Growth stimulation of sulfur oxidizing bacteria for optimization of metal leaching efficiency of fly ash from municipal solid waste incineration

Sulfur oxidizing bacteria were used to leach metals bound in municipal solid waste incineration fly ash. Pure or mixed cultures of Thiobacilli with or without addition of anoxic sewage sludge were incubated with various amounts of fly ash for several weeks. Co-cultures with Thiobacilli and sewage sludge grew faster than pure cultures of T. thiooxidans or cultures with only sewage sludge as inoculum. The addition of sewage sludge increased the rate of growth of sulfur oxidizing bacteria and simultaneously enhanced the acidification of the suspension. As a result, the time for fly ash processing was reduced by 50%. Thiobacilli were able to grow in the presence of up to 8% (w/v) fly ash. Depending on the amounts of fly ash added, the final pH, and the concentration of sulfuric acid formed, different amounts of metals were leached: over 80% for Cd, Cu, and Zn, 60% for Al and approx. 30% for Fe and Ni (totally, approx. 50% of the heavy metals present in the fly ash) were extracted.