Plasma fine bubble generation device, water treatment apparatus using same, and water treatment method

Abstract

The present invention provides a water treatment apparatus and water treatment method with a simple structure and easy-to-operate configuration, for purifying contaminated water or the like mixed with refractory organic substances such as organofluorine compounds.　A water treatment apparatus 1 comprises a treatment tank 10, a plasma fine bubble generator 20, and a high-voltage power supply device 30. A high voltage is applied by a high-voltage power supply device 30 to a fluid F containing fine bubbles FB flowing out from the plasma fine bubble generator 20, to thereby generate discharge plasma inside the fine bubbles FB. The fluid is then mixed with water WD under treatment stored in the treatment tank 10 to decompose refractory organic substances in the water WD under treatment.

Description

Plasma microbubble generator, water treatment device using the same, and water treatment method

[0001] The present invention relates to a water treatment device and a water treatment method, and more particularly to a plasma microbubble generator for treating contaminated water containing persistent organic substances such as organofluorine compounds, and a water treatment device and a water treatment method using the same.

[0002] In recent years, the harmfulness and accumulation of organic fluorine compounds have become clear, and as a result, their production and use have been restricted. Among these, perfluorooctanesulfonic acid (C ８ F １７ SO ３ H, PFOS) and perfluorooctanoic acid (C ７ F １５ Long-chain organic fluorine compounds such as PFOS (COOH, PFOA) are highly stable chemical substances, and therefore are difficult to decompose in the natural environment after being released into the environment. Furthermore, if ingested by the human body, they remain in the body for a long time and are known to bioaccumulate. In recent years, contamination of well water and tap water with PFOS and PFOA has been found in Japan, and the Ministry of the Environment has designated PFOS and PFOA as items requiring monitoring for the protection of human health, and has set provisional target values ​​for public water bodies and groundwater.

[0003] For the treatment of PFOS and PFOA, methods such as photocatalytic decomposition and adsorption using adsorbents have been proposed. Another method has been proposed: generating fine bubbles in the liquid to be treated and generating plasma within the bubbles to purify the liquid. This method has been reported to be capable of rapidly decomposing refractory organic substances, such as organofluorine compounds, which could not be decomposed even by accelerated oxidation using OH radicals, the strongest oxidizing agent except for fluorine. Furthermore, this method significantly reduces the breakdown voltage required for plasma formation by introducing fine bubbles into the liquid, which is expected to dramatically improve the efficiency of radical generation and lead to the resolution of problems associated with underwater plasma treatment.

[0004] A liquid treatment device disclosed in Patent Document 1 has been proposed for purifying liquids through decomposition treatment using plasma generated within such microbubbles. In this liquid treatment device, multiple openings are provided at the bottom of a treatment vessel containing the liquid to be treated, and gas is sent into the treatment vessel from a bubble generator through the openings. Meanwhile, the bubble generator controls the gas flow rate to a constant level and generates periodic pressure fluctuations in the gas, which is then sent into the openings of the treatment vessel. Then, a high voltage synchronized with the pressure fluctuations is generated between multiple anode electrodes positioned overlapping the openings and a cathode electrode located inside the treatment vessel, thereby treating the liquid to be treated.

[0005] According to the liquid treatment device, by applying periodic pressure fluctuations to the gas, bubbles can be generated intermittently, continuously, and reliably from multiple openings, and by generating a high voltage at the optimal timing in synchronization with the pressure fluctuations, plasma can be generated within the bubbles generated at each opening.

[0006] JP 2013-128909 A

[0007] However, in the liquid treatment device, since the multiple openings provided at the bottom of the treatment vessel function as bubble generation sources and also as plasma generation sites, the openings must be formed small enough to prevent the liquid in the treatment vessel from leaking to the outside, but large enough to reliably generate plasma within the bubbles, so the diameter of the openings is set to 0.3 to 0.5 mm. However, since the diameter of the openings cannot be changed, it is difficult to adjust the bubble size when the type or flow rate of the bubble-forming gas changes, and changes in bubble size may change the energy state of the plasma generated within the bubbles, resulting in a decrease in treatment efficiency.

[0008] The present invention has been made in consideration of the above-mentioned object, and has an object to provide a plasma micro-bubble generator that is simple in structure and easy to operate, and that can purify groundwater, tap water, or contaminated water that is contaminated with persistent organic fluorine compounds such as PFOS and PFOA, as well as a water treatment device and water treatment method that use the same.

