Plasma water treatment apparatus, and plasma water treatment method

Abstract

To provide a plasma water treatment system and a plasma water treatment method that enable highly efficient decomposition of harmful substances in treated water.SOLUTION: A plasma water treatment apparatus has a discharge part and a power supply for generating plasma. The discharge part has a high-voltage electrode, a grounding electrode, and a dielectric between the high-voltage electrode and the grounding electrode, and a rare gas is present between the high-voltage electrode and the grounding electrode. The power supply for plasma generation applies an alternating voltage between the high-voltage electrode and the ground electrode to cause a plasma to arc in the rare gas between the high-voltage electrode and the ground electrode, bringing the plasma into contact with the liquid surface of the treated water to generate OH radicals in the treated water, and treat harmful substances in the treated water. The power supply for plasma generation performs a duty operation, repeating an ON time in which AC voltage is applied between the high-voltage electrode and the ground electrode, and an OFF time in which no AC voltage is applied between the high-voltage electrode and the ground electrode.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a plasma water treatment apparatus and a plasma water treatment method.

Background Art

[0002] In the treatment of sewage and water supply, ozone treatment and chlorine treatment are generally known. However, treated water such as industrial wastewater may contain hardly decomposable substances such as dioxins and dioxane that cannot be decomposed by ozone treatment or chlorine treatment. Conventionally, there has been a method of generating hydroxyl radicals (OH radicals) with higher reactivity than ozone treatment or chlorine treatment in the treated water by an advanced oxidation treatment method using a combination of ozone, hydrogen peroxide, ultraviolet rays, etc., and decomposing hardly decomposable substances with OH radicals.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the conventional advanced oxidation treatment method, the equipment cost and the operation cost become extremely high. Therefore, a technology that directly generates OH radicals by the action of plasma and decomposes hardly decomposable substances in the treated water with high efficiency is required.

Means for Solving the Problems

[0005] The plasma water treatment apparatus of the embodiment has a discharge unit and a power source for plasma generation. The discharge unit has a high-voltage electrode, a ground electrode, and a dielectric provided between the high-voltage electrode and the ground electrode, and a rare gas exists between the high-voltage electrode and the ground electrode. The power source for plasma generation applies an alternating voltage between the high-voltage electrode and the ground electrode to arc plasma in the rare gas between the high-voltage electrode and the ground electrode, brings the plasma into contact with the liquid surface of the water to be treated, generates OH radicals in the water to be treated, and treats harmful substances contained in the water to be treated. Further, the power source for plasma generation executes a duty operation in which an ON time for applying an alternating voltage between the high-voltage electrode and the ground electrode and an OFF time for not applying an alternating voltage between the high-voltage electrode and the ground electrode are repeated.

Brief Description of Drawings

[0006]

Figure 1

Figure 2

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Figure 6

Embodiments for Carrying Out the Invention

[0007] Hereinafter, an example of the plasma water treatment apparatus and the plasma water treatment method according to the present embodiment will be described with reference to the accompanying drawings.

[0008] (First Embodiment) FIG. 1 is a diagram showing an example of the configuration of a plasma water treatment apparatus according to the first embodiment. As shown in FIG. 1, the plasma water treatment apparatus according to this embodiment includes a discharge unit 1, a plasma propagation unit 2, and a plasma generation power source 3.

[0009] The discharge unit 1 includes a pair of high-voltage electrodes 1A, a ground electrode 1B, and a dielectric 1C. The dielectric 1C is an example of a dielectric provided between the high-voltage electrode 1A and the ground electrode 1B. In this embodiment, the ground electrode 1B and the dielectric 1C have a coaxial cylindrical structure. Further, a rare gas exists between the high-voltage electrode 1A and the ground electrode 1B. Also, the high-voltage electrode 1A and the ground electrode 1B are connected to the plasma generation power source 3.

[0010] The plasma generation power source 3 applies an alternating voltage between the high-voltage electrode 1A and the ground electrode 1B to arc the plasma 4 in the rare gas between the high-voltage electrode 1A and the ground electrode 1B. Then, the plasma generation power source 3 brings the arced plasma 4 into contact with the liquid surface of the water to be treated 10 to generate OH radicals in the water to be treated 10, and is an example of a plasma generation power source for treating harmful substances contained in the water to be treated 10. For example, the plasma generation power source 3 applies an alternating voltage having a frequency of 15 kHz and a high-voltage peak value of about 4 kV between the high-voltage electrode 1A and the ground electrode 1B.

