Abstract: An aeration tank for purifying sewage water by activated sludge comprising a mixing vessel communicated, according to the principle of communicating vessels, with a sludge separator, and a regenerator of activated sludge communicable with an outlet or recyclable sludge of the sludge separator and with the mixing vessel for feeding thereinto regenerated activated sludge. The mixing vessel is provided with at least one vertically arranged blade-type aerator having a suction tube an inlet end of which communicates with the regenerator, whereas an outlet end of which has a funnel accommodating the blades of the aerator. The sludge separator is equipped with a regulator of dynamic level of sludge medium, the upper edge of the funnel being positioned substantially above the dynamic level of the sludge medium in the mixing vessel.