Effect of chemical species generated by different geometries of air and argon non-thermal plasma reactors on bacteria inactivation in water

Abstract

The aim of this study was to investigate the inactivation of Escherichia coli (E. coli) in water by atmospheric pressure non-thermal plasma using air and argon (Ar) as plasma gas using two reactor geometries for application efficiency purpose. The following reactor geometries were tested: gas phase plasma discharge with an electrode in the liquid phase – RFG and direct plasma discharges in the liquid phase with both electrodes immersed – RFL. Microbiological analyzes of water pre-inoculated with E. coli were carried out before and after non-thermal plasma treatment and also pH, conductivity and temperature of the water were checked during the plasma treatment. The obtained results showed that when air was used as plasma gas the two plasma reactors studied RFG and RFL had similar performance taken about 8 min to inactivate E. coli colony in water. However when Ar was used as plasma gas only the RFG-type reactor showed a decrease in the colony counts of E. coli during treatment. As the RFG-type reactor produces plasma discharges at the gas-liquid-water vapor interface there may be some air acting as contaminant providing the formation of air reactive species that diffuses in the liquid medium resulting in bacteria inactivation. This result indicates that the chemical species formed by non-thermal plasma using air as plasma gas (such hydrogen peroxide and nitrates lowering the pH and increasing the conductivity) are the key species inducing bacteria inactivation in water. Although the two reactors showed a similar results using air as plasma gas the RFL-type reactor with both electrodes immersed has operational advantages since it generates the plasma directly inside the water supply pipes.