Quantitative evaluation of potential ecological risk of heavy metals in sewage sludge from three wastewater treatment plants in the main urban area of Wuxi, China

In this study, the potential ecological risks and eco-toxicity of heavy metals (Cu, Pb, Zn, Ni, Cd, Cr, As and Hg) in sewage sludge were quantitatively evaluated. Sewage sludge samples were collected from three wastewater treatment plants in Wuxi city during five years from 2009 to 2013. The levels of the eight metals temporally varied. The contents of Zn and Cu in the tested sewage sludge were the highest, followed by Cr, Ni, Pb and As, and the contents of Cd and Hg were the least. The Community Bureau of Reference (BCR) sequential extraction results showed that Zn, Ni and Cd had the highest mobilisation potential, while Cu, Pb, Cr and Hg were not stabilised and would release to the environment under oxidising conditions. The removal efficiency of Cu and Cr was higher than 70%, while that of As, Cd and Hg was less than other heavy metals. The overall potential ecological risk index (RI) of heavy metals in sewage sludge was 1376.34, revealing very high risk. However, the environmental risk values based on the index of geo-accumulation (I_geo) and risk assessment code (RAC) were both low. Cd contamination is the major concern while the treated sewage sludge is used for agricultural purposes.