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PULSED ELECTRICAL DISCHARGE IN BUBBLED WATER FOR
ENVIRONMENTAL APPLICATIONS
O. Mozgina1, S. Gershman2, A. Belkind1, K. Becker1, C. Christodoulatos1
1
2
Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ 07030 USA
Department of Physics and Astronomy, Rutgers, the State University of New Jersey, Piscataway, NJ
08854-8019 USA
ABSTRACT
This study investigates water purification and the production of active species by the electrical
discharge in water in the presence of oxygen gas bubbles. The organic dye, Rhodamine WT is used as
a model to assess the effectiveness of the cleaning process for several reactor designs and
experimental parameters. Significant reductions in the dye concentrations have been achieved by
applying 10 – 15 kV, ~1-5s pulses at a rate of 20 and 80 Hz to a reactor with horizontal parallel
mesh electrodes. Water and bubbles pass vertically through the mesh electrodes. The experiments
reported here explore the effect of varying the applied voltage, the electrode design, and oxygen gas
flow rate on the discoloration process and on the production of hydrogen peroxide and ozone.
1. INTRODUCTION
The interest in treating contaminated water using electrical discharge techniques has been growing
consistently since the initiation of this method almost twenty years ago [1]. Pulsed and ac electrical
discharge is used for generating ozone, hydrogen peroxide, O, H and OH radicals and other
chemically active species involved in water sterilization and discoloration [2 – 9]. Pulsed power
techniques allow limiting thermal effects and therefore leading to higher energy efficiency. A higher
percentage of overall input energy is used for the production of energetic electrons capable of
ionization and/or dissociation of molecules leading to the production of active radicals [3, 10, 11].
The generation of active species in water by electrical discharge can be substantially enhanced by
bubbling oxygen through the water [11 – 14]. The presence of gas bubbles lowers the electric fields
needed to initiate the discharge. This has the advantage of decreasing the power used in generating
the discharge. Introducing oxygen bubbles into the water leads to increased concentrations of
hydrogen peroxide and ozone generated by the discharge at lower power. Radical production by
pulsed discharge is affected by various parameters of the discharge and the solution such as the
applied pulse peak voltage, polarity, rise time and width; electrode tip curvature radius; and the
composition, pH and conductivity of the aqueous solution [5, 7, 10, 11, 15].
This investigation applies short (1 – 5 s) pulses to water with oxygen bubbles flowing through the
solution. The discharge is operated at voltages low enough to contain the discharge in the gas within
the bubbles. Several experimental systems have been designed to achieve robust and stable
performance in prolonged use. The effects of the applied voltage, oxygen flow, and overall design
have been investigated. Rhodamine dye has been used as the model contaminant to investigate
discoloration. The concentration of hydrogen peroxide generated by the pulsed discharge has been
measured in distilled water.
2. EXPERIMENTAL METHODS AND RESULTS
2.1.
Apparatus
De-ionized water and water contaminated with Rhodamine is mixed with O2 bubbles and circulated
through a discharge treatment chamber. Rhodamine concentrations, concentrations of active species,
and physical parameters of water such as temperature, conductivity, and pH are monitored during the
treatment process. The discharge voltage and current are recorded using a digital oscilloscope. Three
treatment chambers are designed and tested. Rhodamine WT at an initial concentration of 100 – 200
ppb is used as a model contaminant and its concentration is measured by fluorescence spectroscopy
using the SCUFA Fluorescence meter. H2O2 concentrations are measured by sodium thio-sulfate
titration in experiments with pure de-ionized water.
Electrical discharges are generated in bubbled water using pulsed power. The pulsed power systems
in these experiments generate 8 – 20 kV pulses 1 – 5 s in duration at a rate of 0 – 140 Hz. Two
switching methods have been used, one using a rotational spark-gap switch (20 – 140 Hz) and
another using a Perkin Elmer Optoelectronics HY 3003 Thyratron (0 – 200 Hz) with TM-27
Thyratron driver. All pulse generating circuits are charged using a Hipotronics HV DC power supply.
Voltage between the electrodes is measured using Tektronix P6015A 75 MHz HV probes; a Pearson
high band pass coil is used to measure current to the ground, and the measurements are recorded
using a Tektronix TDS 340A 100 MHz digital oscilloscope. Simon peristaltic pump is used to
regulate water flow. Pure oxygen gas is bubbled through the water using MKS mass-flow controller
with 247 fore-channel readout to control the flow rate in the range of 0 – 40 ml/min. Gas enters the
water treatment chambers through a diffuser used to control the size of gas bubbles. Diffusers with
bubble diameters in the range of 1 mm have been used.
2.2.
