Biological process calculator

Aeration System & Basin Zoning Calculator

Screen aerobic and anoxic basin volumes, biological oxygen demand, field oxygen-transfer efficiency, blower airflow, and thermodynamic isentropic blower power for an MBR biological-treatment system.

Preliminary aeration screen: This tool converts an oxygen-demand mass balance into an altitude-corrected, thermodynamic isentropic blower estimate. Confirm oxygen-transfer performance, alpha and beta factors, diffuser depth, site elevation, process temperature, mixing air, redundancy, and blower curves using supplier data and field or pilot testing.

Input parameters

Define loading, basin zoning, oxygen-transfer, site elevation, and blower assumptions.

Average Influent Flow

m³/day

Carbon and nitrogen oxygen demand

Influent BOD₅

mg/L

BOD₅ Removal

percent

Carbonaceous Oxygen Factor

kg O₂ per kg BOD removed

Influent Ammonia-N

mg/L as N

Target Effluent Ammonia-N

mg/L as N

Influent Nitrate-N

mg/L as N

Target Effluent Nitrate-N

mg/L as N

Peak Oxygen-Demand Factor

peak / average oxygen demand

Basin zoning

Total Biological HRT

hours

Anoxic-Zone HRT

hours; must not exceed total biological HRT

Field oxygen transfer, site elevation, and blower

Site Elevation Above Sea Level

meters; adjusts barometric pressure and inlet air density

Standard Clean-Water OTE

percent; use certified diffuser performance data

Alpha Factor

process-water / clean-water oxygen-transfer ratio

Beta Factor

process-water dissolved-oxygen saturation correction

Mixed-Liquor Temperature

°C

Aerobic Basin DO Setpoint

mg/L

Blower Pressure Rise

Pa; include diffuser, static, and piping losses

Overall Electrical-to-Air Efficiency

percent. Combined wire-to-air efficiency (blower isentropic, motor, VFD).

Calculated results

Average and peak oxygen demand, basin zoning, atmospheric conditions, airflow, and isentropic blower power.

Enter values and select Calculate to view the aeration and basin-zoning screen.

Equations used

Average oxygen demand combines carbonaceous oxygen demand and nitrification demand, less the denitrification oxygen credit. Atmospheric pressure and inlet air density are adjusted for site elevation. Blower power applies thermodynamic isentropic compression work.

Ambient Pressure = 101,325 × (1 − 0.0065 × elevation ÷ 288.15)^5.25588

Inlet Air Density = (Ambient Pressure × 0.02896) ÷ (8.314 × (273.15 + T))

Average O₂ demand = Carbon O₂ + nitrification O₂ − denitrification O₂ credit

Field OTE = Clean-water OTE × α × 1.024^(T−20) × (β × C*₂₀ − DO) ÷ C*₂₀

Required Peak Airflow = Peak O₂ demand ÷ 24 ÷ (Density Ratio × 1.40 × 0.232 × Field OTE)

Isentropic Power = [k ÷ (k − 1)] × P₁ × Q₁ × [(P₂ ÷ P₁)^((k − 1) ÷ k) − 1]

Electrical Blower Power = Isentropic Power ÷ Overall Efficiency

Scope and interpretation

The alpha factor summarizes oxygen-transfer inhibition in process water relative to clean water. Thermodynamic isentropic compression accounts for gas temperature rise and density changes during compression. Activated-sludge aeration design guidance [1] Alpha-factor technical study [2]

Aeration, oxygen-demand, and basin-zoning 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 combines BOD oxygen demand, nitrification oxygen, denitrification credit, HRT-based basin volumes, field oxygen-transfer correction, airflow capacity, and an isentropic blower-power screen. It is separate from membrane-scour and mixing-air design.

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.

U.S. EPA, Nutrient Control Design Manual (2010)
U.S. EPA, Design Manual: Fine Pore Aeration Systems (EPA/625/1-89/023)
U.S. EPA, Fine Bubble Aeration Fact Sheet
ASCE/EWRI Standard 2-06, Measurement of Oxygen Transfer in Clean Water

Assumptions, limitations, and review actions

4.57 and 2.86 are stoichiometric equivalents; net oxygen demand is not a final aeration capacity. The page does not model endogenous respiration, SRT/MLSS, alkalinity, kinetics, minimum mixing/scour air, zone distribution, controls, redundancy, or membrane aeration.
Alpha/beta, SOTE, temperature, DO, pressure rise, fouling, diffuser performance, airflow standard state, and blower efficiency require site and supplier/test data. Verify oxygen-transfer tests and blower curves before procurement.
HRT-derived anoxic/aerobic volumes do not demonstrate nitrogen-removal performance. Check peak/seasonal loads, carbon, DO carryover, pH, temperature, SRT, mixing, and permit conditions separately.

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.