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. If the wastewater flow rate for Question 1 is 0.8 m^3/s, specify the dimensions of the clarifier.

Explanation / Answer

Determine time for 20 min time and 0,5m depth

                              20x 0.5 =10m/day

Determine time for 40 min time and 2,5 m depth

                              40 x 2.5 = 100 m/day

Determine time for 55 min time and 3.5m depth

                              55x3.5=1925=192.5

Overflow rate =H/t1      H=height               t1 defined time

Overflow rate for 20 min time and 0.5m depth

                              =0.5/20 =0.025

Overflow rate for 40min time and 1.5m depth

                              =1.5x40=0.037

Overflow rate of 55 min time and 4,5m depth

                              = 4.5 x 55=0.08

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