The influence of additives (Ca2+, Al3+, and Fe3+) on the interaction energy and loosely bound extracellular polymeric substances (EPS) of activated sludge and their flocculation mechanisms.

The activated sludge (AS) flocculability markedly improved after the addition of Al(3+) and Fe(3+) compared to Ca(2+) at a concentration of 2 mEq/L. Though the energy barrier decreased about 30% when Ca(2+) was added, the AS flocculability did not improve substantially. This indicates that extended DLVO theory can explain AS flocculation with Al(3+) and Fe(3+) as additives but is not appropriate for Ca(2+). 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 Ca(2+) remained in the bulk solution (about 92%), whereas almost all of the Al(3+) and Fe(3+) 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.