Water treatment apparatus and water treatment method

A water treatment apparatus including a central electrode, and an outer periphery electrode, water to be treated being treated by applying a high voltage between the central electrode and the outer periphery electrode to thereby form a discharge in a discharge space between the central electrode and the outer periphery electrode and supplying the water to be treated as water droplets or a water film into the discharge space from above the discharge space, the water treatment apparatus further including a water droplet reformation unit connected to the outer periphery electrode, wherein the water droplet reformation unit captures, as trapped water, a portion of the water to be treated falling in the discharge space, performs water droplet reformation by causing the captured trapped water and gas containing oxygen supplied via a gas pipe to be mixed, and jets out water droplets formed by water droplet reformation into the discharge space.

TECHNICAL FIELD

The present invention relates to a water treatment apparatus and a water treatment method in which water to be treated is treated using ozone, radicals, and the like generated by a discharge.

BACKGROUND ART

Until now, ozone and chlorine have been widely used in water and sewage treatment. However, persistent substances that are not decomposed by ozone or chlorine may be contained in, for example, industrial wastewater, recycled water, and the like. In particular, removal of dioxins, dioxane, and the like is a major problem.

In some areas, a method of removing persistent substances by combining ozone (O3) with hydrogen peroxide (H2O2) or ultraviolet light, thereby causing hydroxyl radicals (OH radicals), which are higher in activity than ozone or chlorine, to be generated in water to be treated, is in practical use. However as equipment and operation costs are very high, this method is not very prevalent.

In view of this, a method has been proposed in which persistent substances are removed with high efficiency by causing OH radicals generated by a discharge to act directly on water to be treated. More specifically, a water treatment apparatus has been proposed in which a streamer discharge is formed by applying a pulse voltage between a linear high-voltage electrode and a cylindrical ground electrode that surrounds the high-voltage electrode, and water to be treated is treated by supplying the water to be treated in a water droplet state to a streamer discharge space from thereabove (see PTL 1, for example).

The water treatment apparatus in PTL 1 is provided with a gas suction supply means for sucking out gas from inside a treatment chamber and, during a preliminary process in which the water to be treated is formed into water droplets, supplying the sucked out gas into the water to be treated in the form of gas bubbles.

Therefore, with the water treatment apparatus in PTL 1, short-lived OH radicals generated in the streamer discharge space can be caused to act on the water to be treated efficiently. Further, ozone generated in the streamer discharge space can be used, without waste, for decomposition of a substance to be treated. As a result, decomposition treatment of the substance to be treated can be performed more efficiently.

In addition, a water treatment apparatus has been proposed in which a streamer discharge is formed by applying a pulse voltage between a linear high-voltage electrode and a cylindrical ground electrode surrounding the high-voltage electrode, and water to be treated is treated by supplying the water to be treated in a water droplet state into a streamer discharge space from thereabove, wherein grids constituted by an insulating material are formed in a plurality of stages in a region where the water droplets fall (see NPL 1, for example).

With the water treatment apparatus in NPL 1, short-lived OH radicals generated in the streamer discharge space can be caused to act on the water to be treated efficiently. Further, as the falling water droplets collide with the grids, falling speed thereof is lost, such that a residence time of the water droplets in the streamer discharge space can be prolonged. For this reason, a water treatment apparatus that realizes a higher decomposition efficiency and a higher decomposition speed can be obtained.

CITATION LIST

Patent Literature

Non Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, the following problems exist in the abovementioned prior art. That is, the conventional water treatment apparatus indicated in PTL 1 has a problem in that, if a reactor height is increased in order to increase an amount of water treated per reactor, organic matter decomposition efficiency reduces as water droplets advance toward a lower part of the reactor. Therefore, a problem exists in that high-speed water treatment and high-efficiency water treatment cannot be realized at the same time.

Further, in the conventional water treatment apparatus indicated in NPL 1, it is necessary to make spacing of the grids wide so that water droplets can fall. For this reason, particle diameters of the water droplets that fall from the grids increase, and a surface area thereof with which OH radicals and the like react is reduced. For this reason, a problem exists in that efficient treatment cannot be performed.

The present invention has been made to solve the abovementioned problems, and an object thereof is to obtain a water treatment apparatus and a water treatment method capable of performing highly efficient and high-speed decomposition of persistent substances or removal of highly concentrated organic contamination.

Solution to Problem

A water treatment apparatus according to the present invention includes a central electrode provided such that a longitudinal direction thereof is set to be vertical, and an outer periphery electrode provided coaxially with the central electrode so as to surround the central electrode, water to be treated being treated by applying a high voltage between the central electrode and the outer periphery electrode to thereby form a discharge in a discharge space between the central electrode and the outer periphery electrode and supplying the water to be treated as water droplets or a water film into the discharge space from above the discharge space, the water treatment apparatus further including a water droplet reformation unit connected to the outer periphery electrode, wherein the water droplet reformation unit captures, as trapped water, a portion of the water to be treated falling in the discharge space, performs water droplet reformation by causing the captured trapped water and gas containing oxygen supplied via a gas pipe to be mixed, and jets out water droplets formed by water droplet reformation into the discharge space.

