Harvesting microalgae by flocculation-sedimentation

Slurries of five marine and freshwater microalgae (Chlorella vulgaris, Choricystis minor, Cylindrotheca fusiformis, Neochloris sp., Nannochloropsis salina) were effectively flocculated using aluminum sulfate and ferric chloride as flocculants. The flocculant dose for 95% removal of the algal biomass by sedimentation in a standardized 62 min treatment depended on the following factors: the type of the flocculant; the algal species and cell diameter; the concentration of the biomass in the algal slurry; and the ionic strength of the suspending fluid. For all algae, the flocculant dose for 95% removal of the cells increased linearly with the concentration of the biomass in the slurry. Aluminum sulfate was a generally better flocculant than ferric chloride. Less flocculant was required for flocculation from a high ionic strength medium for the one alga (C. vulgaris) that could be grown both in freshwater- and seawater-based media. Quantitative relationships are reported for the flocculant dose dependence on the biomass concentration in suspensions of the five algae.