nitrification
biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate
en-US/ˌnaɪtɹəfəˈkeɪʃən/
en-GB/naɪtɹɪfɪkeɪʃn/
biological oxidation of ammonia or ammonium to nitrite followed by the oxidation of the nitrite to nitrate