Fate and removal of microplastics in unplanted lab-scale vertical flow constructed wetlands

Many studies have reported conventional wastewater treatment plants as one of the main sources of microplastics (MPs). However, constructed wetlands (CWs) as a nature-based wastewater treatment system have received little attention. This study investigated the influence of biofilm, media type and earthworms on the fate and removal of MPs in a short-term (45d) experiment with unplanted lab-scale vertical flow CWs (VFCWs). In sand-filled VFCWs, MPs were retained in the first 10 cm, and the removal efficiency was 100%, regardless of the presence of a biofilm. When gravel was used as filling material, the removal efficiency of MPs was stable at 96%, but the MPs were distributed throughout the 80 cm high VFCWs. In the presence of earthworms, the maximum depth that MPs reached within sand-filled VFCWs increased from 10 to 15 cm. Furthermore, the MPs concentration at a depth of 3–6 cm and 6–10 cm increased 2 and 10 fold respectively compared to the same VFCWs without earthworms. Although no MPs were detected in the sand from deep layers (15–80 cm), transport of MPs from top to the bottom by earthworms was found, and a few MPs were detected in the effluent, leading to a removal efficiency of 99.8%. This study indicated that both a higher media grain size and the presence of earthworms have a small effect on the removal efficiency of MPs in VFCWs, but the effect on the distribution of MPs was considerably. Longer-term studies in full-scale CWs are advised to perform under the influence of more practical factors. [Display omitted] •Porous media of vertical-flow constructed wetlands (VFCWs) can remove microplastics.•Small size and spherical microplastics showed higher mobility in lab-scale VFCWs.•Microplastics were distributed throughout the full height of gravel-filled VFCWs.•Sand-filled VFCWs retained more than 98.8% of microplastics in the top 10 cm layer.•Earthworms can transport microplastics to the bottom of VFCWs and egest them.