Co-composting of biodegradable plastic with food waste digestate: Influence of co-composting strategy on plastic fate and nitrogen transformation

Given that biodegradable plastics (BDPs) are progressively replacing petroleum-based packaging materials, they are frequently present in food waste and its digestate (FWD). This study investigated BDP degradation and nitrogen transformation during co-composting of FWD as main materials (FWD-BDP treatment) versus co-composting FWD with dewatered municipal sludge (DMS) (FWD-DMS-BDP treatment). BDPs were added at 1% and 10% (dry weight basis). All the treatments maintained a thermophilic phase (>55 °C) for over 5 days. The FWD-DMS-BDP treatment significantly accelerated BDP degradation, with weight loss rates reaching 50.88–66.53% by day 15, while the FWD-BDP treatment achieved only 8.20–15.21%. In the FWD-DMS-BDP treatment, the DMS addition provided hydrolytic enzymes and diverse microbial communities that enhanced BDP surface erosion. For nitrogen transformation, DMS increased the abundance of amoA, leading to the highest NH₄+ -N levels in T3. The 1% BDP addition elevated the nosZ/nirK ratio and reduced N₂O emissions by 31.93% in T1, while the 10% BDP addition increased N₂O emissions via anaerobic microzones. All amendments reduced cumulative NH₃ emissions by 32.64–42.68% and improved germination indices to > 87% compared to CK. The PLS-PM analysis revealed that functional genes and physicochemical properties directly governed nitrogen conversion. The FWD-DMS-BDP treatment created synergistic effects: the DMS accelerated the BDP degradation, the BDP provided carbon sources and structural support, and their combination optimized the microbial microenvironment. The 1% BDP dose is recommended as it promotes organic matter degradation and ensures compost maturity while minimizing greenhouse gas risk. This study provides a scientific basis for valorizing multi-source organic wastes containing BDP residues.