Bioprocess development on microalgae-based CO2 fixation and bioethanol production using Scenedesmus obliquus CNW-N

•A two-stage process was used to optimize CO2 fixation and carbohydrate productivity.•Dilute acid hydrolysis is very effective for saccharification of S. obliquus CNW-N.•The carbohydrate-rich microalgal biomass was used to produce bioethanol via SHF.•An integrated carbohydrate formation/bioethanol production process was evaluated.

A two-stage cultivation strategy was applied to achieve greater CO2 fixation and carbohydrate productivity with an indigenous microalga Scenedesmus obliquus CNW-N, which was first cultivated using a nutrient-rich medium to promote cell growth, and was then switched to a nutrient-deficient condition to trigger carbohydrate accumulation. The optimal biomass productivity, carbohydrate productivity, and CO2 fixation rate were 681.4, 352.9, and 1192.5 mg L−1 d−1, respectively, with a 51.8% carbohydrate content (based on dry weight). This performance is better than the results in most related studies. The microalgal carbohydrate was mainly composed of glucose, which accounts for nearly 80% of total sugars. Dilute acid hydrolysis with 2% H2SO4 can saccharify the wet microalgal biomass effectively, achieving a glucose yield of 96–98%. Using the acidic hydrolysate of the microalga as feedstock, the separate hydrolysis and fermentation (SHF) process gave an ethanol concentration of 8.55 g L−1, representing a theoretical yield of nearly 99.8%.