Valorization of process water from hydrothermal carbonization of food waste by dark fermentation
Marcos Díez, Emma Barahona, M.A. de la Rubia, A.F. Mohedano, Elena Díaz
Abstract
Hydrothermal carbonization (HTC) of food waste produces hydrochar—a suitable biofuel—and process water (PW), which has a high organic content suitable for material and energy recovery. In this work, we study the effect of pH (4.8, 5.3, and 6.0) and organic loading rate (OLR; 2.5, 5.0, and 7.5 g COD L −1 d −1 ) throughout the dark fermentation (DF) of PW from the HTC of food waste (180 °C, 1 h) in a continuous stirred tank reactor. The highest hydrogen yield (197.5 mL H 2 L −1 d −1 ) was reached at pH 4.8 and OLR 5 g COD L −1 d −1 , associated with a prevalence of Clostridium bacteria. The highest volatile fatty acids concentration (10.2 g COD ·L −1 ) was achieved at pH 4.8 and OLR 7.5 g COD L −1 d −1 , with a dominance of Actinobacteria phylum. The integrated system HTC-DF allowed a potential energy recovery of 11.2 MJ kg −1 . • HTC process water from food waste has suitable characteristics for dark fermentation. • Optimal H 2 production was found at pH 4.8 and organic loading rate of 5 g COD L −1 d −1 . • Highest VFA production was obtained at pH 4.8 and 7.5 g COD L −1 d −1 . • The energy recovery of the coupled process achieved the 63.3% from food waste. • H 2 yield and VFA production rate is driven by dominant microbial consortia.