Process for converting ammonium (NH 4 + ) of a mainstream of a wastewater plant to dinitrogen gas (N 2 ), including the consecutive steps of: i.) removing biodegradable carbon compounds in the mainstream; ii.) converting ammonium (NH 4 + ) in the mainstream to nitrite (NO 2 - ) in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitritation vessel (133a-133d), wherein growth of nitrite oxidizing bacteria (NOB) in step ii.) is prevented by periodically subjecting the bacteria in said nitritation vessel (133a-133d) to water suppressing growth of nitrite oxidizing bacteria (NOB), which is reject water (RW) from a sludge digester ; and iii.) denitrifying the resulting stream from step ii.) to dinitrogen gas in an anammox vessel , which is operated under non-aerated or anoxic conditions; wherein the water suppressing growth of nitrite (NO 2 - ) oxidizing bacteria (NOB) has a concentration of ammonium (NH 4 + ) higher than that of the mainstream of the wastewater plant, wherein the concentration of ammonium (NH 4 + ) lies in the range of 300 mg(N)/l to 1200 mg(N)/l, and/or wherein water suppressing growth of nitrite oxidizing bacteria (NOB) contains concentrations of nitrite (NO 2 - ) higher than that of the mainstream of a wastewater plant, wherein the concentration of nitrite (NO 2 - ) lies in the range from 100 mg(N)/l to 1200 mg(N)/l, characterized in that there are several different parallel nitritation vessels (133a-133d) for performing the nitritation process and there is a valve assembly, which is controlled such that wastewater during some periods is conveyed to some of the nitritation vessels (133a-133d) and reject water (RW) is conveyed to some others of the nitritation vessels (133a-133d), so that a sequencing is achieved for each nitritation vessel (133a-133d) between running it with reject water (RW) and with wastewater; or there are different vessels and there are free-flowing carrier elements onto which the bacteria are grown, wherein the wastewater and reject water (RW) are always connected to the same vessels, and the carrier elements are moved from vessel to vessel, so that once the bacteria on the carrier elements of a vessel connected to wastewater need a time at high ammonium (NH 4 + ) or nitrite (NO 2 - ) concentrations in order to regain the ability to convert ammonium (NH 4 + ) to nitrite (NO 2 - ) rather than nitrate (N0 3 - ), they are moved from a vessel connected to wastewater to a vessel connected the reject water (RW). i.) removing biodegradable carbon compounds in the mainstream; ii.) converting ammonium (NH 4 + ) in the mainstream to nitrite (NO 2 - ) in an aerated biological process containing ammonium oxidizing bacteria (AOB) in a nitritation vessel (133a-133d), wherein growth of nitrite oxidizing bacteria (NOB) in step ii.) is prevented by periodically subjecting the bacteria in said nitritation vessel (133a-133d) to water suppressing growth of nitrite oxidizing bacteria (NOB), which is reject water (RW) from a sludge digester ; and iii.) denitrifying the resulting stream from step ii.) to dinitrogen gas in an anammox vessel , which is operated under non-aerated or anoxic conditions; <EOS>