The activation process is automatically interrupted after a drop of the sewage water level in the equalizing tank below the minimum level setting and the excessive sludge is then pumped off from the activation tank.

After this, the sludge is fed back into the activated-sludge tank (1) and the clarified water withdrawn.

It comprises an activation stage, with at least one activation tank, and at least one final settling stage with one or a plurality of final settling tanks.

To overcome these disadvantages, the invention further proposes to have the drain of the activated sludge tank undergo an incomplete solid/liquid-separation.

There is thus a sludge circuit from the intermediate clarification tank (13) via the anaerobic intermediate stage (44) and the activation tank (11) back to the intermediate clarification tank (13).

According to said method, the waste-water to be treated is fed first into an activated-sludge tank (1), which can be ventilated, and then into a sedimentation tank (2) in which the sludge and clarified water are separated.

With the conventional sludge processes only a limited high dry matter content TSBB can be achieved in an activated sludge tank and a high amount of return sludge has to be pumped.

For the service and inspection of the ventilation system, the activated sludge tanks are emptied every two years.

The raw waste water flowing into the activation tank (11) is aerated together with the sludge from the anaerobic intermediate stage (44) and then flows together with sludge flakes into the intermediate clarifying tank (13).