Utilization of a sewage sludge for rehabilitating the soils degraded by the metallurgical industry and a possible environmental risk involved

Soils collected in the proximity of a steel plant in Krakow were analyzed for their pH, the content of total organic carbon, and the total contents of metals and metalloids. Applying the Commission of European Communities Bureau of Reference (BCR) extraction of selected metals: Cd, Fe, Pb, and Zn, their forms of binding in soils have been determined. The results of the soil analyses of the samples taken in 1996 and 2016 were compared, and then the soil contamination changes were assessed using the GI, contamination factor (CF), and PLI indicators. A rehabilitation of the degraded soils was tested using the communal sewage sludge, which was added into the soil at four various ratios. The sewage sludge applied meets the environmental standards as a material for rehabilitating soils for non-agricultural needs but cannot be used for upgrading strongly contaminated soils. The reason is that the sludge addition increases the soil As, Ca, Cr, Mn, and Zn quantities as well as the easily mobilized fractions of the sludge/soil mixtures that contain Cd, Cr, Mg, Mn, Pb, and Tl. The addition of the communal sludge into soils increases the value of the environmental risk in the case of Pb, Mn, Mg, Fe, Cr, Zn, and Ca, but within the same risk classification ranges.