Nitrifier (AOB/NOB): Sources of inhibition

Stalk Ciliates are a good indicator protozoa for conditions favoring AOB/NOB growth.

Ammonia oxidation to nitrate is a two-step process in wastewater treatment. What we call nitrifiers are actually two groups of bacteria: (1) Ammonia Oxidizing Bacteria (AOB) and (2) Nitrite oxidizing bacteria (NOB). And some organisms can actually do both steps! Before we get to inhibition, here are some important facts on AOB & NOB:

• AOB & NOB have slower growth rates than heterotrophic microbes
• Obligate aerobic organisms
• Require pH between 6.5 - 8.5 best growth between 7.2 - 7.8
• Temperatures 10 - 40 Degrees C. (I have seen activity outside these ranges, but it is a stressor)
• Alkalinity for pH buffering 

The most important factor is the relatively slow growth of AOB/NOB cultures. This is why nitrifying systems require longer sludge ages and lower F/M than non-nitrifying biological treatment units. Now things that can harm an AOB/NOB community in the MLSS:

• Toxic compounds - sulfides, phenol, cyanide, solvents, and surfactants
• High COD - depression of dissolved oxygen inside the floc
• Effect of multiple stress factors - no one factor is completely inhibitory but creating conditions slowing AOB/NOB growth.
• Hydraulic overload (washout)

Most often AOB/NOB problems are related to longer term multiple stressors that lead to slower growth followed by a decrease in overall AOB/NOB populations as the organisms "washout". While this is not as easy to solve as a single compound or environmental condition, AOB/NOB populations can be cultivated by managing MCRT and optimizing the environment for their growth. In some systems with high stress, adding supplemental AOB/NOB cultures can help maintain needed populations for effective ammonia & nitrite oxidation.