The usual detention time through the AT for the conventional activated sludge process is __ to __

• A. 2 to 3
• B. 6 to 8
• C. 8 to 12
• D. 4 to 6

Answer: (D)

Detention time in the aeration tank is the amount of time the wastewater stays in that tank so the microorganisms have adequate contact with the organics to degrade them. In a conventional activated sludge setup, you design this contact time to be long enough to achieve meaningful BOD removal, but not so long that you waste energy and large tank volumes. The usual practice is to keep the aeration basin occupied for a few hours, typically around four to six hours. This range provides a good balance: enough time for significant treatment to occur while keeping reactor size and aeration energy reasonable. If the time is too short, treatment is incomplete; if it’s too long, you may waste energy and capacity without commensurate improvements in effluent quality.