The effects of temperature on nitrification in a polishing biological aerated filter (BAF) were investigated using a 75-mm diameter pilot-scale BAF with a gravel media size of 5 mm and a depth of 1.7 m. Influent soluble chemical oxygen demand (sCOD) and ammonia-nitrogen (NH3-N) concentrations were approximately 50 mg/L and 25 mg/L simulating the effluent from an aerated lagoon system. For an influent wastewater temperature of 6.5 degrees C, approximately 95% of NH3-N was nitrified at a hydraulic retention time (HRT) of 2 hours. By recirculating 200% of the effluent back into the BAF for a HRT of 1 hour and at 6.5 degrees C, NH3-N percentage removal improved from 54% to 92%. For NH3-N loading larger than 0.9 kg NH3-N/m3-day at 24 degrees C, the mass of NH3-N removed in kg NH3-N/ m3-day reached an asymptotic value of 0.63 kg NH3-N/m3-day. The NH3-N concentrations within the column at different temperatures were modelled using zero-order biotransformation rate kinetics. The results showed that gravel BAF operating at an HRT of 1 hour with 100% or 200% recirculation can be used as an add-on technology for nitrification for cold weather conditions.