Continuous two-stage lactate-driven dark fermentation process for enhanced biohydrogen production from food waste

The performance of a continuous lactate-driven dark fermentation (LD-DF) process based on the physical separation of lactate and hydrogen production was evaluated using simulated food waste (FW) as substrate. The optimum pH to maximize lactate production in the lactic reactor (LR) and the optimum hydraulic retention time (HRT) to maximize H 2 production in the hydrogen production reactor (HR) were investigated. The lack of pH control in the LR implied a decrease in pH to values as low as 3.7, with the maximum lactate titer reached at a controlled pH of 4.5. The controlled reduction of the HRT from 12 to 4 h strongly affected the volumetric hydrogen production rate (HPR) in the HR. The stepwise reduction of HRT to 6 h resulted in the maximum recorded HPR of 9.6 ± 0.9 L H 2 /L-d, whereas a reduction to 4 h mediated a process collapse. The total consumption of lactate over carbohydrates in the HR suggested the occurrence of the LD-DF process at this stage. Principal component analysis (PCA) revealed a positive correlation between HPR and butyrate concentration in the HR. Additionally, HPR was negatively correlated with elevated levels of acetate and lactate in the HR broth. Overall, the application of a continuous two-stage LD-DF process allowed a significant reduction in HRT while maintaining high HPRs from FW. • Two-stage LD-DF process showed high potential for bioH 2 production from food waste. • Fixed pH of 4.5 led to the highest lactate concentration of 11.4 ± 0.7 g/L. • Maximum H 2 productivity of 9.6 ± 0.9 L H 2 /L-d was achieved at 6 h HRT and 6.5 pH. • Lactate was the main H 2 precursor rather than carbohydrates. • High H 2 productions entailed high butyrate and low acetate and lactate.