22P077-P
Gas-liquid plasma using nano-sec pulsed discharge
and its application for sterilization
ナノ秒パルス放電を用いた気液混相プラズマ生成と滅菌への応用
Tatsuya Togashi, Ryutaro Shimokawa, Nozomi Tanaka and Akira Ando
富樫達也, 下川龍太郎，田中のぞみ，安藤 晃
Graduate School of Engineering, Tohoku University
6-6-05, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
東北大学工学研究科 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
Plasma discharge in gas-liquid mixed phase was investigated for water treatment applications. A bubbling
type reactor and a high-voltage pulsed power supply were utilized to generate gas-liquid phase discharge.
We applied this technique to sterilize spore-forming bacteria in water. Sterilization effects were examined invarious kinds of working gases. Active species like ozone and OH radicals and UV rays generated in thedischarge worked effectively in inactivation of bacteria.

1. Introduction
Water pollution is one of the most serious
environmental problems. More then 8 million
people cannot use safe water and are facing fatal
problems all over the world. Water purification
technology is urgent required for protection ofwater quality.
Recently, plasma discharge in water has
attracted much attention for water treatments.
Discharge in water enhances active species likeozone (O3) and OH radicals, ultraviolet (UV) rays.
Strong electrical field and shock waves are also
generated in the discharge and they work
effectively for water treatment [1].
In this study, we utilized gas-liquid phasedischarge to sterilize spore-forming bacteria inwater and measured the sterilization effect.

2.2 Gas-liquid plasma reactor
Figure 2 shows the plasma reactor for gas-liquid
mixed phase discharge. The upper part of thereactor was filled with water, and working gas wasfed from the lower part through 25 holes of 1 mm indiameter. In this study, air, oxygen (O2) andnitrogen (N2) were used as working gas with the gas
flow rate of 8.0 l/min. A thin stainless steel wire (φ= 0.3 mm) was placed below the holes and thepulsed high voltage was applied between the wire
and a ground electrode set in the water. Thedischarge was formed between the wire electrodeand surface of bubbles above the small holes inwater.

2. Experimental Setup
2.1 High voltage pulsed power supply
Figure 1 shows an electric circuit of thehigh-voltage pulsed power supply. The circuit
consists of a capacitor C, a gap switch GS, aninductor L, and semiconductor opening switch(SOS) diodes. When the gap switch was shorted
after the capacitor was charged by DC power
supply (11 kV), current started to oscillate in theLC circuit. When the current was interrupted by theSOS diodes with delay of 100 ns, the suddeninterruption of current resulted in the generation ofhigh voltage pulse Vo with a short pulse width of40ns typically [2]. The pulse repetition rate wasvariable and set at 30 Hz in this study.

Fig.1. The electric circuit of the high-voltage pulse
generator utilizing SOS diodes.

Fig.2. Schematic of the gas-liquid plasma reactor.

Number of colonies
Initial number of colonies

(1)

20150
VcVo

10050-10-200-30-40100

Current [A]Survival Ratio =time in various gas species. Sterilization effect wasobserved using various gas species. D-values (atime to decrease the number of bacteria to 1/10) in
air, O2 and N2 gases were about 280 s, 370 s and370 s respectively. Active species and UV
generated with the discharge worked effectively forinactivation. Additionally, pH value decreased in
the discharge with air gas and the solution became
acidic. The pH value may contribute thesterilization.

Voltage [kV]

2.3 Biological indicator
In order to investigate the effect of sterilization,
Spore-forming bacteria Bacillus Subtillis was usedas biological indicator. The initial population of thebacteria was set at 105 in 50 ml solution. After
treatment, samples were diluted appropriately andwere put on Petri film. After incubation at 30 ºC for
24 hours, colonies of the bacteria appeared asshown in Fig.3. Sterilization effect was measured
by counting the number of the colonies andcalculating survival ratio according to eq.(1).

Io-0.4 -0.2 0 0.2
time [µµs]0.4-50Fig.5. Wave forms of voltage and current at a pulsed
discharge.

Fig.3. Sterilization effect on the number of the colonies(a) untreated, (b) 5 min, (c) 10 min and (d) 15 min.

Survival Ratio [-]100airO-1

2

10N

2-2

10-3

10-4

3. Experimental results
3.1 Gas-liquid plasma discharge
Gas-liquid plasma discharge was observed in theplasma reactor as shown in Fig.4. Typical wave
forms of pulsed high voltage and current atdischarge are shown in Fig.5. When charging
voltage VDC was 11 kV, the pulsed voltage of about
20 kV can be applied with the pulse width of 30 –40 ns. Averaged consumed energy was about 11 mJ
per pulse.

Fig.4. Gas-liquid phase plasma discharge

3.2 Sterilization effectFigure 6 shows that survival ratio as a function of

100510 15 20Time [min]2530Fig.6. Survival Ratio as a function of time.

4. Summary
To sterilize bacteria in water, we utilized
gas-liquid plasma discharge, which was generated
by using a pulsed power supply and a plasma
reactor. Sterilization effect was observed withvarious kinds of working gas. Active species andUV generated by the plasma discharge worked
effectively for sterilization. Further experiments
will be performed in order to study the effect of pH
value in the solution.
References[1] M. Sato: Plasma Source Sci. Technol. 17 (2008)
024021.
[2] K.Takaki, et al: IEEE Tran Die Ele Ins. 14 (2007)
834