide was added after the high-pressure RO pump so that the iodide ions were 0.5 mol, 1.0 mol, and 2.0 mol relative to the free chlorine. Free chlorine was measured using a HACH DR3900 spectrophotometer. The results are shown in Table 2.

[0072] (Test conditions) • Testing equipment: Reverse osmosis membrane element testing apparatus • Supply pressure: 0.2~0.35MPa • Water supply: Sagamihara well water (dechlorinated, bacterial count 2 x 10) 3 CFU / mL) ·Water temperature: 16~19℃ pH: 7.3~7.7 • Hypochlorous acid concentration: Added to the treated water so that it becomes 5 mg / L as free chlorine. • Potassium iodide: 99.8% potassium iodide manufactured by Godo Shigen Co., Ltd. • Reverse osmosis membrane: Nitto Denko Corporation, 4-inch reverse osmosis membrane element (ESPA2) ·Water amount: Concentrated water 500L / h, permeate water 125L / h

[0073] [Table 2]

[0074] In Comparative Example 1, when an aqueous potassium iodide solution was added to a solution containing 0.5 mol of iodide ions relative to free chlorine and the system was operated, the EC rejection rate (rejection rate due to electrical conductivity) decreased from 98% to 90% after 1000 hours of operation, and the ionic silica rejection rate decreased from 98% to 85%. This indicated that the oxidizing power derived from hypochlorous acid that remained unreduced by the iodide ions caused oxidative degradation of the reverse osmosis membrane. Example 1- 1, 1- In experiment 2, potassium iodide aqueous solution was added to achieve concentrations of 1.0 mol and 2.0 mol of iodide ions relative to free chlorine, respectively, and the system was operated. After 1000 hours of operation, there was almost no change in the EC rejection rate and the ionic silica rejection rate, indicating that there was almost no degradation of the reverse osmosis membrane. Furthermore, under all conditions, there was almost no increase in the differential pressure of the water flow, indicating that a sufficient slime suppression effect was obtained.

[0075] <Example 2, Reference Example 1> Using the water treatment apparatus shown in Figure 1, under the following test conditions, (free iodine in the treated water (mg) / The free iodine CT value (mg / L·h), expressed as (L) × (iodide ion addition time (h)), was varied as shown in Table 3, and the treatment was carried out. The results are shown in Table 3.

[0076] (Test conditions) • Test water: Sagamihara well water (dechlorinated, bacterial count 2 x 10) 3 CFU / mL) • Chemical: Water treatment agent composition containing free iodine (potassium iodide content: 20% by mass) pH: 7.0 • Reverse osmosis membranes: Nitto Denko ES20, ESPA2, LFC3; Toray TML10D

[0077] [Table 3]

[0078] It was found that by reducing the CT value to 1.25 or less, the concentration of free iodine in the permeate water can be suppressed, thereby preventing deterioration of the permeate water quality. In Reference Example 1, the iodine was added under conditions corresponding to the free iodine CT value in Patent Document 1.

[0079] <Example 3, Reference Example 2> 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 I-Boy wide-mouth bottle (AS ONE) while stirring with a stirrer under the following storage conditions, and the residual percentage of total chlorine after a predetermined time was calculated. The results are shown in Table 4. Example 3: Close the lid and seal tightly. Reference Example 2-1: Remove the lid and open the top. Reference Example 2-2: Remove the lid and aerate with air.

[0080] [Table 4]

[0081] In Example 3, 100% of the total chlorine remained even after 1140 minutes, but in Reference Example 2-1, the residual rate of total chlorine decreased after 10 minutes and became 0 after 1140 minutes. In Reference Example 2-2, similar to Reference Example 2-1, the residual rate of total chlorine decreased after 10 minutes, and the residual rate of total chlorine was lower than in Reference Example 2-1. Thus, it was found that in a closed system, the decrease in total chlorine is almost negligible, and sufficient sterilization effect is maintained.

[0082] <Example 4, Comparative Example 2, Reference Example 3> Under the following test conditions, sodium hypochlorite was added to test water to achieve a free chlorine concentration of 1 mg / L, and potassium iodide was added to a concentration of 0.1 to 10 mol relative to the amount of free chlorine. The oxidation-reduction potential (ORP) was measured using a portable ORP meter (TOA DKK, RM-30P model). The results are shown in Table 5. There was almost no increase or decrease in free chlorine concentration before and after the addition of potassium iodide.

[0083] (Test conditions) • Test water: Sagamihara well water (dechlorinated) • pH: 7.0 (adjusted after adding sodium hypochlorite)

[0084] [Table 5]

[0085] In Reference Example 3, when potassium iodide was not added, the ORP of test water containing 1 mg / L of sodium hypochlorite as free chlorine was 754 mV. In Comparative Examples 2-1 and 2-2, when potassium iodide was added so that the iodide ions were 0.1 mol and 0.5 mol, respectively, relative to free chlorine, the ORPs were high, at 708 mV and 686 mV, respectively, indicating the possibility of degrading the reverse osmosis membrane. In Examples 4-1, 4-2, and 4-3, when potassium iodide was added so that the iodide ions were 1.0 mol, 2.0 mol, and 3.0 mol, respectively, relative to free chlorine, the ORPs were low, at 546 mV, 516 mV, and 507 mV, respectively, indicating a low possibility of degrading the reverse osmosis membrane. Therefore, it is preferable to control the amount of iodide ions added so that the oxidation-reduction potential (ORP) of the treated water is 550 mV or less.

