A method is described for the biological cleaning of waste water, in which the waste water and gas are supplied to a reaction vessel (1) containing micro organisms through a two-component nozzle (2) which consists exclusively of two steady mutually concentric tubes, the nozzle having no additional supply elements and projecting into the reaction vessel and into the waste water therein along its vertical axis. The internally located gas-conveying inner tube of the two-component nozzle (2) is surrounded through the intermediary of a free annular gap by the outer tube conveying the waste water which is delivered by means of a pump (12). The gas ends within the outer tube at a spacing from the outlet opening (3) thereof which is greater by at least a factor of "5" than the internal diameter of the outer tube in the vicinity of the outlet opening (3).
