Currently, no mechanistic model is available in wastewater industry that can accurately describe the batch settling behaviour of activated sludge. Such a model, which is based on the fundamental mass and force balances for water and solids, is extended and applied in this work and excellently describes batch settling experiments for sludges originating from two different wastewater treatment plants. The mechanistic model contains a Kynch batch density function f(bk) (hindered settling) and an effective solids stress function sigma e (compression). Initial settling velocities were obtained from detailed spatio-temporal dynamic solids concentration profiles measured with the aid of a radiotracer [De Clercq, J., Jacobs, J., Kinnear, D.J., Nopens, I., Dierckx, R.A., Defrancq, J., Vanrolleghem, P.A., 2005. Detailed spatio-temporal solids concentration profiling during batch settling of activated sludge using a radiotracer. Water Res. 39(10), 2125-2135]. Moreover, inverse modelling calculations were performed using the same data set. Both calculations showed that (1) the power function of Cole gave acceptable results and (2) a single effective solids stress function could be found when a time-dependent compression solids concentration C C was considered. This compression solids concentration is found just below the sludge blanket and is readily calculated from the solids concentration profiles. Given these time-evolutions, the effective solids stress values exhibit a uniform logarithmic relationship with the difference between the solids concentration and the compression solids concentration. The descriptive power of the model indicates a good potential for wider applicability of the model.