Sedimentation is a water treatment process where suspended solids settle under gravity in tanks called sedimentation basins or clarifiers. It is used to remove coarse particles, coagulated particles, precipitated particles, and biomass. The key principles are that particles with a specific gravity greater than water will settle, and the detention time in the basin must be sufficient for settling. Sedimentation can be plain or assisted by chemicals. The performance is influenced by factors like particle size and density, water viscosity and temperature, detention time, basin depth and area, and flow rate. Rectangular and circular basins are commonly used.

1. SEDIMENTATION
BY.M.Z .J

2. SEDIMENTATION
 Sedimentation is water treatment process, during which water has
little or no movement, and suspended solids sink to the bottom under
the force of gravity and form a sediment, this process is called
sedimentation.

3. PURPOSE OF SEDIMENTATION
 To remove coarse dispersed phase.
 To remove coagulated and flocculated impurities.
 To remove precipitated impurities after chemical treatment.
 To settle the sludge (biomass) after activated sludge process / tricking
filters.

4. SEDIMENTATION PRINCIPLE
 Suspended solids present in water having specific gravity greater than that of water
tend to settle down by gravity as soon as the turbulence is retarded by offering
storage. Inorganic suspended solids having specific gravity of about 2.65;
and Organic suspended solids having specific gravity of about 1.04
 The particles having specific gravity of about 1.20 or so readily settle down at the
bottom of tank. But it is difficult to cause the settlement of lighter particles.
 This phenomenon of settling down of particles at the bottom of sedimentation tank
is known as hydraulic subsidence and every particle has its own hydraulic settling
value which will cause its hydraulic subsidence.
 Basin in which the flow is retarded is called sedimentation basin.Theoretical average
time for which the water is detained in the settling tank is called the detention period

5. TYPES OF SEDIMENTATION
 Plain sedimentation: It is process of settling down of solids and
impurities in the raw water to the bottom of sedimentation basin by
natural gravity force alone, no chemical is added.
 This method is very cheaper and mostly used in all filtration and
purification system of water.
 Sedimentation by using clarifier and contact: In this method
chemicals are mixed in water and that water is rotated by help of
pumps for period of two hours per day, and suspended solids are settled
down in the bottom of reservoir or tank etc.

6. TYPES OF SEDIMENTATION
 Chemically Assisted sedimentation or clarification: this is process in
which chemicals are added to water and through mixing the suspended
solids and other impurities are stick together and form floc, which
settles to the bottom of basin.
 In general most used process is chemically assisted horizontal
sedimentation and following assumptions are based on.
 water generally flows through a tank as an irregular flow thus the
intention of sedimentation is to create condition in which the flow is
uniform as for long enough period, to permit the maximum practical
amount of floc to be settled before the water reaches at the end of
tank.

7. PARAMETERS INFLUENCING
SEDIMENTATION
Size, shape and weight of particles.
Viscosity and temperature of water.
Detention period of tank.
Effective depth of tank.
Area of tank.
Surface over flow rate.
Velocity of flow and outlet design.

8. TYPE OF SEDIMENTATION BASIN
 Sedimentation tank may function either intermittent or continuously.
 The intermittent type tank are those which store water for a certain period
and keep it on completely rest.
 In continuous type tank , the flow velocity is only reduced and water is not
brought to complete rest .
 Sedimentation tank or basin may be either circular or rectangular in plan.
 Long narrow rectangular tanks with horizontal flow are generally preferred to
the circular tank of radial or spiral flow.

9. RECTANGULAR SEDIMENTATION BASIN
 Rectangular basins are the simplest design, allowing water to flow
horizontally through a long tank This type of basin is usually found in
large-scale water treatment plants.
 Rectangular basins have a variety of advantages such as predictability,
cost-effectiveness, and low maintenance. They are the least likely to
short-circuit, especially if the length is at least twice the width.The
inlet and outlet arrangements of rectangular basis are shown below.
 A disadvantage of rectangular basins is the large extent of land area
required.

10. CIRCULAR SEDIMENTATION BASIN
 Square and circular
sedimentation basins with
horizontal flow are often
known as clarifiers.
 This type of basin faces
short-circuiting problems.
below Figure shows the
inlet and outlet
arrangements.

12. DESIGN DETAIL
 Detention period for plain sedimentation 4-8 hours and for coagulated
sedimentation 3-4 hours.
 Velocity of flow not greater than 30cm/ mint( horizontal flow).
 Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m (common) maximum 100 m.
 Breadth= 6 m to 10 m. Circular: Diameter not greater than 60 m. generally 20
to 40 m.
 Depth 2.5 to 5.0 m (3 m).
 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.
Slopes: Rectangular 1% towards inlet and circular 8%.

13. DETENTION PERIOD
 The theoretical (calculated) time required for a given amount of water
or wastewater to pass through a tank at a given rate of flow. In other
words it is average time for which the water is detained in tank.
 t=
𝒗𝒐𝒍𝒖𝒎𝒆
𝒇𝒍𝒐𝒘 𝒓𝒂𝒕𝒆
=
𝑽
𝑸
=
𝑳∗𝑩∗𝑫
𝑨𝑽
=
𝑫
𝑽
 Where t is detention period, Q is flow rate.

14. DETENTION PERIOD
 Detention time is inversely proportional to the incoming flow rate - as
flow rate increases, the detention time decreases.
 Example
 Calculate the detention time for a rectangular sedimentation tank
with the following dimensions, length is 20m, width is 10m, depth is
4m, and the flow rate is 0.5 𝒎 𝟑
per sec.
 detention time = tank volume
incoming water flow rate

15. DETENTION PERIOD
 Note units of measurement are all consistent. ie time is given in seconds
and length is given in meters.
 detention time = (20 x 10 x 4)
0.5
 detention time = 800
0.5
 detention time = 1600 seconds
detention time = 26.7 minutes
 The term detention is often used interchangeably
with retention and hydraulic retention time.

16.  http://nptel.ac.in/courses/105104102/Domestic%20water%20treat.htm
 http://www.thewatertreatments.com/wastewater-sewage-
treatment/types-sedimentation-basin/