Answer:
Detention time will be 9 days
Step-by-step explanation:
We have given diameter of setting tank d = 50 feet
Radius of setting tank

SWD = 9 FEET
Area

So volume

We have given flow Q = 15000 gpd
So Q= 15000×0.13368 =2005.22
per day
Detention time is given by

So detention time will be 9 days