Decontamination of fly ash and used lime from municipal waste incinerator using Thiobacillus ferrooxidans

The purification or incinerator gases produces large quantities of fly ash and used lime [a combination referred to as air pollution control residues (APCR)], both of which contain elevated levels of metals. This paper descri bes biological solubilization assays utilizing Thiobacillus ferrooxidans to leach metals from APCR to render them nonhazardous. The multistage solubil ization process involves an alkaline aqueous phase that removes some Pb. In the second phase, the APCR are acidified to pH 4 with H2SO4, then inoculat ed with a bacterial culture that has been acclimated in the presence of 2% Fe (FeCl3). Several rinses and decantings achieve removal of the leachable metals. The final step involves the addition of Ca(H2PO4)(2) and an increas e in the treatment pH prior to the final filtration. Viability of thiobacil li in APCR was poor. Despite this problem, the removal of Pb was 35.9%, 46. 0%, and 68.7% (for APCR containing 1594, 3026, and 5038 mg Pb/kg, respectiv ely), which demonstrates greater metal removal with increased APCR contamin ation. Zn removal varied from 68.2% (8273 mg Zn/kg APCR) to 79.5% (16,873 m g Zn/kg APCR), which was positively correlated to the level of residue cont amination, whereas Cu was removed in the proportions of 26.9% (495 mg Cu/kg APCR) to 68.2% (465 mg Cu/kg APCR). Cadmium removal appeared to be indepen dent of the level of Cd in the APCR; Cd was removed to the greatest degree, with a variation of 92.0% (129 mg Cd/kg APCR) to 94.7% (267 mg Cd/kg APCR) . The treated APCR were tested using four different leachate tests. The APC R released 43 mg Pb/liter during contact with water, and 7.40 mg Cd/liter d uring TCLP [the toxicity characterization leaching procedure of the United States Environmental Protection Agency (US EPA)]. After biological treatmen t, the leachate from TCLP was within the acceptance criteria of the US EPA, if the pH of the APCR was increased to pH 5 after the biological treatment . In the case of the Transport Canada leaching test, a betterment of the pr ocess is required in order to satisfy the stringent regulatory level of 0.5 mg Cd/liter (0.68 and 0.57 mg/liter).