静電気学会誌，38, 1（2014）34-39

J. Inst. Electrostat. Jpn.

論

文Investigation of Organic Compound Decomposition andMicrobial Inactivation by a Cylinder Style Pulsed Discharge
System in High Conductivity Solution＊, 1

Takanori TANINO, Yukihiro TAMURA and Takayuki OHSHIMA(Received September 10, 2013; Accepted November 19, 2013)

A cylinder style discharge system was applied for organic compound decomposition in high conductivity solution. Effect ofthe submerging electrode and treatment water conductivity on the degradation of the organic compound by pulsed discharge
plasma was investigated by using indigocarmine and oxalic acid. Submerging electrode sheathed with gas flow in thetreatment water facilitated decomposition of these organic compounds. It was demonstrated that decomposition ratios oforganic compound in the high conductivity solutions, that were equivalent to saline（18 mS/cm）and seawater（51 mS/cm）
respectively, were higher than that in the deionized water. And the reachable decomposition ratio was also increased in theadjusted high conductivity solutions. Moreover, application of this system for microbial inactivation was investigated. Similarto the results in the organic compound decomposition, efficient inactivation was also detected in high conductivity solution.

1．Introduction

One of the techniques to treat high conductivity water byDischarge plasma that could generate ultraviolet light anddischarge plasma is to irradiate from above the treatment

active species is one of the applicable advanced technologies

water . The other technique is utilizing the cylinder styleto the water treatment. Because of the high dielectric

discharge system with gas flow and high voltage electrodebreakdown voltage, generation of the discharge plasma ininside of it . In this system, high voltage electrode is placed

7）

8）

water is difficult. Several plasma generation systems havein gas phase and discharge plasma generated in the gas phasebeen developed to generate discharge plasma in water directly.

is irradiated to the water surface. It could be submerged andAs the mechanism to generate plasma in water, gas bubble

used to generate and utilize discharge plasma in highgenerated at the electrodes submerged in water could be a

conductivity water. Except for whether the high voltage

trigger to decrease dielectric breakdown voltage and generate

electrode is situated above or in water, it is same in bothdischarge plasma1-4）, and injection of gas bubble has beeninvestigated as the one of the method to readily generate andtechnique that discharge plasma generated in gas phase isirradiated to the gas-water interface.

utilize discharge plasma in water. We also previously

In this study, the effect of submerging cylinder style

constructed novel submerged style pulse discharge systemdischarge system in water on the decomposition of organic

with gas bubble injection, and reported its characteristics andcompound by the pulsed discharge plasma was investigated.

5, 6）. This system could readily

Moreover, the influence of treatment water conductivity on theallow generating continuous discharge plasma in water.

decomposition of organic compounds and inactivation of

Decomposition of organic compound and inactivation of

microorganism using this system was also investigated.

application to water treatment

bacteria in water were successfully demonstrated. However, it
was difficult to generate and utilize discharge plasma in the2．Materials and Methods

treatment water with high conductivity, because the electrodes

2.1 Cylinder style discharge systemin this style discharge plasma generation systems were in

The cylinder style discharge system was constructed using 4

contact with the treatment water.

mmφ stainless rod for high voltage electrode and innerdiameter 7 mmφ Plexiglas pipe（350 and 900 mm）for

Keywords：pulsed discharge plasma in water，conductivity,decomposition，inactivation＊
Department of Chemical and Environmental Engineering,
Graduated School of Engineering, Gunma University, 1-5-1,Tenjin-cho, Kiryu city, Gunma, 376-8515, Japan1
tohshima@gunma-u.ac.jp

cylinder（Fig. 1）.
As the power supply, a high-voltage pulse generator with astationary spark gap was used. The discharge capacitor, pulse
repetition frequency and applied voltage were 2 nF, 100 Hz
and 20 kV, respectively. The high-voltage pulse generator wasInvestigation of Organic Compound Decomposition and Microbial Inactivation by a Cylinder Style Pulsed Discharge System in High Conductivity Solution（Takayuki OHSHIMA at al.） 35

