Total concentrations and speciation of heavy metals in municipal sludge from Changsha, Zhuzhou and Xiangtan in middle-south region of China

The presence of heavy metals in municipal sludge restricts its use for agricultural purposes. In this paper, the bioavailability and eco-toxicity of heavy metals in municipal sludge was evaluated, taking into consideration both the speciation of metals and the local environmental characteristics. The dewatered municipal sludge samples were collected from five sewage plants in Changsha, Zhuzhou and Xiangtan respectively, which are representative cities with characteristics of the middle-south region of China. Some agricultural significant parameters and total metal concentrations in the sludge were determined and the metal speciation was studied by using BCR sequential extraction procedure. It was found experimentally that in general the municipal sludge collected from the five sewage plants was rich in organics, N and P. Except that the sludge from Xia Wan Sewage Treatment Plant showed higher concentrations of heavy metals, the sludge from other plants all showed a low total content of heavy metals with only Cd slightly exceeding the permitted values of the national application standard of acid soil in China (GB18918-2002). The sequential extraction results showed that Cu and Zn were principally distributed in the oxidizable fraction, which meant a high potential toxicity, but the bioavailability of Zn might be overestimated to the soil of Hunan. Pb was mainly in the residual fraction. The distribution of Cd showed no obvious characteristics.