[0011] The plasma propagation unit 2 is composed of a cylindrical insulator and is an example of a plasma propagation unit connected to the discharge unit 1. Specifically, the plasma propagation unit 2 arcs the plasma 4 in the rare gas inside the plasma propagation unit 2 from the opening 2B by the action of the plasma 4 arced in the rare gas between the high-voltage electrode 1A and the ground electrode 1B, and discharges the plasma 4 from the opening 2A opposite to the opening 2B to the water to be treated 10.

[0012] In the plasma water treatment apparatus according to the present embodiment, the interiors of the discharge unit 1 and the plasma propagation unit 2 are filled with a plasma generation gas containing a rare gas. As the rare gas contained in the plasma generation gas, a rare gas that can efficiently convert the energy input for plasma generation into plasma generation is preferable, and for example, Ar gas or He gas.

[0013] FIG. 2 is a diagram for explaining an example of an application process of an alternating voltage between a high-voltage electrode and a ground electrode in a conventional plasma water treatment apparatus. In FIG. 2, the vertical axis represents the alternating voltage (applied voltage) applied between the high-voltage electrode 1A and the ground electrode 1B, and the horizontal axis represents time. Next, an example of the plasma irradiation process for the water to be treated 10 by a conventional plasma water treatment apparatus will be described with reference to FIG. 2.

[0014] In a conventional plasma water treatment apparatus, as shown in FIG. 2, an alternating voltage is continuously applied between the high-voltage electrode 1A and the ground electrode 1B from a plasma generation power source 3. Then, in the plasma water treatment apparatus, due to the application of this alternating voltage, a discharge is generated between the high-voltage electrode 1A and the ground electrode 1B at a cycle corresponding to the frequency of the alternating voltage through the dielectric 1C, and plasma 4 is generated in the discharge unit 1.

[0015] Then, in the plasma water treatment apparatus, starting from the space electric field formed by the plasma 4 itself, the plasma 4 is formed by advancing from the discharge unit 1 side in the plasma propagation unit 2 connected to the discharge unit 1. Next, in the plasma propagation unit 2, starting from the space electric field formed by the plasma 4 itself, the plasma 4 is emitted from the opening 2A provided in the plasma propagation unit 2 toward the liquid surface of the water to be treated 10. The emitted plasma 4 contacts the liquid surface of the water to be treated 10, and the water to be treated 10 is irradiated with the plasma 4.

[0016] By irradiating the water to be treated 10 with this plasma 4, highly reactive OH radicals, which are chemically active species, are generated by the reaction described below. The generation mechanism of OH radicals by plasma irradiation mainly involves the following three reactions (1) to (3).

[0017] (1) Reaction in which OH radicals generated by plasma 4 in the gas phase dissolve into the water to be treated This reaction (gas-phase reaction) is, as shown in the following formula (1), a reaction in which OH radicals are generated in the gas phase through the dissociation reaction of water molecules (H 2 O) by electron collision in plasma 4 generated in the gas phase, and the OH radicals dissolve into the water to be treated 10. This gas-phase reaction causes the water molecules evaporating at the interface between the water to be treated 10 and the gas phase to dramatically increase the generation amount of OH radicals. Then, these OH radicals dissolve into the liquid phase (the water to be treated 10). Gas-phase reaction: H 2 O + e → OH + H + e ··············· (1)

[0018] (2) Reaction in which ions in plasma 4 react with water molecules (H 2 O) in the water to be treated 10 to generate OH radicals This reaction (liquid-phase reaction) is, as shown in the following formulas (2) and (3), a reaction in which positive ions generated by plasma 4 in the gas phase dissolve into the water to be treated 10 and OH radicals are generated in the water to be treated 10 through charge exchange with water molecules (H2O). Liquid-phase reaction: Positive ions (N 2 + , O 2 + , etc.) + H 2 O → H 2 O + ··· (2) H 2 O + + H 2 O → H 3 O + + OH ··········· (3)

