Occurrence and removal of microplastics in wastewater treatment plants and drinking water purification facilities: A review

Wastewater treatment plants (WWTPs) are important sources of microplastics (MPs), having a high daily discharge of MPs despite a relatively high removal efficiency of MPs. Recently, MPs have been detected in drinking water purification facilities (DWPFs), indicating a direct threat to humans. In this review, we summarize published papers on the occurrence and removal efficiency of MPs, focusing on the status of removal mechanisms through different unit operations and processes. For WWTPs, the mass concentrations of MPs in influent ranged from 61 to 5,600 µg/L, and 0.5–170 µg/L of MPs were found in treated effluent, representing an overall removal efficiency of 93.8%–99.8%. Due to limited publications using mass as MP concentration, the comparison of MP abundance among studies was based on the number concentration. MPs have been found in influents at levels of 1.01–31,400 items/L, and in treated effluents at 0.004–447 items/L, with a high variation of MP removal from 10.2% to 99.9%. Preliminary and primary treatments contribute to MP removal, while the efficiency of secondary and tertiary treatments is highly dependent on the applied techniques. Disinfection has a relatively high potential to breakdown MPs into smaller sizes. For DWPFs, MPs in raw water ranged from 1 to 6,614 items/L, and 1–930 items/L of MPs were found in treated water, representing an overall removal efficiency of 66.9%–100% regardless of the treatment types. Yet, the understanding about the role of DWPFs on MP removal is inadequate. Although many studies on WWTPs have been published, many issues have yet to be resolved, and further research is warranted on standardization of analytical methods and the removal mechanisms behinds to develop effective remediation of MPs in WWTPs and DWPFs.