The general metabolic path of historical assumptions is pretty limited from ammonia to nitrite, to nitrate, and to nitrogen Gas N2, which has been widely acknowledged. In the presence of ammonia-oxidizing bacteria AOB and nitrite-oxidizing bacteria NOB, ammonium is converted to nitrite and further to nitrate. These two reactions are collectively called nitrification. Denitrification, conversely, performed by denitrifying community, is an anaerobic respiration process using nitrate as a final electron acceptor and result in stepwise microbiological reduction of nitrate, nitrite, nitric oxide NO, nitrous oxide N2O and nitrogen gas