Hydrothermal Carbonization of Anaerobic Digestate: Compositional and Textural Characterization of Hydrochar in the Context of Sustainable Soil Amendment Use

Hydrothermal carbonization (HTC) converts digestate into hydrochar for potential soil applications. This study characterized hydrochar produced from anaerobic digestate in a single batch at 180 °C for 6 h with sulfuric acid addition to evaluate properties relevant to sustainable circular resource utilization and potential soil amendment use. Analyses included elemental composition, carbon and nitrogen fractions, pH, electrical conductivity, and pore structure. The hydrochar contained 42.70% total organic carbon and macronutrients, including 2.22% N, 0.51% P, 0.67% K, 1.91% Ca, and 0.39% Mg on a dry matter (DM) basis. The pH(H2O) was 5.75, while electrical conductivity was 42.55 mS·cm−1. Ni (179 mg·kg−1 DM) exceeded the indicative 50 mg·kg−1 benchmark for organic soil improvers under Regulation (EU) 2019/1009, although the source of the elevated Ni was not resolved. Cr (417 mg·kg−1 DM) was not speciated, preventing comparison with the Cr(VI) criterion. N2 physisorption showed a BET surface area of 31 m2·g−1, limited mesoporosity, and no measurable microporosity. The composition supports investigating carbon and nutrient inputs to soil, while acidity, soluble salts, and elemental constraints need to be evaluated under relevant soil conditions before the potential for sustainable soil application can be assessed.