pH requirement for the bioleaching of heavy metals from anaerobically digested wastewater sludge

Bioleaching has been demonstrated to be a feasible technology for removing heavy metals from sewage sludge, but the leaching medium needs to be pre-acidified to less than 4. The objective of the research presented in this paper was to investigate the pH requirement for isolated indigenous Thiobacillus ferrooxidans for bioleaching heavy metals from wastewater sludge in Hong Kong. Isolated sludge-indigenous iron-oxidizing bacteria were used for the bioleaching experiments to investigate the dissolution behaviour of heavy metals (Zn, Cu, Ni and Cr) from sludge set at an initial pH ranging from 3-7 with the purpose to reduce the acid consumption. The results showed that the inoculation of 15% of isolated indigenous iron-oxidizing bacteria and addition of 4.0 g L^-1 Fe²⁺ (in the form of FeSO₄.7H₂O) resulted in a sharp decrease in sludge pH from an initial pH 3-7 to 2.1-2.4 and an increase in ORP (oxidation-reduction potential) from -200-38 mV to > 500 mV within the first 6 days. After 16 days of bioleaching, 50.2-78.4% of Cr, 63.7-74.1% of Cu, 74.9-88.2% of Zn and 15.5-38.6% of Ni can be leached out from the sludge at an initial pH range of 3-7. In contrast, only 1.5% of Cr, 1.7% of Cu, 15.3% of Zn and 15.5% of Ni was solubilized in the control run at pH 7.0. At the end of bioleaching, the dissolution of nutrients N and P from the organic matrix at pH 6 was significantly less than that at pH 3-5. Hence, the bioleaching efficiency could still be maintained at an initial pH of > 4 using the isolated indigenous T. ferrooxidans which would reduce the cost of operation.