Page 233 - Advances in Eco-Fuels for a Sustainable Environment

Advances in bio-oil extraction from nonedible oil seeds and algal biomass 197

products. This method is often recommended to recover high-quality algae without
chemical and bacterial contamination [20]. Recently, several newly designed centri-
fuges have been used in microalgae harvesting for biodiesel production. Nevertheless,
these centrifuges still require a high capital investment and high operating costs
compared to other approaches. As a result, recent research has suggested that the cen-
trifugal energy consumption can be saved by applying other preconcentration methods
prior to the centrifugation.

7.6.2.2 Flocculation
There are several approaches that have been used to flocculate individual microalgal
cells to build algal flocs that are more suitable for separation. This technique can be
used alone or in combination with other methods of harvesting as a preconcentration
technique. Microalgal cells have a negatively charged surface that makes the micro-
algal cells stable in a dilute solution. The negatively charged cells can be neutralized
and destabilized with positively charged coagulants, such as polyvalent cations and
cationic polymers. Several studies have employed aluminum- and iron-based metal
salts as flocculants. However, in metallic salt-induced flocculation, a high dosage
of costly flocculant and an acidic pH are required to achieve a satisfactory result
[55]. Moreover, cell lysis was induced by the addition of aluminum salts [56]. Resid-
ual metal salts after harvesting may negatively affect both the medium recycling and
the quality of the desired products [57–59]. In contrast, organic polymer flocculants,
such as chitosan and grafted starch, exhibited a more acceptable recovery of micro-
algae with both a lower dosage and a reduced impact on the environment compared
with metallic salts [60, 61]. By optimizing the concentration ratio of the flocculating
and oleaginous microalgae, the sedimentation rate and the recovery efficiency were
increased. Autoflocculation is the phenomenon of chemical flocculation of microalgal
cells in the presence of calcium and magnesium ions at a high pH [62]. The effects of
both the base used for flocculation and the acid used for pH neutralization on the
economic feasibility and the environmental impact of the process should be
considered [63].

7.6.2.3 Filtration
Membrane filtration has been widely utilized in biotechnological applications due
to its high separation efficiency, simple and continuous operation, and no require-
ment of chemicals in the process. For microalgae-based biofuel production, mem-
brane filtration can also facilitate recycling of the culture medium used for the
cultivation of microalgae to retain the residual nutrients in the culture medium
and to remove the protozoans and viruses [64]. In addition, membrane filtration
can simplify subsequent processes, for example, extraction, conversion, refining,
and the use of the residual biomass, without the use of coagulants [65].Cross-flow
filtration is widely used to decrease fouling with the tangential flow and performs
more efficiently than does dead-end filtration. In cross-flow filtration, back-
washing and ventilation of the algae medium can help control the fouling and