Biological process calculator

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.

Average Influent Flow

m³/day

Nitrate-N to Be Denitrified

mg/L as NO₃-N available to the anoxic process

Target Effluent Nitrate-N

mg/L as NO₃-N; must be lower than the nitrate-N to be denitrified

Recycle and biomass inputs

Return Activated Sludge (RAS) Ratio

Q_RAS / Q; included in the total recycle requirement

Anoxic-Zone MLVSS

mg/L as MLVSS

Specific Denitrification Rate (SDNR)

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.

Q_RAS = Q × RAS ratio

Q_RAS + Q_NR = Q × (N / N_e − 1)

Q_NR = max[0, Q × (N / N_e − 1) − Q_RAS]

N removed = Q × (N − N_e,estimated) ÷ 1,000

Denitrification alkalinity recovery = N removed × 3.57 kg as CaCO₃/kg NO₃-N

V_anoxic = 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]

Internal-recycle and anoxic-zone basis

Documentation revision: 2026-10-02. This page documents the calculation basis so an engineer can trace the inputs, units, assumptions, and review needs. Results are preliminary screening estimates; they are not a permit determination, OEM guarantee, or construction/procurement approval.

Method and source basis

The page applies an MLE nitrate-recycle relationship to calculate RAS, additional internal recycle, estimated effluent nitrate, nitrate load, and an SDNR-based anoxic volume. HRT is reported on influent and mixed-liquor flow bases.

The sources below provide technical context or analytical/design methodology. They do not endorse this calculator's defaults, and any jurisdiction-specific requirement applies only where that authority has jurisdiction.

New York State DEC, Long Island Sound Nitrogen Removal Training Program, Module 2
U.S. EPA, Nutrient Control Design Manual (2010)
U.S. EPA, Process Design Manual for Nitrogen Control (1993)

Assumptions, limitations, and review actions

This is a steady-state average-flow screen. It does not calculate carbon adequacy, DO ingress, mixing, temperature, pH/alkalinity, SRT, toxicity, recycle-pump capacity, hydraulic short-circuiting, peak conditions, or transient nitrogen species.
SDNR is site-, temperature-, carbon-, DO-, biomass-, and configuration-specific. The default is illustrative, not a design limit. Validate with plant data, testing, or calibrated modelling.
The gross alkalinity recovery output is not a complete net alkalinity balance; it does not include nitrification consumption or process-location effects.

Professional review checkpoint

Before using the result for specification or operation, reconcile it with representative site data, the applicable permit and design standard, the selected equipment or membrane supplier's current data, and a qualified wastewater/process engineer. Record the input basis, date, units, and any pilot, bench-test, or comparable full-scale evidence used to select the assumptions.