How Backwashing Filters Work
Backwashing water filters are large tank-style filters that get their name from the fact that they clean and renew themselves by backwashing. Backwashing consists of reversing the flow of water so that it enters from the bottom of the filter bed, lifts and rinses the bed, then exits through the top of the filter tank.
The fiilter bed itself is a granular substance that is usually referred to as the filter media. . Common media are granular carbon, sand, garnet, anthracite, zeolite, granular manganese dioxide, and greensand. Many media are known by their brand names of the leading product in the category: Activated Carbon, Pyrolox, Greensand, Filter AG Plus, Calcite and KDF, for example.
The picture above shows the filter in "service" position. This is how it works when it is doing the job it is designed to do. The unfiltered water enters from the left and is routed by the control valve into the filter tank. The water then filters slowly through the medium until it reaches the bottom of the tank where it is collected through a specially-designed sieved "basket" at the bottom of the center tube seen in the picture. The filtered water then passes up through the center tube, called a "riser" passes through the control valve and exits the right side of the filter.
Note that there's a drain line in the picture, but no water goes through it during the filter's "service" function.
The Backwash
As the filter operates in the service mode, it collects particles in the filter bed. Also, since water's nature is to follow the path of least resistance, after a time it begins to cut channels through the medium. As channels or holes in the media bed form, water begins to flow around rather than through the medium. This process is called "channeling," and it reduces the effectiveness of the filter considerably.
At a preset time, the automatic control valve initiates a "backwash" to clean the medium of collected particles and to resettle the bed and eliminate channels that have formed.
The backwash is accomplished by sending water down the riser tube from which it enters the filter tank at the bottom. The force of the water is such that it actually lifts the media bed, swirling and tossing the granular medium. The water leaves the filter tank through the control valve, which routes it through the filter's drain line. Particles that were in the bed are washed to drain.
The backwash is an intense rinsing and tossing of the medium that lasts for several minutes. In a standard residential filter, a typical backwash lasts about ten minutes.
After the backwash, the control valve initiates a "rinse" of the bed during which water flows downward through the medium, up through the riser tube, and out the drain. The purpose of this rinse is to rinse and settle the bed and prepare it for return to service flow