2.3 Preparation and analysis of treatment watercontaining microorganism
Escherichia coli K-12 strain was used for inactivation
experiment. The E. coli was cultivated in Luria-Bertain（LB）
medium（1%（w/v）peptone, 0.5% yeast extract and 0.5%
sodium chloride）at 35°
C. The cultivated E. coli cells werecollected by centrifugation for 5 min at 10,000 × g. Appropriate
amounts of collected cells were resuspended in sterilized
deionized water. Conductivity of the treatment water wasadjusted to 18 and 51 mS/cm by adding sodium chloride. Afterthe plasma treatment, aliquot of treatment water wasappropriately diluted with saline, and 100 μl of diluted sample
were inoculated onto LB medium plate（LB mediumFig. 1 Cylinder system discharge system

containing 1% agar）. Plate were incubated at 35°
C for 16 h,
and then colony appeared on the plate was counted toconnected to the high voltage electrode and grounded electrodewas placed in treatment water. Two different size treatment
reactors were constructed with Plexiglas cylinder（225 and800 mm）and plate（21 mmφ）
. The treatment volume ofreactor constructed using 225 or 800 mm Plexiglas cylinder

determine the survival ratio.

2.4 Calculation of decomposition and inactivation
rate constants
Decomposition and inactivation rate constants werecalculated by using following formula（Eq. 1）:

was 50 or 220 ml, respectively. The electrodes covered with（1）350 and 900 mm Plexiglas pipe were combined with 50 and200 ml volume treatment reactors for plasma treatment,

where t is the treatment time, and N 0 and N are therespectively. The cylinder system with 0.5 L/min air flow wasconcentration of organic compounds or viable cells per volume

situated above or in treatment water, and then 20 kV pulsed

before and after discharge plasma treatment, respectively.

voltage was applied to generate pulsed electric discharge.

2.2 Preparation and analysis of treatment watercontaining organic compound
Indigocarmine and oxalic acid were used for thedecomposition experiment, and they were dissolved in

3．Results and discussion
3.1 Effect of submerging cylinder style discharge
system in water on decomposition of organic
compound by pulsed discharge plasma

deionized water, respectively. Conductivity of the treatment

In the cylinder style discharge system with gas flow,

water was adjusted to 18 and 51 mS/cm by adding sodium

discharge plasma is generated at the edge of high voltage

chloride. These are equivalent to physiological saline andelectrode in gas phase and irradiated to the gas-water interface.

seawater, respectively. Initial concentration of indigocarmine

The high voltage electrode could be situated above or in theand oxalic acid solutions was 50 mg/L. Indigocarmine

treatment water, and discharge plasma itself has no difference

concentration in the plasma treated water was measured bywhen high voltage electrode sheathed with gas flow is situatedspectroscopic analysis using spectrophotometer（UVminiabove or in the treatment water（Fig. 2）.

1240, Shimadzu, Kyoto, Japan）. Oxalic acid concentration in

The effect of the submerging high voltage electrodethe plasma treated water was measured high performance

sheathed with gas flow on organic compound decomposition

liquid chromatographic analysis using spectrophotometricby pulsed discharge plasma was investigated usingdetector（SPD-6AV, Shimadzu）. The column used forindigocarmine and oxalic acid when water conductivity wasseparation was ICSep ICE-ORH-801（Transgenomic INC.,

51 mS/cm.

Omaha, NE）. The HPLC apparatus was operated at 35°
C withOn the experiment using indigocarmine in the 50 ml

0.05 N sulfuric acid at a flow rate of 0.8 ml/min as the mobile

treatment reactor, high voltage electrode sheathed with gasphase.

flow was situated at the height of 10 mm above the water

36

静電気学会誌 第38巻 第 1 号（2014）

Fig. 2 Schematic illustration of submerging cylinder styledischarge system in water

Fig. 4 Effect of submerging cylinder style discharge system on
oxalic acid decomposition by discharge plasma

surface or at the depth of 100 mm under the water surface.
Decomposition of indigocarmine, especially initialand above water after 240 min were almost same. Form these

decomposition, was clearly facilitated by submerging highresults, it was demonstrated that submerging high voltage

voltage electrode. The decomposition ratio achieved after 2electrode is also effective on the facilitation of oxalic acidmin discharge plasma treatment in water was 2.6-fold higher

decomposition by pulsed discharged plasma.

than that above water（Fig. 3）.

