Nitrification & Denitrification Calculator
Size combined nitrification/denitrification (CNdN) reactors for biological nitrogen removal — calculates aerobic and anoxic zone volumes, SRT safety factor, HRT, and oxygen demand using the Metcalf & Eddy (5th ed.) kinetic approach for activated sludge systems.
CNdN Reactor Design Parameters
Based on Metcalf & Eddy (5th ed.) Chapters 8 & 9 — aerobic SRT approach for combined nitrification / denitrification sizing. Results update live as you type.
Average daily design flow
Total Kjeldahl nitrogen in raw influent
Discharge standard for total nitrogen
Carbon source for denitrification
Design aerobic sludge age
Operating temperature (θ = 1.072)
Mixed liquor suspended solids
Sizing Results
Q = 500 m³/day | TKN = 45 mg/L | SRT = 15 d | T = 20°C
Nitrification Zone Volume
135 m³
Aerobic zone
Denitrification Zone Volume
125 m³
Anoxic zone
Total Reactor Volume
261 m³
Aerobic + anoxic
Total HRT
12.5 hours
Combined zones
Nitrification Safety Factor
10.1×
Adequate (≥ 1.5)
O₂ Demand for Nitrification
57 kg/day
4.57 g O₂/g NH₄-N
Zone Volume Breakdown
Disclaimer: These are preliminary estimates based on Metcalf & Eddy (5th ed.) default kinetic constants. Actual design requires site-specific treatability studies, wastewater characterisation, and detailed process engineering.
How to Use This Calculator
- 1Enter the design flow rate (m³/day) and influent TKN (total Kjeldahl nitrogen, mg N/L) from your wastewater characterisation. TKN is the sum of organic nitrogen and ammonia nitrogen measured by the Kjeldahl digestion method.
- 2Set the target effluent total nitrogen (mg N/L) from your discharge standard or consent condition. In India, NGT-notified STPs typically require TN ≤ 10 mg/L; CPCB ZLD norms specify NH₃-N ≤ 50 mg/L.
- 3Enter the influent BOD₅ (mg/L). This is the carbon source available for denitrification. If the BOD:NO₃-N ratio falls below 4.2, the calculator will flag carbon-limited denitrification and suggest external carbon addition.
- 4Set the aerobic SRT (days), operating temperature (°C), and MLSS (mg/L). The calculator applies the Arrhenius temperature correction to the Nitrosomonas maximum growth rate and checks the nitrification SRT safety factor — a value below 1.5 triggers a warning to increase SRT.
- 5Review the six metric cards for zone volumes, total HRT, safety factor, and oxygen demand, plus the detailed breakdown table and zone volume chart. Results update live as you adjust parameters.
Biological Nitrogen Removal: Nitrification and Denitrification
Biological nitrogen removal (BNR) from wastewater requires two sequential processes: aerobic nitrification, in which ammonia is oxidised to nitrate by autotrophic bacteria, and anoxic denitrification, in which nitrate is reduced to nitrogen gas (N₂) by heterotrophic bacteria using organic carbon as an electron donor. Together, these processes constitute combined nitrification/denitrification (CNdN), the dominant technology for achieving low total nitrogen effluent concentrations in municipal and industrial wastewater treatment.
Nitrification is performed by two groups of autotrophic microorganisms: ammonia-oxidising bacteria (AOB), principally Nitrosomonas, convert NH₄⁺ to NO₂⁻; and nitrite-oxidising bacteria (NOB), principally Nitrobacter, convert NO₂⁻ to NO₃⁻. Both groups are obligate aerobes with slow growth rates compared to heterotrophic BOD-removing bacteria. This slow growth is the critical constraint: the aerobic SRT must be long enough for nitrifiers to accumulate in the biomass faster than they are wasted — otherwise washout occurs and nitrification fails completely.
Denitrification by heterotrophic bacteria in anoxic zones uses the oxygen bound in NO₃⁻ as a terminal electron acceptor, allowing continued metabolism of organic carbon in the absence of dissolved oxygen. This process can use the influent BOD (pre-anoxic configuration), recycled mixed liquor (MLE process), or external carbon sources (methanol, acetate, glycerol) for post-anoxic polishing. Combined BNR systems integrate both aerobic and anoxic zones in a single bioreactor with targeted internal recycles to manage nitrogen through the treatment train. For complete integrated biofilm nitrogen removal systems, see our MBBR technology page.
SRT-Based Reactor Sizing for Nitrogen Removal (Metcalf & Eddy)
The aerobic SRT is the primary design variable controlling nitrification reliability. Metcalf & Eddy (5th ed., Equations 9-36 and 9-37) define the minimum nitrification SRT as the reciprocal of the net growth rate of Nitrosomonas: SRT_n,min = 1 / (µₙ,max − bₙ), where µₙ,max is the maximum specific growth rate of Nitrosomonas (0.75 d⁻¹ at 20°C) corrected for temperature using the Arrhenius factor θ = 1.072, and bₙ is the endogenous decay coefficient (0.08 d⁻¹). A safety factor of at least 1.5 must be applied to the minimum SRT to account for process variability, peak nitrogen loads, and toxic inhibition episodes — giving the design aerobic SRT.
