PP water filter cartridge.

Virtually all cartridge-style sediment filters follow the "radial flow" pattern seen in the illustration. In a radial flow design, water flows through the wall of the filter into the inner core. This arrangement provides filtering surface that consists of the entire length and circumference of the cartridge. The filter above is a "depth" filter. Pleated filters offer even more filtering surface.

Water filters have many purposes. Some are designed to remove chemicals from water. Others remove metallic contaminants or "dissolved solids" or charged particles.

This article is about filters that remove suspended solids, variously referred to as turbidity, sediment or particulate.

In general terms, filters of this type are sieving devices. They are like nets that catch and hold particles that are too large to pass through the holes in their surface.

There are large backwashing filters that perform this function. They use beds of natural media like sand, garnet, and anthracite to filter out unwanted particles like dirt or iron rust. The newer versions usually rely on specifically designed and manufactured media with brand names like Filter Ag, ChemSorb, or Micro Z to catch particles. When particles are trapped in the bed, the filter "backwashes" by running water backward through the media bed to wash the unwanted particles down the drain.

This article, however, is about cartridge-style filters rather than backwashing filters. Cartridges are for the most part disposable items. Except in rare cases, they are not backwashed. They are used until they stop up, then they are discarded. Certain cartridges can be cleaned and reused. More of that later.