1. DESIGN OF WASTEWATER TREATMENT PLANT

2. Clarifier/Sedimentation Tank Design

3. Design criteria
The following design criteria are generally assumed to design a Primary Settling Tank / Sedimentation
A) GENERAL
No. of Tanks
2 or more (usually)
Types of tanks
Circular or rectangular
Removal of Sludge and Scum
Mechanical (usually)
Tank bottom slope
mm/m
Speed of sludge scraper
0.02 – 0.05 rpm

4. B) DIMENSIONS C) TECHNICAL Range Typical Rectangular Tank Length (m)
15-100 30
Width (m)
3-30 10
Depth
2.5-5 4
Circular Tank
Diameter (m)
3-60
Depth (m)
3-5
Bottom slope, (mm/mm)
0.02 – 0.05
0.03
B) DIMENSIONS
Range
Typical
Detention Time, t (hr)
1.0 – 4.0
2.0
Flow Through velocity (m/min)
0.6 – 3.6
0.9
SLR (m3/m2/hr) at average flow
1.2 – 2.5
1.6
Peak Hourly Flow
2.0 – 5.0
4.2
WLR (m3/m/d)
250
C) TECHNICAL

5. Primarily used in WWT to separate solids from liquids in effluent streams.
Types
light
Clarifiers
Criteria for sizing clarifier (settling tank)
Tank depth at the side wall
Overflow rate (surface loading rate)
Detention time
Scour velocity

8. Criteria For Sizing Clarifier (Settling Tank)
-Overflow Rate (Surface Settling Rate)-
Definition: The average daily flow rate divided by the surface area of the tank.
Clarifiers
Average daily flowrate
(m3/day)
overflow
surface settling rate
(m3/m2d)
Total surface area of the tank
(m2)

9. Criteria For Sizing Clarifier (Settling Tank) Average daily flowrate
-Depth Of Tank-
The water depth at the side wall measuring from the tank bottom to the top of the overflow weir.
Depth of tank
Effluent weir
Exclude the additional depth resulting from slightly sloping bottom that is provided in both circular and rectangular clarifiers.
Influent
Influent H
Occupied
with sludge
Effluent weir loading (typical= 250 m3/m.d) is equal to quantity of WW flowing divided by the total weir length, Lw
Average daily flowrate
(m3/day)
Total weir length
(m)

10. Criteria For Sizing Clarifier (Settling Tank) Average daily flowrate
-Detention Time-
length of time a particle or a unit volume of WW remains in a reactor
Detention time
Tank volume (m3)
Detention time =
(day)
Average daily flowrate
(m3/day)

11. Criteria for sizing clarifier (settling tank)
-Scour velocity
horizontal velocity through the tank to avoid resuspension of settled particles
Scour Velocity
Where:
VH = horizontal velocity that will just produce scour (m/s)
k = cohesion constant that depends on type of material being scoured (unitless)
s=specific gravity of particles
g=acceleration due to gravity (9.81 m/s2)
d=diameter of particles
f=Darcy-Weisbach friction factor (unitless)

12. Example:

13. Stabilization pond design

16. In series

17. Design Parameters

18. Facultative Pond

19. Design Criteria Surface Organic Loading Rate Depth Detention Time
Geometry (L/B ratio)

20. Surface Organic Loading Rate
Related with algae activity and balance between oxygen production and consumption.

22. Depth

23. Depth

24. Depth
Based on volume, V and the required area, A , the depth (H) of pond = V/A.
H = 1.5 to 2.0 m

25. Detention time
In primary detention time,t usually vary from 15 to 45 days.

26. L/B Ratio

27. If K = 0.35d-1 θ=1.085
If K = 0.30d-1 θ=1.05

28. Example

29. Solution

32. ANAEROBIC POND DESIGN

33. Volumetric Loading Rate

34. Volumetric Loading Rate

35. Detention Time

36. Depth

37. L/B Ratio

38. AEROBIC POND DESIGN Design Criteria Detention time, t = 5 to 10 days
Depth, H = 2.5 to 4.0 m
Oxygen Requirement
Power Requirement

39. Oxygen Requirement

40. Power Requirement

41. Activated sludge design