Water treatment method and water treatment device

Abstract

The present invention provides a water treatment method and a water treatment device capable of improving the decomposition rate of an organic fluorine compound such as PFAS in water.　In an example, a water treatment method includes: a mixing step for mixing water to be treated (W1) containing PFAS with a decomposition agent to obtain additive water (W2); and an ultraviolet irradiation step for irradiating the additive water (W2) with ultraviolet rays to obtain UV-treated water. In the mixing step, the additive water (W2) is mixed with a decomposition-promoting catalyst, and the decomposition-promoting catalyst is a compound having one or both of a group 2 metal element and a group 13 metal element in the periodic table.

Description

Water treatment method and water treatment device

[0001] The present invention relates to a water treatment method and a water treatment device. This application claims priority based on Japanese Patent Application No. 2024-84696, filed with the Japan Patent Office on May 24, 2024, the contents of which are incorporated herein by reference.

[0002] Among organic fluorine compounds, there are concerns about the environmental impact of compounds known as PFAS. PFAS is a general term for perfluoroalkyl and polyfluoroalkyl compounds, and it is said that more than 10,000 types of substances fall into the PFAS category. Two representative examples of PFAS are PFOS (perfluorooctanesulfonic acid) and PFOA (perfluorooctanoic acid). PFOS and PFOA are particularly difficult to decompose in nature or in the body, and are known to easily accumulate in the environment, such as soil.

[0003] While some PFAS are buried in landfills or released into the atmosphere, the majority are said to be released into the hydrosphere via wastewater. Therefore, concerns are growing about the impact of PFAS, which are difficult to decompose, on the aquatic environment. In April 2020, Japan established a provisional target value for the combined concentration of PFOS and PFOA in tap water of 50 ng / L or less. Each business operator is implementing water quality management and purification processes in compliance with this target value.

[0004] Several methods for treating PFAS in water have been proposed (for example, Patent Document 1 and Non-Patent Document 1). Patent Document 1 proposes a method for decomposing fluorine-containing organic acids in an aqueous medium in the presence of hydrogen peroxide and / or ozone. Patent Document 1 also proposes a method for treating PFAS in water using Fenton's reagent (H ２ O ２ +FeSO ４and a method for decomposing fluorine-containing organic acid compounds by applying external energy such as ultraviolet light. Non-Patent Document 1 discloses adding potassium persulfate (or potassium iodide (KI)) as a decomposing agent to test water containing PFOA or PFOS, followed by irradiation with ultraviolet light using a germicidal lamp or a low-pressure mercury lamp.

[0005] Japanese Patent Application Laid-Open No. 2006-169146

[0006] Environmental Sanitation Engineering Research Vol. 25 No. 3 (2011) P. 70-P. 73

[0007] However, even if the methods described in Patent Document 1 and Non-Patent Document 1 are adopted, it is still difficult to decompose organic fluorine compounds such as PFAS. Therefore, in order to further increase the decomposition rate of PFAS, further improvement of the conventional methods is desired.

[0008] The present invention provides a water treatment method and a water treatment apparatus that improve the decomposition rate of organic fluorine compounds such as PFAS in water.

[0009] The present inventors have investigated the use of hydrated electrons (e), which are powerful reducing agents generated by ultraviolet irradiation, in order to improve the decomposition rate of organic fluorine compounds. ａｑ － As a result, the inventors have mainly found the following technical means 1, 2, and 3, and have completed the present invention.

[0010] Technical Means 1: By bringing negatively charged hydrated electrons and negatively charged PFAS close to the surface of a positively charged catalyst, the decomposition reaction is promoted. Technical Means 2: By increasing the pH, H, which is an element that interferes with hydrated electrons, is removed. ＋ Technical measure 3: Reducing dissolved oxygen (DO), which is an element that interferes with hydrated electrons.

[0011] Preferred embodiments include, but are not limited to, the following. [1] A water treatment method for purifying water to be treated that contains organic fluorine compounds, comprising: a mixing step of mixing the water to be treated with a decomposition agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein in the mixing step, the added water is mixed with a decomposition-promoting catalyst, and the decomposition-promoting catalyst is a compound containing either or both of a metal element from Group 2 and a metal element from Group 13 of the periodic table. [2] The water treatment method according to [1], wherein in the mixing step, the amount of the decomposition agent added is adjusted so that the concentration of the decomposition agent is 800 to 80,000 mmol / L relative to 1 mmol / L of the organic fluorine compounds. [3] The water treatment method according to [1] or [2], wherein in the mixing step, the amount of the decomposition-promoting catalyst is adjusted so that the concentration of the decomposition-promoting catalyst is 20 to 2,000 mmol / L relative to 1 mmol / L of the organic fluorine compounds. [4] The water treatment method according to any one of [1] to [3], wherein the metal element in the decomposition-promoting catalyst is one or more selected from the group consisting of magnesium, calcium, and aluminum. [5] The water treatment method according to any one of [1] to [4], wherein the temperature of the added water in the ultraviolet irradiation step is 10°C to 80°C. [6] The water treatment method according to any one of [1] to [5], wherein the wavelength of the ultraviolet light in the ultraviolet irradiation step is 160 to 410 nm. [7] The water treatment method according to any one of [1] to [6], wherein the pH of the added water is adjusted to 10 or higher. [8] The water treatment method according to any one of [1] to [7], wherein the dissolved oxygen content of the UV-treated water is adjusted to 3.0 mg / L or less. [9] The water treatment method according to any one of [1] to [8], further comprising a concentration step of concentrating the organofluorine compounds in the water to be treated before the mixing step.

[0012]

[10] A water treatment device comprising: a decomposition agent mixing means for mixing water to be treated containing organic fluorine compounds with a decomposing agent to obtain added water, a catalyst adding means for adding a decomposition-promoting catalyst to the added water, and an ultraviolet irradiating means for irradiating the added water to which the decomposition-promoting catalyst has been added with ultraviolet light, wherein the decomposition-promoting catalyst is a compound having either or both of a metal element from Group 2 and a metal element from Group 13 of the periodic table.

[11] The water treatment device according to

[10] , wherein the catalyst adding means comprises: a catalyst tank for storing the decomposition-promoting catalyst, a mixing tank for storing the added water, a catalyst supply pipe connecting the mixing tank and the catalyst tank, a catalyst supply valve provided in the catalyst supply pipe, and a catalyst supply pump provided in the catalyst supply pipe.

