Water treatment apparatus and water treatment method

Abstract

A water treatment apparatus 1 comprises a storage tank 2 for water to be filtered, an ozone water mixing tank 6, and a pulse plasma discharge tank 7. The storage tank 2 for water to be filtered stores water to be filtered which contains a substance to be treated and which is provided for filtration. The ozone water mixing tank 6 mixes, with ozone water, concentrated discharge liquid of the substance to be treated, in which the substance to be treated that has been separated by the filtration from the water to be filtered is concentrated. The pulse plasma discharge tank 7 provides pulse plasma to a mixed liquid of the ozone water and the concentrated discharge liquid of the substance to be treated.

Description

Water treatment device and water treatment method

[0001] The present invention relates to a water treatment technology for decomposing organic matter contained in water, and more particularly to a water treatment technology for decomposing persistent organic matter.

[0002] Liquid surface plasma discharge is used to purify water quality for treating various industrial wastewater and lake water, and for securing drinking water and industrial water (see, for example, Patent Document 1).

[0003] The water treatment device of Patent Document 1 performs purification treatment such as decomposing organic matter and chemical substances in the water to be treated by placing a cathode in the liquid phase in a reaction tank into which the water to be treated is introduced, while placing an anode in the gas phase in the reaction tank and causing a plasma discharge between the cathode and anode.

[0004] Japanese Patent Application Laid-Open No. 2015-97975 Japanese Patent Application Laid-Open No. 2005-111296 Japanese Patent No. 6702514 Japanese Patent No. 4296393 Japanese Patent Application Laid-Open No. 2006-297240 Japanese Patent Application Laid-Open No. 2018-38955

[0005] Takafumi Iijima, Ryutaro Makise, Takaaki Murata, "OH Radical Generator Aimed at Application to Treatment of Persistent Harmful Organic Matter," Toshiba Review, Vol. 61, No. 8 (2006). S. Horikoshi, S. Sato, M. Abe, and N. Serpone, "A novel liquid plasma AOP device integrating microwaves and ultrasounds and its evaluation in defluorinating perfluorooctanoic acid in aqueous media," Ultrason. Sonochem., 2011, 18, 938-942.

[0006] Patent Document 1 describes an example of a water treatment device applied to the purification of lake water. In this example, raw water is fed to a pretreatment device equipped with a filter, an aeration tank, and an adsorption filtration tank (a charcoal tank and a zeolite tank), and the resulting pretreated water is supplied to the water treatment device.

[0007] However, since raw water must be treated in a pretreatment device and then in a water treatment device, the time (retention time) required for supplying raw water to a water treatment system having the pretreatment device and the water treatment device until it is obtained as recycled water is long. It is desirable that the time required for supplying industrial wastewater to the water treatment system and regenerating it as industrial water is short.

[0008] Furthermore, since there is no process for concentrating organic matter and chemical substances (hereinafter referred to as "substances to be treated") in the raw water, the concentration of the substances to be treated in the pretreated water obtained by the pretreatment equipment is very low. It is inefficient to treat such low-concentration substances by applying large amounts of energy, such as ozone or plasma discharge.

[0009] Furthermore, no consideration has been given to how to treat the substances to be treated that have been adsorbed in the pretreatment device, particularly the adsorption filtration tank. The filtration capacity of the adsorption filtration tank gradually decreases with continuous use. When the filtration capacity decreases to a certain level, the filter material in the adsorption filtration tank is washed with water or water containing an acid or base, such as by surface washing or backwashing, in order to restore the filtration capacity. Furthermore, the water used to wash the filter material contains high concentrations of the substances to be treated that have been separated from the raw water, so it is necessary to detoxify the substances.

[0010] In view of the above, an object of the present invention is to obtain reused water in a relatively short time after subjecting raw water to water treatment, and to treat wastewater in which the target substance has been concentrated during the water treatment process in an energy-efficient manner.

[0011] Therefore, one aspect of the present invention is a water treatment device that includes a filtered water storage tank for storing filtered water containing the treated substance to be subjected to filtration, an ozone water mixing tank for mixing the treated substance concentrated effluent, which is obtained by concentrating the treated substance separated from the filtered water by the filtration, with ozone water, and a pulse plasma discharge tank for applying pulse plasma to the mixture of the treated substance concentrated effluent and the ozone water.

[0012] In one aspect of the present invention, the water treatment device further comprises a power source capable of generating the pulsed plasma that breaks inter-element bonds of the material to be treated that have a bond energy higher than the oxidation potential of hydroxyl radicals.

[0013] In one aspect of the present invention, in the water treatment device, the inter-element bond is a bond between a carbon element and a fluorine element.

