NOVELTY - Biological purification of ammonium-containing wastewater comprises: reacting the ammonium present in the wastewater in an aeration tank (3) to form elemental nitrogen; feeding the excess sludge to a sludge digester; feeding the sludge to a sludge drainage line; feeding the sludge water to a deammonification tank; feeding the excess sludge to the aeration tank, where the ammonium is reacted by aerobic bacteria to form nitrite and then by anaerobic bacteria to form elemental oxygen; and separating excess sludge into a heavy sludge- and a light sludge phase.    USE - The method is useful for biological purification of ammonium-containing wastewater.    ADVANTAGE - The method: is an economically feasible process and is environment friendly; and requires less energy.    DETAILED DESCRIPTION - Biological purification of ammonium-containing wastewater, preferably at 7-25 degrees C comprises: reacting the ammonium present in the wastewater in an aeration tank (3), at a predetermined oxygen concentration to form elemental nitrogen; feeding the excess sludge formed in the reaction at least in parts to a sludge digester, where the organic components of the sludge are converted to gas; feeding the sludge to a sludge drainage line to separate the highly nitrogenous warm sludge water; feeding the sludge water having a nitrogen concentration of 500-2000 mg/l and a temperature of 25-39 degrees C to a deammonification tank, where the nitrogen compounds (ammonium, organic nitrogen) present in the sludge water are converted to elemental nitrogen by deammonification; feeding the excess sludge formed in the deammonification step to the aeration tank, in which a low oxygen concentration of less than 1 mg/l is adjusted, such that the ammonium present in the wastewater is first reacted by aerobically oxidizing bacteria to form nitrite and is then converted to elemental nitrogen by anaerobically oxidizing bacteria, preferably planctomycetes; separating the excess sludge formed in the aeration tank, before feeding it to the sludge digester, into a heavy sludge phase predominantly comprising the anaerobically ammonium-oxidizing bacteria, and a light sludge phase; and recirculating the heavy sludge phase to the aeration tank and feeding the light sludge phase as excess sludge to the sludge digester.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of a plant for the biological purification of ammonium-containing wastewater.    Primary settling tank (2)    Aeration tank (3)    Settling tank (4)    Screening system (5)    Hydrocyclone (10)