What is the preferred dissolved oxygen (DO) concentration range in the activated sludge process?

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

Answer: (C)

Oxygen must be available in enough quantity for the aerobic microbes to efficiently oxidize the waste, but not so high that you waste energy. In the activated sludge process, keeping dissolved oxygen in about two to four milligrams per liter provides a healthy balance: it supports rapid organic oxidation and nitrification while maintaining good mixing and settleability of the sludge. If DO falls to 0–1 mg/L, oxygen becomes limiting, slowing treatment and increasing the risk of poor floc formation and bulking. If DO is kept at higher levels, such as 4–6 mg/L, you don’t gain much in treatment efficiency but you pay more for aeration energy. So, two to four mg/L is the preferred range.