Algae flocculation is a method of separating algae from its medium by using chemicals to force the algae to form lumps or aggregates. The main disad­vantage of this separation method is that the additional chemicals are diffi­cult to remove or recover from the separated algae, thus making it inefficient and uneconomical for commercial use. The cost to remove or recover these chemicals may be too expensive to be commercially viable, unless a techno­logical breakthrough is achieved.

Flocculating agents, or flocculants, are chemicals that promote flocculation by causing colloids and other suspended particles in liquids to aggregate, forming a floc. In general, there are two types of flocculants commonly used: inorganic flocculants and organic polymer/polymer electrolyte flocculants. Alum (hydrated potassium aluminum sulfate (KAl(SO4)212H2O)) and ferric chloride (FeCl3) are the chemical flocculants most frequently used to har­vest algae [20]. A commercial product called "Chitosan," popularly used for water purification, can also be used as a flocculant but is far more expen­sive than other flocculants. The shells of crustaceans, such as shrimp, lobster, crabs, and crayfish, are ground into powder and processed to obtain chitin, a polysaccharide found in the shells, from which chitosan is derived via de­acetylation. Water that is more brackish or saline requires additional chemi­cal flocculant to induce flocculation [20]. High molecular weight organic polymers are considered good flocculants, because several segments of a polymer can attach themselves to the surface of a colloidal particle and the remainder of the segments are extended into the solution [25].

Harvesting via chemical flocculation alone, by current technoeconomic standards, is a method that may be too expensive for large operations. In addi­tion to the chemical flocculation discussed above, there are different methods of flocculation of algae, including autoflocculation [26], bioflocculation [27], and electroflocculation. Bioflocculation has been studied and practiced in wastewater and sewage treatment. The efficacy of algae flocculation depends upon a large number of factors that are usually poorly understood or ignored, including cell size, cell shape, cell wall thickness, cell surface and interfacial properties, and so on. By considering these properties as well as appropri­ate combinations of flocculation with other concentrating techniques, there is room for substantial enhancement in algae flocculation practice.