Alkaline co-hydrothermal valorisation of digestate and PET waste toward upgraded hydrochar fuels

Increasing prevalence of plastic residues in anaerobic digestate presents emerging environmental and operational challenges for waste management systems. Conventional treatment approaches typically require mechanical separation of heterogeneous biomass–plastic mixtures prior to processing, which is labor-intensive, energy demanding, and costly. This study proposes a co-hydrothermal treatment (co-HTT) strategy for the direct co-valorisation of digestate and polyethylene terephthalate (PET), thereby eliminating the need for prior separation and improving overall resource recovery. Unlike most reported hydrothermal co-processing studies that operate at high temperatures, often approaching the supercritical water regime, this work demonstrates effective conversion under mild subcritical conditions (200 °C) using alkaline catalysis, offering a more energy-efficient alternative. At an optimal digestate:PET ratio of 1:1 (w/w), the co-HTT process produced hydrochar with significantly enhanced fuel characteristics relative to digestate-derived hydrochar. Nitrogen, oxygen, and sulfur contents were reduced by 91.71%, 49.36%, and 6.56%, respectively, compared with raw digestate, and by 76.70%, 52.31%, and 19.94%, respectively, relative to hydrochar obtained from sole digestate treatment. These reductions indicate a substantial potential for lowering NO_x and SO_x emissions during combustion. The resulting hydrochar also exhibited improved higher heating value (21.56 MJ kg−1), energy recovery efficiency (22.83%), and favorable combustion indices, demonstrating enhanced energy performance. Importantly, the feasibility of the approach was validated using real PET waste, confirming its applicability for integrated valorisation of mixed biomass–plastic residues. Overall, this study presents the first demonstration of low-temperature co-hydrothermal valorisation of digestate and PET, providing a lower-energy pathway for converting mixed biomass–plastic wastes into value-added solid fuel.