A systematic review on the occurrence and removal of microplastics during municipal wastewater treatment plants

This study uses a systematic review of Scopus, Web of Science, and ScienceDirect databases to investigate the removal rate of microplastics (MPs) at various stages of municipal wastewater treatment plants (WWTPs). In this context, 25 articles were included, which investigated 54 WWTPs. Results showed that the overall removal efficacy of MPs in studied WWTPs is about 88.1 (+/- 18.8) percent. The contribution of preliminary, primary, secondary, tertiary, and disinfection treatment in the removal of entry MPs are 37.4% (+/- 24), 62.7% (+/- 20.8), 63.7% (+/- 24.8), 62.8% (+/- 29.5), and 12.5% (+/- 15.7) (mean (+/- SD)), respectively. The critical aspects about MPs occurrence in WWTPs influent were summarized and on average 2267.2 (+/- 5376.4) MPs particles per liter (P/L) were present with a considerable wide range of 0.28 to 31400 P/L. In the next section, details about frequent shape (i.e., spherical (beads, pellets, granules), lines (filaments, fibers), films, fragments, and foams) of MPs in influent and effluent of included WWTPs and their removal efficacy during treatment steps are given. Finally, this study highlights future challenges in MP removal and suggests key areas for further research, such as improving treatment efficiency and understanding MP characteristics.