[12] The water treatment device according to

[11] , further comprising catalyst control means for controlling either or both of an aperture of the catalyst supply valve and an output of the catalyst supply pump in order to control the amount of the decomposition-promoting catalyst added.

[13] The water treatment device according to

[10] or

[11] , further comprising a cooling means for cooling the ultraviolet light source.

[14] The water treatment device according to any one of

[10] to

[13] , further comprising a concentrating means for concentrating the organic fluorine compounds in the water to be treated, located upstream of the decomposing agent mixing means.

[15] The water treatment device according to

[14] , wherein the concentrating means is an adsorption tower filled with an organic fluorine compound adsorbent and a regeneration system thereof, or a nanofiltration membrane, a reverse osmosis membrane, and a concentrate collection system thereof.

[0013]

[16] A water treatment method for purifying water to be treated that contains organic fluorine compounds, the water treatment method comprising: a mixing step of mixing the water to be treated with a decomposing agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein the pH of the added water is adjusted to 10 or more.

[17] A measured pH value (pH ＭＥＡＳＵＲＥ ) based on the pH target value (pH ＧＯＡＬ

[16] The water treatment method according to

[16] , wherein the amount of pH adjuster added is controlled so that the pH of the treated water is 3.0 mg / L or less.

[18] The water treatment method according to

[17] , wherein the pH adjuster is added to the added water after mixing the water to be treated with the decomposition agent and before performing the ultraviolet irradiation step.

[19] The water treatment method according to any one of

[16] to

[18] , wherein the dissolved oxygen in the UV-treated water is adjusted to 3.0 mg / L or less.

[0014]

[20] A water treatment device comprising: a decomposition agent mixing means for mixing water to be treated containing an organic fluorine compound with a decomposition agent to obtain added water; a pH control means for controlling the amount of pH adjuster added; and an ultraviolet irradiation means for irradiating the added water to which the pH adjuster has been added with ultraviolet light to obtain UV-treated water.

[21] The water treatment device according to

[20] , further comprising: a mixing tank for storing the added water; and a pH meter provided in the mixing tank.

[22] The water treatment device according to

[21] , further comprising: a pH adjuster addition means for adding a pH adjuster to the mixing tank, the pH adjuster addition means comprising: a pH adjuster tank for storing the pH adjuster, a pH adjuster supply pipe connecting the pH adjuster tank and the mixing tank, a pH adjuster supply valve provided in the pH adjuster supply pipe, and a pH adjuster supply pump provided in the pH adjuster supply pipe.

[23] The pH control means controls the pH of the added water to a target pH value (pH ＧＯＡＬ a storage unit for storing in advance the pH measurement value (pH ＭＥＡＳＵＲＥ ) and a receiving unit that receives the pH measurement value (pH ＭＥＡＳＵＲＥ ) to the pH target value (pH ＧＯＡＬ and a control unit that controls either one or both of the opening degree of the pH adjuster supply valve and the output of the pH adjuster supply pump so that the pH adjuster supply valve is opened.

[0015]

[24] A water treatment method for purifying water to be treated that contains organic fluorine compounds, comprising: a mixing step of mixing the water to be treated with a decomposing agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein the dissolved oxygen content of the UV-treated water is adjusted to 3.0 mg / L or less.

[25] The DO measurement value (DO ＭＥＡＳＵＲＥ ) based on the DO target value (DO ＧＯＡＬ The water treatment method according to

[24] , wherein the dissolved oxygen in the UV-treated water is adjusted so that the water content is 0.01%.

[0016]

[26] A water treatment device comprising: a decomposing agent mixing means for mixing water to be treated containing an organic fluorine compound with a decomposing agent to obtain added water; an ultraviolet irradiation means for irradiating the added water with ultraviolet light to obtain UV-treated water; and a DO reducing means for reducing the dissolved oxygen in the UV-treated water.

[27] The water treatment device according to

[26] , further comprising a DO control means for controlling the operation of the DO reducing means.

[28] The water treatment device according to

[27] , wherein the DO control means adjusts and controls the dissolved oxygen in the UV-treated water to 3.0 mg / L or less.

[29] A DO meter provided in a treated water pipe through which the UV-treated water flows; and a DO measurement value (DO ＭＥＡＳＵＲＥ The water treatment device according to

[27] or

[28] further comprises a DO transmitter that transmits a DO target value (DO ＧＯＡＬ ) and a storage unit for storing the DO measurement value (DO ＭＥＡＳＵＲＥ ), and a receiving unit that receives the DO measurement value (DO ＭＥＡＳＵＲＥ ) to the DO target value (DO ＧＯＡＬ

[31] The water treatment device according to any one of

[27] to

[29] , further comprising: a control unit that controls the operation of the DO reducing means so that the DO is equal to the DO measured value (DO ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ

[32] The water treatment device according to any one of

[27] to

[30] , wherein the DO reduction means is an oxygen-consuming chemical supply means that supplies an oxygen-consuming chemical to the added water, and the DO control means controls the DO measurement value (DO ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ

[33] The water treatment device according to any one of

[27] to

[30] , wherein the DO reduction means is a pressure reduction means for reducing the pressure inside the mixing tank so that the mixing tank storing the added water is sealed, and the DO control means is a means for controlling the DO measurement value (pH ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ

[34] The water treatment device according to any one of

[27] to

[30] , wherein the DO reduction means is a degassing means for degassing the added water to remove dissolved oxygen, and the DO control means controls the DO measurement value (pH ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ The water treatment device according to any one of

[27] to

[30] , wherein the output of the degassing means is controlled so that the water concentration becomes equal to or greater than the predetermined value.

[0017] According to the present invention, the decomposition rate of organic fluorine compounds such as PFAS in water is improved.

[0018] Fig. 1 is a schematic diagram showing an example of a water treatment device, Fig. 2 is a schematic diagram showing another example of a water treatment device, and Fig. 3 is a schematic diagram showing another example of a water treatment device.

[0019] The meanings of the terms are as follows: An organic fluorine compound is a general term for organic compounds having one or more bonds between a carbon atom (C) and a fluorine atom (F). For example, organic compounds in which a fluorine atom is directly bonded to carbon are included in the organic fluorine compounds. In organic fluorine compounds, fluorine atoms are generally bonded as substituents to a carbon skeleton such as an alkyl chain or aromatic ring. "PFAS" means either or both of a perfluoroalkyl compound and a polyfluoroalkyl compound, and is a general term for perfluoroalkyl compounds and polyfluoroalkyl compounds. "PFAS concentration" means the total concentration of perfluoroalkyl compounds and polyfluoroalkyl compounds.

