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.