The invention relates to a device for purifying anaerobic biological wastewater, which wastewater is fed in through a feed pipe (4) and the purified wastewater and built-up gas are removed respectively by a discharge pipe (24) and a gas outlet pipe (18), said device comprising a main chamber (1), wherein wastewater, which is to be clarified, is fed to a lower area and at least one lifting pipe (9), which is used to remove the built-up gas and the purified wastewater, which contains sludge particles, emerges from an area which is higher than said lower area, said lifting pipe joining a return tank (2) in order to separate gas and purified wastewater, which contains sludge particles, and the outlet (11) thereof m the return tank (2) being higher than the inlet (10) thereof in the main chamber (1). The return tank (2) is connected to an auxiliary chamber (3), wherein purified wastewater, which contains sludge particles and which was lifted to the return tank (2) via the at least one lifting pipe (9), flows out from the return tank (2) to the auxiliary chamber (3). The purified wastewater flows off to the discharge pipe (24) from an upper region of the auxiliary chamber (3) and the auxiliary chamber (3) is connected in a lower region via a valve, preferably a non-return valve (15), or a pump to a lower region of the main chamber (1).