Water treatment device and water treatment method

Abstract

The water treatment device 1 includes a filtered water storage tank 2, an ozone water mixing tank 6, and a pulse plasma discharge tank 7. The filtered water storage tank 2 stores the filtered water containing the substances to be treated that are to be subjected to filtration. The ozone water mixing tank 6 mixes the ozone water with a concentrated effluent of the substances to be treated that has been separated from the filtered water by the filtration and is thus concentrated. The pulse plasma discharge tank 7 applies pulse plasma to the mixture of the concentrated effluent of the substances to be treated and the ozone water.

Description

[Technical field]

[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. [Background technology]

[0002] 2. Description of the Related Art Liquid surface plasma discharge is used to purify water quality for treating various industrial wastewater and lake water, and for ensuring drinking water and industrial water (for example, Patent Document 1).

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

[0004] [Patent Document 1] JP 2015-97975 A [Patent Document 2] JP 2005-111296 A [Patent Document 3] Patent No. 6702514 [Patent Document 4] Patent No. 4296393 [Patent Document 5] JP 2006-297240 A [Patent Document 6] JP 2018-38955 A [Non-patent literature]

[0005] [Non-Patent Document 1] Takafumi Iijima, Ryutaro Makise, Takaaki Murata, "OH radical generator aimed at application to treatment of persistent hazardous organic matter," Toshiba Review Vol. 61 No. 8 (2006) [Non-Patent Document 2] 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 Summary of the Invention

[0006] Patent Document 1 describes an example of the water treatment device applied to purifying 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 the raw water needs to be treated in a pretreatment device and then in a water treatment device, the time (retention time) from when the raw water is supplied 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 from when industrial wastewater is supplied to the water treatment system and regenerated as industrial water is short.

[0008] In addition, since there is no process for concentrating the organic matter and chemical substances in the raw water (hereinafter also referred to as the treated substances), the concentration of the treated substances in the pretreated water obtained by the pretreatment device is very low. It is inefficient to treat the treated substances with low concentration 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 during continuous use. When the filtration capacity decreases to a certain level, the filter media in the adsorption filtration tank is washed with water or water containing an acid or base, such as by surface washing or backwashing, to recover the filtration capacity. The water used to wash the filter media contains a high concentration 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 reuse 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 with high energy efficiency.

[0011] Therefore, a water treatment device according to one embodiment of the present invention includes a filtered water storage tank for storing filtered water containing the substance to be treated that is to be subjected to filtration, an ozone water mixing tank for mixing a concentrated effluent of the substance to be treated, which is obtained by concentrating the substance to be treated 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 concentrated effluent of the substance to be treated 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 pulse plasma that breaks inter-element bonds of the substance to be treated that have a bond energy higher than the oxidation potential of a hydroxyl radical.

[0013] In one aspect of the present invention, in the water treatment device, the interelement 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 of the substance to be treated is produced in the process of regenerating an adsorbent added to the to-be-filtered water 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 effluent of the treated substance 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 a first filtrate, and further filtering this first filtrate using a second filtration membrane having a pore size smaller than that of the first filtration membrane.

[0018] A water treatment method according to one embodiment of the present invention includes a step of filtering filtered water containing a substance to be treated to separate the substance to be treated from the filtered water, a step of mixing a concentrated effluent of the substance to be treated, in which the substance to be treated is concentrated, with ozone water, and a step of 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. [Brief description of the drawings]

