Nitrification and Denitrification in Wastewater Treatment
What is Nitrification?
Nitrification is the first step of the nitrogen removal process. It involves converting ammonium (NH₄⁺)—the most common form of nitrogen in raw wastewater—into nitrite (NO₂⁻), and then into nitrate (NO₃⁻).
This is an aerobic process, meaning it requires oxygen, and it occurs in two distinct biological stages performed by two groups of autotrophic bacteria.
Stage 1: Nitrosomonas bacteria convert ammonium (NH₄⁺) to nitrite (NO₂⁻).
Stage 2: Nitrobacter bacteria convert nitrite (NO₂⁻) to nitrate (NO₃⁻).
- Nitrosomonas: Ammonia-oxidizing bacteria (AOB). Reproduce by binary fission.
- Nitrobacter: Nitrite-oxidizing bacteria (NOB). Reproduce by budding.
Both Nitrosomonas and Nitrobacter are nitrifying autotrophs. They gain energy by oxidizing inorganic nitrogen compounds and use carbon dioxide (CO₂) as a carbon source for growth.
What is Denitrification?
Denitrification is the final step in the nitrogen removal process. It is an anoxic process (no free dissolved oxygen) where bacteria convert nitrate (NO₃⁻) into nitrogen gas (N₂), which is then released into the atmosphere.
In anoxic conditions, bacteria use the oxygen bound in nitrate (NO₃⁻) instead of dissolved oxygen. The result: nitrogen gas (N₂) bubbles out of the water.
Denitrifying Bacteria
Nitrate-reducing bacteria (NRB) are heterotrophic—they require organic carbon (such as CBOD from wastewater) to grow and carry out denitrification.
- Pseudomonas
- Paracoccus
- Bacillus
Carbon Source for Denitrification
Although denitrifying bacteria use oxygen from nitrate (NO₃⁻), they still need a carbon source—typically from CBOD (carbonaceous biochemical oxygen demand)—as their food.
Without a sufficient carbon source, the denitrification process will stall—even if nitrate is present.