The heavy metal content of Gaborone secondary sewage effluent in Botswana

Abstract

The objective of this study was to determine the heavy metal content of the Gaborone secondary effluent in Botswana before its discharged to the Notwane river. Secondary effluent water was sampled every month for one year, from a breather pipe on the pivot irrigation system at the Botswana College of Agriculture, Notwane Farm, Gaborone, using USEPA procedures and guidelines. The results showed that the heavy metals of most widespread concern to human health Hg, Cd, Pb and As were below detectable limits, 5.75, 4.83 and 5.7 parts per billion (pgL1), respectively, and were below concentrations that could pose pollution of the environment and were within the recommended limits for irrigation water by FAO and the Botswana guidelines for irrigation water. The other trace elements Be, Ti, V, Cr, Co, Cu, Ga, Ge, Se, Sr, Ba, Ce, Bi, Al, Li, Mn, Mo and Fe were all below the recommended limits for irrigation water by FAO and the Botswana guidelines for irrigation water. Most of the trace elements were in concentrations of parts per billion. The current results suggest that the Gaborone secondary effluent is suitable for unrestricted irrigation of field, fodder and horticultural crops. However, heavy metal levels should be monitored continuously either by the Gaborone City Council or any other government institution in order to make sure the treated secondary sewage effluent meets the recommendations for irrigation water, and prevent environmental pollution and reduce health hazards that may be caused by pollutants to the end users of this scarce resource.