To remove heavy metals from sludge and slurries the heavy metals must first be solubilised. In this study, metal bioleaching using sulphuric acid producing microorganisms is investigated. The inhibitory effects of four metals (zinc, chromium, nickel and copper) on acid production were firstly assessed for concentrations of each metal up to 1,000 mg/l. Low concentrations of zinc and chromium (50 mg/L) appear to stimulate the production of sulphuric acid but concentrations of those metals at 500 mg/L and above inhibited acid production (20 to 30% of the control rate at a concentration of 1,000 mg/L). The average sulphuric acid production of the metal free control was 0.78 g/L/day over a 10 day period. At all concentrations of nickel and copper, sulphuric acid production was suppressed and was less than 10% of the control at 250 mg/L or more after 10 days.

Activated sludge was also loaded with the four metals in individual bioleaching tests, in increments from zero up to 5,050 mg of metal/kg solids on a dry basis. In these bioleaching trials the sulphuric acid production averaged 0.9/g/L/day and the pH after 7 days was less than 2.0. Sludge copper concentrations of 3,850 and 5,050 mg/kg partially inhibited acid production (pH after 7 days was 2.4 and 2.7). The solubilisation for each metal after 7 days was Zn: 82–97%, Ni: 12–70%, Cu: 28–55%, and Cr: <20%. Bioleaching of a sludge loaded with a mixture of the four metals gave peak solubilities of 99% for Zn after 3 days, 68% for Ni after 6 days, 57% for Cu after 9 days and 33% for Cr after 9 days.

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