Optimization of the proportion of irradiated sludge for enhancement of sludge activity in biological treatment of wastewater by low intensity ultrasound

The proportion of irradiated sludge (percentage that the irradiated sludge account for the total sludge in the reactor) for enhancement of biological treatment of wastewater by ultrasound (US) was optimized through contrast experiments between two sequenced batch reactors (SBR) with and without US. During the experiment, the US with intensity of 0.3W/cm² and frequency of 35kHz was employed to irradiate a certain percentage of sludge in the US-SBR reactor for 10 minutes every 8 h. The results showed that the removal rate of COD and NH₃-N could be increased of 5% and 0.5% respectively when the proportion of exposed sludge was 10%. A significant increase of sludge activity was also obtained with this proportion and more than 12% increase of OUR (Oxygen Uptake Rate, OUR) was achieved when 10% of sludge was irradiated. Moreover, the growth rate of activated sludge was decreased of 11% when the proportion of irradiated sludge was 10%, which can reduce the subsequent procedure of sludge treatment. With the increase of proportion of the irradiated sludge, SVI (sludge volume index) of the sludge in the US-SBR increased continuously, but it had little influence when the proportion of the irradiated sludge was no more than 10%. Therefore, to improve the treatment efficiency of biological reactor, US with intensity of 0.3W/cm² could be employed to irradiate 10% of the sludge in the biological reactor for 10 minutes every 8 h.