Influence of Al/Fe-based coagulant dosing sequences on floc formation and settling behavior in algae-laden water

A high dosage of aluminum- (AC) or iron-based coagulant (IC) is required for the efficient destabilization of algae during the coagulation process. However, such a high dosage poses a risk of human exposure to residual aluminum or iron in drinking water, would be costly to apply, and would induce the formation of precipitates that could be deposited in the distribution network. In this study, PACl combined with either monomeric FeCl3 or polymeric polysilicate iron (PSI) was implemented with different dosing approaches including PACl → FeCl3, FeCl3 → PACl, PACl → PSI and PSI → PACl for algal (i.e., Chlorella sp.) removal by sedimentation. The results indicated that PACl → FeCl3 and PACl → PSI achieved the highest algae removal of 94% and 97%, respectively. By contrast, FeCl3 → PACl and PSI → PACl resulted in only 81% and 87% of algae removal, which were 10-13% lower than those that resulted when PACl was dosed first. Importantly, PACl → FeCl3 dosing brought about 2.7 times faster floc aggregation as well as the formation of 26% larger flocs than FeCl3 → PACl dosing resulting in a higher floc settling rate for PACl → FeCl3 dosing. Higher filterability and lower residual Al of the supernatant were also observed with PACl → FeCl3 dosing. It was also found that PACl combined with PSI resulted in slightly higher algae removal (3-6%) than when combined with FeCl3, and the former resulted in a more compact floc structure and a higher floc settling rate, thus leading to higher algae removal than the latter application. Hence, the PACl → FeCl3 and PACl → PSI dosing provided more efficient approaches towards the removal of algae from the water.