ign of Filtered Water Channels

Two Filtered Water Channels shall flow below the floor of the Pipe Gallery in front of the Filters. Each channel shall collect flow from its respective four Filters Water Outlet Pipes and carry to the Backwash & Service Water Sump located below the Machine Hall. The size of the channels is worked out as follows:

Flow from the four Filtered Water Outlet Pipes = 2.91 x 4
= 11.64 Cusecs

Velocity of Flow through the channel = 3 Ft / s

Crossectional Area of the Channel = 11.64 / 3

= 3.88 Sft

Width of the Channel = 4.0 Ft

Depth of the Channel = 3.88 / 4
= 1.0 Ft

Free Board = 1.5 Ft

Total Depth of the Drain = 2.50 Ft

2.10 Filter Cleaning Arrangement

Filtration of water is stopped due to the clogging of filter by the turbidity causing particles. Filter cleaning shall be carried out through Backwashing. In Backwashing filtered water is allowed to flow upward through the filter beds by pumping huge volumes of water at a certain head. The discharge and head of the flow in backwashing is so adjusted that the flow completely agitates filtration media and removes the clogging material. The process of Backwashing is supplemented by air wash in which air is pumped through the filter media which breaks the layer blocking the filter media. Air wash is carried out prior to the water wash. The entire process of backwashing completes in 20 minutes. Design Criteria governing the design of filter cleaning system is as follows:


Design Parameter
Ref 1
Ref 2
Ref 3
Ref 4
Adopted
Backwashing water Flow Rate
880 - 1200 m / d
25 m3 /m2/hr
5.0 - 6.5 mm / s
30 - 60 m / hr
25 m3/m2/hr
Air Flow rate

55 m3 /m2/hr
1-1.5 m3/m2/min
36 - 72 m / h
55 m3/m2/hr
Duration of washing



8 - 15 minutes

Head at the Pump or elevated tank



10.0 M
10.0 M
1Introduction to Environmental Engineering by Davis & Cornell (P236)
2 Operational Manual of Water Treatment Plant, Rawalpindi
3Water Supply by A.C Twort, F. M. law (P - 278)
4Water Treatment Principles & Design by MWH (P -810)

2.10.1 Design of Backwashing Pumps

Backwash Pumps shall be installed in the Machine Hall and draw water from the Backwash Water & Service Water Sump provided beneath the Machine Hall. The same backwash water rate has been adopted as used in the backwashing of filter beds at the Rawalpindi Water Treatment Plant.

Backwash Water Rate = 25 M3 / M2 / H
= 82.0 Cft / Sft / H
Area of each Filter Bed = 606.81 Sft
Capacity of Backwashing Pumps = 49748 Ft3 /H
= 13.81 Cusecs

No of backwashing Pumps = 2
Capacity of Backwashing Pumps = 13.81 / 2
= 7.0 Cusecs

It is proposed to install 3 no vertical turbine pumps, 2 working and 1 standby to serve as backwashing pumps.