Two-stage anaerobic digestion of fruit and vegetable waste: optimization of dark fermentation through thermal pretreatment and co-digestion with sugar-rich wastewater

Fruit and vegetable waste (FVW) poses environmental challenges but can be valorized via dark fermentation (DF) and two-stage anaerobic digestion (TSAD) to produce H₂ and CH₄. This study evaluated the effects of culture medium and thermal pretreatment of inoculum in DF, testing various sugars and jam wastewater (JWW) as a renewable sugar source. Thermal pretreatment at 60 °C for 30  min enriched H₂-producers, while culture medium had no significant effect. Glucose and sucrose allow H₂ yield of 6.5 and 45.8  N mL/g_VS, respectively. JWW proved suitable for DF of both raw and pretreated FVW, achieving H₂ yields of 67.1 and 99.5 N mL/g_VS, respectively. Volatile fatty acids after DF indicated active fermentation pathways reaching 7.2 g/L in JWW_FVW_1:1. In the TSAD, CH₄ production reached 184 N mL/g_VS, surpassing conventional single-stage digestion. These results demonstrate the potential of TSAD systems using FVW and JWW for efficient simultaneous H₂ and CH₄ production.