[0020] "Primary side" means the upstream side in the direction of flow of the water to be treated and the treated water. "Secondary side" means the downstream side in the direction of flow of the water to be treated and the treated water. Therefore, the water to be treated and the treated water flow from the primary side to the secondary side. "To" indicating a numerical range means that the numerical values ​​written before and after it are included as the lower and upper limits.

[0021] The treatment target of the water treatment device and water treatment method is not particularly limited as long as it contains PFAS. Examples of the water to be treated include groundwater, surface water, industrial wastewater, and domestic wastewater. In addition, the water to be treated may be concentrated water generated when PFAS-containing water is treated with a separation membrane (nanofiltration membrane, reverse osmosis membrane, etc.), or regenerated wastewater generated when PFAS-containing water is treated with an ion exchange resin and then the ion exchange resin is regenerated.

[0022] The PFAS concentration varies depending on the water source of the water to be treated. The PFAS concentration of the water to be treated is not particularly limited, but for example, 1×10 －１１ mol / L or more, and may be 1×10 －９ mol / L or more, and may be 1×10 －７ mol / L or more, and may be 1×10 －５ It may be mol / L or more.

[0023] In addition to organic fluorine compounds such as PFAS, the water to be treated may further contain impurities such as organic matter, ammonia nitrogen, anions such as bicarbonate ions, nitrate ions, sulfate ions, and chloride ions, cations such as iron ions, manganese ions, calcium ions, and magnesium ions, and bacteria.

[0024] Hereinafter, several embodiments of the present invention will be described with reference to the drawings. However, the following description is for representative examples, and the present invention is not limited to the following description.

[0025] The dimensional ratios in the drawings are for the convenience of explanation and may differ from the actual ones. In the following drawings, the same components are indicated by the same reference numerals, and descriptions of overlapping components may be omitted.

[0026] [First embodiment] (Water treatment device) Fig. 1 shows a preferred example of a water treatment device. The water treatment device 1A in Fig. 1 is for purifying water to be treated W1 containing organic fluorine compounds such as PFAS. The water treatment device 1A includes a water to be treated pipe L1, a mixing tank 2, a decomposing agent tank 3, a decomposing agent pipe L2, a decomposing agent pump P2, a decomposing agent supply valve V2, an added water pipe L3, an added water pump P1, an added water valve V1, an ultraviolet irradiation means 4, a decomposition-promoting catalyst tank 5, a decomposition-promoting catalyst pipe L5, a decomposition-promoting catalyst pump P3, a decomposition-promoting catalyst supply valve V3, a UV-treated water pipe L4, and a catalyst control means 10.

[0027] The water to be treated pipe L1 is used to supply the water to be treated W1 containing PFAS to the mixing tank 2. A first end of the water to be treated pipe L1 is connected to a supply source of the water to be treated W1 (not shown), and a second end of the water to be treated pipe L1 is connected to the mixing tank 2. The water to be treated W1 is supplied into the mixing tank 2 via the water to be treated pipe L1.

[0028] A decomposing agent is stored in the decomposing agent tank 3. The decomposing agent is not particularly limited as long as it is a compound capable of generating radicals or hydrated electrons that contribute to the decomposition reaction of PFAS, or a compound capable of reducing a side reaction in which the radicals or hydrated electrons react with something other than PFAS and are consumed.

[0029] Decomposers are compounds that generate either radicals or hydrated electrons or both when exposed to ultraviolet light. Examples include oxidative decomposers, reductive decomposers, and photocatalysts. Oxidative decomposers are compounds that release hydroxyl radicals when exposed to ultraviolet light. Reductive decomposers are compounds that release electrons when exposed to ultraviolet light.

[0030] The decomposing agent preferably has an absorption spectrum for ultraviolet light having a wavelength in the range of 160 to 410 nm.

[0031] Examples of oxidative decomposition agents include oxygen acids and salts thereof such as sodium hypochlorite and sodium persulfate, oxygen, ozone, hydrogen peroxide, and halogens such as chlorine. However, the oxidative decomposition agents are not limited to these examples. One type of oxidative decomposition agent may be used alone, or two or more types may be used in combination.

[0032] Examples of reducing decomposition agents include sodium sulfite, potassium iodide, ferrous chloride, ferrous sulfite, sulfuric acid, oxalic acid, etc. However, the reducing decomposition agents are not limited to these examples. One reducing decomposition agent may be used alone, or two or more reducing decomposition agents may be used in combination.

[0033] As a photocatalyst, TiO ２ , Ga ２ O ３ Examples of such heterogeneous photocatalysts include metal complexes.

[0034] The decomposing agent may be used alone or in combination of two or more. For example, a combination of sodium sulfite and potassium iodide is effective.

[0035] The decomposing agent may be a solid, liquid, or gas. From the viewpoint of increasing the decomposition rate of PFAS, a solid or liquid is preferable. There are no particular limitations on the solid PFAS decomposing agent as long as it is soluble in the water to be treated, but it may be dissolved or dispersed in the water to be treated W1. The decomposing agent may be fixed to the ultraviolet irradiation means 4 so as to come into contact with the water.

[0036] The decomposing agent pipe L2 is used to mix the decomposing agent with the water to be treated W1. A first end of the decomposing agent pipe L2 is connected to the tank 3, and a second end of the PFAS decomposing agent pipe L2 is connected to the mixing tank 2. The PFAS decomposition accelerator in the tank 3 is supplied to the mixing tank 2 via the decomposing agent pipe L2.

[0037] 1, the decomposer pipe L2 is provided with a decomposer pump P2 for controlling the amount of decomposer supplied from the tank 3. In another example, a valve for controlling the amount of decomposer supplied from the tank 3 may be provided in the decomposer pipe L2 instead of the decomposer pump P2, or may be provided in the decomposer pipe L2 together with the decomposer pump P2.

[0038] The water treatment device 1A includes a catalyst adding means for adding a decomposition-promoting catalyst to the added water W2. In the example shown in Fig. 1, the catalyst adding means includes a decomposition-promoting catalyst tank 5, a decomposition agent catalyst pipe L5, a decomposition-promoting catalyst pump P3, and a decomposition-promoting catalyst supply valve V3, but is not limited to this example.

