Ammonia in source waters can cause water treatment and distribution system problems, many of which are associated with biological nitrification. Therefore, in some cases, the removal of ammonia from water is desirable. Biological oxidation of ammonia to nitrite and nitrate (nitrification) is well understood and common in wastewater processes. The biological filtration to convert ammonia to nitrate in drinking water applications in full-scale systems is limited in the United States. The purpose of this study was two-fold: (1) to monitor and evaluate nitrification in a full-scale iron removal filtration plant with biologically active granular media filters located in Ohio, and (2) to determine how to most efficiently regain nitrification following filter rebedding with new filter media. Results showed that the biologically- active filters consistently oxidized all of the 1.2 mg/L NH3-N to nitrate. Seasonal variations in ammonia oxidation effectiveness were not observed because yearly changes in water temperature and other water quality parameters were minimal. Pilot tests using dual anthracite/sand filters were used to determine the time required to achieve complete nitrification by three different seeding methods of new filters. The results of the pilot tests showed that all three methods took approximately 70 days. Biological oxidation of ammonia is a simple, robust and effective way to convert ammonia to nitrate in full-scale water treatment systems.