2.3.3 Design of Channel / Launder around Flash Mixer

Raw water and Chemicals (Alum) after being mixed in the Flash Mixer shall over flow a rectangular weir installed on the periphery of the Flash Mixer. A channel provided all around Flash Mixer shall receive water and transfer to Clarifier Feeding Channel. The design of channel around the Flash Mixer is as follows:

Diameter of the Flash Mixer = 7.5 Ft
Circumference of the Flash Mixer = 3.14 x 7.5
= 23.55 Ft
Half Circumference of each Clarifier = 11.775 Ft

Head over the Rectangular Weir

H = (3 x Q/2 x Cd x L (√2g))2/3
Flow over the wier (Q) = 35.0 Cusecs
Coefficient of Discharge (Cd) = 0.60
Length of the weir (L) = 23.55 Ft
Gravity (g) = 32.2 Ft / s2

Head over the weir (H) is worked out as follows:

H = (3 x 35 / 2 x o.6 x 23.55 x 8.024) 0.666
= 0.60Ft

Depth of water over the weir shall be 0.6 Ft. A metallic rectangular weir shall be installed on the periphery of the Flash Mixer over which the water will flow before falling in the Channel / Launder provided around the Flash Mixer. The metallic rectangular weir shall project 2 “ above the concrete weir crest.

Design of Channel / Launder Around the Flash Mixer

Width of launder (b) = 3.0 (Assumed)
Flow (Q), In Launder flow splits in to two halves = 35 /2 Cusecs
= 17.5 cusecs
Gravity (g) = 32.2 Ft / s2

Depth of Flow at the lower end (h) = (Q2/ b2 x g)1/3
= ((17.5)2/ (3.0)2 x 32.2)1/3
= 1.0 Ft

Depth of Flow at the Upper End (H) = √(1.0)2+ 2(1.486)2((11.775)2/32.2(1.0)(3.0)2
= 1.76 Ft

Depth of Flow in the Launder = 1.76 Ft
Add friction allowance of 25 % = 0.44 Ft
Add 1.0 Ft to ensure Free Fall = 1.0 Ft
Total Depth of Launder (1.76 + 0.44 + 1.0) = 3.2 Ft
Say = 3.25 Ft (Say)
A rectangular channel 3 Ft wide and 3.25 Ft deep shall be provided around the Flash Mixer. The depth of the channel is measured from the weir crest feeding the channel.