S16-3
Energy efficiency of advanced oxidation processes for the decomposition ofpersistent organic compounds in water using plasmas
プラズマを用いた促進酸化による水中難分解有機物の分解エネルギー効率
Koichi Yasuoka, Yu Kamiya, Ryo Saeki, and Kosuke Tachibana
安岡康一, 神谷佑, 佐伯 亮, 立花孝介
Dept. of Electrical and Electronic Engineering, Tokyo Institute of Technology
2-12-1, S3-3, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
東京工業大学電気電子工学専攻 〒152-85525 目黒区大岡山2-12-1,S3-3
An advanced oxidation process (AOP) using OH radicals is a promising method for decomposing
persistent organic substances in water. The ozone and hydrogen peroxide (H2O2) method is widely used toform OH radials; however, this method needs a continuous supply of O3 and H2O2. We have developed anew plasma AOP system in which H2O2 is continuously supplied by plasmas generated in oxygen bubbles.
Sample wastewater containing highly concentrated persistent substances was completely decomposed
within 6 h of operation. The energy efficiency for the decomposition is a function of the production
efficiency of O3 and H2O2. The highest H2O2 yield of 2.1 g/kWh was obtained at 2.5 kV, 4 kHz, and a 1-spulse width.

1. Introduction
An advanced oxidation process (AOP) is one of
the most effective methods for decomposing
persistent organic pollutants in water using OH
radicals. Typical AOP systems feed ozone gas
and a hydrogen peroxide (H2O2) solution intowastewater. However, the continuous supply of
H2O2 has some difficulty; thus, the in-situ
generation of H2O2 is particularly needed forprocessing high concentrations and a large
amount of wastewater.
This report presents a new plasma-based AOP
system in which both O3 and H2O2 are suppliedby plasmas [1]. This system needs no supply of
H2O2 solution and can mineralize highly
concentrated persistent organics in wastewater.
The generation efficiency of H2O2 [2]-[4] wasstudied to improve the system efficiency.
2. Experimental setup
Fig. 1 shows the experimental setup of theplasma-based AOP system in which O3 and H2O2
are generated by an AC power supply and acapacitor discharging circuit, respectively. The
capacitance of storage capacitor was 2 nF. An
acrylic chamber contained 50 mL of wastewater. A
ceramic having a tiny hole with a diameter of 0.3
mm separated the wastewater and the gas chamber.
Oxygen flowed through the hole at a flow rate of
100 mL/min and generated gas bubbles 3–5 mm indiameter in the water. A pulsed plasma wasgenerated inside the bubbles. H2O2 was generated
through the interaction between the water surface
and the plasma.

The H2O2 concentration cannot be measured inwastewater; therefore, the generation efficiency of
H2O2 was evaluated with a 1% Na2SO4 solution.
The H2O2 concentration was measured by
PACKTEST
Hydrogen
Peroxide [Kyoritsu
Chemical-Check Lab, WAK-H H2O2] with anabsorptiometer (JASCO, V-630).
Mass flow controller
Gas inlet
AC power supply～

O2

Ozone
generator
Gas outlet
O3

Semiconductor
switchAcrylicchamber
Waste
water
PlasmaDCpower
supplyGas inlet

Storage
capacitor

O2

Mass flow controller
GND

StainlessmeshFig. 1. Experimental setup of the plasma-based AOP
system.

3. Experimental results
Fig. 2 shows the results obtained for 50 mL ofwastewater treated by O3 and H2O2, which wasgenerated by the pulsed plasma. The average
discharge input power was 5.7 W, and the ozone
concentration was 105 g/Nm3. After 480 min ofoperation, 99% of the persistent compounds in the2
1.8rate

1.6
1.4
1.2
1800

H2O2 generation rate [mg/h]

1200

results indicate that a low-voltage long-pulse-width
pulsed discharge is the suitable for the efficient
production of H2O2.

H2O2 yield [g/kWh]

TOC concentration [mg/L]

wastewater were mineralized. The energy efficiencyof the decomposition process was estimated to be
0.3 gTOC/kWh. The efficiency was not so high;nonetheless, the complete decomposition of highly
concentrated organic compounds in wastewater wasachieved. To improve the energy efficiency ofdecomposition, the generation efficiencies of OH,
H2O2, or O3 by discharge should be improved inaddition to the dissolution efficiency of ozone.

yield
0100
200
300
Pulse width [ns]400Fig. 3. H2O2 energy yield and generation rate
as a function of the pulse width.

40000100 200 300 400 500
Time [min]Fig. 2. Time variation of the TOC concentration ofwastewater treated using the plasma and ozone system.

The generated H2O2 rates and yields were studied
by varying the plasma parameters of repetitivefrequency, applied voltage, pulse width, and theamount of water vapor in the oxygen gas.
Increasing the pulse frequency could increase thegeneration rate while keeping the energy yield
constant, although the rate of increase in thegeneration rate decreased at high frequencies.
Increasing the applied voltage was inefficient forincreasing the generation rate because the energy
yield decreased. The effect of pulse width wasevaluated with a constant discharge power at 1 kHz.
With a larger pulse width, the generation rate andenergy yield both increased as shown in Fig. 3.
Theseresults
indicatethat
low-voltage,long-pulse-width, and high-frequency operation aresuitable for the generation of H2O2. Introducing
water into the supply gas improved the energy yieldof H2O2 when the amount of water was greater than
the amount in the saturated water vapor. A
maximum generation rate of 61.8 mg/h and an
energy yield of 2.1 g/kWh were obtained at 2.5 kV,
4 kHz, and a 1-s pulse width.
The applied voltages and pulse widths were 3.5,4, and 5 kV and 100, 200, and 300 ns, respectively.
Both the generation rate and energy yield increased
with increasing pulse width. The experimental

4. Conclusion
A plasma-based O3 and H2O2 system for an AOP
was developed for high-conductivity wastewatercontaining a high concentration of persistentcompounds. A wastewater volume of 50 mL with aconcentration of 1180 mgTOC/L was mineralized
after 480 min of operation using a pulsed plasma
operated at 5.7 W in combination with 105 g/Nm3of ozone gas.
The properties of H2O2 generation by pulsed
discharge were investigated by varying the plasma
parameters of voltage, frequency, pulse width, andthe amount of water vapor in the gas. The results
indicate that low-voltage, long-pulse-width, andhigh-frequency operation are the most suitable forthe generation of H2O2. A maximum generation rateof 61.8 mg/h and an energy yield of 2.1 g/kWh
were obtained at 2.5 kV, 4 kHz, and a 1-s pulse
width.
Acknowledgments
This work was supported by JSPS KAKENHI,
grant no. 26249032.
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PlasmaPollutionControl
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