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. Author manuscript; available in PMC: 2019 Jul 23.
Published in final edited form as: Environ Sci Technol Lett. 2018;5(2):110–116. doi: 10.1021/acs.estlett.7b00577

Figure 1.

Figure 1.

Nitrogen cycling in engineered biological nitrogen removal (BNR) systems. In conventional BNR, autotrophic ammonia-oxidizing bacteria (AOB) convert ammonium (NH4+) to nitrite (NO2−) through hydroxylamine (NH2OH), and nitrite-oxidizing bacteria (NOB) convert nitrite to nitrate (NO3−). Ordinary heterotrophic denitrifiers (OHO) then convert nitrate to dinitrogen gas (N2) through nitrite, nitric oxide (NO), and nitrous oxide (N2O). Alternatively, anaerobic ammonia-oxidizing bacteria (AMX) convert ammonium and nitrite directly to dinitrogen gas through hydrazine (N2H4). Complete ammonia-oxidizing bacteria (CMX) have only recently been described and studied, and are capable of converting ammonium to nitrate through hydroxylamine and nitrite in a single organism.