Abstract: Method for purifying waste water in a compact settling plant, particularly for waste water which is biologically very loaded, wherein the final settling tank (23) is arranged inside a scavenging basin (3), wherein the aeration of waste is separated from the aeration of the return sludge, the two flows being mixed and previously degased before entering the scavenging basin and optionally once more contacted with a gas. Due to the star configuration of the aeration, suction and mixing units, a uniform charge and transformation are obtained so that at equal flow rate, the volume may be reduced with respect to conventional plants.