Evaluation of parameters governing dark and photo-repair in UVC-irradiated Escherichia coli

Abstract

After decades of UV disinfection practice and numerous studies on the potential for pathogens to undergo dark or photo-repair after UV exposure, recent advances in UV light emitting diode (LED) technologies prompt renewed attention to bacterial reactivation and regrowth processes after UV exposure. The aspect of photorepair conditions warrants particular attention, because even studies on conventional mercury vapor lamps have not sufficiently characterized these parameters. Wastewater encounters a wide range of environmental conditions upon discharge (e.g., solar irradiation and dissolved organics) which may affect repair processes and ultimately lead to overestimations of pathogen removal. Escherichia coli was used here to investigate the impacts of changing reactivation conditions after UV₂₅₄ and UV₂₇₈ irradiation. UV₂₅₄ and UV₂₇₈ doses of 13.75 ± 0.4 mJ cm⁻² and 28.3 ± 0.8 mJ cm⁻² were required to induce a 3.0 log inactivation of E. coli, respectively. Specifically, photoreactivation conditions were varied across dissolved organic matter (DOM) content and photoreactivation wavelengths and intensities. Photoreactivation achieved higher log recoveries than dark repair, ranging from 0.8 to 1.8 log differences, but a secondary disinfection effect occurred under UVA irradiation. During photoreactivation, humic acid inhibited the initial repair of UV₂₇₈-dosed E. coli, but culture media enhanced recovery for both dosage wavelengths. Photoreactivation profiles under UV₃₉₅, UV₃₆₅, and visible light depended on both fluence and time, with more regrowth observed upon exposure to visible light and the least under 365 nm. The susceptibility of E. coli to UVA was increased by prior exposure to UVC.