Livestock production generates over a billion tons of manure annually, presenting a significant challenge in nutrient management. While manure is rich in nitrogen, phosphorus, and potassium, conventional land application leads to nutrient losses through volatilization and runoff, contributing to environmental pollution. Hydrothermal carbonization (HTC) offers a promising solution by stabilizing nutrients in hydrochar and concentrating them in process liquids, offering a sustainable alternative. Unlike individual HTC of poultry manure, which requires adding additional water as a reaction medium, Co-HTC has the potential to utilize inherent moisture (>95%) of swine manure and dairy manure, reducing water input while potentially improving nutrient recovery. This study investigates the Co-HTC of poultry manure with swine manure and dairy manure at three temperatures (180°C, 220°C, and 260°C) to evaluate the effect of temperature on nutrient retention and redistribution. Individual HTC of each manure was also done at the same temperatures to serve as a baseline for comparison with the Co-HTC process. Higher hydrochar yields from Co-HTC compared to individual HTC indicated a synergistic effect between different manure types, with hydrochar yield decreasing as temperature increased. Elemental analysis showed enhanced nitrogen retention in hydrochar after Co-HTC, where reduction in ammonium concentration was seen from ion chromatography of the process liquids. Further elemental analysis revealed improved carbon stability of hydrochar, with decreasing H/C and O/C ratios, along with increasing fixed carbon from proximate analysis and increasing temperatures, making them more suitable for long-term soil amendment. Fourier Transform Infrared Spectroscopy (FTIR) and Energy Dispersive X-ray Spectroscopy (EDX) confirmed the presence of functional groups and mineral elements in hydrochars with significant structural changes comparing Co-HTC and individual HTC. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was done on hydrochars and process liquids. The concentration of calcium, magnesium, and phosphorus increased in hydrochar. In contrast, the concentration of sodium and potassium increased in the process liquids after Co-HTC, suggesting nutrient-specific application for both the hydrochar and process liquids. These findings demonstrate that Co-HTC can optimize nutrient recovery from animal manure while reducing water input and enhancing hydrochar stability. The temperature-dependent trends provided insight into optimum HTC conditions for specific nutrient recovery goals.
