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Name: Bakhtiyar
Status: Other
Grade: Other
Location: N/A
Country: United States
Date: April 2006

I saw the media inside a filter in a water plant that serves potable water to people. It was sand and gravel (of deferent grades), but did not understand why the sand is at the top and the big gravel (about 38-50mm) is at the bottom. I think that the big gravel should be up to filter the water that goes to the smaller gravel under it and then to the smaller and so on until it reaches the sand, which should be the last level of filtering. Please explain.

The different levels of gravel are to help filter out the water AND keep the sand and gravel in the tank. By having the larger media at the bottom where the water comes out, you allow a large amount of water to be released without that much of a pressure drop. As you go towards the top of the tank, the gravel and sand become finer in grain size, thus increasing the filtering capability and the pressure drop. The piping at the bottom of the tank has large holes in it to allow the water to be released. If the fine sand was in the bottom, then the sand would go out the pipe with the water. You could make the holes smaller, but it would take a large pressure drop to get the water out without letting the sand go out the bottom. By stacking up the gravel and sand at different grain sizes, you gradually build up to a fine grain size while still keeping the sand and gravel in the tank and minimizing pressure drop.

Hope this helps, and thanks for using Newton.


Christopher Murphy, P.E.
Air Force Research Laboratory

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