Modelling and mechanisms of ultrasound-assisted membrane filtration

超滤技术在工业和城市污水处理中得到了广泛的应用。与其他工艺相比，超滤处理废水的好处之一是：电化学氧化、生物处理或芬顿氧化，可以回收出水中的固体，并将滤液循环用作工艺或清洁水。然而，由于膜污染，渗透通量随过滤时间的延长而下降，导致超滤效率降低，膜寿命缩短，清洗频率增加，因此运行成本较高。因此，膜污染的控制在膜过滤中具有重要意义。研究了几种膜污染控制技术，如：进料混凝和絮凝、改变进料的化学/物理性质、膜表面改性和接枝以增加膜的亲水性。然而，由于膜污染的复杂性质，没有一种单一的技术被证明对所有的膜过程都有效。各行各业都使用超声波清洁各种材料的表面，例如：半导体晶圆、塑料、陶瓷和金属零件。超声波可以在液体中诱导形成气泡。气泡破裂可以产生冲击波、局部高温、高压和高流体速度(微喷流)，这为去除膜表面的颗粒提供了一种有效的手段。对陶瓷膜、PVDF膜、PTFE膜和PES膜的超声波清洗进行了研究，超声波频率从28 kHz到100 kHz不等。然而，在这些研究中使用了离线超声波清洗污染的膜。离线清洗的主要缺点是膜过程停机，类似于化学清洗。此外，大多数研究人员在他们的研究中使用了死胡同过滤。这将限制他们的数据和发现在错流过滤中的应用，错流过滤是工业中最常用的膜过滤操作模式。在过滤过程中同时应用超声波控制污垢的机理还不完全清楚。为此，本文提出了超声辅助污垢控制的研究。这将有助于彻底了解超声波在去除和/或破坏膜表面污染颗粒方面的作用，以及控制污染的最佳条件。对于工业应用，污垢控制应该是廉价的。因此，将开展在线集成超声污垢控制的开发，以避免工艺停机。此外，超声诱导的气泡、微射流和微流将改变膜的流体动力学和颗粒-膜相互作用，这在膜污染中起着关键作用。据我们所知，这还没有被研究过。因此，研究超声波作用下的膜污染机理，有助于膜污染控制研究的进一步发展。

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.