Bioleaching for Extraction of Metals and Detoxification of Activated Sludges

Abstract

Samples of anaerobically digested sludges from municipal wastewater treatment installations containing several toxic heavy metals (Cu, Zn, Ni, Co, Cr, Mn, Fe), different organochemicals and biological pollutants (mainly bacteria of the genera Escherichia, Klebsiella, Streptococcus and Clostridium) were subjected to different procedures for detoxification to make possible their use in agriculture. It was found that the bioleaching of these sludges at 70 °C by mixed cultures of extremely thermophilic chemolithotrophic bacteria was the most efficient way to decrease below the relevant permissible levels all above-mentioned pollutants. The bioleaching was carried out in reactors with mechanical stirring and enhanced aeration (by CO2 enriched air) under batch and continuous-flow conditions. The extraction of some metals (mainly of Cu and Zn) which were present in relatively high concentrations in some sludge samples was very efficient and exceeded 90 % within residence times of about 96 - 120 hours. The dissolved copper and zinc were recovered from the pregnant solutions after bioleaching by means of solvent extraction plus electrolysis. At the same time, the bioassimilable forms of N, P, K and the essential microelements were still present in sufficient concentrations in the pretreated sludges making them suitable for use in agriculture. This was confirmed by experiments in green-houses with plants intended for use in the bioremediation of post-mining landscapes.