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Why cold is more of a problem for ammonia oxidation than BOD removal

12/4/2019

 
PictureNitrifier production tank biomass
If I ask people about their problems with low temperatures, the most common problem is maintaining ammonia oxidation (AKA Nitrification) during winter months. So why can we see decent BOD/COD removal with low temperatures and not ammonia/nitrite oxidation to nitrate? 

It all has to do with AOB & NOB having slow growth rates under all conditions that become more exagerated under low temperature conditions. Secondly, genera that oxidize ammonia/nitrite are mesophilic organisms that don't do well at temperatures under 10 - 12 Deg C.

Nitrifier Growth Rates
Ammonia Oxidizing Bacteria (AOB) - often a mix of Nitrosomonas, Nitrospira (the COMAMMOX subset), Nitrosococcus - are the slowest growing of the chemotrophic nitrifiers. That means the initial conversioni of ammonia into nitrite is usually the rate limiting step. The AOB growth under ideal conditions (D.O. > 2 at cell level, pH 7.2 - 8.2, sufficient alkalinity, and no inhibition) maxes out at 10 - 12 hours versus 30 - 60 minutes for many heterotrophic (BOD removing) organisms. AOB ideal growth happens between 25 - 30 deg C.  As you decrease temperature below 20 Deg C, time required for cell replication increases and if operating at a low MCRT, you may see the AOB population "washout".

More on Temperature Impact on Nitrifier Population
Nitrifiers growth slows as you drop below 20 deg C and is so slow that by 10 Deg C that we consider the cultures dormant. Unlike the heterotrophic populations that can adjust to lower temperatures with low range mesophiles and some psychrophiles that thrive at temperatures between 3 - 15 deg C. The increase in lower temperature heterotrophic organisms is the reason that you can keep BOD removal even with low temperature wastewater. In fact, I have worked with cultures that grow well at 5 deg C. 

Ways to keep nitrification at low temperatures
Now that you know why nitrification is a problem at lower temperatures, what can you do to keep ammonia from reaching the effluent? Here are some suggestions:
  • Increase MCRT to avoid washout
  • Keep pH, Alkalinity, and D.O. at ideal levels 
  • Try to keep inhibitory compounds amines, sulfides, phenol, solvents, and other inhibitory compounds out of the biological unit as much as possible
  • Moving from surface aeration to diffusers can help "insulate" and reduce temperature drop across the biological unit


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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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