Heavy metals in sewage sludge were classified into three partitions associated with extracellular and intracellular organic matters and the residue before and after different elution processes of heavy metals from the sludge. Alkalization of the sludge to pH 11 decreased the content of Cu in the three partitions and that of Ni in extracellular organic and residual ones. Acidification of the sludge to pH 3 decreased the content of Cd, Mn, Ni and Zn in all of the partitions and that of Cu in the intracellular organic one. A further decrease in the pH to 2 caused a decrease in the content of Cu in extracellular organic and residual partitions and that of Ni in the inorganic one. The addition of ferric sulfate to the sludge effectively lowered the content of Cd and Cu in extracellular organic and inorganic partitions under acidic conditions. These results would give information on a selection of the method for removing heavy metals from the sludge.

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