Coupling hydrothermal carbonization and anaerobic digestion for cooking plantain peel valorization: enhanced methane production and soil carbon dynamics

The valorization of cooking plantain ( Musa paradisiaca ) peel biomass (MPB) remains underexplored despite its abundance and nutrient-rich composition. This study evaluated an integrated hydrothermal carbonization (HTC)–anaerobic digestion (AD) approach for MPB valorization, targeting methane production and the production of value-added soil improvers. MPB was subjected to HTC at 180 and 200 °C for 1 and 2 h, producing four hydrochars. The hydrochar obtained at 200 °C for 2 h (HC-200-2) was evaluated as an AD additive in batch assays with cow manure under mesophilic conditions at dosages of 6, 12, and 18 g L −1 . Resulting digestates were characterized, and carbon mineralization was assessed during an 83-day incubation of sandy soil amended with HC-200-2 and hydrochar-derived digestates at a rate equivalent to 9 Mg ha −1 . HTC increased total carbon content by up to 36 % and fixed carbon by up to 50 % at the highest process severity. Hydrochar amendment at 18 g L −1 boosted manure-to-methane conversion yields by 35 % (net of hydrochar-only controls), whereas no statistically significant enhancement occurred at 6 and 12 g L −1 . The digestate obtained at 18 g L −1 hydrochar addition showed the highest model-estimated one-year humification coefficient (45.9 %), and effective organic matter (311 kg Mg −1 dry matter). Overall, integrating HTC and AD offers a circular strategy for MPB valorization by enhancing methane yields while producing digestates with lower carbon mineralization under the tested soil conditions.