DEVICE AND METHOD FOR TREATING ORGANIC-MATERIAL-CONTAINING WATER

Abstract

The present invention seeks to decompose and remove various types of organic materials contained in oilfield water by one operation. To this end, this method for treating oilfield water is provided with: a step for obtaining primary treated water containing organic material by removing the solid content and oil content from oilfield water; and a step for decomposing the organic material by exposing the primary treated water to discharge plasma, wherein the discharge plasma is generated by a nanosecond pulse power source for outputting a pulse having a pulse width of 10 ns or less.

Claims

1. A device for treating organic material containing water, comprising: an electrode pair including an outer first electrode and a second electrode extended in an inner space of said first electrode along the longitudinal direction of said first electrode; a pulse power source applying a high pulse voltage to said electrode pair; an inlet and an outlet for water to be treated, said inlet and outlet being in communication with the inner space of the first electrode; an inlet and an outlet for oxygen containing gas, said inlet and outlet being in communication with the inner space of the first electrode; and said pulse power source is a nanosecond pulse power source which generates a pulse having a pulse width of 10 ns or less and a voltage rising speed of 10 kV/ns or more wherein organic materials contained in said water which is sprayed into the inner space from the inlet are decomposed by exposure to a nanosecond pulse discharge plasma generated under an oxygen atmosphere.

2. A device for treating organic material containing water, comprising: an electrode pair consisting of a spacedly opposed first electrode and second electrode; and a power source for applying a high frequency voltage to said electrode pair; wherein water to be treated is sprayed into a space between said electrode pair and a distance between said first electrode and second electrode varies in the propagation direction of high frequency voltage signal.

3. The device of claim 2 wherein said second electrode is extended in an inner space of said first electrode along the longitudinal direction of said first electrode, said second electrode has a first end and a second end, a pulse as said high frequency voltage signal input to said first end side propagates toward said second end, a distance between an inner circumferential surface of said first electrode and said second electrode is gradually decreased from the first end side along the propagation direction of the pulse.

4. The device of claim 2 wherein said second electrode is extended in an inner space of said first electrode along the longitudinal direction of said first electrode, said second electrode has a first end and an open second end, a pulse as said high frequency voltage signal input to said first end side propagates toward said second end, a distance between an inner circumferential surface of said first electrode and said second electrode is gradually decreased from the first end side in the propagation direction of the pulse down to a predetermined portion of the second electrode, and the distance is then gradually increased from the predetermined portion toward the second end.

5. The device of claim 2 wherein said first electrode is a cylindrical electrode and said second electrode extends along the longitudinal direction of said first electrode in an inner space of said first electrode, said second electrode has a first end and a second end, a pulse as said high frequency voltage signal input to said first end side propagates toward said second end, a ratio b/a in an intermediate portion of the second electrode in the longitudinal direction is higher than the ratios b/a in the first end side and the second end side where a is an outer diameter of the second electrode and b is an inner diameter of the first electrode.

6. The device of claim 2 wherein said high frequency voltage signal is a pulse having a pulse width of 10 ns or less and a voltage rising speed of 10 kV/ns or more.

7. The device of claim 1 wherein the oxygen containing gas is sprayed so as to form a vortex along an inner circumferential surface of the outer electrode.

8. A method for treating oilfield water comprising: removing an oil content and a solid content from the oilfield water to obtain primary treated water containing organic substances; decomposing the organic materials by exposing said primary treated water to a discharge plasma wherein said discharge plasma is generated by a nanosecond pulse power source providing a pulse having a pulse width of 10 ns or less and a voltage rising speed of 10 kV/ns or more.

9. The method of claim 8, said decomposing repeating the steps of: spraying the primary treated water into a nanosecond discharge plasma; collecting the primary treated water which is sprayed and exposed to the nanosecond discharge plasma, and spraying the collected primary treated water into the nanosecond discharge plasma.

10. The method of claim 9 wherein said primary treated water and said collected primary treated water are sprayed into the nanosecond discharge plasma together with an oxygen containing gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0075] FIG. 1 is a schematic diagram of a discharge treatment apparatus;

[0076] FIG. 2 shows a nanosecond pulse generator;

[0077] FIG. 3 shows treatment processes of oilfield water;

[0078] FIG. 4 is a schematic diagram of a flocculation magnetic separation system;

[0079] FIG. 5 is a diagram showing treatment results regarding oilfield water PW-1;

[0080] FIG. 6 is diagram showing treatment results (COD removal rate, TOC removal rate) regarding oilfield water PW-1;

[0081] FIG. 7 is diagram showing treatment results (COD removal rate, TOC removal rate) regarding oilfield water PW-2;

[0082] FIG. 8 is diagram showing treatment results (COD removal rate, TOC removal rate) regarding oilfield water PW-3;

[0083] FIGS. 9A, 9B show other embodiments of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0084] FIGS. 10A, 10B show other embodiments of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0085] FIGS. 11A, 11B show other embodiments of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0086] FIGS. 12A, 12B show other embodiments of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0087] FIGS. 13A, 13B show other embodiments of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0088] FIG. 14 shows another embodiment of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0089] FIG. 15 shows another embodiment of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device;

[0090] FIG. 16 shows another embodiment of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device; and

[0091] FIG. 17 shows another embodiment of an electrode pair (an outer electrode+a center electrode) for a discharge treatment device.

DETAIL DESCRIPTION

[0092] [A] Discharge Treatment Apparatus

[0093] FIG. 1 illustrates a schematic diagram of a discharge treatment apparatus. The discharge treatment apparatus is an apparatus for decomposing organic substances by exposing organic-substance-containing water to discharge plasma. A discharge treatment unit is configured with a cylindrical outer electrode 1 and a center electrode 2 provided in a concentric manner inside the outer electrode 1. The outer electrode 1 is grounded, and the center electrode 2 is supplied with a high voltage pulse from the later-described pulse generator. By transmitting a nanosecond pulse generated by the pulse generator to the center electrode 2, the high voltage pulse is applied between the center electrode 2 and the outer electrode 1, and thus, streamer discharge propagates from the center electrode 2 toward the outer electrode 1, so that a nanosecond discharge plasma is generated. The cylindrical outer electrode 1 is arranged in an erect posture so that a cylindrical wall is vertical, and the center electrode 2 also extends vertically.

