Effects of Energy Source Concentration on Bioleaching of Heavy Metals from Undigested Sewage Sludge by Using Iron-Oxidizing Bacterium

摘要

The objective of the present study was to optimize the energy source addition required for bioleaching of heavy metals from undigested sewage sludge after secondary treatment. Bioleaching was conducted in a batch system with both inoculation of iron-oxidizing bacteria and the addition of FeSO_4·7H_2O in the range of 0-17.5 g L~(-1). The results showed that the pH of the sludge decreased with an increase in the ferrous iron concentrations and reached the maximum acidity of pH 2.1-3.0 for treatments receiving both bacterial inoculation and substrate addition. This led to a significant solubilization of metals from the solids fraction of sewage sludge. However, solubilization behavior differed for different metal species. Solubilization of Zn was not affected by the concentration of the substrate in the range of 7.5-17.5 gL~(-1), whereas solubilization of Cu and Cr was highly dependent on substrate concentration. After 10 days of bioleaching the following heavy metal solubilization efficiencies were obtained: Zn 100%, Cu 83-98%, and Cr 9-75%. In contrast, no obvious solubilization of Cu and Cr occurred except that less than 20% of Zn was leached out in the control without substrate addition. The result from the present study showed that 10 g L~(-1) of substrate was sufficient to provide an optimum bioleaching condition. However, the biomass reduction rate was far below the criteria for sludge stabilization and thus, a further stabilization process is necessary prior to land application of the bioleached sludge.