22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Development of a solar powered air plasma system for microbiological
decontamination
Y. Ni1, M.J. Lynch1, M. Modic1, R. Whalley2, D.L. Bayliss3 and J.L Walsh1
1
Department of Electrical Engineering & Electronics, University of Liverpool, U.K.
2
School of Mechanical and Systems Engineering, University of Newcastle, U.K.
3
Campden BRI, Station Road, Gloucestershire, U.K.
Abstract: This contribution details the development and optimisation of a solar powered
plasma decontamination device. The developed system operates independently of any
external gas or power source, thus providing a truly portable means of rapid microbial
inactivation. Experiments involving the inactivation of common waterborne pathogens
demonstrate that the device is able to effectively decontaminate liquids in a short timeframe.
Keywords: atmospheric pressure plasma, plasma liquid, microbial decontamination
1. Introduction
Water is essential to sustain life, access to safe potable
water is crucial for a person’s wellbeing; despite this,
providing safe water remains a global challenge. In 2011,
approximately 768 million people had access only to
unsafe water supplies, which are thought to contain high
levels of chemical and pathogen contamination [1].
Contaminated potable water is one of the greatest risks to
human health and contributes on average to 1.9 million
deaths each year [2]. Out of the many sources of
contamination, the impact of bacterial pathogens, which
can cause devastating diseases, is the most common and
widespread risk [3]. In 2012, over 500,000 people died
through Diarrhoea linked directly to bacterial
contamination in potable water [1][2] [4].
Most waterborne pathogens infect the gastrointestinal
tract [3]; common examples include Salmonella,
Cryptosporidium, norovirus, E. coli, Campylobacter and
Legionella. Non-thermal plasma has shown great
potential in inactivation of microorganisms including
bacterial pathogens on both solid surfaces and aqueous
solutions [5][6][7]. The Reactive Oxygen Species (ROS)
and Reactive Nitrogen Species (RNS) generated by cold
plasma are known to be the primary drivers of microbial
inactivation [6]. A wide variety of cold plasma systems
for biomedical applications have been explored recently,
including corona discharges, electrospray systems, plasma
jets and dielectric barrier discharges (DBD). An
increasing number of studies have focused on the DBD
configuration as their design and operation are both
simplistic and convenient. A key feature of a DBD
generated directly on a surface is the ability to sustain a
wide area plasma directly in ambient air, without
requiring the use of expensive noble gases [8].
In this contribution the development and optimisation of a
solar powered plasma decontamination device, based on a
surface DBD configuration, will be detailed. The
developed system operates independently of any external
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power source or gas flow, thus providing a truly portable
means of rapid microbial inactivation.
2. System design and configuration
To develop a flexible and portable decontamination
device a means to power the plasma system without
reliance on mains electricity was designed. A solar panel
was combined with a high energy density Li-Po battery to
act as a substantial and essentially renewable power
source for the system. The block diagram in figure 1
highlights the key system components. The plasma is
excited using a conventional step-up transformer
operating at 30 kHz. The average dissipated power in the
discharge could be varied from 2 – 10 W under
continuous-wave operating mode. A microprocessor
system was employed to monitor the battery usage /
charging functionality as well as to control plasma power
by varying the applied voltage based on feedback from
the plasma electrodes. Figure 2, shows a photograph of
the system operating at a power of ~7 W. The system is
able to operate continuously under these conditions for ~
2 hours.
Fig. 1. Block diagram of solar powered plasma system.
1
Fig. 2. Photograph showing plasma system in operation
with a 10 cm diameter surface DBD electrode.
3. Afterglow chemistry characterisation
In a surface barrier discharge, samples are typically
placed several mm downstream of the plasma generating
electrodes. While this spatial separation has the
significant advantage of isolating the high-voltage
electrode from the sample; a negative consequence is a
reduction in the mass transport of highly reactive plasma
species. Species such as O and OH have very short
lifetimes, as such they are not transported to the sample; it
is the long-lived ROS and RNS species produced in the
afterglow of the plasma that are of critical importance.
Fourier Transform Infrared Spectroscopy (FTIR) is the
most common method for characterisation and
quantification of long-lived gas phase plasma species.
N 2 O, NO 2 , HNO 3 and O 3 are the commonly identified
species in atmospheric pressure air plasmas [9][10].
