Degradation dynamics of Mater-Bi and crystalline PLA during anaerobic co-digestion of household organic waste and wastewater sludge, and phytotoxicity assessment of digestate

The increasing use of biodegradable bioplastics, such as Mater-Bi (MB) and Crystalline Polylactic Acid (CPLA), as alternatives to traditional polymers has raised concerns about their degradation and environmental impact. This study examines the biodegradability of MB and CPLA during the anaerobic co-digestion of organic municipal solid waste (OFMSW) and thickened sewage sludge in semi-continuous lab-scale reactors operated under mesophilic conditions. Bioplastics were tested at different concentrations (0-8 %) under two operating phases with varying organic loading rates (1 or 3 gVS/L·d for MB and 1 or 2 gVS/L·d for CPLA) and retention times (14 or 21 days). Biomethane production, process stability, and degradation efficiency were monitored. Results show limited degradation, with MB degrading faster than CPLA (23 % vs. 15 % weight loss after six weeks). Most MB degradation is due to abiotic factors. Higher bioplastic concentrations destabilise the process, reducing methane yield and causing volatile fatty acid spikes. Phytotoxicity tests indicate that the digestate may be suitable for agriculture, though bioplastic concentration impacts require further study. These findings stress the need to identify more biodegradable biopolymers, improve waste management, optimise anaerobic digestion, and screen digestate for agricultural use. This study is among the first to combine semi-continuous anaerobic digestion with abiotic degradation tests and phytotoxicity assessment, providing a comprehensive evaluation of bioplastic behaviour and its implications for digestate reuse.