Evaluation of the potential to dispose of sewage-sludge. 2. Potential for off-site movements of solids and solutes

Abstract

This paper reports a study of the potential for off-site movements of pollutants from sewage sludge broadcast onto the soil surface in Pinus plantations established on the coastal lowlands of south-east Queensland. Laboratory studies of size and settling velocity distributions of rainfall-wetted sludge showed that it is relatively coarse and non-erodible. Field rainfall simulation studies at three sites in pine forests near Beerburrum found no significant increase in interrill erosion due to broadcasting of sludge. (Most or all of the sediment was observed to come from mineral soil exposed when a gutter to collect runoff was installed at the downslope ends of the plots.) However, there was considerable movement of solutes in runoff from rainfall simulator plots that had received sludge-either freshly applied or 'consolidated' sludge (that had been broadcast on the plots 6 months prior to the rainfall simulation study and exposed to rain and weathering). Electrical conductivity (EC) of runoff from freshly applied sludge was initially high but decreased steadily during the 30 min rainfall event. Runoff from consolidated sludge had lower ECs, though significantly (P < 0.05) higher than those of runoff from control plots. Concentrations of nitrate-N and ammonium-N were initially high in runoff from freshly applied sludge, but decreased rapidly during the rainfall event. In contrast, concentrations of mineral N in runoff from consolidated sludge were low throughout the rainfall event. These results indicate a significant risk of off-site N pollution if runoff occurs during the first rains following broadcasting of sludge. By contrast, concentrations of total phosphorus, copper, and zinc in runoff from both fresh and consolidated sludge were relatively high, and showed only small decreases during the rainfall event applied. This suggests that sludge will contribute significant quantities of these elements to runoff for extended periods after broadcasting. The implications of these findings become important in terms of the timing and method of sludge application to the soil.