Application of ultrasound in membrane separation processes: A review

Abstract

Membrane fouling is one of the major problems encountered during the application of membrane filtration. While power ultrasound has been used as a cleaning mechanism for many decades it has only recently been applied to membrane systems. This review provides an overview of the application of ultrasound in membrane filtration processes on both flux enhancement and membrane cleaning. The ultrasonic cleaning mechanisms, influence of operating parameters and effects of ultrasound on membrane and feed solutions have been examined. Most reports indicate that ultrasound increases membrane permeation by either reducing the depth of the foulant cake layer or by increasing turbulence in the concentration polarisation layer. There is ongoing debate as to whether these effects arise from acoustic cavitation or microstreaming. The main experimental parameters that alter the efficiency of ultrasonic treatment such as external pressure, power density, cross-flow velocity, frequency, temperature and feed properties have also been reviewed. There is a discrepancy in the literature regarding the integrity of membranes following sonication. Hence larger scale trials of ultrasonic application to different membrane modules over a number of years of operation are required to confirm that these impacts do not reduce the total membrane life. Furthermore, work is required to confirm the ultimate effect of sonication upon process economics and life cycle sustainability.