Abstract: Described is a method and a device for purifying waste water, in particular by means of anaerobic purification. The method is carried out in a waste water- and sludge-receiving container under generation of gas, the developing gas being trapped by a gas collector and the circulation formed by the ascending gas being used for loosening the sludge which has settled on the bottom of the container. Such a method is to be developed in a constructionally simple and more efficient way. To this end the settled sludge should be sucked from the bottom by a gas lifting effect provided by the ascending gas, and should be passed into the upper portion of the container in a condition separated from the waster water in the container, and returned into the waste water.