ABSTRACT
Driven by economic factors, environmental concern, and technological advancement, membrane technology has been playing a major role in various industries. However, membrane productivity and lifetime are severely limited by membrane fouling. Although various methods ranging from process optimization and feed stream pre-treatment to physical and harsh chemical cleaning have been employed to limit its extent, none of these methods was found reliable. An alternative yet very attractive approach to tackle the challenges faced due to membrane fouling is the development of bio-inspired self-cleaning membranes. These membranes take inspiration from nature such as lotus plant and the wings of butterfly. In this chapter, various strategies that can be used to create membrane
with self-cleaning capacity are discussed. These methods are further illustrated with examples where they have shown effectiveness as self-cleaning surfaces. The relation between membrane wettability and self-cleaning property is also highlighted. 9.1 IntroductionMembrane has a versatile application in the separation of particulate matter and biomolecules. However, membrane bio(fouling) severely limits its productivity and lifetime. Depending on the process stream, membrane fouling could be due to salt residue (inorganic fouling), organic fouling due to macromolecules, or biofilm due to microorganisms. Methods to mitigate membrane fouling ranges from feed stream pre-treatment and optimization of operational parameters to application of post-process physical, chemical, or biological membrane cleanings. In particular, for organic fouling and biofilm growth, use of oxidative chemical cleaning reagents such as hypochlorite is the most common approach. Although oxidative reagents proved to be successful in degrading the organic foulant and restoring the membrane performance to a higher degree, they are often nonselective and aggressive to the polymeric constituent of the membrane. Hence depending on the exposure duration and concentration (ppm*h), they tend to degrade the membrane and result in premature membrane module disposal/replacement. Furthermore, since membrane cleaning is a post-damage action, which already allowed direct-membrane foulant interaction, the presence of residual effects even after cleaning is inevitable. In addition, the use of chemical cleaning incurs cost through reduced production during periodic shutdown for cleaning and acquisition and disposal of cleaning reagents. An alternative yet very attractive approach for tackling the challenges faced due to membrane fouling is the development of bio-inspired self-cleaning surfaces. Self-cleaning membranes have recently drawn an ever-growing interest both in academics and industry since they have broad applications ranging from window glass, solar panel cleaning to cements and textiles (Ganesh et al., 2011). The self-cleaning capacity of lotus, often termed as the lotus effect, is hypothesized to be the result of the complex surface
structure of the leaf. Examination of the leaf structure through a scanning electron microscope (SEM) shows two levels of surface structure: micro-scale mound-like structures protruding from the leaf and nanoscale hair-like structures covering the leaf surface (Fig. 9.1). The multi-level surface roughness together with its waxy coatings is thought to be responsible for the lotus leaf’s water repellency. The rolling water on the surface of the leaf collects dirt and other particles along its way, hence self-cleaning. Studies also showed that altering the surface structure without affecting the surface composition, in particular modifying the nanoscale hair-like structure, significantly hampered the membrane surface property and wettability. This clearly indicates the important role of the leaf’s nanoscale hair-like structure on its self-cleaning ability (Cheng et al., 2006).
