A nitrifying MBBR is an aerobic stage specifically designed to convert ammonia (NH₄-N) to nitrate (NO₃-N) via autotrophic nitrifying bacteria. It is typically placed downstream of a carbon-removal stage so that the biofilm is dominated by slow-growing nitrifiers rather than fast-growing heterotrophs. The low surface loading rate and high oxygen demand make it the most size-critical stage in many MBBR plants.
Why nitrification needs its own stage
Nitrifiers (mainly Nitrosomonas and Nitrospira) grow much more slowly than heterotrophic carbon oxidisers. When both are present together, the heterotrophs colonise the carrier surface first and form a thick, oxygen-consuming layer that starves the nitrifiers underneath. Pre-removing the bulk of the organic carbon upstream ensures the nitrification stage receives a low-BOD feed, allowing a thin, well-oxygenated nitrifying biofilm to develop.
The allowable surface loading rate for nitrification is much lower than for carbon removal, which means the nitrification stage typically requires more biofilm area — and therefore more reactor volume — than the upstream carbon stage. Adequate dissolved oxygen is critical; nitrification consumes roughly 4.6 g O₂ per g NH₄-N oxidised (stoichiometric), and the aeration system must keep DO above the minimum the nitrifiers need to function.
Design considerations
Nitrifying MBBR design (indicative — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Biofilm type | Thin, autotrophic | — | Low organic load keeps heterotrophs from dominating. |
| Surface loading | Lower than carbon stage | g NH₄-N/m²·d | Sets the required biofilm area and reactor size. |
| Oxygen demand | ~4.6 g O₂ / g NH₄-N | stoichiometric | Actual demand higher with safety margin. |
| Temperature sensitivity | Significant | — | Rate drops at low temperature; size accordingly. |
Frequently Asked Questions
Why is a nitrifying MBBR stage usually the largest reactor?
What happens if too much BOD enters the nitrification stage?
Does temperature affect nitrifying MBBR performance?
Related Pages
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