Anammox -acronym for anaerobic ammonium oxidation- is the latest addition to the knowledge on the nitrogen cycle. In this biological process, nitrite and ammonium are converted directly into dinitrogen gas. This process makes up a mayor proportion of dinitrogen conversion in the oceans.

The bacteria that perform the anammox process belong to the rare order of the planctomycetes, of which planctomyces and pirellula are the most important members. Current anammox genera are: Brocadia and Kuenenia (both fresh water species), and Scalindua (marine species). Of special interest is the turnover of hydrazine (normally known as rocket fuel, and poisonous to most living organisms) as an intermediate. Another striking feature of the organism is the extremely slow growthrate: the doubling time is nearly two weeks!

The application of the anammox process lies in the removal of nitrogen in wastewater treatment. Instead of the conventional nitrification-denitrification process, only half of the nitrogen has to be oxidized partly to nitrite using the SHARON process, after which the 1:1 nitrite-ammonium mixture can be converted by the anammox-process. The cost reduction compared to conventional N-removal is considerable; the technique however still young and not proven.