What Is Denitrification In Water Treatment

21st May, 2024.

In this post, we will see the concept of denitrification in wastewater treatment.

In sewage water treatment, there are many hazardous chemicals or gases which need to be removed before discharging it in clean water bodies or using it for agricultural purposes. One of them is nitrogen. Nitrogen is the most abundant element of our atmosphere. But, if it is found in abundance in water, then it will damage marine life as well as cause issues to human bodies. So, it is important to remove nitrogen from wastewater before discharging it finally. The process used to do this task is called denitrification. In this post, we will see the process of denitrification in water treatment.

Nitrogen and water quality:

There is no doubt that nitrogen is required for growth of plants and other animal nourishments. 78% of our earth’s atmosphere is made of nitrogen. But, when it comes to water, nitrogen and water quality go hand in hand. The nitrogen amount must be very less in the water for use. First of all, let us see where this nitrogen comes in water from. The very correct answer to this is our urine. Urea has large quantities of nitrogen chemical in it, which becomes ammonia when dissolved in water. Other sources of nitrogen are waste coming from agricultural fertilizers, animal manure, or combustion of fossil fuels. Nitrogen in high levels causes growth of algae in water bodies which causes clogging, damage to marine life and even killing of fishes (because they will not get adequate oxygen to breathe then), and harm to babies and pregnant women by drinking such water. That is why, it is necessary to keep nitrogen levels under control for a pure water quality.

What are nitrification and denitrification in wastewater treatment?

Now that we have seen the role of nitrogen in water quality, let us see two important terms related to it for further study - nitrification and denitrification. As we had seen earlier, nitrogen is converted to ammonia automatically when dissolved in water, due to chemical chain reaction. The process of converting ammonia to nitrate is called nitrification. This is performed with the help of two bacteria types in such water - the first one will convert ammonia to nitrite (NO2) and the second one will convert nitrite to nitrate (NO3). Now, we have to release the nitrate out of the water. But, it cannot be done directly. It needs to be converted in some gaseous form, probably nitrogen gas to enable itself to be thrown out in the atmosphere. This is called denitrification. Both these processes work in tandem to finally convert ammonia into nitrogen gas for easy discharge. Why these both terms are important will be known later in the post, because they are co-related to each other and will help us study denitrification better. 

Denitrification reaction:

Chemicals cannot be dosed in water every time for removal of waste, as it is costly and can also degrade water quality many times. So, there needs to be some natural processes inside the water for treating them. As ammonia cannot be removed naturally through water directly, we saw how it is converted into nitrate and then nitrogen gas for elimination. Ammonia gas chemical reaction is such that it cannot be easily digested by the bacteria inside for decomposition, which is why we are converting it back to nitrogen for disposal. Denitrification reaction, as discussed, is mainly possible through a special type of bacteria inside called heterotrophic bacteria. They break the nitrate molecules into oxygen for their consumption, and decompose it through nitrogen gas, which is released in the outside air (all this happens inside them when they consume nitrate). These bacteria are also called denitrifiers. 

The process happens inside anoxic tanks. It must be known that water treatment is divided into various chambers and zones for keeping the processes isolated and giving the bacteria the required time to grow and decompose. Anoxic refers to an environment where oxygen is depleted. It is usually present in the primary stage, as wastewater will be at higher concentration of waste and less concentration of oxygen there. As the bacteria in our case is using natural oxygen present in the nitrate, it can survive in such an environment without additional oxygen. Some cases require oxygen, depending on the design and waste levels, to keep the microorganisms alive for breathing. This is done through aeration process, where air blowers will blow air in strict quantity to maintain DO (dissolved oxygen) and BOD (biological oxygen demand) levels. The oxygen will be neither high nor low. 

Why is methanol used for denitrification?

Let us come to a very important term in denitrification - use of methanol chemical. The anoxic zone where denitrification happens is often dosed with methanol. Due to the chemical reactions occurring inside this whole process, one common compound which is most used is carbon. Carbon will be required further for aiding the bacteria growth and accelerating the denitrification process. This is done by adding an external carbon source, in the form of methanol chemical. Methanol is used for denitrification because of it’s molecule structure and chemical properties. It accelerates the activity of heterotrophic bacteria by quick digestion and also keeps their population in control by preventing them from depletion. 

I have covered some general principles related to the denitrification process. I have also not attempted to cover all the methods, as it can vary from case to case and there is no end to it. Once you are familiar with this technique, you can easily tackle all types of problems in it.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.