[0014] In one aspect of the present invention, in the water treatment device, the concentrated wastewater containing the substance to be treated is produced in the process of regenerating an adsorbent added to the water-to-be-filtered storage tank.

[0015] In one aspect of the present invention, in the water treatment device, the mixed liquid is aerated.

[0016] In one aspect of the present invention, in the water treatment device, the adsorbent is activated carbon and / or an ion exchange resin.

[0017] In one aspect of the present invention, in the water treatment device, the concentrated wastewater of the treated substance is produced by filtering the filtered water from the filtered water storage tank containing activated sludge using a first filtration membrane to separate first filtrate, and then filtering this first filtrate using a second filtration membrane having a smaller pore size than the first filtration membrane.

[0018] A water treatment method according to one aspect of the present invention comprises the steps of filtering water containing a substance to be treated to separate the substance to be treated from the filtered water, mixing a concentrated effluent of the substance to be treated, which has been concentrated, with ozone water, and applying pulse plasma to the mixture of the concentrated effluent of the substance to be treated and the ozone water.

[0019] According to the present invention, it is possible to obtain reused water in a relatively short time after subjecting raw water to water treatment, and it is also possible to treat wastewater in which the target substances have been concentrated during the water treatment process with high energy efficiency.

[0020] 1 is a schematic diagram of a water treatment device according to a first embodiment of the present invention, which is an aspect of the present invention; FIG. 2 is a schematic diagram of a water treatment device according to a second embodiment of the present invention, which is an aspect of the present invention; FIG. 3 is a schematic diagram of a water treatment device according to a third embodiment of the present invention, which is an aspect of the present invention; FIG. 4 is a diagram showing the relationship between the oxidation potential of various free radicals and the bond energy between various elements; (a) the structural formula of perfluorooctanecarboxylic acid, which is an example of an organic fluorine compound, and (b) an explanatory diagram of the decomposition process of perfluorooctanecarboxylic acid; (a) an example of an embodiment of the present invention applied to a small-scale facility, (b) an example of an embodiment of the present invention applied to a medium-scale facility, and (c) an example of an embodiment of the present invention applied to a large-scale facility; FIG. 5 is a flow diagram of a preliminary test of a pulsed plasma discharge treatment process according to the present invention; and FIG. 6 is a schematic diagram of a water treatment device according to an example of the present invention. The PFOS concentration, fluoride ion concentration, and PFOS reduction rate for water quality evaluation points (1), (2), and (3) after pulsed plasma discharge treatment of PFOS-containing water in a preliminary test. The PFOA concentration, fluoride ion concentration, and PFOA reduction rate for water quality evaluation points (1), (2), and (3) after pulsed plasma discharge treatment of PFOA-containing water in a preliminary test. The reduction rate of the combined total amount of PFOS and PFOA, the combined total amount of PFOS and PFOA, and the total amount of fluoride ions for water quality evaluation points (1), (2), and (3) when the water to be treated is treated using the ozone water mixing treatment process and the pulse plasma discharge treatment process.

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0022] The water treatment method and water treatment device of the present invention decompose the target substances in an energy-efficient manner by mixing pretreated water, in which target substances have been concentrated in advance by solid-liquid separation of raw water (hereinafter referred to as concentrated effluent), with ozonated water, and then exposing the pretreated water to pulsed plasma. The present invention can decompose not only organic substances that can be decomposed by general water treatment, but also persistent organic substances such as PFAS (fluorinated organic compounds) that have been difficult to decompose by conventional water treatment devices.

[0023] [Embodiment 1] A water treatment device 1 according to embodiment 1 of the present invention shown in Figure 1 includes a water storage tank 2, an adsorbent adding means 3, an adsorbent transferring means 4, an adsorbent regenerating device 5, an ozone water mixing tank 6, and a pulse plasma discharge tank 7.

[0024] The unfiltered water storage tank 2 stores unfiltered water containing the substances to be treated and is subjected to filtration. A membrane filtration device 20 is immersed in the unfiltered water. Furthermore, activated carbon is added to the unfiltered water as an adsorbent. The membrane filtration device 20 is equipped with a filtration membrane 21 that sucks the unfiltered water and separates filtrate that does not contain the substances to be treated from the unfiltered water. A ceramic membrane is preferable as the filtration membrane 21. For example, this ceramic membrane can be made of alumina-based ceramic (alumina is the main component). The filtrate water is supplied to treatment equipment not shown. The substances to be treated are adsorbed by the added activated carbon.

