Application of biological method for removing selected heavy metals from sewage sludge

The aim of the study was to evaluate the application of bioleaching technique to reduce content of selected heavy metals (Zn, Cu) in sewage sludge, and hence to indicate possibilities for metals recovery from this type of waste. Bioleaching experiments were carried out with mixed bacteria Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, using leaching media: ferrous sulfate with different concentrations of Fe²⁺ (2 g/dm³ and 9 g/dm³) and sulfuric acid (VI). Dynamics of the increase in zinc concentration in biological systems was almost identical for both 9 g Fe²⁺/dm³ and 2 g Fe²⁺/dm³ in samples. However, higher values of Cu concentration were achieved using a medium with iron(II) salt 9 g/dm³ than in a 2 g/dm³ solution. Bioleaching with 9 g Fe²⁺/dm³ allowed for a nearly 20-fold reduction of zinc content and a 2-fold reduction in copper content in sewage sludge. Using 9 g/dm³ ferrous sulfate bioleaching could dissolve 94.8% Zn and 58.9%, whereas chemical leaching dissolved 47.3% Zn and 4.2% Cu.