A depth filter is a filter that traps particles throughout the body of the filter, not just at the surface. A classic example is a cartridge filter, which should have deposits of particulate material all the way through the cartridge when viewed in cross-section. These filters can be useful for materials that may have large concentrations of sediment, in contrast with a surface filter, which could have difficulties processing the material. Various depth filter designs are readily available on the market along with custom versions for specialized applications.
This filter design includes an increasing porosity from entry to exit. As material enters the filter, the largest particles are trapped and cannot proceed. While the material works through the depth filter, increasingly smaller pores prevent the passage of all but the smallest particles. When the material exits at the other end, it will be largely filtered. The degree of filtering available depends on how small the pores get at the end.
With a surface filter, clogs can be a problem in the case of heavily contaminated materials or materials with a wide range of particle sizes. The pores quickly become blocked as fluid moves through them, and the filter may need to be cleaned or changed. Depth filters are less prone to clogging because of the layers of filtration offered, although the pores will eventually be occluded with particulate materials. At that point, the depth filter can be discarded, or cleaned and prepared for another use.
Depth filtration may be preferable to surface treatments in some applications. The best filter depends on the type of material being processed, the end goal, and the standards for that material. Multiple passes of filtration can be necessary with some materials, like sewage, which moves through filters and processing tanks to remove pathogens and particulates. For activities like home water filtration, a single depth filter may be enough to remove hazards from the water, depending on the contaminants present.
Depth filters can vary widely in size and cost. Companies with the need for industrial filtration may need to order custom products to adequately meet their needs. Industrial filters can come as part of a larger filtration system that may include holding and settling tanks, tubing, and other supplies necessary for filtering. For fine-tuned applications like preparing chemicals and biological specimens, a very high degree of filtration is necessary, and workers may also need to regularly test and calibrate their filters to confirm that they are working properly.