Pilot Study Results for Peracetic Acid and Sodium Hypochlorite Disinfection of Combined Sewer Overflows

The City of Fort Madison (City), Iowa is required by a United States Environmental Protection Agency (USEPA) consent order to mitigate its combined sewer overflows (CSOs). The selected CSO mitigation method is chemical disinfection of the overflows to reduce Escherichia Coli (E. Coli) bacteria to 1073 most probable number per 100 milliliters (MPN/100 mL). The consent order required a full-scale pilot CSO disinfection system be constructed and operated to demonstrate the feasibility of disinfecting highly variable CSO discharges. As part of the design process, a bench study of peracetic acid (PAA) and sodium hypochlorite (SH) disinfectants was performed by the University of Iowa on behalf of Stanley Consultants, the City's wastewater engineer, to establish potential dose rates and contact times. The bench study utilized jar testing to examine dose concentration-time relationships and impact of suspended solids on disinfection effectiveness. A pilot chemical disinfection system consisting of chemical feed systems and disinfection contact pipe was designed and constructed.Bench study work under ideal and well mixed conditions suggest that a SH dose of 20 mg/L with 5 minutes of contact time or a dose of 10 mg/L with 10 minutes of contact time should reduce E. Coli below 126 mpn/100 mL.Bench study work under ideal and well mixed conditions suggest a PAA dose of 15 mg/L or higher with 3 minutes of contact time or a dose of 10 mg/L with slightly greater than 5 minutes of contact time should reduce E. Coli below a target of 126 mpn/100 mL.Preliminary pilot study results indicate that sodium hypochlorite appears to be effective at meeting the CSO disinfection goal of reducing E. coli in CSO water to below 1,073 mpn/100 mL. Unfortunately dose data is not available.Preliminary pilot study results suggest PAA can be an effective disinfectant for meeting CSO E. Coli reductions. The data provides evidence that doses that provide residuals of 0.5 mg/L or greater reduces E. Coli to down to approximately 100 mpn/100 mL.