A new municipal wastewater treatment plant plans to have an average flowrate of 5.3 MGD and a peak daily flow rate of 13.2 MGD. Primary sedimentation tanks are planned prior to secondary treatment processes.

a. Design a system with two primary sedimentation basins in parallel that each have a channel width of 20 ft. Assume an average overflow rate of 1,000 gal/ft2-d. Specify the required surface area, A (ft2), and tank length, L (ft) of each basin. Assuming a sidewater depth of 13 ft, calculate the total tank volume (ft2).
b. Calculate the hydraulic retention time (i. e., detention time) during average flow conditions (hours). Is this value within the acceptable range (hint: refer to a Table in your book)?
c. Determine the hydraulic retention time (h) and overflow rate during peak hourly flow (gal/ft^2-d). Are these values within the acceptable range?
d. The scour velocity is the horizontal velocity in the tank that will just start to produce the resuspension (i. e., scouring) of settled particles. Calculate the scour velocity, assuming that the cohesion constant, k, is 0.05, the specific gravity, s, is 1.25, the average diameter of particles is 85 μm, and the Darcy-Weisbach friction factor is 0.025. Will scouring occur during the peak flow rate?