Wastewater nitrification was carried out using a laboratory-scale upflow biological aerated filter (BAF) packed with a polyurethane-based porous medium. The filtration medium has macropores which provide a greater surface area for the development of biofilms. The macro-pores have both aerobic and anaerobic zones, depending on the depth of oxygen penetration in the medium. Wastewater ammonium was oxidized at a maximum rate of 1.8 kg NH4+ m(-3)d(-1) and showed more than 90% nitrification efficiency in the BAF. During the biological nitrification of wastewater, considerable nitrogen loss was observed in the BAF under oxygen-limited conditions when organic carbons were not provided for denitrification. Most probably, the lost nitrogen was converted to gaseous nitrogen compounds including dinitrogen by autotrophic dentrification and anaerobic ammonium oxidation.