Effect of hydroxyapatite, octacalcium phosphate and calcium phosphate on the auto-flocculation of the microalgae in a high-rate algal pond.

Recovering microalgae is one of the main technological and economic concerns in a high-rate algal pond (HRAP) because of their small size and their low density. This paper emphasizes the characterization (identification and assessment of potential flocculation) of chemical compounds involved in microalgae auto-flocculation in a HRAP. First, thermodynamic simulations were performed, using two models (i.e. Visual Minteq and a simplified thermodynamic model) in order to determine the chemical compounds of interest. Experimental tests were then carried out with these compounds for assessing their flocculation ability. Both models revealed that precipitates of calcium phosphates and their substituted forms were the compounds involved in the auto-flocculation. Moreover, experimental tests showed that the stoichiometric neutralization of algal charges by calcium phosphates (i.e. hydroxyapatite (Ca5(PO4)3OH), octacalcium phosphate (Ca4H(PO4)3) and amorphous calcium phosphate (Ca3(PO4)2)), at a pH within the range 7-10 yields 70-82% recovered algal biomass. The optimum ratio required for algae auto-flocculation was 0.33 Ca5(PO4)3OH/g DM(algae) at pH 10, 0.11 Ca4H(PO4)3/g DM(algae) at pH 7 and 0.23 g Ca3(PO4)2/g DM(algae) at pH 9. Auto-flocculation appears as a simple, sustainable and promising method for efficient harvesting of microalgae in a HRAP.