Disinfection of Escherichia coli by Mixing with Bulk Ultrafine Bubble Solutions

For potential use in wastewater management and health control, this study investigates the disinfection effectiveness of bulk ultrafine bubbles (UFBs) with different bubble number densities and solution pH. Initially, neutral UFB solutions with different bubble concentrations were mixed with E. coli suspension for 120 min, but these solutions did not achieve sterilization. The bubble number density did not affect the disinfection ability of the neutral solution. Next, the pH of the UFB solutions was fixed at 5, 7, and 9. When mixed with E. coli suspension, the acidic UFB solutions reduced the colony counts by 12% after 30 min of cultivation and by 66% after 60 min of cultivation. The colony counts increased slightly in neutral and significantly in alkaline UFB solutions. The acidic UFB solutions had lower zeta potentials and smaller number densities after cultivation, implying that the number density reduced through bubble coalescence rather than increased by bubble collapse. Additionally, the UFBs exhibited insignificant fluorescence intensity, suggesting that the colony counts increased by generated ∙OH radicals. This study revealed that the effect of UFB on E. coli significantly depends on the solution pH. Further, an acidified solvent achieves a bactericidal effect, whereas a neutral or alkaline solvent enhances the growth effect. This result is important when using actual wastewater.