EN8591 WASTE WATER ENGINEERING UNIT 2 PRIMARY …

EN8591 – WASTE WATER ENGINEERING

UNIT 2 – PRIMARY TREATMENT OF SEWAGE

SESSION – 8 – DESIGN OF SEWAGE

TREATMENT UNITS – PRIMARY SEDIMENTATION TANK

2/17/2020 1 MATHANAKEERTHI S / AP / CIVIL

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.

2/17/2020 MATHANAKEERTHI S / AP / CIVIL

ENGINEERING / SNSCE 2

TYPES OF PRIMARY SEDIMENTATION TANK

1. Based on shape • Rectangular Sedimentation Tank

• Circular Sedimentation Tank







Circular Sedimentation Tank

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

2/17/2020

MATHANAKEERTHI S / AP / CIVIL ENGINEERING / SNSCE

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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

7

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)



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



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.





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.



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



ACTIVITY



EN8591 WASTE WATER ENGINEERING UNIT 2 PRIMARY …

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