Sedimentation tanks are used to separate suspended particles from water through gravitational settling. There are different types of sedimentation tanks based on their geometry and flow pattern, including rectangular tanks with horizontal flow, circular tanks with radial flow, and hopper bottom tanks with vertical flow. Key design considerations for sedimentation tanks include maintaining a low water velocity below 30 cm/min, selecting an appropriate tank size and depth, and incorporating inlet and outlet structures to avoid disturbing the flow. Sedimentation is useful for removing inorganic and organic solids from water in a simple and low-cost manner.

1. WATER SUPPLY ENGINEERING
(ENVIRONMENT ENGINEERING I)
SEDIMENTATION
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ASST. PROF. PRACHI DESSAI

2. SEDIMENTATION TANKS
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ASST. PROF. PRACHI DESSAI

3. Sedimentation / Settling / Clarification
o process of separating from water by gravitational settling the suspended particles that are heavier
than water.
oIn water there are mainly two types of Suspended solids:
o Inorganic Solids: Specific Gravity 2.65
o Organic Solids: Specific Gravity 1.0 – 1.4
oMost of them have specific gravity of water greater than 1.
oMost of these particles are held by Turbulence, when water is retained in a tank or a basin and it
is brought to rest the suspended particles settle down and get deposited at the bottom of the tank.
oSedimentation Tanks are classified as:
o Plain Sedimentation (Settled by natural Forces)
o Sedimentation aided with coagulation (Settled by addition of an external agent).
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ASST. PROF. PRACHI DESSAI

4. Types of Settling
TYPE I ‐ Discrete (or Free) settling:
i. No interaction with other particles
ii. No flocculation upon contact
iii. No change in shape, size or mass during settling
iv. E.g. Sand particles.
TYPE II ‐ Flocculent settling:
i. Particles flocculate during settling
ii. Tend to increase in mass and thus settle faster
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ASST. PROF. PRACHI DESSAI

5. Types of Settling
TYPE III ‐ Hindered (or Zone) settling:
i. Inter‐particle forces are sufficient to hinder the settling
of neighboring particles.
ii. The particles tend to remain in a fixed positions with
respect to each others.
iii. Activated sludge process (secondary clarifier).
TYPE IV ‐ Compression settling:
i. Particle concentration is so high that so that particles at
one level are mechanically influenced by particles on
lower levels.
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ASST. PROF. PRACHI DESSAI

6. Types of Sedimentation Tanks
Types
of
Sedimentation
Tanks
Method of Operation
Fill and Draw Type
Continuous Type
Horizontal Flow Type
Rectangular Type with
Longitudinal flow
Circular Tanks with
Radial flow
Vertical Flow Type
Geometry
Rectangular Tanks
Circular Tanks
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ASST. PROF. PRACHI DESSAI

7. Fill and Draw Type Sedimentation Tank
i. Intermittent Type
ii. Rectangular Shaped , Horizontal Bottom
iii. Water from inlet is stored for some time.
iv. The time may be 24 hours.
v. In that time, the suspended particles are
settled at the bottom of the tank.
vi. After 24 hours, the water is discharged
through outlet.
vii. Then settled particle are removed.
viii. Filling + Settling + Cleaning = 30-40
hours
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ASST. PROF. PRACHI DESSAI

8. Fill and Draw Type Sedimentation Tank
Disadvantages:
i. Labour and Supervision
ii. Additional Number of Tanks
iii. Wastage of time.
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ASST. PROF. PRACHI DESSAI

9. Continuous flow type Sedimentation tank
i. Raw Water is continuously admitted into the
tank
ii. Allowed to flow slowly in the tank during
which the particles in suspension settle down
and it flows out continuously from the tank.
iii. Principle- reducing the velocity of flow of
sewage a large amount of suspended particles
present in sewage can be made to settle down.
iv. The velocity of flow of sewage in these tanks
is so adjusted that the time taken by a particle
to move from inlet to outlet of the tank is
slightly more than that required for settling of
that particle & Length.
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ASST. PROF. PRACHI DESSAI