A water treatment method according to the present invention is a water treatment method to be applied in a water treatment apparatus including a central electrode provided such that a longitudinal direction thereof is set to be vertical, and an outer periphery electrode provided coaxially with the central electrode so as to surround the central electrode, water to be treated being treated by applying a high voltage between the central electrode and the outer periphery electrode to thereby form a discharge in a discharge space between the central electrode and the outer periphery electrode and supplying the water to be treated as water droplets or a water film into the discharge space from above the discharge space, the water treatment method including, in the water droplet reformation unit connected to the outer periphery electrode, a step in which a portion of the water to be treated falling in the discharge space is captured as trapped water, a step in which water droplet reformation is performed by causing the captured trapped water and gas containing oxygen supplied via a gas pipe to be mixed, and a step in which water droplets formed by the water droplet reformation are jetted out into the discharge space.

Advantageous Effects of Invention

With the present invention, when water to be treated passes through a discharge space, a concentration of organic matter and a concentration of oxidizing substances such as ozone in the water to be treated are homogenized through repeated formation into water droplets and coalescence. As a result, speed and efficiency of organic matter decomposition does not decline even as the water to be treated proceeds to a lower part of the discharge space. Further, as gas is used to form the water droplets, water droplets having a smaller diameter are formed, such that a reactivity area between the water to be treated and oxidizing substances such as ozone is larger. As a result, a water treatment apparatus and a water treatment method capable of performing highly efficient and high-speed decomposition of persistent substances or removal of highly concentrated organic contamination can be obtained.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG. 1is a cross-sectional view showing a water treatment apparatus according to a first embodiment of the present invention. A water treatment apparatus110is provided with a water treatment reactor100, a water to be treated tank1, and a treated water tank2. The water treatment reactor100is provided with a water receiver50, which is a bottom portion thereof and, thereabove, a discharge tube60, which is hermetically connected to the water receiver50.

The discharge tube60is provided with a cylindrical electrode10, that is, an outer periphery electrode, a longitudinal direction of which is set to be vertical, a ring-shaped upper insulator11and a ring-shaped lower insulator12, which are provided respectively on an upper portion and a lower portion of the cylindrical electrode10, a rod-shaped upper holder13and a rod-shaped lower holder14, which are provided respectively in the upper insulator11and the lower insulator12and extend from an outer periphery of the cylindrical electrode10toward a center thereof, and a wire electrode15, that is, a central electrode, attached between the upper holder13and the lower holder14so as to be coaxial with the cylindrical electrode10.

A gap exists between the cylindrical electrode10and the wire electrode15, and the gap constitutes a discharge space25. Space inside the upper insulator11, the cylindrical electrode10, and the lower insulator12is hermetically connected so as to be continuous in a vertical direction. A top plate16covering the discharge tube60is hermetically connected to an uppermost part of the upper insulator11. Further, a nozzle17is attached, so as to face downward in a vertical direction, at a central vicinity of the top plate16in a position at which the axis thereof is coincident with that of the wire electrode15.

In addition, the discharge tube60is provided with three water droplet reformation units44a,44b, and44c, arranged sequentially in a longitudinal direction. A suction port22is provided in an upper part of the water receiver50. In addition, a circulation pipe23is attached to the suction port22. Further, the circulation pipe23is provided with a compressor24, that is, a gas suction unit.

The circulation pipe23is branched into three lines, namely23a,23b, and23c, at a subsequent stage to the compressor24. The branched circulation pipes23a,23b, and23care respectively connected to the water droplet reformation units44a,44b, and44c.

The water to be treated tank1and the nozzle17are connected to each other by a water supply pipe18, and the water supply pipe18is provided with a water supply pump19.

In addition, a bottom vicinity of the water receiver50and the treated water tank2are connected to each other by a drainpipe20. Further, the drainpipe20is provided with a drainage pump21.

The upper insulator11is provided with an introduction terminal27, and a high voltage power supply28and the wire electrode15are connected to each other by an electric wire29via the introduction terminal27. Further, the cylindrical electrode10is electrically grounded.

A gas supply port30is formed in the water receiver50. The gas supply port30and an oxygen gas supply source31are connected to each other by a gas pipe33via a flow rate regulator32. Further, an exhaust port34is formed at an upper part of the water receiver50. A check valve36is attached to an exhaust pipe35, which is connected to the exhaust port34.

FIG. 2is a cross-sectional perspective view of the discharge tube60according to the first embodiment of the present invention.FIG. 3is a transverse cross-sectional view of a water droplet reformation unit44according to the first embodiment of the present invention. Further,FIG. 4is a longitudinal sectional view of the water droplet reformation unit44according to the first embodiment of the present invention. A specific water treatment in the first embodiment will be described hereinafter usingFIG. 2,FIG. 3, andFIG. 4.

