Text: 08-2 (Show a diagram; mass balance) Municipal wastewater is treated in a wastewater treatment plant with two secondary treatment trains in parallel. Given the following information and assumptions: average flow of 10 million gallons per day; plant influent BOD5 = 200 mg/L; plant effluent BOD5 = 15 mg/L; typical BOD removal in the primary sedimentation system as described in your textbook (MZ 9.5); first-order treatment kinetics in the biological reactor (k = 8.5/d); no soluble BOD removal in the secondary tank (so the aeration basin must do all the treatment); kBOD = 0.2/d.
(a) Calculate the area (in ft2) and volume (in ft3) needed for each primary settling tank assuming the overflow rate is not to exceed 1100 gpd/ft2 and the detention time is to be at least 2 hours. Assume a set depth of 12 ft.
(b) Calculate the hydraulic detention time and volume of each aeration tank if it is a complete mix reactor operating under steady-state conditions.
(c) Calculate the diameter of each circular secondary clarifier if the overflow rate is not to exceed 700 gpd/ft2. Round up to the nearest ft. (Hints: Use the larger area that meets the two criteria for the primary clarifier.)
Answers: (a) 4.64x10^3 ft2, 5.57x10^4 ft3, (b) 653,250 ft3, 23.5 h based on typical BOD removal in the primary clarifier, (c) 96 ft