Membrane and Desalination Technologies - TCE Moodle Website

104 L. Song and K.Guan Tay
Flux
A
A
Flux
Foulant
Membrane
layer. A more compact fouling layer reduces the amount of water that is allowed to reach the
membrane surface and increases the rate of fouling.
The dimensions of membrane channel can affect the rate of membrane fouling by creating
hydrodynamics that does not favor the deposition of foulant on membrane surface. One
popular method, commonly found in spiral-wound and hollow-fiber membrane modules, is to
pass the feed water through a narrow membrane channel, so that the crossflow velocity
generates sufficient wall shear to remove the foulants from the membrane surface. To
enhance the shearing effect and promote mixing, the membrane channel is constructed in a
way such that the feed water is forced to travel through a series of bends within the channel,
generating turbulence that hinders foulant deposition. This method to develop unstable feed
water flow can be achieved with the use of feed spacers commonly found in spiral-wound
membrane modules. However, it is important to note that the use of narrow channel and
generation of turbulence can significantly reduce the downstream driving pressure. In addition,
the presence of feed spacers may trap or capture the suspended foulants, leading to
“clogging” problem. Hydrophobicity of RO membranes is also recognized to affect membrane
fouling in RO processes. It is believed that hydrophobic membranes attract organic
foulants more easily than hydrophilic ones. This is why some hydrophobic RO membranes
are pretreated to give them a hydrophilic property.
Feed water characteristics – such as types and concentration of foulants, ionic strength, and
pH – can have huge impact on membrane fouling in RO processes. A higher foulant
concentration naturally poses a greater fouling problem with the availability of larger pool
of foulants for deposition. The foulant type and composition influence the porosity of the
fouling layer, which in turn determines the hydraulic resistance of the fouling layer. For
example, a fouling layer consisting of organic matters and colloidal particles can have higher
B
B
Time
Flux
Fouling
layer
Fig. 3.1. Decline of permeate flux with the development of fouling layer on membrane surface.