BACKGROUND: For the first time, low intensity ultrasonication was applied to stimulate biological activities during aerobic digestion of municipal waste activated sludge (AS). Specific oxygen uptake rate (SOUR) analysis was performed to investigate the sono-biostimulation effects on AS activity. RESULTS: The greatest improvement of 60% in SOUR was observed at sonication density and irradiation time of 0.2WmL(-1) and 60 s, respectively. A subsequent batch aerobic digestion study showed 74% reduction in total chemical oxygen demand (TCOD) and volatile suspended solids (VSS) after 12 days of aerobic digestion, with 127% and 112% improvement in TCOD and VSS removal, respectively, in comparison with the control reactor. The sono-biostimulation effects on AS lasted 6 h and completely disappeared within 8 h after sonication stopped. CONCLUSIONS: The sonication density, irradiation time and volume and frequency of sludge sonication were observed to play important roles in bio-stimulation and the subsequent aerobic digestion process. Intermittent sonication protocol for 60 s every 8 h was the best treatment condition to improve aerobic digestion efficiency. (C) 2013 Society of Chemical Industry