The settling rate of the sludge is of central importance to the design of the activated sludge plant. This concerns especially the design of the secondary clarifiers and the resulting operation of the plant. Often, the sludge settleability is characterized by the sludge index. A sludge sample is placed in a 1-liter graduated cylinder and the sludge index ISV is calculated from the settled volume after 30 minutes and the dry matter content of the sludge:

SVI = settling volume in ml/l / dry matter content in g/l = ml/g

Sludge indices above 150 ml/g are considered bulking sludge. The described determination of the sludge index according to Mohlmann has been used for many decades yet does not ideally represent the processes in secondary clarification.

Much more meaningful is the measurement of the sludge settling speed (rate). For this purpose, a much longer cylinder with a significantly larger diameter is used. The measurement itself is not finished after a certain time. Instead, it determines the settling time which is passes while the fully suspended activated sludge settles a distance of 20 cm in the upper portion of the cylinder. This ensures that disturbed settling or thickening effects do not distort the true conditions as take place the final clarification. A settling rate of >3.0 m/h can be described as very good settling. In many systems, however, only sedimentation rates between 0.5 and 1.0 m/h are detected.

Aerobic granular activated sludge (AGS) from the S: Select process settles much faster than normal flake sludge with values of 3.0 to 8.0 m/h. The consequences with regard to the dimensioning and operation of an aeration plant are enormous.

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