Iron and manganese in sediments of constructed wetlands with horizontal subsurface flow treating municipal sewage

In 2008, concentrations of iron and manganese in the sediments of seven constructed wetlands (CWs) with horizontal subsurface flow in the Czech Republic were evaluated. The surveyed constructed wetlands varied in the length of operation between 2 and 16 years at the time of sampling. In each constructed wetland three samples of sediment were taken in the inflow, middle and outflow zones to the depth of 0.6 m. The sample was divided into top (0-20 cm) and bottom (20-60 cm) sections, resulting in a total number of 18 samples in each system. In each sample the amount of sediment on dry mass basis was evaluated and concentrations of Fe and Mn in the sediment were determined using ICP-MS. The survey revealed that the amount of sediments in the filtration bed increases with the length of operation. In general, more sediments were located at the bottom layers due to the wastewater distribution near the bottom. Concentrations of manganese in the sediment were highest in the new systems and decreased with length of operation. With the exception of one CW, in all other constructed wetlands Mn concentration in the sediment was significantly higher within the top layer. Mn concentrations in sediments found in this study were found within the concentration range commonly occurring in natural unpolluted wetlands. Iron concentration in the sediment also decreased with the increasing length of operation but the dependence on the operation time was not as clear as for manganese. The concentration of iron was comparable with other studies from constructed wetlands treating sewage but higher as compared to polluted sediments which are usually highly anaerobic. The results clearly demonstrated that in order to evaluate the amount of iron and manganese in the filtration beds it is necessary to take into consideration both concentration of the elements and the amount of sediment accumulated in the filtration bed. Despite the lowest concentrations in the sediment the highest accumulated amount of both iron and manganese was found in the oldest CW Spálené Poříčí because of the highest concentration of sediment in the filtration bed.