After exiting the coagulation$/$flocculation stage, $127000$ USG$/$d of a floc suspension in

water is sent to a clarifier for sedimentation at $11C.$ The clarifier is rectangular, ${24}^{\text{'}}$ long

by $16\text{'}$ wide, and $11\mathrm{.}5$ deep. The composition of the stream is as follows:

$15\mathrm{.}5\frac{w}{v}\%$ flocs

$1\mathrm{.}2\frac{w}{v}\%$ residual alum coagulant, $pH=8\mathrm{.}8$

Chlorine disinfection is performed in the clarifier during the sedimentation process. The

detention time for the solids in the clarifier is the same as the chlorine contact time. What

residual $($free chlorine$)$ concentration, in ppm$,$ would be expected in the water? $[5$ marks$]$