A method for treating ammonium-containing waste water by means of a single sludge system in an SBR reactor, in which ammonium is converted to nitrite in a first reaction and ammonium and nitrite are converted into molecular nitrogen in a second reaction carried out in parallel, the oxygen concentration in the reactor being maintained at a low level. A robust process is achieved by providing that the pH-value of the system is controlled and kept at a target value by controlled intermittent aeration, the fluctuation range of the pH-value amounting to 0.05 at most, and preferably 0.02 at most, and the O2-concentration being kept between 0.2 mg/l and 0.4 mg/l, and preferably between 0.25 mg/l and 0.35 mg/l.