[0094] As a material of the outer electrode 1, stainless steel and brass may be exemplified; and a material of the center electrode 2, tungsten and stainless steel may be exemplified. Of course, the type of a metal material constituting the electrode pair is not particularly limited, but it is appropriately selected by the ordinarily skill in the related art.

[0095] In the illustrated embodiment, an outer wall of a treatment container is configured with the cylindrical outer electrode 1, an inlet 3 for water to be treated is provided at an upper end side of the treatment container, and a reservoir unit 4 for the water to be treated is provided at a lower end side of the treatment container. The inlet 3 and the reservoir unit 4 are in fluid communication with each other by a circulation path (including a pump P) 5 to supply the water to be treated stored in the reservoir unit 4 to the inlet 3, so that the water to be treated in the reservoir unit 4 is repetitively exposed to the discharge plasma.

[0096] The inlet 3 is configured with a two-fluid nozzle, and oxygen from an oxygen source 6 as an oxygen-containing gas source is introduced into the inlet 3. By the two-fluid nozzle, the water to be treated supplied from the reservoir unit 4 and the oxygen supplied from the oxygen source 6 are mixed and ejected as fine particles into an inner space between the outer electrode 1 and the center electrode 2. The oxygen supplied from the oxygen source 6 has a function of increasing an amount of oxygen-derived radicals (oxygen atoms, ozone, or the like) as well as a function of a gas source for the two-fluid nozzle. In the illustrated embodiment, an exhaust path 7 for exhausting a gas from the inner space is provided to the treatment container, and the exhausted gas (including oxygen) is recirculated to the two-fluid nozzle by a pump, so that a consumed amount of oxygen supplied from the oxygen source 6 may be reduced.

[0097] A pulse having a pulse width of 10 ns or less and a voltage rising speed of 10 kV/ns or more is applied from the pulse generator to the center electrode 2, streamers propagates radially from the center electrode 2 toward the outer electrode 1, a nanosecond pulse discharge plasma is generated, particles of the water to be treated ejected from the two-fluid nozzle together with the oxygen fall while exposing to the nanosecond discharge plasma, and the particles are collected in the reservoir unit 4. The collected water is ejected again from the two-fluid nozzle through the circulation path 5, the water falls while exposing to the nanosecond discharge plasma, the water is collected in the reservoir unit 4, the water is ejected from the two-fluid nozzle through the circulation path 5, and the water is exposed to the nanosecond discharge plasma. In this manner, by repetitively treating the water to be treated by using the nanosecond discharge plasma, the organic substances contained in the water to be treated are decomposed.

[0098] The nanosecond pulse discharge can solve both factors of low energy efficiency at the time of non-thermal-equilibrium plasma formation by the pulse discharge including mismatch between the pulse power source and heat loss at the time of glow discharge.

[0099] In the present invention, the content of the nanosecond pulse discharge is not limited. WO 2010/074195 discloses that, by applying a pulse voltage having a rising time being shorter than a streamer head formation time between electrodes of a discharge unit, a streamer head is allowed to propagate from a center electrode toward an outer electrode, and a duration of the pulse and an applied voltage are selected so that the discharge is ended according to a distance between the electrodes when the propagating streamer head approaches the outer electrode (at least within a time of 1.5 times the streamer discharge time). By performing the discharge by using only the streamer discharge, characteristic impedance of the pulse power source may be matched with impedance between the electrodes at the time of the streamer discharge, and the impedance mismatch between the pulse power source and the discharge electrode caused by a rapid change in impedance at the time of transition from the streamer discharge to the glow discharge can be solved. Therefore, it is possible to improve the efficiency of energy transfer from the pulse power source to a discharge reactor.

[0100] The streamer discharge time is streamer head formation time +propagation time of the streamer head between the electrodes. Since the voltage rising time (for example, voltage rising time of 2 ns) of the nanosecond pulse discharge is shorter than the streamer head formation time, the voltage is rising before the propagation of the streamer head between the electrodes is started, and the voltage applied to the electrodes is almost constant when the streamer is propagating. As a result, the streamer head is moved at a constant speed.

[0101] [B] Pulse Generator

[0102] The pulse generator is a high voltage pulse power source which outputs a nanosecond pulse having a pulse width of 10 ns or less. The pulse generator will be described with reference to FIG. 2. The pulse generator is configured to include a pulse shaping line 8 configured with a distributed constant line, a charging unit 9 for charging the pulse shaping line 8, and a switch 10 for allowing the pulse shaping line 8 to output a pulse.

[0103] As the pulse shaping line 8, a triaxial Blumlein line is employed. The triaxial Blumlein line is configured with a cylindrical outer conductor 11, a cylindrical intermediate conductor 12 provided inside the outer conductor 11, and a rod-shaped (cylindrical) center conductor (inner conductor) 13 provided inside the intermediate conductor 12. The outer conductor 11, the intermediate conductor 12, and the center conductor 13 are arranged in a concentric manner to be separated from each other, and thus, spaces are formed between the outer conductor 11 and the intermediate conductor 12 and between the intermediate conductor 12 and the center conductor 13. These spaces are filled with dielectric liquid (constituting a dielectric material of the distributed constant line). In the embodiment, the dielectric liquid is a silicone oil, but any other electric liquid having a predetermined permittivity may be used. The length of the Blumlein line is 500 mm as a non-limitative example. The diameters of the outer conductor 11, the intermediate conductor 12, and the center conductor 13 are 110 mm (inner diameter of the outer conductor), 22 mm (outer diameter of the intermediate conductor), 20 mm (inner diameter of the intermediate conductor), and 4 mm (outer diameter of the center conductor) as non-limitative examples. In the embodiment, these conductors are formed by using brass, but other metals may be used as the conductors.

[0104] An inner wall 14 made of an insulating material is formed in the one end side of the cylindrical outer conductor 11 in the longitudinal direction. The one end side of the cylindrical intermediate conductor 12 in the longitudinal direction is supported on the inner wall 14 in a cantilever manner, and the outer end side thereof in the longitudinal direction is opened. The one end side of the center conductor 13 is supported on the inner wall 14. The one end side of the intermediate conductor 12 and the one end side of the center conductor 13 are insulated from each other by using an insulating material.

[0105] The charging unit 9 for charging the pulse shaping line 8 is a pulse forming circuit (first-stage pulse generating circuit) configured with, for example, a charging capacitor, a thyratron switch, and a pulse transformer, and the charging unit supplies a pulse voltage to the pulse shaping line 8. As the charging unit 9 (a pulse power source), a micro-second pulse generator (voltage rising/falling time: several tens to several thousands of nanoseconds) can be widely used.

