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Anthracite Building

Media Selection (L/D Ratio)

No ideal media configuration applies to all water sources and pretreatment schemes. 

There is a growing acceptance of some minimum summation of the ratio L/D [depth (L)/diameter (D)] as a guideline. A summation L/D ratio of 1200 (dimensionless L/D using ES for D) has been a common guide. 

For conventional pretreatment with sedimentation, a common dual media uses 18 to 24 inches of anthracite of 0.90 mm effective size over 9 to 12 inches of sand of 0.45 mm effective size. The L/D (mm depth/mm ES) for the anthracite would range from 508 to 675 and for sand from 508 to 675. Thus, the summation L/D would range from 1016 to 1350. 

A coarser top size is needed to accommodate higher solids load and algae blooms for direct filtration without sedimentation. For example, the new Los Angeles Department of Water and Power Plant uses 6 feet of 1.5 mm ES anthracite. The L/D ratio is 1219. 

A tri-media for a conventional plant may use 16 inches of anthracite 1 mm ES, 9 inches of sand with 0.5 mm ES and 3 to 4.5 inches of garnet with 0.24 mm ES. This would provide an L/D ratio summation from 1180 to 1339. 

So based on successful experience, the 1200 L/D seems supported. The important issue is to select the top size appropriate to the source water so that Unit Filter Run Volume production goals are achieved. 

The choice of the grain size of the media at the top, where the water enters the filter bed, influences the depth of penetration of solids into the bed. The bigger the grain size, the better the solids penetration and thereby the better the bed utilization. 

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