[0019] (3) Reaction in which plasma light acts on water molecules (H 2 O) in the water to be treated 10 to generate OH radicals This reaction (liquid-phase reaction) is, as shown in the following formula (4), a reaction in which water molecules (H 2This is a reaction in which OH radicals are generated by the photodissociation of O). Usually, the average free path of light in water due to luminescence is several tens of μm. Liquid-phase reaction: Plasma luminescence (hv) + H 2 O → OH + H ········ (4)

[0020] In the plasma water treatment apparatus, the OH radicals generated by the reactions shown in (1) to (3) above act to decompose the hardly decomposable harmful substances contained in the water to be treated 10.

[0021] On the other hand, for the OH radical generation reaction, as shown in the following formula (5), a reaction occurs in which the generated OH radicals disappear. This reaction is a recombination reaction between the generated OH radicals, and stable H 2 O 2 (hydrogen peroxide) is generated. As a result, the action efficiency of the OH radicals acting on the harmful substances in the water to be treated 10 is lowered. Gas-phase·liquid-phase reaction: OH + OH → H 2 O 2 ············· (5)

[0022] Here, a specific example of water treatment in a conventional plasma water treatment apparatus will be described. For example, plasma is formed by pulse discharge in which a pulse voltage is applied between a wire-shaped high-voltage electrode and a flat-shaped ground electrode, and the ground electrode is inclined with respect to the horizontal plane, and the water to be treated is caused to flow along its upper surface to form a thin liquid phase, and a water treatment apparatus has been developed that treats hardly decomposable substances in the water to be treated with OH radicals generated by the plasma by pulse discharge. Furthermore, a water treatment apparatus has been developed that treats the water to be treated with higher efficiency by atomizing at least a part of the water to be treated while forming a pulse discharge between the high-voltage electrode and the ground electrode. According to this water treatment apparatus, the short-lived OH radicals generated by the plasma can act efficiently on the hardly decomposable substances in the water to be treated, and the hardly decomposable substances can be decomposed and treated.

[0023] However, in this water treatment apparatus, since the water to be treated is introduced into the discharge part between the electrodes (with a gap of several millimeters to several tens of millimeters), the plasma is directly affected by the electrical properties of the water to be treated, such as its shape, conductivity, and permittivity, and the discharge forming the plasma tends to be unstable or abnormal discharge (spark discharge). Therefore, it is essential to set the pulse voltage (applied voltage) applied between the high-voltage electrode and the ground electrode to a voltage sufficiently higher than the discharge start voltage (for example, several tens of kV or more) and apply a pulse voltage with a very short pulse width (for example, several hundred nanoseconds). Instead of an inexpensive general-purpose power supply, a plasma generation power supply with special specifications is required. Furthermore, insulation measures for the water treatment apparatus are also necessary.

[0024] Also, for example, a plasma water treatment apparatus has been developed that includes a discharge part having a pair of high-voltage electrodes for arcing plasma and a ground electrode provided via a dielectric with respect to the high-voltage electrodes, a plasma generation power supply for applying an alternating voltage between the high-voltage electrode and the ground electrode, and a plasma propagation part connected to the discharge part and having at least one opening. In this plasma water treatment apparatus, the discharge part and the plasma propagation part are filled with a plasma generation gas containing a rare gas, and the plasma generated in the discharge part is used to generate plasma in the plasma propagation part. Furthermore, in the plasma water treatment apparatus, the plasma generated in the plasma propagation part is discharged from the opening of the plasma propagation part and brought into contact with the liquid surface of the water to be treated to generate OH radicals that are extremely rich in chemical activity in the water to be treated and treat the hardly decomposable harmful substances contained in the water to be treated.

[0025] In this plasma water treatment apparatus, since the plasma generated in the discharge part is irradiated to the water to be treated through the plasma propagation part, it is configured not to introduce the water to be treated into the discharge part. Therefore, in this plasma water treatment apparatus, the plasma in the discharge part is not affected by the electrical properties of the water to be treated, such as its shape, conductivity, and permittivity, and there is no discharge instability or abnormal discharge, enabling stable plasma generation. As a result, a general-purpose alternating current power supply can also be used for the plasma generation power supply.

