Thermophilic Dark Fermentation for Simultaneous Mixed Volatile Fatty Acids and Biohydrogen Production from Food Waste

Abstract

Food waste is categorized as organic solid waste, which has a negative impact on environmental sustainability. Food waste was simultaneously used for the feasible generation of mixed volatile fatty acids (VFAs) and bio-hydrogen by deploying dark fermentation. Original anaerobic digested sludge was prepared via the shock technique with 50 g/L glucose under thermophilic temperature (55 °C). The pretreated inoculum was found capable of converting 10 g VS/L food waste to hydrogen with a rather high yield of 135.2 ± 7 mL H2/VSadded. The effect of various concentrations of food waste, including 10.2, 16.3, 20.3, and 26.4 g VS/L, on mixed VFAs production was subsequently carried out in batch dark fermentation. The highest butyric acid concentration (5.26 ± 0.22 g/L) in soluble metabolites was obtained from batch dark fermentation with 26.4 g VS/L of food waste. The dominant Clostridium thermobutyricum, Clostridium sporogenes, and Octadecobacter sp. found in the batch of dark fermentation of food waste could confirm the effectiveness of the load shock pretreatment method for inoculum preparation. The continuous stirred tank reactor (CSTR) inoculated with mixed cultures, also prepared via the load shock pretreatment method and without the addition of external nutrients, was operated by feeding 26.4 g VS/L food waste at the kinetically designed HRT for 4 days, corresponding to an organic loading rate (OLR) of 7.6 g VS/L·d. Under steady state conditions, promising butyric acid (5.65 ± 0.51 g/L)-rich mixed VFAs were achieved along with the hydrogen yield of 104.9 ± 11.0 mL-H2/g VSadded, which is similar to the upper side of the previously reported yields (8.8 ± 0.6–103.6 ± 0.6 mL-H2/g VS).