10. Continuous flow type Sedimentation tank
Horizontal Flow Type:
i. Rectangular Tanks with horizontal flow:
i. Raw water enters inlet side and passes
slowly through outlet side.
ii. Near Inlet and Outlet baffles are provided to
prevent cross current.
iii. Length : 30 m(common), Largest – 100 m.
iv. Width:– 12 m
v. Depth : 2.5 m – 5 m (3m Common)
vi. Length: Width – 3:1 or 5:1
vii. Slope: 1% ( Outlet to Inlet)
viii. Narrower tanks preferred (avoid cross
currents due to wind)
ix. Sludge Removal: Mechanical or Manually
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ASST. PROF. PRACHI DESSAI

11. Continuous flow type Sedimentation tank
Horizontal Flow Type:
ii. Circular type with Central feed
i. Water enters the center and leaves at the
periphery or vice versa.
ii. Weir is located at the periphery and then into
an effluent channel.
iii. Diameter: 30 m (Largest: 60 m)
iv. Slope: 1 Vertical to 12 Horizontal.
v. Sludge is removed with the help of ranking
arms.
vi. Raking Arms: 1 Revolution per 30 - 40 mins.
vii. Concentrates Sludge to the centre.
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ASST. PROF. PRACHI DESSAI

12. Vertical Flow Sedimentation Tanks
i. Square or Circular.
ii. Hopper Bottom
iii. Flow is vertical.
iv. Water enters through the centrally placed
inlet pipe and travels downwards.
v. Sludge is collected at the bottom and
removed by Sludge pipe connected to a
sludge pump.
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ASST. PROF. PRACHI DESSAI

13. Environment Engineering I
TE Civil – DBCE, Margao
Asst. Prof. Prachi Dessai
Inlet Zone: Provide a smooth transition of water
and distribute the flow uniformly across the inlet
typically using baffles perforated with holes that
gently spread the flow across the inlet and
prevent short circuiting.
Zones in a Settling
Basin
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ASST. PROF. PRACHI DESSAI

14. Environment Engineering I
TE Civil – DBCE, Margao
Asst. Prof. Prachi Dessai
Settling Zone: The largest portion of the
sedimentation basin, which provides the calm
area for the suspended particles to settle.
Zones in a Settling
Basin
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ASST. PROF. PRACHI DESSAI

15. Environment Engineering I
TE Civil – DBCE, Margao
Asst. Prof. Prachi Dessai
Sludge zone: Provides a storage area for the
sludge before it is removed by scraper or vacuum
devices. Flow velocities near the sludge zone
should be minimized.
Zones in a Settling
Basin
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ASST. PROF. PRACHI DESSAI

16. Environment Engineering I
TE Civil – DBCE, Margao
Asst. Prof. Prachi Dessai
Outlet Zone: Provides a smooth transition from the
settling zone to the outlet. Also controls the depth of
water in the basin. Usually, weirs are set at the end of the
tank to control the overflow rate and prevent the solids
from leaving the tank before they settle out.
Zones in a Settling
Basin
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ASST. PROF. PRACHI DESSAI

17. DESIGN ASPECTS OF
SEDIMENTATION TANKS
The design aspects of sedimentary tanks are
1. Velocity of flow
2. Capacity of tank
3. Inlet and outlet arrangements
4. Shapes of tanks
5. Miscellaneous considerations.
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ASST. PROF. PRACHI DESSAI

18. DESIGN ASPECTS OF
SEDIMENTATION TANKS
Velocity of flow: The velocity of flow of water in sedimentation tanks should be sufficient
enough to cause the hydraulic subsidence of suspended impurities.
It should remain uniform throughout the tank and it is generally not allowed to exceed 150mm to
300mm per minute.
Capacity of tank: capacity of tank is calculated by:
i) detention period -The theoretical time taken by a particle of water to pass between entry and
exit of a settling tank is known as the known as the detention period. The capacity of tank is
calculated by
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ASST. PROF. PRACHI DESSAI

