Nitrogen Removal in Wastewater: Nitrification-Denitrification Guide

Short answer: Classical biological nitrogen removal is two-stage: (1) Nitrification — conversion of NH₄-N to NO₃-N under aerobic conditions (Nitrosomonas + Nitrobacter bacteria), (2) Denitrification — conversion of NO₃-N to N₂ gas under anoxic conditions (heterotrophic bacteria, requires carbon source). Efficiency reaches 85-95% total nitrogen removal. Anammox is a more economical alternative for low C/N ratio wastewater.

Nitrogen Forms in Wastewater

Nitrogen is found in wastewater in 4 main forms and can be converted into one another:

• Organic nitrogen (Org-N): Protein, urea, amino acids — converted to NH₄ through bacterial decomposition
• Ammonia/ammonium nitrogen (NH₃/NH₄-N): Mostly found as NH₄⁺ in water; at pH>9, NH₃ gas is dominant
• Nitrite nitrogen (NO₂-N): Temporary intermediate product, toxic (inhibitory to chlorination reactions)
• Nitrate nitrogen (NO₃-N): Fully oxidized form, leads to eutrophication in receiving environments

Total nitrogen (TN) = Org-N + NH₄-N + NO₂-N + NO₃-N. Turkey's SKKY and EU directives set the outlet TN limit in the 10-15 mg/L range.

Nitrification: NH₄ → NO₃

Two different groups of bacteria work in two steps:

• Step 1: NH₄⁺ + 1.5 O₂ → NO₂⁻ + H₂O + 2H⁺ (Nitrosomonas, AOB)
• Step 2: NO₂⁻ + 0.5 O₂ → NO₃⁻ (Nitrobacter, NOB)

Critical parameters for nitrification:

Denitrification: NO₃ → N₂

Under anoxic (oxygen-free, nitrate present) conditions, heterotrophic bacteria reduce NO₃⁻ to nitrogen gas:

NO₃⁻ → NO₂⁻ → NO → N₂O → N₂↑

This process requires electron acceptor (NO₃) and electron donor (carbon). The carbon source is typically:

• Internal: BOD in wastewater (most economical)
• External: Methanol, ethanol, acetate, glucose (in case of carbon deficiency)

Practical rule: At least 4 g BOD / g NO₃-N is required for complete denitrification. External carbon dosing is mandatory for low C/N ratio wastewaters.

Configurations: MLE, A2/O, Bardenpho

MLE (Modified Ludzack-Ettinger)

The simplest and most common nitrogen removal configuration:

Inlet → Anoxic → Aerobic (Nitrification) → Settling/Membrane → Outlet
↑ Internal recycle (nitrate) ←—————————————┘

NO₃ in aerobic is recycled back to the anoxic zone; there it is reduced to N₂. Internal recycle ratio is generally 2-4Q. Maximum nitrogen removal efficiency is typically 75-85%.

A2/O (Anaerobic-Anoxic-Aerobic)

A configuration that removes both nitrogen and phosphorus:

Inlet → Anaerobic (P release) → Anoxic (Denit) → Aerobic (Nit + P uptake) → Settling → Outlet

Both PAO bacteria (phosphorus) and denitrification are active. It is the de facto standard for municipal wastewater treatment plants.

Bardenpho 4 and 5 Stage

Used in sensitive receiving environments where high nitrogen removal is required (TN<5 mg/L):

Anoxic 1 → Aerobic 1 → Anoxic 2 → Aerobic 2 (aeration) → Settling

In the second anoxic zone, additional NO₃ is removed through endogenous denitrification. Efficiency is 90-95%.

Advanced Methods: Anammox and SHARON

Anammox (Anaerobic Ammonium Oxidation)

A revolutionary biological process discovered in the Netherlands in the 1990s. NH₄⁺ and NO₂⁻ are directly converted to N₂:

NH₄⁺ + 1.32 NO₂⁻ → 1.02 N₂ + 0.26 NO₃⁻ + 2 H₂O

Advantages:

• Oxygen consumption decreases by 60% (compared to classical nitrification-denitrification)
• Does not require a carbon source (autotrophic)
• Sludge production is 90% lower
• Greenhouse gas (N₂O) emissions are very low

Disadvantage: Anammox bacteria grow slowly (doubling time 11-20 days), require large SRT, temperature sensitive (optimum >25 °C). Ideal for sludge digesters, high concentration industrial wastewater, and fertilizer production waters.

SHARON (Single reactor High activity Ammonia Removal Over Nitrite)

Partial nitrification — stops NH₄ at NO₂ instead of NO₃. Then proceeds to denitritation to N₂. Advantage: oxygen decreases by 25%, carbon decreases by 40%. Generally combined with Anammox (SHARON+Anammox).

5 Common Problems Encountered in the Field

1. Nitrification stops at low temperature: If the reactor temperature drops below 10 °C in winter months, Nitrosomonas activity halves. Solution: Increase SRT to 25+ days, compensate with heating or high MLSS.
2. Alkalinity depletion → pH drop: Nitrification consumes 7.14 mg CaCO₃ for every 1 mg NH₄. Solution: Stabilize pH at 7.5 with caustic soda (NaOH) or lime dosing.
3. Insufficient carbon — denitrification lacking: When C/N ratio <3. Solution: Methanol or glycerin dosing; reduce internal recycle ratio.
4. Nitrite accumulation (NO₂ > 1 mg/L): Low NOB activity, AOB predominates. If chlorination is done at the outlet, it is toxic. Solution: Check if DO is 2.5+ mg/L, ensure SRT is sufficient.
5. Presence of inhibitors: Galvanized materials, pharmaceuticals, chlorinated solvents kill nitrification bacteria. Solution: Wastewater characterization, pre-treatment at the source if necessary.

Cost and Energy Comparison

Advantages of Nitrogen Removal in MBR

• Long SRT (20-40 days) → safe nitrification even at low temperatures
• High MLSS → compact anoxic zone, small recycle
• Membrane physical barrier → AOB/NOB are not washed out, population remains stable
• Internal recycle ratio can be optimized → nitrogen removal efficiency >90%

Conclusion

Nitrogen removal is one of the most critical and sensitive parameters in wastewater treatment. The correct process selection should be made according to the wastewater's C/N ratio, temperature, discharge limit, and energy cost. For classical wastewater, MLE/A2O is the standard, for sensitive discharge Bardenpho, and for high concentration/low C wastewaters Anammox should be preferred.

Related topics: Phosphorus Removal in MBR, MBR vs MBBR. You can request a TN removal optimization study for your facility.