Photoreactivation and Dark Repair of Coliform Bacteria in Wastewater After UV-C Disinfection Treatment

The disinfection process in wastewater treatment is key to the discharge and/or reuse of high-quality effluent. However, disinfection using ultraviolet (UV) light may be inefficient because bacteria possess mechanisms for repairing damaged DNA. This study aimed to assess the photoreactivation and dark repair of total coliform (TC) in wastewater effluent after UV-C disinfection treatment. Four UV-C doses (28.8, 53.1, 57.6, and 106.2 mJ/cm2) and two post-irradiation conditions (light vs. darkness) were applied. Reactivation was monitored after 2, 4, 6 and 24 h (25 °C). Similar TC inactivation efficiencies were observed for the three lowest UV-C doses, whereas the 106.2 mJ/cm2 dose achieved the greatest reduction (1.1 Log of TC), decreasing TC concentrations from 3.1 × 105 ± 3.5 × 105 to 1.2 × 105 ± 1.4 × 105 MPN/100 mL. Reactivation assays revealed substantial bacterial recovery after UV treatment, with 24 h survival rates up to 2.3 × 103 under light and 9.2 × 102 in darkness. Photoreactivation and dark repair assays revealed substantial variability in bacterial recovery after UV treatment depending on UV-C dose, post-irradiation condition and incubation time. In general, bacterial recovery was still detected even at the 106.2 mJ/cm2 dose, particularly after 24 h of incubation (178–604%). These findings suggest that effective organic matter removal before UV-C disinfection is critical to improve UV transmittance, reduce shielding effects, and limit subsequent bacterial recovery.