Carbon dioxide fixation and lipid storage of Scenedesmus sp. ASK22: A sustainable approach for biofuel production and waste remediation

Abstract

In this study, an airlift reactor (ALR) has been employed to evaluate the carbon dioxide fixation rate (Rc) and lipid yield (LY) of unicellular green microalgae Scenedesmus sp. ASK22, using dairy effluent as a biofuel feedstock. Independent process parameters (PPs) such as light intensity, CO2 concentration, and aeration rate and their effect on Rc and LY were revealed. The central composite design (CCD) was used to optimize the PPs. The best-operating conditions were measured as light intensity −6.24 Klux, CO2 concentration −14.03% (v v−1), and aeration rate −1.02 liter per minute (LPM). Under these conditions, LY and Rc were found to be 4.22 gL-1 and 1.27 gL−1d−1 which were 2.24- and 1.94-fold higher than the value obtained in the control experiment (1.88 gL-1 and 0.656 gL−1d-1) at the end of 12th day. The corresponding values for bioremediation of nitrate, phosphate, as well as chemical oxygen demand (COD), remained within 98–100%. The biochemical, CHN, thermogravimetric, and fatty acid analysis of Scenedesmus sp. ASK22 biomass and lipid confirmed their potential as a clean biofuel feedstock. Furthermore, a comprehensive analysis of lipid-extracted microalgae biomass (LEMB) was carried out suggesting that LEMB could be used as a high-quality cattle and fish feed, fertilizers, and a sustainable source for biogas, bioethanol, and bio-oils. In addition to improving the developed system's efficiency, a semi-continuous regime was implemented which resulted in biomass productivity of 1.89 gL−1d−1 which was 2.6-fold higher than the batch cultivation without hampering lipid productivity (0.377 gL−1d−1). The present results suggest that Scenedesmus sp. ASK22 is a potential candidate for CO2 sequestration from atmosphere/flue gas, biofuel production (biodiesel, bioethanol, biogas, biobutanol, etc.), and waste remediation.