Microplastic abundance, characteristics and removal in large-scale multi-stage constructed wetlands for effluent polishing in northern China

Wastewater treatment plants (WWTPs) are considered to be an important source of microplastics (MPs) to the aquatic environment. Multi-stage constructed wetlands (CWs) are widely used for the advanced treatment of effluent from the WWTPs. Research on the abundance, characteristics, and removal of MPs in multi-stage CWs is limited. Therefore, we studied two large-scale multi-stage CW systems (Lingang Ecological Wetland Park and Konggang CW system) in Tianjin, China. The removal efficiency of MPs in two CW systems was greater than 89.0% (from 8.4 to 28.9 particles L-1 in the influent to 0.2-0.9 particles L-1 in the effluent). During the treatment, the horizontal subsurface flow CWs (HSSFCWs) in Lingang Ecological Wetland Park and the integrated vertical flow CWs (VFCWs) in Konggang CW system removed 82.8%-88.9% of MPs. 27.3%-60.0% of MPs were removed by surface flow constructed wetland (SFCW) in two CW systems. Among detected MPs, five types of polymers were identified by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), including polyvinyl chloride (PVC), polystyrene (PS), polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE). The characteristics of MPs, such as sizes, shapes, and types influenced the removal of MPs in various treatment processes in two CW systems. Specifically, microfibers could be intertwined to form coils by hydraulic action, and effectively removed by HSSFCWs. Results of the present study indicated that large-scale multi-stage CW systems could substantially reduce the MP pollution discharged from WWTPs into the aquatic environments, and provided a theoretical basis for understanding the migration and distribution of MPs in the aquatic environment.