N and P Recovery Pathways in HTC of Solid Digestate from Winery Supply Chain

The solid fraction of digestate, a major by-product of anaerobic digestion, represents a potential source of nutrients and energy. In Europe, digestate solid fraction production is projected to rise from 31 Mt (2022) to 177 Mt in 2050, highlighting the urgency for sustainable valorization strategies. This study explores Hydrothermal Carbonization (HTC) of digestate solid fraction derived from winery supply chain, to recover phosphorus (P) and nitrogen (N) in the liquid fraction while improving the energy content of the solid hydrochar. A systematic investigation was conducted on key HTC parameters: temperature (180-250 °C), reaction time (10-60 min), solid/liquid ratio (1/10-1/20), and acetic acid concentration (0-0.5 M), to assess their impact on nutrient recovery and hydrochar quality. Obtained results showed that P recovery was favored at lower temperatures, shorter times, higher acetic acid concentrations, and lower S/L ratios, while N recovery increased with higher temperature and longer reaction time. Explored HTC process condition allowed the tuning of P/N recovery ratios (0.2-1.4) in the liquid fraction, which could be suitable as a biomass-derived fertilizer. Solid hydrochar was obtained with carbon content and heating value (~16 MJ/kg) higher than those of the starting digestate. This study also demonstrates the use of acetic acid solutions as an HTC media; this confers circularity to HTC, since acetic acid is naturally present in anaerobic digestion and in winery supply chain. Overall, HTC emerged as a possible candidate to offer a dual benefit: generation of a nutrient-rich fertilizer and a sanitized, energy-dense solid, supporting circularity and sustainable management of agro-industrial residues.