Chemical Composition of Municipal Sewage Sludge: A Tool for Risk Assessment of Agricultural Land Application

A number of studies have shown that the application of sewage sludge to agricultural soils has a positive effect on plant nutrition. However, frequent application has been shown to result in soil contamination with potentially toxic metallic elements. The present research was conducted with the objective of establishing the chemical composition of sewage sludge from three urban wastewater treatment plants in Algerian’s north region. We determined chemical parameters that account for their nutrient concentration and total content of Cr, Cu, N and Pb.The analysis revealed that the municipal sludge collected exhibited a high organic content, as well as significant concentrations of nitrogen, phosphorus, and potassium. This finding suggests that the use of sludge for agricultural purposes may be a beneficial application. The total contents of the five metals in the sludges were found to be significantly lower than the limit values that would be considered suitable for agricultural soil. However, it was observed that Ni concentration was already above the standards set by AFNOR, while Cr was found to be equivalent to 50% of the limit. Furthermore, the results obtained by means of the sequential extraction protocol demonstrate that a significant proportion of the total content of Cu and Pb is associated with fractions (acid-soluble, carbonates, oxides, organic matter) exhibiting higher levels of bioavailability under soil conditions. In contrast, Ni and Cr have been found to be bound to the residual fraction in up to 80–90% of the total concentration, making it the least bioavailable form for environmental impact.The results indicated that regulatory guidelines require refinement to account for the bioavailability of heavy metals in relation to soil properties.