Direct upcycling of unrecyclable household waste into biodegradable high‑performance biocomposites

Abstract In the UK, post-consumer waste is sent to materials recovery facilities (MRFs) to separate the metals, plastics, paper and cardboard for subsequent recycling. However, a significant portion of materials sent to MRFs are rejected. Here, we demonstrate a pathway that directly transforms the recovered lignocellulosic pulp (herein termed recovered solids, RS) from MRF rejects and its enzymatic post-hydrolysis solids (PHS) into biodegradable poly(L‑lactide) (PLLA) composites. We found that both RS and PHS act as effective nucleating and reinforcing agents, increasing the crystallinity of PLLA and its tensile modulus to ~6 GPa while maintaining comparable strength ( ~ 60 MPa). By comparison, unreinforced PLLA at a comparable crystallinity possessed only a tensile modulus and strength of 2.5 GPa and 42 MPa, respectively. The resulting biocomposites also show accelerated biodegradation under simulated industrial composting compared to PLLA, achieving complete disintegration within 30 days. This work provides the first demonstration of directly upcycling unrecyclable MRF reject streams into structural, fully compostable materials, revealing a viable, low‑carbon route to integrate post‑consumer and secondary wastes into a closed‑loop circular economy.