Airborne disinfection using microwave-based technology: Energy efficient and distinct inactivation mechanism compared with waterborne disinfection

Microwave has been extensively applied to inactivate microorganisms in liquids, food, and surfaces. However, energy efficiency is a limiting factor for the environmental application. The utilization pathway and energy efficiency of the microwave in different media have not been investigated. In this study, the inactivation performance, energy utilization, and bactericidal mechanisms for microwave-irradiated airborne and waterborne Escherichia coli were compared. A Beer-Lambert law-based model was also developed and validated to compare the inactivation performance in different phases. Microwave had greater inactivation effect on airborne bacteria than waterborne bacteria. The inactivation rate constant for airborne E. coli (0.29 s(−1)) was nearly 20 times higher than that of waterborne species (0.014 s(−1)). Most of the absorbed microwave energy (92.3%) was converted to increase water temperature instead of inactivating the waterborne bacteria, because the microwave photons were easily absorbed by water molecules. By contrast, 45.4% of the absorbed energy could disinfect the airborne bacteria. Finally, the required energies for 1-log inactivation were calculated as 2.3 J and 116.9 J per log-inactivation for airborne and waterborne E. coli, respectively. The airborne and waterborne E. coli samples showed distinct microwave inactivation mechanisms. Waterborne E. coli disinfection was primarily due to thermal effect, while the non-thermal effect was the major mechanism for airborne E. coli inactivation.