treatment of wastewater by underwater discharge …...treatment of wastewater by underwater...

International Journal of

Renewable Energy and Environmental Engineering

ISSN 2348-0157, Volume 03, No 03, July 2015

IJREE 030303 Copyright © 2015 BASHA RESEARCH CENTRE. All rights reserved

Treatment of Wastewater by Underwater Discharge in Gas Bubbling Water

Ruma1, M Ahasan Habib

2, SHR Hosseini

3, T. Sakugawa

3, H. Akiyama

3

1Department of EEE, Dhaka University of Engineering and Technology, Gazipur, Bangladesh

2Department of ME, Bangladesh Army University of Science and Technology, Saidpur, Bangladesh 3Graduate School of Science and Technology, Kumamoto University, Kumamoto 860-8555, Japan

Email: [email protected]

Abstract: This paper describe the high-voltage pulsed discharge generated in water with different types of

bubbling gases as well as no gas in a reactor with a point-mesh electrode configuration. The pulsed discharges

are a promising technique in environmental remediation to treatment of wastewater. Acid orange 7organic dye

solution was treated as a common wastewater sample to elucidate chemical efficiency of bubbling gas types in

discharge reactor. Experimental results showed that, oxygen bubbling gas effectively removed 84.35% of acid

orange 7, while corresponding removal rates were 70.40%, 64.67%, 58.36% and 50.81% with argon, air,

nitrogen and with no bubbling gas, respectively. Our research clarifies that chemical efficiency of the discharge

reactor is significantly influenced by bubbling gases. A magnetic compression pulsed power modulator at 25kV,

100Hz was used as a high voltage pulse source to initiate discharge in reactor.

Keywords: Discharge, Streamer, Acid orange 7, Oxygen, Argon, Air, Nitrogen, Bubble, Magnetic pulsed

compression

Introduction:

High voltage pulse discharge is of technological great

interest for industrial and environmental applications.

Especially for the treatment of water pollution that

occurring in tandem with rapid development of the

textile industry and other such factories causes serious

environmental problems. Different kinds of organic

dyes of complex composition and strong biologic

toxicity are synthesized subsequently producing dye

wastewater [1-4]. Over the past few decades, various

physicochemical and electrochemical methods such as

UV photolysis, photocatalysis, sonochemistry,

supercritical water oxidation,have been examined for

the removal and degradation of dye molecules from

wastewater [5-8]. However, these methods suffer

inherent disadvantages in terms of their applicability

and cost. Therefore, development of an advanced dye

wastewater treatment method is very important to

determine environmental pollution.

Discharge plasma generated directly in water using

high-voltage pulses is known as an effective waste

water treatment method. The high voltage pulse is able

to concentrate a strong electric field at the high voltage

needle enough to easy breakdown of water to initiate

discharge [1-5]. The discharge can be in the form of

corona or streamer, spark and arc make conductive

channels producing high energy electrons in water

which are capable of ionization, dissociation and/or

recombination of water molecules [1-10]. Through

these processes discharge plasma interacts with water

molecules to initiate various physical and chemical

processes in water such as a strong electric field,

intense UV radiation, shockwaves, and the generation

of various active ions such as H+, H3O

+, O

+, H

-, O

-,

reactive radicals such as OH2, O2, OH; and molecular

species such as H2, O3, H2O2 [[1-10]. These chemical

active species generated by the electrical discharge

can attack and then degrade the organic pollutants

contained in the water.

Due to the higher permittivity (εr= 81) and density

(103 kg/m

3) of water, a high electrical field in the order

of several MV/cm is necessary to initiate the discharge

in water [11]. The applications of external bubbling

gas in water can influence plasma chemical activity as

well as the production of radicals or reactive

molecules. In addition, bubbling gas in water lowers

electric field required to initiate discharge, thus

reducing electrode erosion, a common problem in the

direct discharge method. Another advantageous

feature of bubbling in discharged water is easy

initiation of ionization and electron avalanche process,

which influences active species formation and reduces

joule heating losses due to vaporization [11-14].

Different bubbling gases are thus anticipated as a

positive influence on the removal of organic dye from

polluted water.