Alternative methods including UV-absorption are also
very useful in plasma diagnostics, especially in the
quantification of ozone using the Beer-Lambert’s Law
[11].
The composition of reactive species generated by the
plasma can be varied by changing the plasma operating
conditions. It is well known that in a low power mode of
operation, the low intensity air plasma generates
considerable fluxes of ROS, especially O 3 ; while under
high power conditions, the discharge chemistry is
dominated by RNS species with little ROS production. It
is worth noting that both ROS and RNS are known to
have potent antibacterial effects, as such careful
consideration of the operating conditions can lead to an
optimised
discharge
chemistry
for
microbial
decontamination. The reduction in O 3 production at
higher powers can be attributed to the increased thermal
decomposition of O 3 and quenching by NO.
Figure 3 shows FTIR spectra of the plasma system
operating at a low power (3 W) and high power (7 W). as
expected, under low power conditions the spectra is
dominated by O 3 . Under high power operating conditions,
the spectra is dominated by RNS species including NO
and NO 2 .
2
Fig. 3.
FTIR spectra showing differing afterglow
chemistry under high and low power operation.
4. Water decontamination experiments
In order to assess the ability of the plasma system to
decontaminate pathogens in water, the plasma device was
placed 2 mm from the surface of a 25 ml bacterial
soultion. Two common waterborne pathogens were
considered (E. coli & P. Flourescens), in each case a
starting concentration of ~2x108 cfu was suspended in tap
water. Figures 4 and 5 show the impact of high and low
power plasma treatment on the bacterial log reduction
with respect to treatment time.
From figure 4 it is clear that a high power plasma
treatment is extremely effective at inactivating bacteria in
water. With a 3 minutes exposure an 8 log reduction of E.
Coli was achieved, with an 8 log reduction in P.
Flourescens achieved after a 4 minutes. Critically, figure
4 shows the change in pH with treatment time. It is clear
that the high-power plasma treatment causes considerable
acidification of the solution. While this is highly effective
at reducing bacterial contamination, in the context of a
device to decontaminate drinking water it is far from ideal.
Figure 5 shows the impact of a low power plasma
treatment on the concentration of bacteria in water. A 4
minute treatment led to a 3 log reduction of E. Coli, while
it took 16 minutes to achieve a 2 log reduction of P.
Flourescens. While it is clear that a low power treatment
is not as effective at inactivating bacteria as a comparable
high-power treatment, the pH of the low-power treated
solution is only slightly reduced; this situation is far more
preferable in the context of water treatment.
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[6] N. N. Misra et al. Food Eng. Rev., 3, 3–4, (2011).
[7] M. J. Pavlovich et al., J. Phys. D. Appl. Phys., 46,
(2013).
[8] Y. Sakiyama et al. J. Phys. D. Appl. Phys., 45, 42,
(2012).
[9] A. A. Stec et al. , Fire Saf. J., 46, 5, (2011).
[10] G. Laroche et al., Rev. Sci. Instrum., 83, 10, (2012).
[11] J. Orphal and K. Chance, J. Quant. Spectrosc. Radiat.
Transf., 82, 1–4, (2003).
Fig. 4. Graph showing bacterial log reductions and pH of
water containing various pathogens under high-power
exposure.
Fig. 5. Graph showing bacterial log reductions and pH of
water containing various pathogens under low-power
exposure.
5. Summary
In summary, this contribution details the development of
a portable plasma system that is powered by a Li-Po
battery which is continously charged from a solar panel.
The discharge chemistry is characterised using FTIR and
it is demonstrated that the afterglow composition can be
controlled by varying the discharge power. The impact of
high and low power plasma treatments were considered
on the bacterial concentration of tap water samples; it was
shown that under both plasma conditions significant
bacteria log reductions were achieved. Criticaly, under
low power conditions the pH of the soultion remains ~7,
which is highly advantageous in the context of a portable
water treatment system.
6. References
[1] A. Prüss-Ustün, et al. Trop. Med. Int. Heal., 19, 8,
(2014).
[2] B. H. Rosen, Waterborne Pathogens., Elsevier,(2000).
[3] H. G. Gorchev and G. Ozolins, WHO Chron., 38,
(2011).
[4] WHO and Unicef Monit. Program. water supply Sanit,
(2014).
[5] A. Fridman, Plasma chemistry. 2008.
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