The feasibility and mechanisms of anaerobic ammonium oxidation (ANAMMOX) pathway stimulation by ultrasound irradiation were explored. A response surface methodology (RSM) was chosen to optimize ultrasonication parameters according to specific ANAMMOX activity (SAA), and optimum conditions were attained at an ultrasound intensity (UI) of 0.7 W/cm(2) and an exposure time (ET) of 1.9 min. Compared with ET, Ul significantly affected the SAA. Dehydrogenase enzyme activity measured after ultrasonication was 61% higher compared to the control sludge not exposed to ultrasound. After ultrasound irradiation, the half-saturation constant decreased from 174.5 to 168.5 mgTN/L (TN represents total nitrogen), and the IC50 (half-maximal inhibitory concentration) decreased from 633.4 to 571.4 mgTN/L. The dehydrogenase enzyme activity and kinetic analysis of sludge after ultrasound irradiation confirmed that ultrasound treatment under optimal conditions increased sludge activity. (C) 2014 Elsevier B.V. All rights reserved.