Denitrification Explained: The Four Steps That Turn Nitrate into Nitrogen

Denitrification is the step that returns nitrogen to the atmosphere. Without it, the nitrogen fixed by biology and by industry would keep accumulating in water, and nitrate would keep moving from fields into rivers and oceans. The overall reaction is a single line, but it runs as four sequential reductions, each with its own enzyme and each releasing energy.

The stoichiometry, start to finish

The overall denitrification process can be represented by the following equation:

2 NO3 + 10 e + 12 H → N2 + 6 H2O, ΔG = -333 kJ/mol

Behind that summary sit four reduction reactions, run in order by the same physiological group of bacteria:

• Nitrate (NO3) is reduced to nitrite (NO2): 2 NO3 + 4 H + 4 e → 2 NO2 + 2 H2O
• Nitrite (NO2) is reduced to nitric oxide (NO): 2 NO2 + 4 H + 2 e → 2 NO + 2 H2O
• Nitric oxide (NO) is reduced to nitrous oxide (N2O): 2 NO + 2 H + 2 e → N2O + H2O
• Nitrous oxide (N2O) is reduced to nitrogen gas (N2): N2O + 2 H + 2 e → N2 + H2O

All four reactions are exothermic. That is the point of the pathway for the cell: under anoxic or oxygen-free conditions, bacteria use nitrate (NO3) as the terminal electron acceptor (TEA) of the electron transport chain (ETC) and complete material and energy exchange without oxygen. Every step is a chance for the chain to stall, which is why partial denitrification and nitrous oxide emissions appear whenever conditions slip.

Why it matters beyond the reactor

In nature denitrification is a key link in the nitrogen cycle. It removes the NO3- that leaches from soil into rivers and oceans, eliminating the toxic effects of nitrate accumulation on living organisms. Together with anaerobic ammonium oxidation (Anammox) it forms the pathway by which nitrogen fixed in nature returns to the atmosphere.

Agriculture sees the same process as a loss. Denitrification reduces nitrate to nitrogen gas and lowers the nitrogen nutrient content of the soil, which is unfavourable for crop production, so farmers often carry out inter-row loosening of the soil to keep air out and limit it.

In environmental protection the process is the backbone of biological nitrogen removal. Denitrification together with nitrification forms the various process configurations used in wastewater treatment plants worldwide. The denitrifying bacteria employed there are heterotrophs: they grow rapidly but require an external organic carbon source, which is why a small amount of methanol or other organic matter is sometimes dosed to keep the process running smoothly when the influent carbon-to-nitrogen ratio is low.

The organisms that do the work

Denitrifiers are widely distributed and come in more metabolic varieties than the textbook shorthand suggests:

• Paracoccus denitrificans — autotrophic, oxidises hydrogen gas (H2).
• Thiobacillus denitrificans — autotrophic, oxidises sulfide (S) or thiosulfate (S2O3).
• Pseudomonas stutzeri — heterotrophic, oxidises organic carbon.

They are mainly prokaryotes and are abundant in the alpha, beta and gamma classes of Proteobacteria. Known denitrifying genera include Achromobacter, Acinetobacter, Agrobacterium, Bacillus, Chromobacterium, Flavobacterium, Spirillum, Vibrio, Halobacterium, Methanomonas and Pseudomonas, among others.

A few denitrifiers are autotrophs, such as Thiobacillus denitrificans, which obtain energy by oxidising sulfur or nitrate, assimilate carbon dioxide, and use nitrate as the terminal electron acceptor of respiration. Most denitrifying bacteria, however, are heterotrophs such as Micrococcus denitrificans and Pseudomonas denitrificans, which use organic matter as both nitrogen source and energy source and perform anaerobic respiration.

Assimilation is a different process with a similar start

Microorganisms take up nitrate for two entirely different purposes, and confusing them is a common source of design error. One is to use the nitrogen it contains as a nutrient, known as assimilatory nitrate reduction: NO3- → NH4+ → organic nitrogen. Many bacteria, actinomycetes and moulds can use nitrate that way, and it consumes no external electron donor because the nitrogen ends up in biomass rather than in the atmosphere.

Dissimilatory reduction is the denitrification route proper: NO2- and NO3- serve as terminal electron acceptors, nitrate is reduced to nitrogen (N2), and the nitrogen leaves the system. Only a minority of bacteria can carry this out, and this physiological group is what the term denitrifiers refers to. In plant design, biomass assimilation removes a few percent of the nitrogen load; everything else has to go through the four-step chain, which is why carbon availability and anoxic contact time decide performance far more than seeding does.

The engineering consequences

Three design habits follow from the chemistry. Carbon must arrive before nitrate does, so the anoxic zone sits ahead of the aerated one and return sludge carries nitrate into it. Dissolved oxygen entering that zone is wasted electron acceptor competition, which is why tank walls, drop pipes and mixer placement receive more attention than they seem to deserve. And the last step, N2O to N2, is the one that fails when residence time is cut too fine, so a marginal saving in tank volume often reappears as an emission factor in the greenhouse gas inventory. The four reductions are cheap to run and unforgiving to rush.