Removal of Prorocentrum minimum from seawater using dissolved air flotation

Abstract

As world water resources struggle to support a growing population, desalination is attracting more attention. High algal removal rates achieved by dissolved air flotation (DAF) can protect down-stream reverse osmosis membranes from fouling and shutting down during high-biomass harmful algal bloom events. This study was designed to investigate the factors affecting algal cell removal using DAF in a simulated bloom of the dinoflagellate Prorocentrum minimum. The addition of sodium hypochlorite and adjustment of the pH were also investigated as ways to reduce coagulant dosage. Prorocentrum minimum presented a negatively charged surface. The optimum FeCl 3 dose was 60 mg/L at the lowest density tested (∼5×103 cells/mL), removing 90% of cells. At a higher density (40–50×103 cells/mL), 90 mg/L of FeCl 3 removed over 90% of the cells. Results suggest that the dominant mechanism for P. minimum removal was sweeping flocs, with charge neutralization constituting an important step. Adding 1 mg/L of sodium hypochlorite may have reduced surface charge. pH appeared to be a determining factor for total organic carbon (TOC) removal. At a lower pH, TOC removal was maintained at 70% while the coagulant dosage was reduced by half and algae removal was not compromised.