Production of ethanol from pretreated biomass of Chlorella sorokiniana raised at lab and pilot scales

Extortionate demand of global bioethanol has led to a burgeoning interest in finding the alternative sources of biofuel. For this purpose, researchers are interested in microalgae because of its biorefinery applications and ability to produce biofuels, and other value-added products. Bioethanol is the most extensively used renewable fuel and is one of the realistic candidates for addressing environmental issues and mitigating the low base of oil supply. Chlorella sorokiniana can be considered as the most eminent source of bioethanol due to the accumulation of high levels of carbohydrates which can significantly be converted into renewable, cost-effective, and eco-friendly bioethanol. Keeping in view the importance of pretreatment for the efficient production of biofuels, the present study was targeted to determine the optimum conditions required for the chemical pretreatment. The selected microalgae strain was cultivated under mixotrophic and photoautotrophic conditions at laboratory and pilot scales. For mixotrophic cultivation, 0.2% (v/w) of molasses was used as an organic source of carbon. The reducing sugar content of Chlorella sp. was studied by pretreating the dried biomass with dilute H₂SO₄ and NaOH at 3%, 5%, 7%, and 9% (v/v) concentrations. Mixotrophic cultivation conditions along with 3% H₂SO₄ pretreatment showed the most efficient recovery of entrapped sugars from algal cell wall consequently producing maximum amount of bioethanol. At lab scale, the biomass cultivated under photoautotrophic and mixotrophic conditions and pretreated with 3% acid produced maximum of 10.96 and 16.07 g/L ethanol, respectively, while maximum ethanol production of biomass cultivated under mixotrophic conditions in alkaline pretreatment was 12.21 g/L at 9% NaOH and in biomass cultivated under photoautotrophic conditions maximum 9.89 g/L of ethanol production was estimated at 5% NaOH. During pilot-scale cultivation, the mixotrophically cultivated biomass pretreated with acidic medium produced 13.45 ± 4.31 g/L ethanol and photoautotrophically cultivated biomass resulted into 10.8 ± 6.31 g/L of ethanol with 88 and 83% of substrate reduction, respectively. This study proofs that biomass of C. sorokiniana pretreated with H₂SO₄ can act as a possible alternative for the sustainable production of bioethanol. Optimum growth of C. sorokiniana by using molasses in mixotrophic culture conditions also addresses the ability of microalgae to treat wastewaters and use waste materials as a carbon source. For the commercial production of bioethanol from microalgae, results of the present study will be helpful.