Effects of lime treatment on fractionation and extractabilities of heavy metals in sewage sludge

Abstract

Many studies have focused on heavy metal characteristics in sewage sludges and sludge‐amended soils other than lime‐stabilized sewage sludge (LS). Thus, the aerobically digested sewage sludge cake (13.61% solids) sampled from the Min‐Sheng Wastewater Treatment Plant in Taipei City, Taiwan was lime‐stabilized for studying the effects of the lime treatment on fractionation and extractabilities of heavy metals (Cu, Pb, Cr, and Zn) in sewage sludge, using various extraction tests. For comparative objective, oven‐dry sewage sludge without liming (unlimed sludge (S)) was also studied. The results of this study revealed that the percentages of the metals extracted from lime‐stabilized sludge (pH = 12.05) and the amounts of the metals fractionated to the organically‐bound fraction (30% H2O2 + 0.02 M HNO3 (pH2), 85°C) in unlimed sludge were both in the order of Cu > Pb > Cr > Zn, meaning that the more the heavy metals (e.g., Cu) were fractionated to the organically‐bound forms in unlimed sludge, the more unstable they were in the sludge after lime‐stabilized. On the other hand, the metals (e.g., Zn) in lime‐stabilized sludge was more stable when they have less affinity for organics. Single extraction conducted at various pHs revealed that this effect was due to the irreversible dissolution of organics and some metals having a higher affinity for organics (e.g., Cu) at very high pH during processing and air drying of lime‐stabilized sludge. Therefore, examining the fractionation of heavy metals in sewage sludge is useful for expecting heavy metal stability in the sewage sludges after lime‐stabilized.