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 m 2 . 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.
Introduction
In the rural areas in Poland the farm buildings are usually scatered at long distances. Therefore applying an integral sewer system and treatment of sewage is economically unjustified. Only 3.8% of villages are connected to the sewer systems and just a half of these 3.8% have their sewage treated in the wastewater treatment plants.
The local treatment of sewage usually takes place in septic tanks that provide only mechanical treatment. Often these tanks are leaking and the outflowing sewage, in many cases, is not suffciently purified.
The quality standards for sewage discharged to the surface waters are severe in Poland, also in the case of small volumes of sewage. This situation asks for research and application of new technologies and solutions, sometimes not fully established. One of the new methods applied for sewage treatment are the wetland systems or constructed wetland systems.
Constructed wetland systems have become a very popular technology for removal of contaminants from domestic sewage. However, the new application of these systems is their utilization for removal of heavy metals. The potential of wetland systems for removal of heavy metals is the result of the lack of biological barrier in the macrophyte plants, due to which the passive adsorption of heavy metal ions occurs. Retaining of heavy metals in the wetland system may be also the result of binding by microorganisms. There are two mechanisms of heavy metals retention by microorganisms: formation of complex extracellular compounds composed of heavy metals and substances released by the cells and biosorption of the metal ions on the cells' surfaces.
Ozimek and Renman (1996) discovered a simple relationship between the distribution of heavy metals in the wetland environment and their content in the macrophyte plants.
The amount of heavy metals retained as well as the retention rate depend on the environmental factors such as the content of organic and mineral suspended solids in water and bottom sediments. Most of heavy metals are dissolved in water for just a short period of time since they are quickly adsorbed by the mineral and organic fraction of the bottom sediments and by the detritus suspended in the water.
According to Vymazal et al. (1998) the heavy metals precipitate in the form of carbonates and sulphides, produced in the redox reactions. This paper is aimed at evaluation of distribution of three heavy metal ions: Pb, Cu and Cd in the wetland ecosystem inhabitated with reed. Metals are supplied with domestic sewage from the Przywidz village near Gdansk.
The wetland system in Przywidz
The wetland system in Przywidz (the Pommeranian region), inhabitated with reed, was constructed as a pilot-scale one, for treating domestic sewage. Prior to inflowing to the wetland system, the sewage are treated in a conventional wastewater treatment plant consisting of an Imhoff tank and trickling biofilter. The amount of inflowing sewage equals to 22.5 m Gravitational flow of sewage is secured by the filter being situated on slope of a hill. After passing through the VSB filter, the sewage is directed to the first ditch of the upper cascade, where it is aerated. While the ditch is filling, the sewage percolates through the first dam to the lower ditch, again percolates through the second dam and so on. Sewage from subsequent parts of the constructed wetland was drained with a drainage pipe and collected in an inspection chamber. The so obtained outflow was directed to the next part of the wetland. Schematic illustration of the construsted wetland system in Przywidz is given in the Fig. 1. 
Materials and methods
In the period 1996-98 the measurments of efficiency of removal of heavy metals (Cd, Pb and Cu) from sewage were performed at the following sampling points:
1 -the inflow to the VSB (the inflow to the wetland system), 11 -the first ditch of the upper cascade KFTG-1 (effluent from the VSB), 14 -the last ( The amount of sewage was evaluated indirectly, knowing the time of operation of the pump and its average capacity (Q, = Q p s t). The pump was operated automatically, depending on the water level in the well. In order to measure the time of operation of the pump, the counting device (type NOD-LGS) was installed on the switchboard. The average capacity of the pump (Q p s ) was estimated knowing the volume of the control layer in the wet well of the  pumping station (V ,) and the time of draining (t) .
,e = Q p V, i-1
Results
The measured content of cadmium, copper and lead in various parts of reed collected from the VSB filter and from the fourth ditch of the lower cascade (KFTG-2) are presented in Tables I and 2 . The "total" contents in the table refer to the content of metals in samples collected in spring. summer and autumn. The highest accumulation of heavy metals was found in the rhizomes. The highest amounts of Pb and Cu in the rhizomes were present in summer while the amounts of Cd were highest in spring. The accumulation of these elements in subsequent parts of the system, as given in Table 2 .
The yearly average concentration of the suspended solids in sewage after the subsequent compartments of the wetland system and the daily load of the analysed metals are presented in Table 3 . The obtained results indicate significant role of the first part of the wetland systemthe VSB filter in elimination of heavy metals (69.6% Cd, 69.2% Cu and 64.3% Pb). The average efficiency of elimination of the analysed elements in the whole wetland system was equal to: 92.3% Cd, 89.4% Cu and 87.8% Pb.
TABLE 1
The contents of cadmium, copper and lead in different parts of reed (VSB) The results allowed for counting a simplified balance for the analysed heavy metals: Cd, Cu and Pb in the wetland ecosystem. The results are given in Table 4 . They confirm that substantial part of the analysed elements is retained in the filtering materials of the dykes and in the bottom sediments of the ditches. Accumulation in the reed biomass was much smaller and equaled to 14.8% Cd, 22.1% Cu and 13.8% Pb.
The results obtained from the sequential extraction of the filtering material of the dykes and for the bottom sediments are given in Tables 5 and 6 . The content of copper showed the biggest variations, the content of cadmium -smaller, while the content of lead changed slightly. Both the smallest amounts and the smallest variations of analysed elements were found in the filtering materials of the dykes. The biggest variations were noted in the bottom sediments.
The content of heavy metals decreased in the subsequent compartments of the wetland system. The highest amounts of all three metals were retained in the first stage of the system, that is in the VSB filter, while in the next stages (KFTG-1 and KFTG-2) the elimination of analysed elements was much smaller. Both the contents of Pb and Cu decreased slightly in the filtering material of the dykes, while there was almost no change in the contents of analysed elements in the ditches. 
TABLEtS

Sequence extraction in the filtering medium of the dykes in the constructed wetland system in Przywidz Fraction

Cd


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