Enhancing hydrothermal carbonization of drained chicken manure: Effects of process parameters on nutrients...

The use of leaching as a pretreatment is a promising strategy for increasing anaerobic digestion (AD) performance for chicken manure (CM). However, the insoluble residues (i.e., drained CM) from these leaching processes have been undescribed. This study aims to evaluate how the hydrothermal carbonization (HTC) process of drained CM redistributes its energy and nutrient content to its products, being hydrochar and process water. By doing so, an integrated approach combining leaching, HTC, and AD of its process water is proposed. The best energy yield from the drained hydrochar was obtained at 220 °C for 75 min, with 84.98 ± 0.06%. Both hydrochar from non-pretreated CM and drained CM showed potential as phosphorus fertilizer, with concentrations ranging from 23.4-35.8 g/kg to 16.1-28.8 g/kg, respectively. Methane yields of 202.0 ± 5.3 and 205.3 ± 25.0 NmL CH₄/g VS_added were obtained from the process water of drained CM-HTC at 180 °C-120 min and 234 °C-30 min, respectively. The use of hydrochar improved methane production by 15% and 41% under these conditions. The hydrochar and process water produced at 234°C for 30 min had a total energy recovery of 62% of the energy contained in the raw material. Leaching improved the quality of all output streams by enhancing the fuel properties and nutrient content of hydrochar (especially Ca, Mg and P) and increasing the biodegradability of the process water for AD. The integrated leaching-HTC-AD strategy supports circularity by minimizing waste, maximizing resource recovery, and promoting rural development through waste valorization.