Effects of Operating Water Level Regulation on the Microplastic Distribution in Constructed Wetlands

Increasing human activities are introducing large amounts of microplastics (MPs) into the environment, which has proven to be harmful to ecosystems. Although many studies have focused on the removal of MPs from freshwater, research on the distribution of MPs in ecologically friendly, widely-used and cost-effective water treatment facilities with free water surface constructed wetlands (FWS-CWs), is lacking. In this study, we established four FWS-CWs at Shibalianwei constructed wetland with different operating water levels in August and December 2023 to compare the migration of MPs in FWS-CWs under different operating conditions. The removal efficiency of MPs in surface water was 21.93%, 25.51%, 71.19% and 59.34% in summer and 0.29%, 23.91%, 37.53%, and 70.07% in winter for the four FWS-CWs with water levels from low to high, respectively. The results demonstrate that FWS-CWs with higher water levels are more efficient at removing MPs and have shorter settling distances, whereas FWS-CWs with lower water levels can be affected by resuspension, which leads to lower removal efficiency. Removal efficiency is generally greater in the summer due to a more stable sediment–water interface. These factors affected the size distribution of MPs, with large MPs more likely to be resuspended in low-water-level FWS-CWs, whereas shape and type have less of an impact. To remove MPs more efficiently, the operating water level of FWS-CWs should not be lower than 30 cm.