Experimental System #1
2.2.1. Water Treatment chamber #1
The first design of the water discharge reactor consists of a PVC chamber with vertical concentric
cylindrical stainless steel mesh electrodes 5 mm apart. Water flows upward through the reactor.
Oxygen is delivered through the center of the PVC tube with a diffuser at the bottom. (Figure 1) The
diffuser produces bubbles 2 – 3 mm in diameter that pass freely upwards between the mesh
electrodes without attaching to the electrodes. The discharge is observed only sporadically.
Figure 1 (a) Water Discharge Reactor model #1 uses concentric cylindrical mesh electrodes with
oxygen bubbles and water passing between the electrodes.
(b) Pulse generating circuit using a rotating switch used for Systems #1 and #2
2.2.2. Electrical System
High voltage pulses 5 s in duration are generated by charging a 10 nF capacitor bank through a 100
k charging resistor and discharging using a rotating spark-gap switch at a repetition rate of 0 – 140
Hz. The DC high voltage power supply delivered 8 – 15 kV to the system at a maximum current of
25 mA. The tie down resistor is 500  (Figures 1(b) and 2)
Figure #2 Typical voltage and current traces for system #1
2.2.3. Experimental Results for System #1
Rhodamine discoloration tests in this system are performed at an applied voltage of 10 kV, solution
flow rate of 0.3 L/min and oxygen flow rate of 20 mL/min. The initial parameters of the solution,
such as Rhodamine concentration (79 ppb), conductivity (6.4 s) and the pH (6.7) remained
essentially unchanged throughout the treatment. The temperature of the solution increased from 23.8
C to 32.8 C during treatment. Discoloration is not observed in this system as Rhodamine
concentration remains within 97% of its original value after two hours of processing time.
2.3.
Experimental System #2
2.3.1. Water Treatment Chamber #2
The second reactor has horizontal mesh electrodes that allow the water and the gas bubbles to flow
upward through the electrodes. This chamber contains a removable and stackable middle part so that
the electrode pairs shown in the diagram below may be repeated if needed to increase treatment time.
And stainless steel mesh electrodes are 3 mm apart. The diffuser used in this system produces
bubbles 2 – 3 mm in diameter. In this design, gas bubbles get trapped, accumulate and coalesce either
between the electrodes or underneath the bottom electrode. The polarity of the electrodes can be
changed easily if desired. (Figure 3)
Figure 3 Water Treatment Chamber #2 using horizontal stainless steel mesh electrodes
2.3.2. Electrical System #2
This reactor is used with the essentially the same pulse generating circuit as used with the first
reactor. The circuit diagram for system #2 is shown in Figure 1(b). A sample of the oscilloscope
traces corresponding to the applied voltage of 10 kV and one pair of stainless steel electrodes, 3 mm
apart is shown in Figure 4.
Figure 4 Oscilloscope traces for system #2 during treatment
This system shows significant reduction in Rhodamine concentration, but requires frequent
maintenance of the rotating switch to ensure optimum performance. Prolonged continuous operation
of the switch results in the deterioration of the gap and failure to open quickly. The rotating spark-gap
switch is replaced with a Thyratron switch (with appropriate adjustment of the circuit parameters) in
the third version of the experimental system.
2.3.3. Experimental Results #2
Experiments with this reactor include Rhodamine discoloration at various applied voltages and at
various flow rates of Oxygen gas. Solution conductivity, pH, and temperature have been monitored at
fifteen minutes intervals during treatment. Experiments show that discoloration occurs when the
discharge process is present. Over 90% destruction of Rhodamine is achieved by applying 10 kV at a
repetition rate of 80 Hz (Figure 5(a)) 900 mL of solution is circulated through the system at a rate of
600 mL/min. Oxygen is bubbled through the system at 20 mL/min and oxygen bubbles are 1 – 3 mm
in diameter. The discoloration process is strongly dependent on the rate of bubbling oxygen gas as
can be seem from the graphs below (Figure 5(b)).
Figure 5 (a) Reduction of Rhodamine concentration during processing at 0, 8, and 10 kV
(b) The reduction in Rhodamine concentration during processing for various oxygen flow rates
The temperature of the solution remained within one degree Celsius and no significant changes in the
pH of the solution occurred during processing. The conductivity of the solution increased steadily, for
example at 20 mL/min oxygen flow, the change is from about 19 S to 30 S.
2.4.
Experimental System #3
The third experimental system is also employs horizontal mesh electrodes and successfully combines
the lessons learned from the first two systems. It allows bubbles to be trapped on the mesh and
between the electrodes similarly to the conditions leading to the successful discoloration of
Rhodamine in the second system. The design is similar to the second system but the overall
dimensions are smaller. 700 mL of distilled water or Rhodamine solution have been used.
