The invention relates to a method for decomposing nitrogen compounds contained in the sewage of a biological sewage treatment plant. The inventive method comprises a first treatment step during which a gas containing oxygen is fed in a controlled manner into the sewage according to a predetermined O2 specified value for the oxygen content in the sewage, whereby ammonium contained in the sewage is oxidized while resulting in the formation of nitrates. In addition, the inventive method comprises a second step during which the nitrates are eliminated while resulting in the formation of gaseous nitrogen. Based on this fact and while maintaining a high level of operational reliability, the aim of the invention is to reduce the oxygen demand and thus the operational costs. To this end, the invention provides that the ammonium content or a property of the sewage that can be correlated therewith is continuously measured, and the O2 specified value is preset according to the measured ammonium content.