Retention of selected heavy metals: Cd, Cu, Pb in a hybrid wetland system

The budget of heavy metals was investigated in a constructed wetland in a hybrid wetland system near Gdan«sk. It is a pilot wastewater treatment plant (WWTP) designed for 150 PE (person equivalent). The system consists of two sections: a vegetated submerged bed (VSB) with horizontal flow of sewage and a cascade filter situated on a slope of a hill. Total area of the constructed wetland is about 870 m2. Domestic sewage, after a conventional pretreatment (consisting of an Imhoff tank and a trickling filter) is pumped to the VSB filter located on the top of the hill and then flows through subsequent segments of the constructed wetland. In the period 1995-98 the measurements of several heavy metals (Cd, Cu, Pb) were carried out in sewage inflowing, outflowing and collected from the in between sections of the system. Moreover analysis of sediment collected in ditches of the cascade filter, VSB filter and reed were carried out. The content of heavy metals in suspended solids decreased along the course of treatment, starting from VSB filter, through the first ditch to the last ditch. Measurable concentrations of dissolved heavy metals were found in sewage collected from several subsequent ditches. Sorption was deemed the main mechanism of dissolved metals removal in subsequent ditches.