Effective flocculation of target microalgae with self-flocculating microalgae induced by pH decrease

A flocculation method was developed to harvest target microalgae with self-flocculating microalgae induced by decreasing pH to just below isoelectric point. The flocculation efficiencies of target microalgae were much higher than those flocculated only via pH decrease. The mechanism could be that negatively charged self-flocculating microalgal cells became positively charged during pH decrease, subsequently attracted negatively charged target microalgae cells to form flocs and settled down due to gravity. Microalgal biomass concentration and released polysaccharide (RPS) from target microalgae influenced flocculation efficiencies, while multivalent metal ions in growth medium could not. Furthermore, neutralizing pH and then supplementing nutrients allowed flocculated medium to be recycled for cultivation. Finally, Spearman’s Rank Correlation Coefficients (Rs) between flocculation efficiency and key factors were also investigated. These results suggest that this method is effective, simple to operate and allows the reuse of flocculated medium, thereby contributing to the economic production from microalgae to biodiesel.