In the aeration tank, the pollutants are degraded with the help of the biomass. The biomass / activated sludge is then separated from the clean effluent in the final clarification, the clear water enters the receiving body of water and the settled sludge is returned back to the aeration tank as return activated sludge. It is again made available for biological removal. The surplus sludge, the end product of the biological process, is sent to sludge treatment.

In order to be able to maintain the desired sludge concentration in the aeration tank, the sedimented sludge deposited in the final clarification must be returned to the aeration basins. If the sludge in the final clarification thickened to twice the concentration in the activated sludge tank, then the return sludge flow would have to be just as large as the inflow in order to keep the sludge concentration in the activated sludge tank at the target value
Very often, however, the activated sludge thickens a lot less in the final clarification. Then, a lot more volume has to be pumped back in order to be able to transport the required amount of sludge back into the activated sludge basins. If, for example, the activated sludge process is operated at a concentration of 2.7 kg/m3 and the sludge thickens to only 3.5 kg/m3 in the final clarification, then instead of 100% return sludge, now 340% of the feed must now be returned.
This directly translates to considerable energy expenditures (pumping). Compared to "normal" thickening of the sludge to approx. 5.4 kg/m3, an additional 4.5 million Euro in additional expenditures for pumping are required over a period of 30 years.

In addition, it should be noted that the effect of secondary clarification crucially depends on the hydraulic load. Therefore, if instead of a hydraulic load of 2 (inlet = 1 and return sludge = 1) now a hydraulic load of 4.4 (inlet 1 and return sludge 3.4) has to be maintained, then the final clarification is loaded by significantly more than twice the normal flow.

Usually, poor thickening in the final clarification is always accompanied by a poor sludge index, hence a poor settling behavior. If therefore the final clarification does now not only has to cope with a sludge with poor sedimentation, but also at the same time with the hydraulic load that several times greater than normal, then its limits will be reached quickly and sludge losses and poor effluent results are the inevitable consequences.
A poorly settling sludge leads to a higher return sludge ratio and thus to a higher hydraulic load and thus to large problems.
Stable and good sludge settleability is the key factor in a well-functioning activated sludge plant.


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