Question

A utility wants to design a granular media filter using sand and anthracite. Given that the sand has an average diameter of 0.9 mm and the anthracite has an effective diameter of 1.5 mm, how deep should the filter bed be? Note – there is a range of bed depths for media based on particle size – please state your assumptions.

Answer 1

For a granular media filter, using 0.9mm effective diameter sand 560 mm depth of sand can be used. for anthracite of 1.5mm effective diameter 975mm depth can be used.

The L/D ratio for the anthracite is 650 and for the sand it is 622 . Thus, the summation L/D would be 1272. In general filteration systems, authorities and operators work in the range of 1000–1200 L/d ratio as a guideline. The L/d ratio value has been derived from years of filtration design and operation. The general conditions is that an L/d ratio of 1200 is the preferred value.

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