Evaluation of the risk of heavy metals in sewage sludge intended for agricultural application in Swaziland

Sewage sludge generated from municipal wastewater treatment plants is widely acknowledged as an important source of nutrient and soil conditioner for agricultural use. Determination of levels of heavy metals in sewage sludge is necessary prior to application of the sludge to agriculture because of the inherent risk of heavy metal toxicity to soil, plants and humans. At present large amounts of sewage sludge are being generated and stockpiled in several waste water treatment plant premises in Swaziland and agricultural application is being considered as a future viable option of disposal. Sewage sludge generated from seven wastewater treatment plants in Swaziland were analyzed for a range of total metals using ICP-OES. Most of the samples analyzed show compliance with respect to the regulatory limits set by the USEPA, South Africa and EU guidelines for levels of heavy metals in sewage sludge intended for agricultural application. Sewage sludge generated from the Matsapha wastewater treatment plant in general show high levels of heavy metals due to the presence of several industries in the area. No visible trend in heavy metals increase was observed among sewage sludge samples generated at different time periods.