Flocculation–Dewatering Behavior of Microalgae at Different Growth Stages under Inorganic Polymeric Flocculant Treatment: The Relationships between Algal Organic Matter and Floc Dewaterability

An algal bloom is a naturally occurring phenomenon in freshwaters due to discharge of wastewater containing nitrogen and phosphate. Coagulation-flocculation was widely used in removal of cyanobacteria and algae organic matter (AOM) from water. Microalgae and AOM differ greatly in physiochemical properties at different growth stages, which are likely to have important effects on their coagulation behavior. In this study, the interacting mechanisms between polymeric aluminum chloride (PACl) and microalgae cells and AOM at the different growth stages were investigated by characterizing morphology and AOM properties of microalgae flocs formed from PACl treatment. The results showed that PACl coagulation exhibited better removal efficiency for the microalgae cells and AOM in exponential and stationary phase than that in decline phase. The protein and humic acid content in loose bound AOM (LB-AOM) were found to be key constituents affecting floc dewatering behavior, and complexation adsorption of hydrolyzed products for proteins and humic substances in LB-AOM was responsible for microalgae floc dewatering improvement. These results provide a novel insight into optimization of flocculation and dewatering of cyanobacteria at different growth stages with PACl treatment.