Sewage sludge disposal on land – impacts on soils and groundwater quality
Pollutant transport mechanisms in soils and ground water, together with regulatory aspects and design criteria for a safe application of sewage sludge on land have been presented. The impacts of long-term sludge land disposal on soils and groundwater quality have been assessed by means of a specific case study of digested sludge and effluent mixture, disposed beneficially for pasture irrigation on Crowborough farm in Harare, performed during the period 2002–2003. Results show that soils were acidified at selected portions of the pasture, subject to excess irrigation loads, but in general, the soil’s organic carbon and nutrients content has been improved. The metals content of soils were far below recommended maximum values. Adverse impacts on ground water were found with respect to metals – Cr, Cd, Pb and Ni, which present a high environmental risk, and restrict the use of this aquifer for any future beneficial purpose. Recommendations to improve the present practice have been made.