The cylindrical electrode10is provided with the water droplet reformation unit44. The water droplet reformation unit44in the first embodiment is provided with a ring-shaped circulated gas flow path52, which covers an outer surface of the cylindrical electrode10, and a water-droplet-forming member46(a capturing member) constituted by a ring-shaped insulator and attached so as to run along an inner surface of the cylindrical electrode10.

The water-droplet-forming member46is attached to the cylindrical electrode10in such a way that a predetermined gap is formed between the water-droplet-forming member46and the wire electrode15and contact does not occur therebetween. An upper surface side of the water-droplet-forming member46has an inclined surface47(a collecting portion) formed so that a central vicinity thereof is lowest. A plurality of water flow paths48are formed in the longitudinal direction so as to extend from a lowest part of the inclined surface47. Note that the water flow paths48are formed so as not to penetrate a bottom of the water-droplet-forming member46.

The water-droplet-forming member46includes gas flow paths49. The gas flow paths49are formed so as to penetrate obliquely downward from an outer periphery of the water-droplet-forming member46toward an inner periphery thereof. Further, the gas flow path49communicates with a jet port54formed on the inner periphery of the water-droplet-forming member46. The gas flow path49and the water flow path48intersect each other at an intersection51.

The cylindrical electrode10is provided with a plurality of through holes53that penetrate a tube wall. The through holes53are formed at a position continuous to the gas flow paths49, and gas flowing through the circulated gas flow path passes through the gas flow paths49from the through holes53to the discharge space25. Further, a cross-sectional area of the gas flow paths49is smallest at the intersection51.

Next, water treatment operations in the first embodiment will be described usingFIG. 1toFIG. 4. Oxygen gas supplied from the oxygen gas supply source31passes through the gas pipe33and, following adjustment to a predetermined flow rate by the flow rate regulator32, is supplied into the water receiver50from the gas supply port30.

Gas is exhausted from the exhaust pipe35to an exterior at the same flow rate as the flow rate of the supplied oxygen gas. Here, the flow of gas in the exhaust pipe35is restricted to one direction, that is, from the water receiver50to the exterior, by the check valve36.

By driving the compressor24, gas in the water treatment reactor100is sucked out from the suction port22, passes through the circulation pipe23, and is supplied to the water droplet reformation units44a,44b, and44c.

Meanwhile, by driving the water supply pump19, water to be treated3in the water to be treated tank1is sucked out therefrom, passes through the water supply pipe18and is supplied to the nozzle17. The water to be treated3is formed into water droplets by the nozzle17, falls inside the upper insulator11, the cylindrical electrode10, and the lower insulator12, and is accumulated in the water receiver50as accumulated water5.

At this time, a portion of water droplets4falling from the nozzle17adhere to an inner wall of the cylindrical electrode10and flow downward while forming a water film45, run down the inclined surface47of the water-droplet-forming member46in the water droplet reformation unit44aand become trapped water55.

In addition, a portion of the water droplets4formed by the nozzle17, in the process of falling inside the cylindrical electrode10, collide with the upper surface of the water-droplet-forming member46, flow downward along the inclined surface47, and become trapped water55.

Then, the trapped water55runs down through the water flow path48and is jetted out in the form of water droplets from the jet port54via the intersection51by the circulated gas8flowing through the gas flow path49.

At this time, the cross-sectional area of the gas flow path49is smallest at the intersection51. For this reason, negative pressure is generated in the water flow path48by the Venturi effect, which sucks in the trapped water55. As a result, the trapped water55and the circulated gas8are mixed to form water droplets.

In the same way as in the water droplet reformation unit44a, portions of the water to be treated3falling in the discharge tube60are, likewise, repeatedly captured and reformed as water droplets in the water droplet reformation units44band44ctherebelow. The water to be treated3subsequently reaches the water receiver50, is accumulated therein as accumulated water5, passes through the drainpipe20due to the drainage pump21, and is transported to the treated water tank2to become treated water6.

Here, in the process of the water to be treated3falling in the discharge tube60, the high voltage power supply28is operated and a pulsed high voltage is applied to the wire electrode15, thereby forming discharges7in the discharge space25between the wire electrode15and the cylindrical electrode10, with the result that oxidizing particles such as OH radicals and O3are generated. As a result, organic matter in the water to be treated3falling in the discharge tube60reacts with the oxidizing particles such as OH radicals and O3and decomposes, whereby water treatment is performed.

Next, principles by which water treatment of the water to be treated3is performed by the water treatment apparatus indicated in the first embodiment will be described. Here, description is given using decomposition of organic matter as an example, however, it is a well-known fact that O3and OH radicals generated by a discharge are also effective for removal of bacteria, decoloration, and deodorization.