[0026] Incidentally, in this plasma water treatment apparatus, the operation method of applying an alternating voltage between the high-voltage electrode and the ground electrode is an operation method of continuously applying an alternating voltage (for example, frequency: 15 kHz and voltage: about 4 kV) between the high-voltage electrode and the ground electrode. However, in this operation method, OH radicals generated by the action of plasma disappear due to the recombination reaction of OH radicals as shown in the following formula (6), so the action efficiency of OH radicals acting on harmful substances in the water to be treated is low. OH + OH → H 2 O 2 ····················(6)

[0027] Therefore, in the plasma water treatment apparatus according to this embodiment, the plasma generation power source 3 executes a duty operation that repeats an ON time for applying an alternating voltage between the high-voltage electrode 1A and the ground electrode 1B and an OFF time for not applying an alternating voltage between the high-voltage electrode 1A and the ground electrode 1B. As a result, the recombination reaction between OH radicals generated in the water to be treated 10 by the plasma reaction is suppressed, and the generated OH radicals can act efficiently on the harmful substances in the water to be treated 10. As a result, the decomposition of harmful substances in the water to be treated 10 can be performed with high efficiency. In addition, by providing an OFF time for the output of the alternating voltage from the plasma generation power source 3, the power consumption of the plasma generation power source 3 can be reduced.

[0028] FIG. 3 is a diagram for explaining an example of the output process of an alternating voltage by the plasma generation power source in the plasma water treatment apparatus according to the first embodiment. In FIG. 3, the vertical axis represents the alternating voltage (applied voltage) applied from the plasma generation power source 3, and the horizontal axis represents time. Next, an example of the output process of the alternating voltage by the plasma generation power source 3 in the plasma water treatment apparatus according to this embodiment will be described with reference to FIG. 3.

[0029] In this embodiment, as shown in FIG. 3, the plasma generation power supply 3 provides an OFF time with respect to the output (application) of an alternating voltage between the high-voltage electrode 1A and the ground electrode 1B. That is, the plasma generation power supply 3 executes a duty operation in which the ON time and the OFF time of the alternating voltage between the high-voltage electrode 1A and the ground electrode 1B are repeatedly provided.

[0030] Thus, according to the plasma water treatment apparatus according to the first embodiment, the recombination reaction between OH radicals generated in the water to be treated 10 by the plasma reaction is suppressed, and the generated OH radicals can efficiently act on harmful substances in the water to be treated 10. As a result, the decomposition of harmful substances in the water to be treated 10 can be performed with high efficiency. Further, by providing an OFF time for the output of the alternating voltage from the plasma generation power supply 3, the power consumption of the plasma generation power supply 3 can be reduced.

[0031] (Example 1) In this example, it is an example in which the OFF time is set to 100 μsec or more when performing a duty operation of repeating the ON time and the OFF time. In the following description, the description of the same configuration as that of the first embodiment will be omitted.

[0032] FIG. 4 is a diagram showing an example of the configuration of the plasma water treatment apparatus according to Example 1. The plasma water treatment apparatus according to this example includes a discharge unit 1, a plasma propagation unit 2, and a plasma generation power supply 3.

[0033] The discharge unit 1 includes a high-voltage electrode 1A having a diameter of 2 mm, a ground electrode 1B, and a dielectric 1C that is a quartz glass tube having an outer diameter of 6.15 mm. The plasma propagation unit 2 is composed of an insulating silicon tube.

[0034] The power supply 3 for plasma generation is a 15 kHz power supply. According to the ON / OFF command signal input from an external control device or the like, an AC voltage of 4 kV is applied between the high-voltage electrode 1A and the ground electrode 1B to strike an arc of plasma 4 in He, which is an example of a rare gas, between the high-voltage electrode 1A and the ground electrode 1B. In this embodiment, He is used as the plasma generation gas (rare gas) and is introduced between the high-voltage electrode 1A and the ground electrode 1B of the discharge unit 1 via the flow meter 401.

