Abstract: In the embodiments of the invention described in the specification, an arrangement for biological treatment of wastewater includes a basin with an aeration region and a clarification region separated by a movable wall having a passage between the bottom of the wall and the bottom of the basin. The aeration region includes a plurality of diffuser arrangements for aerating the wastewater and the clarification region includes a sludge removal duct and at least one turbulence generator for periodically stirring up sludge particles which have been deposited on surfaces of the clarification region remote from the sludge removal duct and recirculating sludge through the wall passage to the aeration region and permitting sludge to resettle on surfaces closer to the sludge removal duct. The movable wall may be positioned to include one or more diffuser arrangements from the aeration region within the clarification region.