ASSESSMENT OF THE EFFICIENCY OF DIFFERENT MIXES OF MACROPHYTES IN REMOVING HEAVY METALS FROM WASTEWATER USING CONSTRUCTED WETLAND

Abstract

Industrial wastewater has high quantities of heavy metals emanating from industrial processes. Theseheavy metals need to be removed before the wastewater is released to the environment.  Constructedwetlands are inexpensive and highly effective in removal of heavy metals from wastewater.  This study assessed the efficiency of different mixes of macrophytes  Polygonum spp, Typha latifolia and Papyruscyperus (X,Y,Z)  to remove zinc, lead and cadmium from  municipal wastewater using constructed wetlands. The model wetlands were wash basins with an outlet at the bottom. A total of 16 basins were used, 12 of which were planted and 4 unplanted to act as control (4 treatments with 3 replicates). Quarry dust (¼”) was used as the substrate in each basin. 12 plants (X,Y,Z) were planted in mixes of 1:1:1, 2:1:1, 1:2:1 and 1:1:2 respectively. Wastewater was collected from Thika municipal treatment works and analyzed for Zinc, Lead and Cadmium using  Atomic Absorption  Spectrophotometer  before being introduced into the wetlands. Samples from the wetlands were collected after 4, 6 and 8 days for analysis for several runs. Plant samples were harvested and concentration of heavy metals determined.The results showed that treatment with more of  Polygonum  spp achieved highest removal for Zinc (80.5%), treatment with more of papyrus achieved the highest removal for Lead (89.8%) while maximum removal for Cadmium was achieved when the macrophytes were in equal ratios. The treatment with 6 Polygonm spp, 3 Typha  spp  and 3  Papyrus spp  recorded relative high removal rates for all metals and was recommended as the best mix. The analysis of the metals in the macrophytes indicated increasing absorbance;  Papyrus spp >Typha spp >Polygonum  spp.  The concentrations of the Zinc and Cadmium in the effluent sewage were 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.