Mobility Of Heavy Metals Of Sewage Sludge Submitted To Drying Process

Drying treatment of sewage sludge is one of the dewatering techniques widely used, attributing to its notable effect on reducing the amount and volume of sewage sludges. While heavy metals are supposed to enrich in the dried sludge, which will produce threaten to the environmental safety. The aim of the present work was to establish the effect of drying treatment on the distribution and mobility of heavy metals in sludges.Sewage sludge samples from three municipal and two industrial wastewater treatment plants were subjected to air-drying, thermal drying at 105℃, 150℃and 300℃, respectively. Sequential extraction procedure was applied to investigate the fractions of heavy metals in dried sludges. The dried sludges were also subjected to physico-chemical characterization to provide a preliminary assessment of their mobility of heavy metal. All the determinations are of great importance, not only to track the evolution of the chemical forms of heavy metals throughout the drying treatment but also to suggest their potential disposal options.The air-drying test gave out the result that fluctuation of temperature of sludges undergone thermal drying was related to the transformation of components of sludges. For sludges with different preliminary water content, the water content turned out to be quite different with the same drying time, regardless the type of the source. Results showed a clear drop in finally water content in all the sludges under consideration and the volatile substance in Sibao and Shanghai sludges as the drying temperature going up.The total concentration was determined. Zn was the most plentiful metal in sludges except the tannery sludge with extraordinarily great amount of Cr. Concentrations of the same element differed greatly in sludges from different wastewater treatment pants, with the exception of As. As and Cd, both of which have intense toxicity, were of small content. Approximately, the total concentration of Zn ranged the first, which of Cu, Cr just followed, then followed by Ni, Pb, As. Cd was the least abundant element in all the sludges.From the fraction determination of all the dried sludges, conclusions can be drawn that the drying treatment influenced the mobility of heavy metals with different extent.Correlation analysis revealed that there was remarkable relationship between pH and the fractions with higher mobility of heavy metals.