Investigation of polyaluminum chloride (PACl) coagulation to remove cyanobacteria from maintenance to decay stage: Performance and mechanism

Abstract

Cyanobacterial bloom is a global issue affecting drinking water supply, and coagulation process is important to remove cyanobacteria. Cyanobacteria can get into decay stage after a long-term maintenance stage during a successive bloom. Some studies compared cyanobacterial coagulation at both stages, but their removal performance is still an argument. To well address this debate, the performance and mechanism of PACl coagulation to remove cyanobacterial cells at both stages, was compared with or without extracellular organic matters (EOMs) in this study. Results showed that cyanobacterial coagulation performed better at decay stage than that at maintenance stage without EOMs, treated by equal dosages of PACl. However, an opposite result was observed with EOMs, since EOMs at maintenance stage exhibited lower inhibitory effects on the coagulation due to fewer contents and negative charge. Meanwhile, inter-particle bridging for cyanobacterial coagulation at maintenance stage became stronger via elevated high-MW EOMs than that at decay stage. Analyses of Zeta potential and flocs characteristics revealed that electrical neutralization was the main coagulation mechanism at both stages without EOMs, and electrostatic patch was responsible for cyanobacterial coagulation with EOMs. Moreover, inter-particle bridging and sweeping flocculation were involved in cyanobacterial coagulation with EOMs, leading to loosely flocs with a decrease of fractal dimension (Df, 0.6–0.7), intensity factor (If, 0.2–0.7), and recovery factor (Rf, 0.3–0.9). Overall, EOMs could differ the performance and mechanism of cyanobacterial coagulation at both stages, and thus, managing varied EOMs would be vitally necessary to optimize cyanobacterial coagulation from maintenance to decay stage of a successive bloom.