| In recent years, the handling capacity for urban sewage is improving rapidly with the increasing speed of urbanization progress. At the same time, the sludge output of urban sewage factories also has a sharp increase. There is a large number of organic matters and some elements in sludge, such as nitrogen, which are needed in plants growl. All of these matters and elements are good organic fertilizers, their agricultural utilization will be one of the most economically feasible and promising disposal method. But urban sludge contains numerous heavy metal pollutants. If we directly apply the sludge containing heavy metal to the soil, it will result in increasing of the heavy metal content in soil. It will bring about potential danger and risk to ecological environment and human heal. Therefore, before the agricultural utilization of sludge, we must adopt appropriate treatment measures to reduce the hazard of heavy metal pollutants.At the present time, the disposal methods for heavy metal have two types:one is to wipe off heavy metal from sludge; the other one is to make the heavy metal in sludge into stabilization, which can change its toxic, soluble, unstable state into a non-toxic, low soluble or insoluble and stable state. For this, according to the two types of method, different scholars researched into the removal rate and stabilization rate of heavy metal and their influence factors. On the basis of ample comparison of precious research on various methods, this article chooses two methods to practice an experiment-wiping off and stabilization. On the premise of alleviating unfavorable influence made by heavy metal, this test focus on the nutrient elements of organic matter. N. P in sludge and their change of effective ideological content. Moreover, according to the proportion, this test try to match the sludge and soil which are after treatment. And it also will analyze the index of each group.The results of heavy metals removal and nutrients analysis experiment showed that:(1) Citric acid effectively removed heavy metals in sludge, and in the optimum concentration of citric acid, the heavy metal removal rate of Pb, Cd, Cu, Zn, Cr were83.32%,69.23%92.85%,47.11%,45.65%, respectively. The treated sludge can be achieved Class A standards of heavy metals control in the sludge for agricultural. EDTA could also have effectively removal for heavy metals, but overall rate were not as good as citric acid. When use optimal concentrations of EDTA, Pb, Cd, Cu, Zn, Cr removal rates were47.83%,43.11%,38.9%,20.57%,37.73%, respectively. The treated sludge can be achieved Class B standards of heavy metals control in the sludge for agricultural.(2) In the treated sludge removed heavy metals, nutrient contents had been changed, but the overall level was higher than that of other fertilizers. Under the premise of reducing the heavy metal content, the sludge could be used as a soil amendment.(3) After the mix of sludge and soil, the heavy metal content decreased with reduction of sludge/soil proportion, and the risk of heavy metal enrichment and accumulation in the soil also could reduced, but nutrients content also declined. Therefore, when the ratio of sludge/soil was1:2. the amendment works best. |
