POLLUTION EFFECTS OF WASTE-WATER SLUDGE APPLICATION TO SANDY SOILS WITH PARTICULAR REFERENCE TO THE BEHAVIOR OF MERCURY

Hammarsdale, an industrial centre about 40 km inland from the coastal city of Durban, South Africa experiences effluent treatment problems t ypical of highly industrialised centres. The majority of the factories in the area manufacture textiles, with the remainder comprising chemi cal manufacturers and a large chicken processing plant. Effluent produ ced by these industries constitutes approximately 97% of the flow to t he local wastewater treatment works. This is operated by Umgeni Water, a regional water supply authority which has an interest in pollution control as part of its integrated catchment management strategy. The a ctivated sludge treatment process employed at the works produces a slu dge which, following dewatering, is transported to a nearby land dispo sal site which has been engineered to ensure environmentally acceptabl e use is made of the site. The heavy metal concentrations of the sludg es produced by the Hammarsdale Wastewater Works have long been a matte r of concern, in particular the elevated concentrations of Hg. Investi gations of the land disposal site showed that the Hg is bound in the u pper topsoil (0-10 cm), and only very low concentrations are found in the subsoil. As the soils at the disposal site possess low cation exch ange capacities, the difference between Hg concentrations in the top a nd subsoils may be attributed to binding of the Hg by organic matter i ntroduced by sludge addition. Current research on the disposal area is focused on the movement of Hg, N and other elements through the soil profile into plants, surface and groundwater resources, with the ultim ate aim of identifying suitable crops for cultivation at the site foll owing its use as a sludge disposal facility. (C) 1997 Elsevier Science Ltd.