Quantitatively tracing microplastics in sewage sludge using thermodesorption gas chromatography/mass spectrometry combined with pyrolysis

Sewage sludge is a major source of environmental microplastics. To address this issue, a reliable mass-based methodology was developed for the identification and quantification of five frequently detected microplastics in sewage sludge-polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET)-using pyrolysis combined with thermal desorption gas chromatography/mass spectrometry (Py-TD-GC/MS). The method achieved detection limits ranging from 0.14 μg to 3.70 μg and was utilized to assess microplastics removal during wastewater and sludge treatment processes, covering sizes from 0.22 μm to 5.00 mm. Total microplastics concentrations decreased from 8714.59 ± 228.48 μg/g dry weight (dw) in primary sedimentation sludge to 3828.41 ± 557.92 μg/g dw in secondary sedimentation sludge, with PE, PET, and PVC as the dominant polymers. Size- and polymer-dependent deposition patterns were noted during wastewater treatment. Further treatments-including centrifugal dewatering, thermal hydrolysis, anaerobic digestion, and compression dewatering-reduced microplastics concentrations from 6033.18 ± 77.94 μg/g dw in raw sludge to 470.82 ± 48.08 μg/g dw in final sludge. Centrifugal dewatering significantly removed microplastics while thermal hydrolysis and anaerobic digestion had minimal effects. This study establishes a robust methodology for quantifying microplastics in sludge and insights into their removal processes, aiding source prevention for microplastics contamination.