Studies on Waterborne Pathogen Reactivation after Disinfection

Reactivation of different Escherichia coli strains and Aeromonas hydrophila after ultraviolet (UV), ultrasound, and photocatalytic disinfection treatments was addressed in this study. Photocatalytic disinfection was carried out under low pressure ultraviolet (LP UV) irradiation at five titanium dioxide (TiO_(2)) concentrations (1 g/L, 0.5 g/L, 0.75 g/L, and 0.1 g/L) to achieve 5 log_(10) reduction of a laboratory E. coli K-12 strain (ATCC? 10798). Regrowth and reactivation of E. coli in dark and light was studied up to 4 h after disinfection period. During the repair period, flow cytometry had shown 4-5 log_(10) higher cell counts than culture based method. Photocatalysis at 0.1 g/L TiO_(2) had resulted in 50% cells with intact cell membrane during the repair period and has lowered the repair rate of the E. coli (ATCC? 10798) after disinfection.

Then ultrasound (24 kHz) and UV-C irradiation were applied to inactivate four E. coli isolates (ATCC?10798, E. coli isolate from feces of feral hog and deer, and treated wastewater effluent) to nearly 8 log_(10) reduction. Photoreactivation and dark repair of E. coli isolates were studied over a 24 h period after disinfection. In general, ultrasound disinfection had resulted in higher inactivation rate of 0.52 log min^(-1) than UV-C (0.39 log min^(-1)) for E. coli isolates. The extent of percent log repair of ultrasound inactivated E. coli isolates after 24 h of dark repair and photoreactivation were 30% lower than after UV-C. The metabolic activity of E. coli cells was greatly reduced after ultrasound as shown by AlamarBlue? assay. Transmission electron micrographs of ultrasound disinfected E. coli revealed shearing and size reduction of bacterial cells.

Aeromonas hydrophila (ATCC? 35654), an emerging pathogen, was inactivated using a 24 kHz continuous ultrasound and UV-C in combination with three TiO_(2) concentrations (1g/L, 0.1 g/L and 0 g/L). High inactivation rate of 1.52 log min^(-1) was observed for ultrasound disinfection in absence of TiO_(2). Aeromonas hydrophila had showed a net log reduction of 6 log_(10) after ultrasound exposure in comparison to a net 2 log_(10) reduction after UV-C over a 24 h repair period. Metabolic activity of Aeromonas hydrophila was adversely affected by ultrasonication as well. Ultrasound appears to be effective in inactivating environmental E. coli isolates and Aeromonas hydrophila in water through shearing and breaking effects, which decreased the metabolic activity as well as photoreactivation and dark repair.