Purifying Water and Media Size
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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Update: June 2012