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There are two distinct modes of liquid filtration. In one, the filter medium sits across the fluid flow channel, so that all of the liquid must pass through the medium, leaving any separated solids to be held in or on the medium. This is called through-flow or dead-end filtration, and it separates most or all of the suspended solids from a more or less completely clarified liquid.

In the other mode, the flow of suspension is parallel to the medium, and some of the liquid flows through the medium by virtue of there being a pressure difference across it. The remainder of the slurry flows on and out of the filter. Very little
of the suspended solid remains on the medium, and the intention is that the flow of liquid across the surface of the medium should indeed keep it scoured free of any deposit. This is called cross-flow filtration (sometimes tangential-flow), and while
it can give extremely clear filtrates (in this context usually called permeates), it is only a thickener as far as the slurry is concerned.

Cross-flow filtration normally employs a surface filtration medium, since any solids moving into the thickness of the medium would not improve the efficiency of the process, and would reduce the active life of the filter medium. This medium is usually (but not always) a membrane, which is why the two topics are covered in this one chapter. (There is also considerable coverage of the membrane and related filters in Section 2F.)

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