A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources & development

Thesis (M.Eng.) - Asian Institute of Technology

The presence of high concentration of heavy metals in digested sewage sludge strictly limits its possible application to agriculture lands. In order to render the sludge suitable for application to agricultural land and to prevent the environment from contamination, heavy metal concentration should be reduced to an acceptable level. A biological oxidation process called bacterial leaching was used to remove heavy metals from anaerobically digested sewage sludge. According to this study, T. ferrooxidans was found to play a significant role in heavy metal removal. The ferric iron could directly oxidized the metal sulfide to metal sulfate (indirect process) without bacteria participation; and this not only enhances the metal · leaching rate but also increases the metal removal efficiency. The ferrous iron, se1ved as an energy source for bacteria growth, significantly affected the heavy metal removal rate and removal efficiency. The metal removal rate and efficiency increased as ferrous iron concentration increased. However, the bacteria activity was surpressed when pH is high than 4.5. In addition, the orders of heavy metal removal efficiency was found being related to the solubility product of mineral sulfide. Sludge with high solids concentration may interference the mass transfer of 0 2 and C02; and this leads to lower the metal removal efficiency. The biological heavy metal removal efficiency from anaerobically digested sludge at pH 2. 5 and 30°C was observed as follows: copper, 80.71%, zinc, 91.61%, manganese, 92.44%, nickel 95.0%, chromium, 33.09%, and cadmium 41.67%. According to Flynn guidelines, the sludge used in this study was originally not acceptable for agricultural land application because of high zinc contents. It was decontaminated after bacterial leaching.