The two critical stages in product development are the harvesting of the microorgan­isms and the extraction of the product as these unit operations can add considerable costs to the process. For microalgae, there are a number of methods for harvesting

the cells including centrifugation, filtration, flocculation and settling. Flocculation can be used to improve the other methods of harvesting. Flocculation can be carried out using multivalent metal salts (Molina Grima et al., 2003) or cationic polymers. Centrifugal recovery is rapid and expensive but has been used for many microalgae. Filtration using filter presses has proved unsuccessful with the smaller microalgae, and membrane filtration has not been extensively used.

Extraction

Intracellular oils are difficult to extract from wet biomass (Belarbi et al., 2000), but can be more easily extracted from freeze-dried cells or cell paste. Oil has been extracted from Phaeodactylum tricormutum (diatom) and Monodus subterraneus (green alga) with solvents under pressure (Belarbi et al., 2000). In the case of C. protothecoides, the cells were freeze-dried before solvent extraction as were the oils from Isochrysis galbana (Molina Grima et al., 1994). Microalgal cells may be disrupted to extract the oils using a number of microbial cell disruption methods, but these methods can be expensive. Free fatty acids have been extracted from wet biomass using a potassium hydroxide — ethanol mixture (Molina Grima et al., 2003). Whole cells of Dunaliella tertiolecta have been liquefied at 300°C and 10 MPa to form oil comparable to fuel oil. Both supercriti­cal CO2 (Mendes et al., 2003; Gouveia et al., 2007) and thermochemical liquefaction have also been used to produce biodiesel from macroalgae (Aresta et al., 2005). In addi­tion, whole microalgal cells containing high levels of oil have been used directly in diesel and biodiesel in emulsion fuels (Scragg et al., 2003). In general, all methods both mechanical and solvent based are expensive and will affect the cost of the biofuel.