Continuous energy recovery and nutrients removal from molasses wastewater by synergistic system of dark fermentation and algal culture under various fermentation types

Synergistic system of dark fermentation and algal culture was initially operated at batch mode to investigate the energy production and nutrients removal from molasses wastewater in butyrate-type, ethanol-type and propionate-type fermentations. Butyrate-type fermentation was the most appropriate fermentation type for the synergistic system and exhibited the accumulative hydrogen volume of 658.3 mL L⁻¹ and hydrogen yield of 131.7 mL g⁻¹ COD. By-products from dark fermentation (mainly acetate and butyrate) were further used to cultivate oleaginous microalgae. The maximum algal biomass and lipid content reached 1.01 g L⁻¹ and 38.5%, respectively. In continuous operation, the synergistic system was stable and efficient, and energy production increased from 8.77 kJ L⁻¹ d⁻¹ (dark fermentation) to 17.3 kJ L⁻¹ d⁻¹ (synergistic system). Total COD, TN and TP removal efficiencies in the synergistic system reached 91.1%, 89.1% and 85.7%, respectively. This study shows the potential of the synergistic system in energy recovery and wastewater treatment.