Alkalinity is the water's acid-neutralising capacity — its ability to buffer pH changes. In any MBBR system that performs nitrification, alkalinity management is critical because the oxidation of ammonia to nitrate consumes alkalinity at a stoichiometric rate of approximately 7.1 mg CaCO₃ per mg NH₄-N removed. If the influent does not supply enough alkalinity, pH falls, nitrification slows, and a downward spiral can develop.
Alkalinity stoichiometry
The nitrification reaction consumes approximately 7.1 mg CaCO₃ of alkalinity for every 1 mg of NH₄-N oxidised. In practice, a residual alkalinity of at least 50–100 mg/L as CaCO₃ should remain in the effluent to keep pH in the nitrification-safe zone (above ~6.8). A simple alkalinity balance — influent alkalinity minus nitrification demand minus any other acid loads — shows whether supplementation is needed.
Denitrification recovers roughly 3.6 mg CaCO₃ per mg NO₃-N reduced. This means a combined nitrification-denitrification process needs only about half the alkalinity supplementation of a nitrification-only system. Maximising denitrification is therefore also an alkalinity (and cost) management strategy.
Key alkalinity values
Indicative alkalinity data (confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Alkalinity consumed per mg NH₄-N | ~7.1 | mg CaCO₃ | Stoichiometric nitrification demand. |
| Alkalinity recovered per mg NO₃-N | ~3.6 | mg CaCO₃ | Denitrification returns ~50 %. |
| Minimum residual alkalinity | 50–100 | mg/L CaCO₃ | To keep pH >6.8 in the reactor. |
| Typical municipal influent alkalinity | 150–300 | mg/L CaCO₃ | Varies widely by region. |
Supplementation options
When the alkalinity balance shows a deficit, operators can add sodium bicarbonate (safe and easy to handle, provides gentle buffering), sodium hydroxide (fast pH correction, risk of overshoot), or lime (cheapest chemical but introduces calcium that can form scale on diffusers and carriers). The dosing point should be upstream of the nitrifying stage, with feedback from a continuous pH sensor.
In industrial applications where the wastewater itself is highly acidic (e.g. dairy acid whey, citrus processing), the alkalinity demand can be very large. Equalisation and pre-neutralisation before the MBBR is often more economical than trying to buffer inside the reactor.
Frequently Asked Questions
How do I know if my MBBR needs alkalinity supplementation?
Is sodium bicarbonate or sodium hydroxide better?
Does denitrification really save alkalinity cost?
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