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

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ABSTRACT
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 (BIOFORâ„¢) 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.

INTRODUCTION
Westchester County, New York owns and operates four wastewater treatment plants that discharge to the Long Island Sound (Blind Brook, Mamaroneck, New Rochelle, and Port Chester). Water quality issues in the Sound have led to the assignment of nitrogen waste load allocations (WLAs) by the Environmental Protection Agency (EPA) to communities surrounding the Sound. As the permitting authority in New York State, the New York State Department of Environmental Conservation (NYSDEC) modified existing discharge permits to include these WLAs (as well as other parameters). Based on the WLAs and the permitted flow, all four plants are required to meet an average total nitrogen concentration of less than 4 mg/L.

As an integral part of developing a nitrogen management strategy, several pilot studies were performed. One of these pilot studies includes the investigation of the biofiltration process for total nitrogen (TN) removal. The principal objective of the study was to verify the capability of this process to meet the future TN, total suspended solids (TSS), and carbonaceous biochemical oxygen demand (CBOD) limits. Other objectives included evaluating varied process operating conditions to determine target hydraulic and pollutant loading rates.

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