Combined Sewer Overflow pretreatment with chemical coagulation and a particle settler for improved peracetic acid disinfection

•CSO disinfection was investigated by coagulation, settling and peracetic acid dosage.•Particle separation improved peracetic acid exposure and disinfection efficiency.•Applying PAX-XL 100 before the separator further increased disinfection efficiency.•Above 1-log Enterococcus removal in untreated CSO requires 10mg/L peracetic acid.•1-log Enterococcus removal improved with pretreatment to 2ppm peracetic acid. Full scale disinfection by peracetic acid (PAA) was achieved on Combined Sewer Overflow (CSO) water, which was pre-treated physically by a fast settling-filtration unit. Disinfection of untreated CSO water using PAA was compared to treatment using a particle separator (HydroSeparator®) and additional coagulation with poly-aluminum-chloride. Disinfection for Enterococcus increased with the applied dose of PAA and additional improvement was achieved when it was preceded by chemical coagulation with 5mgL−1 poly-aluminum-chloride. When Enterococcus was reduced by treatment in the HydroSeparator, followed by PAA treatment during a CSO event, the treatment was sufficient to maintain microbial quality in the recipient water.