DO concentration in the sludge process is typically maintained within which range?

• A. 0-1 mg/L
• B. 1-2 mg/L
• C. 2-4 mg/L
• D. 4-6 mg/L

Answer: (C)

Maintaining enough dissolved oxygen in the aeration basin is essential so aerobic microbes can efficiently oxidize organic matter. If oxygen is too low, microbial activity slows, reducing treatment efficiency. If oxygen is kept too high, you’re just wasting energy without giving additional treatment benefit.

The typical range used for the activated sludge process is about 2 to 4 mg/L. This level provides plenty of oxygen for the microbes to do their job while avoiding the extra energy cost of higher aeration. In practice, many plants target around 2–3 mg/L, which often meets treatment needs efficiently. Levels below roughly 1–2 mg/L risk poor oxidation and settling, while levels above about 4–5 mg/L offer little advantage and simply waste energy.