22P081-P
Effect of Bubble Formation on Water Treatment Using Gas-Liquid Plasma
気液混相プラズマ生成時における気泡形成と水処理への効果
Ryutaro Shimokawa1, Tatsuya Togashi1, Nozomi Tanaka1, Akira Ando1 and Koichi Takaki1
下川龍太郎1, 富樫達也1，田中のぞみ1、安藤 晃1，高木浩一2
1

1

Tohoku University, Sendai Miyagi 980-8579, Japan
東北大学工学研究科〒980-8579 宮城県仙台市青葉区荒巻字青葉6-6-05
2Iwate University, Iwate Morioka 980-8579, Japan2
岩手大学工学研究科〒980-8579 岩手県盛岡市上田3-19-8

Recently, much attention has been paid to water treatment utilizing a discharge in bubbling water andvarious methods were studied extensively. Radicals with short lifetimes are generated by the discharge andeffective for water purification and sterilization. We have studied gas-liquid mixed phase discharge by using
pulsed high-voltage with several tens of nanoseconds in a duration. The voltage and current characteristics of adischarge in bubbling water were measured. The shape of bubbles and discharge in a bubble were observed invarious gas flow rates and hole diameters. We have investigated decolorization of indigo solution for watertreatment. The relation of the contact area between the plasma and water was studied for a higher efficient ofwater treatment.

1. Introduction
Commercially used water treatments such as
chlorine treatment process, ozone treatment
process, and activated sludge process have various
problems in safety issues for human bodies andenvironment. Water treatment techniques using
plasma discharges have attracted much attention fornext generation water treatment because it isexpected that this process does not have suchproblems and has ability to decompose persistentorganics.
Ultraviolet exposure, ozone, shock wave, andvarious radicals are generated by using a discharge
in or on water and various methods were studied
extensively [1-3]. Radicals and ions with short
lifetime are generated by the discharge, which areeffective for water purification or sterilization. OH
radicals have strong oxidation ability and producedin the discharge in water. They are expected todecompose contaminated materials and persistentorganics and to remove compounds in water.
We have investigated the effect of water treatment
by using pulsed high voltage discharge andevaluated decolorization effect in water using
indigo carmine (blue dye liquid) [4]. The reaction
rate depends on contact area between the plasma
and water. In order to improve the efficiency ofwater treatment, discharge in bubbles wereexamined with various working gases.

2. Experimental apparatus
Figure 1 shows the electric circuit of a highvoltage pulse generator used in the experiments.
The circuit consists of capacitor C, gap switch GS,inductor L, and semiconductor opening switch(SOS) diodes. When the GS is closed, current starts
to oscillate in the LC circuit. When the current
direction reverses, the reverse current wasinterrupted by the SOS diodes with the delay of 100
ns. The sudden interruption of the current resulted
in the generation of high voltage pulse Vo with ashort pulse duration of 40 nanoseconds.
The plasma reactor for water treatment is also
shown in Fig.1. A wire electrode was placed in thegas section (lower part of the reactor) and agrounded electrode was immersed in the watersection (upper part). When the pulsed high voltage
with negative polarity was applied to the wire
electrode, a streamer-like discharge was observed inelongated bubbles above each hole of the separator.
The diameter and number of holes were changed(0.3, 0.5, 0.8, 1.0 mm and 5, 10, 15, 20, 25 holes) inorder to vary the contact area between the plasma
RD.C.

LVC GS

VC0C

SOS
diodes
IO

Separator

VoGND

SampleHole

Bubblegg

Electrodes

gs

H.V.
Gas

Fig.1. Schematic diagram of the pulsed high voltage
circuit and a plasma reactor with bubblingwater.

5mmand water. The discharge characteristics and theeffect of water treatment were measured bychanging the gas flow rate. To evaluate the effect ofwater treatment, the decolorization ratio of indigo
solution were used.

Decolorization Rate[%]3. Experimental results and Discussions
Figure 2 and 3 show the decolorization rate andefficiency with various gas flow rates and holediameters. According to previous study, ozone andoxygen radicals have selectivity for reactions andreact easily with C=C double bond that shows theblue color of indigo carmine [7]. Treatment time is
20 sec with the pulse repletion rate of 30 Hz. These
values increased with the gas flow rate but did notchange with hole diameter.
The decolorization ratio depends on the contact
area between the plasma and water. Figure 4 shows
photographs of bubbles and discharges occurred in
the bubbles. The shape of the bubbles was different
with different gas flow rates and hole diameters.
Larger bubbles appeared with larger flow rate andlarger hole diameter. With a constant flow rate thenumber of bubbles decreased and total area seemed
to be constant. The decolorization rate andT t=20s

1000.3mm
0.5mm
0.8mm
1.0mm

8060(b) Gas flow was large(0.5L/min for single hole)Fig. 4. Photographs of the bubble and the discharge
from a single hole of 1mm in diameter. (left:bubble without discharge. right: with discharge).

efficiency were also evaluated in different numberof holes.
4. Conclusions
Pulsed high voltage with several tens of nanoseconds was generated by using SOS diodes. The
discharge in water with bubbling gas was studied
and applied for the decolorization of the indigo
solution.
The shape of bubbles and discharge in a bubble
were observed in various gas flow rate and holediameters. The decolorization rate and efficiency ofthe indigo solution were improved by increasing thegas flow rate. To enable higher efficiency of water
treatment, the relation of the contact area between
plasma and water to the efficiency should beclarified further.

402000246810

Gas Flow [L/min]Fig. 2. The energy consumption as a function of thegas flow rate and the hole diameter.
1.5

Decolorization Efficiency[g/W・・ h](a)Gas flow was small(0.1L/min for single hole)

5mm

T =20st

0.3mm
0.5mm
0.8mm
1.0mm

10.500246810Gas Flow [L/min]Fig. 3. Decolorization efficiency as a function ofthegas flow rate and the hole diameter.

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