[0039] The decomposition-promoting catalyst tank 5 stores a decomposition-promoting catalyst. The decomposition-promoting catalyst is a metal compound containing an element that dissolves in water to generate polyvalent cations. For example, the catalyst may be a catalyst that promotes the generation of hydroxyl radicals or electrons, or a catalyst that promotes the proximity of hydroxyl radicals or electrons to PFAS, and is not particularly limited.

[0040] For example, compounds having either or both of a metal element of Group 2 and a metal element of Group 13 of the periodic table can be mentioned. From the viewpoint of availability, the metal element in the decomposition-promoting catalyst is preferably one or more selected from the group consisting of magnesium, calcium, and aluminum. From the viewpoint of the PFAS decomposition rate, magnesium is more preferable. One type of decomposition-promoting catalyst may be used alone, or two or more types may be used in combination.

[0041] The decomposition agent catalyst pipe L5 is used to supply the decomposition agent catalyst to the water to be treated W1. A first end of the decomposition agent pipe L5 is connected to the tank 5, and a second end of the decomposition promotion catalyst pipe L5 is connected to the mixing tank 2. The decomposition promotion catalyst in the tank 5 is supplied into the mixing tank 2 via the decomposition promotion catalyst pipe L5.

[0042] The cracking promotion catalyst pipe L5 is provided with a cracking promotion catalyst pump P3 for controlling the amount of cracking promotion catalyst supplied from the tank 5. The output of the cracking promotion catalyst pump P3 and the opening of the cracking promotion catalyst supply valve V3 can be controlled by catalyst control means 10.

[0043] The mixing tank 2 is an example of a mixing means and is used to mix the water to be treated W1 with the decomposing agent. The water to be treated W1 and the decomposing agent, which are respectively supplied into the mixing tank 2, are mixed to obtain added water W2. A decomposition-promoting catalyst can be further added to the added water W2, and the added water W2 to which the decomposition-promoting catalyst has been added can be passed downstream through the added water pipe L3.

[0044] The added water pipe L3 is used to supply the decomposition agent-added water W2 to the ultraviolet irradiation means 4. A first end of the added water pipe L3 is connected to the mixing tank 2, and a second end of the added water pipe L3 is connected to the ultraviolet irradiation means 4. The added water pipe L3 is provided with an added water pump P1 and an added water valve V1.

[0045] The added water pump P1 is for supplying the decomposing agent-added water W2 to the ultraviolet irradiation means 4. The amount of the decomposing agent-added water W2 supplied to the ultraviolet irradiation means 4 may be changed by the output of the added water pump P1.

[0046] The added water valve V1 is used to change the amount of added water W2, to which a decomposition-promoting catalyst has been added, supplied to the ultraviolet irradiation means 4. For example, the amount of added water W2 supplied to the ultraviolet irradiation means 4 may be changed by changing the aperture of the added water valve V1. The aperture of the added water valve V1 may be set depending on the output of the ultraviolet light source or the target PFAS decomposition rate. The aperture of the added water valve V1 may be changed manually or automatically controlled by an electrical signal.

[0047] The ultraviolet irradiation means 4 has an ultraviolet light source (UV-LED light source, mercury lamp, etc.) not shown, a reaction flow path not shown, and a cooling flow path not shown. The ultraviolet irradiation means 4 may be installed inside the mixing tank 2 or may be installed downstream of the mixing tank 2. When installed inside the mixing tank 2, it is possible to perform ultraviolet irradiation while adding the decomposition agent and the decomposition-promoting catalyst.

[0048] The ultraviolet light source (not shown) of the ultraviolet light irradiation means 4 is for irradiating the decomposition agent-added water W2 with ultraviolet light in the range of 160 to 410 nm. The wavelength is preferably 180 nm or more, and more preferably 280 nm or less.

[0049] By irradiating the decomposition agent-added water W2 with ultraviolet light, the PFAS in the PFAS decomposition accelerator-added water W2 can be efficiently decomposed. By decomposing the PFAS in the decomposition agent-added water W2 by ultraviolet irradiation, UV-treated water W3 is obtained.

[0050] The UV-treated water pipe L4 is for collecting the UV-treated water W3 obtained by the ultraviolet irradiation means 4. The UV-treated water W3 can then be subjected to post-stage treatment as necessary to become, for example, drinking water, water for daily use, water for industrial use, etc. Drinking water means water that meets the water quality standard items and standard values ​​(51 items).

[0051] In order to cool the light source of the ultraviolet irradiation means 4, the water treatment device may be provided with a cooling means.

[0052] The water treatment device 1A may further include a concentrating means for concentrating organic fluorine compounds in the water to be treated, located upstream of the mixing means. The concentrating means is preferably an adsorption tower filled with an adsorbent for organic fluorine compounds and a regeneration system for the adsorption tower. The adsorption tower may be an ion exchange resin or activated carbon.

[0053] (Water Treatment Method) Next, an example of a water treatment method using the above-described water treatment device 1A will be described. First, the water to be treated W1 and a decomposing agent are supplied to the mixing tank 2. In the mixing tank 2, the water to be treated W1 and the decomposing agent are mixed to obtain decomposing agent-added water W2 (mixing step). After a decomposition-promoting catalyst is further added to the added water W2, the added water is supplied to the ultraviolet irradiation means 4 through the added water pipe L3.

[0054] In order to protect the hydrated electrons that cause the decomposition reaction of organic fluorine compounds, H ＋ It is preferable to reduce factors that interfere with hydrated electrons, such as H ＋ In order to reduce the pH of the PFAS decomposing agent, it is preferable to adjust the pH of the added water W2 to 10 or higher, and more preferably to adjust the pH to 10.5 or higher. Furthermore, from the viewpoint of the efficiency of generating hydrated electrons from the PFAS decomposing agent, it is preferable to adjust the pH of the added water W2 to 14 or lower, and more preferably to adjust the pH to 13 or lower. When the pH of the added water W2 is within the above-mentioned numerical range, the PFAS decomposition rate is improved.