19. DESIGN ASPECTS OF
SEDIMENTATION TANKS
Capacity of tank: capacity of tank is calculated by:
ii) Overflow rate : in this method it is assumed that the settlement of a particle at the bottom of
the settlement of a particle at the bottom of the tank does not depend on the depth of tank and
depends upon the surface area of the tank.
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ASST. PROF. PRACHI DESSAI

20. Relation between Settling Velocity and Surface Overflow
Rate
Consider a rectangular sedimentation tank of Length ‘L’, Width ‘B’ and Depth ‘H’. It is assumed
that the sediment is uniformly distributed as the water enters with a uniform velocity ‘v’.
If ‘Q’ is the discharge entering into the tank, then Velocity ‘V is given by:
V =
Q
BH
…………………………………………(1)
Every particle is moving with the flowing water and also tends to settle down, it possess a
horizontal velocity V and Settling Velocity Vs in the vertical downward direction. Thus the path
of the particle is given by the vector sum of the flow velocity V and Settling Velocity Vs .
From geometric consideration it can be seen that
V
Vs
=
L
H
…………………………………………... (2)
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ASST. PROF. PRACHI DESSAI

21. Relation between Settling Velocity and Surface Overflow
Rate
Vs =
VH
L
By substituting the value of V in Eq.(1). We get
Vs =
Q
BH
×
H
L
Vs =
Q
BL
Assumption: Criteria for the settling of the particles indicates that all the particles with settling
velocity Vs equal to or greater than
Q
BL
will settle down and removed.
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ASST. PROF. PRACHI DESSAI

22. DESIGN ASPECTS OF
SEDIMENTATION TANKS
INLET AND OUTLET ARRANGEMENTS
The inlet is a device, which is provided to distribute the water inside a tank, and the outlet is a
device, which is meant to collect outgoing water. These arrangements should be properly
designed and located in a such a way that they do not form any obstruction or cause any
disturbance to the flowing water.
SHAPES OF TANKS
Following are the three shapes of settling tank.
(i) Rectangular tanks with horizontal flow
(ii) Circular tanks with radial or spiral flow
(iii) Hopper bottom tanks with vertical flow
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ASST. PROF. PRACHI DESSAI

23. Design Criteria
i. Detention period: for plain sedimentation: 3 to 4 h, and for coagulated sedimentation: 2 to 2.5 h.
ii. Velocity of flow: Not greater than 30 cm/min (horizontal flow).
Tank dimensions:
i. L:B = 3:1 to 5:1.
ii. Generally L= 30 m (common) maximum 100 m.
iii. Width = 6 m to 10 m.
iv. Circular: Diameter not greater than 60 m. generally 20 to 40 m.
v. Depth 2.5 to 5.0 m (3 m).
vi. Surface Overflow Rate: For plain sedimentation 12000 to 18000 L/d/m2 tank area; for thoroughly
flocculated water 24000 to 30000 L/d/m2 tank area.
vii. Slopes: Rectangular 1% towards inlet and circular 8%.
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ASST. PROF. PRACHI DESSAI

24. ADVANTAGES & DISADVANTAGES
ADVANTAGES OF SEDIMENTATION
TANK:
i. It is low cost and simple water pre-treatment
technology.
ii. It has low operating costs and handles a wide
range of flows.
iii. To settle out suspended solids, the coagulants
reduce the time required.
iv. It has high clarification efficiency and easy
sludge removal properties.
v. For plants with constant flow rates and quality, it
is best suited.
vi. For free or at a low cost, natural coagulants can
sometimes be obtained.
DISADVANTAGES OF SEDIMENTATION
TANK:
i. To protect against freezing in a cold climate it
needs covering units.
ii. Mechanical drive unit and shaft bearings unit
require frequent maintenance.
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ASST. PROF. PRACHI DESSAI