Determine the detention time and overflow rate for a settling tank that will red

Question

Determine the detention time and overflow rate for a settling tank that will reduce the influent suspended solids from 100 mg/L to 35 mg/L. The following batch settling column data are available. The data given are percent removals at the sample times and depths shown. You are required to show detailed calculations for at least three lines of constant removal (e.g. 30%, 40%, 50%, etc.) which must exceed the desired per cent solids removal. You cannot extrapolate to get the desired per cent solids removal.

Explanation / Answer

Detention time is the amount of time which is spent by a water in a tank.

Since we need to reduce the influent suspended solids from 100 to 35mg/l which means 100/35=2.85

depth of the column= 4.5 m

We need to find the time at which the settling percentage is close to 2.85. For that we need to plot the graph of the values at the depth of 4.5 m versus time. The time will be plotted on the x axis. We can also directly observe from the table that a value 25 or 0.25 %on the column is close to the value 2.85. So this value is achieved at the 20 minutes.

The detention time will be=20*1.75=35

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