[0035] Also, in this embodiment, the power supply 3 for plasma generation performs a duty operation with an OFF time provided in a 10 msec cycle. The duty ratio in the duty operation is defined by the following formula (7). Duty ratio (%) = (ON time / (ON time + OFF time)) × 100 ·· (7)

[0036] Starting from the space electric field formed by the plasma 4 itself generated in the discharge unit 1, the plasma 4 is formed by advancing from the discharge unit 1 side into the plasma propagation unit 2 connected to the discharge unit 1. Next, in the plasma propagation unit 2, starting from the space electric field formed by the plasma 4 itself, the plasma 4 is discharged from the opening 2A provided in the plasma propagation unit 2 toward the liquid surface of the water to be treated 10. In this embodiment, the water to be treated 10 is the water treatment solution 404 containing acetic acid. Specifically, the water treatment solution 404 is a 0.33 mmol / L sodium acetate solution (40 mL) obtained by mixing sodium acetate and pure water. The initial concentration of acetic acid contained in the water treatment solution 404 is 19.7 mg / L. In this embodiment, the plasma water treatment apparatus is an example in which an experiment on the decomposition of acetic acid is carried out by duty operation. Also, in this embodiment, the water treatment solution 404 is contained in a glass container 403 (for example, a glass container with an inner diameter of 80 mm) installed on the turntable 402. Also, in this embodiment, it is assumed that the turntable 402 is rotating at 80 rpm.

[0037] FIG. 5 is a diagram showing the experimental results of acetic acid decomposition in the duty operation of the plasma water treatment apparatus according to Example 1. Next, an example of the experimental results of acetic acid decomposition in the duty operation of the plasma water treatment apparatus in this example will be described with reference to FIG. 5.

[0038] In FIG. 5, the vertical axis represents the decomposition efficiency of acetic acid during the duty operation at each duty ratio with respect to the decomposition efficiency of acetic acid during the duty operation with a duty ratio of 100%. Also, in FIG. 5, the horizontal axis represents the duty operations with different duty ratios. Note that the decomposition efficiency of acetic acid is a value obtained by dividing the reduction amount (g) of acetic acid by the input plasma power amount (Wh).

[0039] In FIG. 5, the duty operation (1) is a duty operation with a duty ratio of 100% without an OFF time. Also, in FIG. 5, the duty operation (2) is a duty operation with a duty ratio of 10%, an OFF time of 9 msec, and an ON time of 1 msec. Also, in FIG. 5, the duty operation (3) is a duty operation with a duty ratio of 1%, an OFF time of 9.9 msec, and an ON time of 0.1 msec.

[0040] As shown in FIG. 5, the duty operations with the OFF times shown in the duty operations (2) and (3) have an improved decomposition efficiency of acetic acid compared to the duty operation (1) without an OFF time. This is presumably the result of suppressing the recombination reaction between OH radicals generated by the action of the plasma 4 during the ON time when an alternating voltage is applied between the high-voltage electrode 1A and the ground electrode 1B by providing the OFF time, as described above.

[0041] Also, the time when OH radicals disappear due to the recombination reaction between OH radicals (i.e., the lifetime of OH radicals) is estimated to be about several hundred μsec. Therefore, in this embodiment, when the power supply 3 for plasma generation performs duty operation, the OFF time is set to 100 μsec or more. As a result, the recombination reaction of OH radicals can be further suppressed, so that the decomposition efficiency of harmful substances in the water to be treated 10 can be improved.

[0042] (Example 2) This example is an example in which the ON time during duty operation is set to the time for one cycle of the alternating voltage applied between the high-voltage electrode and the ground electrode by the power supply for plasma generation. In the following description, the description of the same configuration as that of the above-described embodiment and example will be omitted.

[0043] FIG. 6 is a diagram for explaining an example of duty operation in the plasma water treatment apparatus according to Example 2. In FIG. 6, the vertical axis represents the alternating voltage (applied voltage) applied between the high-voltage electrode 1A and the ground electrode 1B, and the horizontal axis represents time. In the plasma water treatment apparatus, when performing duty operation, plasma is generated at a cycle corresponding to the frequency of the alternating voltage (power supply frequency) of the power supply 3 for plasma generation during the ON time. As a result, in the plasma water treatment apparatus, OH radicals are generated at a cycle corresponding to the power supply frequency.

