Governing kinetic relationships
The calculator uses an explicit, simplified Monod-style autotroph-growth relationship. The functional form reflects aerobic autotroph growth in ASM1, while the user controls every kinetic and correction assumption.[1]
μmax,T = μmax,20 × θμ(T − 20) and bT = b20 × θb(T − 20)
fNH = SNH ÷ (KNH + SNH) and fDO = DO ÷ (KDO + DO)
μgross = μmax,T × fNH × fDO × site adjustment factor
μnet = μgross − bT → Theoretical minimum SRT = 1 ÷ μnet
Design minimum SRT = theoretical minimum SRT × selected safety multiplier
How to read fNH, fDO, μmax, and decay
These terms describe how favourable the entered conditions are for nitrifier growth. They are not removal efficiencies and do not multiply the ammonia load directly. They reduce the maximum possible nitrifier growth rate to reflect residual ammonia, available dissolved oxygen, and wastewater temperature.
| Term | Definition | Default-case interpretation |
|---|---|---|
| fNH | Ammonia limitation factor: SNH ÷ (KNH + SNH). | 1.00 ÷ (1.00 + 1.00) = 0.500. At 1 mg/L target residual ammonia-N and KNH of 1 mg N/L, the simplified substrate term allows 50% of the maximum growth rate. |
| fDO | Dissolved-oxygen limitation factor: DO ÷ (KDO + DO). | 2.00 ÷ (0.40 + 2.00) = 0.833. At 2.0 mg/L DO and KDO of 0.40 mg O₂/L, oxygen allows about 83.3% of maximum growth. |
| μmax,T | Temperature-corrected maximum specific growth rate: μmax,20 × θμ^(T − 20). | 0.50 × 1.072^(12 − 20) = 0.287 d⁻¹. The entered 20 °C maximum growth rate falls from 0.50 d⁻¹ to 0.287 d⁻¹ at 12 °C. |
| bT | Temperature-corrected endogenous decay rate: b20 × θb^(T − 20). | 0.05 × 1.029^(12 − 20) = 0.040 d⁻¹. Biomass decay also slows in cold water, but less sharply than growth under the selected temperature factors. |
Default-case calculation path
Gross growth = 0.287 × 0.500 × 0.833 × 0.95 = 0.113 d⁻¹
Net growth = 0.113 − 0.040 = 0.074 d⁻¹ → theoretical minimum SRT = 1 ÷ 0.074 ≈ 13.6 days
The fNH term uses the entered target/residual ammonia-N, not influent ammonia-N. As the required residual ammonia becomes lower, the substrate term becomes more restrictive and the calculated minimum SRT increases. This is a useful conservative kinetic screen; the entered growth, decay, half-saturation, temperature, and site-adjustment inputs should be calibrated to plant data where possible.
Interpretation
A positive net growth rate indicates that the selected kinetic assumptions allow theoretical nitrifier retention. A selected-SRT margin below 1.00 means the entered operating SRT is below the selected safety-adjusted threshold; it is not, by itself, a compliance prediction. EPA nutrient-control guidance identifies temperature, oxygen transfer, and solids retention as key nitrification design considerations.[2][3]
Related hand-offs
The calculator stores the current selected-SRT, calculated design minimum SRT, ammonia removal load, oxygen demand, and alkalinity consumption in this browser session for later integration with the Nitrogen Removal and MBR Energy & Operating Cost calculators.