Cyanobacteria in late lag and exponential phases suit different pre-chlorination and coagulation strategies.

Pre-chlorination is usually applied to facilitate coagulation process when treating cyanobacteria-laden source waters. However, the responses of cyanobacteria after chlorination were varied in different growth phases, hence how these differences affect the coagulation is not clear. In this study, Microcystis aeruginosa (M. aeruginosa) in late lag and exponential phases were selected for comparing the remove effect of cyanobacterial coagulation after varied doses (0, 0.5, 1.0 and 2.0 mg/L) of pre-chlorination. Results showed that the M. aeruginosa in late lag phase held higher removal efficiencies (48-86%) than those in exponential phase (1-60%). Lower pH and buoyancy, higher Zeta potential and hydrophobicity were beneficial to its coagulation. The application of 0.5-2.0 mg/L pre-chlorination in late lag phase did not promote its coagulation, mainly due to the impediment by released hydrophilic humus. In contrast, the remove efficiency increased about 40% with the addition of 2.0 mg/L chlorine and 15 mg/L polyaluminum chloride (PACl) in exponential phase. The increased macromolecular organic matters, Zeta potential and decreased buoyancy made for this promotion. In addition, the removal of extracellular organic matters, aromatics and microcystin was not that productive after coagulation in exponential phase, but both of chlorination and coagulation could reduce microcystin in late lag phase. The findings suggest that optimizing the application methods of pre-chlorine and coagulant according to the growth phases is necessary for cyanobacterial control.