Pulsed submerged arc plasma disinfection of water: Bacteriological results and an exploration of possible mechanisms

The pulsed submerged arc is a high-current electrical discharge between two electrodes in a liquid, in which the electrical current is conducted via a plasma bubble consisting of vaporized and partially ionized liquid and electrode material. The submerged arc discharge has the potential to kill harmful pathogens by a combination of several mechanisms. In this preliminary investigation, the ability of the pulsed arc to sterilize water was tested, hydroxyl radical (OH·) production and the radiation spectrum were measured, and shock wave production was estimated. Vessels containing 50 ml of tap water were inoculated with 10² -10⁴/ml of E. coli TG-1 bacteria. A repetitively pulsed submerged discharge was then applied to the inoculated water. The discharge was sustained between pairs of either silver or graphite electrodes, which periodically contacted each other, and then separated, at a frequency of 100 Hz. With each separation, a 20 μs, 50 mJ arc discharge was ignited in the inoculated water sample. Samples were thus treated for times ranging from 5 s to 10 min. Samples of arc treated and control liquids were plated, incubated, and the bacterial colonies were counted. Colonization was observed only in the control samples, from which the bacterial concentrations reported above were determined. No colonies were observed in any of the discharge-treated samples, i.e. even at the minimum 5 s treatment time tested in these experiments, and thus the survival rate after even the shortest arc treatment tested was >5 × 10^-4. The radiation spectrum showed broadened strong H line radiation at 656 nm and broad bands of emission throughout the visible and extending into the ultraviolet region. OH·radical production was measured by a modified Nash technique, and found to be 5 × 10^-10 moles per pulse. Theoretical models indicate that the plasma and adjacent water experience a short duration pressure pulse reaching a peak of 10¹¹ Pa. It is likely that some combination of radiation, radical induced chemical damage, and sonification is responsible for the observed bacterial inactivation.