An assessment of efficiency of heavy metals removal by a constructed wetland at Egerton University, Kenya

Abstract

An assessment of the efficiency of heavy metals removal by a constructed wetland system atEgerton University, Kenya, was conducted between August 2013 and January 2014. The aim ofthe study was to evaluate the physico-chemical characteristics of wastewater and investigateheavy metals retention in the wetland. Water samples were collected monthly in plastic bottles atthe inlet, along the wetland, and at the outlet; sediment samples were collected from the gravelbed and the three wetland cells using a core sampler. Whole plants were randomly collected andpooled together to form composite samples for each species in every site. In every sampling occasion; temperature, pH, electrical conductivity (EC) and Dissolved Oxygen (DO) of water samples were measured in situ. In the laboratory the samples were processed and the concentrations of metals; lead (Pb), cadmium (Cd), copper (Cu) and zinc (Zn) determined using Atomic Absorption Spectrophotometry (AAS). Minitab software was used to determine spatial variations of heavy metals concentrations and physico-chemical characteristics of water using Analysis of Variance (ANOVA). Further, correlation analyses were performed to establish relationships between the physico-chemical parameters and the retention of metals in the wetland. The study results showed significant variations in temperature and conductivity across the wetland (p < 0.05). On the contrary, there was no significant difference in DO across the wetland. Influent levels for lead, copper and zinc were 1.25 ± 0.75 mg/L, 1.09 ± 0.49 mg/L and 0.15 ± 0.11 mg/L respectively while the level of these metals in the effluent were 0.07 ± 0.07 mg/L and 0.32 ± 0.11 mg/L for lead and copper, respectively with zinc being below detection limit. Removal efficiencies of 94%, 70% and 100% for lead, copper and zinc respectively were observed. There was a significant negative correlation between zinc in sediment and pH in water (r = - 0.55), and a moderate positive correlation between copper in plants and pH in water (r = 0.47). These findings indicate that the constructed wetland is efficient in removing heavy metals from the wastewater. The study recommends that the wetland should be rehabilitated to enhance and sustain its function of removing heavy metals from wastewater in order to safeguard human and environmental health.