The aim of this study was to characterize sewage sludge from mechanical-biological wastewater treatment plants (WWTPs) in terms of total concentrations of heavy metals (HMs) and their distribution. Moreover, HM mobility, stability and ecological risk were assessed by calculating the mobility factor (MF), the reduced partition index (Ir) and the ecological risk index (ERI). Hierarchical cluster analysis (CA) was used to group the HMs in the sludges on the basis of their MF, Ir and ERI. The concentration of HMs decreased in the following order: Zn > Cu > Ni > Pb > Cd. HM mobility, as indicated by the MF values, was as follows: 18.7%-30.6% (Zn), 8.1%-36.4% (Ni), 6.8%-11.1% (Cu), 3.2%-32.2% (Pb) and 0% (Cd). Based on the average Ir values, the stability of the metals in the sludge increased in this order: Zn (0.23)=Pb (0.23)<Cd (0.45)=Cu (0.45)<Ni (0.69). Despite the high mobility of some metals, each sewage sludge had low environmental risk, as indicated by the ERI (ERI ≪ 150). The HMs were more similar in terms of their mobility (two clusters) than in terms of their ecological risk (three clusters).
Keywords: Sewage sludge; environmental risk; heavy metals; mobility; stability.