Activated sludge is one of the best investigated and understood suspensions. But up to now we still lack the factors influencing the sedimentation velocities concerning the main sludge characteristics/properties to describe the settleability of different sludges. Different in terms of SVI, organic loading, sludge retention time, composition, content of polymers, iron or aluminium, density, porosity and particle size distribution. But for the design of clarifiers most of these parameters are disregarded, of none importance or neglectable. With the help of PSD and particle shape analysis we can improve our knowledge of the settleability of activated sludge flocs. The measurements, done with the CIS show the influence of the differential sedimentation in comparison to the effluent of the secondary clarifiers under different conditions.

Calculating the sedimentation velocities, the behaviour of activated and precipitated sludge could be explained and conclusions can be drawn to optimize the solid flux in secondary clarifiers.

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