[0025] The adsorbent adding means 3 adds the activated carbon to the water to be filtered in the water to be filtered storage tank 2. As the adsorbent adding means 3, a well-known powder supplying device such as a powder feeder used in wastewater treatment technology is applied.

[0026] The adsorbent transfer means 4 transfers the activated carbon that has adsorbed the substance to be treated from the to-be-filtered water storage tank 2 to the adsorbent regeneration device 5. As the adsorbent transfer means 4, a pump that transfers the liquid phase containing the activated carbon from the to-be-filtered water storage tank 2 to the adsorbent regeneration device 5 is used.

[0027] The adsorbent regeneration device 5 separates the activated carbon from the liquid phase containing the activated carbon and desorbs the target substance from the activated carbon through chemical treatment, thereby regenerating the activated carbon. A separation device well-known in the wastewater treatment technology, such as a screening device, is used to separate the activated carbon. The chemical treatment is, for example, a well-known chemical treatment method in which the activated carbon is brought into contact with alkaline and acidic regenerating chemicals, as described in Patent Document 2.

[0028] The ozone water mixing tank 6 decomposes interelement bonds other than C—F bonds of the treated substance by mixing the concentrated effluent of the treated substance, which is obtained by concentrating the treated substance desorbed from the activated carbon in the adsorbent regeneration device 5, with ozone water in a batch or continuous manner.

[0029] The ozone water can be prepared by dissolving ozone gas in raw water. This dissolution method can be any of the existing methods for producing ozone water, such as a method of producing ozone water by supplying raw water and ozone gas to a gas-liquid mixer.

[0030] The raw water may be any suitable water that can dissolve ozone gas, such as pure water, ultrapure water, etc. If necessary, the purity of the raw water may be increased using a known water purifier.

[0031] The ozone gas can be generated by a known ozone gas generator, such as the pure ozone generator manufactured by Meidensha and described in Patent Document 3. When ozone gas is generated by the pure ozone generator, ozone gas having an ozone concentration of 50% by volume or more and an ozone partial pressure of 30 kPa (abs) or less can be supplied to the gas-liquid mixer, so that high-concentration ozone water (100 ppm or more) can be safely generated.

[0032] The gas-liquid mixer may be, for example, an ejector, an aspirator, a jet pump, or the like, but is not limited thereto and various other embodiments may be applied.

[0033] The gas-liquid mixer has, for example, a raw water flow passage through which the received raw water flows, and an ozone gas inlet passage connected to the raw water flow passage and through which the received ozone gas is introduced into the raw water flow passage.

[0034] In the ozone gas inlet passage, a suction pressure is generated according to Bernoulli's theorem in accordance with the flow rate (flow velocity) of raw water flowing through the raw water flow passage. In the ozone gas inlet passage, steam is generated in accordance with the saturated vapor pressure of the raw water.

[0035] Since the raw water has properties similar to those of water (saturated vapor pressure characteristics and water vapor pressure characteristics), a suction range is derived based on the raw water and the ozone gas acceptance pressure of the gas-liquid mixer, which is the raw water temperature range in which the vapor pressure in the ozone gas inlet passage is lower than the acceptance pressure. The suction range is appropriately set (for example, 25°C or less as described in Patent Document 4) depending on the general solubility characteristics of gases in raw water (the tendency for solubility to increase as the temperature of the raw water decreases). Therefore, if the temperature of the raw water falls outside the suction range, it is adjusted to within the range by adjusting the temperature of the raw water before being fed to the gas-liquid mixer or by adjusting the temperature of the gas-liquid mixer into which the raw water is introduced.

[0036] A concentration-adjusting gas for stabilizing the ozone concentration may be added to the ozonated water produced in the gas-liquid mixer before it is fed to the ozonated water mixing tank 6, or the ozonated water may be circulated or temporarily stored in an ozonated water production unit equipped with the gas-liquid mixer. When a concentration-adjusting gas is used to increase the concentration, for example, carbon dioxide gas or the like may be injected into the ozonated water to acidify the ozonated water.

[0037] According to the above-described method for producing ozone water, it is possible to safely produce ozone water with a high concentration of 100 ppm or more (for example, 300 to 400 ppm).

[0038] In the pulse plasma discharge tank 7, pulse plasma is applied to the mixture of the concentrated wastewater containing the substances to be treated and the ozone water supplied from the ozone water mixing tank 6. Hereinafter, applying pulse plasma to the water to be treated, such as the concentrated wastewater containing the substances to be treated or the mixture, will also be referred to as "pulse plasma discharge treatment" or "pulse discharge treatment".

