2.7 Design of Clarifier with Concentric Flocculation Tank

Design Criteria

Standard design criteria for Clarifiers following coagulation and flocculation with alum are as follows:

Design Parameter
Ref 1
Ref 2
Ref 3
Ref 5
Ref 6
Ref 7
Ref 8
Adopted
Detention Time
2-8 Hrs


2.5 - 3.0 Hr
1.5 - 4.0 Hr
1.5 - 4.0 Hr

2 Hr
Overflow Rate following Coagulation & Flocculation

20-33 m3 /m2/d


40 - 60 m3 /m2/d

1-1.2 m3/m2/h
20 - 40 m3/m2/d
1.25 - 2.5 m3/m2/h

2.3 m3/m2/hr
Water Depth





3-5m

4.35 m
Bottom Slope





1:600
¾ - 2 inches /Ft
1/2 inch / Ft
Weir Overflow rate
250 m3 /m/d

6 – 14 m3/m/h


9-13 m3/m/h

13 m3/m2/hr

1Water Supply & Sewerage by E.W Steel & Terence J.McGhee (P -234)
2Hand Book of Environmental Engineering Calculations by C. C. Lee & Shun Dar Lin (P - 1.370)
3 Environmental Engineering, A Design Approach by ArcadioP.Sincero & Gregoria A. Sincero (P 267)
4Introduction to Environmental Engineering by Davis & Cornell (P227)
5Water Supply Engineering by BC. Punmia, Ashok jain & Arun Jain (P - 282)
6 Water Supply by A.C Twort, F. M. law (P - 268)
1. Water Treatment Principles & Design by MWH (P -810)
2. Wastewater Engineering by Metcalf & Eddy (P – 477)

Ref 4 recommends the following:
Types of flocs
Type of Waters
Weir Overflow Rates (m3/m/d)
Light Alum Flocs
Low turbidity waters
143 - 179
Heavier Alum Flocs
Higher Turbidity Waters
179 – 268

Typical Sedimentation Tanks Overflow rates
Application
Type of Tanks
Sedimentation Tanks Overflow Rates (m3/m/d)
Turbidity Removal
Long Rectangular or circular
40
Color Removal
“
30
High Algae
“
20
Source: American Water Works Association, Water Treatment Plant Design




Design

In addition to the above mentioned references, experience has been drawn from the Rawalpindi Water Treatment Plant, where an Overflow rate of 2.3 m3 /m/hr has been adopted and the Clarifiers are successfully functioning. Design calculations proceed as follows:

Flow = 35.0 Cusecs
O. F. R = 7.544 Ft3 / Ft2 / H (2.30 m3/m2/h)
No of Clarifiers = 3

Flow through each Clarifier = 35.0 /3
= 11.67
Area of each Clarifier = 11.67 x 3600 / 7.544
= 5567.34 Sft

Diameter of Flocculation tank = 33.15 Ft
Area of Flocculation tank = 862.65 Sft
Let thickness of the Diffuser wall = 1’-0”
Area of the Diffuser Wall = 3.14 (35.152 - 33.152) / 4
= 107.231 Sft

Total Area of each Clarifier cum Flocculator (5567.34 + 862.65 +107.231) = 6537.221 Sft

Dia of each Clarifier with Concentric Flocculation Tank = (6537.221 x 4/3.14)1/2
= 91.26 Ft
= 91’ - 6”

Floor of the Flocculation tank shall be flat, whereas floor between the diffuser wall and the Clarifier Wall shall be sloped at a rate of 1/2 inch / Ft for the smooth sliding of sludge to Sludge Pits.

Floor width between Diffuser Wall & Clarifier = 91.50 - 33.15 - 2
= 56.35 Ft
Proposed Slope = 1/2 inch / Ft

Difference of Floor Level between outer & inner walls of the Clarifier = 0.5 x 56.35
= 28.175 inches
= 2.34 Ft
Water Depth in the Flocculation Tank = 16.5 Ft
Side Water Depth in the Clarifier = 16.5 - 2.34
= 14.16 Ft
= 14’ - 2”
Check for Detention Time in the Clarifier

Average Water Depth in the Clarifier = (16.5 +14.17) / 2
= 15.335 Ft
Effective Volume of Clarifier = 5567.34 x 15.335
= 85375.16 CFt

Detention Time = 85375.16 /11.67
= 7317.9 Seconds
= 2.04 Hrs (Okay)

Provide 3 no Clarifiers cum Flocculators each having diameter of 91’- 6” with concentric flocculation tank having 33’ - 2” diameter. Two Clarifiers cum Flocculators will be developed under Ph I & II whereas 3rd will be developed under Ph III.

Sludge Scrapper

Clarifier shall be fitted with a Sludge Scrapper. The flocs formed shall settle down on the floor of the Clarifier to be scrapped by the Sludge Scrapper towards the sludge pits. Two Sludge Pits have been provided in each Clarifier. A Sludge Pipe shall connect the Sludge Pit to the outer Manhole for the removal of the sludge. Sludge shall be intermittently removed by opening valves installed on the Sludge Pipe. A Sludge pit shall also be provided in the Flocculation Tank for the removal of the sludge particles if settled in the Flocculation tank.