The anammox process is a promising technique for the treatment of ammonium-rich wastewaters. After the biological oxidation of half of the ammonium to nitrite (partial nitritation), the produced ammonium nitrite is converted in the anammox process. The partial nitritation and the anammox process can be both performed in separate reactors or in one (aerated) reactor. In this thesis, growth (Chapters 2 and 3) and metabolism (Chapters 4, 5 and 6) of the anammox bacteria are treated on full-scale (Chapters 2 and 5) as well as on laboratory scale (Chapters 3, 4 and 6)