Water cleaning in a heterogeneous pulsed corona reactor
L. Grabowski 1, E.M. van Veldhuizen1, G. Peemen2, W.R. Rutgers1
2
1
Department of Physics ,Technische Universiteit Eindhoven (TU/e),The Netherlands
Department of Electrical Engeneering ,Technische Universiteit Eindhoven (TU/e),The Netherlands
Abstract
This article discusses a new water treatment system using electrical discharges. The method is based
on corona discharges in air-water reactor. The discharges are created above the water and is an effective
source of ozone but also creates lots of other oxidizers, such as hydroxyradicals and UV photons. This so
called CAW Reactor (Corona Above the Water) is promising to be an effective and cheap technique for
water treatment.
Introduction
Although the Earth is covered in 75% by water, most of it is useless for people. Resources of potable
water are shrinking in hectic pace. With the growing commitment to improve water quality, there is a need
for new processes for environmental cleanup. Discovery of those processes is necessary for both existing and
ongoing wastes. Studies of plasma processing of waste have shown good looking possibilities for more
effective treatment and for lower treatment cost.
Plasma created in corona discharges are used nowadays for many purposes.[1,2] Its oldest
applications is probably ozone generation. Water and gas cleaning experiments started in 1980’s and are
becoming the biggest part of using plasma in environmental objectives. Gas and water cleaning using corona
discharges makes use of the radicals created during discharges and are similar in principle to ozone
generation. Existing applications concerns removal of NOx, SO2 and various VOC’s from flue gases. Water
cleaning is also an option but is reported less frequent [3]. The main reason are difficulties in efficient
transport radicals to treated water and relatively big cost of large scale proceses.
Recently a European project on corona discharge gas and water cleaning, called ytriD, has
started.[4] Within this project the two different techniques for water cleaning will be studied. Aerosol
reactor build by Soreq Israel will be compared with CAW reactor from TU/e Eindhoven. Optimisation and
upscaling the system for water cleaning by pulsed corona discharges will be studied. This implies a
parametric study of a reactor module and testing a large reactor system with a continuous water flow.
1. Wastewater treatment with the Corona Above the Water Reactor
In our work we apply the electric discharge water treatment in a heterogenous medium.
The process is based on using advanced oxidation processes to create in-situ strong oxidizers. The
discharge is created in air and the reaction products diffuse into thin layer of contaminated water where the
conversion of contaminants takes place. The discharge creates ozone, ions, (UV)-photons and metastables.
The short-life radicals created during discharge (such as ozone, hydroxy-radicals (OH), atomic oxygen and
hydrogen peroxide (H2O2) )are very useful for water cleaning purposes because of their non-toxicity after
treatment. The use of corona discharges for water treatment is more effective then simply ozonization
techniques because from ozone, during discharges, the vide range of different radicals is created.
In CAW technique, in addition to hydroxyl radicals and ions, first of all the corona produces ozone.
Ozone diffuses into the liquid phase and oxidises the target compound. The hydroxyl radicals cannot reach
the liquid surface, due to their high reactivity, but could produce hydrogen peroxide.
One of the adventages of using CAW techniqe is fact that highly reactive zone take place only in the
streamer, while the bulk of gas and water are in ambient conditions.In contra to aerosol reactor, in which
water is sprayed in to aerosol and the products of discharge diffuse into the droplets [8], in the CAW reactor
the reactivable surface is much stable. With the proper electrode design the CAW technique provides
uniform distribution of discharge over the whole interelectrode gap.
As a model compound phenol (hydroxybenzene), is used, which is an important precursor in organic
chemical synthesis. Besides during test, to prove reactor suitable, the different solutions were used i.e.
polyethyleenglycol(PEG), 2-butyloethyl.
Previous studies shows that the conversion efficiency of corona discharges can likely be improved
by the application of additives. Adding of Fe(II,III) salts, NO2 [1,3] and NaOH [7] , and also increase of pH
level, can significantly improve the decomposition of phenol in corona discharges.
According to [3] and [7] for increase the efficiency of reactor there is a need to use several additions.
Both, to waste water and to the air. In future the water flow system and the air flow system will be installed.
2. CAW treatment system
The main task of the project is to build system for water treatment which can continously clean up to
0.2m3/h.
The main elements of CAW treatment unit are a rectangular reactor made of Perspex with the system
of electrodes, a high-voltage pulse generator and a diagnostic system.
A: Reactor
The reactor is an hermetic construction, to prevent the corona products from escape. The reactor is
made of Perspex where inside dimensions are 8x5,5x38cm and. The side walls thickness is 10mm and w
bottom wall of 4mm. The 4 electrodes, 0.2 mm in diameter, are placed in cover of the reactor. The cathode is
placed outside the reactor, at the bottom side.
electrodes
8 cm
38 cm
2 cm
4 cm
5,5 cm
4,8 cm
cover
a
cathode
b
Fig. 1.Reactor view (with a system of electrodes): a-side view, b- front side view
The horizontal distance between the electrodes is 2cm. The gap between cathode and anodes is
4,5cm. Water volume used for test was 250ml, therefore the water layer thickness is about 1 cm.
B: Electric circuit
Repetitive pulsed power generators are the enabling technology for successful industrial applications
of pulsed corona discharges. Prototypes of efficient nanosecond pulse generators are available and have been
demonstrated in laboratory and in field trials [2]. These generators employ a heavy-duty spark-gap switch
combined with a transmission-line-transformer. Lifetime and reliability are adequate for industrial
demonstrations. Treatment of gas flows up to 100.000 Nm3/hr is possible. We expect to reach an average
corona output power of 100 kW within the next year [9].
During the firsts tests the circuit shown on Fig.2 was used. The power supply works at maximal
voltage 40kV with repetition rate from 20 up to 50Hz. The current at the DC source was about 200A. This
simple source functioned very well in previous measurements with a very small reactor, but with the larger
reactor of fig. 1 it delivered almost no real corona current into the discharge.
C= 2pF
R= 50M Ohm
V-probe
high
voltage
source
triggered
sparkgap
R=6M
Ohm
reactor
+
I-probe
Fig 2. The high voltage circuit to generate corona discharges
The second pulsed power generator used in this work is shown in Fig. 3. Voltage, current, power and
energy waveshapes are given in Fig. 4. Capacitor C is discharged over a transmission-line-transformer (TLT)
via a sparkgap switch, with a repetition rate up to about 100 pps. Detailed descriptions of a TLT are given in
[1,10]. The TLT used here consists of four 50 ohm coaxial cables. At the generator-side, the lines are
connected in parallel, thus providing a low discharge impedance for the capacitor (12.5 ohm). At the reactorside, the lines are connected in series. This output impedance of 200 ohm provides better matching with a
corona reactor. In addition, the output voltage will be increased. The main functions of the TLT are: (i)
impedance transformation for faster switching and for better matching with a corona reactor, (ii) increase the
output voltage, and (iii) protection of the switch against short-circuits and breakdowns. The performance of a
TLT depends to a large extent on the choice of magnetic cores. For this work, no magnetic cores were used.
As a result, the low secondary mode impedance limits the voltage transformation (about 1.3 instead of 4) and
oscillations on the voltage will occur (Fig. 4). Due to these oscillations, a second, negative corona discharge
will occur after about 200 ns. For the coming experiments, Metglas cores with a microgap will be added to
the TLT. This will significantly increase the voltage (upto about 50 kV) and will prevent the oscillations.