[0086] As described above, the examples demonstrate that in the reverse osmosis membrane treatment of water to be treated containing at least one of a chlorine-based oxidizing agent and a bromine-based oxidizing agent, it was possible to suppress the deterioration of the reverse osmosis membrane and suppress slime formation.

[0087] <Examples 5, 6> Sodium hypochlorite (free chlorine concentration of 1 mg / L) and potassium iodide (1.5 mol relative to the amount of free chlorine) were mixed in pure water, and the oxidation-reduction potential (ORP) was measured over time. In Example 5, potassium iodide was added to the sodium hypochlorite solution, and in Example 6, sodium hypochlorite was added to the potassium iodide solution. The oxidation-reduction potential was measured using a portable ORP meter (TOA DKK, model RM-30P). The results are shown in Figure 4.

[0088] In Example 5, the redox potential before mixing was 700mV or higher, but it gradually decreased after mixing, and after 15 seconds or more, the redox potential stabilized at around 550mV. In Example 6, the redox potential before mixing was 400mV or lower, but it gradually increased after mixing, and after 15 seconds or more, the redox potential stabilized at around 550mV.

[0089] As described above, in both Examples 5 and 6, the oxidation-reduction potential settles around 550 mV, but in Example 5, a high oxidation-reduction potential is maintained until around 15 seconds. For this reason, when injecting iodide ions after injecting hypochlorous acid, it is preferable to set the injection point so that the time it takes for the iodide ions to reach the reverse osmosis membrane is 15 seconds or more. When injecting sodium hypochlorite after injecting iodide ions, there are no particular restrictions on setting the injection point. [Explanation of Symbols]

[0090] 1 water treatment device, 10 tanks to be treated, 12 membrane filtration device, 14 reverse osmosis membrane device, 16 pump, 18, 20 piping for water to be treated, 22 piping for membrane filtered water, 24 piping for permeate water, 26 piping for concentrated water, 28 piping for additives, 30 piping for reducing agent addition.

Claims

1. The process includes a reverse osmosis membrane treatment step in which a reverse osmosis membrane is used to obtain concentrated water and permeate from the water to be treated. To the water to be treated, which contains at least one of hypochlorous acid, hypobromous acid, and their salts, 1 mol or more of iodide ions are added per 1 mol of free chlorine and free bromine in the water to be treated, and the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the water to be treated with the added iodide ions × the time of iodide ion addition (h), is 1.25 (mg / L·h) or less, or To the water to be treated containing iodide ions, at least one of hypochlorous acid, hypobromous acid, and their salts is added such that the amount of free chlorine and free bromine is 1 mol or less per 1 mol of iodide ions in the water to be treated, and the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the water to be treated to which at least one of the hypochlorous acid, hypobromous acid, and their salts has been added × the addition time (h) of at least one of the hypochlorous acid, hypobromous acid, and their salts, is 1.25 (mg / L·h) or less. A water treatment method characterized by the following:

2. A water treatment method according to claim 1, A water treatment method characterized in that, when adding the iodide ions to the water to be treated which contains hypochlorous acid, hypobromous acid, and salts thereof, the time from when the iodide ions are added until they reach the reverse osmosis membrane is set to 15 seconds or more.

3. A membrane filtration process in which the water to be treated is subjected to membrane filtration using a separation membrane, A reverse osmosis membrane treatment step is performed using a reverse osmosis membrane to obtain concentrated water and permeate from the membrane-filtered water obtained in the membrane filtration step, Includes, In the aforementioned membrane filtration process, to the treated water containing at least one of hypochlorous acid, hypobromous acid, and their salts, 1 mol or more of iodide ions are added per 1 mol of free chlorine and free bromine in the treated water, and the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which the iodide ions have been added × the time of iodide ion addition (h), is 1.25 (mg / L·h) or less, or In the membrane filtration process, at least one of hypochlorous acid, hypobromous acid, and their salts is added to the treated water containing iodide ions in such a way that the amount of free chlorine and free bromine is 1 mol or less per 1 mol of iodide ions in the treated water, and the free iodine CT value, expressed as the free iodine concentration (mg / L) generated in the treated water to which at least one of the hypochlorous acid, hypobromous acid, and their salts has been added × the addition time (h) of at least one of the hypochlorous acid, hypobromous acid, and their salts, is 1.25 (mg / L·h) or less. A water treatment method characterized by the following:

4. A water treatment method according to claim 3, A water treatment method characterized in that, when adding the iodide ions to the water to be treated which contains at least one of the hypochlorous acid, hypobromous acid, and salts thereof, the time from the addition of the iodide ions until they reach the reverse osmosis membrane is set to 15 seconds or more.

5. A water treatment method according to any one of claims 1 to 4, A water treatment method characterized in that, when adding iodide ions to the water to be treated which contains at least one of the hypochlorous acid, hypobromous acid, and salts thereof, the process from the point where the iodide ions are added to the reverse osmosis membrane treatment step is carried out in a closed system.

6. A water treatment method according to any one of claims 1 to 5, A water treatment method characterized by adding iodide ions to the water to be treated, which contains at least one of the hypochlorous acid, hypobromous acid, and salts thereof, in such a way that the amount of iodide ions added is controlled so that the oxidation-reduction potential of the water to be treated after the addition of iodide ions is 550 mV or less.

7. Used in the water treatment method according to any one of claims 1 to 6, A water treatment agent composition characterized by containing water and an iodide salt.

8. A water treatment agent composition according to claim 7, A water treatment agent composition characterized by further containing iodine.

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