Abstract:
This study was undertaken to assess the efficiency of macrophyte plants in removing heavy metal in municipal
wastewater using laboratory scale quarry dust vertical constructed subsurface wetlands. Plants used were
Typha latifolia, Phragmites australis and Polygonam spp. The control treatment was not vegetated. The
parameters evaluated were the concentration of lead, cadmium and zinc in the influent and effluent sewage
after different retention periods in the wetland. At the end of experiment the macrophytes were harvested
and the concentration of the heavy metals in shoots and roots determined. Polygonam spp. absorbed highest
amounts of Zinc while Phragmites australis absorbed the highest amount of lead and cadmium. The roots had
a higher concentration of the heavy metals than the shoots. Planted beds differed in the removal rates of the
heavy metals which were found to be dependent on the plant species. The percentage removal of the heavy
metals from the raw sewage by the vegetated system was 87%, 83% and 84% for lead, cadmium and zinc
respectively after a retention period of 8 days. The control surprisingly showed a percentage removal in excess
of 60%, indicating that the removal of heavy metals was mainly accomplished by the quarry dust medium. The
concentrations of the three heavy metals in the effluent sewage were all below the maximum allowable
concentrations for discharge into the environment. The constructed wetlands were therefore effective in
reducing the heavy metals concentrations from the raw sewage to tolerable levels.
