Impact of Sewage Sludge Leaching on Soil Constituents and Quality

Abstract

Activated sludge treatment plants generate large quantities of sludge each year, thereby posing a serious environmental problem. This study aims to experimentally assess the effect of rainwater on the leaching of sludge components. In this context, a percolation test was set up, and composed of PVC cylinders into which the solid substrate was introduced. Five modalities of the solid substrate were used: a sludge modality, a soil modality and three modalities with increasing percentage of sludge (1%, 5% and 25%) in the soil. The percolation water is collected during the rainy months in bottles placed below each column. Solid substrate samples were taken before the test and after one year. The physicochemical analysis of the percolation water showed an increase in the electrical conductivity, BOD₅, COD, nitrogen compounds and phosphate compounds which were proportional to the percentage of sludge. The pH of the sewage sludge leachates varies from 7.61 to 7.98. Zinc and copper were the most mobilized metals. A year following the installation of the percolation test, electrical conductivity, total phosphorus (TP) and orthophosphate (PO₄) contents decreased for the solid substrates using the five modalities. Furthermore, ammonium (NH₄) and nitrates (NO₃) levels decreased in soil mixed with 1 to 25% of sludge due to their leaching by rainwater. Collectively, these data show that the leachates through the soil mixed with sludge are stable and loaded with NO₃, a plant nutrient that can contaminate the groundwater as well as the surface waters inducing their eutrophication. Furthermore, addition of sludge to the soil improves the levels of carbon, total nitrogen, TP and PO₄ in the soil and thereby soil fertility. The addition of sludge, however, is not without soil contamination with heavy metals. Such soil contamination would cause pollution of surface and ground water. Reaching certain severity, it should call for the adoption of prompt measures for the protection of environment and human health.