Enhanced Inactivation of <i>E. coli</i> and Enterococci through Dual Disinfection with PAA and Chlorine: Two Full-Scale Wastewater Treatment Case Studies

This study reports on two full-scale pilot investigations conducted at wastewater treatment facilities in Cincinnati, Ohio (USA). It demonstrates two aspects: first, to assess the comparative disinfection performance of a dual disinfection treatment method employing peracetic acid (PAA) and chlorine [sodium hypochlorite (NaOCl)] versus individual disinfection with either PAA or chlorine on secondary effluent; and second, to determine the optimal PAA/chlorine combination dosage in the dual disinfection regimen that achieves the regulatory E. coli limit (<126 CFU/100 mL) in the final effluent while ensuring total oxidant residuals remain below 0.33 mg/L. Both wastewater treatment facilities currently use chlorine as the primary disinfectant; however, they differ significantly in plant design. The first pilot study compared PAA alone at doses from 0.6 to 1.2 mg/L with a dual disinfection regimen of 0.4 mg/L chlorine with 0.7 mg/L PAA. The second pilot study compared three treatments: chlorine alone (1.1 to 1.9 mg/L); PAA alone (1.2 and 1.5 mg/L); and dual disinfection (0.5 mg/L PAA combined with either 0.5 or 0.6 mg/L NaOCl). Disinfection efficiency was evaluated by measuring E. coli and enterococci inactivation after the treatment. In both investigations, the dual disinfection approach consistently yielded greater inactivation of E. coli and enterococci than either PAA or chlorine applied individually. These results highlight the potential of dual disinfection using PAA and chlorine to enhance wastewater treatment efficacy, support regulatory compliance, and advance environmental protection efforts. This dual disinfection strategy offers a viable method for meeting the Ohio Environmental Protection Agency’s effluent standard of 126 E. coli colony-forming units per 100 mL during the recreational season. This method provides a cost-effective solution for managers by optimizing chemical usage and streamlining the treatment process, resulting in improved compliance with environmental regulations.