Disinfection By-Product Formation in Combined Sewer Overflow under Wet Weather Conditions

Disinfection of combined sewer overflow (CSO) effluent is necessary to protect surface water sources but can lead to the formation of carcinogenic disinfection by-products (DBPs). For CSOs, higher chlorine doses and/or longer contact times may be needed to meet bacterial effluent requirements. The objective of this work is to determine the DBPs formed at the minimum chlorine dose and contact time that would result in CSO effluent meeting current and proposed bacterial requirements for two facilities with different water quality characteristics. The dominant DBP species that formed at both facilities (A and B) were haloacetic acids (HAAs), which showed higher concentrations at higher initial chlorine doses. Haloacetonitriles (HANs) were only found after chlorination of influent (Facility A) that had higher ammonia concentrations, likely caused by the formation of chloramines. Both facilities were able to meet the current and proposed bacterial requirements at an initial chlorine dose of 6 mg/L and a contact time of 15 minutes. As a measure of risk to downstream users, percent DBP brominated was calculated for both facilities at the same chlorine dose and contact time. Despite the formation of higher total HAA concentration at Facility B, the percent HAA brominated was 69% at both Facility A and B, indicating similar risk to downstream users despite differing total HAA concentrations.