[0106] The outer conductor 11 is provided with a charging port 15 located on the inner wall 14, and the charging unit 9 and the intermediate conductor 12 are electrically connected to the outer conductor. The intermediate conductor 12 is charged with pulsed power from the pulse forming circuit. The center conductor 13 and the outer conductor 11 are connected to each other through a charging inductor 16, and the outer conductor 4 is grounded. The charging inductor 16 is an inductor having inductance of, for example, 0.5 to 3.0 pH.

[0107] The switch 10 for allowing the pulse shaping line 8 to output a pulse is a high-voltage spark gap switch (SGS) as a high-speed self-closing switch with low inductance. The space between the end portion 17 of the one end side of the outer conductor 11 and the inner wall 14 is filled with an SF.sub.6 insulating gas. The one end side of the intermediate conductor 12 protrudes from the inner wall 7 into the space. A first electrode is formed in the end portion of the one end side of the intermediate conductor 12, and a second electrode is formed in the inner surface of the end portion 17 of the one end side of the outer conductor 11. The gap distance between the first electrode and the second electrode is 1 mm. The pressure of the SF.sub.6 insulating gas is in a range of, for example, 0.1 to 1.0 MPa. The magnitude of the output voltage from the pulse generator can be adjusted by selecting the pressure value of the SF.sub.6. If the gap switch is short-circuited, the outer conductor 11 and the intermediate conductor 12 are conducted.

[0108] The outer conductor 11 is longer than the intermediate conductor 12. The other end side of the outer conductor 11 extends over the other end of the intermediate conductor 12, and an insulating end wall 18 is formed on the other end of the outer conductor 11. The one end side of the center conductor 13 is supported on the inner wall 14, and the other end side thereof extends from the other open end of the intermediate conductor 12. The extension portion 130 becomes an energy transmission line from the Blumlein line to the load. The extension portion 130 of the center conductor 13 extends through the inner space of the outer conductor 4 filled with the dielectric liquid in the longitudinal direction of the outer conductor 11 and penetrates the end wall 18 to supply a nanosecond pulse of a high voltage between the cylindrical outer electrode 1 and the center electrode 2 of the discharging unit, so that the nanosecond pulse discharge is performed.

[0109] In one embodiment, the unit inductance and unit capacitance of the triaxial Blumlein line are 322 nH/m and 76 pF/m, respectively, and a characteristic impedance of 130 is provided. The length of the Blumlein line is 500 mm, and a pulse having a duration of 5 ns is provided. Since the pulse width (duration) of the pulse formed in the distributed constant line depends on the length of the distributed constant line and the permittivity and permeability of the inner dielectric material thereof, the pulse width can be changed by selecting the length of the Blumlein line. In one embodiment, the triaxial Blumlein line is charged with a negative pulse voltage, and a positive electrode is applied to the load. In addition, the pulse generator may be configured so as to apply a negative electrode to the load. As the energy transmission line from the Blumlein line to the load, a concentric transmission line having characteristic impedance of 130 is used.

[0110] [C] Treatment Process for Oilfield Water

[0111] A treatment process for the oilfield water as a typical treatment object of the present invention will be described. Since the oilfield water contains oil and solids (including metals), it is preferable that the oil and solids are removed. In the specification, treatment of removing the oil and solids from the oilfield water is called primary treatment. As illustrated in FIG. 3, primarily treated water is obtained by removing the oil and solids from the oilfield water through the primary treatment, and the nanosecond pulse plasma discharge is applied to the primarily treated water to decompose and remove the organic substances contained in the primarily treated water, to obtain the treated water. Typical means of the primary treatment are configured to include a step of flocculating the oil and solids by using a flocculent and a step of separating and collecting floc.

[0112] As an example of the primarily treatment means, a flocculation magnetic separation system is illustrated in FIG. 4. The flocculation magnetic separation system flocculates the oil and solids (including heavy metals) in the oilfield water by mixing a flocculent containing magnetic powder into the oilfield water and removes the oil and solids, and the details thereof may be referred to Patent Document 3. In addition, Patent Document 5 discloses means where oilfield water is transported into a flocculation tank, treatment of flocculating suspended solids by aggregation is performed by using a flocculent such as ferric sulfate or polyaluminum chloride or magnetic particles such as magnetite (Fe.sub.3O.sub.4) or hematite (Fe.sub.2O.sub.3), and the oil and the floc is separated from the oilfield water by a rotary filtration device. This method may be used as the primary treatment. As the primarily treatment means, other well-known methods (including a combination of a plurality of methods) may be used. For example, the methods disclosed in Patent Documents 1 and 2 may be used.

[0113] [D] Experimental Results

[0114] An experiment of the treatment of organic substances contained in the oilfield water was performed. The treatment objects are three types of oilfield water (PW-1, PW-2, and PW-3). As listed in Tables 1, 5, and 8, the oilfield water PW-1 has a relative large amount of propionic acid in comparison with other oilfield water; the oilfield water PW-2 has a relatively large amount of benzene in comparison with other oilfield water; and the oilfield water PW-3 has a relatively large amount of naphthenic acid in comparison with other oilfield water. Each oilfield water contains various types of organic substances. In addition, besides the substances listed in Tables 1, 5, and 8, each oilfield water may contain various types of metals including heavy metals and various types of polycyclic aromatic hydrocarbons. Specifically, some or all of Cd, Pb, As, T-Hg, Cr6, Se, B, F, Cu, Zn, Fe, Mn, T-Cr, 1, and T-S may be contained. As the polycyclic aromatic hydrocarbons, naphthalene, acenaphthene, fluoroene, and phenanthrene are exemplified.

[0115] The treatment method is a nanosecond pulse discharge treatment (treated water-circulation showering method) using the discharge treatment apparatus illustrated in FIG. 1 and the pulse generator illustrated in FIG. 2. By a showering method of spraying the water to be treated from the two-fluid nozzle, the water to be treated can be uniformly applied to discharge treatment. The water to be treated is primarily treated water obtained by treating raw water of the three types of oilfield water (PW-1, PW-2, and PW-3) by the flocculation magnetic separation system illustrated in FIG. 4. As an experiment method, discharge conditions (discharge voltage, voltage time width, discharge frequency, and discharge electrode shape) were fixed except for discharge frequency. The treatment state of organic substance was checked according to COD (chemical oxygen demand, biochemical oxygen demand), and TOC (total organic carbon). The treatment state of the dissolved organic compounds was checked by measuring COD. The treatment state of the organic substance (status of simultaneous treatment of large amount of components) was checked by measuring TOC.

