Effects of sulfur dosage and inoculum size on pilot-scale thermophilic bioleaching of heavy metals from sewage sludge

Abstract Land application of sewage sludge has received significant attention in recent years but the presence of elevated heavy metals in the sludge limits its land application. The purposes of this study were to investigate the effects of sulfur dosage and inoculum size on the thermophilic bioleaching of heavy metals from sewage sludge in a pilot-scale bioreactor. The microbial communities in this thermophilic bioleaching process were also identified using real-time polymerase chain reaction (real-time PCR). The results showed that the oxidation of sulfur and metal solubilization decreased with the increasing sulfur dosage. When the sulfur dosage was greater than 2% (w/v), the sulfur oxidation and metal solubilization rates decreased, indicating that the thermophilic bioleaching was hindered by high levels of substrate. However, it was found that the efficiency of metal solubilization and solid degradation was increased with the increase of inoculum size in the range from 5% to 20%. At the end of bioleaching, the efficiency of Mn, Zn, Ni, Cu and Cr from the sewage sludge reached 73–100%, 51–60%, 38–52%, 17–43% and 1–38%, respectively, while SS and VSS were degraded by 33–48% and 47–67%, respectively. Based on the analysis of real-time PCR, Sulfobacillus acidophilus was observed to be the predominant species (13–67% of total bacteria), whereas the populations of Sulfobacillus thermosulfidooxidans and Acidithiobacillus caldus were accounted relatively low (