This study aimed at assessing the impact of ultrasonication on the anaerobic digestibility of thickened waste activated sludge (TWAS) in an anaerobic fluidized bed reactor (AnFBR). Two lab-scale AnFBRs treating TWAS were studied to explore the impact of ultrasonication (US) in the dispersing and reuse of scum for methane production. Our current work applied ultrasound energy at 120-600 kJ/d for 2 s per 30 s corresponding to ultrasonication densities of 6-15.7 MJ/kg dry-scum-d. At an organic loading rate (OLR) of 5.1 kg COD/m(3)-d and US energy of 11.9 MJ/kg dry-scum-d, scum decreased by 82% from 20.2 gm/d to 3.7 gm/d, and COD and VSS destruction efficiencies were 65% and 63%, respectively roughly 20% higher than the control reactor without US. Scum reduction varied linearly with US energy about 25 kJ/g TS was required to break the scum. The energy balance also indicated that the aforementioned US energy of 11.9 MJ/kg dry-scum-d was optimum because of energy neutrality. Specific methanogenic activity (SMA) tests showed that the activity-based sludge retention time (SRT) is higher for the ultrasonicated AnFBR (U-AnFBR) (7.1 days) compared to AnFBR (5.1 days). Furthermore, a higher rate of maximum specific biogas production (R-m) was observed in the U-AnFBR of 26.7 ml/gmVSS-hr as compared to 15.7 ml/gmVSS-hr for the control AnFBR, with respective suspended biomass activities of 2 ml/gmVSS-hr and 4.1 ml/gmVSS-hr i.e. confirming that US improved methanogenic activity in the reactor and reduced the detachment of active methanogens. (C) 2017 Elsevier Ltd. All rights reserved.