Abstract
In this study, we propose an integrated process to valorize the residual wet streams from food processing industries and their high water content considering apple pomace as the model substrate. The process consists in three combined steps: Subcritical water pretreatment (SubH2O), hydrothermal carbonization (HTC) of the pretreated solid fraction and anaerobic digestion (AD) of the pretreated liquid fraction combined with the HTC solid product (i.e., hydrochar). Both SubH2O and HTC were run in series in the same vessel, a 250 mL pressurized batch reactor equipped with one thermocouple and a pressure transducer. The SubH2O step was optimized using robust parameter design to assess the most effective factors affecting the treatment (temperature, pressure, and biomass-towater ratio). According to the preliminary results, subcritical water deconstructs the apple pomace matrix, solubilizing the organic compounds and increasing the percentage of volatiles. Such increase is the consequence of the enhanced reactivity of the substrate, which is now more likely to be treated. Moreover, it is expected that adding the hydrochar from HTC to the AD process will improve the biogas quality in terms of methane concentration per unit of volume.