Sedimentation of algal biomass is a further method of biomass removal but generally requires prior flocculation for high removal efficiencies. Sedimentation can be carried out with some species without flocculation, but removal efficiency is generally considered poor. Flocculation can be used to increase cell dimensions allowing improved sedimentation. If car­ried out in conjunction with flocculation, a sedimentation tank can provide a reliable solution for biomass recovery [69].

2.2.5.2 FLOTATION

Flotation was a method of harvesting considered in the 1960s [60][70] however the recoverability of biomass was generally found to be poor with a wide range of reagents tested. It has been reported that using dis­solved air flotation mixed algal species could be harvested up to a slurry of 6% total solids; using electro-flotation, which creates air bubbles through electrolysis which then attach to the algal cells, mixed algal species could be harvested up to a slurry of 5%, but this approached required a signifi­cant energy input; using dispersed air flotation which uses froth or foam to capture the algal cells resulted also in similar results [69]. Existing research indicates that flotation offers a quicker alternative to sedimen­tation following algal flocculation, but more energy is required and thus cost is higher whilst providing a final product with lower total solids content.