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To evaluate the use of bioflocculants as potential alternatives to chemical flocculants in improving the harvesting efficiency of buoy-bead flotation (BBF) process, harvesting experiments for Scenedesmus obliquus ( S . obliquus ) and Chlorella vulgaris ( C . vulgaris ) were first carried out by the BBF method combined with pre-flocculation in a bench scale flotation column. Chitosan and ferric chloride were used as pre-flocculants in this study. Measurements of zeta potential and algal cell surface architecture were performed to reveal the mechanisms of action of ferric chloride and chitosan as pre-flocculants. The results showed that the BBF method with chitosan pre-flocculation provided harvesting efficiencies as high as 83.77% and 92.47% for S . obliquus and C . vulgaris , respectively. The harvesting efficiency was found to be related directly to the characteristics of microalgae-microsphere aggregates formed. The addition of a small number of flocculants could significantly change the size and density of aggregates and increase the rising velocity of aggregates. The characteristics of aggregates were dependent on the effects of different pre-flocculants. The effect of chitosan was stronger than that of ferric chloride in improving the rising velocity. The faster the rising velocity, the more the microalgae enriched on the suspension surface. Therefore, the harvesting efficiency could be improved by enhancing the rising velocity. These results suggest that bioflocculants like chitosan are good substitutes for chemical flocculants to improve the harvesting efficiency in BBF process. [ABSTRACT FROM AUTHOR]