The discharges7are formed in the discharge space25by applying a pulse voltage to the wire electrode15. At this time, oxygen molecules (O2) and water molecules (H2O) collide with high-energy electrons, and dissociation reactions indicated by the following formulas (1) and (2) occur. Here, e is an electron, O is atomic oxygen, H is atomic hydrogen, and OH is an OH radical.
e+O2→2O  (1)
e+H2O→H+OH  (2)

Much of the atomic oxygen generated in the above formula (1) becomes ozone (O3) due to the reaction of the following formula (3). Here, M is a third body of the reaction and represents any molecule or atom present in air.
O+O2+M→O3(3)

Further, a portion of the OH radicals generated by the above formula (2) become hydrogen peroxide (H2O2) due to the reaction of the following formula (4).
OH+OH→H2O2(4)

Oxidizing particles (O, OH, O3, and H2O2) generated by the reactions of the above formulas (1) to (4) react, by the following formula (5), with organic matter on water surfaces of the water droplets4and the water film45and oxidatively decompose the organic matter into carbon dioxide (CO2) and water. Here, R is organic matter to be treated.
R+(O,OH,O3,H2O2)→CO2+H2O  (5)

Meanwhile, a portion of the O3and H2O2generated by the above formulas (3) and (4) is dissolved into the water to be treated3from the water surfaces of the water droplets4and the water film45by the following formulas (6) and (7). Here, (l) indicates a liquid phase.
O3→O3(l)  (6)
H2O2→H2O2(l)  (7)

Further, OH radicals are generated in water by a reaction between O3(l) and H2O2(l) as indicated by the following formula (8).
O3(l)+H2O2(l)→OH(l)  (8)

O3(l), H2O2(l), and OH (l) generated by the above formulas (6) to (8) decompose organic matter through a reaction in water as indicated by the following formula (9).
R+(O3(l),H2O2(l),OH(l))→CO2+H2O  (9)

As described above, decomposition of organic matter the in water to be treated3by the first embodiment can be thought of as proceeding through both decomposition of organic matter on surface layers of the water droplets4and the water film45by the reaction indicated by the above formula (5) and decomposition of organic matter in water by the reaction shown in the above formula (9).

Next, reasons for which decomposition of persistent substances and removal of highly concentrated organic contamination can be performed with high efficiency and high speed by the water treatment apparatus according to the first embodiment will be described.

When the water droplets4fall in the discharge space25or when the water film45flows down the inner wall of the cylindrical electrode10, organic matter on the surface layers of the water droplets4and the water film45is decomposed by the reaction of the above formula (5). As a result, the amount of organic matter on the surface layers of the water droplets4and the water film45is reduced, the reaction frequency between oxidizing substances such as OH radicals and organic matter decreases, and a proportion of oxidizing substances such as OH that is ineffectively consumed increases.

Organic matter in the water droplets4and the water film45is also decomposed by the reaction of the above formula (9). However, as diffusion of O3(l) and H2O2(l) in water is slow, O3(l), H2O2(l), and OH (l) concentrate near the surface layers of the water droplets4and the water film45, and do not diffuse into an interior of the water droplets4or into a deeper part of the water film45.

For this reason, as organic matter near the surface is decomposed immediately after the water droplets4start to fall down in the discharge space25or immediately after the water film45starts flowing down the inner wall of the cylindrical electrode10, concentrations of O3(l), H2O2(l), and OH (l) on the surface layers of the water droplets4and the water film45increase while a concentration of organic matter decreases and, conversely, concentrations of O3(l), H2O2(l), and OH (l) in the interior of the water droplets4and in the deeper part of the water film45decrease while a concentration of organic matter increases. For this reason, reaction frequency of the above formulas (5) and (9) decreases in both regions and efficiency of water treatment is reduced.

However, in the first embodiment, the water droplets4and the water film45formed by the nozzle17are subjected, until the water droplet reformation unit44ahas been reached, to the reactions of the above formulas (5) and (9), such that organic matter near the surfaces thereof is decomposed. Next, a portion of the water droplets4and the water film45collide with the inclined surface47of the water droplet reformation unit44a, thereby coalescing with other water droplets as the trapped water55. In this process, organic matter, O3(l), and H2O2(l) are agitated and concentrations thereof homogenized such that the reaction of the above formula (9) occurs throughout all of the trapped water55.

Further, new water droplets4formed by the water droplet reformation unit44a, an organic matter concentration thereof having been homogenized, are re-supplied into the discharge space25. Therefore, new organic matter appears on the surfaces of the water droplets, the reaction of the above formula (5) occurs actively again, and decomposition of the organic matter proceeds. The same process is repeated thereafter as portions of the water droplets4and the water film45pass through the water droplet reformation units44band44c.

In other words, in the first embodiment, water droplet formation and coalescence is repeated as the water to be treated3falls in the discharge space25. Each time, O3(l) and H2O2(l) are agitated and homogenized, and new organic matter appears on the surfaces of the water droplets4. As a result, ineffective consumption of oxidizing substances such as OH radicals is suppressed and decomposition of organic matter by the above formulas (5) and (9) takes place efficiently.