[0039] The pulsed plasma can be generated, for example, by applying a pulsed voltage between a pair of electrodes immersed in the mixed solution, thereby discharging the pulsed plasma into the mixed solution.

[0040] In another embodiment of the method for supplying the pulsed plasma, a pulsed voltage is applied between one electrode placed in the gas phase near the stable liquid surface of the mixed liquid in the pulsed plasma discharge vessel 7 and the other electrode immersed in the mixed liquid, thereby discharging the pulsed plasma into the mixed liquid.

[0041] The other electrode may be in the form of the pulsed plasma discharge vessel 7 shown in Fig. 8. In the pulsed plasma discharge vessel 7, a pulsed voltage is applied between the electrode 72 serving as the one electrode and the conductive pulsed plasma discharge vessel 7 functioning as the other electrode, and pulsed plasma discharged from one end of the electrode 72 near the liquid surface is supplied to the mixed liquid via the gas phase near the liquid surface.

[0042] The power supply for applying the pulse voltage is capable of generating the pulse plasma that decomposes inter-element bonds (e.g., bonds between carbon and fluorine elements) of the material to be treated that have bond energy higher than the oxidation potential of hydroxyl radicals (hereinafter, referred to as OH). As the electrode and the power supply, for example, the electrode and power supply device described in Patent Document 5 are applicable.

[0043] The mixed solution in the pulsed plasma discharge vessel 7 may be aerated by an aeration device 71 as shown in Fig. 8. This aeration stabilizes the discharge of the pulsed plasma and improves the reduction rate of PFOS and PFOA.

[0044] An example of the function and operation of the water treatment device 1 of the first embodiment will be described with reference to FIG.

[0045] The activated carbon is added to the water to be filtered introduced into the water to be filtered storage tank 2 by the adsorbent adding means 3. The substances to be treated in the water to be filtered are adsorbed by the activated carbon. The filtration membrane 21 of the membrane filtration device 20 immersed in the water to be filtered has pores small enough to block and capture the activated carbon, so the activated carbon is removed by the filtration membrane 21. The adsorption of the substances to be treated and the separation of the activated carbon can be applied to large-scale water treatment, and filtered water by the filtration membrane 21 can be obtained quickly and in large quantities. The liquid phase containing the activated carbon that has adsorbed the substances to be treated is transferred to the adsorbent regeneration device 5 by the adsorbent transfer means 4. Meanwhile, the filtrate that does not contain the substances to be treated separated by the filtration membrane 21 is transferred to the outside of the system.

[0046] In the adsorbent regeneration device 5, the activated carbon separated from the liquid phase by the screen or the like comes into contact with the regeneration chemical, causing the target substance to be desorbed from the activated carbon and regenerating the activated carbon. While easily decomposable organic substances may be decomposed during the regeneration process, if the target substance is a persistent organic substance such as PFAS, it is not decomposed and is concentrated to a high concentration in the regeneration effluent. This concentrated target substance effluent is supplied to the ozone-water mixing tank 6.

[0047] In the ozone water mixing tank 6, the interelement bonds other than C—F of the substance to be treated are decomposed by contact between the concentrated wastewater of the substance to be treated and the ozone water. That is, the interelement bonds (e.g., C—C bond, C═O bond, C═C bond, C—H bond) having a bond energy lower than the oxidation potential of OH shown in FIG. 4 (Non-Patent Document 1) are converted into active species (.OH, .O, O) of the ozone water. ３ ) is oxidatively decomposed. For example, when the substance to be treated is PFOA (perfluorooctanecarboxylic acid), a type of PFAS shown in FIG. 5(a) (Non-Patent Document 2), it is decomposed by repeated decarboxylation and oxidation of the linear carbon terminals enclosed by the dashed lines due to the oxidizing power of the active species. However, PFOA or its decomposition products containing C—F bonds cannot be completely decomposed by the oxidizing power of the active species, and the liquid phase containing these is supplied to the pulse plasma discharge chamber 7.

[0048] In the pulse plasma discharge tank 7, the mixed solution introduced from the ozone water mixing tank 6 is aerated. The pulse plasma generated by the application of the pulse voltage decomposes inter-element bonds of the material to be treated, which have a bond energy higher than the oxidation potential of -OH. For example, as shown in Figure 5(b), decomposition products of PFOA containing C-F bonds contained in the liquid phase supplied from the ozone water mixing tank 6 are decomposed into fluorine ions and carbon dioxide gas by the pulse plasma.

