Importance of Recovery of E-coli in Water Following Ultraviolet Light Disinfection

Escherichia coli photorepair and dark recovery was investigated in two different water sources. Sterilized drinking water and wastewater effluent treated for reuse were spiked with either wild E. coli isolate or a laboratory-purchased E. coli strain and ultraviolet (UV)-irradiated at fluences from 40 to 120mJ/cm2. Although no threshold UV fluence was established after which E. coli photo and dark repair did not proceed, the extent of repair was generally low, not exceeding 0.5 log. Contrary to the repair results, the 48-h regrowth of E. coli disinfection survivors reached up to 3 log, likely due to the remaining inactivated E. coli cells that served as a source of biodegradable carbon postdisinfection. Although it is proposed that concerns regarding bacterial repair under drinking and reuse water UV disinfection conditions are overestimated based on experiments at low UV fluences, the long-term postdisinfection safety of water may be underestimated for conditions of low or no disinfection residual, or where nutrients are present, and regrowth could occur.