Cyanobacterial biomass: a striking factor to decrease polyaluminium chloride (PACl) coagulation efficiency during a successive bloom

Abstract

Occurrence of cyanobacterial blooms in source waters challenges water treatment processes. During a successive bloom, typical characteristics of elevated cell-density and pH were observed from development to maintenance stage. However, studies about their influences on the coagulation process have been limited. Here, PACl coagulation experiments were conducted to investigate Microcystis removal with varied pH and cell-density. Results showed that PACl coagulation alone was sufficient to remove Microcystis with low cell-density (105–106 cells mL−1), since an elevated pH value (8.5–9.5) can promote PACl coagulation, possibly ascribed to sweeping cells via neutral gelatinous precipitate of alum. Nevertheless, elevated cyanobacterial biomass was a striking factor in decreasing Microcystis removal (80%–100%) by PACl coagulation, since its inhibitory effects on the coagulation process could not be offset by in situ elevated pH value. Chlorination-assisted (1 mg L−1) coagulation was recommended to treat cyanobacteria-laden source waters with high cell-density of >107 cells mL−1, as it promoted cyanobacterial removal and achieved the highest removal ratio of DOC and turbidity among these treatments. These findings will provide an important reference for water supplies to choose the proper water treatment process to treat cyanobacteria-laden source waters during a successive bloom. HIGHLIGHTS Elevated pH (8.5–9.5) enhanced cyanobacterial removal by PACl coagulation.; Elevated cell-density (107 cells mL−1) hindered cyanobacterial removal by PACl coagulation.; Coagulation alone was sufficient to remove cyanobacteria of 105–106 cells mL−1.; Pre-chlorination was necessary to treat elevated cyanobacterial biomass of 107 cells mL−1.;

Keywords

• cell-density
• microcystis
• ph values
• polyaluminium chloride (pacl) coagulation
• pre-chlorination
• successive bloom