The activated sludge (AS) flocculability markedly improved after the addition of Al3+ and Fe3+ compared to Ca2+ at a concentration of 2 mEq/L. Though the energy barrier decreased about 30% when Ca2+ was added, the AS flocculability did not improve substantially. This indicates that extended DLVO theory can explain AS flocculation with Al3+ and Fe3+ as additives but is not appropriate for Ca2+. In addition, no matter which cation was added, the AS flocculability was highly correlated to the loosely bound extracellular polymeric substances (LB-EPS) content. The majority of added Ca2+ remained in the bulk solution (about 92%), whereas almost all of the Al3+ and Fe3+ added was found in the pellet (about 98%). The cation' ability to bind to the AS is closely related to the energy barrier and LB-EPS contents, therefore it is the core reason behind the AS flocculation changes observed upon the addition of multivalent cations. (C) 2012 Elsevier Ltd. All rights reserved.