Content and Fractionation of Heavy Metals in Water Treatment Sludges

The composition and distribution of Cd, Cu, Cr, Ni, Pb, and Zn in eight alum and FeCl 3 coagulation sludges were investigated using a five‐step chemical fractionation procedure to assess potential impacts following soil incorporation. The mean total metal levels (mg kg −1 dry wt.) for these sludges were Cd (<2), Cu (234), Cr (187), Ni (102), Pb (230), and Zn (557), which are well below maximum allowable levels for land‐applied wastes. Two individual FeCl 3 sludges had Ni levels slightly above maximum allowable concentrations. Mean total Cr and Ni concentrations in the FeCl 3 sludges were, respectively, 5.4 and 3.1 times the corresponding alum sludge levels, reflecting elevated concentrations in the FeCl 3 coagulant. The Cu, Cr, Ni, Pb, and Zn in sludges were predominantly bound within the oxide or silicate components. A small portion of the sludge Cd was in a readily exchangeable form, whereas the dilute acid extractable and organically bound fractions contained the majority of non‐residual Cd. Fractionation results suggest sludge Cd would potentially be mobile under acidic soil conditions, but the total Cd levels were very low (1–2 mg kg −1 compared with an allowable maximum of 25 mg kg −1 ). Although fractionation data must be interpreted cautiously, the results suggest most metals in water treatment sludges occur predominantly in weakly mobile, nonbioavailable forms.