PE Civil Engineering Exam Prep

Answer: C - The total volume of the grit chamber is 2,079 ft³ when required to handle an average flow of 3 million gallons per day (mgd), with a peak flow ratio of 2.5 and a detention time of 3 minutes.

Solution: volume of grit chamber = peak flow x detention time

Where peak flow = average flow x peak flow ratio

= 3 mgd x 2.5

= 7.5 mgd

Given 1 mgd = 1.54 ft³/sec

Where detention time = 3 min x 60 sec/min = 180 sec

Volume = 7.5 mgd x 1.54 ft³/sec x 180 sec

= 2,079 ft³