2.4.1. Electrical System #3
The rotating spark gap switch has been replaced with a thyratron, and the electrodes are almost
entirely surrounded by water minimizing the contact points with the solid plastic and eliminating any
electrical discharges along the surface of the solid dielectric. The electrodes may be changed and they
may be positioned at several different distances from each other. In the experiments reported here, the
electrodes are stainless steel with a mesh of 12/in positioned 3 mm apart. The parameters of the pulse
generating circuit shown below are adjusted to produce pulses approximately one microsecond in
duration. The pulse repetition frequency is adjustable from 0 – 200 Hz with 20 Hz used in this
experiment. A 1 M limiting resistor, a 500  tie down resistor, and a 3.3 nF capacitor bank is used
as shown in Figure 6.
Figure 6 Pulse generating circuit for system #3
The oscillograms taken during the discharge are significantly different from those observed with the
previous two systems (Figure 7)
Figure 7 Oscilloscope traces for System #3 during treatment
The traces show a drastic reduction in voltage drop across the system at the same time with a
significant current measured by the coil. This is consistent with a sharp reduction in the resistance
between the electrodes and may indicate the formation of a conducting path between the mesh
electrodes.
2.4.2. Experimental Results #3
Discoloration experiments in system #3 are conducted using 700 mL of Rhodamine solution at the
initial concentration of 200 ppb flowing through the reactor at a rate of 300 mL/min. Oxygen flow
rate for these experiments is 40 mL/min and the applied voltage is 6 and 11 kV. The 11 kV applied
voltage leads to about 85% destruction of Rhodamine during treatment (Figure 8(a)). Solution
parameters monitored during treatment are shown for the 11 kV trial (Figure 8(b)).
Figure 8 (a) Reduction in Rhodamine concentration during treatment in System #3 at 6 and 11 kV
(b) Solution parameters during Rhodamine discoloration in System #3 at 11 kV applied voltage
2.4.3. Hydrogen Peroxide Measurements in System #3
Measurements of the concentration of hydrogen peroxide confirm the production of OH radicals
during the discharge since the mechanism for producing H2O2 involves the dissociation of water
molecules as the first step. De-ionized water is used for these experiments and subjected to the same
treatment conditions as Rhodamine solution. Hydrogen Peroxide Test Kit (HACH, Model HYP-1) is
used to measure the concentration of H2O2 during treatment. The experiment shows an increase in
both the concentration of hydrogen peroxide produced and the conductivity of the de-ionized water
used in these experiments (Figure 9 (a) and (b)). The rate of increase and final concentration of
increases with applied voltage. The effect is stronger on the conductivity too when a higher voltage is
applied. These results demonstrate the production of active species during discharge.
Figure 9 (a) Increase in the conductivity of the DI water during treatment at 11, 15, 13 kV
(b) The production of H2O2 during discharge at the applied voltages of 11, 15, and 13 kV
3. Discussion Summary and Future Study
The voltages used here are at the limit of being enough to produce the discharge in the gas in the
bubble but not high enough to lead to streamer or spark discharges in the water. This is observed in
all experiments conducted here since a. the observed discharge pulses do not destroy the gas bubbles
as the bubbles remain attached to the electrodes; b. the discharge process occurs only when bubbles
are actually attached to the metal electrodes suggesting the discharge process initiated in the gas
phase; c. the bubbles remain intact and leave the water eventually as the gas-water mixture circulates
through the reactor. Comparing the three systems and also using other preliminary observations, it is
possible to suggest that the discharge occurs only when gas-metal interface is present (bubbles
attached to an electrode) under the conditions of these experiments.
The experimental results presented here demonstrate technological feasibility of producing active
radicals and destroying organic dyes using pulsed electrical discharge confined to gas bubbles in
water. The advantages of this method include lower voltages needed to initiate the discharge as
compared to streamer discharges in water even in the presence of bubbles (6 – 10 kV as compared to
20 – 25 kV for similar distances between the electrodes [8, 9]) and more efficient transfer of the
reactive species as compared to the discharges above the surface of the solutions or other methods
involving externally produces species (ozonization or addition of hydrogen peroxide, for example).
The experimental system #3 has the flexibility and the robustness to allow for more systematic study
of the preliminary suggestions made based on the present investigation. The role of the metal
electrode may be investigated using different electrodes; the role of the bubble attachment may be
investigated further by changing the size of the bubbles, and the type of discharge may be
investigated further by adding optical study including direct imaging of these discharges.
ACKNOWLEDGEMENT
This work has been supported in part by the US Army TACOM/ARDEC (Picatinny Arsenal)
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