Here, decomposition of organic matter in water to be treated flowing downward as a water film is less effective than decomposition of organic matter in water to be treated falling as water droplets. This is because, when compared using the same volume of water, a surface area of the water film45is smaller than that of the water droplets4and, accordingly, the frequency at which the reactions of the above formulas (5) to (7) take place is lower.

However, as shown inFIG. 3, in the first embodiment, all of the water film45flowing down the inner wall of the cylindrical electrode10is captured in the water droplet reformation units44a,44b, and44c, and then jetted into the discharge space25as water droplets. Therefore, the water film45, in which organic matter decomposition efficiency is low, is converted into the water droplets4, in which organic matter decomposition efficiency is high, such that water treatment efficiency increases as a whole.

Moreover, when compared using the same volume of water, a surface area of the water droplets4is larger when a size thereof (a water droplet diameter) is smaller, such that organic matter is decomposed more efficiently. In other words, a large number of water droplets4having a small diameter allows water treatment to occur more efficiently than with a small number of water droplets4having a large diameter.

With the first embodiment, the trapped water55is formed into water droplets in the water droplet reformation units44a,44b, and44cby the circulated gas8. For this reason, the water droplets4can be formed so as to have a smaller diameter such that water treatment efficiency improves in comparison to when, as in NPL 1 for example, the water to be treated3having been captured by a grid falls due to gravity.

Further, as shown inFIG. 1, in the first embodiment, the water droplets are jetted obliquely downward by the water droplet reformation units44a,44b, and44c. For this reason, in contrast to when the water droplets fall perpendicularly downward, travel also occurs in the lateral direction. As a result, a distance traveled in the discharge space25is longer in comparison to when the water droplets fall perpendicularly downward, and contact between the water droplets and oxidizing substances such as OH radicals occurs more, such that decomposition of organic matter proceeds quickly.

Also, as the water droplets4are jetted obliquely downward, a portion thereof collides with and adheres to the wire electrode15. For this reason, an effect is obtained in which the wire electrode15, a temperature thereof having risen due to discharges, is cooled by the adhesion of the water droplets4thereto. As a result, breakage of the wire electrode15can be prevented even when a high discharge power is applied, and the apparatus can be stably operated for a long time.

Moreover, in the first embodiment, water droplets4are formed by supplying the circulated gas8to the water droplet reformation units44a,44b, and44c. It is therefore unnecessary to prepare a separate gas. For this reason, an amount of oxygen gas supplied from the oxygen gas supply source31need only be enough to compensate for an amount thereof consumed by decomposition of organic matter and an amount consumed by dissolution thereof into the water to be treated3, such that it is possible to perform efficient, high-speed water treatment without increasing a gas cost.

Further, in the first embodiment, a separate gas for water droplet reformation is not supplied. For this reason, the supplied oxygen gas stays in the discharge space25for a long period of time, and an ozone gas concentration increases. Here, the ozone dissolution reaction of the above formula (6) proceeds more readily when the ozone gas concentration is higher. As a result, a dissolved ozone concentration (O3(l)) also increases, such that decomposition of organic matter by the above formula (9) occurs efficiently and at high speed.

Further, in the first embodiment, oxidizing particles such as ozone generated by the discharges7are contained in the circulated gas8. Accordingly, when water droplets are formed by the water droplet reformation units44, the circulated gas8containing oxidizing particles and the water to be treated3contact each other, thereby facilitating dissolution of ozone into the water to be treated3. As a result, high-speed and efficient water treatment can be performed.

Note that, in the first embodiment, the water droplets4are formed by the nozzle17. However, a means for forming the water droplets4is not limited to the nozzle17. For example, a showerhead can be used as a means for forming the water droplets4, as can ultrasonic atomization or electrostatic atomization.

Further, there is no specific limit on the particle diameter of water droplets to be formed. In general, however, the area of the gas-liquid interface increases when the particle diameter is smaller, allowing water treatment to proceed efficiently and at high speed.

However, more energy is required to form water droplets that have a small particle diameter than to form water droplets that have a large particle diameter. Therefore, it is preferable to appropriately determine the particle diameter of the water droplets upon consideration of water treatment efficiency and speed.

Although the wire electrode15is used in the first embodiment, a material used for or a wire diameter thereof can be appropriately determined in accordance with specifications of the high voltage power supply28, an operating ratio of the water treatment apparatus110and, further, a water quality of the water to be treated3. For example, by making the wire electrode15thinner, the electric field is concentrated and a discharge can be formed with a lower voltage, with the result that performance required of the high voltage power supply28can be lowered.

However, by making the wire electrode15thinner, the possibility of breakage increases. As a result, it is necessary to frequently replace the wire electrode15, which brings about an increase in maintenance costs and a decrease in the operating ratio of the apparatus.

