2.10.4 Capacity of Backwash & Service Water Sump

Backwash & Service Water Sump shall be provided in the Machine Hall. Sump shall be fed from the Filtered Water Channels flowing below the floor of the Pipe Gallery and carrying water from the Filtered Water Outlet Pipes to the Sump. Storage in the Backwash Sump is achieved by building an Over flow weir at the end of the Sump. The level of the weir is so adjusted that the required volume of water for backwashing is always available in the Sump and the surplus water overflows the Weir and goes to the Clear Water Tank through a pipe.

According to Surface Water Treatment for Communities in Developing Countries by Christopher R. Shulz & Daniel A. Okun (1984, P - 161), Wash Water Tank should have sufficient capacity to wash at least two filters for at least 10 minutes at the maximum backwashing flow rate without refilling.


Water required for backwashing of one Filter (From 14.1 above) = 14 Cusecs
Time required for backwashing of one Filter = 10 Minutes
Storage of Backwash Water Sump = 2 x 14 x 60 x 10
= 16800 Cft
Length of the Sump = 54.33 Ft
Width of the Sump = 40.33 Ft
Depth of water to ensure required storage = 16800 / 54.33 x 40.33
= 7.66 Ft
Depth of water below the suction pipe of Backwash Pumps = 2 Ft

Free Board = 2 Ft

Total Depth of Wash Water Sump (7.66 + 2’ + 2’) = 11.67 Ft

Height of Overflow Weir above the floor of the Sump = 9.66 Ft