[0009] The plasma micro-bubble generator according to the present invention is a plasma micro-bubble generator that receives a fluid and generates plasma inside micro-bubbles contained in the fluid, and is composed of a fluid holder that receives the fluid, an impact receiver that is disposed at a predetermined gap from the fluid holder, and a high-voltage power supply device, wherein a flange is formed at one end of the fluid holder, a first metal electrode is disposed on a surface of the flange opposite to the surface that faces the impact receiver, and a second metal electrode is disposed on a surface of the impact receiver opposite to the surface that faces the fluid holder, and wherein a voltage is applied between the first metal electrode and the second metal electrode by the high-voltage power supply device, thereby generating plasma inside micro-bubbles contained in the fluid that flows out from the gap between the fluid holder and the impact receiver.

[0010] Furthermore, the water treatment device according to the present invention comprises: a treatment tank for storing water to be treated; one or more plasma micro-bubble generators disposed within the treatment tank, for receiving fluid from outside the treatment tank, and for generating plasma inside micro-bubbles contained in the fluid; and a high-voltage power supply unit, wherein the plasma micro-bubble generator comprises a fluid holder for receiving the fluid, and an impact receiver disposed at a predetermined gap from the fluid holder, wherein a flange is formed at one end of the fluid holder, and a first metal electrode is disposed on a surface of the flange opposite to the surface facing the impact receiver, and a second metal electrode is disposed on a surface of the impact receiver opposite to the surface facing the fluid holder, and wherein the high-voltage power supply unit applies a voltage between the first metal electrode and the second metal electrode, thereby generating plasma inside micro-bubbles contained in the fluid flowing out from the gap between the fluid holder and the impact receiver.

[0011] Furthermore, the present invention provides a water treatment method comprising: a treatment tank for storing water to be treated received from the outside; one or more plasma micro-bubble generators disposed within the treatment tank, for receiving the water to be treated stored in the treatment tank, and for generating plasma inside micro-bubbles contained within the water to be treated; a high-voltage power supply; and a flow path for discharging the treated water from the treatment tank to the outside, wherein the plasma micro-bubble generator comprises a first metal electrode and a second metal electrode, the first metal electrode and the second metal electrode being disposed on either side of a flow path through which the micro-bubbles flow out, and the first metal electrode and the second metal electrode being electrically insulated, and wherein the high-voltage power supply applies a voltage between the first metal electrode and the second metal electrode, thereby generating plasma inside micro-bubbles contained in the water to be treated that flows out from the gap between the fluid holder and the impact receiver.

[0012] According to the present invention, plasma is efficiently generated in the microbubbles by the plasma microbubble generator, so that water to be treated that contains persistent organic matter can be efficiently treated.

[0013] Fig. 3 is a flow diagram of a water treatment device according to an embodiment of the present invention. Fig. 4 is a cross-sectional schematic diagram showing a plasma micro-bubble generator constituting the water treatment device. Fig. 5 is a longitudinal cross-sectional schematic diagram (Fig. 3(a)) showing another plasma micro-bubble generator constituting the water treatment device, and Fig. 3(b) is a view taken along the arrows A-A in Fig. 3(a). Fig. 6 is a flow diagram of a water treatment device according to a first modified embodiment of the present invention. Fig. 7 is a flow diagram of a water treatment device according to a second modified embodiment of the present invention.

[0014] (Embodiments of Water Treatment Device) An embodiment of a water treatment device according to the present invention will be described below. Fig. 1 is a flow diagram of a water treatment device according to an embodiment of the present invention, and Fig. 2 is a cross-sectional schematic diagram showing a plasma micro-bubble generator constituting the water treatment device. It should be noted that the present invention is not limited to these embodiments.

[0015] The water treatment device 1 according to the present invention comprises a treatment tank 10, a plasma bubble generator 20, and a high-voltage power supply 30. The treatment tank 10 is a container for accommodating water to be treated WD, and is preferably formed from a material that is stable with respect to the water to be treated WD contained therein, such as plastic. A plurality of plasma micro-bubble generators 20 are disposed within the treatment tank 10, and a fluid F is supplied to each plasma micro-bubble generator 20 from the outside via a supply pipe 11. An on-off valve 12 is disposed at the inlet for the fluid F to each plasma micro-bubble generator 20.