[0020] [Figure 1] 1 is a schematic configuration diagram of a water treatment device according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic configuration diagram of a water treatment device according to a second embodiment of the present invention. [Diagram 3] FIG. 1 is a schematic configuration diagram of a water treatment device according to a third embodiment of the present invention. [Figure 4] A diagram showing the relationship between the oxidation potential of various free radicals and the bond energy between various elements. [Diagram 5] (a) Structural formula of perfluorooctanecarboxylic acid, an example of an organic fluorine compound, and (b) an explanatory diagram of the decomposition process of perfluorooctanecarboxylic acid. [Figure 6] (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. [Figure 7] FIG. 2 is a flow diagram of a pre-test of the pulse plasma discharge treatment process of the present invention. [Figure 8] 1 is a schematic configuration diagram of a water treatment device according to an embodiment of the present invention. [Figure 9] PFOS concentration, fluoride ion concentration, and PFOS reduction rate at water quality evaluation points (1), (2), and (3) after pulse plasma discharge treatment of PFOS-containing water in a preliminary test. [Figure 10] PFOA concentration, fluoride ion concentration, and PFOA reduction rate for water quality evaluation points (1), (2), and (3) after pulse plasma discharge treatment of PFOA-containing water in a preliminary test. [Figure 11] 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 in 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 substances to be treated in an energy-efficient manner by mixing pretreated water (hereinafter, concentrated wastewater of substances to be treated) in which the substances to be treated have been concentrated in advance by solid-liquid separation of raw water with ozone water, and then exposing the water to pulse plasma. The present invention can decompose not only organic substances that can be decomposed by general water treatment, but also refractory organic substances such as organofluorine compounds (hereinafter, PFAS) that were difficult to decompose by conventional water treatment devices.

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

[0024] The filtered water storage tank 2 stores the filtered water containing the substances to be treated that are to be subjected to filtration. A membrane filtration device 20 is immersed in the filtered water. Furthermore, activated carbon is added to the filtered water as an adsorbent. The membrane filtration device 20 is equipped with a filtration membrane 21 that sucks the filtered water and separates filtrate water that does not contain the substances to be treated from the filtered water. A ceramic membrane is preferable as the filtration membrane 21. For example, this ceramic membrane is made of alumina-based ceramics (mainly composed of alumina). The filtrate water is supplied to a treatment facility 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 by chemical treatment, thereby regenerating the activated carbon. A well-known separation device in wastewater treatment technology, such as a screen device, is used for the separation of the activated carbon. For the chemical treatment, a well-known chemical treatment method is used in which the activated carbon is brought into contact with an alkaline and acidic regenerating chemical, as described in Patent Document 2, for example.

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

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

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

[0031] The ozone gas can be generated by a known ozone gas generator. An example of the ozone gas generator is the pure ozone generator manufactured by Meidensha and described in Patent Document 3. When the 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 (100 ppm or more) ozone water 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 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 for introducing the received ozone gas into the raw water flow passage.

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

[0035] Since the raw water has the same characteristics as water (saturated vapor pressure characteristics and water vapor pressure characteristics), a suction range is derived based on the raw water and the ozone gas receiving pressure of the gas-liquid mixer, which is a raw water temperature range in which the vapor pressure of the ozone gas introduction passage is smaller than the receiving pressure. The suction range is appropriately set (for example, 25°C or less as described in Patent Document 4) according to the general solubility characteristics of gas in raw water (the solubility tends to increase as the temperature of the raw water decreases). Therefore, if the temperature of the raw water is out of the suction range, it is adjusted to the range by adjusting the temperature of the raw water before it is 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 ozone water generated in the gas-liquid mixer before being fed to the ozone water mixing tank 6, or the ozone water may be circulated or temporarily stored in an ozone water generating section equipped with the gas-liquid mixer. When attempting to increase the concentration by using a concentration adjusting gas, for example, a method of injecting carbon dioxide gas or the like into the ozone water to acidify the ozone water may be mentioned.

[0037] According to the above-described method for generating ozone water, it is possible to safely generate 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 of 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 of the substances to be treated or the mixture, is also referred to as "pulse plasma discharge treatment" or "pulse discharge treatment".

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

[0040] In another embodiment of the method for supplying the pulsed plasma, a pulsed voltage is applied between one electrode disposed 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 pulse voltage is applied between the electrode 72 as the one electrode and the conductive pulsed plasma discharge vessel 7 functioning as the other electrode, and the 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 breaks inter-element bonds (e.g., bonds between carbon and fluorine elements) of the material to be treated that have a bond energy higher than the oxidation potential of a hydroxyl radical (hereinafter, OH). As the electrode and the power supply, for example, the electrode and power supply device described in Patent Document 5 are applied.

