In an effort to enhance water quality in the Long Island Sound, New York State Department of Environmental Conservation implemented stringent nitrogen and Total Residual Chlorine discharge limits for Wastewater Treatment Plants that discharge to the Sound. Four Wastewater Treatment Plants owned and operated by Westchester County, New York are required to meet an average effluent total nitrogen discharge concentration of less than 4 mg/L. Total Residual Chlorine limits are plant specific and ranged from 0.5 to 1.2 mg/L. Due to significant land constraints and cold wastewater temperatures, Westchester County initiated several pilot studies targeted at meeting the new limits. Savin Engineers were selected to identify, design, operate, and evaluate two pilot scale facilities to meet the limits.

One of the technologies included a pilot-scale Break Point Chlorination (BPC) facility constructed at the New Rochelle WWTP. BPC is a multi-step chemical process where chlorine oxidizes ammonia to form free nitrogen gas. Sodium hypochlorite was used as the chlorine source for the study and was controlled using an ORP probe and controller. 

A pH probe and controller was used to control a caustic soda feed to maintain the process pH.  Bisulfite was used as the dechlorinating agent and was controlled by an ORP probe and controller. The results showed that ORP successfully controlled the process to achieve a total residual chlorine concentration below 0.5 mg/L and expected nitrogen    performance in the range of 4 to 5 mg/L (without filtration). The project identified significant process considerations with respect to the interaction of chlorine and nitrogen species.

The second pilot study included the assessment of the biofiltration (BIOFORä) process. Three process configurations (C-N-DN, add-on N-DN, and predn-N-DN) were evaluated using the three separate biofilter units over the year long testing period.   Results showed that the biofiltration process achieved an average total nitrogen concentration of less than 3 mg/L under average conditions as well as during critical cold periods. Performance with respect to hydraulic and pollutant loading parameters was evaluated.