CONTRIBUTION OF SEWAGE-SLUDGE TO EROSION CONTROL IN THE REHABILITATION OF LIMESTONE QUARRIES

The aim of this study was to evaluate the effects of high doses of mun icipal sludge on soil aggregation and to assess its value for soil ero sion reduction, both under natural and in simulated rainfall condition s. Doses of 200 and 400 Mg ha(-1) sewage sludge (dry weight) were appl ied to the soil of experimental plots situated on a 28 degree slope. T wo sludge application procedures were tested: pre-mixing into the soil before disposal on the slope, and direct application on the soil surf ace. Sheet erosion was measured by collecting the sediment carried dow n to a Gerlach trough situated at the base of the plots. Simulated rai nfall, with an intensity of 64 mm h(-1), was applied to evaluate soil erodibility. When the soil had no vegetation, the erosion measured on plots treated with sludge represented less than 10 per cent of the ero sion from the control plot. Even when the vegetation was well develope d, the erosion was also lower in the plots where sludge had been appli ed. In simulated rainfall, the soil loss was inversely proportional to the sludge dose, and when sludge was applied directly on the soil sur face the erosion rates and particle mobilization caused by raindrop im pact were minimal. Sludge amendments increase infiltration rates and i mprove soil structure, increasing the mean weight diameter of aggregat es and their water resistance.