This paper describes the application of population balance models to activated sludge flocculation. It presents the development and selection of appropriate expressions for aggregation and breakage mechanisms within the population balance framework to describe the evolution of mean size and mass distribution of flocs under shear-induced conditions. To describe the flocculation process, 16 models with different size dependent aggregation and breakage expressions were compared and the kinetic parameters of aggregation rate constant and selection rate constant were extracted by fitting each model to the experimental data. Of the 16 models, the shear-induced aggregation and size-dependent selection model was found to best describe the experimental data, however there were some discrepancies between the model and experimental results at long experimental times. A size dependent collision efficiency was introduced into the aggregation expression and this improved the fitting of the model with the experimental data. However, the relationship between the kinetic parameters and shear rate did not follow expected physical relationships. Further improvements to the model were made by setting the aggregation rate constant proportional to shear rate and the selection rate size independent whilst still including the size-dependent collision efficiency. The aggregation rate constant, selection rate constant and critical size were extracted by fitting the model to the experimental data. This model was able to follow the change in mean size and evolution of mass and was used to predict other experimental data successfully. The modelling results indicated that the population balance model is a useful tool to describe the dynamics of activated sludge flocculation. (c) 2005 Elsevier Ltd. All rights reserved.