In this study, low-strength ultrasonication was applied at various ultrasonication densities (UD) (0-0.1 W/mL) and ultrasonication time (UT) (0-30 min) to methanogenic granules on the purpose of increasing their activity, and eventually, enhancing the performance of upflow anaerobic sludge blanket reactor (UASBr). Batch test results showed that 5 min of ultrasonication at 0.05 W/mL was found to be the optimal conditions, resulting in the increase of dehydrogenase activity and adenosine triphosphate content by 257%, and 374%, respectively, compared to the control. These increments were confirmed by specific methanogenic activity test. When ultrasonication CUD 0.05 W/mL, UT 5 min) was irradiated every 8 h during the continuous operation of UASBr, it caused a gradual drop of methanogenic activity, complete loss after 20 days. At further operation, UT was decreased to 1 s but irradiated every 1 min, which resulted in a 43% higher specific CH4 production rate. (c) 2012 Elsevier Ltd. All rights reserved.