Cell viability changes coagulation efficiency of polyaluminium chloride for cyanobacteria-laden source water treatment

Abstract

Cyanobacterial blooms challenge drinking water safety, and polyaluminium chloride (PACl) coagulation is crucial in treating algal-laden source water in drinking water treatment plants (DWTPs). The cell viability of cyanobacteria declines at the bloom decay stage, which may affect the coagulation process. Most studies have focused on high-viability cells, not low-viability cells. Herein, PACl coagulation experiments were conducted to compare the removal efficiencies of high- and low-viability Microcystis, and the effects of cellular properties and extracellular organic matter (dEOM) on their removal efficiency were discussed. The removal efficiency of low-viability cells (38.3–99.3 %) was higher than that of high-viability cells (14.5–85.9 %) after coagulation-sedimentation, owing to the declining cellular buoyancy and enhanced adsorptive interaction with Al(OH)3(am) via increased aromatics/peptides containing functional groups of aromatic, OH, NH, CN on their hydrophobic cellular surface. By contrast, together with in situ dEOM, the removal efficiency of low-viability cells (1.3–90.5 %) became much lower than that of high-viability cells (2.2–98.2 %) even with a higher ratio of [PACl: DOC], due to elevated low molecular weight (MW) peptides of <10 kDa in dEOM impeding the coagulation removal of low-viability cells via chelation and negative charge supply. Additionally, a decrease in the removal ratio of dEOM was observed, and peptides (<10 kDa) in the residual dEOM increased by 93–100 %. Overall, this study demonstrated that the declining cell viability could strongly affect the coagulation efficiency of algal-laden source water, and the optimization of the coagulation process by water managers should depend on the cell viability of cyanobacteria.