Enhancement of energy production efficiency from mixed biomass of Chlorella pyrenoidosa and cassava starch through combined hydrogen fermentation and methanogenesis

•Mixed biomass of C. pyrenoidosa and cassava starch was used for H2 fermentation.•Mixed biomass at C/N molar ratio of 25.3 gave the highest dark H2 yield.•Mixed biomass at C/N molar ratio of 15.6 gave the highest dark H2 production rate.•H2 yield was improved to 664.2 mL/g TVS via combined dark and photo fermentation.•Energy production efficiency was enhanced to 67.2% via cogeneration of H2 and CH4.

To enhance energy production efficiency (EPE) from microalgae Chlorella pyrenoidosa (CP), cassava starch (CS) was mixed with CP to optimise the carbon/nitrogen (C/N) ratio and facilitate efficient dark hydrogen fermentation, followed by photo hydrogen fermentation and methanogenesis. Steam heating with dilute acid was a preferred pre-treatment method to hydrolyze mixed biomass. The maximum dark hydrogen yield of 276.2 mL/g total volatile solids (TVS) from the mixed biomass at C/N molar ratio of 25.3 showed 3.7-fold and 1.8-fold increases, respectively, compared with those from only CP and only CS. The maximum dark hydrogen production rate of 31.96 mL/g TVS/h from the mixed biomass at the C/N molar ratio of 15.6 showed 3.4-fold and 3.7-fold increases, respectively, compared with those from only CP and only CS. The dark and photo hydrogen yield of 664.2 mL/g TVS and the methane yield of 126.0 mL/g TVS corresponded to a total EPE of 67.2%.