Optimization of continuous hydrogen fermentation of food waste as a function of solids retention time independent of hydraulic retention time

Four anaerobic sequencing batch reactors (ASBRs) were used for hydrogen fermentation and fed with food waste (4.4 ± 0.2% volatile solids (VS) containing 27 g carbohydrate-COD/L). The aim of this study was to investigate the effects of solids retention time (SRT) in the range 24-160 h and hydraulic retention time (HRT) in the range 24-42 h. Achieving high SRT independent of HRT with internal sludge retention contributed to higher H₂ production than in previous studies using continuous stirred-tank reactor systems. The maximum H₂ production rate of 2.73 L H₂/(L d) was estimated at an SRT of 126 h and HRT of 30 h, while the maximum H₂ yield of 80.9 mL H₂/g VS (1.12 mol H₂/mol hexose_added) occurred at an SRT of 126 h and HRT of 33 h. Furthermore, hydrogen fermentation facilitated organic acid conversion, alcohol conversion, and volatile suspended solid removal with efficiencies ranging from 29.6 to 46.3%, 4.1 to 14.6%, and 54.9 to 75.8%, respectively, which were comparable to conventional acidogenic fermentation without H₂ production.