The nitrified activated sludge in a clarifier can begin forming nitrogen gas bubbles if the sludge becomes which condition?

• A. Aerobic
• B. Anoxic
• C. Microaerophilic
• D. Anaerobic

Answer: (B)

Nitrified activated sludge has nitrate produced during the nitrification step. When the sludge environment becomes anoxic, meaning oxygen is absent but nitrate is still present, denitrifying bacteria use that nitrate as the terminal electron acceptor and convert it to nitrogen gas. The buildup of this nitrogen gas forms bubbles in the clarifier. So, the condition that leads to nitrogen gas bubble formation is anoxic, because it provides the necessary lack of oxygen while nitrate remains available for denitrification. Aerobic conditions prevent denitrification by supplying oxygen; microaerophilic is only a low-oxygen state and doesn’t reliably drive full denitrification; anaerobic conditions often lack nitrate as an electron acceptor, making the standard denitrification pathway less likely.