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Denitrifying MBBR

A dedicated biofilm stage that converts nitrate to nitrogen gas — completing the nitrogen-removal pathway.

A denitrifying MBBR is an anoxic stage whose sole purpose is to convert nitrate (or nitrite) to harmless nitrogen gas (N₂). It works with a preceding nitrification stage: the nitrifier converts ammonia to nitrate, and the denitrifier converts that nitrate to gas. Together they achieve biological nitrogen removal without chemicals.

Pre- vs post-denitrification

In a pre-denitrification layout the anoxic reactor is placed before the aerobic nitrification reactor and receives raw wastewater rich in organic carbon. Nitrified effluent is recirculated back to the anoxic stage to supply nitrate. The advantage is that the influent carbon serves as electron donor, avoiding external dosing. The trade-off is that denitrification efficiency depends on the recirculation ratio.

In a post-denitrification layout the anoxic stage follows the nitrification stage. The nitrified effluent already carries the nitrate, so no recirculation is needed, but the carbon in the wastewater has been consumed upstream. An external carbon source (e.g. methanol, acetate, glycerol) must therefore be dosed, adding operational cost. Post-denitrification can achieve very low total nitrogen because 100% of the nitrified nitrate passes through the denitrification stage.

Design factors

Denitrifying MBBR design (indicative — confirm per project)

ParameterTypical ValueUnitNotes
Pre-DN carbonInfluent BOD—No external dosing; limited by recirculation ratio.
Post-DN carbonExternal dosing—Methanol/acetate; adds cost, achieves very low TN.
MixingMechanical—Must maintain anoxic conditions (no air).
AlkalinityPartial recovery—Denitrification recovers ~50% of alkalinity consumed by nitrification.

Frequently Asked Questions

What is the difference between anoxic MBBR and denitrifying MBBR?
They describe the same fundamental process — nitrate reduction under anoxic conditions. "Anoxic MBBR" emphasises the reactor environment; "denitrifying MBBR" emphasises the treatment objective. In practice the terms are often used interchangeably.
Which layout achieves lower total nitrogen?
Post-denitrification can achieve very low TN because all of the nitrified nitrate passes through the denitrification stage. Pre-denitrification is limited by the recirculation ratio and typically achieves moderate TN removal without external carbon.
Does denitrification recover alkalinity?
Yes. Denitrification produces approximately half the alkalinity consumed by nitrification. In a combined nitrification–denitrification plant this partial recovery reduces or eliminates the need for external alkalinity dosing.

Related Pages

Anoxic MBBR

The anoxic reactor environment.

Nitrifying MBBR

The upstream nitrification stage.

Denitrification

Denitrification applications.

Biological Nitrogen Removal

Combined nitrification–denitrification.

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