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 °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 °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 °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 °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.

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The performance of two laboratory mesophilic anaerobic digesters was studied to determine if the claims for improved mesophilic anaerobic digestion following autothermal pre-treatment are valid and if the benefits claimed are affected by the mesophilic digester SRT. One digester was fed sludge taken after a full-scale autothermal reactor and the other was fed sludge from the same plant just before the autothermal treatment unit. Autothermal treatment did not significantly increase volatile solids destruction kinetics during subsequent anaerobic digestion. The VS destruction rate was 2.7 to 2.8% per day with or without autothermal treatment. Autothermal pre-treatment did not affect methane production per unit VS destroyed at anaerobic digester SRTs ranging from 6 to 12 days. In the digester fed autothermal sludge, higher alkalinity and lower VFA concentration produced a lower VFA to alkalinity ratio, indicating the autothermal pretreatment has the potential to make anaerobic digester operation more stable compared to anaerobic digestion only. Autothermal pretreatment consistently reduced fecal coliform to below detection limits and fecal coliform remained below detection limits during anaerobic digestion.

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