To increase the recovery of cells via sedimentation, a flocculant is added to the system. Flocculation is the first step of the harvesting; this process aims to aggregate the microalgal cells and thereby increase the particle size (Grima et al., 2003). The microalgae have a negative charge on the surface to prevent cell aggregation. The loads on the surface of algae can be altered by the addition of flocculant (Harun et al., 2010).

Flocculation may be accomplished by three methods: chemical flocculation,

bioflocculation, and electroflocculation. The most common flocculants are aluminum sul­phate, aluminum chloride, and ferric chloride. The addition of sodium hydroxide raises the pH of the culture to 8-11, coagulating the cells in just a few minutes. However, the floc — culants are toxic in high concentration. Flocculants should be inexpensive, nontoxic, and ef­fective at low concentrations. Chitosan is an organic cationic polymer, a nontoxic flocculating agent that is used in wastewater treatment and in the food industry (Pires et al., 2012).