Three Stage Biological Filter Process to Achieve Extremely Low Total Nitrogen Discharge to the Long Island Sound

In an effort to enhance water quality in the Long Island Sound, New York State Department of Environmental Conservation has implemented stringent nitrogen 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. Due to significant land constraints and cold wastewater temperatures, Westchester County initiated several pilot studies targeted at meeting the new limit. One of the pilot studies included the assessment of the biofiltration (BIOFOR™) 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. This paper shows 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.