Organic loading rate can be expressed volumetrically (kg BOD or COD per m³ of reactor per day) or, more usefully for MBBR, as a surface area loading rate (g BOD/m²·d) referenced to the carrier area. The loading level chosen determines whether the reactor performs high-rate roughing, full carbon removal, or combined carbon removal and nitrification.
Loading Regime Determines Function
At high organic loading, heterotrophic bacteria dominate the biofilm and the reactor acts as a roughing or high-rate carbon-removal stage. As loading decreases, more of the carrier area becomes available for the slow-growing nitrifiers, allowing nitrification to establish. This is why combined carbon-and-nitrogen plants stage a higher-loaded carbon stage ahead of a lower-loaded nitrification stage.
High heterotrophic activity outcompetes nitrifiers for oxygen and space, so nitrification only proceeds reliably once the organic load reaching that stage is low enough. The organic loading rate is therefore the key lever for allocating carrier area between competing objectives.
Indicative Organic Loading Regimes (literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| High-rate / roughing | > 15 | g BOD/m²·d | Partial carbon removal |
| Normal carbon removal | 5 – 15 | g BOD/m²·d | High BOD removal efficiency |
| Carbon removal + nitrification | < 5 | g BOD/m²·d | Frees area for nitrifiers |
| Volumetric equivalent | project-specific | kg BOD/m³·d | Depends on fill ratio & carrier |
Volumetric vs. Surface Area Loading
Volumetric loading is convenient for comparing overall reactor duty, but it depends on fill ratio and carrier type and so is not directly transferable between designs. Surface area loading normalises to the biofilm area actually available and is the more robust basis for MBBR design and for comparing carriers. Both should be reported, with the surface area loading rate governing the sizing.
Frequently Asked Questions
What is the difference between volumetric and surface area loading?
Why does high organic loading prevent nitrification?
How is loading used to design combined C and N plants?
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