Assessment of Heavy Metals in Treated Wastewater Used for the Irrigation of Vegetable Plants in Arusha City

Abstract

The scarcity of fresh water has promoted a growing interest in the reuse of treated wastewater for irrigation activities. However, treated wastewater reuse has been associated with contamination of the soil and plants by heavy metals. This study investigates the levels of Chromium (Cr), Manganese (Mn), Iron (Fe), Zinc (Zn) and lead (Pb) in the treated wastewater from Arusha city waste stabilization ponds and its suitability for the irrigation of edible crops. The influent to the facultative pond and the effluent of the last maturation pond were sampled and analysed for Cr, Mn, Fe, Zn and Pb by using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Similarly sludge from the facultative, first maturation and the last maturation ponds were collected and analysed for the same heavy metals using Energy Dispersive X-ray fluorescence spectrometer (EDXRF). The results indicated that the concentration of Cr, Zn and Pb in the sludge decreases from the facultative to the last maturation pond while that of Mn and Fe increases from facultative to last maturation pond. The heavy metal contents in the sludge were significantly higher than in the water column. Furthermore it was revealed that there was a significance difference of Cr, Fe and Zn concentration between the influent to facultative pond and the effluent of last maturation pond while the difference of Pb and Mn concentration between the influent to facultative pond and the effluent of last maturation pond was insignificant. The concentration of Cr, Mn, Fe, Zn and Pb in the treated wastewater was 0.19 mg/L, 0.16 mg/L, 0.17 mg/L, 0.03 mg/L and 0.1 mg/L for Cr, Mn, Fe, Zn and Pb respectively. The ability of Arusha waste stabilization ponds on the removal of heavy metals from the wastewater is relatively low leading into the discharge of partially treated wastewater with the Cr concentration above the prescribed limit (0.1 mg/L) of  Food and Agriculture Organization of United Nations (FAO) for irrigation water. Thus the use of treated wastewater from Arusha waste stabilization ponds for irrigation may results into the contamination of soil and cultivated crop by heavy metals.