Heavy metal concentrations and their decreasing trends in sewage sludges of China
中国城市污泥的重金属含量及其变化趋势

Analysing the reported data about heavy metals in sewage sludges between 1994-2001 in 60 cities of China, it is concluded that Zn, Cu and Cr concentrations of heavy metals in sewage sludges of China was velativel high while the concentrations of the high toxic metals, Hg, Cd, As, were lower than 20 mg/kg.The concentrations of neavy metals in 70% of statistical samples were found to be lower than 20 mg/kg for As, 2.8 mg/kg for Cd, 250 mg/kg for Cr, 417 mg/kg for Cu, 5 mg/kg for Hg, 75 mg/kg for Ni, 130 mg/kg for Pb and 1701 mg/kg for Zn, respectively. There were about 11% of Ni, 30% of Cu, and 45% of Zn in all the statistical samples which had overran the heavy metal limits of sewage sludge for land application in China. This suggests that Zn was the most primary heavy metal pollutant in sewage sludge in China. The overrunning percent of Zn would be 9% according to the new limits set by China National Standard of Pollutant Release from Sewage Treatment Plant. Because all the heavy metal concentrations in China's sewage sludges were lower than the limits of US EPA, heavy metal pollution in China causing from land application of sewage sludges may not be a serious problem. It is also proved that the heavy metal concentrations in sewage sludge have being decreased over time both in developed countries and in China in the last two decades. The average of heavy metal concentrations has being decreased by 2.7%-4.9% in Beijing City and 0.1%-2.4% in Tianjing City. The heavy metal limits of China are too low that most of sewage sludges (ca. 55% of the total) can not be used for land application. In fact, about 51.5% of China's crop soils were reported to have Zn deficiency. It is suggested that he heavy metal limits in sewage sludge for land application should be modified to promote the safety reuse of sewage sludges in China.