Inactivation of pathogenic bacteria under mesophilic and thermophilic conditions

This paper concerns field survey and laboratory experiment on the inactivation of pathogenic bacteria during sludge treatment processes with particular emphasis on anaerobic digestion process. We surveyed the inactivation of pathogenic bacteria processes by sampling various types of sludges from 17 wastewater treatment plants located in Japan and counting the number of bacteria in the sampled sludges. The bacteria we counted included fecal coliform groups, enterococcus and salmonella. The median number of fecal coliform groups in primary sludge was found to be 10(5) MPN/g, while the number of fecal coliform groups in digested sludge decreased to 10(3) MPN/g. We also confirmed that the treatments of dewatering using inorganic coagulant, drying and composting are also effective in inactivating pathogenic bacteria. In addition, we studied the performance conditions of anaerobic digestion and the degree of inactivation of pathogenic bacteria in the experiment of anaerobic treatment of the primary sludge. This study showed that the number of fecal coliform groups in mesophilic digestion sludge was in the range of 10(2) to 10(4) MPN/g regardless of the HRT, whereas the number of bacteria in thermophilic digestion sludge was of the order of 10(0) MPN/g, clearly indicating that the number of bacteria substantially decreases when the sludge is digested at thermophilic temperature, The number of enterococcus in digested sludge was in the range of 10(2) to 10(5) MPN/g after the sludge was subjected to mesophilic digestion while the number decreased to 10(0) MPN/g after the sludge was digested at thermophilic temperature. The number of salmonella in digested sludge was in the range of 1.8 to 30 MPN/4g after the sludge was digested at mesophilic temperature, but the number decreased to less than 1.8 MPN/4g after the sludge went through thermophilic digestion process. The thermophilic digestion is thus effective in inactivating pathogenic bacteria. (C) 1997 IAWQ. Published by Elsevier Science Ltd.