Inactivation of Klebsiella michiganensis B199A using plasma-activated mist (PAM): Computational fluid dynamics and microbial inactivation

The purpose of this study was to evaluate how exposing a fine mist of water to air-based plasma affects reactive oxygen and nitrogen species (RONS) concentrations in plasma-activated mist (PAM) and the ensuing microbial inactivation on the surfaces that are exposed to PAM. Tryptic soy agar plates containing Klebsiella michiganensis B199A were tested within a 33.5 L PAM chamber using a dielectric barrier discharge (DBD) plasma device. Inactivation up to 4 log CFU/plate was achieved when treated with PAM for 20 min. Equivalent inactivation occurred in DBD plasma treatments without mist; however, gaseous ozone concentrations were reduced by 87% when mist was incorporated. Microbial inactivation was independent on the location within the chamber, demonstrating the chamber configuration resulted in homogenous treatment. Concomitantly, a computational fluid dynamics model was created in COMSOL Multiphysics® to simulate three-dimensional mist flow within the chamber. PAM droplets ranged in diameter from 2 µm to 20 µm and were traced to determine their transport. Simulation results revealed that droplet size affected how PAM flows through the chamber and the extent that target surfaces were treated. Gaseous and liquid ozone and hydrogen peroxide concentrations were simulated, and the microbial inactivation rates were modeled based on the RONS load delivered to target surfaces. The results demonstrated that the PAM droplets contributed less than 1 log CFU/plate to the overall microbial inactivation. The results indicated that the introduction of mist serves more to alter the plasma reaction mechanism versus increasing the antimicrobial effectiveness of non-thermal plasma treatments without mist. Tweetable Abstract Plasma-activated mist (PAM) inactivated Klebsiella michiganensis up to 4 log CFU/plate in 20 min. PAM reduced gaseous ozone concentrations by 87% and was microbiologically as effective as DBD plasma. • Plasma-activated mist inactivated Klebsiella michiganensis by 4 log CFU/plate. • Mist droplets altered the plasma chemistry and reduced ozone concentrations. • With-mist and without-mist treatments resulted in similar microbial inactivation. • CFD model showed that PAM droplet diameter variation caused non-uniform transport. • Microbial inactivation due to PAM droplets was minor relative to gas phase species.