Le cycle de l'azote
Nitrogen removal is one of the essential steps in wastewater treatment. In fact, there are regulatory standards for nitrogen concentrations at the plant outlet in many countries. Therefore, to meet these concentrations, nitrification and denitrification must be optimally controlled.
Nitrogen in water can be found in mineral (ammonia, nitrate) or organic form. Its organic or ammoniacal presence results in a consumption of oxygen in the natural environment and alters the living conditions.
In sanitation, the nitrogen cycle follows the different stages of biogeochemical evolution of the component. It leads to the formation of nitrogen gas (nitrogen N2) starting with organic nitrogen, and passing by: ammonia, nitrite, nitrate.
In wastewater treatment plants, several forms of nitrogen are present:
- Nitrites and nitrates: oxidized nitrogen
- the non-oxidized states: Kjeldhal nitrogen including organic nitrogen and ammoniacal nitrogen (NH4+)
- Ammonifiable organic nitrogen
- Refractory organic nitrogen
The measurement of all forms represents the total nitrogen.
The wastewater is mainly composed of :
- ammonifiable or refractory organic nitrogen (in soluble and particulate form)
- ammoniacal nitrogen
Ammonifiable organic nitrogen
It is the most common form and ammoniacal nitrogen (NH4-H). Indeed, it is the one present in the urine.
The wastewater coming into a wastewater treatment plant contains a large part of organic nitrogen (albumin, carbamide, etc.). In addition, the nitrogen released from a home is originally slightly more concentrated in ammonifiable organic nitrogen.
Organic nitrogen is said to be ammonifiable when it can be transformed by enzymatic hydrolysis into ammoniacal nitrogen.
In other words, the ratio of ammonium to ammonia may depend on factors such as temperature and incubation time (depending on the length of the network) because this is where the transformation of organic nitrogen to NH4-H begins.
During the treatment in a wastewater treatment plant, ammonification continues until the majority of the nitrogen is transformed into NH4-N.
Soluble and particulate refractory nitrogen known as "hard nitrogen
Soluble refractory nitrogen is the non-biodegradable element in nitrogen. Moreover, this part is already detected when it enters the station.
On the other hand, the orders of magnitude of this nitrogen for a standard urban water are between 1.5 to 2.5 mg/liter. The hard or refractory particulate nitrogen has the same concentrations but it will be trapped in the mud.
In some cases, the order of hard soluble nitrogen may be more important. This is usually because of the sludge returns or a network with a long residence time.
Among the technologies that impact the hard organic nitrogen rate, there are the backflows from thermal drying.
This is the biological cycle of transformation of reduced nitrogen into the oxidized form nitrate (NO3-). And micro-organisms play a major role in this process.
Nitrification occurs in two stages:
- the transformation of ammonia into nitrite by oxidation
- then the evolution of nitrite (NO2-) into nitrate (NO3-). This is called nitritation and then nitration.
Micro-organisms responsible for nitrification are Nitrosomonas and Nitrobacter. They are very fragile and require a constant temperature (above or equal to 12°).
However, an adequate oxygen supply and a favorable C/P/N supply are also essential.
Maximum growth rate of nitrifying bacteria is significantly lower than that of heterotrophic bacteria (those feeding on carbonaceous substrate). In a treatment plant, the mass of nitrifying bacteria is directly related to the amount of substrate it receives and the temperature of the water. Nitrifying populations grow at a rate that is more or less adapted to the mass of nitrogen to be treated.
Biological nitrification of 1 kg of ammoniacal nitrogen :
- requires nominally 4.2 kg of oxygen, 80% of which is included in the nitrates formed,
- is associated with a reduction in alkalinity (compensated by 3.9 kg of quicklime CaO),
- produces 170 g of nitrifying bacteria which is very low compared to the production of sludge generated during the degradation of the carbonaceous organic load.
To summarize, to ensure proper nitrification without additional chemical additions, one must:
- A minimum temperature of 12°C
- Sufficient natural alkalinity in the raw water
- A lot of oxygen
- Sufficient nitrifying bacteria
- pH between 7.2 and 8.5
The purpose of biological denitrification is to completely remove nitrogen from wastewater. During this treatment process, the nitrogen evaporates into the atmosphere in its molecular form N2.
Denitrification is an anaerobic mechanism that enables a large number of heterotrophic bacteria to cover their energy needs from nitrates when dissolved oxygen is lacking.
To put it simply, since they lack O2, these bacteria must use the oxygen contained in the nitrates to breathe.
Indeed, they breathe nitrates. And to ensure a good denitrification, it is necessary to avoid having O2 in the water to be treated.
Bacteria involved in the denitrification cycle are also involved in carbon alteration. Since they eat a ” healthy ” diet, they need a source of carbon to complete their nitrogenous menu. In wastewater treatment plants, it is common to add methanol or vinegar to compensate for carbon deficiencies.
Another advantage of denitrification is the recovery of alkalinity (part of that lost in the nitrification stage). Indeed, denitrification ensures a restitution of alkalinity equal to half the consumption necessary for nitrification. i.e. 1 kg of denitrified nitrate nitrogen is equivalent to the addition of 1.95 kg of quicklime CaO.
In addition, it is interesting to implement a recirculation system in the water treatment process to reuse this alkalinity.
In conclusion, to ensure proper denitrification, one must:
- No oxygen
- Enough denitrifying bacteria
- A supplementary carbon source that can be easily assimilated (aim for a minimum BOD5 / NO3- pollution ratio greater than 2).
As explained earlier, bacteria have a healthy diet. The COD/NTK ratio is used to measure this balance.
However, a low COD/NTK ratio has a negative impact on the biotransformation of nitrogen into oxidized and gaseous nitrogen. In other words, this type of effluent cannot promote good denitrification.
By contrast, a high ratio will lead to a total assimilation of nitrogen but there will be a carbonaceous pollution residue.
For a municipal wastewater, the COD/NTK ratio is therefore between 7 and 20. That is to say, the lower the ratio, the more it will be necessary to add a complementary carbon source during the denitrification step.