Insights into the enhanced effect of Low-Intensity Ultrasound on anammox granular sludge by relieving the embolism

Abstract

Like the lethal cardiovascular embolism in humans, the serious blockage of the mass transfer channel can inactivate anammox granular sludge (AnGS). Also like the therapeutic thrombolysis in humans, the remediatory dredging of the mass transfer channel can sustain the function of AnGS. In this study, the ultrasound was thus explored to remediate the embolism of AnGS. It turned out that Low-Intensity Ultrasound (LIU) with an energy density of 5.6 kJ/(g-VSS·cm2) was optimal to achieve an obvious increase in specific anammox activity of AnGS by 34.8% to 420.3 mg-N/(g-VSS·d). The improved mass transfer of AnGS and cell membrane permeability of anammox bacteria (AnAOB) was revealed to mainly contribute to the enhanced activity. After treatment with LIU under optimal conditions, the mass transfer efficiency of AnGS was improved by 26.9% with enhanced permeability, and the proportions of pores larger than 50 μm in the surface layer of AnGS increased by 13.7% evidenced by the micro-CT observation. Correspondingly, the permeability of AnAOB was increased by inducing instantaneous pores on the cell membrane, thus stimulating the metabolism by 6.9%. Further analysis demonstrated that LIU was capable of breaking the hydrogen bonds of protein in the EPS and loosening its spatial structure, which would benefit the partially exfoliation and dissolution of EPS and resulted in enlarging and dredging pores, thus relieving the embolism of AnGS. Consequently, the results of this study could deepen the understanding of granules treated by ultrasound and also provide a promising technique for raising the ceiling of nitrogen removal performance by AnGS.