Study On The Methods Of Removing Heavy Metals From Sewage Sludge

With the development of city and increase of treatment ratio of wastewater, a large amount of sludge has been produced. The sludge directly enter into environment will cause secondary pollution to waters, air, soil. Land application of sludge on agricultural lands, forest and disturbed lands was considered a best method to treat municipal sludge. Unfortunately, pollutants such as toxic organic compounds, heavy metals and pathogens in the sludge, especially heavy metals limit its use on land application. Now, the main problem is how to remove or control the heavy metals in sludge effectively and economically. The aim of this study is to look for some new methods which can remove heavy metals fast and effectively and are friendly to environment.Through analyzing the mechanism and factors which affect the removal of heavy metals from sludge by bioleaching and chemical method, In this work the ferric sulfate and citric acid were used to remove heavy metals in sludge.In the first experiment , The removal of heavy metals from two different kinds of sewage sludge(W1&W2) was conducted by using ferric sulfate at room temperature and sludge solid concentration is 20g·L-1. It was found that the addition of ferric sulfate to the sludge causes the acidification of the tested sludge and the dissolution of heavy metals from the sludge. For W1, the best amount of ferric ion added was 1.5g·L-1 with a low pH value 3.02 and high removal percents of Cd,Cu,Zn which are as follows: 82.5%,70.9%,89.0% respectively; while for W2 the best amount of ferric ion is 4.5g·L-1 and the pH values is 3.19 and the removal percents of heavy metals——Cd,Cu,Zn are 80.0%,88.2%,92.1% respectively. Ferric ion dissolved Cd,Cu,Zn more effectively than sulfuric acid at same pH value. The removal percents of Cd, Cu and Zn using ferric sulfate was equivalent to that by bioleaching and shorted the experiment time.In the second experiment, CTA was used to treat heavy metals of sludge(W2). The results indicated that both CTA concentration and sludge-liquid ratio have a positive relation with the elution quantity of heavy metals, while just at the early phase of extraction, the extracting time has a positive relation with the elution quantity, with the time prolonging, there is a little decrease in the quantity of extracted metals. And CTA can effectively extract Pb, Cd, Cu, Zn from sewage sludge. At room temperature, the remove percent of Pb, Cd, Cu, Zn can reach 56%,85.2%,65%,88.5%, respectively, by extracting with 0.2 mol·L-1CTA and a sludge-liquid ratio 1:20. CTA can extract not only exchangeable metals, adsorbed metals, organically bound metals and metal carbonates, but also some metal sulfides, and a little residual metals.