Engineering
Denitrification
Biological conversion of nitrate to nitrogen gas under anoxic conditions. Essential for total nitrogen removal.
Process Description
Denitrification is the biological reduction of nitrate (NO₃⁻) and nitrite (NO₂⁻) to nitrogen gas (N₂) by heterotrophic bacteria under anoxic conditions — where dissolved oxygen is absent but nitrate serves as the terminal electron acceptor.
The process requires a carbon source (electron donor). In wastewater treatment, this can be provided by the incoming wastewater itself (endogenous carbon) or by an external carbon source such as methanol, ethanol, or glycerol.
Key Parameters
DO in Anoxic Zone
Must be near zero
Carbon Requirement
For denitrification
TN Removal
Typical pre-anoxic config
Alkalinity Recovery
Per mg NO₃-N denitrified
Typical Configurations
Pre-Anoxic (Ludzack–Ettinger)
Anoxic zone precedes aerobic zone. Internal mixed liquor recirculation returns nitrified flow to the anoxic zone. Uses influent wastewater as carbon source. Most common configuration for municipal plants.
Post-Anoxic
Anoxic zone follows aerobic zone. Uses endogenous respiration or external carbon. Can achieve lower effluent TN but requires additional carbon dosing. Often combined with pre-anoxic in 4-stage BNR.
Simultaneous Nitrification/Denitrification
Occurs within the same reactor at low DO levels (0.3–0.8 mg/L). Anoxic micro-zones within activated sludge flocs support denitrification while the bulk liquid supports nitrification.
Step-Feed
Influent wastewater is distributed to multiple anoxic zones along the treatment train. Maximizes use of influent carbon for denitrification. Good retrofit option for existing long aeration tanks.
Engineering Note
Frequently Asked Questions
Denitrification is the biological reduction of nitrate to nitrogen gas by facultative bacteria under anoxic conditions (no dissolved oxygen but nitrate present). It is the process that removes total nitrogen from wastewater after nitrification has converted ammonia to nitrate.
Denitrifying bacteria need biodegradable carbon as an electron donor to reduce nitrate. In pre-denitrification, incoming wastewater supplies this carbon; in post-denitrification, an external carbon source (such as methanol or acetate) may be dosed when the influent carbon is insufficient.
Pre-denitrification places the anoxic zone before the aerobic zone and uses influent carbon, with nitrate returned via internal recirculation. Post-denitrification places the anoxic zone after nitrification and usually requires an external carbon source but can achieve very low effluent nitrate.
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