Principle of Membrane Filtration
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Principle of Membrane Filtration book
Depending on where the particle rejection occurs, filtration can be classified as surface filtration or depth filtration, as illustrated in Figure 1.1.
In surface filtration, particles are rejected by the filter surface. This type of filtration is also called absolute filtration because any particle larger than the pore size is rejected. Because particles accumulate in two-dimensional spaces on the filter surface, the particle-holding capacity of the surface filter is generally low. To control the particle deposition, deliberate antifouling measures are typically required by generating shear stress on the filter surface. Various methods are available, for example, high crossflow velocity, aeration, rotating/vibrating filter assembly, turbulence generation using spacers in the feed channel, rotating turbulence generator, feed flow pulsing, and others, but their use is restricted by the economics depending on the application. In surface filtration, particles are not lost to the depth of the filter unlike in depth filtration, which is a beneficial characteristic for recovering valuable materials from the feed water. All microfiltration (MF)/ultrafiltration (UF)/ reverse osmosis (RO) membranes, whereas stainless steel screens, and strainers are examples of surface filters. The vast majority of commercial membranes have an asymmetric structure that consists of a thin skin layer with small pores and a thick support layer with large pores. Whereas the skin layer separates particles from water, the support layer provides mechanical strength to the membrane without causing significant filtration resistance.