[0116] [Experiment 1]

[0117] The experiment conditions of Experiment 1 are as follows:

[0118] Treatment Object: PW1

[0119] Amount of Treatment Liquid: 5,000 mL

[0120] Circulation Flow Rate of Treatment Liquid: 200 mL/min

[0121] Treatment Time: 77 hours

[0122] Flow Rate of Oxygen: 5 L/min at 0.2 MPa

[0123] Pulse Width of Applied Pulse: 5 ns

[0124] Voltage of Applied Pulse: 60 kV

[0125] Repetition Frequency of Applied Pulse: 300 pps

[0126] Main materials contained in the oilfield water PW1 are listed in Table 1.

TABLE-US-00001 TABLE 1 Oilfield Water Sample: PW-1 Classi- Primary fication Item Unit Raw Water Treated Water Ions Cl.sup. mg/L 9,380 12,000 Br.sup. mg/L 67 130 SO.sub.4.sup.2 mg/L <100 2,100 TPH Total mg/L 88 20 Water- formic acid mg/L <50 <50 Soluble acetic acid mg/L 1900 Organic propionic acid mg/L 350 310 Material naphthenic acid mg/L 56 46 phenols mg/L 26 25 Volatile benzene mg/L 9.1 4.8 Organic toluene mg/L 11 4.4 Material xylene mg/L 5 2.3 ethylbenzene mg/L 0.38 <0.05

[0127] Besides, the oilfield water PW-1 contains heavy metals and polycyclic aromatic hydrocarbons. The treatment using the nanosecond pulse discharge plasma was performed on the primarily treated water of the oilfield water PW-1. The COD.sub.Mn of the oilfield water PW-1 was measured. The result of measurement is listed in Table 2 and FIG. 5. The reduction of the COD.sub.Mn of the oilfield water PW-1 by the nanosecond pulse discharge was demonstrated, and it was observed that the reduction was proportional to time until the reduction ratio reached about 70%.

TABLE-US-00002 TABLE 2 Time, h COD.sub.Mn, mg/L COD.sub.Mn removal ratio, % 0 494 0 12 415 16.0 20 385 22.1 41 287 41.9 73 195 60.5 77 165 66.6

[0128] [Experiment 2]

[0129] The experiment conditions of Experiment 2 are as follows:

[0130] Treatment Object: PW1

[0131] Amount of Treatment Liquid: 600 mL

[0132] Circulation Flow Rate of Treatment Liquid: 400 mL/min

[0133] Treatment Time: 12 hours

[0134] Flow Rate of Oxygen: 5 L/min at 0.2 MPa

[0135] Pulse Width of Applied Pulse: 5 ns

[0136] Voltage of Applied Pulse: 60 kV

[0137] Repetition Frequency of Applied Pulse: 300 pps

[0138] The treatment using the nanosecond pulse discharge plasma was performed on the primarily treated water of the oilfield water PW-1. The COD.sub.Mn and TOC of the oilfield water PW-1 were measured. The result of measurement is listed in Tables 3 and 4 and FIG. 6. The reduction of the COD and TOC of PW-1 by the nanosecond pulse discharge was demonstrated, and it was observed that the reduction was proportional to time until the reduction ratio reached about 70%. In addition, the result using only the showering was that COD: 358.fwdarw.543, TOC: 1,284.fwdarw.1,403. It is considered that water evaporation by the showering occurs. In addition, the treatment of phenols and other undetermined contained materials by the nanosecond pulse discharge was observed by GC-MS (gas chromatography).

TABLE-US-00003 TABLE 3 Time, h COD.sub.Mn, mg/L COD.sub.Mn removal ratio, % 0 358 0 4 275 23.2 8 150 58.1 12 101 71.5

TABLE-US-00004 TABLE 4 Time, h TOC, mg/L TOC removal ratio, % 0 1,085 0 4 707 34.8 8 365 66.4 12 183 83.1

[0139] [Experiment 3]

[0140] The experiment conditions of Experiment 3 are as follows:

[0141] Treatment Object: PW2

[0142] Amount of Treatment Liquid: 600 mL

[0143] Circulation Flow Rate of Treatment Liquid: 400 mL/min

[0144] Treatment Time: 12 hours

[0145] Flow Rate of Oxygen: 5 L/min at 0.2 MPa

[0146] Pulse Width of Applied Pulse: 5 ns

[0147] Voltage of Applied Pulse: 60 kV

[0148] Repetition Frequency of Applied Pulse: 300 pps

[0149] Main materials contained in the oilfield water PW2 are listed in Table 5.

TABLE-US-00005 TABLE 5 Oilfield Water Sample: PW-2 Classi- Primary fication Item Unit Raw Water Treated Water Ions Cl.sup. mg/L 33 250 Br.sup. mg/L 9 4.2 SO.sub.4.sup.2 mg/L 10 76 TPH Total mg/L 228.3 26 Water- formic acid mg/L <50 <5 Soluble propionic acid mg/L <100 14 Organic acetic acid mg/L 79 Material naphthenic acid mg/L <0.1 <1.0 phenols mg/L 18 14 Volatile benzene mg/L 61 22 Organic toluene mg/L 19 6.8 Material xylene mg/L 4.5 1.5 ethylbenzene mg/L <1 <0.25

[0150] Besides, the oilfield water PW-2 contains heavy metals and polycyclic aromatic hydrocarbons. The treatment using the nanosecond pulse discharge plasma was performed on the primarily treated water of the oilfield water PW-2. The COD.sub.Mn and TOC of the oilfield water PW-2 were measured. The result of measurement is listed in Tables 5 and 6 and FIG. 7. The reduction of the COD and TOC of PW-2 by the nanosecond pulse discharge was demonstrated, and it was observed that the reduction was proportional to time until the reduction ratio reached about 70% and the reduction was saturated after that. In addition, the result using only the showering was that COD: 155.fwdarw.204, TOC: 125.fwdarw.140. It is considered that water evaporation by the showering occurs. In addition, the treatment of phenols by the nanosecond pulse discharge was observed by GC-MS (gas chromatography).