[0016] As shown in FIG. 2 , the plasma micro-bubble generator 20 is composed of a fluid holder 21 and an impact receiver 22. The fluid holder 21 is composed of a body 211, a ceiling 212, a fluid introduction portion 213, and a flange 214. The body 211 is generally cylindrical, and a generally hemispherical ceiling 212 is formed at one end of the body 211. A cylindrical fluid introduction portion 213 for receiving a fluid F is formed from the center of the ceiling 212 toward the outside. A brim-shaped flange 214 is formed at the other end of the body 211 so as to expand radially outward. The impact receiver 22 is disposed opposite the flange 214 with a narrow gap G between them. A first metal electrode 215 is disposed in an annular shape on the surface of the flange 214 opposite the impact receiver 22. The fluid holder 21 is formed of an insulating material such as ceramics.

[0017] The impact receiver 22 is a substantially circular flat plate, and a second metal electrode 221 is disposed in an annular shape on the surface of the impact receiver 22 opposite the flange portion 214 of the fluid holder 21. The outer diameter of the impact receiver 22 and the outer diameter of the flange portion 214 are substantially the same, and as will be described later, when the fluid F flows out of the gap G, a large flow resistance is applied to the fluid F, reducing its pressure and causing cavitation, so that a substantially uniform narrow gap G is maintained between the impact receiver 22 and the flange portion 214 over the entire circumference. The impact receiver 22 is formed from an insulating material such as ceramics.

[0018] Since the first metal electrode 215 disposed on the flange portion 214 and the second metal electrode 221 disposed on the impact receiver 22 need to be electrically insulated as described below, the fluid holder 21 and the impact receiver 22 are supported by separate means within the treatment tank 10. Furthermore, in order to control the pressure loss of the fluid F when the fluid flows out from the gap G between the flange portion 214 of the fluid holder 21 and the impact receiver 22, at least one of the fluid holder 21 and the impact receiver 22 may be supported within the treatment tank 10 via a slide mechanism (not shown) or the like so that its position in the height direction can be adjusted. For example, the impact receiver 22 can be disposed near the bottom of the treatment tank 10, and the fluid holder 21 can be supported relative to the impact receiver 22 via a slide mechanism, thereby making its position in the height direction adjustable. By changing the pressure loss experienced by the fluid F, the size and number of fine bubbles FB generated within the fluid F can be changed.

[0019] A case where water to be treated WD containing persistent organic matter is treated using the water treatment device 1 having the above configuration will be described. First, fluid F is supplied from the supply pipe 11 through the on-off valve 12 to each plasma micro-bubble generator 20 disposed in the treatment tank 10 filled with the water to be treated WD. The fluid F then flows into the plasma micro-bubble generator 20 from the fluid inlet 213 of the fluid holder 21 and is temporarily held in the space S surrounded by the body 211, ceiling 212, and impact receiver 22. The fluid F then flows out of the narrow gap G between the flange 214 of the fluid holder 21 and the impact receiver 22. During this flow, the fluid F encounters significant flow resistance, resulting in a significant drop in pressure and cavitation. As a result, micro-bubbles FB are generated within the fluid F.

[0020] When the fluid F containing the microbubbles FB flows out of the gap G, a high voltage is applied by the high-voltage power supply 30 between the first metal electrode 215 disposed on the flange portion 214 of the fluid holder 21 and the second metal electrode 221 disposed on the impact receiver 22. Meanwhile, because the fluid holder 21, including the flange portion 214, and the impact receiver 22 are made of insulating materials, the flange portion 214 and the impact receiver 22 act as a dielectric barrier. Therefore, a dielectric barrier discharge occurs within the microbubbles FB contained in the fluid F, generating high-speed electrons present in the discharge plasma and active species in a high-energy excited state within the microbubbles FB. The fluid F containing the microbubbles FB containing these active species then flows into and mixes with the water to be treated WD, thereby decomposing the organic fluorine compounds and other refractory organic matter contained in the water to be treated WD. The treated water WE is then discharged to the outside via the drain pipe 13.

[0021] In the above, it is preferable that the high-voltage power supply 30 generates a pulsed voltage, because generating a pulsed voltage makes the current flowing through the fluid F pulsed, reducing the conductor loss of the fluid F, and enabling capacitive coupling of the fluid F, thereby enabling highly efficient treatment.

[0022] The fluid F supplied from the outside to the plasma micro-bubble generator 20 may be a liquid alone, or a gas-liquid mixture fluid containing bubbles such as microbubbles FB in a liquid. While water is typically used as the liquid, this is not limiting. Furthermore, the gas is not limited to air, and may be a gas containing oxygen. By using a gas containing oxygen, dissociated oxygen atoms and excited oxygen molecules are generated, which facilitates the decomposition of organic fluorine compounds such as PFOS and PFOA.

