Nitrogen removal from dilute wastewater in cold climate using moving‐bed biofilm reactors

Abstract

A moving‐bed biofilm reactor has been developed, where the biofilm grows on small, free‐floating plastic elements with a large surface area and a density slightly less than 1.0 g/cm3. Nitrogen removal, based on nitrification and denitrification, was studied in a pilot plant with an active, specific biofilm surface area of approximately 310 m2/m3. Temperatures ranged from 7 to 18°C, and untreated wastewater had concentrations typically less than 100 mg soluble chemical oxygen demand (COD)/L and 25 mg total N/L. Both predenitrification, using untreated wastewater as carbon source, and post‐denitrification of pre‐precipitated wastewater, using acetate as an external carbon source, were examined. The predenitrification process was carbon limited, and only 50% to 70% total N removal was obtained, at a recirculation ratio of approximately 2.0 and a total empty bed hydraulic residence time of approximately 6 hours in the biofilm reactors. With post‐denitrification and an external carbon source, 80% to 90% total N removal could easily be reached at total empty bed hydraulic residence times less than 3 hours.