An assessment of metal removal by a laboratory scale wetland

This paper presents a preliminary assessment of the heavy metal removal performance by a laboratory scale wetland based on the design of a constructed wetland treating surface runoff. Laboratory scale gravel-substrate subsurface-flow wetlands in a continuous recirculating mode were studied regarding the removal of Cu, Pb and Zn from contaminated water. After establishing the flow characteristics, the wetland was planted with Typha latifolia, Phragmites australis, Schoenoplectus lacustris and Iris pseudacorus. The wetland system was dosed with increasing concentrations (1, 5 and 10 mg/l) of Cu, Pb and Zn. Finally a shock load of metals (concentration 20 mg/l) was introduced to simulate a storm event. In each experiment water samples collected from the outlet at timed intervals were analysed and loadings calculated in order to assess the metal removal efficiency of the system. The removal efficiences and rates for these different doses ranged from 81.7% to 91.8% and 36.6 to 372.7 mg/m2/d for Cu, 75.8% to 95.3% and 30.8 to 387 mg/m2/d for Pb and 82.8% to 90.4% and 33.6 to 362.1 mg/m2/d for Zn respectively. Results for the storm simulation showed that in the time taken for the water level to subside to its original level (just below the substrate surface) at established outlet flow rates, the metal loadings leaving the system remained very low with the wetland system retaining over 99% of the metals. These results indicate the ability of the system to act as an efficient sink for heavy metals. The macrophytes were collected and segregated by species and subsurface tissue type (rhizomes, roots and root tips) and analysed for Cu, Pb and Zn. The results are discussed with respect to the surrounding peat substrate, biomass accumulation and the overall removal performance of the laboratory scale wetland.