Plain sedimentation tanks ,Types & Design considerations

Sedimentation is an effective techniques involved for treatment of waste water . Various sedimentation techniques are employed world wide for the purpose.
Plain sedimentation is the simplest technique involving quiescent settling or storage of water, such as would take place in a reservoir, lake, or basin, without the aid of chemicals, preferably for a month or longer, particularly if the source water is a sewage-polluted river water.
This presentation covers various plain sedimentation tanks & design considerations of the same .

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1. Plain sedimentation tanks

-Theory design & types of tanks
By :
ANSHIKA SAXENA
B.Tech (FPE)2015027006
INDIAN INSTITUE OF FOOD PROCESSING TECHNOLOGY
TAMIL NADU

2. Sedimentation
•The process of settling or being deposited as a
sediment.
•Physical water treatment process
•Uses gravity to remove suspended solids from
water.

3. Sedimentation tanks
Waste water
Filled
Stored for
some time
Settled sludge
(SS)OR (SP)
Clear Water
(SS removed)
•Settled suspended solids are removed by using scrapers.
•May be continuous / intermittent.

•NOTE - If the suspended particles have low specific gravity than water, they settle at the top of the tank.
•Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation.

4. 4 major zones in sedimentation tank
•I. Inlet zone II. Outlet zone III. Settling zone IV. Sludge zone

6. •The settlement of a particle iis obstructed or opposed by following 3
forces :
•Velocity of flow
•Size & shape of par66ticles
•Viscosity of water

7. Stokes’ law
•Applicable to the discrete particles (which donot change in size,shape
or mass during settling).

•�=vsz5−5s∗�
2
∗u6+yr/srr
•for particle dia < 0.1mm
•Where
•d=diameter of discrete particles
•T= temperature(deg. Celsius)
•V=velocity of settlement (mm/sec)
•S=specific gravity of particle
s1=sp. Gr. Of water

8. Types of sedimentation tanks
•Depending upon various factors sedimentation tanks are classified as follows :
Based on methods of operation
Intermittent / Quiescent type
Continuous type
 Based on shape
Circular tank
Rectangular tank
Hopper bottom tank
Based on location
Primary tank
Secondary tank

9. Intermittent / Quiescent Type
•The intermittent tanks also called quiescent type tanks
•They are those which store water for a certain period and keep it in
complete rest.

10. Continuous Flow Type
•Only the flow velocity is reduced.
•Water is keeps running.
•Settling basins may be either long rectangular or circular in plan.




*Long narrow rectangular tanks with horizontal flow are generally preferred
to the circular tanks with radial or spiral flow(depends upon requirement)

11. Fill and Draw Type Sedimentation Tank

12. Fill and Draw Type Sedimentation Tank
•Water from inlet is stored for some time.
•The time may be 24 hours. In that time, the suspended particles are
settled at the bottom of the tank.
•After 24 hours, the water is discharged through outlet.

•Then settled particle are removed. This removal action requires 6-12
hours.
•Here 1 complete action of sedimentation requires 30-40 hours.

13. Continuous Flow Type Sedimentation Tank
•water is not allowed to rest.
•Flow has very small velocity.
•During this flow, suspended particles are settle at the bottom of the
tank.
•The flow may be either in horizontal direction or vertical direction.

14. Horizontal flow type sedimentation tank
•Generally rectangular in shape.
•They have more length twice its width. (Because they need to flow
more distance to settle all suspended particles)
•Maximum permissible velocity in this case is 0.3m/sec.

15. Vertical flow type sedimentation tank
•The vertical flow type sedimentations tanks are generally in circular
shape
•Flow takes place in vertical direction.
•Hopper bottom is provided at the bottom of the tank to dispose the
collected sludge.

16. 3 Types of tanks
Rectangular- large
Circular- small & medium
Square(similar to rectangular)

High-Rate Tube Settlers
Water inlet with baffles
Lamellar separators

17. Rectangular Sedimentation Tank
Easy to operate.
Low maintenance costs.
Large capacity plants.
Used in municipal & industrial applications.
Sometimes baffle walls are provided to
prevent short circuiting.

18. Hopper Bottom Tank
•A deflector box is located at the top which deflects the influent
coming from central pipe downwards.
•Sludge is collected at the bottom
•It is disposed through sludge pump.

19. Circular Sedimentation Tank
Easy sludge removal.
High clarification efficiency.
 Small to medium sized applications.
For plants with constant flow rates & quality.
Uneconomical as compared to rectangular tanks.
high clarification efficiency.

20. Design & Technology
•Detention period : 1 – 3 hours
•Velocity of flow : Not greater than 30 cm/min (horizontal
flow)
•Tank dimensions : L:W = 3:1 to 5:1
•Depth : 2.5 to 5.0 m (3 m).
•Slopes : Rectangular 1% towards inlet and circular 8%
(For plain sedimentation tank)

21. Thank you

22. •Very large surface area onto which particles may fall and become stabilized
•Flow is temporarily accelerated between the plates and then immediately slows
down
•this helps to aggregate very fine particles that can settle as the flow exits the plates

High-Rate Tube Settlers
Water inlet with baffles
Lamellar separators
WORKING of

23. Primary Sedimentation Tank
•Primary sedimentation tank is a normal sedimentation tank in which
water is stored at rest for some time .
•Sludge collected at bottom
•Oily matter collected at top are removed.
•After primary sedimentation process the wastewater is discharged
into aerobic filter where activated sludge process take place.

24. Secondary Sedimentation Tank
•After activated sludge process the wastewater enters secondary
sedimentation tank in which suspended particles containing microbes
are removed.
•Then are reflected towards aerobic filter to maintain high microbe
concentration in aerobic filter.

25. Secondary Sedimentation Tank