Submerging high voltage electrode sheathed with gas flowin the treatment water, i.e. generating pulsed discharge plasma
in water on a macroscopic meaning, facilitated decompositionof the indigocarmine and oxalic acid. One of the reasons forthese facilitations of organic compound decomposition might
be efficient diffusion and utilization of relatively long-life
active species such as ozone in the treatment water. The
facilitation of the decomposition of indigocarmine withsubmerging high voltage electrode（Fig. 3）seemed to be withozone solved in the treatment water. However, it is reported9）that oxalic acid could not be decomposed by ozone , and we
Fig. 3 Effect of submerging cylinder style discharge system on
indigocarmine decomposition by discharge plasma

also confirmed no decomposition of oxalic acid by injection of
10 ppm ozone into the treatment water at the flow rate 0.5 L/
min（data not shown）. Therefore, ozone did not play an

The effect of depth to submerge on the decomposition ofimportant role for oxalic acid decomposition. Directorganic compound was also investigated on the experimentinteraction of discharge plasma and water surface, in whichusing oxalic acid in the 220 ml treatment reactor（Fig. 4）.

short-life active species such as hydroxyl radical are produced

Compared to situating high voltage electrode at the height ofand contribute to decomposition of oxalic acid, should be

10 mm above the water surface, situating high voltage

necessary, and major reason of facilitation in oxalic acid

electrode at the depth of 100 mm under the water surface diddecomposition must be an improvement of mixing condition

not show clear facilitation in the decomposition of oxalic acid.

by the injected bubble in the treatment water（Fig. 4）.

However, increasing the depth of submerging high voltage

3.2 Influence of treatment water conductivity onelectrode to 300 and 500 mm facilitated decomposition, anddecomposition of organic compound by pulsed

final decomposition ratio was also increased. Especiallydischarge plasma

situating high voltage electrode at 500 mm under the water

In the cylinder style discharge system with gas flow, highsurface clearly facilitated the oxalic acid decomposition. The

voltage electrode is not in contact with treatment water. So

decomposition ratios achieved by discharge plasma treatment

regardless of whether the conductivity of treatment water isat the depth of 500 mm under the water surface after 120 min

high or low, the pulsed discharge plasma itself could be readily

Investigation of Organic Compound Decomposition and Microbial Inactivation by a Cylinder Style Pulsed Discharge System in High Conductivity Solution（Takayuki OHSHIMA at al.） 37
generated in water on a macroscopic meaning. However,shown that increasing conductivity of treatment water

strength and input energy of the pulsed discharge plasma

facilitated decomposition of indigocarmin by pulsed electric

might be changed by the conductivity of treatment water. Anddischarge.

those factors affect the amount of the radical produced at gas-Similar tendencies to the decomposition of indigocarmine

water interface. The influence of treatment water conductivity

were also observed on the decomposition of oxalic acid thoughon organic compound decomposition by pulsed discharge

more treatment time was necessary for the decomposition ofplasma was investigated using indigocarmine（Fig. 5）andoxalic acid than that of indigocarmine. The oxalic acid

oxalic acid（Fig. 6）. These experiments were carried out in 50

decomposition ratios in the high conductivity solutions（18ml treatment reactor, and high voltage electrode sheathed withand 51 ms/cm）were almost same, and those are higher thangas flow was situated at the depth of 100 mm under the waterthat in deionized water. In the high conductivity treatment

surface. The indigocarmine decomposition ratios in the highwater, the required time to decompose 50% of oxalic acid wasconductivity solutions（18 and 51 mS/cm）were higher thanalmost half of that in low conductivity treatment water.

that in deionized water（0.26 mS/cm）. However, significant

Decomposition rate constants of discharge plasma treatment indifference of decomposition ratio was not observed between

0.26, 18, 51mS/cm conductivity treatment water are 2.68×10 ,the conductivities of 18 and 51 ms/cm. Indigocarmin was

4.42×10 and 7.31×10 /sec, respectively. And the reachable

almost completely decomposed after 4 and 12 min in the highdecomposition ratio was increased in the high conductivity

and low conductivity treatment water, respectively.

solutions.