[0043] In addition, the mixed liquid in the pulse plasma discharge tank 7 may be aerated by an aeration device 71 as shown in Fig. 8. This aeration stabilizes the discharge of the pulse plasma, and can improve 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 filtered water introduced into the filtered water storage tank 2 by the adsorbent adding means 3. The substances to be treated in the filtered water are adsorbed by the activated carbon. The filtration membrane 21 of the membrane filtration device 20 immersed in the filtered water has pores small enough to block and capture the activated carbon, so that 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 are applicable 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. On the other hand, 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 liquid, and the treated substance is desorbed from the activated carbon, and the activated carbon is regenerated. Easily decomposable organic matter may be decomposed during the regeneration process, but if the treated substance is a persistent organic matter such as PFAS, it is not decomposed and is concentrated to a high concentration in the regeneration effluent. This concentrated effluent of the treated substance is supplied to the ozone water mixing tank 6.

[0047] In the ozone water mixing tank 6, the interelement bonds of the substance to be treated other than CF 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 bonds, C=O bonds, C=C bonds, and CH bonds) having a bond energy lower than the oxidation potential of ·OH shown in FIG. 4 (Non-Patent Document 1) are oxidized and decomposed by the active species (·OH, ·O, O3) of the ozone water. For example, when the substance to be treated is PFOA (perfluorooctane carboxylic acid), a type of PFAS shown in FIG. 5(a) (Non-Patent Document 2), the straight-chain carbon terminal surrounded by the dashed line is repeatedly decomposed by the oxidizing power of the active species. However, PFOA containing CF bonds or its decomposition products are not completely decomposed by the oxidizing power of the active species, and the liquid phase containing them is supplied to the pulse plasma discharge tank 7.

[0048] In the pulse plasma discharge tank 7, the mixed liquid introduced from the ozone water mixing tank 6 is aerated. In the mixed liquid, the inter-element bonds of the substance to be treated, which have a bond energy higher than the oxidation potential of ·OH, are decomposed by the pulse plasma generated by application of the pulse voltage. For example, as shown in Fig. 5(b), decomposition products of PFOA containing CF 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 and C=O bonds in addition to C-F bonds (i.e., concentrated wastewater from the substances to be treated), the energy is also consumed in decomposing C-C and C=O bonds, and it may take a lot of energy and time to decompose the C-F bonds.

[0050] Therefore, in the present embodiment, among the inter-element 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 of the ozone water are decomposed by pulse plasma. This allows the decomposition of the refractory organic matter with high energy efficiency. The treated water after the pulse plasma discharge treatment is rendered harmless by the decomposition of the refractory organic matter, making it possible to reuse the wastewater.

[0051] According to the above embodiment, reusable water can be generated by the first filtration using a combination of activated carbon and a ceramic membrane, so that the residence time in the system can be shortened compared to the conventional method (Patent Document 1). In addition, the substance to be treated is decomposed using ozone water and pulse plasma after being concentrated, so that energy efficiency is improved. Furthermore, it becomes possible to reuse the substance (activated carbon) that adsorbs the substance to be treated. In addition, it becomes possible to decompose the inter-element bonds of the substance to be treated that have a higher bond energy than the oxidation potential of hydroxyl radicals, so that decomposition and detoxification of refractory organic substances such as PFAS, which are difficult to decompose by normal water treatment, can be achieved.

[0052] [Embodiment 2] The water treatment device 1 of the second embodiment shown in Fig. 2 is the same as that of the first embodiment, except that an ion exchange resin is used as an adsorbent instead of activated carbon. As the ion exchange resin, for example, a well-known ion exchange resin described in Patent Document 6 is used, and a regenerating liquid containing a regenerant such as an acid or alkali described in the same patent document is used to regenerate the ion exchange resin. It is clear that the above-mentioned embodiment also has the same effects as the first embodiment. The ion exchange resin may be used together with the activated carbon of the first embodiment.

