Water Programs
Nitrification and denitrification are processes used to remove nitrogen from wastewater to meet discharge requirements. Both are part of the natural nitrogen cycle. Both processes are carried out by bacteria. So, how are they different?
First, let's talk briefly about why nitrogen is removed from wastewater. Nitrogen stimulates eutrophication, an increase in the nutrient levels of a body of water that usually enhances the growth of aquatic animal and plant life. However, eutrophication can cause harmful algal blooms, dead zones, and fish kills. Also, nitrogen in the form of ammonia can exert a direct demand on dissolved oxygen (DO) and be toxic to aquatic life. Nitrogen in the form of nitrate can cause serious health effects in humans and animals. For these reasons, nitrogen limits are part of many discharge permits. Nitrogen occurs in wastewater as ammonia (NH3), ammonium ion (NH4+), nitrite (NO2), nitrate (NO3–), and organic nitrogen. Most wastewater discharge limits are in terms of ammonia as nitrogen (NH3–N), nitrite as nitrogen (NO2–N), and nitrate as nitrogen (NO3––N). Usually, the amounts of all these forms are reported as nitrogen.
This is a lot of terms for operators to keep straight. The best way to understand and remember them is to see how the nitrification and denitrification processes work. This involves some basic chemistry.
Nitrification is a 2-step aerobic process. It is carried out by 2 different bacterial species. First, ammonia (NH3) is converted to nitrite (No2–) by ammonia-oxidizing bacteria (AOB). Then nitrite (No2–) is converted to nitrate (NO3–) by nitrite-oxidizing bacteria (NOB). Both of these reactions require oxygen (O2). A simplified chemical reaction for this step looks like:
NH3 + H+ + 1.5O2 → NO2– + H2O + 2H+
ammonia + hydrogen + oxygen → nitrite + water + hydrogen
The second reaction converts nitrite to nitrate. Again, oxygen is required for this reaction. A simplified version of this step looks like:
NO2– + 0.502 → NO3–
nitrite + oxygen → nitrate
Some wastewater treatment plants are allowed to discharge nitrate. Others are not because nitrate can stimulate algae and phytoplankton growth in receiving waters and interfere with blood oxygen levels in infants who consume the water. Check your discharge permit or contact your regulatory agency to find out if your plant is allowed to discharge nitrate.
In denitrification, a variety of naturally occurring bacteria convert nitrate (NO3–) to nitrite (NO2–) to gaseous forms of nitrogen in the absence of oxygen (anaerobic). The gaseous nitrogen—comprised of nitrogen gas (N2), nitric oxide (NO), and nitrous oxide (N2O)—is then released to the atmosphere. It is important that there is no oxygen (or at least very low levels) and that there is a carbon food source (such as organic matter or methanol) to encourage the microorganisms to obtain the oxygen they need by breaking down the nitrate molecule. The steps of reducing nitrate to nitrogen gas are:
NO3– → NO2– → NO + N2O → N2
nitrate → nitrite → nitric oxide + nitrous oxide → nitrogen gas
So, together, the nitrification and denitrification processes convert nitrogen in an objectionable form (as a nutrient, nitrate, or nitrite) to one that has no significant impact on environmental quality. Nitrogen is the dominant gas in the atmosphere and is 78 percent of the air we breathe. Nitrogen gas is stable in the atmosphere and isn't involved in many chemical reactions.
The end results and oxygen requirements (or lack thereof) for nitrification and denitrification are the factors that can help you remember which is which: Nitrification produces nitrate (NO3–) and requires oxygen (aerobic), while denitrification produces gaseous nitrogen (N2, NO, and N2O) and requires little to no oxygen (anaerobic).
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