CHLORINATION – IS IT TIME FOR A CHANGE? NEW YORK CITY REASSESSES ITS WASTEWATER DISINFECTION PRACTICES

New York City operates 14 water pollution control plants (WPCPs) with a combined dry weather treatment capacity of 1.8 billion gallons per day (BGD). Currently, New York City uses sodium hypochlorite to disinfect treated wastewater. However, proposed new effluent limits for total residual chlorine (TRC) are forcing the City to reconsider their disinfection practices. The proposed new limits range from 0.45 to 0.76 mg/L TRC for all but one WPCP. Meeting the lower limits may be met at some WPCPs through operational changes and/or process optimization; however others could require dechlorination or an alternative disinfection technology. At the same time, the City is also working to address other challenges which may impact their disinfection practices now or in the future. Future pathogen TMDLs for local receiving waters and the proposed use of enterococcus to set recreational water quality may limit the ability of the City's WPCPs to further reduce effluent TRC levels while meeting more stringent bacterial water quality requirements. Additionally, the City is working separately to reduce nitrogen discharges from several of its WPCPs through upgrades to biological nitrogen removal (BNR). While this is required to meet nutrient TMDLs, low effluent ammonia levels and increased nitrite formation can increase chlorine dosage requirements, resulting in higher effluent TRC. Beyond its WPCPs, the City may also need to address disinfection from CSO. The same challenges that apply to conventional wastewater treatment are magnified by the high solids content and high peak flows inherent to CSOs and the limited space for CSO facilities. Due to the magnitude of a new system-wide approach to disinfection and the potential cost implications, the City needed to perform a thorough review of the available options.This paper presents the results of the bench scale testing program that was performed to evaluate disinfection technologies (chlorination-dechlorination and UV) and to establish basic design parameters for a range of wastewater effluent quality representative of the City's WPCPs. As most of the City's WPCPs serve combined sewer areas, testing included disinfection under both dry weather and wet weather conditions.