You are given the task of building detention ponds to remove settleable solids from a small stream prior to discharge into a lake. The ponds must be 2 m deep, the flow in the stream is 20 cfs, the solids settle at a rate of 0.2 m/day, and you must achieve a steady-state removal of 65% of the solids. a. Determine the size of a single CSTR to achieve the desired removal.
b. Determine the sizes of a pair of identical CSTRs in series needed to achieve the desired removal.

a) 513775.5 m^3  ≈ 51.38 * 10^4 m^3

b)  256887.75 m^3 ≈ 25.69 * 10^4 m^3

Step-by-step explanation:

Given data :

Depth of pond = 2m

flow in the stream = 20 cfs = 4.8931 * 10^4 m^3/day

settling rate = 0.2 m/day

rate constant ( k )= 0.2 / 2 = 0.1 / day

steady-state removal of the solids required = 65%

A) determine size of a single CSTR to achieve 65%

First determine the time required to achieve this using the relation below

C =

∴ t =

     = ( -1 / 0.1 ) * (- 1.05 ) = 10.5 day

The size of a single CSTR required = ( 4.8931*10^4 ) * (10.5 ) = 513775.5 m^3

B) determine the sizes of a pair of identical CSTRs in series needed to achieve the desired removal

sizes of a pair = 513775.5 / 2 = 256887.75 m^3

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