Abstract
The effect of sludge retention time (SRT) on chemical and biological leaching of heavy metals from anaerobically digested sewage sludge was investigated by using afill and draw reactor. The reactor was operated at SRTs of 0.75, 1, 1.5 and 3 days. Cu was effectively eluted in the biological leaching than the chemical one. The optimum SRT for the removal percent of heavy metals was obtained at SRT of 1.5 days in the biological leaching: 70% of Cd, 95% of Ni, 1% of Pb, 5% of Cr, 50% of Cu, 95% of Mn and 70% of Zn.
Metal forms in sludge were studied by a sequential selective extraction method. The major form of Cu in the sludge was sulfide. Cu was not eluted by the chemical leaching but eluted by the biological one. Then, the elutional mechanism of Cu from CuS was investigated by batch experiments. Elution of Cu was not carried out by the decrease of pH to 3. However, the addition of ferric sulfate or the inoculation of Thiobacillus ferrooxidans resulted in about 60% of elution
