In the context of sedimentation tank, the ratio of the 'flowing through period' to the detention period is called _____. 

Concept:

Sedimentation tank

Displacement efficiency 

• Displacement efficiency of sedimentation tank is the ratio of flowing through the period to detention period.
• For the sedimentation tank, the displacement efficiency varies from 0.25 to 0.3.

​Note:

​In actual practice, a certain amount of short-circuiting always exists, and therefore the actual average time which a batch of water takes in passing through a settling tank called the flowing through the period is always less than the detention period which is the corresponding theoretical time.

​Detention Period 

​Detention period is defined as the time for which the water was in the tank and overflow rate is the velocity with which the tank fills.

In a rectangular sedimentation tank,

The velocity of flow, Vf = Q/(B × H)

Detention time, td = L/Vf = Volume of tank/Discharge = (L × B × H)/Q  

Surface overflow rate, Vs = Q/(B × L)

Important Points Some design Parameters for Plain sedimentation tanks are:

1. Detention period:

• In water sedimentation tank using coagulated raw supplies is 2 to 4 hours.
• In the case of plain sedimentation, detention time is 4 to 8 hours. 

2. Horizontal flow velocity: Not greater than 30 cm/min

3. Length: Breadth ratio: (3 to 5) : 1

4. Surface overflow rate: 12000 to 18000 L/m2/day or 500 to 750 litres/hour/m2

5. Depth: 2.5 to 5 m

Additional Information 

The efficiency of the sedimentation tank indicates the overall percentage removal of suspended matter at a given overflow rate or surface loading rate.

The efficiency of the sedimentation tank increases if the overflow rate reduces (more time available to particles for settle).

overflow rate, 

From the equation, it is clear that if the surface area (B × L) of the tank increases, the overflow rate reduces, and efficiency increases for a given discharge.