The invention relates to a method and device for purifying waste water, especially by means of anaerobic purification. The inventive method is carried out in a tank (19) containing waste water and sludge and involves the production of gas. The gas thus formed is collected by a gas collector (3) and the circulation system that arises as a result of the ascending gas is used to loosen the sludge that has settled on the bottom of the tank (19). The aim of the invention is to provide a more effective method with a simple constructive design. To achieve this, the settled sludge is suctioned away from the bottom using the lifting effect provided by the ascending gas and is conducted to the upper part of the tank separately from the waste water contained in said tank before being reintroduced into the waste water.