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

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

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

[0056] The filtered water introduced into the filtered water storage tank 2 is aerated and agitated. The organic matter in the liquid phase in the filtered water storage tank 2 that is decomposable by normal biological treatment is decomposed by the activated sludge. The filtration membrane 21 of the membrane filtration device 20 in the filtered water storage tank 2, which corresponds to the first filtration membrane in this embodiment, has pores that are small enough to block and capture the activated sludge, so that 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 into a second filtrate not containing the treated substance (hardly decomposable organic matter) and a treated substance concentrated effluent in which the treated substance is concentrated by a reverse osmosis membrane (hereinafter, RO membrane) 81 corresponding to the second filtration membrane of this embodiment. The first filtrate does not contain solid matter that is alienated from the RO membrane 81 or organic matter that is easily decomposed, but still contains the treated substance such as PFAS. The RO membrane 81 has pores with a pore size smaller than that of the filtration membrane 21 that are sufficient to block and capture the treated substance, so that the treated substance is removed from the first filtrate. The treated substance concentrated effluent is sequentially supplied to the ozone water mixing tank 6 and the pulse plasma discharge tank 7 as in the first and second embodiments. On the other hand, the second filtrate is transferred to the outside of the system.

[0058] It is clear that the same effects as those of the first and second embodiments can be obtained with 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 in the figure (a) includes a batch-type ozone water mixing tank 6 with a treatment water amount 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 include 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 in FIG. 2(b) is equipped with a batch-type ozone water mixing tank 6 and a pulse plasma discharge tank 7 with a water treatment capacity of, for example, 100-1000 L / h, and can be installed in an existing medium-sized facility. It can also be equipped with power sources for pumps, etc., so that it can operate as a standalone device.

[0062] The water treatment device 1 in Fig. 2(c) is equipped with a batch or continuous ozone water mixing tank 6 and a pulse plasma discharge tank 7 for treating water with a capacity of, for example, 1000 L / h or more, and is applied to a large-scale facility assuming total treatment of an existing or newly constructed water purification plant 9, etc. The ozone water mixing tank 6 in this embodiment is supplied with concentrated effluent of the substance to be treated that has been generated in advance, for example, in the water purification plant 9 by 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 submerged type filtration membranes of the first to third embodiments, but an external type filtration membrane may also be used. EXAMPLES

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

[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 treatment target water. The outline of the preliminary test will be described with reference to FIG.

[0066] PFOS-containing water is tap water that has been dissolved to a PFOS concentration of 7,200μg / L. Similarly, PFOA-containing water is tap water that has been dissolved to a PFOA concentration of 11,000μg / 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 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 equipped with an aeration device 71, the aeration device 71 is equipped with a blower 74 for introducing air from the atmosphere and an aeration pipe 75 connected to the blower 74 for aerating the water to be treated in the pulsed plasma discharge tank 7. The electrode 72 is made of a round rod electrode for high voltage output, and the tip is placed in the gas phase near the liquid surface in the pulsed plasma discharge tank 7, and generates pulsed plasma between the pulsed plasma discharge tank 7 and the pulsed plasma discharge tank 7 by applying a pulsed voltage from the pulsed power supply 73. 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 the ground (earth) through the pulsed plasma discharge tank 7.

[0068] The tank body 70 of the pulse plasma discharge tank 7 was configured as a stainless steel cylindrical container 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 round rod electrode 72 with a diameter of 3 mm 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 source 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 in the air bubbles. The conditions for pulse plasma discharge of the water to be treated are shown in Table 1 below.

[0069] [Table 1]

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

[0071] The water quality evaluation point (1) in Figure 7 shown on the horizontal axis of Figure 9 is water containing PFOS. The water quality evaluation point (1) in Figure 7 shown on the horizontal axis of Figure 10 is water containing PFOA. In Figures 9 and 10, treated water 1 with water quality evaluation point (2) is water treated with pulse plasma discharge "without aeration", and treated water 2 with water quality evaluation point (3) is water treated with pulse 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) than at 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. In addition, it was experimentally clarified that in pulse discharge treatment, a stable pulse streamer discharge can be generated on the liquid surface in the pulse plasma discharge tank 7 by placing 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] As the treated water of Example 1, water containing 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) was used. This treated water was prepared by dissolving reagent-grade PFOS and PFOA in tap water, as in the preliminary test. Then, this treated water was mixed with ozone water in the ozone water mixing tank 6 shown in FIG. 8 and then pulse plasma discharged in the pulse plasma discharge tank 7 to obtain the treated water of Example 1.

