Flocculation is used to amass microalgae cells from the broth. Flocculation can be used as an initial dewatering step that will significantly enhance the ease of further processing. Microalgae carry a negative charge which prevents them from self aggregation within suspension. The surface charge on the algae can be countered by the addition of chemicals known as flocculants. These cationic moieties flocculate the algae without affecting the composition and toxicity of the product. Types of fl occulants include Al2(SO4)3 (aluminium sulphate), FeCl3 (ferric chloride) and Fe2(SO4)3 (ferric sulphate). These multivalent salts are commonly used and vary in effectiveness, which is directly related to the ionic charge of the flocculant.

The other types of flocculants used are polyelectrolytes, which are cationic poly­mers. Polymer flocculants have the advantage of physically linking cells together. The extent of aggregation by the polyelectrolyte depends on the specific properties of the polymer. Key polymer characteristics include charge, molecular weight and concentration. Increasing the molecular weight and charge on the polymers has been shown to increase their binding capabilities.

Lubian [18] showed that at a pH of approximately 4.5 and 6.5, the algal species Rhodomonas baltica achieved flocculation efficiencies of 68 and 77% at chitosan concentrations of 80 and 160 mg/L respectively. Tetraselmis suecica attained 42% efficiency at 80 mg dosage. When the pH of the cultures was pre-adjusted to 8, the algal species displayed efficiencies above 75%. At the moment, there is no reliable correlation between algal taxonomic groups and the concentration of chitosan required for effective flocculation [14]. Lubian [18] observed that pH control is very critical to the performance of microalgal flocculation.