The nitrification zone volume is sized using the aerobic SRT, the observed yield for heterotrophic biomass (Y_obs = Y / (1 + kd × SRT), with Y = 0.6 g VSS/g BOD and kd = 0.06 d⁻¹), and the design MLSS. The biomass production rate (kg VSS/day) determines the sludge inventory needed in the aerobic zone at the target SRT. The anoxic zone volume is calculated from the fraction of total nitrogen that must be denitrified and the design anoxic fraction, limited to 50% of total reactor volume to maintain adequate aerobic conditions for nitrification.
The oxygen demand for nitrification is calculated from the stoichiometric oxygen consumption: 4.57 g O₂ per gram of NH₄⁺-N oxidised. This is a significant fraction of the total oxygen demand in combined BNR systems — typically 20–40% of total aeration capacity — and must be included in the blower and diffuser design for the aerobic zone. Correctly accounting for nitrification oxygen demand prevents blower undersizing, which is a common cause of process instability in Indian ETPs that were originally designed without nitrogen removal and are subsequently upgraded to meet stricter discharge norms.
# M&E Kinetic Parameters for Nitrification (20°C)
µₙ,max(T) = 0.75 × 1.072^(T − 20) [d⁻¹]
SRT_n,min = 1 / (µₙ,max − 0.08) [days]
Design SRT ≥ 1.5 × SRT_n,min
Y_obs = 0.6 / (1 + 0.06 × SRT)
Biomass (kg VSS/d) = Y_obs × Q (m³/d) × BOD (mg/L) / 1000
V_nitrification (m³) = Biomass × SRT × 1000 / MLSS
O₂_nitrification (kg/d) = N_nitrified × 4.57 × Q / 1000
Nitrogen Discharge Standards in India: NGT & CPCB Norms
Effluent nitrogen standards in India have tightened significantly over the past decade under National Green Tribunal (NGT) orders and CPCB directives. The NGT order of 2015 (Original Application No. 200/2014) mandated that all STPs on the Ganga main stem achieve total nitrogen ≤ 10 mg/L and total phosphorus ≤ 1 mg/L in treated effluent — a standard that requires full BNR capability and goes far beyond the earlier Schedule I standard that only specified BOD and TSS limits.
For industrial effluent, the CPCB Effluent Standards (EPA 1986, Schedule I) specify ammonia nitrogen limits of 50 mg/L (as NH₃-N) for inland surface water discharge and 100 mg/L for public sewers, applicable to most industries including pharmaceuticals, fertiliser plants, petrochemicals, and food processing. Zero Liquid Discharge (ZLD) mandates for red category industries (textile, distillery, paper, electroplating) require nitrogen removal as part of the treated water reuse system — high residual ammonia causes fouling of membrane concentrate evaporators and scaling of cooling towers if reuse is attempted.
State pollution control boards (SPCBs) in Maharashtra, Gujarat, Tamil Nadu, Andhra Pradesh, and Uttar Pradesh have issued plant-specific directives for industrial clusters with combined effluent treatment plants (CETPs), often setting TN targets of 15–20 mg/L. For greenfield projects requiring environmental clearance under EIA Notification 2006, Category A and B projects must demonstrate compliance with the most stringent applicable standard at design stage — making this calculator a useful tool for early feasibility assessment.
Process Configurations: Pre-Anoxic, Post-Anoxic, and Step-Feed
The Modified Ludzack-Ettinger (MLE) or pre-anoxic process is the most widely used CNdN configuration in India and globally. In MLE, the bioreactor is divided into an upstream anoxic zone (30–50% of total volume) and a downstream aerobic zone. Influent enters the anoxic zone, where denitrification uses the incoming BOD as carbon source. An internal recycle pump transfers nitrate-rich mixed liquor from the end of the aerobic zone back to the inlet of the anoxic zone. At a recycle ratio of 4:1 (Q_internal = 4 × Q), approximately 80% of the influent nitrate load can be denitrified using influent BOD alone, achieving effluent TN of 8–12 mg/L from typical municipal wastewater without external carbon addition.
Post-anoxic (endogenous denitrification) processes place the anoxic zone downstream of the complete aerobic zone, enabling very low effluent TN (below 3–5 mg/L) but requiring external carbon addition since influent BOD has already been oxidised in the aerobic zone. Methanol is the most common external carbon source (3.7 g methanol/g NO₃-N), though sodium acetate, glycerol, and acetic acid are also used. The Bardenpho and 5-stage Bardenpho processes combine pre-anoxic and post-anoxic zones with a final aerobic zone to strip nitrogen gas bubbles and improve settleability.
