Abstract: An apparatus for the purification of waste water in accordance with the activated-sludge process which comprises a buffer vessel downstream of a settling tank with a mechanical cleaner, at least one oxidation trough or vessel provided with at least one aerating device and a device for subdividing the input into the latter receptacle in the region of the aeration device, an afterclarifying receptacle and a sludge recycling path from the afterclarifier. The buffer vessel is provided in addition with an aeration device and the waste water or sewage is treated therein to partially reduce its biological oxygen demand. Between the buffer vessel and the oxidation receptacle, a high-efficiency stage is provided which is formed with an aeration device. The sludge recycle from the afterclarifier is introduced into the high-efficiency receptacle which is provided in the region of the aeration device of the latter with a distributor for the recycled sludge and a distributor for the discharge from the buffer vessel.