Aerobic digestion has been recently studied to further treat anaerobically digested sludge (ADS) at sewage treatment plants. However, the process, which is called post aerobic digestion (PAD), only achieves limited degradation of the ADS. Here, we present a new study using ultrasonic pretreatment to enhance full-scale ADS degradation during PAD. The experiments examined the aerobic digestion of ultrasonically pretreated ADS and untreated ADS using activated sludge as digesting sludge. The ADS was degraded by 28%, 35% and 44% within the 5 day PAD period, when pretreated ultrasonically (20 kHz, 10 min) at 25 W, 50 W and 100 W, respectively. In contrast, in the absence of prior sonication, the ADS was only degraded by 24% in the same PAD period. Increased inorganic nitrogen generation and increased percentage of dead cells occurred in the ultrasonically pretreated ADS, indicating that endogenous respiration resulted in the reduction of volatile solids. The ultrasonic pretreatment significantly improved the aerobic digestion efficiency, which resulted in higher ADS degradation. Additionally, we show that the ultrasonic pretreatment could be an economically favorable technology when the cost of sludge transport and disposal is above $55 per wet tonne.