Design parameters affecting metals removal in horizontal constructed wetlands for domestic wastewater treatment

• 8 wetland configurations were assessed for metal removal under the same conditions.
• As, Fe and Mn, sensitive to redox state, were released from subsurface flow systems.
• Free water surface flow favoured the removal of arsenic, iron and manganese.
• Subsurface flow slightly enhanced the removal of copper and lead.
• Unplanted wetlands promoted long-term removal thanks to steady reduced conditions.

As regards constructed wetlands (CWs), there is a great deal of research on metals removal, although comparison of different parameters under the same conditions is scarce. The aim of this study was to determine the most important factors affecting the removal efficiency and dynamics of metals and metalloids according to different configurations of horizontal CWs. An experimental plant, including the most commonly used CWs, was analysed for several metals (Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Se, Sn and Zn). Arsenic, which was under the detection limits at the influent, presented a release in those wetlands with subsurface flow (SSF) and followed the same pattern as iron and manganese. The presence of vegetation and flow type were key design factors affecting metals removal from urban wastewater. Free water surface (FWS) flow provided favourable conditions for the removal of As, Fe and Mn, which are sensitive to redox changes, whereas SSF slightly enhanced the removal of other metals, such as Cu or Pb. On the other hand, vegetation was not able to maintain steady oxidised conditions to guarantee redox dependent metals removal by combination with oxides in SSF systems. In contrast, constant reduced conditions promoted the long-term removal of metals by sulphide combination and precipitation.