Investigation of Combined Fenton/Bioleaching and Bio-Acidification/Fenton-Like Processes in the Removal of Sludge-Borne Metals

Physical and chemical processes involved in wastewater treatment cause heavy metals that are present in raw sewage to accumulate in sludge, which limits utilization of sludge as a fertilizer. Large quantities of these metals negatively affect ecosystems and human health. The present study aimed to investigate the possibility of leaching heavy metals from wastewater sludge by bioleaching (Acidithiobacillus ferrooxidans), Fenton/bioleaching, and bioleaching/Fenton-like processes. Bioleaching tests with iron concentrations of 1 - 4 g/L were carried out for 9 days. Combined Fenton/bioleaching and Bio-acidification/Fenton-like methods were also conducted with Fe2+ concentrations of 0.5 - 2 g/L and H2O2 concentrations of 0.25 - 1 g/L. The results showed that the bioleaching method with Fe2+ concentration of 2 g/L led to removal of 87.1% of zinc, 9% of cadmium, 69.9% of lead, and 69.9% of copper from sludge in 5 days. In the combined methods, the highest amount of removal was related to Fe2+ 2 g/L and H2O2 1 g/L in 2 days. Under these circumstances, 93% of Zn, 97.06% of Cd, 87% of Pb, and 94.5% of Cu in the bio-acidification/Fenton-like were removed. The results suggested that in comparison to single Fenton and bioleaching methods, the combined methods consumed less hydrogen peroxide and did not require pH adjustment, thus, resulting in higher efficiency in the removal of heavy metals from sludge.