What is an Automatic Back-washing Filter?

Simply stated – Automatic back-washing filters are those that are able to reverse flow of water through the media to clean, regenerate, re-stratify or refresh themselves.

Backwashing filters are often the unsung hero in the water treatment world. Backwashing filters do the “heavy lifting” in water treatment. They solve a broad range of quality problems. Problems solved with backwashing filters include:

  • Sediment / particles
  • Iron and manganese
  • Taste and odor
  • Hydrogen sulfide (rotten egg)
  • Low pH adjustment
  • And removal of specific contaminants such as arsenic, fluoride, chlorine

Back-washing filters have a long service life, are very dependable, and have many of the same components as salt softeners. They have media (often called the filter bed), a media tank, and a filter control valve. Since they don’t use ion exchange they don’t have a brine or salt tank.

The filter control valve sits on top of the media tank and automatically directs flow of water through the media and tank. Most filter control valves have 3 cycles;

  1. Service – water is being filtered
  2. Backwash – the first of two cleaning cycles and
  3. Rapid rinse

How does a Backwashing Filter Work?

During the service cycle water is treated by one or more processes. Processes include; separation, adsorption, and addition of sacrificial media to the water each having its specific place and purpose in treatment. Many backwashing filters are specialized, designed to treat one water quality problem. A “filtration system” may have more than one filter, think multiple filters for multiple problems.

During the service cycle contaminants are filtered from the water as it flows through the media bed from the top to the bottom of the filter tank. If the filter did not have a mechanism for cleaning itself it would, over time, foul, plug up or otherwise stop working and fail. To prevent this from happening the control valve periodically directs the flow of water to clean the filter bed after which the filter is returned to service. This filter cleaning process is known as “backwashing” or “regenerating”.

During the backwash cycle water flows in reverse from the service flow, water flows from the bottom of the filter bed to the top lifting the media and separating the filtered contaminants which flow out of the control valve to drain. Following the backwash cycle is the rapid rinse cycle. During the rapid rinse cycle water again flows from the top to the bottom of the media bed. This is the same direction as the service flow, but the water is directed to the drain. The rapid rinse cycle serves two primary purposes; it re-packs the media bed and cleans leftover contaminants from the bottom of the bed. Once the rapid rinse cycle finishes the control valve places the filter back into the service mode.

Backwashing filters requirements:

  • This Placement on a solid level surface
  • Power
  • Adequate flow and volume of water for through backwash
  • An adequate drain
  • Protection from freezing and direct sunlight

Most backwash filters pass untreated water during the backwash and rapid rinse cycles. Because of this, water used while the filter is backwashing or rinsing is untreated. This is typically called “hard water bypass” and has to do with fire code. Imagine what would happen if there was a fire but no water to put it out because the backwash filter was busy cleaning itself?

Full Time Operation:

Because some jobs require treated water 24/7 thought has to be given in the design to provide treated water during the backwash cycle. There are ways to make sure that happens. The most common are:

  1. For continuous flow more than one backwashing filter can work together. These designs allow for “on-line” filters to handle the required flow rate while another is “off-line” during its backwash cycle.
  2. Use a treated water storage tank. Use a storage tank with enough capacity to provide the needed water for backwash and for fighting a fire. Because you use a storage tank to provide water a filter can backwash without interrupting flow.
  3. Using large modern cartridge filters. Properly maintained cartridge filters do not pass untreated water because they do not have a backwash cycle.

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 filter bed itself is a granular substance that is usually referred to as the filter medium. Media (media is plural, medium is singular) are numerous and varied. 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: Centaur, Filox, Birm, Filter Ag, 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” or a “dip tube,” 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.


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