Nitrification and denitrification

Nitrification/denitrification is the most common biological process for removing nitrogenous substances from wastewater. MBBR biological treatment is used to clean nitrogen-rich wastewater. Most treatment systems rely on microbial activity. The purpose is to remove undesirable nitrogen and mineral compounds:
  • ammonia
  • nitrites
  • nitrates
To remove these compounds, bacteria must follow a two-step process.

Nitrification step

Nitrification is the first stage in the biological treatment of nitrogen. It is carried out by aerobic bacteria that oxidise ammonia (NH4+) into nitrite (NO2-) and then nitrate (NO3-). This reaction is catalysed by aerobic bacteria such as Nitrosomonas europaea and Nitrobacter hamburgensis.

In this phase, bacteria oxidize ammonium to nitrite under aerobic conditions.


Nitrification takes place in two successive stages:

  1. Oxidation of ammonia to nitrite: Bacteria of the Nitrosomonas genus oxidise ammonia to nitrite under aerobic conditions. This stage is important because it eliminates ammonia, a toxic pollutant for aquatic environments.
  2. Oxidation of nitrite to nitrate: Bacteria of the Nitrobacter genus then oxidise nitrite to nitrate. This reaction also requires aerobic conditions and a sufficient quantity of dissolved oxygen.

Denitrification step

Denitrification is the second stage in the biological treatment of nitrogen. It consists of converting nitrates into nitrogen (N2), a harmless gas. This reaction is catalysed by anaerobic bacteria such as Paracoccus denitrificans.

In the absence of oxygen, microbes use nitrate from ammonium to release electrons.

In fact, nitrous oxide is harmless, making up around 80% of the air we breathe. Nitrification/denitrification processes naturally eliminate ammonium and nitrate. These compounds contribute to the pollution of natural waters, without harming our health or the planet.

Denitrification takes place in two steps:

  1. Reduction of nitrates to nitrite: Bacteria reduce nitrates to nitrite under anaerobic conditions. This stage is important because it eliminates nitrates, which can be toxic to aquatic organisms.
  2. Reduction of nitrite to nitrogen: Bacteria reduce nitrite to nitrogen. This reaction is also anaerobic and requires a total absence of oxygen in the water.

Importance of denitrification

Denitrification is essential to completely eliminate the nitrogen contained in wastewater. It recovers the alkalinity lost during nitrification and reduces nitrate concentrations in treated water. Denitrification is also important for preserving the quality of drinking water and the environment.

To ensure good denitrification, it is important to respect certain conditions:

  1. Absence of oxygen: Anaerobic bacteria require a total absence of oxygen to function.
  2. Presence of nitrates: Anaerobic bacteria need nitrates to feed.
  3. Presence of denitrifying bacteria: Anaerobic bacteria must be present in the medium to catalyse denitrification.
  4. Constant temperature: The temperature must be constant to allow bacteria to develop.
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