Abstract: A method for the biological purification of ammonium-containing wastewater. The wastewater is purified in an aeration tank in which a low oxygen concentration of less than 1.0 mg/l is set so as to first convert, using aerobically oxidizing bacteria, ammonium contained in the wastewater to nitrite and then to convert, using anaerobically oxidizing bacteria, ammonium and nitrite to elemental nitrogen. At least a part of surplus sludge formed in the aeration tank is separated into a heavy sludge phase and a light sludge phase. The light sludge phase is fed as surplus sludge to a sludge digestion. Sludge water that is separated off from the sludge in a sludge dewatering is fed to a deammonifying tank. Nitrogen compounds in the sludge water are converted, by deammonification in the deammonifying tank, to elemental nitrogen. Surplus sludge formed in the deammonifying tank is fed to the aeration tank.

Abstract: A method for treating wastewater containing ammonium in a de-ammonifying activated-sludge system includes converting a portion of the ammonium into nitrite using aerobically oxidizing bacteria and converting another portion of the ammonium and the nitrite into elementary nitrogen using anaerobically oxidizing bacteria so as to generate a surplus sludge. The surplus sludge is removed from the waste water and separated into a heavy phase including primarily anaerobic ammonium-oxidizing bacteria and a light phase. The heavy phase is collected and/or returned to the system.