Toxicity assessment of a dye industry treatment sludge

Industrial treatment sludges often contain heavy metals and a variety of other hazardous substances which can cause soil and underground water pollution. In the present work, sludge samples from a dye industry treatment plant were analysed for their heavy metal contents, and three different leaching procedures were used to assess the potential toxicity of the sludge. The sludge samples were also incinerated at 600°C, and the remaining inorganic residues were also tested for their leaching characteristics by using the same leaching procedures. The treatment sludge investigated in this study contained about 55 grams of Fe per kilogram of dry sludge because iron salts are used in the chemical treatment of the wastewater. Excluding iron, the decreasing order of abundance for heavy metals in the sludge samples was found to be: Zn> Mn> Cr> Cu> Ni> Pb> Co> Cd. The average concentration of each of these heavy metals varied between 25-740 mg/kg of dry sludge. The Fe content of the inorganic residues was found to be about 140 g/kg, and the heavy metal content varied between 40-2800 mg/kg. In all the leaching tests, Pb was the most readily extracted metal from the sludge, whereas from the inorganic residue, Cd was leached the most easily. The toxicity of the sludge is estimated according to pertinent legislation.