Enhanced bioenergy recovery from oil-extracted microalgae residues via two-step H₂/CH₄ or H₂/butanol anaerobic fermentation

Algae-based biodiesel is considered a promising alternative energy; therefore, the treatment of microalgae residues would be necessary. Anaerobic processes can be used for treating oil-extracted microalgae residues (OMR) and at the same time for recovering bioenergy. In this study, anaerobic batch experiments were conducted to evaluate the potential of recovering bioenergy, in the forms of butanol, H₂, or CH₄, from pretreated OMR. Using pretreated OMR as the only substrate, a butanol yield of 0.086 g/g-carbohydrate was obtained at carbohydrate of 40 g/L. With supplemented butyrate, a highest butanol yield of 0.192 g/g-carbohydrate was achieved at pretreated OMR containing 25 g/L of carbohydrate with 15 g/L of butyrate addition, attaining the highest energy yield of 3.92 kJ/g-OMR and energy generation rate of 0.65 kJ/g-OMR/d. CH₄ production from pretreated OMR attained an energy yield of 8.83 kJ/g-OMR, but energy generation rate required further improvement. H₂ production alone from pretreated OMR might not be attractive regarding energy yield, but it attained a superb energy generation rate of 0.68 kJ/g-OMR/d by combining H₂ production from pretreated OMR and butanol production from pretreated OMR with supplementary butyrate from H₂ fermentation supernatant. This study demonstrated an integrated system as an option for treating OMR and recovering bioenergy.