Dissolved air flotation (DAF) is a process known to efficiently remove high concentrations of cyanobacteria under low turbidity. In applying DAF, coagulation and flocculation (C/F) are commonly used together to improve the processes. However, the performance is often dependent on cyanobacteria species, and coagulant types and doses. In this study, C/F process was combined with sedimentation (S) and DAF and two types of coagulants, aluminum sulfate (AS) and ferric chloride (FC), to examine the removal of two cyanobacteria species, a unicellular Microcystis aeruginosa and a filamentous Cylindropsermopsis raciborskii.

Results showed that performance of C/F/S and C/F/DAF processes relies on coagulation pH, coagulant type, and coagulant dose. Removal efficiency of M. aeruginosa and C. circinallis cells were very distinct. The optimum dosages for M. aeruginosa cell removal with C/F/S were 40 mg/L for AS and 30 mg/L for FC, with removal efficiency of 96.5-98.0 %. C/F/DAF outperformed C/F/S in M. aeruginosa cell removal. With 30 mg/L AS and FC as optimum doses, 97.3-99.2% of removal can be achieved. When treating C. raciborskii with C/F/S, removal efficiencies were low, only 10-20% and 20-30% for the cases of using AS and FC as coagulants, respectively. Higher removal efficiencies of C. raciborskii cells were observed when using C/F/DAF, with 70-80% for using FC and 50-60% for AS. C/F/S and C/F/DAF were proved to effectively remove unicellular M. aeruginosa cells, but not filamentous C. raciborskii cells. During treatment, the two processes remove the whole cells, and no significant amount of intracellular toxins were released into the water.

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