Reaction rates in tertiary nitrifying trickling filters have been significantly increased by the development of the biofilm-controlled nitrifying trickling filter (BCNTF). The use of cross-flow media in the BCNTF permits higher oxygen transfer to the biofilm than does the older vertical media. The use of a flooding cycle prevents the growth of predator organisms that have consumed the biofilm in other plants and disrupted nitrification. A backwash feature controls biomass inventory and eliminates the need for subsequent clarification. Operation and maintenance and capital costs are often less than for other processes. A design model has been developed that accurately predicts performance.