[0044] For example, when the power supply frequency of the power supply 3 for plasma generation is 15 kHz, the plasma water treatment apparatus generates OH radicals at a cycle of every 67 μsec. Therefore, when one cycle of the power supply frequency is shorter than 100 μsec, which is the lifetime of OH radicals, a recombination reaction occurs between the OH radicals generated in the previous cycle and the OH radicals generated in the next cycle.

[0045] Therefore, in this embodiment, when one cycle of the power supply frequency of the plasma generation power supply 3 is shorter than 100 μsec, as shown in FIG. 6, the plasma generation power supply 3 sets the ON time when performing duty operation to the time of one cycle of the AC voltage applied between the high voltage electrode 1A and the ground electrode 1B by the plasma generation power supply 3. Further, the plasma generation power supply 3 provides an OFF time for each cycle of the AC voltage.

[0046] As a result, when one cycle of the power supply frequency of the plasma generation power supply 3 is shorter than 100 μsec, the recombination reaction between OH radicals can be suppressed, so that the decomposition efficiency of harmful substances in the water to be treated 10 can be improved. In addition, by increasing the power supply frequency of the plasma generation power supply 3, the plasma generation power supply 3 itself can be miniaturized.

[0047] As described above, according to the first embodiment and Examples 1 and 2, the harmful substances in the water to be treated 10 can be decomposed with high efficiency.

[0048] Although the embodiments of the present invention have been described, this embodiment is presented as an example and is not intended to limit the scope of the invention. This novel embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the gist of the invention. This embodiment and its modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope.

Explanation of reference numerals

[0049] 1 Discharge part 1A High voltage electrode 1B Ground electrode 1C Dielectric 2 Plasma propagation part 2A, 2B Openings 3 Plasma generation power supply 4 Plasma 10 Water to be treated 401 Flow meter 402 Turntable 403 Glass container 404 Treated solution

Claims

1. A high-voltage electrode, a ground electrode, and a dielectric provided between the high-voltage electrode and the ground electrode, and a discharge part in which a rare gas exists between the high-voltage electrode and the ground electrode. An alternating voltage is applied between the high-voltage electrode and the ground electrode to strike an arc in the rare gas between the high-voltage electrode and the ground electrode, and the plasma is brought into contact with the liquid surface of the water to be treated to generate OH radicals in the water to be treated, and a power source for plasma generation for treating harmful substances contained in the water to be treated. The power source for plasma generation executes a duty operation of repeating an ON time for applying an alternating voltage between the high-voltage electrode and the ground electrode and an OFF time for not applying an alternating voltage between the high-voltage electrode and the ground electrode. A plasma water treatment apparatus.

2. A cylindrical insulator, and by the action of plasma that is struck in the rare gas between the high-voltage electrode and the ground electrode, plasma is struck in the rare gas inside from the first opening, and the plasma is discharged from the second opening on the side opposite to the first opening to the water to be treated. The plasma water treatment apparatus according to claim 1, further comprising a plasma propagation part.

3. The OFF time is 100 μsec or more. The plasma water treatment apparatus according to claim 1 or 2.

4. The ON time is the time for one cycle of the alternating voltage applied between the high-voltage electrode and the ground electrode by the power source for plasma generation. The plasma water treatment apparatus according to any one of claims 1 to 3.

5. A plasma water treatment method executed by a plasma water treatment apparatus including a discharge part having a high-voltage electrode, a ground electrode, and a dielectric provided between the high-voltage electrode and the ground electrode. A step of applying an alternating voltage between the high-voltage electrode and the ground electrode by a power source for plasma generation to strike an arc in the rare gas between the high-voltage electrode and the ground electrode. A step of bringing the plasma into contact with the liquid surface of the water to be treated to generate OH radicals in the water to be treated and treating harmful substances contained in the water to be treated. A step of executing a duty operation of repeating an ON time for applying an alternating voltage between the high-voltage electrode and the ground electrode by the power source for plasma generation and an OFF time for not applying an alternating voltage between the high-voltage electrode and the ground electrode by the power source for plasma generation. A plasma water treatment method including the above steps.

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