Question

A water treatment plant has 6 settling tanks that operate in parallel (the flow gets split into six equal flow streams), and each tank has a volume of 600m^3. If the flow to the plant is 10mgd, what is the retention time in each of the settling tanks? If, instead, the tanks operated in series (the entire flow goes first through one tank, then the second, and so on), what would be the retention time in each tank?

Answer 1

mgd stands for million gallons per day

Now 1 mgd will be 3785.4 m³/day

so 10 mgd will be 37854 m³/day

6 settling tanks each of volume 600 m³

So total settling volume is 3600 m³

a) So as per this the input low is 37854 m³/day this is split into 6 streams so each stream is

37854 m3/6 = 6309 m³/day which is 6309/24 = 262.875 m³/hr when it goes to a tank of capacity 600 m³

The filling time will be 600 m³/262.875 m³/hr = 2.28 Hr. But since the flow is continous after it is filled it has to be immediately entied to be filled again.

So the retention times after filling in each tank will be 0

b) Now the entire flow is 37854 m³/day or 1577.25 m³/Hr

So each tank of capacity 600 m³ gets filled in 600 m³/1577.25 m³/Hr = 0.38 Hr.

going from one tank to th next it takes 0.38 Hr x 6 to fill 6 tanks that is 2.28 Hr to fill 6 tanks before it returns to the first.

So the settling time in each tank after it is filled will be 2.28 Hr. - 0.38 Hr = 1.48 Hr