Civil Engineering - Water Supply Engineering - Discussion

I can't find the difference between two, someones please help to differentiate.

I think more than the detention period.

So please clarify me.

This option is wrong. Flow through period should always be equal or greater than the detention period so that all the suspended particles having detention period can easily settle.

Yeah, this answer is wrong as flow through period should be more to avoid any short-circuiting in the system and to avoid travel of objectionable particles to further units.

Flow through period is the period "during this period the particles in suspension settle down and clear water flows out continuously from the tank. ".

Detention period -- Time taken by water to pass between entry and exit.

So the time taken to settle down i.e. flow through time should be larger than detention period. Hence, answer is correct.

Flowing through period is the time taken by a batch of water to travel from the entry to exit of settling tank. Because of short circuits produced in water current, flowing through period is less than the detention period.

Displacement efficiency = (flow through period/theoretical detention time )*100.

Detention time is calculated on basis of geometry, but in actual, there is turbulence at the top layer and drag, friction at the bottom layer, which are not considered in calculations. So the top layer has more velocity so detention timeless so unable to settle and the bottom layer has less velocity so detention time more and easily settle. Practically the average detection time called the flow through period is less than the geometrically calculated detention time.

So, given answer is correct.

The answer given is right if you consider comparison between Settling velocity and flow velocity.

Displacement efficiency is the ratio of flowing through the period to the detention period. Flowing through period is the time taken by a batch of water to travel from the entry to exit of the settling tank. Because of short circuits produced in the water current, flowing through period is less than the detention period. So, the given answer is true.