This study has primarily established ultrasound disintegration as one of the many methods for the pre-treatment of thickened waste activated solids (TWAS) from Denver, CO and Marshalltown, IA WWTPs ranged from 4% to 4.8% of total solid concentration to control foaming in mesophilic anaerobic reactors. A secondary objective of the study was to evaluate the performance of ultrasound-conditioned (sonicated) TWAS for minimizing the retention time and enhancing the destruction of volatile solids. The sonication duration was adjusted to achieve energy inputs equivalent to 1.25 and 1.95 kWs/g of TWAS. Three reactors; control, full-stream reactor that received 100% sonicated TWAS and part-stream reactor that received 30% sonicated TWAS, were operated at various solids retention times (SRTs) of 15, 10, and 6 days.;The effect of ultrasound disintegration on TWAS was demonstrated in terms of the increase in COD solubilization, the reduction of solids particle size as well as the largest increase in turbidity, and microscopic photographs shown the disintegration of the bacterial cells.;During the lower energy input of 1.25 kWs/g TWAS, the 100% TWAS sonicated unit was able to control foam for both Denver and Marshalltown feed solids effectively. While the reactor that received 100% sonicated Marshalltown TWAS showed the lowest tendency to foam, the reactor that received 100% sonicated Denver TWAS showed the greatest foaming potential during the higher energy input of 1.95 kWs/g TWAS.;Contrary to other reported studies, there was no significant difference in the performance of the three reactors with regard to volatile solids reduction (VSR). All three reactors achieved 47-53% VSR at 15 day HRT, 42-48% VSR at 10 day HRT, and 41-47% VSR at 6 day HRT. Sonication of TWAS increased the rate of substrate degradation, but did not improve the ultimate degree of degradation.