Mass-based fates of microplastics throughout wastewater treatment processes

Sewage treatment plants (STPs) are crucial in removing microplastics from wastewater but also serve as essential point sources for microplastic pollution in environments. To address this issue by improving the microplastic removal efficiency, unveiling the mass-based fates of microplastics during treatments in STPs is essential, as these treatment systems are not specially designed to target microplastics. Herein, a thermodesorption-gas chromatography-mass spectrometry (TD-GC/MS) was developed to quantify the mass of five microplastics: polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethyleneglycol terephthalate (PET) in the range of 0.22 mu m-5.00 mm in size. Then, the approach was further applied to evaluate the mass-based fates of these microplastics throughout the whole STP treatment process. Results showed that PE was the most common polymer detected in influent (1313.11 +/- 336.96 mu g/L) and effluent (25.84 +/- 3.75 mu g/L), while PP was the least prevalent in influent (52.47 +/- 11.49 mu g/L) and PS in the effluent (2.74 +/- 0.26 mu g/L). Moreover, the proportion of the smallest particles (0.22-20 mu m) was dominant in all samples. Based on mass estimations, an average of 95.82 % +/- 2.02 % of microplastics were efficiently removed from the influent, with removal rates ranging from 92.55 % for PP to 98.03 % for PE. Moreover, the removal of microplastic mainly occurred in the primary treatment unit, and their removal efficiencies decreased as the microplastic concentrations decreased, regardless of their sizes and polymer types. To the best of our knowledge, this is the first report on the mass-based fates of microplastics in STP throughout wastewater treatment processes, enabling the comparison of microplastic removal efficiencies among studies to direct future improvements in the treatment process.