Pollution potential and ecological risk of heavy metals in municipal wastewater treatment plants sludge

Application of sewage sludge contaminated with heavy metals may cause health and ecological concern. Ecological risk and heavy metals content of sewage sludge from wastewater treatment plants of East Azerbaijan province were evaluated in the present study.

Nine composite samples were taken and analyzed for heavy metals. The geo-accumulation index (Igeo), contamination factor(CF) , and potential ecological risk index were calculated.

Variation of the heavy metals concentrations were in the following order: Zn>Cu>Pb>Cr>Ni>As>Hg>Cd. The contents of some heavy metals were several order of magnitude higher than the crustal average (CA) values. Considering the Maximum Permissible Standards (MPS) and the Mullerchr('39')s index, the Cd, Cu, and Zn pollution were found to be in the moderate range, whereas As and Pb were in strongly polluted category; and Hg was in extremely polluted category. Considering MPS, the single-factor pollution index (PI) and the Nemerow’s synthetic pollution index (PN) were lower than 1 and 0.7 respectively. This indicated that the sludges were unpolluted with most heavy metals and could be safely applied for agricultural uses. However, a high potential ecological risk for As and Cd; very high risk for Pb; and extremely high risk for Hg were calculated when considering the CA values. There were significant differences between the levels of heavy metals and MPS (p <0.05).

Although the metal concentrations were below the MPS according to the US EPA regulations, sludge samples showed a high degree of potential ecological risk for the environment in comparision with the CA values,. Periodic monitoring of sludge quality are strongly recommended.