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Denitrification - the challenge of removing nitrite/nitrate in biological wastewater treatment systems

5/14/2020

 
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From: Vaishnav, Anukool & Sharma, Sushil & Choudhary, Devendra & Sharma, Kanti & Ahmad, Ees & Sharma, Mahaveer & Ramesh, Aketi & Saxena, Anil. (2018). Nitric Oxide as a Signaling Molecule in Plant-Bacterial Interactions. 10.1007/978-981-10-5514-0_8.
On paper denitrification should be easy. All you need is a "food" source, common facultative heterotrophic bacteria, and anoxic conditions (no free D.O.). However, in practice you need to have a balance of conditions to ensure denitrification. If you are having problems maintaining denitrification, it helps to look at the process from an ecological perspective. 

What is required for bacteria to use nitrate/nitrite as electron acceptor (AKA alternative oxygen source)
  • Many bacteria can use nitrate but not all can use nitrite. Your goal is to develop cultures that an convert all NOx into N2 gas. This process actually h as multiple steps that include 4 enzyme pathways.

  • Denitrification happens with many substrates (electron donors) - this includes soluble organics (sugars, alcohols, starches) but can also include chemoautotrophic organisms such as sulfur and metal oxidizing bacteria. This last group is being researched as Autotrophic Denitrification (AuDen). However, most systems denitrify best with typical wastewater heterotrophic bacteria including Thauera, Zooglea, & Pseudomonas.

  • Anoxic conditions are necessary to trigger denitrification. When D.O. is present, the bacteria obtain more energy using oxygen as an electron acceptor. Inside dense floc, biofilms, or aerobic granular sludge can have anoxic zones that denitrify inside aerobic basin. This is part of the appeal of MBBR and aerobic granular sludge systems. You do not have to manage the anoxic zone redox potentials (ORP) or residence time as closely as you would in conventional aerobic activated sludge.

  • Always consider residence time, temperature, and availability of suitable electron donor (food) for the bacteria in the denitrification zone. Failure in any variable can be a problem.  Denitrification to N2 gas often takes action by several different organisms (although a few can take it to N2 gas inside a single cell).

Aster Bio research in AOB & NOB inhibition

5/7/2020

 
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From: https://en.wikipedia.org/wiki/Anammox
Ammonia Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB) are often grouped into what we call nitrifiers.  These organisms grow by converting ammonia into nitrite and finally nitrate.  Organisms working in the nitrogen cycle are diverse and include the following:
  • Heterotrophs that degrade organic nitrogen compounds (proteins, amines, etc) into ammonia. They also use nitrogen to build proteins used in cellular activities and reproduciton. Under anoxic conditions, many heterotrophs use nitrate and nitrite as alternative electron acceptors converting the NO3/NO2 into N2 gas.
  • Chemoautotrophs - this includes AOB/NOB, COMAMMOX (funtions as both AOB & NOB in one organism), ANAMMOX (under anoxic conditions takes NO2 + NH4+ directly to N2 gas)
  • Heterotrophic ammonia oxidation (slower than obligate AOB/NOB but does take place).
So given such diversity, toxicity screens become a complex undertaking.  So, here is what we are doing (while the COVID19 virus has travel and field work at minimum levels).
  • Sampling diverse WW systems to see makeup of their nitrifying populations using Mirobial Community Analysis (MCA)
  • Using qPCR tests created from MCA data for individual systems to develop baseline data for operating wastewater treatment plants - this involves desigining and refining oligos used in qPCR tests.
  • Testing suspected compounds using concentrated AOB cultures for acute toxicity using Tox-N testing. In this we spike buffered water with suspect compounds and add AOB. We compare ammonia oxidation rates versus a control to get % inhibition. 
  • More advanced simulation in pilot activated sludge units where we use a healthy MLSS and subject it to influents spiked with compounds of interest. We then monitor ammonia, nitrite, nitrate, and use molecular testing to see drifts in AOB/NOB populations.


    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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