Inactivation of bacteria using discharge plasma under liquid fertilizer in a hydroponic culture system

We developed a discharge plasma reactor under liquid fertilizer for inactivating bacteria in the recirculation system of hydroponics. The plasma reactor consisted of a wire electrode that was placed in an insulating circular cylinder and a grounded electrode on a cylinder outside. The reactor was sunk under liquid fertilizer when used. Atmospheric air was injected into the cylinder using a gas pump and released through arrayed tiny holes of the reactor. Repetitive nanosecond high-voltage pulses were applied to the wire electrode using a magnetic pulse compression pulsed-power generator. The performance of the developed reactor was evaluated using tomato (Solanum lycopersicum L., Rinka 409) seedlings in hydroponics. In this study, 15 L of liquid fertilizer was contaminated with Ralstonia solanacearum, a plant pathogenic bacterium, after 40 min of discharge plasma treatment. The discharge plasma treatment was then continued for 100 min. Results showed that the number of colony forming units (CFU) of R. solanacearum in the liquid fertilizer decreased from 10⁷ to 10² CFU/mL using the discharge plasma treatment. Seedlings with discharge plasma treatment were relatively healthy; in contrast, all seedlings in the positive control wilted and died from infection of R. solanacearum after 12 d. Disease severity was also suppressed after discharge plasma treatment.