Hydrothermal carbonization and liquefaction (HTC/L) are thermochemical conversion technologies that valorize wet wastes into sustainable fuels. In HTC/L, subcritical water is both the solvent and a reactant for hydrolysis, decarboxylation, and decarbonylation reactions that convert biomass to solid hydrochar, biocrude, and aqueous phase (AP). However, the leftover AP process water is recalcitrant and cytotoxic, which limits the implementation of HTC/L technologies beyond pilot scale. While anaerobic digestion can treat this waste, it requires orders of magnitude more time than is used to produce the AP. Aerobic digestion is an intriguingly faster alternative. Gluconobacter oxydans is an aerobic bacterium with potential industrial applications in fermentation and biomining. G. oxydans undergoes an incomplete citric acid cycle to oxidize alcohols to organic acids, which enables its survival at low pH, including that of HTC/L AP (3-4). Using a model food waste as a hydrothermal feedstock, we separate biocrude from HTC/L AP produced at six different temperatures. A subset of AP is extracted with dichloromethane to recover additional biocrude. Then we separately mix extracted and unextracted AP with yeast peptone mannitol growth media (YPM) at varying concentrations to grow G. oxydans. We find that G. oxydans can grow in as-carbonized AP concentrations up to 50 % (volume) for higher temperature-produced AP, and that growth is increased over a YPM-pure water blank at as-carbonized concentrations up to 25 %. Extraction of organic biocrude further increases G. oxydans survivability, particularly for higher temperatures. After bacterial digestion, concentrations of organic acids increase, while furfurals and phenols decrease; residual 5-hydroxymethylfurfural in the AP decreases by over 90 %. Overall, this work demonstrates the viability of using aerobic digestion to process fugitive carbon in HTC/L AP and reduce the cost of G. oxydans production by replacing conventional YPM with up to 25 % as-processed, or 50 % post-DCM extracted HTC/L process wastewater.

Matteo Pecchi

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