In previous works, several common organic dyes such

as acid orange 7 [15-19], indigo [20], methyl orange

[21], Chicago sky blue [21], direct red 79 [22], direct

blue 106 [22] and basic blue [22] were treated using

pulsed discharge in water. These studies confirmed

that dye molecules breakdown due to the efficiency of

OH radicals, O radicals, ozone [20-24] and dye

removal rates increased with the addition of hydrogen

peroxide in discharged water [21]. Li et al showed

above 85% of dye removal is possible using TiO2

photo catalyst with streamer discharge [18].Shen et al

reported that acid orange 7 removal was higher for

spark discharge (57.2%) than for streamer discharge

(40.4%) and corona discharge (27.6%) [25].Burlica et

al evaluated effects of various gases (N2, O2, air and

argon) on the removal of reactive blue 137 dye using

GlidArc reactor, finding that degradation was higher

for N2 and O2 gases [19]. Clements et al confirmed

ozone production and removal of indigo dye with

oxygen bubbling gas in a reactor, finding that removal

increased with O2 gas flow rates [20].

RUMA, M AHASAN HABIB, SHR HOSSEINI, T. SAKUGAWA, H. AKIYAMA

International Journal of Renewable Energy and Environmental Engineering

ISSN 2348-0157, Volume 03, No 03, July 2015, pp 189-194

In this work, we observed the removal of acid orange

7 organic dye using pulsed streamer discharge in

bubbling water. Four different types of gases were

used to generate bubbles in water: oxygen, argon, air

and nitrogen; also a no gas condition was evaluated. A

point-mesh electrode configuration and a magnetic

pulsed compression (MPC) pulsed power modulator at

25kV, 100Hz were employed to generate discharge

across electrodes in reactor. A non-conductive porous

ceramic filter was used to generate bubbles in the

presence of applied gases in water. The effects of

bubbling gases on the chemical efficiency of the

reactor were evaluated by treatment of acid orange 7

organic dye solutions.

Materials and Methods:

A schematic of the experimental setup is shown in

Figure 1. A cylindrical glass jacket reactor (volume:

200 ml) filled with water or dye solution was used as a

discharge reactor. The initial concentration of acid

orange 7 organic dye solution, was 20 mg/l

(C16H11N2NaO4S, molecular weight: 350.32 g/mol)

prepared from deionized water. Absorption spectra of

dye solution were measured using a spectrophotometer

(AMTOH, FL-2000, U-2900) [2]. A discharge sample

of 3 ml was used to measure at the range of

200~800nm. The concentration of degraded dye was

then determined at maximum peak of initial dye at 484

nm. The solution conductivity was adjusted by mixing

a desired amount of potassium chloride (KCl) with

dye solution at 100µS/cm as measured by a

conductivity tester (CD5021A, Custom).A magnetic

pulse compression pulsed power modulator was used

as the high voltage source (Suematsu Elect. Co. Lt.,

Japan) to supply 25kV at 100Hz. Electrical

measurement was performed using a digital

oscilloscope (DPO4054B, Tektronix) with a high

voltage probe (P6015A, Tektronix) and current

monitor (model no. 3972, Pearson).

A non-conductive ceramic filter was used to generate

bubbles in dye solution as shown in Figure 1(b), which

consists of three parts such as: (i) a glass tube

(170mm), non-porous glass surface (10mm) and

porous ceramic surface (30mm).This porous surface

was made of aluminum oxide (Al2O3, polycrystalline

material with grain size 1-5μm, pore size 1-100μm)

and offered excellent dielectric properties, zero water

absorption, high thermal stability with low expansion,

high resistivity (>1014

ohm-cm), and non-conduction

of electrical current during discharge. To generate

bubbles, four different types of gases: oxygen, argon,

air and nitrogen was injected through the glass tube at

a 1.0 l/min flow rate and 0.2 MPa pressure. Bubbles

were produced in solution through the porous ceramic

surface and then tended to move through the water

towards the ground electrode. Their initial size was

about 0.1mm and then grew to 3-5mm before

dispersing and finally collapsing. The bubbles

movement area varied over the reactor volume

surrounding the ceramic surface in water. The tip of

point electrode was placed in the vicinity of porous

ceramic surface in solution. Thus, discharge can easily

inject to gas bubbles during their propagation and may

enhance the chemical activity in discharge dye

solution.

(a)

(b)

Figure 1: Schematic of (a) experimental setup and (b)

schematic layout of ceramic filter to generate pulsed

streamer discharge in gas bubbling water

Results and Discussion:

Visualizations of discharge streamer propagation from

the high voltage needle tip to water under (a) Oxygen,

(b) argon, (c) air, (d) nitrogen and (e) no bubbling gas

are shown in Figure 2. In addition discharge injected

in a gas bubble is also shown in Figure 2(f). When the

high-voltage pulse was applied across the electrodes,

discharge initiated as a streamer from the needle tip

and then propagated with good branching in water.