[0023] The time for which the high voltage is applied between the first metal electrode 215 and the second metal electrode 221 can be changed depending on the flow rate of the fluid F, the generation state of the microbubbles FB, or the width of the gap G between the flange portion 214 and the impact receiver 22. These can be adjusted depending on the type and concentration of hardly soluble organic matter contained in the water to be treated WD, the physical properties and composition of the water to be treated WD, the type of chemical reaction induced by the plasma, etc.

[0024] The plasma micro-bubble generator 20 is not limited to the structure shown in Fig. 2, and may have a structure similar to that of a plasma micro-bubble generator 20a shown in Fig. 3. The plasma micro-bubble generator 20a has an outlet flow path 231 for the micro-bubbles FB, which is made of an insulator, attached to the outlet of a micro-bubble generator 23 that generates micro-bubbles FB by, for example, ultrasonic waves, and is integrated with the outlet of the generator 23. A first metal electrode 215a is provided on the outer periphery of the outlet flow path 231. A disk-shaped impact receiver 22a made of an insulator is disposed at the end of the outlet flow path 231, maintaining a narrow gap G between the impact receiver 22a and the end of the outlet flow path 231. A second metal electrode 221a is provided on the surface of the impact receiver 22a opposite to the end of the outlet flow path 231.

[0025] In this plasma micro-bubble generator 20a, when a high voltage is applied between the first metal electrode 215a and the second metal electrode 221a, the outlet flow path 231 and the impact receiver 22a act as a dielectric barrier. As a result, a dielectric barrier discharge occurs in the micro-bubbles FB generated in the fluid F when the fluid F passes through the narrow gap G, generating high-speed electrons and active species in a high-energy excited state. These species flow into the water WD to be treated and mix therewith, thereby decomposing the persistent organic matter in the water WD. Various types of micro-bubble generators, such as an ultrasonic type, a swirling flow type, a pressurized dissolution type, and a micro-pore type, can be used as the micro-bubble generator 23.

[0026] In the plasma micro-bubble generator 20a, in order to control the flow resistance applied to the fluid F by adjusting the width of the gap G between the end of the outlet flow path 231 and the impact receiver 22a, at least one of the micro-bubble generator 23 and the impact receiver 22a may be supported in the treatment tank 10 so that its position in the height direction can be adjusted via a slide mechanism (not shown). For example, the impact receiver 22a may be disposed near the bottom of the treatment tank 10, and the micro-bubble generator 23 may be supported via a slide mechanism, or conversely, the micro-bubble generator 23 may be disposed near the bottom of the treatment tank 10, and the impact receiver 22a may be supported via a slide mechanism.

[0027] (First Variation of the Embodiment) As a first variation of the water treatment device, a water treatment device 1A may be provided with a circulation pump 14 for circulating water to be treated WD, as shown in Fig. 4. In the water treatment device 1A, a portion of the water to be treated WD is circulated by the circulation pump 14, and mixed with a fluid F supplied from an external source to form a mixed fluid WM, which is then supplied to a plasma micro-bubble generator 20. The plasma micro-bubble generator 20 then generates plasma in micro-bubbles FB generated in the mixed fluid WM to treat persistent organic matter contained in the mixed fluid WM, and the treated water to be treated WD is then discharged from a drain pipe 13 as wastewater WE.

[0028] In this case, a flow control valve 16 may be provided on the outlet pipe 15 of the circulation pump 14. By controlling the flow rate of the circulating water WD to be treated with the flow control valve 16, it is possible to adjust the pressure loss of the mixed fluid WM passing through the gap G between the flange portion 214 of the fluid holder 21 and the impact receiver 22 in the plasma fine-bubble generator 20. For example, when the concentration of persistent organic matter in the water to be treated WD is high, the circulation flow rate from the circulation pump 14 can be increased and the amount of fine bubbles FB generated can be increased by adjusting the gap G between the flange portion 214 of the fluid holder 21 and the impact receiver 22 in the plasma fine-bubble generator 20, thereby promoting the decomposition of the persistent organic matter.

[0029] By configuring the water treatment device 1A to circulate the water to be treated WD as described above, it becomes possible to operate the water treatment device 1A in accordance with the type and concentration of persistent organic matter contained in the water to be treated WD.

