Estimate the oxygen the process needs, then convert it to blower air flow.
Oxygen Requirement (AOR)
Formula
AOR, lb/d = (f x BOD removed, lb/d) + (4.57 x NH3-N nitrified, lb/d) - (2.86 x NO3-N denitrified, lb/d)
Inputs used
- Flow to aeration (MGD)
- Aeration influent BOD (mg/L)
- Effluent soluble BOD (mg/L)
- Ammonia-N nitrified (mg/L)
- Nitrate-N denitrified (0 if none) (mg/L)
- lb O₂ per lb BOD removed
Worked example (White River WRRF, fictional)
- BOD removed = 8.0 x 145 x 8.34 = 9,674 lb/d
- Carbon = 1.1 x 9,674 = 10,642 lb/d
- Nitrification = 4.57 x (8.0 x 30 x 8.34) = 9,147 lb/d
- Credit = 2.86 x (8.0 x 20 x 8.34) = 3,816 lb/d
- AOR = 10,642 + 9,147 - 3,816 = 15,973 lb/d
Air Required from AOR
Formula
SOR = AOR / (AOR/SOR ratio) Air, scfm = SOR, lb/d / (SOTE x 0.0173 lb O₂/ft³ air x 1,440 min/d)
Inputs used
- AOR (lb/d)
- AOR/SOR ratio (field to clean water)
- Standard oxygen transfer efficiency (SOTE) (%)
Worked example (White River WRRF, fictional)
- SOR = 15,745 / 0.5 = 31,490 lb/d
- Air = 31,490 / (0.29 x 0.0173 x 1,440) = 4,359 scfm
Tips
- 1.1 lb O2/lb BOD is a typical middle value; longer SRT uses more.
- Compare peak demand with firm blower capacity.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.