[0076] (Ozone water mixing treatment process) The ozone water mixing treatment process is carried out in the ozone water mixing tank 6 shown in FIG. 8. The water to be treated and high-concentration ozone water were supplied to the ozone water mixing tank 6 and mixed. This treatment was a batch process, and the ozone dissolved in the high-concentration ozone water and the active oxidizing species generated when the ozone decomposed were used to oxidize and decompose the organic matter containing PFAS in the water to be treated. As described above, it is expected that the decomposition of the C-F bond of PFAS is difficult with the oxidation potential of ozone and the active oxidizing species, so we expected to break or decompose the bonds such as C-C of organic matter, or to mineralize the organic matter. The high-concentration ozone water produced by the pure ozone generator was used. The conditions of the ozone water mixing treatment process of this embodiment are shown in Table 2 below.

[0077] [Table 2]

[0078] (Pulse discharge treatment process) The pulse discharge treatment process is carried out in the pulse plasma discharge tank 7 in Fig. 8. The treated water (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. The aeration device 71 aerates the mixed liquid with air by a blower 74 via an aeration tube 75. The tip of the electrode 72 is placed 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 mixed liquid through the gas phase near the liquid surface, flows along the liquid surface of the mixed liquid, and flows to the ground (earth) through the pulse plasma discharge tank 7.

[0079] The treated water in the ozone water mixing treatment process was light purple even immediately before the pulse discharge treatment process, and the pulse discharge treatment was carried out in the presence of dissolved ozone. In this embodiment, the pulse discharge treatment was carried out while aerating the mixed liquid 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 embodiment 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 was treated using the ozone water mixing treatment process and the pulse discharge treatment process.

[0082] In Fig. 8, the 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). The water quality evaluation point (2) is the treated water from the ozone water mixing treatment process. The water quality evaluation point (3) is the treated water from the pulse discharge treatment process (pulse plasma discharge tank 7).

[0083] As described above, in the ozone water contact treatment process, the water to be treated and high-concentration ozone 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 order to evaluate the oxidative decomposition of the substances contained in the water to be treated by ozone in this embodiment and the decomposition of the substances by pulse 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%. In addition, 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 broken by the pulse discharge treatment. In addition, since there was no increase in fluoride ions in the ozone water contact treatment, it is thought that pulse discharge treatment is more effective at breaking the C—F bonds of PFOS and PFOA than ozone water contact treatment.

[0085] As is clear from the results of Example 1 above, 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 effect of reducing PFOS with or without ozone water contact treatment before pulse discharge treatment, in Comparative Example 1, pure water was mixed with the water to be treated instead 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 liquid, and the amount of fluoride ions are shown in Table 3 below.

[0088] [Table 3]

[0089] As can be seen 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. [Explanation of symbols]

[0090] 1...Water treatment equipment 2...filtered water storage tank, 20...membrane filtration device, 21...filtration membrane 3. Means for adding adsorbent 4...Adsorbent transport means 5…Adsorbent regeneration device 6…Ozone water mixing tank 7...pulse plasma discharge chamber, 71...aeration device, 72...electrode, 73...pulse power source, 74...blower, 75...aeration pipe 8...Reverse osmosis membrane filter, 81...RO membrane 9…Water purification plant

Claims

1. A filtered water storage tank for storing filtered water containing a substance to be treated, an ozone water mixing tank for mixing a concentrated effluent of the treated substance separated from the filtered water by the filtration with ozone water; a pulse plasma discharge tank for applying pulse plasma to the mixture of the concentrated wastewater of the material to be treated and the ozone water; Equipped with The concentrated wastewater of the treated material is produced by filtering the filtered water from the filtered water storage tank containing activated sludge through a first filtration membrane to separate a first filtrate, and then filtering the first filtrate through a second filtration membrane having a smaller pore size than the first filtration membrane. The water treatment device comprises:

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

3. 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 , wherein the mixed liquid is aerated.

5. A step of subjecting the filtered water containing the substance to be treated to filtration to separate the substance to be treated from the filtered water; mixing the concentrated effluent of the substance to be treated with ozone water; A step of applying pulse plasma to the mixture of the concentrated wastewater of the material to be treated and the ozone water; having The concentrated wastewater of the treated material is produced by filtering the filtered water from the filtered water storage tank containing activated sludge through a first filtration membrane to separate a first filtrate, and then filtering the first filtrate through a second filtration membrane having a smaller pore size than the first filtration membrane. A water treatment method comprising:

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