Anoxic Volume Fraction Calculator

Biological nitrogen removal needs the right split between anoxic and aerobic volume. Enter your basin volumes, flow, and internal recycle rate to check the anoxic fraction and anoxic-zone detention time.

Total basin volume

Anoxic zone volume

Volume unit

Influent flow

Flow unit

Internal recycle ratio (% of influent Q)Recycle from aerobic to anoxic zone. Typical 100–400% for MLE. Default 200.

Anoxic Zone Check

Item	Value

How the calculation works

Formulas
Anoxic fraction (%) = 100 × anoxic volume ÷ total basin volume
Flow through anoxic zone = influent Q × (1 + internal recycle ÷ 100)
Anoxic HRT (hr) = 24 × anoxic volume (gal) ÷ anoxic-zone flow (gpd)
The internal recycle carries nitrate from the aerobic zone back to the anoxic zone, so the anoxic zone sees influent flow plus recycle flow.

Process	Typical anoxic fraction	Typical internal recycle
MLE (pre-denitrification)	15–40%	100–400%
A²O (with anaerobic zone)	20–30%	100–400%
4-stage Bardenpho (both anoxic zones)	20–35%	200–400%

Notes & limitations

Typical anoxic fractions and recycle ranges follow Metcalf & Eddy, Wastewater Engineering. Actual targets depend on effluent nitrogen limits, wastewater characteristics, and temperature — verify against the current edition and your design basis.

This is a volume-split screening tool. Full BNR design also checks nitrate load, readily biodegradable COD availability in the anoxic zone, and alkalinity.
Denitrification needs carbon: roughly 4–6 mg of BOD per mg of NO₃-N denitrified. Carbon-limited anoxic zones denitrify poorly no matter how large they are.
Cold wastewater slows denitrification; winter operation may need a larger effective anoxic fraction or higher MLSS.
The anoxic HRT shown uses influent plus internal recycle flow. RAS flow also passes through but is already counted inside the influent term for this screening.

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