TABLE-US-00006 TABLE 6 Time, h COD.sub.Mn, mg/L COD.sub.Mn removal ratio, % 0 155 0 4 38 75.5 8 15 90.3 12 9 94.2

TABLE-US-00007 TABLE 7 Time, h TOC, mg/L TOC removal ratio, % 0 92 0 4 11 88.0 8 8 91.3 12 11 88.0

[0151] [Experiment 4]

[0152] The experiment conditions of Experiment 4 are as follows:

[0153] Treatment Object: PW3

[0154] Amount of Treatment Liquid: 600 mL

[0155] Circulation Flow Rate of Treatment Liquid: 400 mL/min

[0156] Treatment Time: 12 hours

[0157] Flow Rate of Oxygen: 5 L/min at 0.2 MPa

[0158] Pulse Width of Applied Pulse: 5 ns

[0159] Voltage of Applied Pulse: 60 kV

[0160] Repetition Frequency of Applied Pulse: 300 pps

[0161] Main materials contained in the oilfield water PW3 are listed in Table 8.

TABLE-US-00008 TABLE 8 Oilfield Water Sample: PW-3 Classi- Primary fication Item Unit Raw Water Treated Water Ions Cl.sup. mg/L 15,100 14,000 Br.sup. mg/L <100 <100 SO.sub.4.sup.2 mg/L <100 <100 TPH Total mg/L 88 2.5 Water- formic acid mg/L <50 <50 Soluble propionic acid mg/L <100 <100 Organic naphthenic acid mg/L 110 91 Material phenols mg/L 0.7 0.33 Volatile acetone mg/L <0.5 0.51 Organic benzene mg/L 1.1 0.34 Material toluene mg/L 2 0.55 total xylene mg/L 1.5 0.45 ethylbenzene mg/L 0.26 0.048

[0162] Besides, the oilfield water PW-3 contains heavy metals and polycyclic aromatic hydrocarbons. The treatment using the nanosecond pulse discharge plasma was performed on the primarily treated water of the oilfield water PW-3. The COD.sub.Mn and TOC of the oilfield water PW-3 were measured. The result of measurement is listed in Tables 9 and 10 and FIG. 8. The reduction of the COD and TOC of PW-3 by the nanosecond pulse discharge was demonstrated, and it was observed that the reduction was proportional to time until the reduction ratio reached about 70% and the reduction was saturated after that. In addition, the result using only the showering was that COD: 260.fwdarw.335 and TOC: 160.fwdarw.171. It is considered that water evaporation by the showering occurs. In addition, the generation of chlorides by the nanosecond pulse discharge was observed by GC-MS (gas chromatography).

TABLE-US-00009 TABLE 9 Time, h COD.sub.Mn, mg/L COD.sub.Mn removal ratio, % 0 260 0 4 65 75.0 8 12 95.4 12 2 99.2

TABLE-US-00010 TABLE 10 Time, h TOC, mg/L TOC removal ratio, % 0 152 0 4 35 77.0 8 6 96.1 12 0 100.0

[0163] The experimental results are summarized as follows:

[0164] It was observed that, with respect to PW-1, 2, and 3, moisture was vaporized by showering, and as a result, the COD and TOC were increased.

[0165] It was observed that, with respect to the oilfield water PW-1, 2, and 3, the organic substances were decomposed by the nanosecond pulse discharge, and as a result, the COD and TOC were reduced.

[0166] It was observed that, a higher reduction ration was obtained with the oilfield water having lower initial COD and TOC.

[0167] It was observed that the decomposition of the organic substances proceeded in proportion to time until the reduction ratio reached about 70%.

[0168] It was observed that, with respect to the oilfield water PW-1 and 2, the decomposition of phenols proceeded.

[0169] With respect to the oilfield water PW-3, the generation of chlorides together with the decomposition of the organic substance was observed.

[0170] [E] Other Embodiments of Discharge Treatment Unit

[0171] For the treatment with a good efficiency, it is preferable to form more uniform discharge in the discharge space by considering the attenuation of the discharge energy in the propagation direction of the pulse. FIGS. 9 to 17 illustrate other embodiments of the outer electrode 1 and the center electrode 2 in the inner side of the outer electrode 1 constituting the discharge treatment apparatus. Similarly to FIG. 1, the outer electrode 1 and the center electrode 2 of the discharge treatment unit of FIGS. 9 to 17 extend in the height direction, and the water to be treated is introduced from the upper side into the treatment space between the outer electrode 1 and the center electrode 2. In FIGS. 9 to 17, only the configurations of the outer electrode 1 and the center electrode 2 are illustrated, but other elements are omitted. With respect to the structure for treatment of the water to be treated, FIG. 1 and the description associated with FIG. 1 may be referred to. In FIGS. 9 to 17, the support structures or the like of the outer electrode 1 and the center electrode 2 are omitted. However, the support structures of the outer electrode 1 and the center electrode 2 are matters appropriately made by the ordinarily skilled in the related art.

[0172] In FIGS. 9(A) to 13(B), the left-side (A) diagrams illustrate embodiments where the lower end side of the center electrode 2 becomes the pulse input side and the pulse progresses or propagates from the lower end toward the upper end of the center electrode 2. The right-side (B) diagrams illustrate embodiments where the upper end side of the center electrode 2 becomes the pulse input side and the pulse progresses or propagates from the upper end toward the lower end of the center electrode 2. FIGS. 14 to 17 illustrate embodiments where the lower end side of the center electrode 2 becomes the pulse input side and pulse progresses or propagates from the lower end toward the upper end of the center electrode 2. FIGS. 9 to 17 are schematic diagrams illustrating the shapes of the outer electrode 1 and the center electrode 2 and the propagation direction of the pulse, and in each diagram, the position of the pulse power source does not indicate the positional relationship between the discharge treatment unit and the pulse power source (a pulse generator). In the ordinarily skilled in the related art, the pulse generator is arranged at an appropriate position, and the pulse generator is appropriately electrically connected to the discharge treatment unit so that the pulse generated by the pulse generator is applied to the discharge treatment unit.

[0173] In the embodiments illustrated in FIGS. 9(A) and 9(B), attention is paid to the fact that the energy is attenuated as the discharge generated between the electrode pair propagates in the longitudinal direction of the electrodes, and the distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased over the propagation direction of the pulse. Herein, in the specification, the distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is a distance in the direction perpendicular to the longitudinal direction (propagation direction of the pulse) of the center electrode 2 and, in the illustrated embodiment, the distance is a horizontal distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2. In the embodiment, the water to be treated is sprayed in the outer electrode 1 from the inlet (not shown) provided in the upper side of the figure, falls through the space between the inner side of the outer electrode 1 and the center electrode 2, and is collected in a water receiving tank (not shown) provided in the lower side of the figure.