[0049] The pulsed plasma can decompose C—F bonds, but does not selectively decompose C—F bonds. Therefore, when pulsed plasma is generated in water to be treated that contains substances such as PFOA that have C—C bonds, C═O bonds, etc. in addition to C—F bonds (i.e., concentrated wastewater of the substances to be treated), the energy is also consumed in decomposing C—C bonds, C═O bonds, etc., and it may take a lot of energy and time to decompose the C—F bonds.

[0050] Therefore, in this embodiment, among the interelement bonds of the organic matter, those with an oxidation potential lower than that of ozone water are decomposed by the ozone water, and those with an oxidation potential higher than that are decomposed by pulsed plasma. This allows the persistent organic matter to be decomposed energy efficiently. After the pulsed plasma discharge treatment, the treated water is rendered harmless by the decomposition of the persistent organic matter, making it possible to reuse the wastewater.

[0051] According to the present embodiment, reusable water can be produced by the initial filtration using a combination of activated carbon and a ceramic membrane, thereby shortening the residence time in the system compared to the conventional method (Patent Document 1). Furthermore, energy efficiency is improved because the target substance is decomposed using ozone water and pulse plasma after being concentrated. Furthermore, the substance (activated carbon) that adsorbs the target substance can be reused. Furthermore, it is possible to decompose inter-element bonds in the target substance, which have a bond energy higher than the oxidation potential of hydroxyl radicals, thereby achieving the decomposition and detoxification of persistent organic substances such as PFAS, which are difficult to decompose using conventional water treatment.

[0052] [Embodiment 2] The water treatment device 1 of Embodiment 2 shown in Figure 2 is the same as Embodiment 1, except that an ion exchange resin is used as the adsorbent instead of activated carbon. The ion exchange resin may be a well-known ion exchange resin, such as that described in Patent Document 6, and the ion exchange resin is regenerated using a regenerating solution containing a regenerant such as an acid or alkali, as described in the same patent document. It is clear that the above-described embodiment also achieves the same effects as Embodiment 1. The ion exchange resin may be used in combination with the activated carbon of Embodiment 1.

[0053] [Embodiment 3] A water treatment device 1 according to embodiment 3 of the present invention shown in Figure 3 is the same as those according to embodiments 1 and 2, except that it employs a membrane bioreactor (hereinafter referred to as MBR) method, activate sludge is added to the water-to-be-filtered storage tank 2 instead of an adsorbent, and a reverse osmosis membrane filter 8 is provided instead of the adsorbent regeneration device 5.

[0054] When the material to be treated is concentrated by the reverse osmosis membrane filter 8, the water-to-be-filtered storage tank 2 serves as pretreatment for the reverse osmosis membrane filter 8. Therefore, in this embodiment, it is not necessary to add the adsorbent to the water-to-be-filtered storage tank 2. MBR, which treats organic matter that can be decomposed by normal biological treatment using activated sludge in the water-to-be-filtered storage tank 2, is effective from the viewpoint of improving the efficiency of subsequent treatments (reverse osmosis membrane filter 8, ozone water mixing tank 6, and pulse plasma discharge tank 7).

[0055] The action and operation example of the water treatment device 1 of the third embodiment will be described with reference to the same figure.

[0056] The unfiltered water introduced into the unfiltered water storage tank 2 is aerated and agitated. The organic matter in the liquid phase in the unfiltered water storage tank 2 that can be decomposed by normal biological treatment is decomposed by the activated sludge. The filtration membrane 21 of the membrane filtration device 20 in the unfiltered water storage tank 2, which corresponds to the first filtration membrane in this embodiment, has pores that are small enough to block and capture activated sludge, so the activated sludge is separated from the first filtrate by the filtration membrane 21. The first filtrate is supplied to the reverse osmosis membrane filter 8 by a pump.

[0057] In the reverse osmosis membrane filter 8, the first filtrate is separated by a reverse osmosis membrane (hereinafter, RO membrane) 81, which corresponds to the second filtration membrane of this embodiment, into a second filtrate that does not contain the target substance (refractory organic matter) and a concentrated target substance effluent in which the target substance is concentrated. The first filtrate does not contain solids that would be rejected by the RO membrane 81 or easily decomposed organic matter, but still contains the target substance, such as PFAS. The RO membrane 81 has pores with a smaller pore size than the filtration membrane 21, which is sufficient to block and capture the target substance, so the target substance is removed from the first filtrate. The concentrated target substance effluent is sequentially supplied to the ozone-water mixing tank 6 and the pulse plasma discharge tank 7, as in embodiments 1 and 2. Meanwhile, the second filtrate is transported outside the system.

[0058] It is clear that the same effects as those of the first and second embodiments can be obtained in this embodiment.

