The nitrification characteristics of fertilizer wastes were investigated in a biofilm system using a submerged aerated filter. The attachment of biomass on packing media was studied. Supplement of organic carbon in the form of glucose and yeast extract enhanced biofilm formation although the nitrifiers did not require organic carbon for growth. After an attachment period, continuous operation of the reactor at different loading rates and dissolved oxygen levels was investigated. The maximum achievable nitrification rate was strongly dependent on the dissolved oxygen. In the dissolved oxygen range of 3.2-3.5 mg dm(-3), the maximum ammonia removal rate was about 0.17 kg NH4-N m(-3) day(-1). When the dissolved oxygen was increased to 4.9 mg dm(-3), removal rates as high as 0.41 kg NH4-N m(-3) day(-1) could be obtained. Nitrite accumulation depended on the bulk nitrogen and dissolved oxygen concentrations.