Microalgal lipid extraction strategies for biodiesel production: A review

Microalgae have gained widespread interest all over the globe as potential substitutes to petroleum-based fuels. The biodiesel production from microalgae has been investigated intensively with a focus on optimization of various upstream and down-stream operations involved in the process. Downstream operations in the production of biodiesel consist of various steps whereof microalgal lipid extraction holds utmost importance for the overall efficiency of the process. The conventional method of extraction involves direct use of organic solvents to extract the intracellular lipids, making the process more time consuming and less eco-friendly. This review describes various modifications that have emerged to the conventional approach of lipid extraction, with a focus on potential biodiesel microalgae strains, namely, Chlorella sp., Nannochloropsis sp., and Scenedesmus sp. Various pre-treatment approaches in conjunction with solvent based extraction namely mechanical, chemical and biological cell disruption along with their comparative efficiencies on the basis of lipid yield, extraction time, energy intake, solvent use, biomass status (dry and wet) and scalability are discussed. The other promising approaches i.e. use of greener solvents, viz. bio-based solvents, supercritical fluids, ionic liquids, and switchable solvents are also reviewed along with the simultaneous extraction, and trans-esterification approach. Further, various challenges and solutions to the lipid extraction processes have also been analysed.