2.11 Filter Inlet / Backwashing & Drainage Arrangement

The central channel that divides each filter bed in to two parts shall serve as an inlet channel for the clarified water for filtration as well as Backwash channel for discharging backwash water out of the filter bed. Since the volume of backwash water (14.0 Cusecs) is much higher than incoming water (3.0 cusecs), therefore the channel shall be designed for backwash water.

Backwash Flow Rate (q) = 14 Cusecs
(Sum of the Flow Rate of 2 No Backwash Pumps)
No of Troughs for each filter = 1 No
Width of Channel (b) Assumed = 2.0 Ft
Depth of water in the Channel (y) = 1.73 (Q2 / g x b2)1/3
= 1.73 (14.02 / 32.2 x 22)1/3
= 2.0 Ft
Free board = 0.5 Ft
Depth of Washwater Channel at the Upper End = 2.5 Ft

Velocity in the Channel when Flat (v) = 14 / 2 x 2
= 3.5 Ft / s
Find the slope of the Channel using Manning Formula
Roughness Coefficient (n) = 0.013
Hydraulic Radius (R) = 2 x 2 / (2 x 2 + 2)
= 0.67
Slope = (n x v / 1.486 r2/3)2
= (0.013 x 3.5 / 1.486 x 0.672/3)2
= 0.00160 Ft / Ft
= 2 Ft / 1000 Ft (Say)
Length of Filter Bed = 31.0 Ft
Max Possible Friction Losses = 31.0 x 2/1000
= 0.062 Ft
Depth of Wash Water Channel at the lower end = 0.062 + 2.50
= 2.562 Ft
= 2’-7” (2.58’)


2.11.2 Design of Backwash Water Drainage Channels