Membrane bioreactors (MBRs) have received increasing attention in the field of wastewater treatment in recent years. However, membrane fouling is the main problem of MBRs, limiting their widespread and large applications. Membrane cleaning methods can be mainly classified into four types including chemical, physical, physico-chemical and biological clean the fouled membrane. In recent years, ultrasonication has been reported as a promising cleaning technique for the membranes fouled in MBRs. Ultrasonic irradiation can clean the fouled membrane by creating important physical phenomena including microjets, microstreams and shock waves. Moreover, the ultrasonic method can be combined with other cleaning methods e.g. chemical cleaning and backwashing in order to improve the cleaning efficiency. It should be noted that the application of ultrasonic in the MBR system is not limited to the cleaning of membrane. The pretreatment of the wastewater by ultrasonic irradiation or ultrasound coupled with other methods, e.g. ozonation, prior to MBR system, can decrease the organic loading of the wastewater and subsequently postpone the fouling of the membrane. This paper critically reviews the recent advances in the applications of ultrasound in MBR systems. Emerging issues associated with application of on-line ultrasound and also hybrid on-line ultrasound for controlling the membrane fouling in MBR systems are critically reviewed. Moreover, application of the ultrasound in ex-situ form for cleaning the fouled membranes and pretreatment of wastewater prior to the MBR system is discussed.
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