Effect of sewage sludge digestion on the mobility of heavy metals on the example of the wastewater treatment plant in Sitkówka-Nowiny

In sewage sludge, heavy metals can be dissolved, precipitated, co-precipitated with metal oxides, adsorbed or associated on particles of biological residue. Heavy metals come as oxides, hydroxides, sulphides, sulphates, phosphates, silicates, organic connections in the form of humic complexes, and compounds with complex sugars. Sequential extraction procedures based on Tessier et al. are widely used for examining sewage sludge. The method makes it possible to extract the following fractions that are operationally defined: exchangeable, bound to carbonates, reducible, oxidisable (bound to organic matter) and residual. For the investigations, communal sewage sludge was collected from mechanical-biological sewage treatment plant located in Sitkówka-Nowiny; the Świętokrzyskie Province, Poland. Heavy metal content [ppm] in the sewage sludge was collected before and after digestion process. After the digestion process, for a majority of metals, the content of reducible form is clearly decreased and the concentration of organic and residual fractions is increased. As a result of redistribution, copper, zinc and nickel mobility in the sludge decreased, and the strength of bond, measured with index I_R increased for all metals.