The metal removal from municipal sewage sludge was studied by acid treatment and two microbial leaching processes in laboratory reactors. The leaching potential of the three processes tested were compared from results obtained on twenty-three sludges, and the influence of solids content and types of sludges was also investigated. The use of a microbial leaching process with elemental sulfur and or ferrous sulfate as substrate permits to reduce considerably the quantity of acid required for metal extraction with a reduction of 100 and 83% respectively. Bioleaching process with sulfur as substrate and sulfur-oxidizing bacteria (bioreaction time of 5 days) was revealed to be better than the acid treatment process and microbial leaching with ferrous sulfate and iron-oxidizing bacteria (bioreaction time of 10 days) for solubilization of all metals tested. Microbial leaching process with ferrous sulfate as substrate permits a better solubilization of cadmium, copper, manganese and zinc than the acid tratment with sulfuric acid (1 day, at pH 1.5). However, the solubilization of aluminium, chromium, nickel and lead was less than acid leaching.

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