Dedicated land disposal of wastewater sludge in south africa: leaching of trace elements and nutrients.
Most of the wastewater treatment facilities in South Africa (80%) dispose of their sewage sludge on dedicated land disposal (DLD) sites. The impact of this practice on the environment is believed to be negative, but very little research has been carried out to determine the extent of the damage to the soil and water resources. Forty wastewater treatment facilities using DLD, with different soil properties, application techniques, metal concentrations and period of sludge application, were studied. Soil and groundwater samples were collected at each of the selected facilities. Three extraction methods (aqua regia, NH4EDTA and NH4NO3) were used and samples were analysed for total N, P and K, pH, organic carbon and their metal content (Cr, Co, Ni, Cu, Zn, Cd, and Pb). Some degree of leaching of the heavy metals (especially Co and Ni) occurred at some of the sampling sites and the average depth of leaching was 100-200 mm. Deeper than 300 mm, the metal concentrations in most soil samples reached background concentrations. Seven of the nine groundwater samples that could be obtained had high NO3 concentrations (> 6 mg L(-1)). Statistical analyses of the data indicate no significant differences between sludge type (wet or dry) and leaching, or age of the disposal sites and leaching. Taking into account the age of the disposal sites, the frequency of sludge application and the metal load of the sludge, the depth of leaching is surprisingly shallow in most soils, in spite of the low soil pH(H2O) and clay content.
Herselman
J E
JE
Golder Associates Africa, P.O. Box 6001, Halfway House, 1685, South Africa. eherselman@golder.co.za
Steyn
C E
CE
Snyman
H G
HG
D002244:Carbon / Q000032:analysis; D019216:Metals, Heavy / Q000032:analysis*; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis; D010758:Phosphorus / Q000032:analysis; D012037:Refuse Disposal / Q000379:methods*; D012722:Sewage; D012989:Soil Pollutants / Q000032:analysis*; D013019:South Africa; D014874:Water Pollutants, Chemical / Q000032:analysis*
D019216:Metals, Heavy; D009566:Nitrates; D012722:Sewage; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D010758:Phosphorus; D002244:Carbon; D009584:Nitrogen