Visual built from typical rapid gravity filter dimensions (10 × 6 m cell, 0.6 m anthracite over 0.3 m sand on a Type S underdrain with I.M.S. cap); not manufacturer CAD.
Anthracite · sand · I.M.S. cap / underdrain
Deposit front (stored solids vs. capacity)
Water (brown = dirty wash water)
Bubbles = air scour, or air trapped in the bed
Valve open · closed