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Nitrifier (AOB/NOB):  Sources of inhibition

5/16/2018

 
Picture
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.

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    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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