Improvement of the activity of activated sludge by low intensity ultrasound

Aerobic activated sludge from domestic wastewater treatment plant was used as an experimental material. Oxygen Uptake Rate (OUR) was detected as the index to indicate the changes of sludge activity induced by ultrasound (US) at 35 kHz in 0-40 min with the US intensities of 0-1.2 W/cm². The best results were achieved at a US intensity of 0.3 W/cm² and irradiation time of 10 min. Improper US intensity and irradiation time lead to a decrease of sludge activity. Therefore, the stimulating effects of US on the sludge activity could be enhanced at an optimized US intensity and irradiation time. In addition, changes of sludge activities (within 48 hours) after US irradiation at 0.3 W/cm² in 10 min were examined. The activity of sludge took 8 hours after irradiation to reach peak level, and the activity was as double as that of the initial level after irradiation. In 24 hours after irradiation, the enhancement effects induced by ultrasound disappeared. Therefore, to improve the treatment efficiency of biological reactor, ultrasound with intensity of 0.3 W/cm² could be employed to irradiate part of the sludge in the biological treatment system for 10 minutes every 8 hours. It also presented the hypothetical explanation of the mechanism of biological activity enhancement stimulated by ultrasound.