Nutrients dissolved in the wastewater are converted into bacterial mass. This bacterial mass (activated sludge) passes from the aeration basins into the secondary clarifier, where the activated sludge is separated by sedimentation and the clear supernatant is discharged to the receiving body of water. The settled sludge is pumped from the secondary clarification back into the aeration basin in order to have as high a bacterial mass as possible in this biologically relevant zone. Once the target concentration has been reached, the remaining sludge is now released as surplus sludge to sludge treatment.
With increasing sludge retention time, very different removal objectives can be achieved - carbon degradation, nitrification / denitrification, sludge stabilization.
The activated sludge process is by far the most widely used method of wastewater treatment.


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