Unlocking the high-rate continuous performance of fermentative hydrogen bioproduction from fruit and vegetable residues by modulating hydraulic retention time

Harnessing fruit-vegetable waste (FVW) as a resource to produce hydrogen via dark fermentation (DF) embraces the circular economy concept. However, there is still a need to upgrade continuous FVW-DF bioprocessing to enhance hydrogen production rates (HPR). This study aims to investigate the influence of the hydraulic retention time (HRT) on the DF of FVW by mixed culture. A stirred tank reactor under continuous mesophilic conditions was operated for 47 days with HRT stepwise reductions from 24 to 6 h, leading to organic loading rates between 47 and 188 g volatile solids (VS)/L-d. The optimum HRT of 9 h resulted in an unprecedented HPR from FVW of 11.8 NL/L-d, with a hydrogen yield of 95.6 NmL/g VS fed. Based on an overarching inspection of hydrogen production in conjunction with organic acids and carbohydrates analyses, it was hypothesized that the high FVW-to-biohydrogen conversion rate achieved was powered by lactate metabolism.