In Poland, the amount of municipal sewage sludge that contains both organic and inorganic pollutants increases steadily. As a result of penetration of atmospheric precipitations through sludge layers, products of biochemical decomposition of organic matter and soluble mineral compounds are washed away and form contaminated leachates (Arain, J. Hazard. Mater. 154:998-1006, 2008; Fang, J. Hazard. Mater. 310:1-10, 2016; Ignatowicz, Environ. Res. 156:19-22, 2017). Metals contained in these leachates may be particularly burdensome and dangerous, which is due to the toxic nature that disturbs the natural biological balance (Fytili and Zabaniotou, Renew. Sust. Energy Rev. 12 (1): 116-140, 2008). In order to check bio-availability of metals in sewage sludge and find out resulting risks to the environment and human health, apart from the determination of total metal content, speciation analysis is often used. It makes possible a quantitative determination of various chemical forms of metals which are bound in the sewage sludge and finding which of them poses the greatest threat to the environment (Amir, Chemosphere 59:801-810, 2005; Ciba, Waste Manage. 23:897-905, 2003; Hei, Procedia Environ Sci 31:232-240, 2016; Liu, Chemosphere 67(5):1025-1032, 2007).The degree of immobilization of selected metals in sewage sludge has been determined using one of the sequential extraction methods designed to identify groups of compounds with which the metal is bound. Such a method is the Tessier procedure (Janas, Pol J Environ Stud 26(5A):37-41, 2017). Results of this research were used to assess the threat resulting from the increase in the amount of sewage sludge; the management of which is subject to constant restrictions (storage of sewage sludge has been prohibited since January 1, 2016).As a result of the conducted research, it was found that metals in sewage sludge, which undergo various transformations, are very difficult to immobilize. The addition of calcium oxide and an agent supporting the composting process to the sludge does not affect radically the increase of leaching of the analyzed elements from the sludge. Graphical abstract The schema of proceedings in leaching of heavy metals from sewage sludge after various modifications.
Keywords: Calcium oxide; Composting; Metals; Sequential extraction; Sewage sludge; Tessier procedure.