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Automatic backwashing filters are used to filter turbidity from the water. The
turbidity may be natural sand and sediment from the erosion of the aquifer or minerals that have oxidized. This
oxidation could occur naturally, or, as the result of the addition of an oxidant such as chlorine or oxygen.
These filters can contain a number of different types of media. All the media is made of the particles ranging
in size from fine to coarse sand. The media will typically be either - AG (aluminum silicate), Carbon, AN (calcite),
Birm (manganese dioxide), Greensand (manganese dioxide) or a Blend of silicate sands.
All of these filter consist of the following major components -
Item
|
Purpose
|
Construction
|
| Tank |
Water pressurized vessel that holds the media in place |
Typically fiberglass |
| Head |
Controls the frequency of backwash and controls the flow of the water during
service |
Noryl or Brass |
| Distributor and Riser |
Provides the return route to the head for the treated water |
Plastic with a slotted basket at the bottom |
| Media |
Mechanically filter particle for the water. In some cases also provides
chemical reaction sites |
Typically 10- 30 mesh |
| Underbed Gravel |
Provides support for the media and a collection space for the treated water. |
#20 or greater flint gravel |
Operation (see figure)
A filter has three different operation that it performs. The head controls these
functions and they are as follows-
Service - During service (filtration)
the water is directed down through the filter media where the particles in the water are trapped. Once the water has reached the bottom of
the tank, it returns to the top of the tank through the distributor.
Backwash - The head will determine
when the system needs to be backwashed or regenerated based on elapsed time or water usage. During backwash the
head will direct water down through the distributor and up through the media. This flow will be of enough force
to actually lift the media and shake lose any trapped particles. The water that leaves the top of the tank is directed
to the drain.
Rinse - Once the media has been
backwashed, it is rinsed by again sending water down through the media and then to drain. This will rinse any lose
material at the bottom of the bed and in the riser to drain. Once rinsed, the filter will return to service.
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