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.