Total nitrogen removal converts nitrogen into inert nitrogen gas that leaves to the atmosphere. It requires two complementary biological processes: aerobic nitrification (ammonia → nitrate) and anoxic denitrification (nitrate → nitrogen gas). MBBR achieves this by staging aerobic and anoxic reactors, each with its own carriers and conditions. This page gives the total-nitrogen overview; nitrification and denitrification are detailed on their own pages.
Two Steps, Opposite Conditions
Nitrification needs oxygen; denitrification needs the absence of oxygen plus a carbon source. Because the two require opposite conditions, they are carried out in separate zones or reactors. In pre-denitrification the anoxic stage comes first and uses the influent's own carbon, with nitrate recycled from the downstream aerobic stage. In post-denitrification the anoxic stage follows nitrification and usually needs an added carbon source, but can reach lower effluent nitrate.
The achievable total-nitrogen removal depends on how completely each step proceeds and, in pre-denitrification, on the internal recycle ratio, which sets how much nitrate is delivered to the anoxic stage.
Nitrogen Removal Configurations (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Nitrification step | aerobic | — | Ammonia → nitrate |
| Denitrification step | anoxic | — | Nitrate → nitrogen gas |
| Pre-denitrification carbon | influent carbon | — | Nitrate recycle needed |
| Post-denitrification carbon | added source | — | Lower effluent nitrate |
Designing for a Total Nitrogen Limit
When a consent limits total nitrogen, the design must size both the nitrification and denitrification stages and select the configuration (pre-, post- or combined) that meets the limit at least cost. The carbon balance is central: pre-denitrification conserves influent carbon and avoids chemical addition, while post-denitrification buys lower effluent nitrogen at the cost of a supplemental carbon source. The choice is made against the specific effluent target and the wastewater's carbon-to-nitrogen ratio.
Frequently Asked Questions
Why does total nitrogen removal need two steps?
What is the difference between pre- and post-denitrification?
What limits how much nitrogen can be removed?
Related Pages
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