The ideal filtration material to fill the filter bed is gravel, with sizes varying between 2/6 and 4/14. An important characteristic is the hydraulic conductivity. The root and rhizome development increases the hydraulic conductivity over time, but he sedimentation of suspended solids and especially the development of the biofilm will decrease this value. The latter is specifically true for the inlet zone.
In the inlet and outlet zone, coarse gravel (8/32) or evenly graded rock (40/60 or larger) is used.
Horizontal Subsurface Flow CW's: Filtration materials