中国城市污泥的重金属含量及其变化趋势

Heavy metal concentrations and their decreasing trends in sewage sludges of China

对相关研究结果的系统统计表明 ,Zn是我国城市污泥中平均含量最高的重金属元素 ,其次是Cu ,再次是Cr,而毒性较大的元素Hg、Cd、As含量往往较低 ,通常在几个到十几个mg kg范围内 ;70 %统计样本的As含量在 2 0mg kg以内 ,而Cd、Cr、Cu、Hg、Ni、Pb、Zn含量则分别在 2 8、2 5 0、4 17、5、75、130、170 1mg kg以内 .我国城市污泥中的重金属含量普遍低于英美等发达国家 ,而且其中的重金属含量呈现逐渐降低的趋势 .按照 1984年颁布的《农用污泥污染物控制标准》(GB4 2 84 84 ) ,Zn是我国城市污泥中超标问题最突出的重金属 ,其超标率为 5 5 % ;但按照 2 0 0 2年新修订的《城镇污水处理厂污染物排放标准 (报批稿 )》 ,Zn的超标率仅为 9% .一方面 ,我国的土壤和儿童普遍缺Zn ,另一方面 ,我国现行的农用污泥污染物控制标准对城市污泥中的Zn控制过于严格 ,而且也不尽合理 ,急需对其进一步完善 ,以合理解决城市污泥的处理处置途径 。

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.