Speciation of Nutrients in Hydrochar Produced from Hydrothermal Carbonization of Poultry Litter under Different Treatment Conditions

In this study, the effects of operating parameters on the behavior of nutrients during hydrothermal carbonization (HTC) of poultry litter (PL) were investigated. A number of HTC experiments were carried out using PL at different treatment temperatures, residence times, and initial pH. The standard measurement and testing protocol was adapted to determine the phosphorus (P) species in the solid hydrochar (HC), while the Ca, K, Mg, Na, Al, Zn, Mn, Fe, Cu, Cr, and Pb contents in each fraction were quantified and related to the P fractions. The results indicate that HTC can effectively reduce the solubility of most of the measured elements. The treatment temperature and initial pH can significantly influence the speciation of P and other nutrients, while the residence time effects were apparent at low treatment temperature. The majority of the measured nutrients remained in the HC when the hydrochar was produced without additional acid. Addition of acids, particularly H₂SO₄, provides a good approach for nutrient recovery.