Valorization of process water from hydrothermal carbonization of food waste by dark fermentation

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 gCOD 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 H2 L- 1 d- 1) was reached at pH 4.8 and OLR 5 gCOD L- 1 d-1, associated with a prevalence of Clostridium bacteria. The highest volatile fatty acids concentration (10.2 gCOD⋅L- 1) was achieved at pH 4.8 and OLR 7.5 gCOD 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.