PURPOSE: To separate sludge efficiently from org. waste water by mixing said waste water together with activated sludge in an aeration tank then separating the sludge in a settling tank provided with an ultrasonic generator near the surface of the sludge.

CONSTITUTION: Org. waste water is mixed together with activated sludge in an aeration tank, where the mixture is aerated. Thence, the mixed liquids are introduced into a settling tank 1 installed with an ultrasonic generator 6 near the sludge level L, where they are irradiated with ultrasonic waves to separate the sludge. The sludge is scraped with a sludge scraper 3, and is discharged through a sludge discharge port 5. Ultrasonic waves of around 20kHz oscillation frequencies are adequate and the intensity thereof should be such that only the foam is separated and the sludge is kept settled without upward scattering. Plural pieces of the ultrasonic generators 6 may be provided according to need so that the ultrasonic waves are applied over the entire part of the sludge interface.