Mechanisms of ultrasound irradiation for enhancing the ANAMMOX process

•RSM was used to optimize ultrasound irradiation on an ANAMMOX consortium.•Enzyme activity was enhanced after ultrasonication.•Mass transfer was accelerated after ultrasonication. 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 0.7W/cm2 and an exposure time (ET) of 1.9min. Compared with ET, UI 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 168.5mgTN/L (TN represents total nitrogen), and the IC50 (half-maximal inhibitory concentration) decreased from 633.4 to 571.4mgTN/L. The dehydrogenase enzyme activity and kinetic analysis of sludge after ultrasound irradiation confirmed that ultrasound treatment under optimal conditions increased sludge activity.