Vertical Subsurface Flow CW's: Filtration materials
Coarse sand should be used as filter medium. The sand should meet following requirements: d10 between 0.25 and 1.2 mm, d60 between 1 and 4 mm, uniformity coefficient (U = d60/d10) < 3. The contents of clay and silt (particles less than 0.125 mm) must be less than 0.5% to avoid clogging.
To prevent the filtersand from clogging the drainage layer, a filter fabric is placed between the filter column and the drainage layer.
In some cases, for example when the vertical stage acts as first stage in a hybrid system, gravel is used as substrate.
In the distribution and drainage layer coarse gravel (8/16 or 8/32) is used as cover.