Membrane filtration enhanced by ultrasound: a review

Fouling is one of the main problems in membrane filtration. The use of additional forces to aid filtration has gained increasing attention in recent years. One additional force is ultrasound. This paper reviews the theory of ultrasound-induced effects, apparatuses used in ultrasound-aided membrane filtration, effects of different process parameters on flux and effects of ultrasound irradiation on membranes. Ultrasound irradiation does not influence the intrinsic permeability of membranes. It increases the flux by breaking the concentration polarisation and cake layer at the membrane surface. The liquid jet serves as the basis for cleaning, but there are also other cavitational mechanisms which have been discovered to lead to particle release from the fouled membrane. There are several parameters that influence the effectiveness of ultrasound treatment. The effects of frequency, power intensity, feed properties, membrane properties, cross-flow velocity, temperature and pressure are reviewed. Membrane durability during ultrasound treatment is important to take into account. Damage due to ultrasound irradiation on the membrane surface has been discovered in some research, whereas in other studies even a frequent use of ultrasound did not affect the membranes. Ultrasonically enhanced membrane filtration has not yet been widely commercialised. The main reasons for prevention of a break-through are the development stagnation of transducer technology for membrane filtration and the control of membrane erosion.