Harvesting of Scenedesmus obliquus in wastewaters: auto- or bioflocculation?

Autoflocculation and bioflocculation are considered to be the most promising means for the economical harvesting of microalgae. We have therefore studied these phenomena with cultures of Scenedesmus obliquus produced during biological tertiary wastewater treatment. The quantity of extracellular polymers produced during ageing of the cultures proved insufficient to initiate bio-flocculation while the concentration of Ca(2+) and PO(4) (3-) of the treated effluent were too low to induce autoflocculation. It has been shown, however, that the algae sediment more readily upon ageing, possibly as a result of increased cell density. The use of density gradients made with Percoll (a colloidal solution of silica particles) allowed measurement of the true cell density and showed that this increases when cultures enter the declining growth phase. The quality of the biomass thus harvested is, however, considerably impaired, protein content decreasing from 62.7% (dry wt) during the exponential growth phase (day 5) to 14% at the end of cultures (day 21).