Nitrified activated sludge requires which type of conditions to operate effectively?

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

Answer: (C)

Nitrification in activated sludge requires oxygen to fuel the nitrifying bacteria. These microbes are aerobic and oxidize ammonia first to nitrite and then to nitrate, using oxygen as the terminal electron acceptor. Because of this, maintaining adequate dissolved oxygen in the mixed liquor is essential for robust nitrification rates. If the system becomes anoxic or anaerobic, the nitrifying bacteria cannot function, and the process stalls. Extremely low oxygen levels (microaerophilic) can slow nitrification dramatically, since nitrifiers are not adapted to thrive under such conditions. Therefore, aerobic conditions are necessary to operate nitrified activated sludge effectively.