[0174] In FIG. 9(A), the discharge treatment unit has a generally circular truncated cone shape as a whole. More specifically, the outer electrode 1 has a cylindrical shape as a whole, and the diameter thereof is gradually decreased from the lower end toward the upper end, and the outer circumferential surface and the inner circumferential surface have a tapered shape or a slanted shape. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction). In the embodiment, since the inner diameter of the outer electrode 1 is increased downward, the water to be treated sprayed into the outer electrode is hard to adhere to the outer electrode 1 while falling down, so that high treatment efficiency can be obtained.

[0175] In FIG. 9(B), the discharge treatment unit has a generally inverted circular truncated cone shape as a whole. More specifically, the outer electrode 1 has a cylindrical shape as a whole, and the diameter thereof is gradually increased from the lower end toward the upper end, and the outer circumferential surface and the inner circumferential surface have a tapered shape or a slanted shape. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the upper end toward the lower end (namely, in the pulse propagation direction). In the embodiment, the center electrode 2 can be suspended from the upper side, and the water receiving tank and the holding mechanism for the center electrode are independently provided, so that the equipment can be easily designed.

[0176] The embodiments illustrated in FIGS. 10(A) and 10(B) are modification of the embodiments illustrated in FIGS. 9(A) and 9(B). In the embodiments illustrated in FIGS. 10(A) and 10(B), attention is paid to the fact that the energy is attenuated as the discharge generated between the electrode pair propagates in the longitudinal direction of the electrodes, and the distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased over the propagation direction of the pulse.

[0177] In FIG. 10(A), the outer circumferential surface of the outer electrode 1 is a cylindrical surface having the same diameter in the height direction, and the diameter of the inner circumferential surface thereof is gradually decreased from the lower end toward the upper end, so that the inner circumferential surface has a tapered shape or a slanted shape. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction).

[0178] In FIG. 10(B), the outer circumferential surface of the outer electrode 1 is a cylindrical surface having the same diameter in the height direction, and the diameter of the inner circumferential surface thereof is gradually increased from the lower end toward the upper end, so that the inner circumferential surface has a tapered shape or a slanted shape. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the upper end toward the lower end (namely, in the pulse propagation direction).

[0179] In the embodiment illustrated in FIGS. 11(A) and 11(B), attention is paid to the fact that the energy is attenuated as the discharge generated between the electrode pair propagates in the longitudinal direction of the electrodes and the energy is increased as the pulse voltage is inverted and superposed (voltage-reflected) at the open end of the electrode. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased over the pulse propagation direction, and the distance is gradually increased from a portion exceeding the middle position in the longitudinal direction toward the end.

[0180] In FIG. 11(A), the outer electrode 1 has a shape where the diameter of the outer electrode 1 in the upper end side illustrated in FIG. 9(A) is increased. More specifically, the outer electrode 1 has a cylindrical shape as a whole, and the diameter thereof is gradually decreased from the lower end toward the upper end up to a position of the upper side of the middle portion in the height direction. The diameter is gradually increased from the position toward the upper end. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction) up to a position of the upper side of the middle portion in the height direction, and the distance is gradually increased from the position toward the upper end. In the illustration, the diameter of the outer electrode 1 in the lower end side is larger than the diameter thereof in the upper end side, but not necessarily limited thereto, for example, the diameter thereof in the lower end side and the diameter thereof in the upper end side may be substantially the same as each other.

[0181] In FIG. 11(B), the outer electrode 1 has a shape where the diameter of the outer electrode 1 in the lower end side illustrated in FIG. 9(B) is increased. More specifically, the outer electrode 1 has a cylindrical shape as a whole, and the diameter thereof is gradually decreased from the upper end toward the lower end down to a position of the lower side of the middle portion in the height direction. The diameter is gradually increased from the position toward the lower end. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the upper end toward the lower end (namely, in the pulse propagation direction) down to a position of the lower side of the middle portion in the height direction, and the distance is gradually increased from the position toward the lower end. In the illustration, the diameter of the outer electrode 1 in the upper end side is larger than the diameter thereof in the lower end side, but not necessarily limited thereto, for example, the diameter thereof in the upper end side and the diameter thereof in the lower end side may be substantially the same as each other.

[0182] In the outer electrode 1 illustrated in FIGS. 11(A) and 11(B), in the pulse propagation direction, the portion exceeding the middle position of the outer electrode 1 in the longitudinal direction has the smallest diameter. However, for example, as illustrated in FIGS. 12(A) and 12(B), the diameter of the outer electrode 1 at the middle position in the longitudinal direction may be minimized, and the diameter may be gradually increased from the central portion in the height direction upward and downward. In the illustration, the diameter of the outer electrode 1 in the lower end side and the diameter thereof in the upper end side are substantially the same as each other. However, the present invention is not necessarily limited thereto.

[0183] The embodiments illustrated in FIGS. 13(A) and 13(B) are modification of the embodiments illustrated in FIGS. 11(A) and 11(B) and are different from the embodiments of FIGS. 11(A) and 11(B) in that the outer circumferential surface of the outer electrode 1 is a cylindrical surface having the same diameter in the height direction. In the embodiments illustrated in FIGS. 13(A) and 13(B), attention is paid to the fact that the energy is attenuated as the discharge generated between the electrode pair propagates in the longitudinal direction of the electrodes and the energy is increased as the pulse voltage is inverted and superposed (voltage-reflected) at the open end of the electrode. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased over the pulse propagation direction, and the distance is gradually increased from a portion exceeding the middle position in the longitudinal direction toward the end. In the outer electrode 1 illustrated in FIGS. 13(A) and 13(B), in the pulse propagation direction, the portion exceeding the middle position of the outer electrode in the longitudinal direction has the smallest diameter. However, the portion of the outer electrode having the smallest diameter is not limited to the illustrated embodiment, and for example, the diameter of the outer electrode at the middle position in the longitudinal direction may be minimized, and the diameter may be gradually increased from the central portion in the height direction upward and downward (refer to FIGS. 12(A) and 12(B)).

