Composting of anaerobically treated bioplastic-containing organic waste: behavior of polylactic acid (PLA) and poly(butylene 2,5-furanoate) (PBF) and quality of final compost

The increasing use of bioplastics in food packaging and consumer goods requires a clear understanding of their fate within real waste management systems in order to avoid environmental pollution. This study investigated the composting process of digestates obtained from the anaerobic digestion (AD) of polylactic acid (PLA) and poly(butylene 2,5-furanoate) (PBF) co-treated with the organic fraction of municipal solid waste (OFMSW), focusing on polymer transformation, compost quality, and environmental implications. After mesophilic AD, the residual digestates containing degraded PLA and nearly intact PBF fragments were subjected to controlled aerobic composting for 90 days under simulated full-scale conditions. Temperature and aeration were monitored to ensure the proper succession of mesophilic, thermophilic, cooling, and maturation phases. The resulting composts were characterized for physicochemical parameters, including C/N ratio, total organic C, total Kjeldahl N, NH₄⁺-N, water-extractable organic C (WEOC), and water-extractable N (WEN), while residual polymer fragments were examined using FTIR-ATR spectroscopy and optical microscopy. Germination tests were performed to assess phytotoxicity and agronomic suitability. Results showed that the sequential AD-composting process ensured complete mineralization of PLA, with no detectable residues already at the end of the AD stage. The compost derived from PLA-containing digestate exhibited stable organic matter, showing a C/N ratio of about 22, a WEOC/WEN ratio around 10, and low NH₄⁺-N. Conversely, PBF displayed strong recalcitrance, persisting as visible fragments even after composting. FTIR-ATR analysis revealed only minor surface modifications, suggesting that aerobic treatment did not significantly alter the polymer's molecular structure. Nevertheless, the compost obtained from the PBF-containing digestate showed good stabilization, displaying a C/N ratio of approximately 21, a WEOC/WEN ratio of about 10, along with limited NH₄⁺-N content. Germination assays revealed noticeable phytotoxicity at compost concentrations above 25%, whereas at 25% dilution the germination index reached 73% and 57% for the PLA- and PBF-derived composts, respectively. These results indicate that composts from PLA-containing digestates may be suitable for agricultural application after adequate dilution or blending with mature compost, whereas those derived from PBF require careful management due to the persistence of undegraded residues. From a sustainability perspective, the integrated AD-composting approach supports energy recovery from OFMSW while generating partially stabilized composts. However, the resistance of PBF to both anaerobic and aerobic degradation highlights the need for polymer redesign or tailored end-of-life strategies to prevent long-term environmental accumulation. Overall, this study underscores the value of combining physicochemical and agronomic evaluations to accurately assess the biodegradability and environmental fate of emerging bioplastics within circular organic waste management systems.

 

This work has been funded by the European Union – NextGenerationEU under the Italian Ministry of University and Research (MUR) National Innovation Ecosystem grant ECS00000041 - VITALITY - CUP J97G22000170005.