Effect of the hydrothermal carbonization temperature on the conversion of process water produced from food waste in a mesophilic up-flow anaerobic sludge blanket digester

The conversion of process water (PW) produced during the hydrothermal carbonization (HTC) of organic waste in anaerobic digestion (AD) represents an interesting opportunity to reduce the organic load of this aqueous phase and increase the overall energy yield of the process. The AD of PW produced at two different temperatures (180 °C and 240 °C) in up-flow anaerobic sludge blanket digesters was carried out to determine the effect of HTC temperature on AD performance and the associated microbial communities. A stable operation at up to 3 g COD Lrx −1 d −1 was achieved for both temperatures. PW produced at 180 °C resulted in a 22 % higher methane yield and a more stable operation, indicating a lower content of inhibiting and recalcitrant organic compounds. A more pronounced shift from acetoclastic to hydrogenotrophic methanogens was observed with the PW produced at 240 °C, likely due to the harsher AD conditions caused by the higher concentrations of organic inhibitory and recalcitrant compounds generated at elevated temperatures. Feeding PW to the digesters led to the stratification of the granular sludge, independently of the used HTC temperature. While Archaea proliferated at the bottom of the digesters, the abundance and the diversity of bacteria from taxa typically associated with hydrolysis were higher at the top. • Conversion of process water produced at low and high temperatures is compared. • Low-temperature process water leads to higher stability and performance in AD. • Methane yield is 22 % higher for process water produced at 180 °C vs 240 °C. • Feeding process water causes microbial community stratification in UASB digesters.