Stainless steel or titanium, which have excellent resistance to corrosion, are desirable as the material to be used for the wire electrode15, although appropriate selection should be made upon consideration of cost and lifespan. In addition, the central electrode is not required to have a wire shape as in the first embodiment, and a rod shape, a rod shape having projections, a screw shape, or the like, for example, may also be used.

Further, although oxygen gas is supplied into the water treatment reactor100in the first embodiment, the supplied gas is not limited to oxygen. As long as in a gas containing oxygen, the reactions of the above formulas (1) to (9) occur such that water treatment can be performed. A mixed gas constituted by oxygen and a rare gas such as argon, for example, can be used as the gas to be supplied into the water treatment reactor100.

In general, when oxygen is mixed with a rare gas, the voltage necessary for forming a discharge decreases. For this reason, the voltage output from the high voltage power supply28can be reduced, which leads to a reduction in the cost of the apparatus. Moreover, if a mixed gas constituted by oxygen and nitrogen, or specifically, air is used, the gas supply source is no longer necessary, and water treatment can be performed under normal atmospheric conditions.

However, the speed at which ozone is generated by the discharges increases as the concentration of oxygen in the gas rises. Therefore, in general, the speed of water treatment improves when the oxygen concentration is higher. Accordingly, it is desirable to determine gas composition upon consideration of gas cost and speed of water treatment.

Further, in the first embodiment, three of the water droplet reformation units44are provided, however, a number water droplet reformation units44is not limited thereto, and the effect of the present invention can be obtained as long as one or more of the water droplet reformation units44are provided. In general, formation and coalescence of water droplets is repeated at a higher frequency when a larger number of the water droplet reformation units44are used, with the result that efficiency and speed of water treatment are improved. On the other hand, if the number of water droplet reformation units44is made excessive, an effect thereof tends to saturate. Accordingly, it is preferable to determine the number of water droplet reformation units44upon consideration of the flow rate of the water to be treated3, the height of the discharge tube60, the particle diameter of the water droplets4, and the like.

Further, in the first embodiment, it is preferable to set a pressure in the water treatment reactor100so as to be in the vicinity of atmospheric pressure to make supply and drainage of the water to be treated3easier. However, the pressure in the water treatment reactor100can also be set to positive pressure or negative pressure as required. When pressure in the water treatment reactor100is set to positive pressure, contamination by air from the exterior is suppressed such that the atmosphere therein can be easily managed.

Further, when the pressure in the water treatment reactor100is set to negative pressure, discharges are formed at a relatively low voltage, such that it is possible to downsize and simplify the high voltage power supply28. Moreover, as discharges are more inclined to disperse when the pressure is lower, the water to be treated3contacts the discharges7over a wider area, such that efficiency and speed of water treatment are improved.

Further, in the first embodiment, the direction at which water droplets are jetted from the water droplet reformation unit44does not necessarily have to be obliquely downward as shown inFIG. 2, and can be set arbitrarily. For example, if the gas flow path49is provided at an incline so as to be lower at the outer peripheral side of the water-droplet-forming member46and higher at the center side thereof, the water droplets4jetted from the jet port54will be directed obliquely upward.

In such a case, the formed water droplets having initially advanced obliquely upward then begin to fall due to gravity such that, in comparison to when jetted downward, a residence time thereof in the discharge tube60is extended. As a result, efficiency and speed of water treatment are improved.

Note that, in the first embodiment, a pulse voltage is output from the high voltage power supply28to form the discharges7. However, as long as discharges can be stably formed, a pulse voltage is not necessarily required, and an AC voltage or a DC voltage, for example, may also be used. Further, a polarity, a peak voltage value, a repetition frequency, a pulse width, and the like of a pulse voltage can be appropriately determined in accordance with various conditions such as electrode structure and gas type.

In general, a peak voltage value of 1 kV to 50 kV is desirable. This is because a stable discharge is not formed if the voltage is less than 1 kV, and cost increases markedly if the voltage set to more than 50 kV due to enlargement of the power supply and difficulties involved in electrical insulation.

Further, it is preferable to set the repetition frequency at 10 pps (pulse-per-second) to 100 kpps. This is because if the repetition frequency is lower than 10 pps, a very high voltage is required to apply sufficient discharge power, whereas if the repetition frequency is higher than 100 kpps, the effect of water treatment is saturated and power efficiency decreases. Further, the voltage, the pulse width, and the pulse repetition frequency may be adjusted in accordance with the flow rate of the water to be treated3or a concentration of a substance to be treated.

Further, in the first embodiment, the trapped water55is formed into water droplets by supplying the circulated gas8to the water droplet reformation units44. However, the gas supplied to the water droplet reformation units44does not necessarily have to be circulated gas. For example, the oxygen gas from the oxygen gas supply source31may be supplied from the water droplet reformation unit44instead of being supplied from the gas supply port30. Alternatively, a gas source for supplying the water droplet reformation units44may be separately arranged and used.

