In an anoxic MBBR the reactor is kept free of dissolved oxygen so that denitrifying bacteria on the carriers use nitrate (or nitrite) as the electron acceptor instead, converting it to harmless nitrogen gas. This is the biofilm equivalent of the anoxic zone in a conventional denitrification process, but without suspended sludge and without sludge return.
Process principles
Denitrification requires an organic carbon source as electron donor and nitrate (or nitrite) as electron acceptor, in the absence of dissolved oxygen — because oxygen is preferred and suppresses nitrate use. In wastewater the carbon can come from the influent BOD (pre-denitrification layout, where the anoxic stage is before the aerobic stage) or from an external source such as methanol or acetate (post-denitrification).
Because there is no aeration, mechanical mixers keep the carriers circulating. The mixer must be powerful enough to fluidise the media without entraining air from the surface, which would raise the dissolved oxygen and inhibit denitrification.
Key design factors
Anoxic MBBR design factors (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Electron acceptor | Nitrate / nitrite | — | Dissolved oxygen must be absent. |
| Carbon source | Influent BOD or external | — | Pre-DN uses influent; post-DN needs dosing. |
| Mixing | Mechanical | — | Must avoid surface air entrainment. |
| Layout | Pre- or post-anoxic | — | Position determines carbon availability. |
Frequently Asked Questions
Why must the anoxic reactor be free of dissolved oxygen?
Does anoxic MBBR need a carbon source?
How are carriers mixed without aeration?
Related Pages
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