Interest in the nitrifying trickling filter (NTF) process has been increasing in recent years as many wastewater treatment agencies, both public and private, face the prospect of stringent new ammonia discharge standards. NTFs are appealing because they are simple to operate and typically have lower energy and maintenance costs than other alternatives. The tertiary nitrification process permits full utilization of existing secondary treatment capacity. Separate stage NTFs can be added to conventional activated sludge, TF/SC or secondary trickling filter plants with minimal disruption of routine operations. The nitrifying trickling filter is also readily adaptable to partial and seasonal nitrification requirements. Research and development of the NTF process has been progressing, albeit slowly, for over 15 years. Pilot plant studies conducted at Midland, Lima, and Sunnyvale in the early 1970s established the feasibility of separate stage fixed film reactors to achieve nitrification. Empirical design relationships developed from these studies were presented in the Nitrogen Control Manual1 published by USEPA in 1975. Since that time, 20 full-scale nitrifying trickling filters have been constructed in the United States on the basis of the EPA design curves. We have conducted a survey of these facilities, shown on Figure 1, to advance our understanding of the NTF process through full-scale experience. Where cogent comparisons can be made to full-scale performance, relevant pilot scale NTF results have been included in the discussion of survey results.