Fate of Salmonella typhimurium and total coliforms during bacterial leaching

Abstract

ABSTRACT: A biological metal extraction (bacterial leaching) process has been developed to extract metals from sludge. Although the removal of metals from sludge is a significant step, any biological hazard must be eliminated before the sludge can be used on land. Earlier studies established that indicator bacteria were not reduced significantly during bacterial leaching. Whether pathogens also would survive or be destroyed in the acidic environment of the bacterial leaching process was not determined. This study was intended to answer this question. A series of batch units performing bacterial leaching was operated with varying sludge concentrations spiked with attenuated Salmonella typhimurium. The concentrations of Salmonella typhimurium and total coliforms in the sludge were determined. Results showed that, although the Salmonella typhimurium population was eliminated within 7 hours (at pH 4.0, an aeration rate of 100 mL of air per minute per litre of sludge, and at a room temperature of 20 to 25 °C), the total coliforms persisted during the leaching period of 10 days. The inactivation of the Salmonella typhimurium appeared to be independent of the suspended solids concentration in the leaching units, unlike the survival of total coliforms which was found to be a function of the suspended solids concentration. At suspended solids concentrations of about 40 g/L, the total coliforms persisted, whereas after 5 days, coliforms were not detected in units containing about 10 g SS/L.