top of page
  • Writer's pictureTim Coyne

Water Operator Certification Tips: Nitrification and Denitrification processes

In this series of Sterling Insights we will cover some of the most challenging questions that could appear on your Operator Certification Test and the easy ways to remember these answers.


Describe the nitrification and denitrification processes in the context of biological nutrient removal in wastewater treatment.


Nitrification and Denitrification are crucial biological processes used in the treatment of wastewater to remove nitrogen compounds.


· This is a two-step aerobic process carried out by nitrifying bacteria.

· First step: Ammonia (NH3) is oxidized to nitrite (NO2-) by bacteria such as Nitrosomonas.

· Second step: Nitrite is further oxidized to nitrate (NO3-) by bacteria like Nitrobacter.

· Requires oxygen, so aeration is essential in this phase.


· This is an anaerobic process where nitrate (NO3-) is reduced to nitrogen gas (N2), which escapes into the atmosphere.

· Carried out by denitrifying bacteria, such as Pseudomonas and Bacillus.

· Occurs in the absence of oxygen, often in anoxic tanks.

The two processes together form a complete nitrogen removal system, with nitrification converting ammonia to nitrate, and denitrification reducing nitrate to harmless nitrogen gas.

Easy Way to Remember:

Think of these two processes as a journey from harmful compounds to harmless gas:

Nitrification - Think of it as "Nitrogen in the air":

Two-step: Nitroso (ammonia to nitrite) and Nitro (nitrite to nitrate).

"In the air" reminds you that it's an aerobic process.

Denitrification - Think of it as "De-Nitrogen in the water":

"De" as in "removing" or "reducing" nitrate to nitrogen gas.

"In the water" reminds you that it's anaerobic or without oxygen.

This way, you can link the prefix "Nitr-" with nitrogen and remember that the process starts with nitrification (oxygen needed) and ends with denitrification (no oxygen needed), forming a complete pathway from harmful nitrogen compounds to harmless nitrogen gas.

Nitrification and Denitrification: A Comprehensive Guide

Now that you know the answer and the memorization tip, it’s time to delve deeper into the Nitrification and Denitrification process so that you fully grasp the concept.

Nitrification: A Two-Step Aerobic Transformation

1. Ammonia to Nitrite Conversion

The nitrification process starts with ammonia, a toxic compound often present in wastewater. In the first step, specific bacteria called Nitrosomonas oxidize ammonia (NH3) into nitrite (NO2-), a slightly less toxic form. This reaction requires oxygen and a specific pH range (usually between 7.0 to 8.0).


NH3 + 1.5O2 → NO2^- + H2O + 2H^+

2. Nitrite to Nitrate Conversion

The second stage involves the transformation of nitrite to nitrate (NO3-), a more stable compound. This step is performed by another group of bacteria, Nitrobacter, again in the presence of oxygen.


NO2^- + 0.5O2 → NO3^-

Importance of Nitrification

The nitrification process is vital for:

· Transforming toxic ammonia into a less harmful form.

· Preparing nitrogen compounds for further processing through denitrification.

· Complying with regulatory standards for nitrogen content in treated wastewater.

Denitrification: An Anaerobic Process

Nitrate to Nitrogen Gas Conversion

Denitrification is the reduction of nitrate into nitrogen gas (N2), the most prevalent gas in the atmosphere. This process is conducted by facultative anaerobic bacteria like Pseudomonas in the absence of oxygen.


NO3^- + organic matter → N2 + CO2 + H2O

Importance of Denitrification

Denitrification serves several crucial functions:

· Reducing nitrate to harmless nitrogen gas.

· Mitigating the risk of eutrophication in natural water bodies.

· Achieving compliance with stringent environmental regulations regarding nitrogen discharge.

Challenges and Considerations

Oxygen Control: Nitrification requires oxygen, while denitrification necessitates anoxic conditions. Balancing these opposing requirements is a critical aspect of effective treatment.

Temperature and pH Sensitivity: Both processes are sensitive to changes in temperature and pH, necessitating careful monitoring and control.

Toxicity: The intermediate compound nitrite is still toxic and must be effectively converted to nitrate.

If you are having challenges balancing the conditions in your system for the Nitrification or denitrification processes, reach out to a Sterling Specialist to help.

20 views0 comments


bottom of page