Design a 10 unit rapid sand filter system for a flow of 6.3 MGD with a filtration rate of 2 gpm/sq ft. and an L: W ratio of 1.3.
Design flow (Q) = 6.3 MGD = 6.3x106
gpd = 4375 gpm
Assume filtration rate of 2 gpm/ft2
Total filter area required = 4375 gpm/2 gpm/ft2
= 2187.5 ft2
Total of 10 filter units (but design is based on 9 filters being in operation and 1 in backwashing). In a problem you will be trying to match this with the answer
Area required per each filter unit = 2187.5/9 = 243 ft2
L:W ratio is 1.3 so LW = 1.3WxW = 1.3W2
= 243 --- this gives W = 13.7 ft and L
= 17.8 ft.