NITROGEN REMOVAL FROM DILUTE WASTE-WATER IN COLD CLIMATE USING MOVING-BED BIOFILM REACTORS

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-degrees-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 examined. The predenitrification process was carbon limited, and only 50% to 70% total N removal was obtained, at a recirculation ratio 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.