However, the effect in which dissolution of ozone in water to be treated3is facilitated by contact between the circulated gas8containing oxidizing particles and the water to be treated3, and the effect in which efficient, high-speed water treatment is performed at low cost by reducing an amount of gas used, are obtained by using circulated gas. Accordingly, it is more appropriate to form the water droplets4in the water droplet reformation units44using the circulated gas8as in the first embodiment.

Second Embodiment

FIG. 5is a cross-sectional perspective view of a discharge tube60according to a second embodiment of the present invention.FIG. 6is a transverse sectional view of a water droplet reformation unit44according to the second embodiment of the present invention. Upon comparison, a configuration of the water droplet reformation unit44in the second embodiment differs from that of the first embodiment. InFIG. 5andFIG. 6, the water droplet reformation unit44in the second embodiment is provided with a circulated gas flow path52, through holes53, and a baffle plate58, that is, a capturing member.

A plurality of the through holes53are formed in a circle of uniform height around and so as to penetrate an outer periphery of a cylindrical electrode10. A baffle plate58is provided immediately below the through holes53on an inner side the cylindrical electrode10. Here, the baffle plate58is disposed via a gap so as not to contact a wire electrode15. The circulated gas flow path52is provided so as to cover the through holes53from the outer periphery of the cylindrical electrode10, and is hermetically connected to the cylindrical electrode10. Other configurations are the same as those of the first embodiment.

Next, water treatment operations in the second embodiment will be described usingFIG. 5andFIG. 6. A portion of water droplets4and a water film45falling in the cylindrical electrode10are trapped on an upper surface of the baffle plate58, that is, a collecting portion, and accumulated on the baffle plate58as trapped water55. Here, circulated gas8flows from the circulated gas flow path52and through the through hole53toward the interior of the cylindrical electrode10. As a result, the trapped water55mixes with the circulated gas8, is flicked off the baffle plate58, and falls into the cylindrical electrode10as water droplets. Other operations are the same as those of the first embodiment.

With the second embodiment, repetition of water droplet formation and coalescence of the water to be treated3can be realized using a simpler configuration than that of the first embodiment. As a result, highly efficient and high-speed water treatment can be achieved similarly to as in the first embodiment.

Note that, in the second embodiment, it is not necessarily required that the baffle plate58is attached horizontally as indicated inFIG. 5andFIG. 6. For example, an upper surface of the baffle plate58may have an incline so as to be lower at an outer peripheral side of the cylindrical electrode10and higher at a center side thereof. In such a case, the water droplets formed by the water droplet reformation unit44are jetted obliquely upward and are thus present in the interior of the discharge tube60for a longer period of time. As a result, efficiency and speed of water treatment can be increased.

Third Embodiment

FIG. 7is a cross-sectional view showing a water treatment apparatus according to a third embodiment of the present invention. The third embodiment differs from the first embodiment in that a water treatment reactor100is provided with three discharge tubes60a,60b, and60cand one water receiver50. InFIG. 7, a water supply pipe18connected from the water to be treated tank1is branched into three lines (18a,18b, and18c), which are respectively connected to the discharge tubes60a,60b, and60c.

That is to say, the respective discharge tubes60a,60b, and60care connected in parallel to the water to be treated tank1. In addition, the discharge tubes60a,60b, and60care respectively provided with circulated gas flow paths52a,52b, and52c, all of which are provided so as to cover cylindrical electrodes10a,10b, and10c.

Moreover, circulation pipes23a,23b, and23care respectively connected to the circulated gas flow paths52a,52b, and52c, and the circulation pipes23a,23b, and23care all connected to a circulation pipe23, which is connected to the water receiver50.

Further, wire electrodes15a,15b, and15crespectively provided in the discharge tubes60a,60b, and60care connected in parallel to a high voltage power supply28(not shown). In addition, the cylindrical electrodes10a,10b, and10care all electrically grounded. Other configurations are the same as those of the first embodiment.

Next, water treatment operations in the third embodiment will be described usingFIG. 7. Water to be treated3in the water to be treated tank1is transported by a water supply pump19, flows through the water supply pipes18,18a,18b, and18c, and is supplied to each of the discharge tubes60a,60b, and60c, which are provided in parallel, in the form of water droplets. At this time, the high voltage power supply28(not shown) is driven and a high voltage pulse voltage is applied to the wire electrodes15a,15b, and15c, thereby forming discharges in the discharge tubes60a,60b, and60c.

Further, by operating a compressor24to suck out gas in the water receiver50and supplying the sucked out gas to the discharge tubes60a,60b, and60cthrough the circulated gas flow paths52a,52b, and52c, water to be treated3is formed into water droplets. Details concerning water droplet reformation units and other operations are the same as those of the first embodiment.

With the third embodiment, discharges can be formed in each of the three discharge tubes60a,60b, and60cconnected in parallel to the single high voltage power supply28. As a result, water treatment speed is improved in comparison to when only one discharge tube is provided as in the first embodiment.

