What is a typical sludge age requirement for effective nitrification/denitrification?

• A. 5 days
• B. 2 days
• C. More than 10 days
• D. 20 days

Answer: (C)

Sludge age, or mean cell residence time, is how long the biomass stays in the activated-sludge system. Nitrifying bacteria grow slowly, so you need enough time for them to establish and maintain an active population to oxidize ammonia to nitrate. In practice, achieving both nitrification and denitrification requires a sludge age over about 10 days, which provides the microbial maturity and stability needed for sustained nitrification and, with the proper carbon source and anoxic zones, denitrification. Short sludge ages, such as around 5 days or 2 days, wash out the slow-growing nitrifiers and hinder nitrification (and thus limit denitrification). A much longer sludge age, like 20 days, isn’t typically necessary for most plants and can reduce process efficiency.