Effect of Drying Process on Heavy Metals Fractionation of Contaminated Sludge for Land Application

Metal fractionation studies on metals in sludge are usually done on dried sludge. Although there are advantages in this form of sludge, it is possible that fractionation of metals in sludge may be influenced by the drying process, which consequently affects mobility of metals at disposal. In this study, sequential chemical extraction was done to assess the effect of drying on fractionation of cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn) on wet and dewatered (air-dried and oven-dried) anaerobically digested sludge samples in Bangkok, Thailand. Results revealed an insignificant difference in the fractionation profile of metals for both wet and dewatered sludge. The higher percentage of residual fraction for Cu and oxidizable fraction for Zn in the wet sludge, however, makes this form more suitable for direct land application. In the case of oven-dried and air-dried sludge, the forms of some metals (e.g., Pb and Zn) seem to vary significantly as the sludge is oven-dried, with Pb decreasing its residual phase by 15%, and Zn increasing in oxidizable phase by 41%. The results seem to indicate that drying affects bioavailability of metals in sludge and air-drying seems to favor sludge for land application.