Wastewater Operator Level 1 Practice Test

What is the typical range for dissolved oxygen (DO) in aeration tanks used in activated sludge processes?

Approximately 0 to 1 mg/L. Approximately 2 to 4 mg/L. Approximately 6 to 8 mg/L. Approximately 10 to 12 mg/L.

Correct Answer:

Approximately 2 to 4 mg/L.

Explanation:

Maintaining enough oxygen in the aeration tank is essential for the aerobic microbes to effectively oxidize organic matter. A dissolved oxygen level around 2 to 4 mg/L provides reliable biodegradation while balancing energy use; it keeps the mixed liquor aerobic and supports stable sludge characteristics without wasting energy. If DO is too low (near 0–1 mg/L), anaerobic zones can form, leading to odors and poor treatment performance. Conversely, keeping DO much higher (6–8 mg/L or more) wastes blower energy and offers diminishing returns for treatment efficiency.

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