Magnetic Cores
C -charger
Spargap
switch
C AW - reactor
B
A
C
Fig.3. Schematic diagram of the pulsed power generator
100
Voltage
20
Current
60
0
20
0
-10
120
2
100
1.5
Power (MW)
40
Current (A)
Voltage (kV)
10
2.5
80
-40
-30
-100
100
300
500
Time (ns)
700
-60
900
Energy
1
80
60
0.5
40
0
20
-20
-20
Power
-0.5
Energy (mJ)
30
0
-1
-100
100
300
500
Time (ns)
700
-20
900
Fig.4. Pulse voltage, current, power and energy
From fig. 4 it is seen that an energy pulse of ~100 mJ is obtained. The capacitive current is estimated to be
~10 A, so only a small fraction of the total current.
C: Diagnostics
To measure reactor efficiency the on-line diagnostics system, similar as in [5] and [6] , has been
installed.
For concentration measurements of the model compounds a new LIF setup is constructed using a
Quantel Brilliant Ultra YAG laser with 4 mJ/pulse output at 266 nm and an Ocean Optics HR2000
spectrometer. This system is tested with phenol and polyethyleenglycol (PEG), see fig.3.
LIF (au)
4500000
2600
4000000
2400
2200
3500000
2000
LIF ( au)
3000000
1800
2500000
1600
2000000
1400
1200
1500000
1000
1000000
800
600
500000
400
0
200
250
300
350
wavelength (nm)
400
450
500
200
0
200
300
wavelength (nm)
400
500
600
Fig. 3: First examples of LIF spectra of solutions of 1mM phenol (left) and 0.1% PEG (right) in water.
Ozone concentration in air is measured by absorption using the spectrometer and a deuterium lamp.
Soon the ozone concentration will be measured in water.
4. Obtained results
To study the reactor ozone creation efficiency several test were performed. Absorption technique,
used the deuterium lamp, was applied.
The ozone concentration is roughly 1016 cm-3, similar to previous results [5], see fig.5.
den. (cm-3)
3E+16
3E+16
2E+16
2E+16
1E+16
5E+15
0
0
5
10
15
20
25
time (min)
Fig. 5. Ozone density measured using deuterium lamp
The figure above shows that the ozone concentration increase rapidly during first 10 min, than the
concentration becomes stationary. Results are similar to studies performed earlier [5].
Reactor functioning tests were performed using different compounds. Phenol, PEG, waste water
from textile industry and from mattress industry.
All of CAW treatment were realized using the power supply of fig. 1 at a voltage of 35-40kV, with
current from 100-200 μA. Estimated energy of discharge was ~0.1W. For different times of treatment the
changes in efficiency were noticed.
Both waste water used in tests, are characterised by small BOD/COD demand, unpleasent odour and
intensive colour. The main task of treatment was to decrease the level of colour and increase BOD/COD
demand.
Acording to obtained results, in both samples there was visible decrease in colour. BOD/COD demand was
increased that means that after corona treatment those waste waters can be easier decomposed in biological
reactors and natural environment.
PEG sample treatment shows the increase in BOD/COD demand from starting 0.053 to 0.37 after 30
min discharge of ~0.01 kWh/ltr.
A chemical model will be set up. It will help to find the speed-limiting step of the conversion and to
obtain rules for optimization and up-scaling.
4. Conclusions
Performed tests confirm the efficiency of CAW technique.
This technique is suitable for treatment of different kinds of water wastes. It can be used for treatment of
phenol and PEG contaminated water and also for cleaning chemical wastes from different types of industries.
First measurements using LIF spectroscopy and deuterium light absorbtion are suitable for in-situ
detection of model compounds decomposition and ozone creation.
After the improvements in reactor conditions, such as residence time of treated wastes, electrode
configurations, additives, the increase of reactor efficiency is expected.
References:
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Applications, Nova Science Publishers, New York, 1999, ISBN 1-56072-743-8, 420 pages.
[2] Keping Yan- Corona Plasma Generation, PhD thesis TU/e, nov. 2001.
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32(1999)L133-L137.
[4] www.phys.tue.nl/EPG/YTRID
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