Fate of indicator organisms in sludge during bacterial leaching of metals

Biological extraction (bacterial leaching) of heavy metals from digested sewage sludge has been shown to be a practical means for decontaminating sludge for use on agricultural land. However, it was not known whether pathogenic organisms would survive or be destroyed in the acidic environment necessary for the bacterial leaching process. The purpose of the research reported herein was to assess the effect of bacterial leaching on various bacteria commonly used to indicate the possible presence of pathogenic organisms. Although pathogenic viruses, bacteria, protozoa, and helminths may be present in sludge, this study is concerned only with bacterial pathogens. The concentrations of the four indicator bacteria selected to represent the bacterial pathogens were determined in a series of laboratory bacterial leaching units. Results showed that, although about 80–90% metal removal efficiencies were achieved (at a pH of 4.0, an aeration rate of 100 mL of air per minute per litre of sludge) at a temperature of 20–25 °C, the indicator bacteria were not reduced during bacterial leaching at high suspended solids concentrations. The survival of indicator bacteria was found to be a function of the suspended solids concentration in the leaching units, regardless of the source or type of indicator bacteria. At suspended solids concentrations greater than 10 g/L, the indicator bacteria were able to survive, apparently unaffected by the unfavourable environmental conditions (10 days retention at pH 4.0). Below this suspended solids concentration of 10 g/L, significant reductions of total coliforms, faecal coliforms, and faecal streptococci were achieved. Total heterotrophs did not follow the same trends as the other indicator bacteria. Key words: bacterial leaching, Thiobacillus ferrooxidans, indicator bacteria, enteric pathogens removal, sewage sludge, sludge disposal.