Non-Thermal Plasma And Pulsed Electric Field Induced Disinfection

As emerging disinfection techniques, Pulsed Electric Field (PEF) and Non-Thermal Plasma (NTP) are environmentally friendly and of low temperature. PEF is realized by applying a high voltage pulse over two electrodes inside water to generate electric field without electrical breakdown. NTP is generated by high voltage discharges inside water or above water surface. During the NTP process, physical and chemical effects such as reactive species, ultraviolet radiation, strong electromagnetic fields, shock wave, and etc. are produced. In this dissertation, we studied three NTP/PEF processes for water disinfection. The aim of this work is to develop non-thermal decontamination devices.A homemade pulsed plasma system includes a pulsed power source and a2.5L stainless steel reactor filled with1.6L deionized water. Air plasma is generated from the tip of high voltage electrodes to the grounded reactor wall via the water surface. The typical vales of peak voltage, peak current, energy per pulse, and repetition rate are18kV,2.5kA,10J, and2pulses per second (pps), respectively. There is a positive correlation between injected energy density and plasma disinfection efficiency. When energy density is2J/mL, the4-6order reduction can be obtained for microorganism such as Escherichia coli, Bacillus subtilis, Bacillus subtilis spores and Candida albicans. Initial cell density and water conductivity are other important factors. Cells are easily sterilized with a lower initial density. For instance, when the cell density is below7×104cfu/mL, all of them can be disinfected after60pulses with an energy consumption of about0.1kWh/m3. However, for the initial density higher than106cfu/mL, there is a so called "shield effect" of the dead cells to the live, which can lead the high-density autoclaved cells to inhibit the plasma treatment. Water conductivity can significantly affects the characteristics of pulsed discharges. When the conductivity is higher, the intensity of light emission decreases and corresponding aqueous products such as hydrogen peroxide, ozone and nitric acid becomes less. According to UV emission spectrum and disinfection of cell suspension inside quartz tubes, it is deduced that germicidal UVC of NOβ and NOγ radicals plays an important role in plasma disinfection. UV absorbents such as uridine can inhibit the process. Inactivated cell morphology almost remains the same shape, their intracellular protein like green fluorescent protein (GFP), however, is destructed rapidly according to fluorescence observation.The homemade pulsed plasma system can be operated continuously with a water flow rate of up to500L/h. It is found that the disinfection efficiency is much lower than that for the stationary processing. The efficiency can be improved by increasing the pulse repetition rate or decreasing the water flow rate, since both of them can increase the injected energy density. Compared to the deionized water, it gives a less efficiency to the tap water due to its organic impurities. The order reduction of E. coli by single pass the reactor is1-2, with an energy density of less than2J/mL and cell density of103-106cfu/mL.A low intensity PEF device is developed for freshwater disinfection. A repetitive bipolar pulsed power source is applied. The values of peak voltage, pulse width and pulse repetition rate are2-6kV,25μs and200-6000pps, respectively. The PEF intensity is0.6-1.7kV/cm in a1.2L rod-cylinder reactor. It is observed that water conductivity has a significant effect on disinfection efficiency. When conductivity is25μS/cm, it shows best efficiency with a square waveform. After120000pulses, the order reduction of E. coli is2-4with an energy density of70J/mL. Water temperature is below25℃during the PEF processing.Argon (Ar) plasma jet is also realized by the bipolar pulsed power source. It can rapidly inactivate E. coli bacteria on the agar plate, with bactericidal spot of few centimeters. When the peak voltage is2-4kV and repetition rate is2000-6000pps, the power of generated Ar plasma jet is1-5W. For250μL of aqueous E. coli in microtiter plates, up to6orders reduction is obtained after0.5-2.5minute treatment. The efficiency of plasma jet disinfection is enhanced by raising the pulsed voltage or frequency as well as inducing the treatment distance and liquid depth. Although the E. coli cells in water are rapidly inactivated by plasma jet, the required energy density is as high as400-2000J/mL. Therefore the presented plasma jet is suitable for surface decontamination, but not for water treatment.