Evaporation of aerosol droplets from contaminated cooling tower water

The evaporation dynamics of water-based aerosol droplets carrying pathogens, such as Legionella from cooling towers, is critical for assessing the risks of airborne transmission. Yet, the evaporation of contaminated aerosol droplets remains poorly understood and is often overlooked by current risk assessment models. Changes in water properties, such as viscosity and surface tension, induced by the presence of nonvolatile solids or contaminants, affect the evaporation time, the droplet nuclei size, and the time resolved size evolution. The effect of these parameters was experimentally and analytically studied. Surfactants lowering surface tension introduced non-linearity in droplet size evolution, extending evaporation time by up to 14% and halting it at high concentrations. Increased viscosity delayed evaporation onset without affecting nuclei size, which remained around 8–9 μm compared to 0.5 μm for reference water droplets. High concentration of solids, covering over 60% of the droplet surface, nearly doubled the evaporation time and increased nuclei size to 20 μm. Existing evaporation models do not fully account for temporal size changes and the variability in nuclei size due to solids concentration. Improving evaporation models and incorporating them into microbial contamination risk assessments are critical to develop effective mitigation strategies, such as using efficient drift eliminators for cooling towers.