Evaluation of bioethanol and biodiesel production from Scenedesmus obliquus grown in biodiesel waste glycerol: A sequential integrated route for enhanced energy recovery

The present study aimed to evaluate the energy recovery through biodiesel and bioethanol production from Scenedesmus obliquus in a sequential route of lipid extraction followed by fermentation, with recycling of waste glycerol (WG) as a nutrient supplement into the culture. Low WG concentrations significantly enhanced the cellular dry weight over the control, recording the maximum significant value of 3.63 g L-1 using 2.5 g L-1 of WG (WG2.5). In addition, the maximum carbohydrate and lipid contents were recorded in WG2.5, which represented 16.4% and 21.7%, respectively, over the corresponding control, with simultaneous reduction in protein content. Moreover, total fatty acid methyl esters (FAMEs) recovery from biomass increased after WG2.5 supplementation, recording an increase of 24.6% over the control. Fermentation of lipid-free biomass increased the rate of bioethanol production, reaching its peak of 4.82 g L-1 at the 27th day. However, using of WG2.5 for microalgal growth and the residual lipid-free biomass for fermentation showed the highest bioethanol production peak of 5.58 g L-1 at day 27. Due to the accumulation of carbohydrates under WG, the biomass treated with WG2.5 showed increase in maximum bioethanol productivity up to 0.185 g L(-1)h(-1). However, sequential fermentation after lipid extraction enhanced the maximum bioethanol productivity by 32.3% and 15.1% over the whole cells from synthetic wastewater (WW) and lipid-free biomass from WW, respectively. The highest gross energy output of 21.4 GJ ton(-1) dry microalgae was estimated from the integrated route where S. obliquus was grown in WG-enriched medium and sequential fermentation was applied for the residual biomass after lipid extraction, with the highest recorded energy conversion efficiency of 62.9%. These findings provide an innovative practical integrated approach for waste recycling and high conversion efficiency of microalgal biomass for liquid biofuel production.