One of the most straightforward methods for harvesting algal cultures is to let the cells settle naturally. This has been done for many different algal strains and has broad applicability in wastewater treatment facilities where the high bacterial load and nutrient levels tend to favor clumping and settling. Cell density and the radius of the algal cells influence its utility as well as flow rate of the system, which has been enhanced by lamellar separators and sedimentation tanks. Settling is frequently deployed with added flocculant/coagulants (Chen et al. 2011). Reliance on settling is time-consuming and is less useful in situations where one would like the harvested algal biomass or its cellular components to remain intact and/or to maintain cellular function or product integrity for further downstream processing. While it is also the slowest separation option, it is also the one with the lowest energy requirement. Therefore, settling/sedimentation is not employed for algal biofuel application without at least some type of flocculation or settling accelerant.

One of the focuses of microalgal lipid biofuel research is high lipid-containing algae. As has been reported in the past, low-density cells tend to not settle well (Edzwald 1993). The best oleaginous algae used for biofuel feedstock have between 37 and 70 % of their biomass by weight as lipid under induction. High lipid content translates into cells of low density (Eroglu and Melis 2009) that can remain sus­pended and make settling not useful. This type of buoyant biomass can also complicate other methods that rely on gravity for separation.