Effects of biodegradable and conventional plastics on sulfur transformation during aerobic sludge composting

The increasing substitution of conventional plastics with biodegradable alternatives in wastewater systems raises important questions about their functional impacts on sludge composting processes. This study investigated how biodegradable polylactic acid (PLA) and conventional plastic polyethylene (PE) plastics influence sulfur transformation during a 35-day aerobic sludge composting system. Sulfur fractionation showed that available sulfur increased in all treatments during composting, while PLA consistently maintained significantly higher levels through the maturation stage compared with both PE and the plastic-free control. This pattern was primarily reflected by higher water-soluble and HCl-soluble sulfur fractions, suggesting that PLA was associated with sulfur fraction dynamics consistent with more oxidation-related sulfur transformation. Microbial community analysis revealed sustained compositional divergence under PLA, characterized by enrichment of sulfur-oxidizing Proteobacteria (e.g., Rhodanobacteraceae and Xanthobacteraceae), together with higher predicted abundances of sulfur oxidation-related functional groups, including sulfite and thiosulfate oxidation. In contrast, PE was associated with higher relative representation of sulfur reduction-related functional groups. Biofilm analyses further showed that PLA surfaces harbored higher abundances of sulfur-oxidizing bacteria (e.g., Thiobacillus and Halothiobacillus), consistent with functional predictions. Surface characterization confirmed pronounced polymer-specific differences, with PLA undergoing surface erosion and chemical modification during composting, while PE remained largely inert. Overall, these results suggest that replacing conventional plastics with biodegradable PLA may influence sulfur-related transformation patterns during sludge composting, highlighting that biodegradable plastic substitution may not be functionally neutral and may have implications for nutrient dynamics during compost maturation.