Total concentrations and different fractions of heavy metals in sewage sludge from Guangzhou, China

Dewatered municipal sludge samples were collected from five municipal wastewater treatment plants (WWTPs) and one industrial WWTP in Guangzhou, China. A number of agricultural parameters and total metal concentrations in the sludge were determined. Metal speciation was also studied. The results showed that sewage sludge had high organic carbon, and was rich in such nutrients as N and P. The concentrations of Mn, Zn, and Cu were the highest, followed by Ni, Pb, and Cr, Cd had the lowest concentration. In addition, the concentrations of the aforementioned heavy metals in the sludge samples were higher than those recorded in the background data for crop soils. With the exception of Cu and Cd from site S1, and Ni from sites S1, S2, and S5, all other metal concentrations conformed to permissible levels prescribed by the national application standard of acid soil in China (GB 18918-2002). The results of the BCR sequential extraction showed that the concentrations of Mn and Zn were predominant in acid-soluble/exchangeable and reducible fractions. Cu was principally distributed in oxidizable and residual fractions, whereas Cr was present in oxidizable and residual fractions, Pb was found in the state of residual fractions, and the distribution of Ni and Cd did not show significant characteristics.