SALR is defined as the daily pollutant load divided by the total protected carrier surface area (typically g BOD/m²·d for carbon removal or g NH₄-N/m²·d for nitrification). Because biofilm activity scales with available area, SALR relates directly to achievable removal rate and effluent quality, which is why MBBR sizing is built around it rather than around volumetric load.
Why SALR Governs the Design
The removal rate achievable per unit area (the surface area removal rate) has an upper limit set by diffusion and kinetics. Designing at an SALR below that limit ensures the target effluent quality is met; designing above it leaves pollutant unremoved. The design SALR is chosen from established relationships for the treatment objective, temperature and required effluent concentration, then used to calculate the required carrier area.
Nitrification SALR is strongly temperature-dependent and also limited by dissolved oxygen; carbon-removal SALR is higher and more forgiving. Where both objectives coexist, the nitrification SALR usually controls the overall design.
Indicative Design SALR (literature ranges — confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| BOD removal (to low effluent BOD) | 5 – 15 | g BOD/m²·d | Lower for stricter effluent |
| Nitrification (warm) | 0.7 – 1.0 | g NH₄-N/m²·d | Adequate DO required |
| Nitrification (cold) | 0.4 – 0.7 | g NH₄-N/m²·d | Rate falls with temperature |
| Denitrification | 1 – 4 | g NO₃-N/m²·d | Carbon-source dependent |
SALR and Effluent Quality
Lower SALR generally yields lower effluent concentrations because the biofilm is under-loaded relative to its capacity. Tightening an effluent target therefore increases the required carrier area (lower design SALR) and hence the reactor size. This direct link between SALR, effluent quality and cost is central to optimising an MBBR design against its discharge consent.
Frequently Asked Questions
What is SALR in MBBR design?
Why is nitrification SALR lower than carbon removal SALR?
How does a stricter effluent target affect SALR?
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