Flocculation is used to separate microalgal cells from broth by the addition of one or more chemicals. Microalgal cell walls carry a negative charge that prevents self­aggregation within the suspension. This negative charge is countered by the addition of polyvalent ions called flocculants. These can be cationic, anionic, or nonionic, and they flocculate the cells without affecting their composition and/or being toxic. These flocculants have been classified into two groups, namely (1) inorganic agents, including polyvalent metal ions such as Al3+ and Fe+3 that form polyhydroxy com­plexes at suitable pH; and (2) polymeric flocculants, including ionic, nonionic, natural, and synthetic polymers. Among the former group, aluminum sulfate, ferric chloride, and ferrous sulfate are commonly used multivalent flocculants whose efficiency is directly proportional to the ionic charge. Fe3+ has been reported to be 80% effi­cient in harvesting different types of algae (Knuckey et al., 2006). The mechanism of polymer flocculation involves ionic interaction between polyelectrolyte and algal cells, resulting in the bridging of algae and formation of flocs. The extent of aggrega­tion depends on the charge, molecular weight, and concentration of polymers. It has been observed that binding capability increases with an increase in molecular weight and charge on the polymer. Algal properties such as the pH of broth, concentration of biomass, and its charge are equally important to consider when selecting a polymer. Tenney et al. (1969) found effective flocculation in Chlorella when using a cationic polyelectrolyte, whereas an anionic polyelectrolyte failed to do so. Divakaran and Pillai (2002) successfully used chitosan as a bioflocculant for Spirulina, Oscillatoria, Chlorella, and Synechocystis spp. The efficiency of the method is affected by media pH, and best results were recorded at pH 7.0 for freshwater and a lower pH for marine species. Organic flocculants are reported to be beneficial in terms of their lower sensitivity to media pH, low dosage requirements, and wider range of applications. Heasman et al. (2000) also studied chitosan as a flocculant for Tetraselmis chui, Thalassiosira pseudonana, and Isochrysis sp., and they found that only 40 mg L-1 of chemical was needed for complete aggregation, whereas 150 mg L-1 was needed for Chaetoceros muelleri. Microbial flocculants (AM49) were also studied by Oh et al. (2001) for the harvesting of Chlorella vulgaris. This flocculant was found to be bet­ter than other commonly used flocculants. Recovery of more than 83% solids when operating in the pH range 5 to 11 was recorded; this is higher than that when using aluminum sulfate (72%) or the cationic polymer polyacrylamide (78%).

Algae also have the property of auto-flocculation when supplemental CO2 supply is removed. Disruption of the CO2 supply during photosynthesis increases the pH, which results in the precipitation of magnesium, calcium, phosphate, and carbonate salts along with algal cells. The positively charged ions interact with the negatively charged algal surfaces and bind them, resulting in auto-flocculation. Sukenik and Shelef (1984) conducted a study on auto-flocculation in pond and laboratory-scale experiments, and reported some very promising results. The unavailability of condu­cive conditions—especially light and CO2—can, however, limit this process.