The nitrogen transformations may occur in biological treatment systems. Systems can be designed and operated to influence this nitrogen transformation scheme so as to achieve a desired effluent composition. Transformation of ammonia and organic nitrogen to the oxidized form of nitrate is accomplished through biological nitrification. Several physical-chemical processes have been used in the past for nitrogen removal. Municipal wastewater of predominantly domestic origin contains nitrogen in the organic and ammonium forms. In some treatment plants, nitrogen may also be introduced from recycle streams, such as supernatant from anaerobic digestors. Removal of nitrogen is obtained by assimilation and by conversion to nitrogen gas through nitrification and denitrification. Nitrification is the biological oxidation of ammonia to nitrate with nitrite formation as an intermediate. The optimal pH for nitrification varies between 6 and 7.5, depending on the formation of free ammonia and free nitrous acid.