Work out rapid sand filter area instantly with clear inputs, formula shown and shareable results.
Total filter area is design flow divided by the filtration rate over 24 hours. That area is split into units small enough to backwash conveniently, usually under 40-50 square metres each, with at least four units so that one can be washed without pushing the others past their maximum rate.
Filter area
Total area (m2) = flow (m3/day) / (24 x filtration rate (m/h))
Unit count
Filters = ceil(total area / maximum area per filter) + standby
So that removing one for backwash raises the rate on the others by only a third. With two filters, washing one doubles the rate on the other, which is unacceptable.
Discarding the first few minutes of filtrate after a backwash, when turbidity spikes as the bed re-ripens. It is an important pathogen barrier and is required by some regulators.