Many wastewater treatment operators monitor dissolved oxygen (DO) levels carefully and often find that bulk water DO remains within the recommended range. Yet, nitrification performance may still decline, ammonia removal becomes unstable, and unexpected nitrite accumulation appears.

Why does this happen?

The answer lies inside the biofilm.

In MBBR systems, microorganisms do not live freely in the water. They grow as a biofilm attached to the carrier surface. While oxygen may be abundant in the surrounding water, it must first diffuse through the biofilm before reaching the microorganisms located deeper inside.

As the biofilm becomes thicker, oxygen transfer resistance increases. The outer layer of microorganisms consumes oxygen first, leaving less available for the inner layers. This creates oxygen gradients within the biofilm.

As a result:

✔ Bulk water DO may measure 3–4 mg/L or higher

✔ Outer biofilm layers remain aerobic

✔ Inner biofilm layers can become oxygen-limited or even anoxic

When oxygen cannot adequately penetrate the biofilm, nitrifying bacteria are often the first to be affected because they require stable aerobic conditions. The consequences may include: