Study On The Change Of Heavy Metal Forms During Composting Of Sewage Sludge

Sewage sludge contains a lot of water, be rich in organic matter and N, P, K, and other nutrients. But it also contain heavy metals,pathogens and other harmful substances. It may not only pollute the environment, but also make the resources wasted, if not treated before discharge. The agricultural utilization of sludge compost is the most economical and available way for disposal and resources recovery of sludge, and it has been widely used in many countries. The major limitation of soil application of sludge compost is the total heavy metal contents and their bioavailability to the soil–plant system. However, it is widely accepted that the determination of total elements does not give an accurate estimation of the potential environmental impact. So, it is necessary to apply sequential extraction techniques to obtain suitable information about their bioavailability or toxicity.In this paper, the contents of nutrition (N, P, K as well as OM) and the heavy metals (Cd, Pb, Zn, Cu, Cr, As and Hg) of the sludge of Yangling Waste Water Treatment Plant were investigated, and the change trend of the heavy metals have been grasped. Then the change of heavy metal forms in sludge during compost process were determined by use the BCR (Community Bureau of Reference) sequential extraction procedure and the Sposito sequential extraction procedure. The conclusion was showed as follows:1. The results showed that the sludge of Yangling Waste Water Treatment Plant contended high organic matter, total nitrogen (TN) and total phosphor (TP), the contents of organic matter, total nitrogen (TN) and total pHospHor (TP) were as follows: 361.41-529.38, 20.13 -35.42, 3.11-7.48 g/kg, respectively. And the contents of Ni, Cd, Pb, Zn, Cu, Cr, As were 22.86-38.03, 0.68-5.07, 29.53-76.16, 297.4-313.7, 124.37-200.66, 43.23-255.00, 6.02-15.97 mg/kg, respectively. However, the contents of Hg in sewage sludge samples were not determined in near a year time. All the heavy metals'contents were lower than the heavy metal limits for agricultural application of China, and far below USEPA503 standard. So it is believed that the influence that agricultural utilization of sludge compost is little. It is suitable to apply it to farmland if the sludge were treated with some methods. The bioavailability of heavy metals was determined using simple (DTPA) extraction. The active order of heavy metals was as follows: Ni >Cd > Pb >Zn> Cu>As. 2. In the pure sludge compost process, the results of BCR indicated that the exchangeable fraction of heavy metals all tended to decline, the descend range of Cd is very small; the content of residual fraction tended to increase except Cd. The result of Sposito showed that the mobile fractions tended to decrease except Cd; most heavy metals existed in residual fraction and bound to organic matter, the content of these two kinds of forms are all over 50% (Cu, Cd and Pb about 80%) and tended to increase, too. So the results of the two methods all showed that the stability of Zn,Cu,Ni,Pb and Cr increased after compost except Cd.3. During the course of the sludge and sawdust mix compost, there are some same conclusion: the two methods all showed that the mobile fractions'content of Cu,Ni,Pb and Cr tended to decrease except Cd. On the other hand, the result of BCR showed that the bioavailability of Zn decrease, but the Sposito method is on the contrary; the result of the two methods indicated that the bioavailability of Pb decrease, the range was large in BCR method but very small in Sposito method.4. The content of total heavy metal by BCR is larger than that of Sposito. As was not determined in every form of the compost, the As content was very little in raw sludge.