fraction of anammox biodiversity. For instance, there are currently over 2000 16S rRNA gene sequences affiliated with anammox bacteria that have been deposited to the Genbank (https://www.ncbi.nlm.nih.gov/genbank/), representing an overlooked continuum of species, subspecies, and strains, each apparently having found its specific niche in the wide variety of habitats where anammox bacteria are encountered. Species microdiversity is particularly impressive for the marine representative Scalindua. A question that remains to be investigated is which environmental factors determine species differentiation among anammox bacteria. The sequence identities of the anammox 16S rRNA genes range from 87 to 99%, and phylogenetic analysis places them all within the phylum Planctomycetota, which form the PVC superphylum together with Verrucomicrobia and Chlamydiae. Within the Planctomycetota, anammox bacteria deeply branch as a monophyletic clade. Their phylogenetic position together with a broad range of specific physiological, cellular, and molecular traits give anammox bacteria their own order Brocadiales. Application in wastewater treatment The application of the anammox process lies in the removal of ammonium in wastewater treatment and consists of two separate processes. The first step is partial nitrification (nitritation) of half of the ammonium to nitrite by ammonia oxidizing bacteria: 2 + 3 → 2 + 4 + 2 The resulting ammonium and nitrite are converted in the anammox process to dinitrogen gas and ~15% nitrate (not shown) by anammox bacteria: + → + 2 Both processes can take place in 1 reactor where two guilds of bacteria form compact granules. For the enrichment of the anammox organisms a granular biomass or biofilm system seems to be especially suited in which the necessary sludge age of more than 20 days can be ensured. Possible reactors are sequencing batch reactors (SBR), moving bed reactors or gas-lift-loop reactors. The cost reduction compared to conventional nitrogen removal is considerable; the technique is still young but proven in several fullscale installations. The first full scale reactor intended for the application of anammox bacteria was built in the Netherlands in 2002. In other wastewater treatment plants, such as the one in Germany (Hattingen), anammox activity is coincidentally observed though were not built for that purpose. As of 2006, there are three full scale processes in The Netherlands: one in a municipal wastewater treatment plant (in Rotterdam), and two on industrial effluent. One is a tannery, the other a potato processing plant. Advantages Conventional nitrogen removal from ammonium-rich wastewater is accomplished in two separate steps: