Abstract: A method and a device for purifying waste water is described, in particular for an anaerobic purification, in a container receiving a medium under the development of gas, wherein the gas developing is caught by a gas collector, ingredients of the medium entrained by the gas are separated and the ingredients separated are passed back into the medium below the gas collector. In order to improve the purification effect in a constructively simple manner, it is suggested to take in medium from the upper portion of the container by a gas lifting effect of the rising gas and to conduct it back into the container.