Harvesting of microalgae for biofuels: Comprehensive performance evaluation of natural, inorganic, and synthetic flocculants

Microalgal biomass is considered as one of the most suitable alternative feedstocks for the renewable biofuels. Microalgae have several advantages such as ability to grow in harsh environment, comparatively very high productivity, and high lipid contents. Due to such potentials, microalgal biomass is preferred over the convention biofuel feedstocks. The concentration of microalgal biomass typically ranged between 0.5 and 1 kg/m³ in the raceways or open pond type cultivation systems and around 5-10 kg/m³ in the closed photobioreactor-type cultivation systems. The bottleneck of the algal biofuels is the harvesting of microalgae biomass from diluted culture media. Irrespective of the density of the algal biomass, the water content in microalgal culture exceeds 99% that makes the separation process lengthy and energy intensive. This largely determines the economic viability of microalgae-based biofuels and by-products. Among various techniques used for the harvesting of microalgal biomass, coagulation and flocculation have been found very effective and inexpensive; however, the choice of the coagulant depends on the use of harvested biomass for desired end products. The success of microalgae harvesting by flocculation requires thorough understanding about the nature of the flocculants, its molecular weight, mode of interaction, etc., along with the understanding about the algae species to be harvested. Harvesting of microalgae by coagulation and flocculation has its own advantages and disadvantages; however, being simple and cost-effective, it is one of most preferred techniques especially if the biomass is used for biofuels.