−4−4−4Decomposition rate constants of discharge plasma treatment in

3.3 Influence of treatment water conductivity on⊖3inactivation of microorganism by pulsed

0.26, 18, 51mS/cm conductivity treatment water are 4.39×10 ,⊖2discharge plasma⊖21.13×10 and 1.73×10 /sec, respectively. It was clearly

Inactivation of microorganism by pulsed discharge plasma
in high conductivity treatment water was investigated using E.
coli as the model microorganism. It was examined that E. coli
is not inactivated in high conductivity water（51 mS/cm）
before the experiment using pulsed discharge plasma（data notshown）. Figure 7 shows the results of E. coli inactivation bypulsed discharge plasma in different conductivity treatment
waters.
In high and low conductivity treatment waters, almost
complete inactivation of treatment water（no colony formation
on LB plate）was confirmed after 2 and 8 min discharge
Fig. 5 Influence of conductivity on indigocarmine decomposition
by discharge plasma

Fig. 6 Influence of conductivity on oxalic aicd decomposition bydischarge plasma

plasma treatment, respectively. Inactivation rate constants ofdischarge plasma treatment in 0.26, 18, 51mS/cm conductivity

Fig. 7 Influence of conductivity on E. coli inactivation bydischarge plasma

静電気学会誌 第38巻 第 1 号（2014）38−2−1−1treatment water are 3.20×10 , 1.41×10 and 1.49×10 /conductivity treatment water was calculated with thesec, respectively. Similar to the results of organic compound

observation of discharge voltage and current（Fig. 9）. The

decomposition, inactivation of E. coli was also facilitated inelectric energy stored in the capacitor in our pulse generator

high conductivity treatment water. Comparing with thewas 0.4 J, therefore, 75-80 % of stored energy was consumed

decompositions of indigocarmine and oxalic acid, inactivation

as the discharge plasma generation in high conductivityof E. coli is most sensitive for the pulsed discharge plasma

treatment water. The energy per pulsed discharge plasma intreatment.

high conductivity treatment water was at least 6-fold higher

3.4 Energy of pulsed discharge plasma

than that in low conductivity treatment water. This high-energy

Bright discharge plasma with loud sound was observed in

input into the treatment water as the discharge plasma could

the high conductivity treatment water. Discharge voltage andgenerate a lot of active species and facilitate decomposition ofcurrent waveforms in high and low conductivity treatment

oxalic acid.

water were observed with oscilloscope（Fig. 8）. Change ofwaveform between high and low conductivity treatment water
was observed in both discharge voltage and current.
Energy of pulsed discharge plasma in high and low

The decomposition of oxalic acid plotted against the input
energy was shown in Fig. 10. Decomposition efficiency per
input energy in low conductivity treatment water isapproximately 3-fold superior to that in high conductivity
treatment water. This means that active species generated bypulsed discharge plasma were not sufficiently utilized todecompose oxalic acid in high conductivity treatment water.
The rate-limiting factor might be diffusion of oxalic acid togas-water interface.
In our study, the cylinder style discharge system was appliedfor high conductivity water treatment. High conductivity ofthe treatment water allows efficient utilization of stored
electric energy in capacitor for discharge plasma generation
though energy efficiency in high conductivity water was lower
than that in low conductivity water. To improve the energy
consumption efficiency in decomposition of organic compoundin water by pulsed discharge plasma, facilitation of organic

Fig. 8 Discharge voltage waveforms in high（A）and low（C）
conductivity treatment water, and current waveforms inhigh（B）and low（D）conductivity treatment water.

Fig. 9 Calculated energy per discharge plasma in various
conductivity treatment watersmaterial diffusion to gas-water interface by mixing andadoption of suitable capacitor for high-voltage pulse generator
with a stationary spark gap should be important.

Fig. 10 Oxalic acid decomposition plotted against the energy ofdischarge plasmaInvestigation of Organic Compound Decomposition and Microbial Inactivation by a Cylinder Style Pulsed Discharge System in High Conductivity Solution（Takayuki OHSHIMA at al.） 39

4．Conclusion

References

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Acknowledgement
This research was supported by a Grand- in-Aid for
Scientific Research（C）from Japan Society for the Promotionof Science（22510082）.

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