[0059] [Other Aspects of the Present Invention] The present invention is not limited to the above-described embodiment, but can be applied to various industries and service formats as exemplified in FIG. 6, and the same effects as those of the above-described embodiment can be obtained.

[0060] The water treatment device 1 shown in Fig. 1(a) includes a batch-type ozone water mixing tank 6 with a treatment rate of, for example, 100 L / h, to which the concentrated wastewater of the substance to be treated that has been collected in advance is supplied, and a pulse plasma discharge tank 7. The water treatment device 1 of this embodiment does not have a power source such as a pump, and can be installed in a small facility such as a laboratory.

[0061] The water treatment device 1 shown in Figure 1(b) is equipped with a batch-type ozone water mixing tank 6 and a pulse plasma discharge tank 7 with a treatment water volume of, for example, 100-1000 L / h, and can be installed in an existing medium-sized facility, and can be operated standalone by installing power sources such as pumps.

[0062] The water treatment device 1 shown in Fig. 1(c) is equipped with a batch or continuous ozone water mixing tank 6 and a pulse plasma discharge tank 7, for example, with a treatment water volume of 1000 L / h or more, and is applied to large-scale facilities intended for total treatment, such as existing or newly constructed water purification plants 9. The ozone water mixing tank 6 of this embodiment is supplied with concentrated wastewater of the substance to be treated, which has been produced in advance, for example, at the water purification plant 9 using the adsorbent and membrane filtration.

[0063] The filtration membrane applied to the storage tank for water to be filtered in the water treatment device of the present invention is not limited to the immersion type filtration membranes of the first to third embodiments, but an external type filtration membrane can also be applied.

[0064] Examples of the present invention will be described below.

[0065] Prior to the examples of the present invention, a preliminary test of pulse discharge treatment was carried out using PFOS (perfluorooctanesulfonic acid)-containing water and PFOA-containing water as the water to be treated. An outline of the preliminary test will be described with reference to FIG. 7.

[0066] PFOS-containing water was prepared by dissolving PFOS in tap water to a PFOS concentration of 7,200 μg / L. Similarly, PFOA-containing water was prepared by dissolving PFOA in tap water to a PFOA concentration of 11,000 μg / L.

[0067] The pulsed plasma discharge tanks 7 "without aeration" and "with aeration" shown in FIG. 7 are equipped with an aeration device 71, an electrode 72, and a pulsed power supply 73, as in the pulsed plasma discharge tank 7 shown in FIG. 8. The tank body 70 of the pulsed plasma discharge tank 7 functions as a GND electrode. When the pulsed plasma discharge tank 7 is "with aeration," the aeration device 71 is equipped with a blower 74 that introduces air from the atmosphere and an aeration pipe 75 connected to the blower 74 to aerate the water to be treated in the pulsed plasma discharge tank 7. The electrode 72 is a round rod electrode for high voltage output, with its tip positioned in the gas phase near the liquid surface in the pulsed plasma discharge tank 7. A pulsed voltage is applied from a pulsed power supply 73 to generate pulsed plasma between the electrode 72 and the pulsed plasma discharge tank 7. The pulsed plasma is supplied to the mixed liquid through the gas phase near the liquid surface, flows along the liquid surface of the mixed liquid, and flows to ground (earth) through the pulsed plasma discharge tank 7.

[0068] The tank body 70 of the pulse plasma discharge tank 7 was a stainless steel cylindrical vessel with an inner diameter of 100 mm and a capacity of 700 mL. 300 mL of the water to be treated was placed in the tank body 70, and a stainless steel electrode 72, a 3 mm diameter round rod, was placed in the gas phase near the liquid level in the center of the tank body 70 (1 mm above the liquid level). A high-voltage pulse was then applied between the electrode 72 and the tank body 70 using a pulse power supply 73 to generate a pulse plasma discharge. In the pulse plasma discharge tank 7 with aeration, air aeration was also used to agitate the water to be treated in the tank body 70 and generate discharge within the bubbles. The pulse plasma discharge conditions for the water to be treated are shown in Table 1 below.

[0069]

[0070] The PFOS concentration, fluoride ion concentration, and PFOS reduction rate of PFOS-containing water after pulse discharge treatment in this preliminary test are shown in Figure 9. The PFOS concentration, fluoride ion concentration, and PFOS reduction rate of PFOA-containing water after pulse discharge treatment are shown in Figure 10.

[0071] The water quality evaluation point (1) in Fig. 7 shown on the horizontal axis of Fig. 9 is PFOS-containing water. The water quality evaluation point (1) in Fig. 7 shown on the horizontal axis of Fig. 10 is PFOA-containing water. In Figs. 9 and 10, treated water 1 at water quality evaluation point (2) is water treated with pulsed plasma discharge "without aeration," and treated water 2 at water quality evaluation point (3) is water treated with pulsed plasma discharge "with aeration."

