Harnessing the aqueous phase from hydrothermal liquefaction of plastic and non-plastic waste for the production of carboxylic acids

The accumulation of plastic waste, with global recycling rates below 10 %, underscores the urgent need for alternative valorisation strategies. A major challenge is “Plasmix,” a heterogeneous mixture of non-recyclable plastics typically sent to incineration or landfill. Hydrothermal liquefaction (HTL) is a promising technology for processing such waste, producing crude oil, gas, char, and an organic-rich aqueous phase (HTL-AP). The latter, typically regarded as a waste stream, holds potential to be valorised through biotechnological processes due to its high nitrogen and carbon content. This study investigates the feasibility of anaerobic fermentation (AF) to generate carboxylic acids (CAs) from HTL-APs derived from Plasmix and its main thermochemically decomposable components (polyurethanes, cellulose, polyethylene terephthalate-PET), and from polystyrene. Two HTL processing times (2 and 4 h) applied to Plasmix were compared, showing minor effects on the conversion yields into CAs, which reached up to 33 % on COD (Chemical Oxygen Demand) basis. In contrast, feedstock identity strongly influenced fermentability. PET-derived HTL-AP performed best, obtaining 327 ± 67 mgCOD/L of CAs (with a conversion yield of 53 ± 11 %, COD/COD). Polyurethane- and cellulose-derived HTL-APs reached lower yields (up to 20 %, COD/COD), while polystyrene was largely non-convertible (13 ± 2 %, COD/COD). This research represents the first application of HTL to Plasmix along with the assessment of HTL-AP fermentability. Results demonstrate that plastic-derived HTL-APs can be converted into CAs, with yields reflecting polymer type, while the overall acid composition remains broadly consistent. Combining HTL with AF therefore offers a promising route for resource recovery from non-recyclable plastics.