Simultaneous municipal sludge digestion and metal leaching

Abstract

The presence of toxic metals in sewage sludge is one of the most important factors preventing the safe application of this sludge to agricultural land. A few chemical and microbiological methods have been developed for the removal of heavy metals from sewage sludges. A process which removes heavy metals and simultaneously digests the sludge is evidently 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 sewage sludge digestion. The results obtained in batch culture with the sludges from different wastewater treatment plants showed that pH reduction from an initial value of six to eight to less than two in eight days time resulted in the elimination of indicator bacteria (total coliform, fecal coliform, and fecal streptococci) to a level below 10³ CFU/100 mL. The process also achieved a substantial reduction of volatile suspended solids and odor. The pH reduction also resulted in solubilization of toxic metals (Cu 47–95%, Zn 65–98%, Ni 48–97%, Pb 7–63%, Cd 51–93%, Cr 16–58%).