During propagation of discharge in bubbling water,

streamer branches passed through gas bubbles. The

propagation of the streamer branches were

simultaneous and could not inject in all bubbles. There

was no significant variation of physical appearance of

discharge under the bubbling gas and no gas

conditions. An increase of the light emission intensity

has been observed in the full development of branches.

In the presence of gas bubbling, streamer branches

were propagated easily in all direction from the needle

tip and their length were longer than no gas bubbling

condition; because the density of medium in the gas

bubble is much lower than direct liquid water.

Streamer branching is primarily influenced by the

behavior of bubbles in the reactor liquid. Immediately

gas bubbles collapsed after injecting streamer channel

inside it.The length of streamer length was varied at




20-25mm in gas bubbling condition, where it was 15-

20mm in no gas bubbling condition. Some micro

bubbles were produced at no gas bubbling condition

by local heating of field emission current or by ionic

current in the conduction channel between electrodes

[24].

(a) (b)

(c ) (d)

(e) (f)

Figure 2: Visualization of discharge streamer under

(a) oxygen (b) argon, (c) air, (d) nitrogen gas

bubbling, (e) no bubbling gas and (f) discharge

injected in a gas bubble

Figure 3 shows typical waveforms of voltage and

current during discharge under (a) oxygen bubbling

gas and (b) no bubbling gas. The amplitude of current

is seen to be a little higher and the pulse width is lower

under the bubbling gas water than no bubbling gas

condition.

In order to evaluate effects of bubbling gases on the

chemical efficiency of discharge reactor, the removal

of acid orange 7 organic dye was studied. This dye is

characterized by an azo group consisting of two

nitrogen atoms (-N = N-), which is very sensitive to

OH radicals, H2O2 and ozone (O3) [15-18]. This dye

constituting the largest class among the synthetic

colorants, are considered as the widespread

environmental pollutants associated with many

important industries such as textile, food colorants,

printing and cosmetic manufacturing. Typical UV-

visible absorption spectra of treated dye at (a) oxygen

bubbling gas and (b) at no bubbling gas are shown in

Fig. 4.The pulse was applied for 60 min. It is seen that

the absorption spectra of acid orange 7 dye decreases

with time in discharge water, where the absorbance

peak is mainly around 484nm for azo double bond

(N=N). The reduction of these peaks indicates the

breakdown of azo double bond by pulsed discharge in

reactor.

(a)

(b)

Figure 3: Typical waveforms of voltage and current

during discharge when (a) oxygen gas and (b) no gas

were bubbled in water

Experimental results of dye removal rate using

oxygen, argon, air, nitrogen and no bubbling gas are

shown in Figure 5, where the removal rate was much

faster for the case of oxygen bubbling gas than others.

These results indicate the plasma chemical activity of

discharge is strongly depend on the bubbling gas

types.

The color removal rate was calculated by the

following formula:

Removal rate

(1)

Where was the initial dye concentration and was

the treated dye concentration after pulse treatment.

Respective removal ratio were 84.35%, 70.40%,

64.67%, 58.36% and 50.81% under oxygen, argon, air,

nitrogen and at no bubbling gas. Figure 6 shows the

image of acid orange 7 dye solution: (a) before

treatment, (b) after oxygen gas bubbling treatment and

(c) after no gas bubbling treatment. It is seen that the

color of acid orange 7 dye solution is converted to

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

-1 0 1 2 3 4 5

Voltage (kV)

Current (A)

Volt

age

(kV

)

Cu

rren

t (A

)

Time(s)

-30

-20

-10

0

10

20

30

-30

-20

-10

0

10

20

30

-1 0 1 2 3 4 5

Voltage (kV)

Current

Volt

age

(kV

)

Cu

rren

t (A

)

Time(s)

Bubble

Streamer

discharge

5mm 5mm

5mm 5mm

5mm 5mm




almost transparent by pulse treatment in oxygen gas

bubbling condition.

