Bioleaching of toxic metals from anaerobically digested sludge without external chemical addition

Anaerobically digested (AD) sludge is widely applied to agricultural land as fertilizer. However, heavy metals in AD sludge potentially pose a significant threat to environment. This study reports a novel bioleaching approach, with no need for externally added chemicals. Sludge acidification was achieved using the protons produced from microbial oxidation of the inherent ammonium in AD sludge. An acid-tolerant microbial consortium, dominated by ammonia-oxidizing bacteria from the genus Candidatus Nitrosoglobus (i.e. relative abundance of 72.5 ± 2.3% based on 16S rRNA gene sequencing), was enriched after 120 days incubation in a laboratory sequencing batch reactor. The consortium oxidizes ammonium even at pH 2.5, at approximately 30% of its maximum rate, measured at pH 5.5. Inoculating the consortium at a solid ratio of 1:20, caused the pH of the AD sludge to decrease from 7.5 to 2.0 over five days under aerobic conditions. As a result, metals in the AD sludge were efficiently extracted into the liquid phase. In particular, two of the most abundant toxic metals, Cu and Zn, were solubilized with high efficiencies of 88 ± 4% and 96 ± 3%, respectively. Overall, the results of this study enable the economical and safe reuse of excess sludge generated during biological wastewater treatment.