KINETICS OF HEAVY-METAL BIOLEACHING FROM SEWAGE-SLUDGE .1. EFFECTS OFPROCESS PARAMETERS

The bioleaching of heavy metals from sewage sludges can be accomplishe d by indigenous sulfur-oxidizing bacteria present in the sludge. The p erformance of the bioleaching process is influenced by various paramet ers such as sludge solids concentration, pH, acid concentration etc. E ffects of such parameters on system performance were experimentally ev aluated. It was found that the sludge suspended solids concentration p lays a major role in regulating the acid production indirectly, by inf luencing the nature of the pH drop in the system; the sludge solids co ncentration does not directly affect the metal solubilization process. An initial sludge pH of 4.0 produced the maximum sulfate production r ate, but the initial sludge pH had little effect on the final metal so lubilization levels achieved. Growth rates of the two bacterial specie s present in the system were found to have a linear relationship with the natural logarithm of the system pH. To take this dependence into a ccount, a new system parameter called the buffering capacity index of the sludge was defined. BCI was found to bear a linear relationship wi th sludge solids concentration, for a given type of sludge (non-digest ed, aerobically digested or anaerobically digested).