Performic acid disinfection in wastewater effluent and combined sewer overflows: A review

Performic acid (PFA) has emerged as a promising disinfectant for wastewater effluents and combined sewer overflows (CSOs), offering strong microbial inactivation with minimal formation of harmful disinfection byproducts (DBPs). This review systematically evaluates existing research on PFA performance across primary effluent, CSOs, and secondary effluent, emphasizing microbial inactivation kinetics, exposure modeling, DBP formation, and toxicity, while identifying knowledge gaps. Compared with traditional disinfectants such as free chlorine, chlorine dioxide, chloramines, and peracetic acid, PFA demonstrates comparable or superior bacterial inactivation while generating substantially lower concentrations of regulated halogenated DBPs. PFA also generates diverse oxygenated and nitrogen-containing transformation products, with PAA behaving similarly, whereas chlorine and chloramines form more persistent halogenated and nitrogenous byproducts. Disinfection efficacy of PFA varies across microbial groups, with enterococci, spores, and protozoan cysts demonstrating greater resistance, consistent with patterns observed for other disinfectants. However, PFA achieves faster inactivation at equivalent doses. Studies indicate that integral CT (ICT)-based models more accurately describe PFA inactivation under real wastewater conditions by capturing disinfectant decay and non-ideal hydraulics. DBP formation during PFA disinfection remains low, with halogenated DBPs detected only at high PFA concentrations (≥30 mg/L), and no nitrosamine formation reported. Acute toxicity assays show low ecotoxicity, although responses differ among species. Despite these promising findings, research gaps remain regarding PFA reactivity with complex wastewater matrices, inactivation of human enteric viruses and resistant bacterial spores, environmental fate of transformation products, and long-term ecological impacts. Overall, this review highlights PFA's strong potential as a safer, effective disinfectant for wastewater treatment and identifies critical areas for future investigation to support full-scale implementation.