Nitrification and denitrification
- ammonia
- nitrites
- nitrates
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:
- 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.
- 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 causing harm to health or the planet.
Denitrification takes place in two steps:
- 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.
- Reduction of nitrite to nitrogen: Bacteria reduce nitrite to nitrogen. This reaction is also anaerobic and requires a complete absence of oxygen in the water.
Importance of denitrification
Denitrification is essential to completely remove nitrogen from wastewater. It recovers the alkalinity lost during nitrification and reduces nitrate concentrations in the treated water. Denitrification is also important to preserve the quality of drinking water and the environment.
To ensure good denitrification, it is important to respect certain conditions:
- Absence of oxygen: Anaerobic bacteria require a total absence of oxygen to function.
- Presence of nitrates: Anaerobic bacteria need nitrates to feed.
- Presence of denitrifying bacteria: Anaerobic bacteria must be present in the medium to catalyse denitrification.
- Constant temperature: The temperature must be constant to allow bacteria to develop.
FAQ
What are the effects of excessive nitrification on the environment?
Excessive nitrification can lead to nitrate build-up in aquatic environments, promoting eutrophication. This phenomenon causes a proliferation of algae that consume dissolved oxygen, thus threatening aquatic biodiversity and creating “dead zones” where life is absent.
How do MBR systems improve nitrification and denitrification?
Membrane bioreactors (MBRs) combine biological processes with advanced membrane filtration. This integration allows for precise control of environmental conditions, thus optimizing the activity of nitrifying and denitrifying bacteria. MBRs offer a compact and efficient solution for nitrogen removal, ensuring superior effluent quality.
What factors influence the performance of nitrification and denitrification?
Several operational factors affect these processes:
pH : An optimal pH around 7.5 is crucial. Values that are too low can inhibit nitrification.
Temperature : The optimal range is between 28 and 32 °C. Temperatures outside this range reduce microbial activity.
Dissolved oxygen : An adequate concentration is required for nitrification, while denitrification requires anoxic conditions.
Presence of chemical contaminants : Some compounds can inhibit or kill the microorganisms responsible for these processes.
What microorganisms are involved in nitrification and denitrification?
Nitrification is mainly carried out by autotrophic bacteria such as Nitrosomonas (oxidizing ammonia to nitrite) and Nitrobacter (oxidizing nitrite to nitrate). Denitrification is carried out by heterotrophic bacteria such as Pseudomonas and Thiobacillus, which reduce nitrates to nitrogen gas in the absence of oxygen.
What are the risks of nitrification in drinking water networks using chloramines?
In drinking water distribution systems disinfected with chloramines, nitrification can occur when bacteria convert ammonia into nitrites and then nitrates. This can lead to loss of disinfectant residue, biofilm formation, and taste and odor issues. Prevention strategies include optimizing the chloramination process, reducing water age, preventive maintenance and implementing a Nitrification Action Plan (NAP).