[0184] In FIG. 13(A), the outer circumferential surface of the outer electrode 1 is a cylindrical surface having the same diameter in the height direction, and the diameter of the inner circumferential surface thereof is gradually decreased from the lower end toward the upper end up to a position of the upper side of the middle portion in the height direction. The diameter is gradually increased from the position toward the upper end. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction) up to a position of the upper side of the middle portion in the height direction, and the distance is gradually increased from the position toward the upper end.

[0185] In FIG. 13(B), the outer circumferential surface of the outer electrode 1 is a cylindrical surface having the same diameter in the height direction, and the diameter of the inner circumferential surface thereof is gradually decreased from the upper end toward the lower end down to a position of the lower side of the middle portion in the height direction. The diameter is gradually increased from the position toward the lower end. The center electrode 2 is a wire or rod which is located at the center of the cylindrical outer electrode 1 and extends vertically. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the upper end toward the lower end (namely, in the pulse propagation direction) down to a position of the lower side of the middle portion in the height direction, and the distance is gradually increased from the position toward the lower end.

[0186] In the embodiment illustrated in FIG. 14, attention is paid to the fact that the energy is attenuated as the discharge propagates in the longitudinal direction of the electrodes, and the distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased over the pulse propagation direction. In contrast to FIGS. 9(A) and 9(B) and FIGS. 10(A) and 10(B) where the inner circumferential surface of the outer electrode 1 is tapered in the longitudinal direction, in FIG. 14, the outer circumferential surface of the center electrode 2 is tapered in the longitudinal direction.

[0187] In FIG. 14, the outer electrode 1 is a cylinder having the same diameter in the height direction, and the center electrode 2 is a rod-shaped member or a rod of which diameter is gradually increased from the lower end toward the upper end. The distance between the inner circumferential surface of the outer electrode 1 and the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction).

[0188] In the embodiment illustrated in FIG. 15, attention is paid to the fact that the energy is attenuated as the discharge propagates in the longitudinal direction of the electrodes, and the distance between the inner circumferential surface of the outer electrode 1 and the center electrode is gradually decreased over the pulse propagation direction. The center electrode is configured with an axial portion 2 formed with a wire or rod extending in the height direction and a plurality of needle-shaped portions 20 protruding from the axial portion 2 in the direction perpendicular to the longitudinal direction of the axial portion 2. The needle-shaped portions 20 are provided at an interval over the longitudinal direction of the axial portion 2 and extend radially from the axial portion 2 as a center thereof. In the embodiment of FIG. 14, the distance between the inner circumferential surface of the outer electrode 1 and the center electrode is a distance between the inner circumferential surface of the outer electrode 1 and the tip of the needle-shaped portion 20. In addition, instead of the needle-shaped portions, a plurality of blades or plates (circular plates, square plates, rhombic plates, or the like) extending in the direction perpendicular to the longitudinal direction of the axial portion 2 may be used.

[0189] In FIG. 15, the outer electrode 1 is a cylinder having the same diameter in the height direction, and the protrusion size of the needle-shaped portion formed to protrude in the center electrode 2 over the height direction is gradually increased from the lower end toward the upper end. The distance between the inner circumferential surface of the outer electrode 1 and the tip of the needle-shaped portion of the center electrode 2 is gradually decreased from the lower end toward the upper end (namely, in the pulse propagation direction).

[0190] In the embodiment illustrated in FIG. 16, attention is paid to the fact that the electric field generated in the discharge space between the outer electrode 1 and the inner center electrode 2 is influenced by the ratio b/a (actually, ln(b/a)) of the diameter b of the outer electrode to the diameter a of the center electrode, and the higher the ratio b/a is, the larger the electric field is generated at the same voltage. The center electrode 2 is recessed in the middle portion in the longitudinal direction while the outer electrode 1 has a constant diameter in the longitudinal direction, so that the electric field at the recessed portion is strengthened.

[0191] In FIG. 16, the outer electrode 1 is a cylinder having the same diameter in the height direction, and the center electrode 2 is a rod-shaped member or rod of which diameter is gradually decreased from the lower end toward the middle portion in the height direction and gradually further increased toward the upper end. The ratio b/a of the diameter b of the outer electrode 1 to the diameter a of the center electrode 2 is configured so that the ratio in the middle portion of the discharge space in the height direction is higher than the ratios in the upper and lower end areas of the discharge space.

[0192] In the embodiment illustrated in FIG. 17, similarly to FIG. 16, attention is paid to the fact that the electric field generated between the outer electrode 1 and the inner center electrode 2 is influenced by the ratio b/a (actually, ln(b/a)) of the diameter b of the outer electrode to the diameter a of the center electrode, and the higher the ratio b/a is, the larger the electric field is generated at the same voltage. The center electrode 2 is recessed in the central portion in the longitudinal direction, so that the electric field at the recessed portion is strengthened. The center electrode is configured with an axial portion 2 formed with a wire or rod extending in the height direction and a plurality of needle-shaped portions 20 protruding from the axial portion 2 in the direction perpendicular to the longitudinal direction of the axial portion 2. The needle-shaped portions 20 are provided at an interval over the longitudinal direction of the axial portion 2 and extend radically from the axial portion 2 as a center thereof. In the embodiment of FIG. 17, the diameter of the center electrode is a diameter of a virtual circle having the axial portion 2 as a center thereof and passing through the tips of the needle-shaped portions 20. In addition, instead of the needle-shaped portions, a plurality of blades or plates (circular plates, square plates, rhombic plates, or the like) extending in the direction perpendicular to the longitudinal direction of the axial portion 2 may be used.

[0193] In FIG. 17, the outer electrode 1 is a cylinder having the same diameter in the height direction, and the protrusion size of the needle-shaped portion formed to protrude in the center electrode 2 over the height direction is gradually decreased from the lower end toward the middle portion in the height direction and gradually further increased toward the upper end. The ratio b/a of the diameter b of the outer electrode 1 to the diameter a of the center electrode 2 is configured so that the ratio in the middle portion of the discharge space in the height direction is higher than the ratios in the upper and lower end areas of the discharge space.

[0194] The technical ideas relating to the embodiments of FIGS. 9(A) and 9(B), 10(A) and 10(B), 14, and 15 can be described as follows:

[0195] In an electrode pair including an outer electrode configured with a cylindrical electrode and a center electrode extending in a longitudinal direction of the cylindrical electrode in an inner space of the cylindrical electrode,

[0196] the center electrode has a first end and a second end, a pulse input to the first end side propagates toward the second end, and

[0197] the distance between the inner circumferential surface of the cylindrical electrode and the center electrode is gradually decreased from the first end side in the pulse propagation direction.

