Common Design Criteria for Sedimentation Tank:
While designing a sedimentation tank, following guidelines should be observed for its satisfactory performance.
Velocity of Flow
Velocity of flow of water in sedimentation tank should be sufficiently low to cause settling of suspended impurities. It should be uniform throughout the tank. Normally, velocity of flow should not be greater than 30 cm/min.
Tank Dimensions L : B should be 3 to 5 : 1
Generally, length provided is 30 m and in extreme case, it may be taken as 100 m. Breadth should be between 6 m to 10 m. Circular tank: Diameter not greater than 60 m. Common size is 20 m to 40 m.
Depth of the Tank varies from 2.5 m to 5.0 m (3 m, typical).
Detention Period
This is the theoretical time for which the water is detained in a settling basin. It is evaluated as the volume of the tank divided by the rate of flow and is denoted as Dt=V/Q. Thus, it is that time which would be required by the flow of water to fill the tank if there was no outflow.
Detention period t for a rectangular tank
= Volume of the Tank/ Rate of Flow = V/Q
= BLH /Q
And detention period for a circular tank
= D2 (0.11D + 0.78H)/Q
where, D = diameter of the tank,
H = vertical depth at wall or side water depth, and
B and L = width and length of the rectangular tank respectively.
The formula for circular tank is based on the fact that the volume of the shape of circular tank used for sedimentation, which is having a cone shaped bottom with slope 1:1 and its volume is D2 (0.11 D + 0.78 H).
The detention time usually ranges between 4 to 8 hours for plain sedimentation and 2 to 4 hours when coagulants are used.
Surface loading or overflow rate or surface overflow rate (SOR) is defined as the quantity of water passing per hour per unit area of settling tank:
SOR = Q/(L × B)
For the plain sedimentation for normal water 12,000 to 18,000 1/d/m2 tank area is required and for thoroughly flocculated water 24,000 to 30,000 1/d/m2 of tank area should be provided.
Slopes should be kept as 1% toward inlet of the circular tank. The total amount of flow from the tank within 24 hours, generally equals the maximum daily demand of water. For the efficient removal of sediment in the tank, the flow should be uniformly distributed throughout the cross-section of the tank. If current permits a substantial portion of the water to pass directly through the tank without being detained for the intended time, the flow is said to be short-circuited. To reduce the tendency of short-circuiting, proper design of the inlets and outlets near the entrance and exit is necessary. It is to be kept in the mind that relatively narrow tank are less affected by the inlet and outlet disturbance.