Distribution of nutrients and heavy metals in a constructed wetland system

The distribution of nutrients and heavy metals was measured in a constructed wetland system designed for processing domestic sewage (150 person equivalent). The system consisted of a reed root zone filter of the vegetation submergent bed (VSB) type with horizontal flow of sewage, and a combination of ditches and soil filters. The facility was situated on the slope of a hill. Sewage was pretreated in an Imhoff tank and a trickling filter. Nitrogen, phosphorus and a number of heavy metals, including Mn, Cu, Zn, Cd, Pb, Cr, were measured in the various components of the constructed wetland system in the period 1995–1997. Nutrients were removed mostly in the VSB filter section and in the first dam of the cascade filter. Sediments found in ditches after several years of operation consisted of plant debris. In the first ditch the depth of sediments was 5–8 cm, while in the last one it was only 2–3 cm. The heavy metal contents of suspended matter decreased along the course of treatment, starting from the VSB filter, through the first ditch to the last ditch. The main mechanism for removal of dissolved metals was sorption on particulate matter suspended in subsequent ditches. Manganese concentration was controlled by redox conditions in the various sections of the system, and was substantial in ditches rich in organic matter. Nutrients analyses in reed plant samples indicated that accumulation in biomass played an insignificant role in the distribution of analysed elements in the system.