Environmental Engineering Reference
In-Depth Information
CHAPTER 12
Jiangyong Hu, Lai Yoke Lee, Junhong Shan, Say Leong Ong, and
How Yong Ng
Today, membrane processes are used in a wide range of applications, and their
numbers will certainly increase. A general definition of membrane could be: a selective
barrier between two phases, the term 'selective' being inherent to a membrane or a
membrane process (Mulder, 1996). The definition is at the macroscopic level and says
nothing about membrane structure and function, while separation should be considered
as the microscopic level. Nanomembrane describes the nanostructured materials that
could separate the contaminants with molecular sizes in nano-scale. This chapter
discusses the general application of nanomembrane for mitigation of water
environmental issues.
The basic principle for membrane filtration is to use semi-permeable membranes
to separate fluids, gases, particles and/or solutes. For separations from the water phase,
the membrane is water-permeable, but less permeable to solutes and other particles,
which may occur through various mechanisms, such as gradients in pressure,
concentration, electrical potential or temperature. Pressure-driven membrane processes,
distinguished from the particle size of the solute and, consequently, related to the
membrane structure, include microfiltration (MF), ultrafiltration (UF), nanofiltration
(NF) and reverse osmosis (RO). A comparison of the properties and the application
range of those mostly used membranes is listed in Table 12.1 (Mulder, 1996; Thorsen,
1999; Qasim et al., 2000). Based on the separation capacity, NF and RO could be
considered as nanomembrane. NF membranes selectively reject substances and have
advantages in comparison to RO because they enable the retention of nutrients in water.
Therefore, NF is more widely applied for drinking water treatment, while the more
powerful RO is effective for water reclamation and reuse as well as desalination. This
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