Nitrification efficiency in an activated sludge system is most efficient at which pH range?

• A. Strongly acidic
• B. Neutral to slightly alkaline
• C. Highly alkaline
• D. Strongly basic

Answer: (B)

Nitrification in an activated sludge plant is carried out by bacteria whose enzymes work best near neutral pH. The ammonia-oxidizing and nitrite-oxidizing organisms prefer a pH around 7 to 8, where their metabolic processes operate efficiently and growth is robust. If the pH is strongly acidic, nitrifiers slow down because ammonia is mostly present as ammonium (NH4+), which is not as readily used by the bacteria, and enzyme activity declines. If the pH is strongly alkaline, nitrification can still be hampered: free ammonia (NH3) increases with pH and can become inhibitory to the nitrifiers, and other stresses can reduce rates. Remember also that nitrification consumes alkalinity and tends to lower pH, so maintaining buffering helps keep conditions in this optimum range.