2.5 Design of Flocculation Tank

The design of Flocculation Tank is also governed by the Velocity Gradient (G) and Detention Time (t). Varying values of G and t given in the literature and the values adopted are given below:








Design Criteria


Design Parameter
Ref 1
Ref 2
Ref 5
Ref 6
Ref 7
Adopted
Velocity Gradient (G)
20 – 65
20 - 70 1/s
20 - 75 1/s
20 – 75
10 – 80
50
Detention Time (t)
20 – 30 Min
20 - 30 Min
10 – 30 Min
10 - 60 Min
20 - 30 Min
20 Min
Gt
-
-
20000 – 60000
12000 - 270000

60000
Drive




Variable Speed, variation of G value by a factor of 2 to 3
Turbine Type variable speed Flocculator

Rotational Speed



2 - 15 rpm
10 - 30 rpm


1Water Supply & Sewerage by E.W Steel & Terence J.McGhee (P -233)
2Hand Book of Environmental Engineering Calculations by C. C. Lee & Shun Dar Lin (P - 1.363)
3 Environmental Engineering, A Design Approach by ArcadioP.Sincero & Gregoria A. Sincero (P 267)
4Introduction to Environmental Engineering by Davis & Cornell (P203 - 205)
5Water Supply Engineering by BC. Punmia, Ashok jain & Arun Jain (P - 278)
6Water Supply by A.C Twort, F. M. law (P - 252)
7. Water Treatment Principles & Design by MWH (P -743)

Ref (3) recommends the following G & t values:

Type of Raw Water
G (1/s)
(Gt) - Dimensionless
Low Turbidity, Color Removal Coagulation
20 – 70
50000 - 250000
High Turbidity Solids Removal Coagulation
70 -150
80000 - 190000

Ref (4) recommends the following G & t values

Type of Raw Water
G (1/s)
(Gt) - Dimensionless
Low Turbidity
20 – 70
60000 - 200000
High Turbidity Solids Removal
30 – 80
36000 - 96000

2.5.1 Sizing of Flocculation Tank

Design Flow = 35.0 Cusecs
No of Flocculators = 3
Flow per Flocculator = 11.67 Cusecs
Detention Time = 20.0 Min
Volume = 14004 Cft
Water Depth = 16.50 Ft

Crossectional Area of each Flocculation Tank = 848.73 Sft

2.5.2 Sizing of Inlet Pipe

Velocity through inlet Pipe = 2.5 Ft /s
Crossectional Area of inlet pipe = 11.67 Ft /s
= 4.67 Sft
Dia of inlet pipe = 2.43 Ft
= 2.5 Ft (Say)

Pipe shall be wrapped around with a concrete wall (12” thick). The objective of this wall is to serve as a support for the scrapper bridge. Openings shall be provided in the inlet pipe and the concrete wall from where the water will enter the flocculation tank.