Abstract: Described is a two-stage sewage treatment plant (10), in particular for domestic sewage, wherein the first biological purification stage (12) is dimensioned so that besides the carbon load, the mean nitrogen load is maximally degraded. Any peak loads that may appear are degraded in the second biological purification stage (13). This makes possible a space-saving construction and an economical operation. The second biological purification stage (13) is always a two-stage filtering installation with a filter stage (20) for nitrification and a filter stage (21) for denitrification. The first biological purification stage (12) can be an activated sludge stage as per A 131, a pure oxygen activated sludge or likewise a two-stage filtering installation.