[0055] The supply amount of the decomposing agent may be changed by the output of the decomposing agent pump P2. If a valve for controlling the supply amount of the decomposing agent from the tank 3 is provided in the decomposing agent pipe L2, the supply amount of the PFAS decomposition accelerator from the tank 3 may be changed by changing the opening degree of the valve. The opening degree of the valve may be changed depending on the PFAS concentration in the water to be treated W1 and the target PFAS decomposition rate.

[0056] The decomposing agent may be used alone or in combination of two or more. For example, a combination of sodium sulfite and potassium iodide is effective.

[0057] The total amount of decomposition agent added is preferably adjusted based on the concentration of the organic fluorine compound. For example, for 1 mmol / L of the organic fluorine compound, the total amount of decomposition agent added is preferably adjusted to 800 to 80,000 mmol / L, more preferably 1,600 to 40,000 mmol / L, and even more preferably 3,200 to 20,000 mmol / L.

[0058] The decomposition-promoting catalyst is a compound containing either or both of a metal element of Group 2 and a metal element of Group 13 of the periodic table. From the viewpoint of availability, the metal element in the decomposition-promoting catalyst is preferably one or more selected from the group consisting of magnesium, calcium, and aluminum. From the viewpoint of PFAS decomposition rate, magnesium is more preferred.

[0059] The cracking-promoting catalyst may be used alone or in combination of two or more kinds.

[0060] The total amount of decomposition-promoting catalyst added is preferably adjusted based on the concentration of the organic fluorine compound. For example, for 1 mmol / L of the organic fluorine compound, the total amount of decomposition-promoting catalyst added is preferably adjusted to 20 to 2000 mmol / L, more preferably 50 to 1000 mmol / L, and even more preferably 100 to 500 mmol / L.

[0061] When the water to be treated itself contains either or both of metal elements of Group 2 and metal elements of Group 13 of the periodic table, the total amount of decomposition-promoting catalyst to be added includes the amount of existing metal.

[0062] Next, the added water valve V1 is opened, and the decomposing agent-added water W2 is irradiated with ultraviolet light of 160 to 410 nm from an ultraviolet light source in the ultraviolet irradiation means 4 (ultraviolet irradiation step). The PFAS in the decomposing agent-added water W2 is decomposed by ultraviolet irradiation, thereby obtaining UV-treated water W3.

[0063] In order to protect the hydrated electrons that promote the decomposition reaction of organic fluorine compounds, it is preferable to reduce factors that interfere with the hydrated electrons, such as dissolved oxygen (DO). For example, it is preferable to adjust the dissolved oxygen (DO) of the UV-treated water W3 to 3.0 mg / L or less, more preferably 2.0 mg / L or less, and even more preferably 1.0 mg / L or less. If the dissolved oxygen (DO) of the UV-treated water W3 is within the above range, the PFAS decomposition rate is further improved.

[0064] (Mechanism of Action of the First Embodiment) In the first embodiment described above, the decomposition-promoting catalyst is further added to the added water W2 obtained by mixing the water to be treated W1 with the decomposing agent, and then ultraviolet light can be irradiated. Here, hydrated electrons (e ａｑ －The surface of the catalyst is negatively charged, and the surface of organic fluorine compounds such as PFAS is also often negatively charged. Therefore, it is thought that the decomposition reaction of PFAS can be accelerated because hydrated electrons and PFAS approach the decomposition-promoting catalyst, which has a positively charged surface, or because fluoride ions generated by the decomposition of PFAS bond with the decomposition-promoting catalyst.

[0065] Second Embodiment A preferred example of a water treatment device is shown in Figure 2. The water treatment device 1B shown in Figure 2 is for purifying water to be treated W1 containing organic fluorine compounds such as PFAS. The water treatment device 1B includes a water to be treated pipe L1, a mixing tank 2, a decomposing agent tank 3, a decomposing agent pipe L2, a decomposing agent pump P2, an added water pipe L3, an added water pump P1, an added water valve V1, an ultraviolet light irradiation means 4, a decomposition-promoting catalyst tank 5, a decomposition-promoting catalyst pipe L5, a decomposition-promoting catalyst pump P3, a pH adjuster tank 6, a pH adjuster supply pipe L6, a pH adjuster supply valve V5, a pH adjuster supply pump P4, a UV-treated water pipe L4, a pH meter 9, and a pH control means 11.

[0066] The water treatment device 1B in Fig. 2 is equipped with a pH adjuster adding means for adding a pH adjuster to the mixing tank 2. In the example shown in Fig. 2, the pH adjuster adding means includes a pH adjuster tank 6 for storing a pH adjuster, a pH adjuster supply pipe L6 connecting the pH adjuster tank 6 and the mixing tank 2, and a pH adjuster supply valve V5 and a pH adjuster supply pump P4 provided on the pH adjuster supply pipe L6, but is not limited to this example.

[0067] The pH meter 9 is provided in the mixing tank 2 and can measure the pH of the added water W2 in the mixing tank. ＭＥＡＳＵＲＥ ) to the pH control means 11.

[0068] The pH control means 11 controls the pH of the added water W2 to a target value (pH ＧＯＡＬ ) and a memory unit for storing the pH measurement value (pH ＭＥＡＳＵＲＥ ) and a receiving unit that receives the pH measurement value (pH ＭＥＡＳＵＲＥ ) to the target value (pH ＧＯＡＬ) may be provided with a control unit that controls either or both of the opening degree of the pH adjuster supply valve V5 and the output of the pH adjuster supply pump P4.

[0069] The water treatment device 1B of FIG. 2 preferably includes a catalyst adding means like the water treatment device 1A of FIG. 1, but does not necessarily have to include such a means.

[0070] In order to protect the hydrated electrons that promote the decomposition reaction of organic fluorine compounds, H ＋ It is preferable to reduce factors that interfere with hydrated electrons, such as H ＋ In order to reduce the pH, it is preferable to adjust the pH of the added water W2 to 10 or more, and more preferably to adjust the pH to 10.5 or more. Furthermore, it is preferable to adjust the pH of the added water W2 to 14 or less, and more preferably to adjust the pH to 13 or less. If the pH of the added water W2 is within the above range, the PFAS decomposition rate is further improved. When the pH adjustment step is performed, the measured pH value (pH ＭＥＡＳＵＲＥ ) based on the target value (pH ＧＯＡＬ It is preferable to control the amount of pH adjuster added so that the pH of the added water W2 is 1 / 2 or less. Furthermore, it is preferable to add the pH adjuster to the added water W2 after mixing the water to be treated with the decomposition agent and before performing the ultraviolet irradiation step.