[0030] 5 , the water treatment device 1A may be configured as a water treatment device 1B in which only the water to be treated WD is continuously supplied without supplying the fluid F from the outside, the water to be treated WD is treated by the plasma micro-bubble generator 20 while being circulated by the circulation pump 14, and the treated water WE is continuously discharged from the drain pipe 13. With this water treatment device 1B, the water to be treated WD can be continuously treated, and therefore persistent organic matter in the water to be treated WD can be efficiently treated.

[0031] The embodiments disclosed above are merely examples, and the scope of the present invention is defined not by the above embodiments but by the claims, and includes all modifications and variations within the meaning and scope of the claims. (Industrial Applicability)

[0032] The present invention is applicable to a plasma microbubble generator for treating contaminated water containing, for example, persistent organic substances such as organic fluorine compounds, and a water treatment apparatus and water treatment method using the same.

[0033] DESCRIPTION OF SYMBOLS 1, 1A, 1B Water treatment device 11 Supply pipe 12 On-off valve 13 Drain pipe 14 Circulation pump 15 (Circulation pump) Outlet pipe 16 Flow control valve 20, 20a Plasma micro-bubble generator 21 Fluid holder 211 Body 212 Ceiling 213 Fluid inlet 214 Flange 215, 215a First metal electrode 22, 22a Impact receiver 221, 221a Second metal electrode 23 Micro-bubble generator 231 Outlet flow path 30 High-voltage power supply F Fluid FB Micro-bubbles G Gap S Space WD Water to be treated WE Wastewater WM Mixed fluid

Claims

1. A plasma micro-bubble generator that receives a fluid and generates plasma inside micro-bubbles contained in the fluid, said plasma micro-bubble generator comprising a fluid holder that receives the fluid, an impact receiver that is disposed at a predetermined gap from the fluid holder, and a high-voltage power supply, wherein a flange is formed at one end of said fluid holder, a first metal electrode is disposed on the surface of said flange opposite to the surface that faces the impact receiver, and a second metal electrode is disposed on the surface of said impact receiver opposite to the surface that faces the fluid holder, and wherein a voltage is applied between said first metal electrode and said second metal electrode by said high-voltage power supply, thereby generating plasma inside micro-bubbles contained in the fluid that flows out from the gap between said fluid holder and said impact receiver.

2. A water treatment device comprising: a treatment tank for storing water to be treated; one or more plasma micro-bubble generators disposed within the treatment tank, which receive fluid from outside the treatment tank and generate plasma inside micro-bubbles contained in the fluid; and a high-voltage power supply, wherein the plasma micro-bubble generator is composed of a fluid holder which receives the fluid and an impact receiver disposed at a predetermined gap from the fluid holder, a flange portion is formed at one end of the fluid holder, a first metal electrode is disposed on the surface of the flange opposite to the surface facing the impact receiver, and a second metal electrode is disposed on the surface of the impact receiver opposite to the surface facing the fluid holder, and the high-voltage power supply applies a voltage between the first metal electrode and the second metal electrode, thereby generating plasma inside micro-bubbles contained in the fluid flowing out from the gap between the fluid holder and the impact receiver.

3. The water treatment device according to claim 2, further comprising a circulation pump for circulating a portion of the water to be treated in the treatment tank to the inlet of the plasma micro-bubble generator.

4. A water treatment device according to claim 2 or 3, characterized in that one of the fluid holder and impact receiver constituting the plasma micro-bubble generator is arranged to be movable relative to the other, and the gap between the fluid holder and impact receiver is adjustable.

5. A water treatment method comprising: a treatment tank for storing water to be treated received from outside; one or more plasma micro-bubble generators disposed within the treatment tank, which receive the water to be treated stored in the treatment tank and generate plasma inside micro-bubbles contained within the water to be treated; a high-voltage power supply; and a flow path for discharging the treated water from the treatment tank to the outside, wherein the plasma micro-bubble generator comprises a first metal electrode and a second metal electrode, which are disposed on either side of the flow path through which the micro-bubbles flow out, and which are electrically insulated; and wherein the high-voltage power supply applies a voltage between the first metal electrode and the second metal electrode, thereby generating plasma inside micro-bubbles contained in the water to be treated that flows out from the gap between the fluid holder and the impact receiver.

6. A water treatment method according to claim 5, characterized in that it comprises a circulation pump for circulating a portion of the water to be treated and / or the treated water in the treatment tank to the inlet of the plasma micro-bubble generator.

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