Mesophilic and thermophilic acidogenic fermentation of process water from food waste hydrothermal carbonization

The process water from hydrothermal carbonization of food waste (180 °C, 1 h), characterized by a high organic matter content, represents a suitable substrate for acidogenic fermentation which was optimized by evaluating four inocula, under mesophilic and thermophilic batch conditions. The highest hydrogen yield (44.4 mL H 2 /g COD ) was achieved with a thermophilically adapted mixed sludge at pH 5.5, where Prevotellaceae, Bacteroidaceae, Lachnospiraceae and Coprothermobacteracea were dominant. This inoculum was used under thermophilic continuous operation, where pH (4.8 and 5.5), hydraulic retention time (HRT; 3.5 and 5.0 d) and organic loading rate (OLR; 2.5, 5.0 and 7.5 g COD /L·d) were evaluated. Optimal performance was achieved at pH 5.5, HRT 5 d, and OLR 5 g COD /L·d, reaching ∼57 mL H 2 /g COD and ∼7800 mg/L of volatile fatty acids. This operating condition selectively enriched hydrogen-producing bacteria ( Thermoanaerobacterales III , Clostridiaceae ) promoting lactate-driven metabolic routes conducted by Lactobacillaceae , thereby emphasizing the functional microbial consortia underlying process performance. • Inoculum source and its pretreatment play a key role in acidogenic fermentation. • Optimal inoculum was the thermophilically adapted mixed sludge after 14 d. • H 2 yield on continuous mode was optimized at 55 °C, pH 5.5, HRT 5 d, OLR 5 g COD /L d. • VFA production was enhanced at 55 °C, pH 5.5, HRT 5 d, OLR 7.5 g COD /L d. • Optimal H 2 performance is determined by lactate-driven microbial consortia.