Problem:
(Figure should show filter 400mm deep, not 200mm. Error corrected 27Mar12)
A sand filter is to be used to purify water.
The sand filter bed is 400mm deep and has an area of 1.0m2.
Water is supplied to the top of the filter bed from a header tank which provides a constant head of 2.9m to the outlet of the filter (assume this is the total head loss across the filter).
Previous tests with this sand show that the coefficient of permeability of sand when it is clean is 8.1 x 10-4m/s and when the sand is dirty and needs backwashing the coefficient of permeability of the dirty sand n is 5.2 x 10-6m/s
(a) Calculate the expected rate of flow (in liters/sec) when the sand is clean
(b) Calculate the expected rate of flow (in liters/sec) when the sand needs to be backwashed.