Kinetics of heavy metal bioleaching from sewage sludge—I. Effects of process parameters

Abstract

Abstract The bioleaching of heavy metals from sewage sludges can be accomplished by indigenous sulfur-oxidizing bacteria present in the sludge. The performance of the bioleaching process is influenced by various parameters such as sludge solids concentration, pH, acid concentration etc. Effects of such parameters on system performance were experimentally evaluated. It was found that the sludge suspended solids concentration plays a major role in regulating the acid production indirectly, by influencing the nature of the pH drop in the system; the sludge solids concentration does not directly affect the metal solubilization process. An initial sludge pH of 4.0 produced the maximum sulfate production rate, but the initial sludge pH had little effect on the final metal solubilization levels achieved. Growth rates of the two bacterial species present in the system were found to have a linear relationship with the natural logarithm of the system pH. To take this dependence into account, a new system parameter called the buffering capacity index of the sludge was defined. BCI was found to bear a linear relationship with sludge solids concentration, for a given type of sludge (non-digested, aerobically digested or anaerobically digested).