Step-feed configurations (step-feed AS or step-feed SBR) distribute a fraction of the influent to multiple anoxic zones in series, effectively improving the C:N ratio seen by each anoxic zone and reducing or eliminating external carbon requirements. For wastewater with BOD:TKN ratios below 4:1 — common in industrial wastewater after upstream biological pre-treatment — step-feed or external carbon supplementation is typically required to achieve TN below 10 mg/L. To discuss which configuration is optimal for your project, please contact our design team.
Frequently Asked Questions
What is nitrification in wastewater treatment?
Nitrification is the two-step aerobic biological oxidation of ammonia nitrogen (NH₄⁺-N) to nitrate (NO₃⁻-N) by autotrophic bacteria — first Nitrosomonas converts ammonia to nitrite, then Nitrobacter converts nitrite to nitrate. The process consumes 4.57 g O₂ per gram of NH₄⁺-N oxidised and reduces alkalinity by 7.14 g CaCO₃/g NH₄⁺-N. Nitrification is the essential first step in biological total nitrogen removal and requires an adequate aerobic SRT to maintain a stable population of slow-growing nitrifying bacteria.
What is denitrification and why is it important?
Denitrification is the biological reduction of nitrate (NO₃⁻-N) to nitrogen gas (N₂) by heterotrophic bacteria in anoxic conditions, using organic carbon as the electron donor. It is the only biological process that physically removes nitrogen from wastewater. Without denitrification, nitrification converts ammonia to nitrate but effluent total nitrogen remains unchanged — total nitrogen removal requires the full nitrification-denitrification sequence. Denitrification also recovers approximately 2.86 g O₂ equivalent per gram of NO₃-N reduced, partially offsetting aeration costs.
What is the minimum SRT for reliable nitrification?
Per Metcalf & Eddy (5th ed.), the minimum nitrification SRT = 1 / (µₙ,max − bₙ). At 20°C with µₙ,max = 0.75 d⁻¹ and bₙ = 0.08 d⁻¹, the minimum SRT ≈ 1.5 days. A safety factor of at least 1.5× is required, giving a design SRT of about 2.3 days at 20°C. At 15°C the minimum design SRT rises to approximately 5 days; at 10°C, to 12–15 days. The calculator automatically computes the temperature-corrected minimum SRT and flags insufficient safety factors.
How does temperature affect the nitrification rate?
The maximum growth rate of Nitrosomonas is strongly temperature-dependent, following the Arrhenius equation: µₙ,max(T) = 0.75 × 1.072^(T−20). For every 10°C drop, the rate roughly halves. At 15°C, µₙ,max ≈ 0.53 d⁻¹; at 10°C, ≈ 0.37 d⁻¹. Cold-climate designs must use substantially longer aerobic SRTs — typically 12–20 days at 10°C — to maintain reliable nitrification throughout the year. This calculator applies the full temperature correction and recalculates the minimum SRT and safety factor at the entered temperature.
How much carbon (BOD) is needed for denitrification?
Approximately 4.2 g BOD is required per gram of NO₃-N denitrified using influent wastewater BOD as the carbon source (Metcalf & Eddy, 5th ed.). If the available BOD:NO₃-N ratio is below this threshold, denitrification will be carbon-limited and require external carbon supplementation — typically methanol (3.7 g/g NO₃-N), acetate, or glycerol. The calculator displays the actual BOD:NO₃-N ratio and alerts the user when it falls below 1.5 — a warning that full denitrification of the target nitrogen load may not be achievable without external carbon.
What is the difference between pre-anoxic and post-anoxic denitrification?
Pre-anoxic (MLE process) places the anoxic zone before the aerobic nitrification zone, using influent BOD as the carbon source and internal nitrate recycle. It achieves approximately 70–85% total nitrogen removal without external carbon, making it cost-effective for most municipal and industrial applications. Post-anoxic denitrification follows the aerobic zone; since influent BOD has already been consumed, external carbon must be added. Post-anoxic can achieve TN below 3–5 mg/L but incurs additional chemical costs. Step-feed is a hybrid that distributes influent to multiple anoxic zones to improve the C:N ratio.
When is nitrogen removal required under CPCB/NGT norms in India?
Nitrogen removal is required for STPs discharging to NGT-notified rivers (TN ≤ 10 mg/L, TP ≤ 1 mg/L under the 2015 NGT order for Ganga basin STPs). CPCB Schedule I standards specify ammonia nitrogen ≤ 50 mg/L for inland surface water discharge from most industries and ≤ 100 mg/L for public sewers. ZLD-mandated industries (textile, distillery, paper) must remove nitrogen before treated water reuse to prevent membrane fouling and cooling tower scaling. Greenfield projects requiring environmental clearance under EIA Notification 2006 must demonstrate compliance with applicable nitrogen standards at design stage.
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Design Your Nitrogen Removal System
Spans Envirotech designs and commissions combined nitrification/denitrification systems — MLE, Bardenpho, MBBR-based BNR, and SBR configurations — for municipal STPs and industrial ETPs across India. Contact our process engineering team for a site-specific nitrogen removal design review.
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