In this paper was evaluated the effect of different pretreatments on the solubilization, biodegradability and microbiological quality of waste activated sludge. Six pre-treatments were assayed: thermal, alkaline, ultrasonic, thermal-alkaline, alkaline ultrasonic, ultrasonic-alkaline. Pretreatment with that achieved the highest solubilization was ultrasonic-alkaline pretreatment with 85% and this had the highest biodegradability of 65%. Besides, we observed a reactivation of pathogens in all cases, except thermal-alkaline, which implies a decrease in their microbiological quality and biosolids do not fulfill with the classification type A. The phenomenon of reactivation of pathogens may involve a review of the classification criteria of the biosolids or the inclusion of a parameter storage time prior to final disposal, which would raise the costs of disposal.