[0071] (Mechanism of Action of Second Embodiment) In the second embodiment described above, the pH of the added water W2 obtained by mixing the water to be treated W1 with the decomposing agent can be adjusted before being irradiated with ultraviolet light. Hydrated electrons (e ａｑ － One of the factors that hinders ＋ By adjusting the pH of the added water W2 to a high level before ultraviolet irradiation, ＋ This can reduce the amount of hydrated electrons, and as a result, it is believed that the decomposition reaction of PFAS can be promoted.

[0072] [Third Embodiment] Figure 3 shows another preferred example of a water treatment device. The water treatment device 1C shown in Figure 3 is for purifying water to be treated W1 containing organic fluorine compounds such as PFAS. The water treatment device 1A includes a water to be treated pipe L1, a mixing tank 2, a decomposer tank 3, a decomposer pipe L2, a decomposer pump P2, an added water pipe L3, an added water pump P1, an added water valve V1, an ultraviolet irradiation means 4, a decomposition-promoting catalyst tank 5, a decomposition-promoting catalyst pipe L5, a decomposition-promoting catalyst pump P3, a UV-treated water pipe L4, a DO reduction means 8, a DO control means 12, and a DO meter 13.

[0073] The DO reducing means 8 is for reducing the dissolved oxygen in the UV-treated water, and is not particularly limited as long as it can perform this function. An example of the DO reducing means 8 is an inert gas supply means that supplies an inert gas to the mixing tank 2 that stores the added water W2. In the case of the inert gas supply means, the DO measured value (DO ＭＥＡＳＵＲＥ ) to the target value (DO ＧＯＡＬ The amount of inert gas supplied can be controlled so that the inert gas is 0.1% or less. As the inert gas, nitrogen or argon gas is preferred.

[0074] Another example of the DO reducing means 8 is an oxygen-consuming chemical supplying means for supplying an oxygen-consuming chemical to the added water W2. In the case of the oxygen-consuming chemical supplying means, the DO measurement value (pH ＭＥＡＳＵＲＥ ) to the target value (DO ＧＯＡＬ The supply amount of the oxygen-consuming agent can be controlled so that the oxygen-consuming agent satisfies the above-mentioned reducing decomposition agent, and sodium sulfite is more preferred as the oxygen-consuming agent.

[0075] Another example of the DO reducing means 8 is a pressure reducing means for reducing the pressure in the mixing tank 2 storing the added water W2 while the mixing tank 2 is sealed. In the case of the pressure reducing means, the DO measurement value (pH ＭＥＡＳＵＲＥ ) to the pH target value (DO ＧＯＡＬ The output of the pressure reducing means can be controlled so that the pressure is equal to or less than the pressure of the liquid. A vacuum pump is preferred as the pressure reducing means.

[0076] Another example of the DO reducing means 8 is a degassing means for degassing the dissolved oxygen in the added water W2. In the case of the degassing means, the DO measurement value (DO ＭＥＡＳＵＲＥ ) to the target value (DO ＧＯＡＬ The output of the degassing means can be controlled so that the degassing membrane is preferably used as the degassing means.

[0077] The water treatment device 1C includes a DO control means 12 that controls the operation of the DO reduction means 8. The DO control means 12 may control the dissolved oxygen in the UV-treated water W3 to be 3.0 mg / L or less.

[0078] The DO meter 13 is provided in the treated water pipe L4. The DO meter 13 can measure the DO of the UV-treated water W3 flowing through the treated water pipe L4. The DO meter 13 outputs the DO measurement value (DO ＭＥＡＳＵＲＥ ) to the DO control means 12.

[0079] The DO control means 12 controls the DO target value (DO ＧＯＡＬ ) and a storage unit for storing the DO measurement value (DO ＭＥＡＳＵＲＥ ) and a receiving unit that receives the DO measurement value (DO ＭＥＡＳＵＲＥ ) to the DO target value (DO ＧＯＡＬ ) and a control unit that controls the operation of the DO reduction unit 8 so that

[0080] (Mechanism of Action of the Third Embodiment) In the third embodiment described above, the dissolved oxygen (DO) in the UV-treated water W3 can be reduced by irradiating the added water W2 obtained by mixing the water to be treated W1 with a decomposing agent from an ultraviolet light source in the range of 160 to 410 nm. The dissolved oxygen (DO) in the UV-treated water W3 is reduced by hydration electrons (e), which are a strong reducing agent generated by ultraviolet irradiation. ａｑ － One of the factors that hinders the decomposition of PFAS is dissolved oxygen (DO). By reducing the dissolved oxygen (DO) in the UV-treated water W3, it is possible to protect hydrated electrons. As a result, it is believed that the decomposition reaction of PFAS can be promoted.

[0081] Hereinafter, the embodiments will be described in more detail with reference to examples, but the present invention is not limited to the following description in any way.

[0082] [Example A1] PFAS-containing water was treated using the water treatment device 1A in Figure 1. Detailed treatment conditions are as follows. - Ultraviolet irradiation device: PAQ-9D manufactured by Nikkiso Co., Ltd. - PFAS-containing water to be treated: PFOA, a representative PFAS, was used in pure water, and a PFOA standard reagent was added so that the PFOA concentration was 1 ppm. The decomposition agents added were sodium sulfite at 10 mM and potassium iodide at 10 mM. The decomposition-promoting catalyst added was calcium chloride at 0.5 mM. - Water temperature: room temperature, 23°C - PFOA analysis method: LC-MS / MS (liquid chromatography and mass spectrometry)

[0083] Example A2 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example A1, except that the decomposition-promoting catalyst was changed to magnesium chloride.

[0084] Example A3 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example A1, except that the decomposition-promoting catalyst was changed to aluminum chloride.

[0085] Comparative Example A1 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example A1, except that the decomposition-accelerating catalyst was not added.

[0086] Comparative Example A2 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example A1, except that the decomposition-promoting catalyst was changed to iron chloride.

[0087] Example B1 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example A1, except that the pH of the added water was adjusted to 13.0.

[0088] Example B2 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example B1, except that the decomposition-promoting catalyst was changed to magnesium chloride.

[0089] Example B3 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example B1, except that the decomposition-promoting catalyst was changed to aluminum chloride.

