Wastewater nitri®cation was carried out using a laboratory-scale up¯ow biological aerated ®lter (BAF) packed with a polyurethane-based porous medium. The ®ltration medium has macropores which provide a greater surface area for the development of bio®lms. 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 NH 4 m À3 d À1 and showed more than 90% nitri®cation ef®ciency in the BAF. During the biological nitri®cation of wastewater, considerable nitrogen loss was observed in the BAF under oxygen-limited conditions when organic carbons were not provided for denitri®cation. Most probably, the lost nitrogen was converted to gaseous nitrogen compounds including dinitrogen by autotrophic dentri®cation and anaerobic ammonium oxidation.

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