Depth Filtration: process that traps particulates both within the matrix and on the surface of the filter media.

Depth filters are composed of random mats of metallic, polymeric, or inorganic materials. These filters rely on the density and thickness of the mats to trap particles, and generally retain large quantities of contaminants within the matrices. Media migration, which is the shifting of the filter medium under stress, and particulate unloading, however, can be common problems.

Advantages: 　　　　　　　　　　　     　　Disadvantages:
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Lower cost                                          Potential for media migration
High throughputs                                Particulate unloading at high pressures
Protects final filters                             Nominal rating
High dirt-holding capacity
Removes variety of particle size

Membrane filtration: process that traps particulates larger than the pore size on the top surface of the membrane.

In membrane, contaminants smaller than the specified pore size may pass through the membrane or may be captured within the membrane by some other mechanism.  Membrane filters are used for critical applications such as removing microorganisms from a fluid.

Advantages: 　　　    　　　　      　       　Disadvantages:
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Low extractables                             Lower particulate-holding capacity
Integrity testable                             Lower flow rates than depth media
Can be particle retentive  
Absolute submicron pore sizes
Sterilizable

Combination filtration combines several different membrane pore sizes, or combines depth media and membrane filters to create self-contained serial filtration units. They can offer an economical alternative to using individual prefilters and final filters.