Modelling and mechanisms of ultrasound-assisted membrane filtration

超滤技术已广泛应用于工业废水和城市污水的处理。通过UF处理废水的优点之一是可以回收流出物中的固体并将滤液作为工艺用水或清洁用水再循环。UF处理废水的优点之一是：电化学氧化、生物处理或芬顿氧化。然而，由于膜污染，渗透通量随过滤时间下降，导致UF效率降低，膜寿命缩短，清洗频率提高，因此操作成本较高。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，没有单一的技术已被证明是有效的所有膜过程。各种行业已经使用超声波来清洁各种材料的表面，例如：半导体晶片、塑料、陶瓷和金属部件。超声波可以诱导液体中气泡的形成。气泡破裂可以产生冲击波、局部高温、高压和高流体速度（微射流），这可以提供从膜表面去除颗粒的有效手段。研究了在28 kHz ~ 100 kHz的不同超声频率下对陶瓷、PVDF、PTFE和PES膜的超声清洗。然而，在这些研究中使用了污染膜的离线超声波清洗。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死端过滤。这将限制他们的数据和发现应用于交叉流过滤，这是工业中膜过滤最常用的操作模式。与过滤过程同时应用的超声波污垢控制的机制尚未完全理解。因此，提出了超声辅助污垢控制的研究。这将阐明一个透彻的理解的作用，超声波的去除和/或破坏的污染颗粒的膜表面和最佳条件的污染控制。对于工业应用，结垢控制应该是廉价的。因此，将进行在线集成超声波污垢控制的开发，以避免工艺停机时间。另外，超声作用下产生的气泡、微射流和微流会改变流体力学特性和颗粒与膜的相互作用，对膜污染起着重要作用。据我们所知，这还没有被研究过。因此，研究超声波改变流场条件下的膜污染机理，将有助于膜污染控制技术的进一步发展。

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.