BACTERIAL LEACHING OF HEAVY-METALS FROM AEROBIC SLUDGE

Fifty percent of sludges produced by wastewater treatment plants in the Province of Quebec (Canada) cannot be spread on agricultural land because of their heavy metal content. In this study, leaching of heavy metals by bacteria (Thiobacillus) was achieved for the first time using an aerobic digested sludge. This was done in batch experiments with the addition of a substrate. Different options were tested with reference to time periods required for metal solubilization and sludge digestion. Metal solubilization simultaneous with sludge digestion proved to be a process both slow and difficult to apply. In aerobic digested sludges, metal solubilization took place within one day at a rate of 7.7 mg Cu per liter per day and at initial concentration of 1022 mg/kg dry sludge. Oxidation-reduction potential and pH affect copper solubilization. The importance of the ORP in copper solubilization explains the great rapidity with which this element is made soluble in aerobic sludges. Metal solubilization in aerobic sludges is thus possible and the high rates obtained in this study suggest that 1 day in a continuously-stirred tank reactor (CSTR) or 1 to 2 days in a batch reactor would suffice to achieve high solubilization efficiencies.