Use of ferrous iron to solid content ratio as a controlling parameter for bioleaching Zn and Cu from anaerobically digested sludge

The effects of solid content on iron oxidation and metal solubilization during the bioleaching of sludge were investigated in the present study with the purpose of establishing a stable bioleaching process using ferrous iron to solid content ratio as a controlling parameter. In comparison with complete iron oxidation on day 1 in sludge with 1% solid content there was a one-day lag time before ferrous iron oxidation occurred and 2-3 days were required for complete iron oxidation in sludge with solid content raised to 2%-4%. As a result the bioleaching time to achieve a pH decrease to <2.5 and an ORP increase to >550 mV increased from 1 day in the sludge containing 1% of solid content to 2 days in sludge containing 2%-4% of solid content. When bioleaching was performed at a ferrous iron (%) to solid content (%) ratio of 2:1, metal solubilization remained almost unchanged and after three days 100% of Zn and 90% of Cu could be leached out from anaerobically digested sludge with 1%-3% solid content, confirming the feasibility of using ferrous iron (%) to solid content (%) ratio as the controlling parameter for bioleaching heavy metals from sewage sludge.