Preventing growth of pathogens in pasteurized digester solids

The goal of this research was to determine conditions that could promote or inhibit pathogen growth during storage or application of pasteurized digester solids. Growth of enterobacteriaceae and Salmonella after pasteurization (70 degrees C for 30 minutes) of mesophilic anaerobic digester solids was reported for numerous facilities in Switzerland but the cause was not known. However, our work found no growth of indigenous Salmonella or fecal coliform in well-mixed pasteurized samples in bench-scale conditions. Salmonella typhimurium was then added in batch tests to determine growth or die-off at 20 and 35 degrees C incubation temperatures. In addition, pasteurized digester solids were fed for 3 months to a laboratory 6-day solids retention time (SRT) secondary mesophilic digester to determine if pathogen growth could occur. Fecal coliform die-off occurred in inoculated pasteurized solids under all conditions, but Salmonella growth readily occurred within 5 days and died off within 30 days of incubation. The Salmonella grew better at 20 than at 35 degrees C, and substrate addition did not improve growth. When the pasteurized solids were mixed and oxygenated, Salmonella die-off instead of growth occurred. Experiments at 35 degrees C with both batch bottles and the secondary digester showed that the presence of other active anaerobic bacteria apparently caused Salmonella die-off instead of growth. Pathogen growth can be prevented and Class A biosolids can be ensured by storing pasteurized digester solids in a secondary mesophilic digester with a 6-day SRT.