Study On Fraction And Bioavailability Of Heavy Metal Elements In Sewage Sludges

With the rapid increase of sewage treatment plants, the disposal and the environmental impact of municipal sludge have been paid more and more attention. Now, land application of sewage sludge has become an important way for sludge disposal due to its lower cost and higher efficiency. But heavy metals are the key factor that affects the land application of sewage sludge. So we have done some researches on the metal mobility and bioavailability of heavy metals in sewage sludges from different wastewater treatment plants and sewage sludges in the different point of the plants during the wastewater treatment process to get more information for the reasonable disposal of sludge. Also researched the pH impact of on the metal mobility and bioavailability of heavy metals in sewage sludges. The fraction and bioavailability of heavy metals in a yellow-brown loamy treated with municipal sludge was also studied.9 sewage sludges from different wastewater treatment plants all over the country was selected for the research. The results are as follows: (1) Municipal sludges had high organic carbon content, high content of N,P and trace elements, and can be used as good organic fertilizer. But heavy metals affected their land application. The total metal concentrations of heavy metals in sludges varied greatly. The contents of Zn and Cu were the highest. (2) in neutral sludges, Cu, Cd, Pb and Cr were principally distributed in between the oxidizable fraction. As was principally distributed in the acid soluble/exchangeable, oxidizable fraction and the residual fraction. The percentage of Ni, As, B and Zn in the acid soluble/exchangeable was all beyond 10%, their mobility and bioavailability were more higher than other metals, so their potential toxicity should receive more attention. (3) There was no significant difference in the contents of heavy metals in different point sludges during the wastewater treatment process. Non-hazardous treatment of municipal sludge could change the metals' fractions, but couldn't reduce their contents in municipal sludge. It is suggested that a statewide survey for compilation of data on the production of sludges, their heavy metal composition and speciation should be carried out for their environmentally safe disposal. (4) Sludge pH was found to have profound effect on the chemical fractions of heavy metals. Acidic sludges (Xiamen and Jinlin Petrochemical Group Co. wastewater treatment plant) had higher proportion of theacid soluble/exchangeable fractions than in neutral sludge. (5) Through pot experiment of winter wheat in yellow-brown soil treated with municipal sludge, Contents of heavy metals accumulated in roots were much higher than shoots. The regress analysis showed that, among the four extractants, Mehlich 3 was the best extraction procedure for the chemical extraction of heavy metals in the studied sludges.