Comparing dissolved air flotation and conventional sedimentation to remove cyanobacterial cells of Microcystis aeruginosa: Part I: The key operating conditions

Abstract

Dissolved air flotation (DAF) is generally considered more effective than sedimentation (S) in the treatment of algal-rich water. However, the type and dose of coagulant, as well as the coagulation (C), flocculation (F) and DAF operating conditions are key parameters for removing intact cyanobacterial cells. This study compares C/F/DAF and C/F/S performances for removing, without causing damage, cultured cells of Microcystis aeruginosa, a surrogate for overall removal efficiency of cyanobacteria. Two coagulants (alum and an aluminium pre-polymerised coagulant of high basicity (WAC)) and the most relevant operating conditions (velocity gradients for coagulation and flocculation, flocculation retention time and DAF pressurised recycle ratio) were investigated for two scenarios of influent cell concentration, expressed as chlorophyll a (10–35 μg/L and higher than 50 μg/L). Results showed that C/F/DAF is the best process to remove single cells of M. aeruginosa, yielding very high chlorophyll a removal (93–98%), with no toxin release to water (8–15%), using a low recycle ratio (8%) and lower coagulant doses (3 mgAl 2O 3/L versus 5 mgAl 2O 3/L of WAC), slower coagulation (380 s −1 versus 743 s −1), stronger but shorter flocculation (8 min at 70 s −1 versus 15 min at 24 s −1) than C/F/S. WAC performed better than alum, for both processes. For either coagulants cell removal efficiency increased with the influent concentration, although higher doses were necessary to reach the same residuals.