Generally, streamer branches contains high energy

electrons at their head which are reacts with water

molecules during propagation in water. As a result

various types of reactive radicals and active species

(OH, O, H, HO2, O2, NO, NO2, H2O2, O3) are formed

in discharge region; subsequently, these radicals react

with dye molecules to break them down. However, the

production of these active species influenced by

bubbling gas types which is very clear from results in

Figure 5.Gas molecules were energized when

discharge injected in gas bubble and form plasma

channel, which enhanced the production of reactive

radicals in the contact region [18]. It is suggested that

the breakdown process of acid orange 7 dye mainly

occurs due to direct action of OH radicals or ozone

generated during discharge in water. The possibility of

the formation of O3, OH and O radicals are higher

during oxygen bubbling gas than others in water. The

oxidation power of OH radical, atomic oxygen, ozone

and hydrogen peroxide is 2.80, 2.42, 2.07 and 1.78

respectively [12]. It is suggested that H2O2 can attack

to dye molecules directly or via OH radicals, as H2O2

formed by direct combination of OH radicals and their

dissociation also gives OH radicals in discharge water

[7, 14-16].

The production process of various species may be

more obvious in the presence of oxygen gas bubbling

than others. Because atomic oxygen could be formed

by the discharged bubble and react with water

molecules to form H2O2 and O3. As a result the dye

removal rate increase in the presence of oxygen gas

bubbling than other conditions.

Previous research have shown that the decolorization

of azo dye was increased with increasing the ozone

dose in reactor [5-6]. Ozone and H2O2generation was

higher at oxygen gas bubbling than that in air, argon

and nitrogen [8-10]. Argon is a chemically inert gas.

When pulsed discharges take place; argon is

dissociated to excited electrons. These actively react to

water molecules to produce OH radicals, or O atoms

in water [19-20]. H2O2 was produced in our previous

work by discharge in argon gas bubbling water [1].

During nitrogen gas bubbling, several nitrate products

(NO, NO2-, NO3

-) are formed in discharge water [8,

10, 17], these are lead to formation of HNO2, HNO3

through reaction to OH and O radicals [7-10].For this

the production of O3 and H2O2 may be reduced during

nitrogen gas bubbling in water. As a result the removal

rate of acid orange 7 dye is lower in nitrogen gas

bubbling than other gases.

At no gas bubbling condition, only production of OH

radical and H2O2 was confirmed in water by previous

works [1-5]. So dye removal rate is lower in this case

compared to gas bubbling conditions.The study

confirmed the byproducts of acid orange 7 dye after

pulse treatment consist of acetic acid, p-benzoquinone,

phenol, 2-naphthalone, coumarin, benzoic acid,

phthalic anhydride [15-19, 24-25].

(a)

(b)

Figure 4: Typical UV-visible absorption spectra acid

orange 7 organic dye under (a) oxygen bubbling gas

and (b) no bubbling gas in water

Figure 5: Removal rates of acid orange 7 organic dye

under oxygen, argon, air, nitrogen and no bubbling

gas in water

0

0.4

0.8

1.2

1.6

200 250 300 350 400 450 500 550 600

0 min7.5 min15 min22.5 min30 min37.5 min45 min60 min

Ab

sorb

an

ce [

a.u

]

Wavelength [nm]

0

0.4

0.8

1.2

1.6

200 250 300 350 400 450 500 550 600

0 min7.5 min15 min22.5 min30 min37.5 min45 min60 min

Ab

sorb

an

ce [

a.u

]

Wavelength [nm]

0

20

40

60

80

100

0 10 20 30 40 50 60 70

OxygenArgonAirNitrogenNo gas

Rem

oval

rat

e [%

]

Time [min]




(a) (b) (c)

Figure 6: Image of acid orange 7 dye solution: (a)

before treatment, after treatment under (b) oxygen gas

bubbling and (c) no gas bubbling in reactor

Conclusion:

The effects of bubbling gases on wastewater treatment

were studied by high voltage underwater discharge in

gas bubbling water. It can be concluded that, the

physical appearance of discharge did not significantly

change with variation of bubbling gases and at no

bubbling gas condition. But chemical activity of

discharge was significantly varied by bubbling gas

types. Discharge propagation was much easier in

bubbling water than that at no bubbling water. The

removal rate of acid orange 7 is influenced by the

discharge of bubbling water. Production of radicals

and reactive molecules could be enhanced in the case

of oxygen gas bubbling than for the others: argon, air,

nitrogen and no bubbling gas. Therefore, the removal

rate of dye was higher under oxygen bubbling gas than

that under other conditions. It is mentionable that the

removal rate of acid orange 7 is more than 50% in all

conditions. Finally, it is clear that bubbling gas types

are an important parameter of discharge reactors to

improve chemical efficiency of wastewater treatment

methods.

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treatment of wastewater by underwater discharge …...treatment of wastewater by underwater...

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