Polyelectrolyte flocculants in harvesting microalgal biomass for food and feed applications

Harvesting microalgal biomass at industrial scale is a techno-economic bottleneck for the algal biomass industry, compounded by the small cell size of microalgae and dilute biomass concentrations in culture. As a result, large volumes of water need to be removed during harvesting, making the process energy and cost intensive, accounting for up to 30% of the total cost of biomass production. Among the various harvesting techniques adopted commercially, flocculation is convenient and cost effective. The choice of a flocculant depends on its effectiveness on multiple microalgal strains, efficiencies at low biomass concentrations, its environmental footprint, being inexpensive and non-toxic for end application of the recovered biomass. Of the various flocculants, polyelectrolyte flocculants are widely utilised for various industrial applications such as wastewater treatment and mining, but also for effective harvesting of mass cultures of microalgae. Polyelectrolyte flocculants are polymers that are either branched or linear, but carrying ionic charge along their chain. They are accordingly classified as cationic, anionic or non-ionic polymers. These flocculants neutralise surface charges on cells and bind particles together by physical or chemical forces. The efficiency of polyelectrolyte flocculants depend on the type of polymer used, its molecular weight and charge density, dosage concentrations, cell concentration in the medium, type of strain, ionic strength and pH of the medium, and other parameters. Bulk harvesting of toxicant free microalgal biomass by polyelectrolyte flocculants is regarded to be one of the most economically viable techniques, with the cost of flocculants ranging between US$1.50 and 7.50 kg− 1, and requiring very low dosage for effective harvesting. This review focusses on polyelectrolyte flocculants to harvest cultivated microalgae for non-toxic residue free applications of the harvested biomass in the food and the feed industry and evaluates various commercial polyelectrolyte flocculants, their properties and application in harvesting microalgal biomass from high density cultures.