A very simple method to achieve a stabilised sludge is the simultaneous aerobic stabilisation. To provide growth conditions for the microorganisms for an adequate wastewater treatment, a certain solid retention time ( sludge age) is necessary. It differs due to the temperature and the demanded nitrogen removal between approx. 4 and 12 days. If the sludge age is extended up to more than 20 days (Germany: 25 due to low temperatures) the content of organic matter, especially easily degradable organic matter, is reduced and the sludge is stabilised. This concept generally eliminates a primary sedimentation, solids from the effluent are treated in the aeration tank. This process approach is attractive for smaller communities, where space is not an issue and less complex operation is preferred. The large aeration tank volume provides good equalisation at high flow and loading occurrences and a high quality effluent is produced.

A disadvantage is the higher energy demand caused by the extension of the aerated time. The increased capital costs for the larger aeration tanks are in general more than compensated by the omitted facilities for an external stabilisation.