Depending upon the particular modification of the activated sludge process used, enough air must be supplied to produce a DO content of __ to __ mg/L

• A. 1 to 4
• B. 0.5 to 1.5
• C. 2 to 6
• D. 4 to 8

Answer: (A)

In activated sludge, the amount of air supplied is tuned to keep the dissolved oxygen (DO) in the aeration tank at a level that supports aerobic microorganisms without wasting energy. Across the different modifications of the process, a practical operating DO range is about 1 to 4 mg/L.

Why this range works: around 1 mg/L provides enough oxygen for microbial metabolism in many configurations, ensuring steady oxidation of organics. Going up to about 4 mg/L keeps the oxygen supply sufficient for effective treatment, including nitrification where present, but pushing beyond 4 mg/L tends to yield diminishing returns while dramatically increasing energy use. Conversely, DO much below 1 mg/L can lead to incomplete treatment and odor problems. Therefore, 1 to 4 mg/L is the typical, balanced target for most activated-sludge systems.