Abstract: A solid-bed reactor for biological sewage treatment has an intermediate floor which carries floor jets and by means of which the solid bed can be backwashed. The intake for the sewage is above the solid bed. Aeration orifices are integrated into the floor jets, and gas bubbles can be introduced by the orifices in the upper portions of the floor jets without thereby giving rise to instability phenomena. To ensure stable uniform distribution of aeration throughout the reactor, the clear cross-section in the entire intake region of each floor jet must be such that above the aeration orifices the flow of outgoing water does not impede the ascent of the gas bubbles.