A process for converting ammonium (NH4+) of a mainstream of a wastewater plant to dinitrogen gas (N2), including the consecutive steps of i.) removing biodegradable carbon compounds in the mainstream, ii.) converting ammonium (NH4+) in the mainstream to nitrite (NO2−) in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitration vessel (133a-133d); and iii.) denitrifying the resulting stream from step ii.) to dinitrogen gas in an anammox vessel (200). Growth of nitrite oxidizing bacteria (NOB) in step ii.) is prevented by periodically subjecting the bacteria in said nitration vessel (133a-133d) to water suppressing growth of nitrite oxidizing bacteria (NOB).