The three-line circulation pipes23a,23b, and23cand the circulated gas flow paths52a,52b, and52care connected to the single compressor24. Therefore, by driving the compressor24, water droplets can be formed from the water to be treated3in each of the three discharge tubes60a,60b, and60c. Accordingly, in comparison to a case in which three of the water treatment apparatuses as in the first embodiment are used, for example, the apparatus can be greatly simplified.

Further, with the third embodiment, the circulated gas flow paths52a,52b, and52care provided so as to cover the cylindrical electrodes10a,10b, and10c, and have a double pipe configuration. For this reason, although three of the water droplet reformation units44are provided for each of the discharge tubes, only one circulation pipe23is required for each of the discharge tubes, in contrast to the first embodiment, with the result that the configuration of the apparatus is simplified.

InFIG. 7, the circulation pipes23a,23b, and23care connected in series to the compressor24, however, the circulation pipe23may also be branched into three lines, which are connected to the compressor24in parallel. In other words, any configuration in which the circulated gas flow paths52a,52b, and52care connected by pipes to form one continuous space is sufficient.

Fourth Embodiment

FIG. 8is a cross-sectional view showing a water treatment apparatus according to a fourth embodiment of the present invention. In the fourth embodiment, a suction port22is provided in the vicinity of an uppermost part of a cylindrical electrode10. A gas-liquid separator56is provided in a circulation pipe23between the suction port22and a compressor24. A gas outlet of the gas-liquid separator56is connected to the compressor24, and a liquid outlet of the gas-liquid separator56is connected to a drainpipe57. Further, the drainpipe57is connected to a water receiver50. Other configurations are the same as those of the first embodiment.

Next, water treatment operations in the fourth embodiment will be described. By operating the compressor24, gas in an interior of a discharge tube60is sucked out therefrom. At this time, water to be treated3may be sucked out together with the gas from the suction port22. However, in the fourth embodiment, the water to be treated3is separated from the gas by the gas-liquid separator56and sent to the water receiver50through the drainpipe57.

Meanwhile, the gas separated from the water to be treated3by the gas-liquid separator56is sent through the compressor24to water droplet reformation units44a,44b, and44cas circulated gas8. Other operations are the same as those of the first embodiment.

With the fourth embodiment, the suction port22is formed in the vicinity of the uppermost part of the cylindrical electrode10. Therefore, a flow of the gas in the discharge tube60is an upward flow directed from the water droplet reformation units44c,44b, and44atoward the suction port22. As a result, the gas flow flows counter to the falling water to be treated3, which facilitates contact between the water to be treated3and ozone or the like in the discharge tube60, such that speed and efficiency of water treatment can be improved.

Fifth Embodiment

FIG. 9is a cross-sectional view of a discharge tube60according to a fifth embodiment of the present invention.FIG. 10is a cross-sectional view of a unit electrode70that constitutes the discharge tube60in the fifth embodiment of the present invention shown inFIG. 9.

InFIG. 9, the discharge tube60is provided with four unit electrodes70a,70b,70c, and70dcoupled to each other in a longitudinal direction by bolts73, an upper insulator11attached to a top of the unit electrode70a, a lower insulator12attached to a bottom of the unit electrode70d, and a wire electrode15held by an upper holder13assembled on the upper insulator11and a lower holder14assembled on the lower insulator12.

InFIG. 10, the unit electrode70is provided with a cylindrical tube portion71constituted by metal, coupling portions72provided at upper and lower ends of the cylindrical tube portion, a water-droplet-forming member46formed on an inner side of the cylindrical tube portion, and a circulated gas flow path52attached so as to cover the cylindrical tube portion71from the outer periphery thereof. The water droplet reformation unit44is constituted by the water-droplet-forming member46and the circulated gas flow path52.

In the fifth embodiment, the discharge tube60is formed by coupling together a plurality of the unit electrodes70, each provided with the water droplet reformation unit44and the cylindrical tube portion71. Therefore, in comparison to a case in which the discharge tube60is formed by a single cylindrical electrode10, as in the first embodiment, assembly is easier and mass productivity is improved.

Further, a number of the unit electrodes70to be installed can be determined as appropriate. For this reason, an optimal height of the discharge tube60can be formed in accordance with a type and concentration of organic matter to be decomposed in water to be treated3.

Note that, in the present invention, a gas suction unit is not limited to a compressor, and may be, for example, a blower, a pump, or the like. Further, in the present invention, water droplets indicates an aggregate of water molecules in a liquid state that exist in air, and a particle size and a number density thereof are not specifically limited.

Further, in the present invention, although there is no limitation on a material to be used for a water-droplet-forming member46or a baffle plate58, that is, a capturing member, it is preferable to form the capturing member from an insulator such as glass, ceramic, resin, or the like. This is because, when the capturing member is formed from a conductive material such as a metal, a distance thereof to the wire electrode15may be shortened and strong localized discharges formed, with the result that efficiency of water treatment is reduced.