SIMULTANEOUS MUNICIPAL SLUDGE-DIGESTION AND METAL LEACHING

The presence of toxic metals in sewage sludge is one of the most impor tant factors preventing the safe application of this sludge to agricul tural land. A few chemical and microbiological methods have been devel oped for the removal of heavy metals from sewage sludges. A process wh ich removes heavy metals and simultaneously digests the sludge is evid ently advantageous. Therefore, the approach followed in this research was to verify if the process of microbial metal leaching developed in our laboratory could eventually replace the conventional process of se wage sludge digestion. The results obtained in batch culture with the sludges from different wastewater treatment plants showed that pH redu ction from an initial value of six to eight to less than two in eight days time resulted in the elimination of indicator bacteria (total col iform, fecal coliform, and fecal streptococci) to a level below 10(3) CFU/100 mL. The process also achieved a substantial reduction of volat ile suspended solids and odor. The pH reduction also resulted in solub ilization of toxic metals (Cu 47-95%, Zn 65-98%, Ni 48-97%, Pb 7-63%, Cd 51-93%, Cr 16-58%).