COMPARISON OF MICROBIAL SULFURIC-ACID PRODUCTION IN SEWAGE-SLUDGE FROM ADDED SULFUR AND THIOSULFATE

Microbial leaching is one of the most attractive methods of removing t oxic metals from sewage sludge. Sulfuric acid produced by indigenous m icroflora by the oxidation of elemental sulfur and sulfur compounds so lubilizes toxic metals. The oxidation of sulfur compounds can be achie ved by the direct oxidation to sulfates or by indirect oxidation, thro ugh the production and accumulation of soluble intermediate (S2O32-, S 3O62-, S4O62-) compounds. The production of these intermediates may cr eate a potential danger of acidification of the receiving waters or th e agricultural soil where the leached sludge is ultimately destined, v ia slow oxidation of the intermediates with subsequent sulfuric acid p roduction. The objective of this research was to investigate the forma tion of S2O32-, S3O62-, and S4O62- during metal bioleaching using elem ental sulfur and thiosulfate as energy substrates for growth of indige nous thiobacilli (sulfur-oxidizing microorganisms) in sludge. The resu lts obtained showed that intermediates were not formed when elemental sulfur was used as a substrate, whereas trithionate and tetrathionate accumulated in the sludges when thiosulfate was used as substrate. Mor eover, the metabolism of thiosulfate was much slower than that of elem ental sulfur in sludge medium. Therefore, the utilization of thiosulfa te for the growth of indigenous thiobacilli in sewage sludge is a less attractive alternative for the metal bioleaching.