Which of the following pairs correctly describes the oxygen requirement for each bacteria type used in activated sludge?

• A. Aerobic - air; Facultative - air; Anaerobic - no air
• B. Aerobic - air; Facultative - both; Anaerobic - no air
• C. Aerobic - air; Facultative - no air; Anaerobic - air
• D. Aerobic - no air; Facultative - both; Anaerobic - air

Answer: (B)

Bacteria in activated sludge vary by how they use oxygen. Aerobic bacteria require oxygen, which is supplied by air through the aeration process, so they thrive in the aerated tank. Facultative bacteria can grow with or without oxygen; they use oxygen when available but can switch to other metabolic pathways in low-oxygen conditions, making them adaptable to changing environments. Anaerobic bacteria do not require oxygen and may be inhibited by it, so they predominate in zones without air, such as anoxic pockets or digesters. Therefore, the correct description is aerobic—air; facultative—both; anaerobic—no air.