[0072] As shown in the results of Figure 9, the PFOS concentration was lower and the fluoride ion concentration was higher at water quality evaluation point (3) than at water quality evaluation point (2). The PFOS reduction rate by pulse plasma discharge treatment with aeration was 40% higher than that by pulse discharge treatment without aeration.

[0073] As shown in the results of Figure 10, the PFOA concentration was lower and the fluoride ion concentration was higher at water quality evaluation point (3) compared to water quality evaluation point (2). The PFOA reduction rate by pulse discharge treatment with aeration was 7% higher than that by pulse discharge treatment without aeration.

[0074] The above preliminary tests of pulse discharge treatment with and without aeration showed that air aeration stabilizes the pulse plasma discharge and increases the reduction rate of PFOS and PFOA. Furthermore, it was experimentally demonstrated that, during pulse discharge treatment, stable pulse streamer discharge can be generated on the liquid surface in the pulse plasma discharge tank 7 by positioning the tip of the high-voltage electrode in the gas phase near the liquid surface in the pulse plasma discharge tank 7. Example 1 described below is based on the results of the above preliminary tests.

[0075] Example 1 Water to be treated in Example 1 contained PFOS and PFOA (PFOS concentration: 3,300 μg / L, PFOA concentration: 5,300 μg / L, combined concentration of PFOS and PFOA: 8,600 μg / L). This water to be treated was prepared by dissolving reagent-grade PFOS and PFOA in tap water, as in the preliminary test. This water to be treated was then treated with ozone water in the ozone water mixing tank 6 shown in FIG. 8 and then pulse plasma discharge treated in the pulse plasma discharge tank 7, resulting in the treated water of Example 1.

[0076] (Ozone Water Mixing Treatment Process) The ozonated water mixing treatment process was carried out in the ozonated water mixing tank 6 shown in Figure 8. The water to be treated and high-concentration ozonated water were supplied to the ozonated water mixing tank 6 and mixed. This treatment was performed in a batch system, aiming for oxidative decomposition of organic matter, including PFAS, in the water to be treated using ozone dissolved in the high-concentration ozonated water and the active oxidizing species generated when the ozone decomposed. As mentioned above, decomposition of the C-F bond in PFAS was expected to be difficult with the oxidation potential of ozone and the active oxidizing species. Therefore, the aim was to cleave and decompose bonds such as C-C and C=C in the organic matter, or to mineralize the organic matter. The high-concentration ozonated water used was produced by the pure ozone generator. The conditions for the ozonated water mixing treatment process in this example are shown in Table 2 below.

[0077]

[0078] (Pulse Discharge Treatment Process) The pulse discharge treatment process is carried out in the pulse plasma discharge tank 7 of Figure 8. Treated water (a mixture of water to be treated and high-concentration ozone) from the ozone-water mixing treatment process (ozone-water mixing tank 6) is supplied to the pulse plasma discharge tank 7. An aeration device 71 aerates the mixture with air using a blower 74 via an aeration pipe 75. An electrode 72 has its tip positioned in the gas phase near the liquid surface in the pulse plasma discharge tank 7, and generates pulse plasma between the electrode 72 and the pulse plasma discharge tank 7 by applying a pulse voltage from a pulse power supply 73. The pulse plasma is supplied to the mixture through the gas phase near the liquid surface, flows along the liquid surface of the mixture, and flows to ground (earth) via the pulse plasma discharge tank 7.

[0079] The treated water in the ozone-water mixing treatment process was pale purple even immediately before the pulse discharge treatment process, indicating that the pulse discharge treatment was carried out in the presence of dissolved ozone. In this example, the pulse discharge treatment was carried out while aerating the mixed solution in the pulse plasma discharge tank 7 with air by the aeration device 71.

[0080] The conditions for the pulse discharge treatment process in this example are the same as those shown in Table 1.

[0081] FIG. 11 shows the reduction rate of the combined total amount of PFOS and PFOA, the combined total amount of PFOS and PFOA, and the total amount of fluoride ions at the water quality evaluation points (1), (2), and (3) in FIG. 8 when the treated water is treated using the ozone water mixing treatment process and the pulse discharge treatment process.

[0082] In Figure 8, water quality evaluation point (1) is the water to be treated before being subjected to the ozone-water mixing treatment process (ozone-water mixing tank 6). Water quality evaluation point (2) is the treated water from the ozone-water mixing treatment process. Water quality evaluation point (3) is the treated water from the pulse discharge treatment process (pulse plasma discharge tank 7).

