The biological process, involving bacteria, in which nitrite ions and ammonium ions are converted into nitrogen molecules and water:
Anammox is a key feature of the nitrogen cycle, being responsible for between a third and a half of the nitrogen produced in the oceans. Several mechanisms have been suggested for the reaction, including the idea that hydrazine is produced as an intermediate stage. The anammox process has been used for the removal of ammonium from waste water.