What is the aeration tank detention time for the conventional activated sludge process?

• A. 2 to 4 hours
• B. 4 to 6 hours
• C. 6 to 8 hours
• D. 8 to 10 hours

Answer: (B)

In a conventional activated sludge process, the aeration tank detention time is the time the wastewater spends in the aeration basin while being aerated for biological treatment. This time must be long enough for the microorganisms to oxidize a substantial portion of the biodegradable organics (BOD/COD) present.

Typically, designers target about 4 to 6 hours of detention in the aeration tank. This range provides a good balance: enough contact time for effective organic removal without unnecessarily lengthening the process or raising energy use. Shorter times, like 2 to 4 hours, usually don’t give enough contact for adequate oxidation, leading to poorer treatment performance. Longer times, such as 6 to 8 hours or more, can be used in certain setups but are less common for standard conventional plants and may not improve performance commensurately with the added costs. The 4 to 6 hour range is the standard operating guideline for CAS.