[0083] As mentioned above, in the ozonated water contact treatment process, the water to be treated and the high-concentration ozonated water are mixed at a 1:1 ratio, so the concentration of substances in the raw water is halved due to the dilution effect. In this example, to evaluate the oxidative decomposition of the substances contained in the water to be treated by ozone and the decomposition of the substances by pulsed plasma discharge, the substances contained in the water quality evaluation points (1), (2), and (3) were displayed and evaluated as total amounts rather than concentrations.

[0084] As shown in the results of Figure 11, the combination of ozone water contact treatment and pulse discharge treatment reduced the combined total amount of PFOS and PFOA by 43%. Furthermore, the amount of fluoride ions increased after pulse discharge treatment. It is presumed that the increase in fluoride ions is the result of the C-F bonds of PFOS and PFOA contained in the treated water being cleaved by the pulse discharge treatment. Furthermore, since the amount of fluoride ions did not increase with the ozone water contact treatment, it is believed that pulse discharge treatment is more effective than ozone water contact treatment in cleaving the C-F bonds of PFOS and PFOA.

[0085] As is clear from the results of Example 1, it was confirmed that the combination of ozone water contact treatment and pulse discharge treatment is effective in removing PFOS and PFOA.

[0086] Comparative Example 1 In order to verify the PFOS reduction effect depending on whether or not ozone water contact treatment was performed before pulse discharge treatment, in Comparative Example 1, pure water was mixed with the water to be treated in place of high-concentration ozone water in the ozone water mixing tank 6, and then pulse discharge treatment was performed in the pulse plasma discharge tank 7. The mixing ratio (volume ratio) of the water to be treated to the pure water was set to water to be treated:pure water = 1:1.

[0087] The PFOS reduction rates of the treated water in the pulse discharge treatment processes of Example 1 and Comparative Example 1, the PFOS amount in each treated solution, and the fluoride ion amount are shown in Table 3 below.

[0088]

[0089] From the results in Table 3, the PFOS reduction rate in Comparative Example 1 was 56%, while the PFOS reduction rate in Example 1 was 74%, confirming that performing ozone water contact treatment before pulse discharge treatment promotes the decomposition and removal of PFOS in the pulse discharge treatment.

[0090] DESCRIPTION OF SYMBOLS 1... Water treatment device 2... Filtered water storage tank 20... Membrane filtration device 21... Filtration membrane 3... Adsorbent adding means 4... Adsorbent transferring means 5... Adsorbent regeneration device 6... Ozone water mixing tank 7... Pulse plasma discharge tank 71... Aeration device 72... Electrode 73... Pulse power supply 74... Blower 75... Aeration pipe 8... Reverse osmosis membrane filter 81... RO membrane 9... Water purification plant

Claims

1. A water treatment device comprising: a filtered water storage tank for storing filtered water containing a substance to be treated that is to be subjected to filtration; an ozone water mixing tank for mixing ozone water with a concentrated effluent of the substance to be treated, which is obtained by concentrating the substance to be treated that has been separated from the filtered water by the filtration; and a pulse plasma discharge tank for applying pulse plasma to the mixture of the concentrated effluent of the substance to be treated and the ozone water.

2. The water treatment device according to claim 1, further comprising a power source capable of generating the pulsed plasma that breaks down inter-element bonds of the substance to be treated that have a bond energy higher than the oxidation potential of hydroxyl radicals.

3. The water treatment device according to claim 2, wherein the inter-element bond is a bond between a carbon element and a fluorine element.

4. The water treatment device according to claim 1, characterized in that the concentrated wastewater containing the treated substances is produced in the process of regenerating an adsorbent added to the filtered water storage tank.

5. The water treatment device according to claim 4, wherein the adsorbent is activated carbon and / or an ion exchange resin.

6. The water treatment device described in claim 1, characterized in that the concentrated wastewater from the treated material is produced by filtering the filtered water from the filtered water storage tank in which activated sludge is retained using a first filtration membrane to separate first filtrate, and then filtering this first filtrate using a second filtration membrane having a smaller pore size than the first filtration membrane.

7. The water treatment device according to claim 1, wherein the mixed liquid is aerated.

8. A water treatment method comprising the steps of: filtering filtered water containing a substance to be treated to separate the substance to be treated from the filtered water; mixing a concentrated effluent of the substance to be treated, in which the substance to be treated is concentrated, with ozone water; and applying pulse plasma to the mixture of the concentrated effluent of the substance to be treated and the ozone water.

Images