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 degreesC 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. (C) 2002 Elsevier Science Ltd. All rights reserved.