Denitrification is carried out by heterotrophic bacteria that, in the absence of dissolved oxygen, use nitrate as their terminal electron acceptor while oxidising organic carbon. The nitrate is reduced stepwise to nitrogen gas, which leaves the water to the atmosphere — achieving genuine nitrogen removal. In MBBR this takes place in anoxic reactors whose carriers host the denitrifying biofilm.
The Carbon Source Requirement
Denitrification needs a source of biodegradable carbon as the electron donor. Where the anoxic stage is placed first (pre-denitrification), the carbon in the incoming wastewater serves this purpose, and nitrate produced downstream is recycled back to it. Where the anoxic stage follows nitrification (post-denitrification), most of the carbon has already been consumed, so an external carbon source is usually dosed. The carbon-to-nitrogen ratio of the wastewater is therefore central to choosing the configuration.
Denitrification also recovers oxygen bound in nitrate and restores part of the alkalinity consumed during nitrification, which can ease the oxygen and buffering demands of the overall process.
Denitrification Design Context (indicative literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Condition | anoxic | — | No dissolved oxygen |
| SALR | 1 – 4 | g NO₃-N/m²·d | Carbon-source dependent |
| Oxygen credit | ~2.86 | g O₂/g NO₃-N | Nitrate oxygen recovered |
| Carbon source | influent or dosed | — | Pre- vs post-denitrification |
Mixing Without Aeration
An anoxic MBBR must keep the carriers moving without introducing oxygen, so mechanical mixers are used instead of aeration. Maintaining truly anoxic conditions — excluding oxygen while keeping the media fluidised — is a defining design consideration for denitrification reactors, distinct from the aeration-driven mixing of aerobic stages.
Frequently Asked Questions
What is denitrification?
Why does denitrification need a carbon source?
How are carriers mixed in an anoxic MBBR?
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