Tertiary nitrification is a polishing stage that converts ammonia to nitrate after most of the carbon has already been removed upstream. It is added when a plant faces a new or tightened ammonia consent that its existing biology cannot meet. A tertiary MBBR is well suited to this duty: with low BOD in the feed, slow-growing nitrifiers dominate the biofilm and achieve reliable ammonia removal in a compact reactor.
Why Low Upstream BOD Helps
Nitrifying bacteria grow slowly and are easily out-competed by faster-growing heterotrophs when biodegradable carbon is abundant. Because a tertiary stage receives an already-treated, low-BOD effluent, the biofilm is not dominated by heterotrophs, so a larger fraction of the carrier surface supports nitrifiers. This makes tertiary nitrification rates higher and more stable than combined carbon-and-nitrogen duty at the same location.
The stage is typically added as a new reactor downstream of the existing works, making it a clean way to add nitrification without disturbing the upstream process.
Tertiary Nitrification Design Context (indicative literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| SALR — nitrification | 0.5 – 2 | g NH₄-N/m²·d | Temperature-dependent |
| Design DO | 3 – 5 | mg/L | Nitrifiers are DO-sensitive |
| Upstream BOD | low | — | Favours nitrifiers over heterotrophs |
| Alkalinity demand | ~7.1 | g CaCO₃/g NH₄-N | Confirm buffer availability |
Design Considerations
Nitrification consumes alkalinity and is sensitive to temperature and dissolved oxygen. A tertiary MBBR must therefore be designed with temperature-corrected rates, adequate DO, and confirmation that enough alkalinity is present or dosed. Because rates fall in cold weather, the reactor is sized for the critical low-temperature condition.
Frequently Asked Questions
What is tertiary nitrification?
Why is MBBR effective as a tertiary nitrification stage?
What limits tertiary nitrification performance?
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