Treated wastewater disturb the distributions of microplastics in their receiving watersheds

Microplastics (MPs) are ubiquitous in aquatic environments, threatening the security of aquatic organisms. Identifying the emission node and hot spot of MPs holds significant importance in the pollution control of MPs. Wastewater is widely recognized as sink and source of MPs, while the direct evidence is insufficient. To confirm the impact of the treated wastewater on the distribution of MPs in their receiving watersheds, MPs in surface river water and sediment samples collected around a typical industrial (petrochemical) and a municipal wastewater treatment plant (PWWTP and MWWTP) were identified. Results revealed that effluent from the PWWTP and water near the outfall of the MWWTP owned the most abundant MPs of 1280, and 723 items/L, respectively. Moreover, abundances of MPs in samples collected downstream of WWTPs (795 items/L in surface river water, 2142 items/kg in sediment for PWWTP, and 517 items/L, 1057 items/kg for MWWTP) consistently exceeded those of upstream (639 items/L, 926 items/kg for PWWTP, and 417 items/L, 700 items/kg for MWWTP). In addition, the discharge of treated petrochemical wastewater elevated the diversity of MPs while treated municipal wastewater elevated the relative content of narrow MPs or fibers in their downstream watersheds. In summary, treated wastewater do influence the distribution of MPs within their receiving watersheds.