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SEDIMENTATION
TANKS...
Pakistan Council of Scientific & Industrial Research
Laboratories Complex, Karachi
Off: University Road, Karachi-75280.

Solid liquid separation process in which a suspension is
separated into two phases –
Clarified supernatant leaving the top of the sedimentation
tank (overflow).
Concentrated sludge leaving the bottom of the
sedimentation tank (underflow).

Purpose of Settling
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.

Principle of Settling
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.
Basin in which the flow is retarded is called settling
tank.
Theoretical average time for which the water is
detained in the settling tank is called the detention
period.

Types of Settling
Type I settling (free settling)
Type II settling (settling of flocculated
particles)
Type III settling (zone or hindered
settling)
Type IV settling (compression settling)

Design parameters for settling tank
Types of settling
Overflow rate
m3m2/day
Solids loading
kg/m2/day
Depth
Detentio
n time
Average Peak Average Peak
Primary settling only 25-30 50-60 - - 2.5-3.5 2.0-2.5
Primary settling followed by
secondary treatment
35-50 60-120 - - 2.5-3.5
Primary settling with activated
sludge return
25-35 50-60 - - 3.5-4.5 -
Secondary settling for trickling
filters
15-25 40-50 70-120 190 2.5-3.5 1.5-2.0
Secondary settling for activated
sludge (excluding extended
aeration)
15-35 40-50 70-140 210 3.5-4.5 -
Secondary settling for extended
8-15 25-35 25-120 170 3.5-4.5 -

Design Details
Detention period: for plain sedimentation: 3 to 4 h, and for
coagulated sedimentation: 2 to 2.5 h.
Velocity of flow: Not greater than 30 cm/min (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%.

Problem:
Design a rectangular sedimentation tank to treat 2.4
million litres of raw water per day. The detention period
may be assumed to be 3 hours.

Solution: Raw water flow per day is 2.4 x 106 L . Detention
period is 3h.
Volume of tank = Flow x Detention period = 2.4 x 106 x 3/24 =
300 m3
Assume depth of tank = 3.0 m.
Surface area = 300/3 = 100 m2
L/B = 3 (assumed). L = 3B.
3B2 = 100 m2 i.e. B = 5.8 m
L = 3B = 5.8 X 3 = 17.4 m
Hence surface loading (Overflow rate) = 2.4 x 106 =
100
24,000 L/d/m2

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SEDIMENTATION

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