ASSESSING THE EFFICIENCY OF SUBSURFACE CONSTRUCTED WETLANDS PLANTED WITH DIFFERENT MACROPHYTES IN REMOVING HEAVY METALS FROM WASTEWATER

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.