Modelling and mechanisms of ultrasound-assisted membrane filtration

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Ultrafiltration (UF) has been widely used in both industrial and municipal wastewater treatment. One of the benefits of treating wastewater by UF over other processes, such as: electrochemical oxidation, biological treatment, or Fenton oxidation, is the potential to reclaim the solid in the effluent and to recycle the filtrate as process or cleaning water. However, due to membrane fouling, the permeate flux declines with filtration time, resulting in a reduced UF efficiency, shorter membrane lifetime, higher cleaning frequency, and hence, a higher operational cost. Therefore, control of membrane fouling is of great importance in membrane filtration. Several membrane-fouling control techniques have been studied, such as: feed coagulation and flocculation, alteration of chemical/physical properties of the feed, membrane surface modification and grafting to increase the membrane hydrophilicity. However, no single technique has been shown to be effective for all membrane processes due to the complicated nature of membrane fouling. Various industries have used ultrasound to clean surfaces of a variety of materials, such as: semiconductor wafers, plastics, ceramic, and metal parts. Ultrasound can induce the formation of bubbles in liquid. Bubble collapse can produce shock-wave, localized high temperature, high pressure, and high fluid velocity (micro-jet), which can provide an effective means to remove particles from the membrane surface. Ultrasonic cleaning of ceramic, PVDF, PTFE, and PES membranes has been studied at varied ultrasonic frequencies from 28 kHz to 100 kHz. However, off-line ultrasonic cleaning of fouled membranes was used in those studies. The main disadvantage of off-line cleaning is the membrane process downtime, similar to that of chemical cleaning. In addition, most researchers used dead-end filtration in their studies. This would limit the application of their data and findings to cross-flow filtration, which is the most commonly-used operational mode for membrane filtration in industries. Mechanisms of fouling control by ultrasound applied concurrently with filtration process are not fully understood. Therefore, a study on ultrasound-assisted fouling control is proposed. This would elucidate a thorough understanding of the role of ultrasound on the removal and/or breakage of fouling particles on the membrane surface and optimal conditions for fouling control. For industrial applications, fouling control should be inexpensive. The development of an on-line integrated ultrasonic fouling control will thus be carried out so as to avoid process downtime. In addition, ultrasound-induced bubbles and micro-jet and micro-streaming would alter the hydrodynamics and the particle-membrane interaction, which play a key role in membrane fouling. To our best knowledge, this has not been studied. Therefore, fouling mechanisms will be studied under flow field alteration by ultrasound, so to facilitate advancement in membrane fouling control.