The influence of multivalent cations on the flocculation of activated sludge with different sludge retention times

The mechanism governing the flocculation of activated sludge (AS) with different sludge retention times (SRTs) was studied in this paper. AS samples were cultivated in 8 lab-scale reactors with SRTs of 5 d, 7.5 d, 10 d, 12.5 d, 15 d, 20 d, 30 d, and 40 d. The bulk solution, loosely bound extracellular polymeric substances (LB-EPS), tightly bound EPS (TB-EPS), and pellet were extracted for all 8 AS samples. There was a clear trend that the effluent turbidity decreased as the SRT increased, and we deduced that this is because AS samples with longer SRTs have lower interaction energy barriers and lower LB-EPS content. Furthermore, the concentrations of multivalent cations (especially trivalent cations) in the pellets were found to be closely correlated to the AS flocculability, total interaction energy (Wtot), and LB-EPS content. The multivalent (especially trivalent) cations possess greater binding ability, and this ability to bind tightly to AS in large quantities is responsible for the superior flocculability of AS samples with longer SRTs. Hence, the concentrations of multivalent cations in the pellets are an important indicator of AS flocculability. We deduced that variations in the quantities of multivalent cations that tightly bind with the AS rather than remaining in the influent are the core reason behind observed fluctuations in the AS flocculability with different SRTs.