It depends on the size of the ...

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It depends on the size of the sand, the depth and what you are trying to remove. Use the L/d ratio;

L/d ratio—A dimensionless value that is obtained by dividing the length (depth) of a granular media filter bed by the weighted effective diameter “d” of the filter media. The weighted effective diameter of the media is calculated based on the percentage of the total bed depth contributed by each media layer. eg 750 mm depth and a 0.5 mm sand = L/d1500. In general you want to be > 1200 in your L/d.