Comparative speciation and multi-index risk assessment of heavy metals in drinking-water, municipal wastewater, and industrial wastewater treatment sludges: implications for sustainable reuse

This study compared the concentrations, operationally defined chemical fractions, and screening-level environmental risks of Cu, Cr, Zn, Cd, Ni, and Pb in 20 dewatered sludge samples collected from drinking-water treatment plants (DWTPs; n = 3), municipal wastewater treatment plants (MWWTPs; n = 11), and industrial wastewater treatment plants (IWWTPs; n = 6) in Taiwan. A modified BCR sequential extraction procedure was used to separate acid-extractable (F1), reducible (F2), oxidizable (F3), and residual (F4) fractions. MWDWTP sludge contained comparatively high Zn and Cd concentrations, whereas several IWWTP samples showed elevated Cu, Cr, Ni, and Pb. Cu, Cr, Ni, and Pb were frequently associated with F3 and/or F4, while Zn and Cd showed substantial F2 contributions at several sites. Concentration-based indices identified marked enrichment at selected facilities, whereas fraction-based indices indicated that mobility depended on both metal and sludge category. High Igeo values therefore represent enrichment relative to background and do not necessarily indicate high immediate mobility when metals occur mainly in F3 or F4. These results support site-specific evaluation of total metal loads and chemical fractionation before sludge reuse, treatment, or disposal.