Distribution and removal efficiency of heavy metals by a conventional activated sludge at a municipal wastewater treatment plant in Kisumu City

Abstract

The aim of this study was to evaluate the occurrence and the instantaneous overall efficiency of the municipal wastewater treatment facility in Kisumu City-Kenya, a highly populated City, and compare the effluent quality parameters to the National Environmental Management Authority (NEMA) regulations. The heavy metals concentrations (Cu, Pb, Zn, Fe, Mg and Mn) were determined from the inflow and at each stage of the water treatment process including sludge to the effluent discharged to the recipient river. Sample preparation and analysis were done according to the recommended methods. The findings on site characteristics show that pH and chemical oxygen demand (COD) in the treated effluent exceeded the allowable limits. All the selected metal ions (Cu, Pb, Zn, Fe, Mg and Mn) were detected with 100% frequency in the influent water except for Pb which was below the instrumental detection limit (0.001 mg/L). The levels of the heavy metals recorded in the sediment samples were significantly higher than those in the corresponding water samples. The ascending order of the metal percentage removal efficiency (%R) from the treatment plant was: Zn (-127.77%) < Fe (3.66%) < Mn (16.64%) < Cu (24.26%) < Mg (46.97%) indicating that the removal efficiency was directly proportional to the initial metal ion levels in the influent. It is concluded that the plant is a point source for Zn loading into the recipient waters and biosorption and dissolution of the metal ions in the liquid fraction of the sludge were the key modes of metal elimination from the wastewater.