Heavy metal speciation in municipal sewage sludge depending on treatment method

Heavy metal content of sludge is one of the most important factors determining their final usage. However, total metal content of sludge is not a suitable indicator for assessing its bioavailability. A more substantial assessment is to determine the occurrence of metals bound by sludge components, which means fractions. Three types of sludge (excess sludge, post-fermentation sludge and dried sludge, which were taken in four quarters of 2019/2020 from one sewage treatment plant, were studied in this paper. Determinations of total Zn, Cu, Cr, Ni, Pb, Cd and their fractions were determined using Tessier method. Zinc was the most abundant in sludge, followed by copper, then chromium, nickel, lead, and the least cadmium. Total content of determined metals depended on how sludge was processed and, to some extent, on the sampling date. All analysed sludge types, due to their total content, can be used in agriculture. It was found that the distribution of metal fractions varied more with sludge type than sampling date. In the analysed sludges, irrespective of sampling date and sludge type, the distribution of metals in the fractions was as follows: Zn: reducible > exchangeable > residual > carbonate > oxidisable > soluble, Cu: oxidisable > exchange-able > residual > soluble > carbonate > reducible, Cr: reducible > residual > exchangeable > carbonate > oxidisable > soluble, Ni: reducible > carbonate > exchangeable > residual > soluble > oxidisable, Pb: residual > exchangeable > reducible > oxidisable > carbonate > soluble >, Cd: residual > reducible > exchangeable > carbonate > oxidisable > soluble. The shift of the mobile to non-mobile metal fraction was influenced by the sludge treatment method. These shifts were the most influenced by fermentation, and slightly less by sludge drying.