Atmospheric Pressure Non-Thermal Plasma: A Tool for Inactivating Airborne Pathogens

Abstract

Pathogens spread by airborne transmission represent a persistent threat to economic stability and human/animal health. These pathogens are particularly prevalent in the agricultural sector, especially in animal rearing facilities. However, the agricultural industry currently lacks an efficient and cost effective means of controlling airborne pathogens. The present study explored the possibility of developing a new type of antimicrobial air treatment system based on non-thermal plasma technology. The study consisted of an initial laboratory testing stage followed by a pilot scale study performed in a local poultry rearing facility. During the laboratory testing stage two prototype non-thermal plasma reactors were developed and challenged in a closed air circulation system with artificially aerosolized Newcastle Disease Virus and avian influenza virus. The results indicated that both viruses could be rapidly inactivated below the limit of detection after sub second exposure to non-thermal plasma. Specifically, Newcastle Disease Virus was completely inactivated after 7.7x10-3 seconds of direct plasma treatment with a specific energy input of 171 J/L of air. Although the high virus inactivation effects are believed to predominately be attributable to direct non-thermal plasma exposure, preliminary experiments revealed that liquid-based virus collection strategies (e.g. use of an SKC BioSampler) were susceptible to liquid-based inactivation of collected viruses via indirect non-thermal plasma exposure. Attempts were made to circumvent the issue of liquid-based inactivation by employing a gelatin-filter based virus collection strategy. However, due to the high ozone emissions (80ppm) of the non-thermal plasma reactors, surface-based inactivation effects of viruses collected on the filters could not be ruled out as a contributing mechanism to the high virus inactivation rates. Due to biosafety concerns, flow rates >28LPM were not tested and the upper limit of non-thermal plasma’s virus inactivation efficiency was not determined. Additionally, virus samples taken from the 6-jet collision nebulizer, used to aerosolize viruses in this study, revealed that nebulization stress did not contribute to virus inactivation. Finally, the effects of relative humidity on virus losses within the closed air circulation system were explored. Findings showed a strong correlation (R2>0.99) between increasing relative humidity and decreasing airborne virus concentrations. This relationship is thought to predominately be due to humid aerosols experiencing greater condensation losses within the closed air circulation system when compared with less humid aerosols. However, the small volume of condensed solution visualized within the system was not sufficient to account for all of the viruses lost between the nebulizer and sampling port (gelatin filters). Correcting for adhesion losses and possible low gelatin filter sampling efficiencies did not completely account for non-plasma treated viral losses. Therefore, it is possible that the viruses used in the present study experience a decrease in infectivity with increasing humidity levels. The second stage of this study involved the design and fabrication of a pilot scale non-thermal plasma air treatment system and challenging it with ambient aerobic bacteria at a local turkey barn. Technical complications with a commercial high voltage power supply resulted in the pilot scale system operating at approximately 10% of its specified input power resulting in a specific energy input of only ~19.4 J/L. As a result, the pilot system was not able to reduce airborne bacteria concentrations relative to air samples that did not receive plasma treatment. Overall, the findings from the present study indicate that non-thermal plasma based air treatment technologies can be an effective tool for controlling airborne pathogens. However, the successful industrial implementation of this technology will require appropriate power supplies and methods to mitigate ozone emissions.