Optimization of the proportion of the activated sludge irradiated with low-intensity ultrasound for improving the quality of wastewater treatment

The proportion of irradiated sludge for the enhancement of the biological nitrogen and phosphorus removal from wastewater by low-intensity ultrasound was optimized through contrast experiment between two anaerobic-aerobic-anoxic- aerobic sequencing batch reactors ((AO)₂SBR) with and without ultrasound. Ultrasound with the intensity of 0.2 W∈cm^-2 was employed to irradiate a certain proportion of the condensed sludge in one of the (AO)₂SBRs for 10 min every 12 h. Results showed that the optimal sludge proportion for ultrasonic promotion of chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) removal was from 12.0% to 13.5%. In addition, the effluent COD, TP, and TN of the ultrasonic reactor could be decreased of 37%, 40%, and 5.8% on average compared with the control reactor (without ultrasonic irradiation) under the optimal condition. Furthermore, the settling ability of the sludge was affected but not deteriorated by ultrasonic irradiation.