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Ultrasound (US) and chlorine dioxide (ClO(2)) were combined sequentially in an aim to improve wastewater disinfection. Results demonstrated that the combined effect of US and ClO(2) on Escherichia coli (E. coli) and total coliform (TC) inactivation in raw wastewater was synergistic involving high removal of bacteria from the solution. A sequential combination of US (150 or 300 W/L) and ClO(2) (2 mg/L) provided about 3.2-3.5 log reduction in the number E. coli and TC in raw wastewater, while the sum of log reductions by the individual treatments were 1.4-1.9. However, the measured inactivation rate with the combination of ultrasound and ClO(2) in synthetic wastewater or secondary effluent was the same as the sum of the log inactivations with individual treatments. The enhancement attained by combined US and ClO(2) disinfection methods was attributed to the presence of high concentration of particles in raw wastewater and their break up under shock sound waves.

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