Nutrient recovery from Co-Hydrothermal carbonization of animal manures: Synergistic effects on hydrochar properties and agronomic potential

This study evaluates the effects of co-hydrothermal carbonization (Co-HTC) of poultry (PM), dairy (DM), and swine manure (SM) at 180, 220, and 260 °C on hydrochar yield, nutrient recovery, phytotoxicity, and heavy metal retention. Co-HTC of PM with DM (PMDM) and PM with SM (PMSM) were assessed for synergistic effects by comparing observed values to predicted additive outcomes based on individual HTC data. Hydrochar yield decreased with increasing temperature across all treatments; however, PMSM and PMDM exhibited synergistic enhancement in yield at 180 °C (synergistic coefficient: 1.13-1.23). Elemental analysis indicated that nitrogen retention was maximized in PMDM hydrochars (4.71 ± 0.61% N at 180 °C), while PMSM hydrochars exhibited superior retention of phosphorus, magnesium, and calcium. Most potassium leached into the co-HTC process liquid. Heavy metals (Zn, Cu, and Ni) were effectively immobilized, with Co-HTC resulting in lower concentrations of Mo, Pb, and Cr compared to individual HTC. Seed germination index (GI), used to assess phytotoxicity, revealed temperature- and feedstock dependent trends, with PMSM hydrochar produced at 180 °C achieving the highest GI (140.91 ± 7.05%), indicating synergistic reduction in phytotoxicity. These findings demonstrate that Co-HTC can optimize recovery of nutrients into hydrochar and enhance the agronomic and environmental quality of hydrochars through tailored feedstock interactions and process tuning.