Identification and assessment of microplastics in wastewater treatment plants using laser direct infrared spectroscopy and depolymerization-coupled liquid chromatography–tandem mass spectrometry

The pollution levels and distribution characteristics of microplastics (MPs) in wastewater treatment plants (WWTPs) need to be well explored and evaluated for better control of MPs. The use of a single method may be inadequate to achieve comprehensive information on MPs. Studies using combinations of different analytical methods are rare and challenging. In this study, we combined laser direct infrared spectroscopy and depolymerization-coupled liquid chromatography-tandem mass spectrometry, for the first time, to explore and evaluate variation and distribution characteristics of MP number and mass concentrations in wastewater treatment systems. The removal rates (RRs) based on mass and number concentrations were inconsistent. The overall RRs for system A and B calculated by mass concentration were 96.8 % and 96.5 % respectively, whereas those by number concentration were 95.7 % and 97.0 %, respectively. However, the trend of MP removal across different treatment stages in the two wastewater treatment systems was consistent, as estimated using both applied methods. A significant correlation (R² > 0.88) was observed between the mass concentrations of polyethylene terephthalate (PET), polycarbonate (PC), and polyamide (PA) MPs and number concentration of total MPs. The proportion of MPs varied by polymer type, size range, and shape throughout the treatment process. Based on the annual wastewater discharge estimate, approximately 1.7 × 10 MP particles are released into the aquatic environment through effluent each year, with the total mass of PET, PC, and PA microplastics amounting to approximately 0.369 tons. Overall, this study provided valuable information for MP estimates, management planning, and control measures.