A novel two-stage immobilized bioreactor for biohydrogen production using a partial microalgal-bacterial (Chlorella vulgaris and wastewater activated sludge) co-culture

Biohydrogen (bioH₂) has the potential to be a sustainable and carbon–neutral source of bioenergy, as it does not produce any greenhouse gas emissions. Various strategies and techniques such as algal-bacterial co-culturing and bioreactor configurations have been applied to improve bioH₂ yield and regulate process inhibitors (low pH and high organic acids yield). Therefore, in current study, the co-culturing of Chlorella vulgaris (C. vulgaris) and domestic wastewater activated sludge (WWAS) was exploited in an innovative way by immobilization in two separate bioreactors. The C. vulgaris and WWAS were immobilized in sodium alginate and Z-medium was utilized as a source of nutrient supplemented with 10 g/L of glucose as external carbon source. The maximum bioH₂ production of 9.8 and 222.7 mL/L was obtained in microalgal and bacterial chambers, respectively. Acetic acid accumulation was reduced to below 150 mg/L which is 90 % less compared to previous studies and subsequently the pH raised ranging from 7.0 to 10.0 during six days of incubation period. The results suggest that immobilized microalgae and WWAS in partial co-culture tend to reduce process inhibitors by increasing pH via microalgal photosynthesis and bioH₂ production via sugar fermentation, respectively. The strategy to split microalgal-bacterial co-culture into separate chambers provides the ability to work microalgae and bacteria separately but in a symbiotic manner.