STEP FEED BNR: DESIGN AND OPERATION CONSIDERATIONS

Step feed biological nitrogen removal (BNR) is an effective process that can be readily adopted in existing activated sludge plants to achieve significant nitrogen removal. The process has been demonstrated in bench scale, pilot plants and full scale treatment facilities. Step-feed BNR incorporates alternating oxic and anoxic zones. The anoxic zones coinside with the feed locations of primary settling tank effluent. As the flow of wastewater being treated travels through the tank, nitrification takes place in the oxic zones and denitrification in the anoxic zones. In a full scale demonstration facility at the Wards Island WPCP in New York, the process achieved 50% nitrogen removal at an average operating temperature of 15°C, a SRT of 13.6 days, and without adding external sources of alkalinity or organic carbon. The average specific nitrification and denitrification rates measured were 0.081 and 0.026 mg NOX-N per mg MLVSS per day, respectively. An important challenge that must be overcome for a successful operation of the process is adequate control, removal, and disposal of froth which typically develops in BNR treatment plants.