Nationwide characterization of microplastics in sewage sludge as a baseline for sludge quality assessment and valorization planning.

Wastewater treatment plants (WWTPs) are recognized as major sinks for microplastics (MPs), transferring a substantial proportion of particles from wastewater to sewage sludge. However, nationwide assessments of MP contamination in sewage sludge remain scarce. This study investigated the occurrence, characteristics, diversity, and environmental risks of MPs in sewage sludge collected from 65 municipal WWTPs across Taiwan. MP concentrations ranged from 76 × 103 to 519 × 103 items/kg dry weight, with an average concentration of 224 × 103 ± 104 × 103 items/kg. Polyethylene (4.3-99.1 %) and polypropylene (0-87.5 %) dominated the polymer composition, while fragments (46.9-94.2 %) and particles between 51 and 300 μm represented the predominant morphological fractions. Microplastic abundance was positively correlated with the smallest particle size fraction (25-50 μm), overall particle diversity, ash content, and Pb and Mn concentrations, indicating the co-accumulation of particulate contaminants and increasing heterogeneity with higher contamination levels. Environmental risk assessment demonstrated that all sewage sludge samples were classified as polluted, with most WWTPs falling into heavily or extremely polluted categories according to the Pollution Load Index. Furthermore, elevated Polymeric Hazard Index and Potential Ecological Risk values highlighted considerable environmental risks associated with sewage sludge. These findings demonstrate that sewage sludge represents an important reservoir of microplastics and associated contaminants. As Taiwan continues to promote circular economy strategies and resource recovery from sewage sludge, future sludge management should incorporate microplastic abundance and polymer-specific hazards alongside conventional pollutants to ensure environmentally sustainable sludge reuse.