Hydrothermal carbonization of bioplastics and plastics: a promising approach for handling mixed waste

The growing production of biodegradable plastics and the frequent mismanagement of their end-of-life often lead to mixed waste streams containing both bioplastics and conventional plastics, which create challenges for mechanical recycling. Hydrothermal carbonization (HTC) has been proposed as a pre-treatment enabling chemical recycling or anaerobic digestion of bioplastics, but its application to bioplastic-plastic mixtures has not been investigated. In this study, seven plastics—two compostable bioplastics items (based on PLA, PBAT, and starch) and five conventional plastics (PET, HDPE, LDPE, PS, PP)—were subjected to HTC, individually and in mixtures. Results show that PLA and starch hydrolyse readily at 180 °C, PBAT requires at least 200 °C, and PET hydrolyses to monomers at 220 °C, while the three polyolefins and PS remain unaltered even at 250 °C. Processing mixtures shows no major effects on yields and compositions, but presents a notable practical advantage: bioplastics disintegrate and hydrolyse, dispersing or dissolving in the liquid phase, while conventional plastics coalesce into a compact solid block atop the liquid. This enables effective qualitative separation of bioplastics and plastics, allowing the former to be directed to anaerobic digestion or chemical valorisation and the latter to thermochemical processes.