[0090] Comparative Example B1 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example B1, except that the decomposition-promoting catalyst was not added.

[0091] Comparative Example B2 PFAS-containing water was treated using the water treatment device 1A of Fig. 1. The PFAS-containing water was treated under the same conditions as in Example B1, except that the decomposition-promoting catalyst was changed to iron chloride.

[0092]

[0093]

[0094] As described above, because the hydrated electrons and PFAS approach the decomposition-promoting catalyst with a positive surface, when a compound having either or both of a Group 2 metal element and a Group 13 metal element was added (Examples A1 to A3 and Examples B1 to B3), the decomposition rate of PFOA was improved compared to when no decomposition-promoting catalyst was added (Comparative Examples A1 and B1) or when other metal salts were added (Comparative Examples A2 and B2).

[0095] [Example C1] PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example A1, except that the pH of the added water was adjusted to 10 without adding a decomposition-promoting catalyst.

[0096] Example C2 PFAS-containing water was treated using the water treatment device 1B of Fig. 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 10.5.

[0097] Example C3 PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 11.

[0098] Example C4 PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 12.

[0099] Example C5 PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 13.

[0100] Example C6 PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 13.4.

[0101] Comparative Example C1 PFAS-containing water was treated using the water treatment device 1B of Figure 2. The PFAS-containing water was treated under the same conditions as in Example C1, except that the pH was adjusted to 9.

[0102]

[0103] As described above, hydrated electrons (e ａｑ － ) is one of the factors that hinder ＋ Since the pH was reduced, the decomposition rate of PFOA was higher when the pH was adjusted to 10 or higher (Examples C1 to C4) than when the pH was low (Comparative Example C1). － An excessive increase in concentration reduced the efficiency of generating hydrated electrons from the decomposition agent, and there was a tendency for the PFOA decomposition rate to decrease slightly, but compared to Comparative Example 1, the PFOA decomposition rate still showed a high value.

[0104] Example D1 Using the water treatment device 1C of FIG. 3, PFAS-containing water was treated under the same conditions as in Comparative Example B1, except that the dissolved oxygen concentration of the UV-treated water treated with PFAS was adjusted to 0.0 mg / L.

[0105] Example D2 PFAS-containing water was treated using the water treatment device 1C of Figure 3. The PFAS-containing water was treated under the same conditions as in Example D1, except that 2 mM of a decomposition-promoting catalyst (calcium chloride) was added to the water to be treated and the dissolved oxygen concentration of the UV-treated water was 0.84 mg / L.

[0106] Example D3 PFAS-containing water was treated using the water treatment device 1C of Figure 3. The PFAS-containing water was treated under the same conditions as in Example D1, except that the dissolved oxygen concentration of the UV-treated water was 2.4 mg / L.

[0107] Comparative Example D1 PFAS-containing water was treated using the water treatment device 1C of Figure 3. The PFAS-containing water was treated under the same conditions as in Example D1, except that the dissolved oxygen concentration of the UV-treated water was 3.4 mg / L.

[0108]

[0109] As described above, hydrated electrons (e ａｑ － ), the dissolved oxygen (DO), which is one of the factors that interfere with the decomposition of PFOA, was reduced. Therefore, when the dissolved oxygen (DO) was reduced to 3.0 mg / L or less (Examples D1 to D3), the decomposition rate of PFOA was higher than when the DO was more than 3.0 mg / L (Comparative Example D1).

[0110] While the present invention has been described above with reference to specific embodiments, these embodiments are presented as examples and do not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways within the scope of the effects of the invention, and can be combined with features described in other embodiments within the scope of feasibility.

[0111] According to the present invention, the decomposition rate of organic fluorine compounds such as PFAS in water is improved.

[0112] REFERENCE SIGNS LIST 1 Water treatment device 2 Mixing tank (mixing means) 3 Decomposition agent tank 4 Ultraviolet light irradiation means 5 Decomposition promotion catalyst tank 6 pH adjuster tank 8 DO reduction means 9 pH meter 10 Catalyst control means 11 pH control means 12 DO control means 13 DO meter

Claims

1. A water treatment method for purifying water to be treated that contains organic fluorine compounds, comprising: a mixing step of mixing the water to be treated with a decomposition agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein in the mixing step, the added water is mixed with a decomposition-promoting catalyst, and the decomposition-promoting catalyst is a compound having either or both of a metal element from Group 2 and a metal element from Group 13 of the periodic table.

2. The water treatment method according to claim 1, wherein the amount of the decomposition agent added in the mixing step is adjusted so that the concentration of the decomposition agent is 800 to 80,000 mmol / L relative to 1 mmol / L of the organic fluorine compound.

3. The water treatment method according to claim 1 or 2, wherein in the mixing step, the amount of the decomposition-promoting catalyst is adjusted so that the concentration of the decomposition-promoting catalyst is 20 to 2,000 mmol / L relative to 1 mmol / L of the organic fluorine compound.

4. The water treatment method according to claim 1 or 2, wherein the metal element in the decomposition-promoting catalyst is one or more selected from the group consisting of magnesium, calcium and aluminum.

5. The water treatment method according to claim 1 or 2, wherein the temperature of the added water is 10°C to 80°C in the ultraviolet irradiation step.

6. The water treatment method according to claim 1 or 2, wherein in the ultraviolet irradiation step, the ultraviolet light has a wavelength of 160 to 410 nm.

7. The water treatment method according to claim 1 or 2, wherein the pH of the added water is adjusted to 10 or higher.

8. The water treatment method according to claim 1 or 2, wherein the dissolved oxygen content of the UV-treated water is adjusted to 3.0 mg / L or less.

9. The water treatment method according to claim 1 or 2, further comprising a concentration step of concentrating the organic fluorine compounds in the water to be treated before the mixing step.

10. A water treatment device comprising: a decomposition agent mixing means for mixing water to be treated containing organic fluorine compounds with a decomposition agent to obtain added water; a catalyst adding means for adding a decomposition-promoting catalyst to the added water; and an ultraviolet irradiation means for irradiating the added water to which the decomposition-promoting catalyst has been added with ultraviolet light, wherein the decomposition-promoting catalyst is a compound having either or both of a metal element from Group 2 and a metal element from Group 13 of the periodic table.

