Tertiary nitrifying trickling filters (NTFs) at the Littleton/Englewood wastewater treatment plant provide for nitrification to meet seasonally varying effluent requirements for ammonia nitrogen. Operation of the full-scale facilities during the past two years demonstrates highly efficient oxidation of ammonia and the effectiveness of biofilm control strategies. A decline in nitrification performance caused by predators was successfully corrected by the use of a special alkaline backwash feature which controlled the level of larval development within the NTFs.