New insights into the improved contaminants removal in SBR by intermittently weak ultrasound

The combination of intermittently weak ultrasound and sequencing batch reactor was thoroughly investigated to elucidate the relationship between enhanced contaminants removal and activated sludge characteristics, microbial composition, and regulation of differentially expressed genes (DEGs). At 12 °C, irradiation with an ultrasound intensity of 9.68 W/L, an irradiation time of 10 min, and an interval time of 24 h led to significant increases in COD, NH₄⁺−N, and TP removals with the rates of 93.10 ± 1.51%, 95.75 ± 0.76%, and 92.52 ± 0.95%, respectively. The intermittently weak ultrasound enhanced contaminants removal was primarily attributed to the stimulated microbial metabolism, in which the mechanical oscillation rather than free radical oxidation facilitated the loosening of activated sludge flocs and promoted microorganism proliferation. Elevating the ultrasound intensity or irradiation time could weaken the effect of enhancing ammonia−oxidizing bacteria activity and suppressing nitrite−oxidizing bacteria activity. The results revealed that intermittently weak ultrasound primarily affected the extracellular polymeric substances (EPS), with protein nitrogen playing a more significant role than polysaccharide within EPS against ultrasound−induced stress. Furthermore, ultrasound irradiation elevated the energy barrier in total−binding EPS interaction energy curves, thereby inhibiting activated sludge aggregation. Over prolonged operation, the relative abundance of the prevalent denitrifying genus Thauera increased by 90.3%, whereas that of the fully aerobic denitrifier and nitrite producer Dokdonella increased by 68.7%. The intermittently weak ultrasound induced enhancement of microbial metabolism−related DEGs pathways, which served as the main contributor to the improved contaminants removal. These findings provide novel insights into the mechanisms by which intermittently weak ultrasound enhances the effectiveness of biological wastewater treatment.