EN8591 – WASTE WATER ENGINEERING
UNIT 2 – PRIMARY TREATMENT OF SEWAGE
SESSION – 8 – DESIGN OF SEWAGE
TREATMENT UNITS – PRIMARY SEDIMENTATION TANK
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ENGINEERING / SNSCE
PRIMARY SEDIMENTATION TANK
• Sedimentation is a physical water treatment process using gravity to remove suspended solids from water.
• Solid particles entrained by the turbulence of moving water may be removed naturally by sedimentation in the still water of lakes and oceans.
• Settling basins are ponds constructed for the purpose of removing entrained solids by sedimentation.
• Clarifiers are tanks built with mechanical means for continuous removal of solids being deposited by sedimentation.
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ENGINEERING / SNSCE 2
TYPES OF PRIMARY SEDIMENTATION TANK
1. Based on shape • Rectangular Sedimentation Tank
• Circular Sedimentation Tank
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ENGINEERING / SNSCE 3
Sedimentation Tank Design Parameters
To design Sedimentation tank following elements are required in the
consideration:
• Over flow velocity
• Detention period
• Flow through velocity
• Dimensions of the tank
• Sludge zone depth
• Efficiency
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MATHANAKEERTHI S / AP / CIVIL ENGINEERING / SNSCE
6
1.Overflow Velocity (Vo)
• The volume of water applied per unit time per unit horizontal surface area
is called over flow velocity.
• It is also called as surface loading rate or surface over flow rate.
• Over flow velocity must be less than settling velocity of particle otherwise
the particle or sediment will escapes without settling.
The over flow velocity is calculated from the formula
Vo = Discharge / surface area = Q / (BxL)
In general, over flow velocity of sedimentation tank = 12 to 18 m3/day/m2.
2/17/2020
MATHANAKEERTHI S / AP / CIVIL ENGINEERING / SNSCE
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2.Flow Through Velocity (V)
• The velocity of water with which it travels from inlet to outlet of
sedimentation tank is called flow through velocity (V).
• The allowable flow through velocity of water in sedimentation tank is
0.005m/s. Flow through velocity, V = discharge / area = Q / (BxH)
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ENGINEERING / SNSCE 8
3.Detention Period (T)
• The time taken by water to travel from inlet to outlet is termed as
detention period or detention time.
• The allowable detention period in the sedimentation tank is 4 to 8 hours.
Detention Time = volume of tank / discharge = V/Q
2/17/2020 MATHANAKEERTHI S / AP / CIVIL
ENGINEERING / SNSCE 9
4.Dimensions of Sedimentation Tank
• The basin dimensions length, breadth and depth of tank are derived from the
volume of the tank and over flow velocity.
i. The area of tank:
ii. Area (A) = Volume of water/ over flow velocity
iii. After obtaining the area assume length and depth based on allowable limits
iv. The breadth of sedimentation tank should be provided is 10 to 12 meters
while the length of sedimentation tank should be at least 4 times the
breadth of sedimentation tank. The depth of tank should be 3 to 4.5
meters.
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ENGINEERING / SNSCE 10
2/17/2020 MATHANAKEERTHI S / AP / CIVIL
ENGINEERING / SNSCE 11
D = 3 to 4.5 m
Dimensions of Sedimentation Tank
5. Sludge Zone Depth
• Sludge zone depth is used to collect the sludge at the bottom which is nothing
but settled particles.
• It is provided only when removal of the sludge takes place manually.
• Generally it is limited to 0.8 to 1.2 meters.
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ENGINEERING / SNSCE 12
Efficiency
• Efficiency is nothing but the performance of sedimentation tank.
• It is maximum when the maximum amount of suspended particles in the
raw water are separated.
• In sedimentation tanks, efficiency purely depends upon the settling
velocity and over flow velocity.
Efficiency of Sedimentation Tank (η) = (Vs/Vo) x 100
2/17/2020 MATHANAKEERTHI S / AP / CIVIL
ENGINEERING / SNSCE 13
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