Chlorine-UV hybrid disinfection: a review on mechanisms and efficiency towards emerging pollutants

This review comprehensively evaluates the direct and indirect disinfection mechanisms and performance of chlorine-UV hybrid disinfection (CUV-HD) for the removal of microbial and emerging contaminants, including post-disinfection regrowth and formation potential of disinfection byproducts (DBPs), to ensure safe reuse of treated effluents. Direct disinfection reactions are mediated by HOCl and OCl⁻, as well as UV-induced photolytic damage. In contrast, indirect inactivation is driven by reactive oxygen species (ROS), particularly ∙OH, and reactive chlorine species (RCS) such as Cl∙, ClO∙, and Cl₂∙⁻. The order of reactive species generation indicates UV/chlorine > chlorine-UV > UV-chlorine, which is supported by the effective reduction potential and, consequently, the disinfection efficacy against microbial pollutants in treated wastewater. However, the water matrix, including pH, temperature, organic matter, and total suspended solids (TSS), significantly influences both the disinfection efficiency of these hybrid strategies and the potential formation of disinfection byproducts (DBPs). The formation potential of DBPs follows the order: UV/chlorine > UV-chlorine > chlorine-UV. Further studies quantifying reactive chlorine species (RCS) and elucidating their roles in CUV-HD are needed to fully understand the underlying indirect disinfection mechanisms.