In the activation process, secondary clarifier tanks and aeration basins are interdependent units. The wastewater-sludge mixture flows from the aeration basins into the secondary clarifier where the sludge settles and is conveyed back to the aeration basins via the return sludge pumping station. Cleaned wastewater, the clear supernatant in the secondary clarifier, is directed to the receiving body of water.
By separating sludge and water, clarifiers play a crucial role in the activation process. The better this separation works, the better the effluent quality. 1 mg/l of filterable substances in the effluent of secondary clarification correspond to 0.5 - 1.2 mg BSB5 or 1.0 to 2.5 mg/l COD.
The final clarification only works as well as the activated sludge does settle. However, very often there is bulking sludge, a sludge that settles very slowly due to its structure (filamentous organisms). Then, the operation of the activated sludge plant is determined by this very parameter. One tries counteract with very generous dimensioning of secondary clarifiers. In existing plants, only massive additions of chemicals and precipitants mitigate but with a number of disadvantages.
Poor sludge settling properties also require high recirculation of sedimented sludge into the aeration basins. This in turn leads to an additional hydraulic load of secondary clarification; a vicious circle.
Poor sludge settling properties mean poor sludge thickening and high polymer consumption. As a result, the retention time in the digestion and thus the degree of stabilization of the sludge is reduced, often with the result of poor sludge dewatering.

Permanently good sludge settling properties are essential for a well-functioning activation system.


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