Short answer: Aerobic MBBR systems are inherently low-odour because the biological process operates under aerated, oxidising conditions that suppress hydrogen sulphide and other malodorous compounds. Odour risk increases with anaerobic conditions, high-strength industrial wastewater, or inadequate aeration.
Why aerobic MBBR is low-odour
In an aerated MBBR reactor, continuous air supply keeps dissolved oxygen above zero, preventing the formation of H₂S and volatile organic acids that cause strong odours. The turbulent mixing also minimises dead zones where anaerobic conditions could develop.
When odour can occur
- Inlet works and screening: Raw wastewater handling (before the MBBR) is the most common odour source at any treatment plant.
- Overloaded reactor: If the organic load exceeds the aeration capacity, DO drops to zero and anaerobic zones form — producing H₂S and mercaptans.
- Anoxic MBBR zones: Denitrification stages operate without dissolved oxygen. While anoxic (not anaerobic), high-sulphate wastewater can generate some H₂S under these conditions.
- High-strength industrial wastewater: Certain industries (food processing, rendering, tanning) generate influent with high volatile organic compound concentrations that can be stripped by aeration.
- Sludge handling: Settled sludge from the clarifier can produce odour if stored too long before dewatering or disposal.
Odour mitigation strategies
Odour control measures (indicative)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Adequate aeration | Maintain DO above 1.5–2 mg/L | — | Prevents anaerobic conditions in aerobic zones. |
| Covering tanks | Where odour is sensitive | — | With air extraction to a biofilter or carbon adsorber. |
| Chemical oxidation | At inlet works | — | Iron salts or peroxide to control H₂S in the sewer. |
| Prompt sludge removal | From clarifier | — | Prevent anaerobic sludge decomposition. |
Frequently Asked Questions
Is MBBR suitable for residential areas?
Yes. With proper design (covered tanks, odour extraction where needed, adequate aeration), MBBR plants operate in urban and peri-urban locations without nuisance. Many small community MBBR plants are located near residential areas.
Does aeration strip volatile compounds from the wastewater?
Vigorous aeration can strip some VOCs from the liquid phase, which may cause localised odour above the reactor. For industrial wastewaters with high VOC content, covering the reactor with air extraction to a treatment unit (biofilter, scrubber or carbon adsorber) is recommended.
How does MBBR compare to trickling filters for odour?
MBBR is generally better because it operates in a closed or semi-closed tank, while trickling filters are open structures that ventilate naturally, dispersing odour more easily. Covering a trickling filter is more difficult and expensive than covering an MBBR tank.
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