Accumulation of Metals in the Sediment and Reed Biomass of a Combined Constructed Wetland Treating Domestic Wastewater

This study assessed the accumulation of Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn in the sediment and biomass of P. australis (Cav.) Trin. ex Steud. in a combined constructed wetland (CW) designed for the treatment of domestic wastewater of 750 population equivalents. The CW consists of two vertical subsurface flow (VSSF) reed beds followed by two horizontal subsurface flow (HSSF) reed beds. The sediment in the VSSF reed bed was contaminated with Cu (201 ± 27 mg kg-¹ DM) and Zn (662 ± 94 mg kg-¹ DM) after 4 years of operation. Concentrations of Cd, Cu, Pb and Zn in the sediment generally decreased along the treatment path of the CW. On the contrary, higher Al, Cr, Fe, Mn and Ni concentrations were observed in the sediment of the inlet area of the HSSF reed bed. Redox conditions were presumably responsible for this observed trend. Metal concentrations in the reed biomass did not show excessive values. Accumulation in the aboveground reed biomass accounted for only 0.5 and 1.4% of, respectively, the Cu and Zn mass load in the influent. The sediment was the main pool for metal accumulation in the CW.