Nitrification is the biological oxidation of ammonia to nitrate in two steps: ammonia-oxidising bacteria convert ammonia to nitrite, then nitrite-oxidising bacteria convert nitrite to nitrate. These autotrophs grow slowly and are sensitive to temperature, dissolved oxygen, pH and inhibitory compounds. MBBR is particularly effective for nitrification because the slow growers are held in the biofilm and cannot be washed out, giving stable performance under variable flow.
The Nitrifying Organisms and Their Needs
Because nitrifiers grow far more slowly than heterotrophs, they only establish where the organic load is low enough that they are not out-competed for oxygen and space. Nitrification therefore requires the carbon load to be reduced first, and a relatively high dissolved-oxygen concentration to drive oxygen into the biofilm where the nitrifiers live. It also consumes alkalinity, so sufficient buffering must be present or the pH will fall and inhibit the reaction.
Nitrification rate is strongly temperature-dependent, dropping markedly in cold water. The minimum design temperature therefore usually governs the required carrier area, making the cold-weather case the critical design condition.
Nitrification Design Sensitivities (indicative literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| SALR — warm | 0.7 – 1.0 | g NH₄-N/m²·d | Adequate DO required |
| SALR — cold | 0.4 – 0.7 | g NH₄-N/m²·d | Rate falls with temperature |
| Design DO | 4 – 6 | mg/L | Diffusion-limited process |
| Alkalinity consumed | ~7.1 | g CaCO₃/g NH₄-N | Buffering must be adequate |
Why MBBR Suits Nitrification
In suspended-growth systems, maintaining nitrification requires a long enough sludge age to prevent nitrifier washout — difficult under variable flow or at low temperature. MBBR removes this constraint: nitrifiers grow attached to the carriers and are retained independently of hydraulic conditions. This makes nitrifying MBBRs robust and compact, and is a key reason they are used to add or secure nitrification capacity in upgrades.
Frequently Asked Questions
What is nitrification?
Why does nitrification need low organic load and high DO?
Why is the cold-weather case critical for nitrification?
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