Total oxygen demand combines the carbonaceous demand (to oxidise BOD/COD) and the nitrogenous demand (to nitrify ammonia). Nitrification is oxygen-intensive: oxidising one gram of ammonia-nitrogen to nitrate requires substantially more oxygen than removing a gram of BOD. The oxygen demand, together with the aeration system's oxygen transfer efficiency, determines the required air flow and blower capacity.
Carbonaceous and Nitrogenous Demand
The carbonaceous oxygen demand is estimated from the BOD load removed, with an allowance for biomass synthesis. The nitrogenous demand follows nitrification stoichiometry — approximately 4.3 g O₂ per g NH₄-N oxidised to nitrate. Where denitrification is included, a portion of oxygen (as nitrate oxygen) is recovered, reducing the net aeration demand. These stoichiometric figures are established literature values, but the loads they are applied to must come from validated project data.
Peak oxygen demand, not average, sizes the blowers: the aeration must satisfy demand during peak load while remaining turndown-capable for low-load periods.
Oxygen Demand Stoichiometry (established literature values)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Nitrification O₂ demand | ~4.3 | g O₂/g NH₄-N | Ammonia → nitrate |
| Carbonaceous O₂ demand | ~1.0 – 1.5 | g O₂/g BOD | Incl. synthesis allowance |
| Denitrification O₂ credit | ~2.86 | g O₂/g NO₃-N | Nitrate oxygen recovered |
| Design DO (nitrification) | 4 – 6 | mg/L | Higher than for carbon only |
From Demand to Air Flow
The oxygen that must be delivered to the water (the actual oxygen requirement) is converted to a standard oxygen requirement and then to an air flow using the aeration system's standard oxygen transfer efficiency and site correction factors for temperature, pressure, salinity and the process alpha factor. In MBBR the same air often serves both oxygen transfer and carrier mixing, so the air flow is checked against both requirements.
Frequently Asked Questions
How much oxygen does nitrification need?
Does denitrification reduce aeration demand?
What determines the required air flow?
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