Abstract: The present invention comprises a method for biological oxidation of nitrogen in water, e.g., wastewater, nitrification, by using ammonia-rich water for the selection and enrichment of nitrifying micro-organisms in a separate aerated reaction volume, where organic matter already has been reduced. The nitrifying micro-organisms will get a competing edge compared to other species, as they are being able to obtain energy from the oxidation of ammonia into nitrate. The method can be applied to the activated sludge process by the introduction of a sludge aeration zone on the recycled sludge, and also be combined with a sludge anoxic zone, for denitrification, enabling a more reliable operation of biological nitrogen as well as phosphorus removal, by maximal use of reaction volumes with lower detention time and lower costs. It can also be applied to a fixed bed process.