[0198] In one embodiment, the center electrode is a line-shaped or rod-shaped member made of a wire or a rod, and the inner circumferential surface of the cylindrical electrode has a tapered shape or a slanted shape so as to be gradually close to the center electrode in the pulse propagation direction (FIGS. 9(A) and 9(B) and FIGS. 10(A) and 10(B)). In the embodiments of FIGS. 9(A) and 9(B), similarly to the inner circumferential surface, the outer circumferential surface of the cylindrical electrode also has a tapered shape or a slanted shape, and in the embodiments of FIGS. 10(A) and 10(B), the outer circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction.

[0199] In other words, in the embodiments illustrated in FIGS. 9(A) and 9(B) and FIGS. 10(A) and 10(B), the diameter of the discharge space formed between the cylindrical electrode and the center electrode is decreased from the first end side toward the second end in the pulse propagation direction.

[0200] In one embodiment, the inner circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction, the center electrode is a rod-shaped member, the diameter of the cylindrical circumferential surface of the center electrode is increased in the pulse propagation direction, and the cylindrical circumferential surface of the center electrode has a tapered shape or a slanted shape so as to be gradually close to the inner circumferential surface of the cylindrical electrode (FIG. 14).

[0201] In one embodiment, the inner circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction, the center electrode is configured with an axial portion (axial portion 2) extending in the longitudinal direction of the cylindrical electrode and a plurality of protrusion portions (needle-shaped portions 20) extending over the longitudinal direction of the axial portion and in the radial direction from the axial portion as a center thereof toward the inner circumferential surface of the cylindrical electrode, and the protrusion size of the protrusion portion from the axial portion is gradually increased in the pulse propagation direction(FIG. 15).

[0202] The technical ideas relating to the embodiments of FIGS. 11(A) and 11(B), 12(A) and 12(B), and 13(A) and 13(B) can be described as follows:

[0203] In an electrode pair including an outer electrode configured with a cylindrical electrode and a center electrode extending in a longitudinal direction of the cylindrical electrode in an inner space of the cylindrical electrode,

[0204] the center electrode has a first end and a second end, a pulse input to the first end side propagates toward the second end, the second end is an open end, and

[0205] the distance between the inner circumferential surface of the cylindrical electrode and the center electrode is gradually decreased from the first end side in the pulse propagation direction down to a predetermined portion of the center electrode, and the distance is gradually increased from the predetermined portion toward the second end.

[0206] In one embodiment, the center electrode is a line-shaped or rod-shaped member made of a wire or a rod, and the diameter of the cylindrical electrode in the inner circumferential surface is gradually decreased from the first end side in the pulse propagation direction down to a predetermined portion of the center electrode, and the diameter is gradually increased from the predetermined portion toward the second end (FIGS. 11(A) and 11(B), 12(A) and 12(B), and 13(A) and 13(B)). In the embodiments of FIGS. 11(A) and 11(B) and FIGS. 12(A) and 12(B), the cylindrical electrode 1 has an overall concave shape where the smallest diameter exists in the middle portion in the longitudinal direction, and in the embodiments of FIGS. 13(A) and 13(B), the outer circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction.

[0207] If FIGS. 11(A) and 11(B) are described as an example, the pulse incident from the pulse power source loses the energy while the pulse propagates through the center electrode 2, and the pulse is reflected on the second end, so that the pulse obtains large energy in the vicinity of the second end again. Therefore, by adjusting the distance between the outer electrode 1 and the center electrode 2 according to the energy distribution of the pulse, the energy of the pulse can be efficiently used, so that the length of the outer electrode 1 can be set to be larger, so that the water to be treated sprayed into the outer electrode 1 has a long stay time inside the outer electrode 1 and it is possible to obtain a higher treatment efficiency.

[0208] The technical ideas relating to the embodiments of FIGS. 16 and 17 can be describe as follows:

[0209] In an electrode pair including an outer electrode configured with a cylindrical electrode and a center electrode extending in a longitudinal direction of the cylindrical electrode in an inner space of the cylindrical electrode,

[0210] the center electrode has a first end and a second end, a pulse input to the first end side propagates toward the second end, and

[0211] a ratio b/a of an inner diameter b of the cylindrical electrode to an outer diameter a of the center electrode in a middle portion of the center electrode in the longitudinal direction is higher than the ratios in the first end side and the second end side.

[0212] In other words, in the embodiments illustrated in FIGS. 16 and 17, the discharge space formed between the cylindrical electrode and the center electrode is configured with a first portion of which diameter is decreased from the first end side in the pulse propagation direction down to the middle portion of the center electrode in the longitudinal direction and a second portion of which diameter is increased from the middle portion toward the second end side.

[0213] In one embodiment, the inner circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction, and the center electrode is a rod-shaped member, the diameter of the center electrode in the outer circumferential surface is gradually decreased from the both end sides toward the central portion so that the outer circumferential surface of the center electrode is recessed in the middle portion of the center electrode in the longitudinal direction (FIG. 16).

[0214] In one embodiment, the inner circumferential surface of the cylindrical electrode has the same diameter in the longitudinal direction, the center electrode is configured with an axial portion (axial portion 2) extending in the longitudinal direction of the cylindrical electrode and a plurality of protrusion portions (needle-shaped portions 20) extending over the longitudinal direction of the axial portion and in the radial direction from the axial portion as a center thereof toward the inner circumferential surface of the cylindrical electrode, and the protrusion size of the protrusion portion from the axial portion is gradually decreased from the both end sides toward the central portion, so that the middle portion of the center electrode in the longitudinal direction is recessed as a whole (FIG. 17).

[0215] In FIGS. 9 to 17, the vertical-type discharge electrode portions extending in the height direction are illustrated. However, the electrode pair is not limited to the electrode pair of which longitudinal direction is the vertical direction, but an electrode pair extending in a direction slanted with respect to the vertical axis or an electrode pair extending in a direction slanted with respect to the horizontal direction may be used.

[0216] Although the present invention is intended to target the organic-substance-containing water, the configurations of the electrode pairs illustrated in FIGS. 9 to 17 are applied in general to the treatment apparatuses employing discharge with a pulse power source. In addition, the treatment object is not limited to liquid, but the present invention can be applied in general to fluids including gas.