Effects of low-intensity ultrasound on anammox granular sludge

Low-intensity ultrasound can effectively shorten the start-up time of the anaerobic ammonia oxidation process and improve nitrogen removal performance. However, the effect of low-intensity ultrasound on the anammox granular sludge structure and the functional bacteria metabolism is still unclear. Herein, the effects of ultrasonic irradiation with different ultrasonic power densities and treatment times on nitrogen removal performance and anammox granular sludge structure were studied. The activity of anammox granular sludge increased by 20.81 % after irradiation (0.2 W/mL ultrasonic power density, 2 min treatment time). When further increased to 0.3 W/mL or 4 min, respectively, sludge activity was inhibited. Under optimal ultrasound parameters, the activity of Nar, NirS, and HZO was increased by 43.98 %, 17.92 %, and 16.49 %, respectively, and the cell membranes permeability was improved. Extracellular polymers and cell cluster shedding improved the mass transfer performance. Excessive ultrasonic energy lead to cell membrane damage or cell rupture.