Sedimentation tanks, also known as clarifiers, are used in sewage treatment plants to reduce the strength of sewage through sedimentation. The key processes are similar to water treatment plants, with some minor modifications. Sedimentation reduces settleable solids in sewage by 80-90% and biochemical oxygen demand by 30-35%, producing effluent that is suitable for further treatment. Clarifiers are either primary or secondary. Primary clarifiers directly treat raw sewage while secondary clarifiers treat effluent from secondary treatment units like filters. Common tank designs include rectangular, circular and hopper bottom tanks. Design considerations include inlet/outlet arrangements, capacity, scum removal, and maintaining equal inflow/out

1. Dr. M. Prasad

3. Also known as settling tanks or clarifiers
Theory of sedimentation and overall
features are same as water treatment
plants with minor modifications.

4. As follows..
 The process of sedimentation reduces the strength of
sewage to the extent about 30-35%.
 The quantity o settleable solids in the sewage reduced
up to 80-90%.
 Reduction of BOD up to 30-35%
 Sewage after sedimentation becomes fit for further
treatment works.

5. Same principle will be applied as in the
water treatments plant.
Sludge : material collected at the bottom of
the sedimentation tank.
Effluent : the partially treated sewage in
sedimentation tanks.

6. According to the nature of working.
According to the location.
According to the nature of working
1. Fill and draw type
2. Continuous flow type.

7. Primary clarifiers
Secondary clarifiers.
Primary clarifiers: provided just after the grit
chamber and they have to treat raw
sewage.

8. Secondary clarifiers: located the after the
secondary treatment units such as filters
and activated sludge process units.
Secondary clarifiers are provided to treat
the treated sewage. They can be treat the
sewage such an extent such that the
effluent can be directly discharged in to
natural waters.

9. Various aspects of PC….
1. Inlet and outlet arrangements
2. Shape of the tank
3. Capacity of tank
4. Scum baffles
5. Scum removal devices
6. Sludge removal
7. Miscellaneous conditions.

10. Same as water treatment plants…
Following points should be noted.
 Inflow and out flow rates should be equal
 The flow of sewage should be diffused equally across
the entire section of the tank.
 The flow of outlet should come out as a thin as possible

11. Rectangular tanks
Circular tanks
Hopper bottom tanks

12. RT with horizontal flow.
Ratio of length to width is about 4-5
Ratio of width to depth is about 2-3

13. CT with vertical flow
Fitted with mechanical scrappers

17. These can be of vertical and horizontal
flow type.
Horizontal type tank..

19.  Capacity of the tank determined by the
detention period and overflow rate.
 Primary clarifiers designed for 1-3 hrs
detention period
 Overflow rate in primary clarifiers generally
40-50 m3/m2-day.
 Depth not much important but for mechanical
scrappers it should be at least 2 metrs.

20. Scum baffles are provided in the form of
troughs or boards near the out let to
prevent the entry of floating substances in
to the out let channel
Scum removal devices (SRD)
The primary clarifiers should be provided
with suitable SRD , To remove the floating
material.

21. This can be collected at the bottom of the
sedimentation tank.
Removed periodically by following
methods
Hand cleaning : obsolete
Hydraulic cleaning: suitable for hopper tank
Mechanical cleaning : suitable for
rectangular tanks

22. Velocity of flow should be 300 mm/ min
All other arrangements should be made as
per water treatment plant.

23.  Design a primary clarifier for a town
having population of 34000. The formation
of sewage may be assumed at 150 lpcd.
Assume tank effective depth as 2.5m
Note : overflow rate(SOR) should be b/w
40-50m3/m2-day