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Is it the Diatomaceous Earth (D.E.), sand pool filters or the cartridge filter?
Preferences may vary.
The fact of the matter is that any filter system can be used for proper maintenance of a pool. Perhaps the oldest and most common method of filtration is via sand pool filters. Filtration is driven by gravity as it pulls water down through the sand.
As water passes through the sand pool filter, sand catches debris and only allows clean water to come out of the outlet pipe. This process is likened to a filtering process in your coffee maker. Eventually, accumulated debris and dirt in the sand filter will cause significant pressure drop between filter inlet and outlet. Water flow will slow down and this indicates that it is time to perform maintenance work.
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From: (sand pond filters) http://ezinearticles.com/?Sand-Pool-Filters—Is-it-Time-For-You-to-Get-One?&id=3735096