Metal concentration (particularly: cadmium, nickel, chromium and zinc) in municipal sewage sludge from wastewater treatment plants in large agglomerations in Poland exceed the recommended limit for use in agriculture. In addition the concentration of metals varies according to the characteristics of wastewater, sludge treatment and weather. The highest concentration was observed in the case of anaerobically digested sludge. Copper and cadmium bioleaching from polluted sludge was studied under different cultivation conditions. A pure strain of T. ferrooxidans and a mixed culture of T. ferrooxidans, T. thiooxidans and indigenous sulphur oxidising bacteria were used. Various sewage sludges, including primary and digested sludge, exhibit different metal solubilization efficiencies. Anaerobically digested and dehydrated sludge seems to be the most appropriate for metal bioleaching. After 21 days of cultivation the maximal extraction of about 40-50% of Cu and Cd was achieved. The mixed culture of different sulphur and iron-oxidising strains solubilized more metals than T. ferrooxidans alone. The addition of powdered elemental sulphur enhanced the acidification of the sludge and increased the metal extraction. The experiment confirmed the effectiveness of bioleaching for the removal of copper and cadmium to acceptable levels for agricultural use. This is important because sewage sludge is a very good soil conditioner and fertiliser and its application to agricultural land is a very economical method for final sludge disposal. The methods of metal recovery from leaching solution were described earlier by several authors including our team.
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