Total nitrogen (TN) removal requires both nitrification (aerobic ammonia oxidation) and denitrification (anoxic nitrate reduction). MBBR achieves this through staged reactor configurations — aerobic and anoxic zones in series, or through simultaneous nitrification-denitrification (SND) within thick biofilms. The approach is chosen based on the TN target, available carbon and plant layout.
Project drivers
- Tightened total-nitrogen discharge limits (often 10–15 mg/L TN or lower).
- Receiving waters sensitive to eutrophication requiring nutrient load reduction.
- Existing nitrification-only plant needing denitrification capacity added.
- Industrial pre-treatment with nitrogen-rich wastewater (e.g. food processing, fertiliser production).
Configuration options
MBBR nitrogen removal configurations (indicative)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Pre-denitrification + nitrification | Anoxic → aerobic MBBR | — | Uses influent carbon; most common. |
| Post-denitrification | Aerobic → anoxic MBBR | — | External carbon needed; lower TN achievable. |
| Four-stage (Bardenpho-type) | Anoxic → aerobic → anoxic → aerobic | — | For very low TN targets. |
| SND in thick biofilm | Single aerobic stage | — | Anoxic inner biofilm denitrifies; limited TN reduction. |
| Achievable effluent TN | 5–15 | mg/L | Depends on configuration and carbon availability. |
Key design considerations
- Carbon source: Denitrification needs organic carbon. Pre-denitrification uses influent BOD; post-denitrification may need external carbon (methanol, ethanol, acetate).
- DO control: The anoxic zone must be truly anoxic (DO below 0.2–0.5 mg/L) for denitrification. DO intrusion from the aerobic zone or from carrier transfer must be minimised.
- Internal recycle: Pre-denitrification requires nitrate-rich mixed liquor to be recycled from the aerobic to the anoxic zone.
- Alkalinity recovery: Denitrification recovers approximately half the alkalinity consumed by nitrification, reducing the need for chemical alkalinity supplementation.
Frequently Asked Questions
Can MBBR achieve TN below 10 mg/L?
Yes, with a well-designed multi-stage configuration and adequate carbon supply. Achieving below 5 mg/L TN typically requires post-denitrification with external carbon and careful DO management.
Is external carbon always needed?
Not always. Pre-denitrification uses the organic carbon in the influent wastewater. External carbon is only needed when the influent COD:TN ratio is too low (typically below 4–5) or when very low TN targets require post-denitrification.
Does nitrogen removal add significant cost?
It adds an anoxic reactor (or zone), possibly internal recirculation pumps, and potentially external carbon. These costs are moderate compared to the total plant cost, especially when the anoxic stage is integrated into existing infrastructure.
Related Pages
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