How it works
- why filter — Sedimentation leaves 5–20 mg/L of fine floc that will not settle in any reasonable time. Filtration is the barrier that takes that to below 1 mg/L — and for potable duty it is also the physical barrier against protozoa, which is why the turbidity target is regulatory, not aesthetic.
- why two media — Sand alone filters at the surface: the top few millimetres blind, headloss rockets, and the bed depth below is wasted. Put coarse anthracite on top of fine sand and solids penetrate deeper, so the whole bed stores them. Anthracite is less dense, so after backwash it re-stratifies back on top.
- the run — Through the run, solids fill the pore space. Headloss climbs and the floc front works downward through the bed. The filter is doing its job right up until either headloss reaches the available head or floc reaches the underdrain — whichever comes first.
- backwash — The run ends and flow reverses. Air scour breaks the cake, then upflow water at around 40 m/h fluidises the bed and carries the solids to waste. Too little and the bed stays dirty; too much and you wash the media out of the tank.
- what governs it — Filtration rate is the master variable — push above roughly 15 m/h and floc drives straight through. Then bed depth against run length: a deeper bed stores more, so it runs longer between backwashes. Push the rate and watch run length fall away.
Design parameters
The panel opposite runs the same correlations as the F-400 design sheet, so the animation and the calculator cannot disagree. Drag any of them and the picture responds.
- Feed flow — m³/h
- Influent SS — mg/L
- Filter area (each) — m²
- Filters in service — off
- Anthracite depth — mm
- Sand depth — mm
Open the full F-400 design sheet for the governing equations, the accepted design envelopes and the worked calculation.