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Current IssueApril 23, 2014
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Ask The Expert: Filtration and Small Systems
01/02/2001 
 
Question: I am working on filter systems that use gravity flow to filter water. The systems I am looking for basically utilize a combination of sand, gravel and activated charcoal with a pre-treat with chlorine. I am looking specifically for the order, from top to bottom, how those layers would be installed. Also looking for information as to if it is advisable to have layers of a membrane between those layers and what those membranes might be made from.

Randy Long 

Answer: In any filter, the densest media has to be on the bottom and the lightest media on top, otherwise the media won't stay in position during backwashing (they'll wind up classified according to density and particle size anyway). Therefore, the gravel goes on the bottom, sand in the middle and charcoal on top. Any attempt to place a barrier between the layers (coarse mesh netting or screen) is likely to cause more problems than it solves, as these types of devices never stay put. If it's required, your best bet is a fine mesh screen, either stainless steel or preferably nylon. It may be difficult to keep them from classifying. If your thought about a membrane layer is to provide ultrafine filtration, it would probably not work because you could not get enough pressure from gravity flow to force a reasonable quantity of water through the membrane.

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