Comparison of bioleaching of heavy metals from sewage sludge using iron- and sulfur-oxidizing bacteria

The aim of the present study was to compare the bioleaching efficiency of Cu, Zn and Cr from anaerobically digested sewage sludge using iron- and sulfur-oxidizing bacteria. Bioleaching was performed on sewage sludge collected from the Yuen Long wastewater treatment plant. A 15% (v/v) inoculation of either iron- or sulfur-oxidizing bacteria with 4 g FeSO₄ l^-1 and 0.75% elemental sulfur, respectively, was added to sewage sludge with or without autoclaving in the bioleaching experiment. The mixtures were shaken continuously in an incubator at 30 °C for 16 days and samples were tested at 2-day intervals for pH, ORP and metal determination. The results showed that the iron-oxidizing system required only 2 days as compared to 4 days for the sulfur-oxidizing system to reduce the sludge pH from 7 to 2. Both systems achieved a maximum Cr removal of 52-58% after 12 days of bioleaching, but for iron-oxidizing bacteria with iron-sulfate as an energy source it was 20% higher at the beginning of leaching process. Although it took only 2 days to solubilize Cu by iron-oxidizing bacteria as compared to 8 days for sulfur oxidizing bacteria, the iron-oxidizing system removed only 80% of the total Cu, which was 20% lower than that of the sulfur-oxidizing system. Both iron- and sulfur-oxidizing bacteria achieved 95% Zn removals after 4 days of bioleaching. The results demonstrated that the iron-oxidizing system had a faster removal rate than the sulfur-oxidizing bacteria. Nevertheless, further work should be done to improve the bioleaching efficiency of iron-oxidizing bacteria, especially for Cu and Cr.