Mobility of Heavy Metals in Municipal Sewage Sludge from Different Throughput Sewage Treatment Plants

Sewage sludge heavy metals can be dissolved, precipitated, co-precipitated with metal oxides, and adsorbed or associated on the particles in biological debris. Heavy metals are found in the form of oxides, hydroxides, sulphides, sulphates, phosphates, silicates, organic bindings forming complexes with humic compounds, and complex sugars. Polish regulations specifying the maximum levels of heavy metals in municipal sewage sludge used for agricultural purposes refer to the total content of lead, cadmium, mercury, nickel, zinc, copper, and chromium. The aim of our study was to evaluate the mobility of heavy metals in sewage sludge from wastewater treatment plants in Swietokrzyskie Province. Stabilized sewage sludge from wastewater treatment was analyzed in accordance with the extraction method proposed by the Community Bureau of Reference (BCR). Zinc, cadmium, lead, and nickel were determined by means of the standard addition with the use of a Perkin-Elmer 3100-BG FAAS atomic absorption spectrophotometer. Chromium and copper were tested using the FAAS technique. The sequence analysis revealed the presence of heavy metals in all fractions (F-I, F-II, F-IV). It should be strongly emphasized that organometallics and aluminosilicates constitute the most prevalent forms of metals under consideration. Those, according to BCR, make fraction III and fraction IV, respectively. The results for stabilized sewage sludge confirmed a trend being observed in heavy metals concentrations in the immobile fractions, here in combination with aluminosilicates. On the basis of the investigations, it can be concluded that the dominant forms of heavy metals are immobile. It was shown that the total content of heavy metals in the sewage sludge does not provide an objective criterion for environmental risk evaluation. Bearing that in mind, however, it should be noted that heavy metals immobilized in the fraction F-III, may pose a potential hazard to soil in the aeration zone.