11. The water treatment device according to claim 10, wherein the catalyst adding means comprises: a catalyst tank that stores the decomposition-promoting catalyst; a mixing tank that stores the added water; a catalyst supply pipe that connects the mixing tank and the catalyst tank; a catalyst supply valve provided in the catalyst supply pipe; and a catalyst supply pump provided in the catalyst supply pipe.

12. The water treatment device according to claim 11, further comprising catalyst control means for controlling either or both of the opening of the catalyst supply valve and the output of the catalyst supply pump in order to control the amount of decomposition-promoting catalyst added.

13. The water treatment device according to claim 10 or 11, further comprising a cooling means for cooling the ultraviolet light source.

14. The water treatment device according to claim 10 or 11, further comprising a concentrating means for concentrating the organic fluorine compounds in the water to be treated, located upstream of the decomposing agent mixing means.

15. The water treatment device according to claim 14, wherein the concentrating means is an adsorption tower filled with an organic fluorine compound adsorbent and a regeneration system thereof, or a nanofiltration membrane, a reverse osmosis membrane and a concentrate collection system thereof.

16. A water treatment method for purifying water to be treated that contains organic fluorine compounds, comprising: a mixing step of mixing the water to be treated with a decomposing agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein the pH of the added water is adjusted to 10 or higher.

17. The pH value of the added water (pH ＭＥＡＳＵＲＥ ) based on the pH target value (pH ＧＯＡＬ The water treatment method according to claim 16, wherein the amount of pH adjuster added is controlled so that the pH value satisfies the above formula (1).

18. The water treatment method according to claim 17, wherein after the water to be treated and the decomposition agent are mixed, the pH adjuster is added to the added water before the ultraviolet irradiation step is performed.

19. The water treatment method according to claim 16 or 17, wherein the dissolved oxygen in the UV-treated water is adjusted to 3.0 mg / L or less.

20. A water treatment device comprising: a decomposition agent mixing means for mixing water to be treated containing organic fluorine compounds with a decomposition agent to obtain added water; a pH control means for controlling the amount of pH adjuster added; and an ultraviolet irradiation means for irradiating the added water to which the pH adjuster has been added with ultraviolet light to obtain UV-treated water.

21. The water treatment device according to claim 20, further comprising: a mixing tank for storing the added water; and a pH meter provided in the mixing tank.

22. The water treatment device of claim 21, further comprising a pH adjuster adding means for adding a pH adjuster to the mixing tank, wherein the pH adjuster adding means comprises a pH adjuster tank for storing the pH adjuster, a pH adjuster supply pipe connecting the pH adjuster tank and the mixing tank, a pH adjuster supply valve provided in the pH adjuster supply pipe, and a pH adjuster supply pump provided in the pH adjuster supply pipe.

23. The pH control means controls the pH of the added water to a target value (pH ＧＯＡＬ a storage unit for storing in advance the pH measurement value (pH ＭＥＡＳＵＲＥ ) and a receiving unit that receives the pH measurement value (pH ＭＥＡＳＵＲＥ ) to the pH target value (pH ＧＯＡＬ 23. The water treatment device according to claim 22, further comprising: a control unit that controls one or both of an opening degree of the pH adjuster supply valve and an output of the pH adjuster supply pump so that the pH adjuster supply valve opens and outputs the pH adjuster supply pump.

24. A water treatment method for purifying water to be treated that contains organic fluorine compounds, comprising: a mixing step of mixing the water to be treated with a decomposing agent to obtain added water; and an ultraviolet irradiation step of irradiating the added water with ultraviolet light to obtain UV-treated water, wherein the dissolved oxygen content of the UV-treated water is adjusted to 3.0 mg / L or less.

25. The DO measurement value of the UV-treated water (DO ＭＥＡＳＵＲＥ ) based on the DO target value (DO ＧＯＡＬ The water treatment method according to claim 24, wherein the dissolved oxygen in the UV-treated water is adjusted so that the dissolved oxygen in the UV-treated water is 0.01g / L or less.

26. A water treatment device comprising: a decomposition agent mixing means for mixing water to be treated containing organic fluorine compounds with a decomposition agent to obtain added water; an ultraviolet irradiation means for irradiating the added water with ultraviolet rays to obtain UV-treated water; and a DO reduction means for reducing the dissolved oxygen in the UV-treated water.

27. The water treatment device according to claim 26, further comprising a DO control means for controlling the operation of the DO reduction means.

28. The water treatment device according to claim 27, wherein the DO control means adjusts and controls the dissolved oxygen in the UV-treated water to 3.0 mg / L or less.

29. A DO meter provided in a treated water pipe through which the UV-treated water flows, and a DO measurement value (DO ＭＥＡＳＵＲＥ The water treatment device according to claim 27, further comprising: a DO transmission unit that transmits a DO control signal to the DO control means.

30. The DO control means controls the DO target value (DO ＧＯＡＬ ) and a storage unit for storing the DO measurement value (DO ＭＥＡＳＵＲＥ ), and a receiving unit that receives the DO measurement value (DO ＭＥＡＳＵＲＥ ) to the DO target value (DO ＧＯＡＬ 28. The water treatment device according to claim 27, further comprising: a control unit that controls operation of the DO reduction means so that the DO reduction amount is 0.01%.

31. The DO reduction means is an inert gas supply means for supplying an inert gas to a mixing tank storing the added water, and the DO control means is a means for controlling the DO measurement value (DO ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ The water treatment device according to claim 30, wherein the supply amount of the inert gas is controlled so that 32. The DO reduction means is an oxygen-consuming chemical supply means for supplying an oxygen-consuming chemical to the added water, and the DO control means is a means for controlling the DO measurement value (DO ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ The water treatment device according to claim 30, wherein the supply amount of the oxygen-consuming chemical is controlled so that the oxygen-consuming chemical is 0.01% or less.

33. The DO reduction means is a pressure reducing means for reducing the pressure inside the mixing tank so that the mixing tank storing the added water is sealed, and the DO control means is a means for controlling the DO measurement value (pH ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ 31. The water treatment device according to claim 30, wherein the output of the pressure reducing means is controlled so that the pressure is equal to or greater than the predetermined pressure.

34. The DO reduction means is a degassing means for degassing the added water to remove dissolved oxygen, and the DO control means is a means for controlling the DO measurement value (pH ＭＥＡＳＵＲＥ ) is the DO target value (DO ＧＯＡＬ 31. The water treatment device according to claim 30, wherein the output of the degassing means is controlled so that

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