Background: Partial oxidation of ammonium to nitrite, process known as nitritation, has important advantages over complete nitrification, including energy savings due to a reduced aeration, a lower organic carbon requirement for denitrification, a reduced production of sludge, and the potential use of nitrite in anammox and anoxic biogas purification technologies. Dissolved O 2 (DO) concentration commonly has been regarded as the most important parameter to be controlled for achieving a successful nitritation process.Results: The present study showed that setting average DO values below 1 g O 2 m −3 was insufficient to achieve a high and stable nitritation at ammonium loading rates between 2.5 to 5.1 g N-NH 4 + m −3 h −1 . Under these conditions, the predominance of nitrite oxidizing bacteria (NOB) resulted in ammonium oxidation selectivity to nitrite from 5.2 ± 3.6% to 42.8 ± 21%. Increasing the ammonium loading to 10.1 gN-NH 4 + m −3 h −1 together with an average DO value of 0.9 ± 0.6 gO 2 m −3 resulted in an ammonium oxidation efficiency of 94 ± 1.1% and ammonium oxidation selectivity to nitrite >90%. Conclusion:Continuous nitritation was achieved in a single reactor provided with an easy-to-implement aeration strategy. The ammonium loading rate was the most important parameter to be controlled, which inhibited NOB above a threshold value. NOB inhibition was mediated mainly by free nitrous acid, and by free ammonia to a lesser extent. Increasing the ammonium loading rate led to an enrichment of ammonium oxidizing bacteria of the Nitrosomonas genus.