Production of Class A biosolids depends mainly on the effective control of thermotolerant coliforms and Salmonella spp. Thermal pre-treatment followed by anaerobic mesophilic digestion may be a simpler and more sustainable option to accomplish this compared with other process arrangements. In this work, removal and inactivation of thermotolerant coliforms and Salmonella spp. in waste municipal sludge were studied at three thermal pre-treatment conditions in batch tests (60, 70 and 80 °C). Effective removal was obtained with 70 °C and one-hour pre-treatment, but reactivation of both pathogens was noticed in the following step using an anaerobic mesophilic digester. Predictive microbial decay models (Weibull and sigmoidal-empirical) were used for fitting the experimental data. The sigmoidal-empirical model showed better results at the final counts, suggesting thermal adaptation of some microbial subpopulations. In order to overcome this unexpected result, the influence of the cooling phase was also studied applying three different conditions. For effective treatment, a fast cooling step (in this case, an ice-bath with sodium chloride) should follow thermal pre-treatment (70 °C and 60 min). At these conditions, no reactivation or re-growth of both pathogen indicators were seen during the following 48 h under anaerobic mesophilic digestion.

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