What is the proper pH range for activated sludge operation?

• A. 5.5-6.5
• B. 6.0-6.5
• C. 6.8-7.2
• D. 7.5-8.0

Answer: (C)

Microbial activity in the activated sludge process works best at near-neutral pH. Enzymes and the mixed-culture community responsible for breaking down organics perform optimally around 6.8 to 7.2, and this range also supports nitrifying bacteria that convert ammonia to nitrate. If the pH drops too low, the microbes slow down and nitrification can stall; if it rises too high, some bacteria become less active and ammonia can start to strip from the liquid, destabilizing the process. Maintaining pH in roughly 6.8–7.2 provides the best balance between robust biological performance and system stability, which is why this range is preferred. The other ranges are either too acidic or too alkaline to sustain optimal activity.