Distribution of heavy metals in various compartments of the constructed wetland system in Przywidz

Abstract

In the last decade constructed wetlands have become a very popular technology for removal of contaminants from domestic sewage. They are also assesed from the point of view of their capacity for removal of toxic heavy metals and organic substances resistant to degradation. Constructed wetland in Przywidz localized about 60 km from Gdansk is a pilot wastewater treatment plant (WWTP) designed for 150 PE (person equivalent). The system consists of two sections: vegetated submerged bed (VSB) with horizontal flow of sewage and a cascade filter situated on a slope of a hill. Domestic sewage after a conventional pretreatment ( consists of an Imhoff tank and a trickling filter) is pumped to the VSB filter located on slope of the hill. Total area of constructed wetland is about 870 m². In the period 1995-98 the measurments of several heavy metals (Cd, Cu, Pb) were carried out. These measurments were carried out in inflowing and outflowing sewage and as well in samples collected from each section of constructed wetland system. In particular analysis of sediment collected in ditches of the cascade filter, filling material of the dykes, VSB filter and plants were carried out. It was found out that content of heavy metals in suspended soil decreased along the course of treatment, starting from VSB filter, through the first ditch to the last ditch. Measurable concentration of dissolved heavy metals were found in sewage collected from several subsequent ditches. The main mechanism of removal on particular matter in subsequent ditches was sorption.