Water Disinfection with Dual-Wavelength (222 + 275 nm) Ultraviolet Radiations: Microbial Inactivation and Reactivation

Emerging mercury-free ultraviolet (UV) sources, such as krypton chloride excimer (KrCl*) lamps and UV light emitting diodes (UV-LEDs), emit diverse wavelengths with distinct inactivation mechanisms. The combined application has the potential to improve disinfection effectiveness through synergism. In this study, a mini-fluidic photoreaction system equipped with a KrCl* lamp (222 nm) and a strip of UV-LEDs (275 nm) was developed, which could individually/simultaneously deliver accurate UV radiation(s) at 222 nm (0.32 mW cm^-2) or/and 275 nm (0.50 mW cm^-2). Dual-wavelength UV (DWUV) radiations demonstrated a substantial synergistic effect on Escherichia coli (E. coli) inactivation with synergistic coefficients reaching up to 1.92, while no synergistic effect was observed for PR772 bacteriophage (PR772) inactivation. Moreover, DWUV radiations significantly (p < 0.05) suppressed the reactivation of E. coli and PR772 in the subsequent light/dark treatment. For E. coli, the underlying mechanism could be ascribed to the increased level of reactive oxygen species induced by DWUV radiations, which not only enhanced inactivation by damaging proteins and lipids, but also suppressed the reactivation by damaging DNA repair enzymes. For PR772, although the DNA and protein damages caused by DWUV radiations did not yield a synergistic effect, the protein damage prevented the viral DNA from entering host cells for repair, thereby suppressing reactivation. This study helps develop more effective UV disinfection technologies by using DWUV radiations.