The primary particles would erode from the sludge surface under shear conditions. As the primary particles have significant effects on the solid-liquid separation process, the erosion behaviors of activated sludge in biological wastewater treatment processes under shear conditions were investigated using a chemical-equilibrium-based model. The equilibrium dispersed mass concentration of the primary particles in the sludge solution was found to nonlinearly increase with the solid content and shear intensity, and could be well described by the model. Compared with other sludge reported in literatures, the activated sludge used in this study was found to be more stable during the shear test, with a high equilibrium constant K-0 of 30.2 and a low Gibbs' free energy of adhesion (Delta G(0)) of -3.41 at a shear intensity of 800 s(-1). The two parameters could be used to evaluate the strength of the sludge. The negative value of Delta H indicates the energy demand for the erosion process. The low value of Delta H for the activated sludge used in this study indicates that the erosion process was more energy demanding and the erosion process was less Aear dependent for the activated sludge used in this study. (C) 2007 Elsevier B.V. All rights reserved.