MAJOR FACTORS INFLUENCING BACTERIAL LEACHING OF HEAVY-METALS (CU AND ZN) FROM ANAEROBIC SLUDGE

Anaerobically digested sewage sludges were treated for heavy metal rem oval through a biological solubilization process called bacterial leac hing (bioleaching). The solubilization of copper and zinc from these s ludges is described in this study: using continuously stirred tank rea ctors with and without sludge recycling at different mean hydraulic re sidence times (1, 2, 3 and 4 days). Significant linear equations were established for the solubilization of zinc and copper according to rel evant parameters: oxygen reduction potential (ORP), pH and residence t ime (t). Zinc solubilization was related to the residence time with a r2 (explained variance) of 0.82. Considering only t = 2 and 3 days exp lained variance of 0.31 and 0.24 were found between zinc solubilizatio n as a function of ORP and pH indicating a minor importance of those t wo factors for this metal in the range of pH and ORP experimented. Cu solubilization was weakly correlated to mean hydraulic residence time (r2 = 0.48), while it was highly correlated to ORP (r2 = 0.80) and pH (r2 = 0.62) considering only t of 2 and 3 days in the case of pH and O RP. The ORP dependence of Cu solubilization has been clearly demonstra ted in this study. In addition to this, the importance of the substrat e concentration for Cu solubilization has been confirmed. The hypothes is of a biological solubilization of Cu by the indirect mechanism has been supported. The results permit, under optimum conditions, the draw ing of linear equations which will allow prediction of metal solubiliz ation efficiencies from the parameters pH (Cu), ORP (Cu) and residence time (Cu and Zn), during the treatment. The linear regressions will b e a useful tool for routine operation of the process.