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Internal Recycle & Anoxic Zone Calculator

Estimate the internal mixed-liquor recycle requirement and preliminary anoxic-zone volume for a pre-denitrification MBR or modified Ludzack–Ettinger (MLE) process.

Preliminary MLE screening tool: The recycle relation is based on a nitrate mass balance and assumes that the anoxic zone receives influent carbon and recycled nitrified mixed liquor. The selected specific denitrification rate (SDNR) must reflect actual temperature, carbon availability, dissolved oxygen, and site conditions.

Input parameters
Enter average design values for a pre-denitrification process.
m³/day
mg/L as NO3-N available to the anoxic process
mg/L as NO3-N; must be lower than the nitrate-N to be denitrified
Recycle and biomass inputs
QRAS / Q; included in the total recycle requirement
mg/L as MLVSS
g NO3-N / (g MLVSS·day); enter a site-specific or conservatively selected value
Calculated results
Recycle flows are expressed as average hydraulic flows; nitrogen terms are as NO3-N.

Enter values and select Calculate to view the recycle and anoxic-zone checks.

Equations used

The calculator uses a simplified pre-denitrification recycle relationship in which RAS is included in the total flow returned to the anoxic zone.

QRAS = Q × RAS ratio
QRAS + QNR = Q × (N / Ne − 1)
QNR = max[0, Q × (N / Ne − 1) − QRAS]
N removed = Q × (N − Ne,estimated) ÷ 1,000
Denitrification alkalinity recovery = N removed × 3.57 kg as CaCO3/kg NO3-N
Vanoxic = N removed ÷ (SDNR × MLVSS)

Scope and interpretation

The recycle relation is presented in the New York State nitrogen-removal training material for determining total recycle rate, including RAS. Anoxic volume is then estimated from the actual nitrate-N removal load, the selected SDNR, and the anoxic-zone MLVSS concentration. Source: NYS DEC nitrogen-removal training module [1]