Green Route for the Upcycling of Urban Bioplastic Waste: A Biotechnological Resource Recovery Strategy toward Medium-Chain Fatty Acids Production

Abstract

Bioplastics are expected to represent an expanding fraction of urban waste. Recycling and upcycling remain still limited, yet bioplastics are suitable feedstocks for producing value-added products. Developing circular valorization pathways is essential to prevent resource loss and environmental degradation. This study presents an integrated chemo-biotechnological strategy to upcycle bioplastic waste toward medium-chain fatty acid production. Hydrothermal pretreatments were optimized to chemically recycle typical domestic items, such as thermoplastic starch-based bags and poly(lactic acid)-based cups. The yield of carbohydrates extracted from the bag was 86 wt % at 134 °C, 20 min, while the yield of lactic acid from the cup was around 40 wt %. The resulting enriched hydrolysates were utilized in cascade fermentation batch tests using a mixed microbial culture. The system successfully converted the recycled carbohydrates into ethanol, acetate, and butyrate, subsequently leveraging the recycled lactate as an electron donor for anaerobic chain elongation toward medium-chain fatty acids. This process yielded caproate and caprylate concentrations of up to 3000 mgCOD/L and 1300 mgCOD/L, respectively. By decoupling chemical deconstruction from microbial synthesis, this integrated approach enables the recovery of strategic building blocks that are otherwise lost in incineration or landfilling practices. Overall, this study establishes a robust groundwork for transitioning from fossil-dependent chemical production to a waste-based circular bioeconomy, offering a proof of concept to demonstrate the feasibility of upcycling mixed urban bioplastics into platform chemicals.