What dissolved oxygen concentration range is generally required for microorganisms in wastewater treatment?

• A. 2-5 mg/L
• B. 6-9 mg/L
• C. 0-1 mg/L
• D. 10-15 mg/L

Answer: (A)

Microorganisms in wastewater need enough oxygen to perform aerobic metabolism and effectively oxidize organic matter. Maintaining dissolved oxygen in roughly 2 to 5 mg/L provides enough oxygen for steady microbial activity without wasting energy on excessive aeration. If DO drops below about 2 mg/L, microbial growth slows, treatment performance can decline, and odors or bulking issues may arise. Keeping DO well above 5 mg/L offers little additional treatment benefit and just increases energy use for aeration. In practice, activated sludge systems often operate around 2–3 mg/L, with 2–5 mg/L serving as a general, safe range for effective aerobic treatment.