Enhancing sludge dewatering and heavy metal removal by bioleaching with Na2S2O3 as substrates

Bioleaching has been demonstrated to be an effective technology for the removal of heavy metals and sludge dewaterability. Since bacteria gain nutrients by diffusion of soluble compounds, the insolubility of elemental sulfur may slow the growth of bacteria. Thus, it is very important to find an energy substance quickly become available to Acidithiobacillus thiooxidans. This paper studies the improvement of sludge dewaterability and heavy metal removal with sodium thiosulfate as the source of energy for Acidithiobacillus thiooxidans. Through orthogonal experiments with specific resistance to filtration (SRF) as the target index, four factors (FeSO4 dosage, Na2S2O3 dosage, sludge reflux ratio and sludge moisture content) were identified to be the important influencing parameters. The optimal conditions were: FeSO4 dosage, 8 g/L; Na2S2O3 dosage, 1.5 g/L; sludge reflux ratio, 30%; sludge moisture content, 97%. Results indicated that the SRF of the sludge decreased from 9.89 × 1012 to 1.03 × 1011 m/kg. The removal efficiencies of heavy metals Cu, Zn, Pb and Cr could reach 83%, 78%, 31% and 38% within 3 days, respectively. These results confirm the potential of sodium thiosulfate as an alternative energy substance in bioleaching to improve sludge dewaterability as well as removal of metals.