At an activated sludge plant for biological pretreatment of rendering plant effluent, incomplete nitrification was repeatedly observed, despite low loading and high SRT. It is hypothesised that inadequate dosage of phosphorus leads to nutrient deficiency, although the effluent phosphate concentration was mostly above 1 mg/l. Evaluation of the ‘actual’ autotrophic maximum growth rate $μ^A,T$ revealed a nitrification inhibition of almost 50% under the operational conditions of this plant, whereas the inhibitory effect caused by the wastewater compounds was found to be much lower. Thus, the poor performance of the nitrification process is rather generated in the plant than caused by the wastewater. Another possible reason - besides phosphorus limitation - for incomplete ammonia oxidation may be found in the anaerobic state of the pre-denitrification tank. In sludge samples from this tank, the nitrification rate was substantially reduced in some cases. At elevated temperatures, nitrification and nitrogen removal can be achieved in relatively small reactor volumes. However, several potential problems have to be considered in the operation of such plants.