Tech Blog

Denitrification and decarbonization of melamine production wastewater

As is well known, it is difficult to treat melamine production wastewater, which produces high-concentration wastewater characterized by high ammonia nitrogen content (up to nearly 1000 mg/L) and a low carbon-to-nitrogen ratio. Traditional methods of complete nitrification and denitrification are often difficult to treat such wastewater, as the organic carbon required for denitrification is insufficient and the energy demand for nitrification is high.

This article introduces an experimental study that combines short-range nitrification with an A/O system for treating real melamine production wastewater. The results indicate that the domesticated activated sludge can achieve ammonia removal rates of over 87% and COD removal rates of 60%, even at influent ammonia nitrogen concentrations. This process provides a more energy-efficient alternative solution for treating high-nitrogen, low-carbon industrial wastewater.

Characteristics of Melamine Production Wastewater

Melamine is produced by reacting urea at 380–400 °C with ammonia as a carrier gas and silica gel as a catalyst. The crude melamine is quenched, purified, crystallized, centrifuged, and dried. The mother liquor is partially recycled, but a significant wastewater stream is discharged. The key parameters of the wastewater used in this study:

pH	~7
COD	874 mg/L
NH₃‑N	965.7 mg/L

The low C/N ratio (COD/NH₃‑N ≈ 0.9) makes conventional biological denitrification difficult because denitrifiers require readily biodegradable organic carbon. Shortcut nitrification (nitrite shunt) reduces the demand for organic carbon and saves energy for aeration.

Shortcut Nitrification: Principles and Advantages

Conventional biological nitrogen removal consists of:

Nitrification: NH₄⁺ → NO₂⁻ (by ammonia‑oxidizing bacteria, AOB) → NO₃⁻ (by nitrite‑oxidizing bacteria, NOB)
Denitrification: NO₃⁻ → N₂ (requires organic carbon)

In shortcut nitrification, the second step (NO₂⁻ → NO₃⁻) is suppressed, so nitrite accumulates. Then denitrification proceeds from nitrite directly to nitrogen gas:

NO₂⁻ → N₂ (instead of NO₃⁻ → NO₂⁻ → N₂)

Benefits of shortcut nitrification for high‑ammonia wastewater:

25% less aeration energy (oxidation of NH₄⁺ to NO₂⁻ consumes 25% less oxygen than to NO₃⁻)
40% less organic carbon demand for denitrification (since nitrite is a more electron‑accepting intermediate)
Lower sludge production
Higher nitrogen loading rates possible

The SHARON process (Single reactor system for High‑activity Ammonium Removal Over Nitrite) is a well‑known example, operated at elevated temperature (30–40 °C) and short hydraulic retention time to wash out NOB.

Denitrification and decarbonization of melamine production wastewater Experimental Setup

Reactor and Operating Conditions

A completely stirred tank reactor (CSTR) made of plexiglass with an effective volume of 10 L was used. The reactor was equipped with aeration tubing, online DO and pH control, and a heating rod to maintain temperature.

Process flow: Influent → CSTR (shortcut nitrification) → Settler → Effluent (then further treated by A/O system)

Key operating parameters for shortcut nitrification:

Temperature	35 ± 1 °C
Dissolved oxygen (DO)	1.0–2.0 mg/L
pH	7.5 ± 0.5
Hydraulic retention time (HRT)	1 day
Alkalinity addition	Na₂CO₃ solution

Seed Sludge and Acclimation

Activated sludge was taken from the aeration tank of the East China University of Science and Technology wastewater treatment plant. It was washed and then acclimated using synthetic high‑ammonia wastewater (with (NH₄)₂SO₄, NaH₂PO₄, and trace elements) for 50 days. After successful acclimation, the feed was switched to real melamine wastewater, initially diluted and then gradually increased to full strength.

Results: Nitrogen Removal by Shortcut Nitrification

Sludge Acclimation and Nitrite Accumulation

During the 50‑day acclimation period, nitrite (NO₂⁻) began to accumulate from day 29, while nitrate (NO₃⁻) remained low. By day 50, nitrite dominated, and nitrate was almost undetectable – a clear indication of stable shortcut nitrification (nitrite shunt). The system achieved high ammonia removal even with fluctuating influent NH₃‑N.

Ammonia Removal Performance

After switching to real melamine wastewater, the ammonia removal efficiency remained consistently above 83%, reaching 87.7% when the influent NH₃‑N was 965.7 mg/L.

Up to 965.7

~120

87.7%

The volumetric nitrogen loading rate achieved was 0.98 kg NH₃‑N/(m³·d) – nearly double the typical loading of conventional nitrification systems (usually <0.5 kg/(m³·d)). This demonstrates that the acclimated AOB culture is exceptionally robust and well‑suited for high‑strength industrial wastewater.

Nitrogen Mass Balance Note

The authors observed a discrepancy between total nitrogen in influent and effluent, suggesting possible nitrogen losses via other pathways (e.g., anaerobic ammonia oxidation or stripping). This was not further investigated, but indicates that the actual total nitrogen removal might be even higher than ammonia removal alone.

A/O System for Carbon Removal

After shortcut nitrification, the effluent still contained a significant amount of organic carbon (COD). The study then passed the nitrified effluent through an anoxic/oxic (A/O) system to remove residual COD.

A/O System Configuration

Anoxic zone (A) : Oxic zone (O) volume ratio = 1:1
Total HRT = 12 h (6 h anoxic + 6 h oxic)
Flow rate = 0.67 L/h
No sludge recycle was used in this experiment (the authors note that adding sludge recirculation would likely improve COD removal).
Biofilm carriers were pre‑coated and then acclimated for 30 days until the denitrification rate exceeded 75% and the nitrification rate exceeded 60%.

COD Removal Efficiency

The A/O system achieved:

Maximum COD removal: 72%
Average COD removal: approximately 60%
Influent COD (after shortcut nitrification): average 874 mg/L
Effluent COD: around 300–350 mg/L

Separate tests on the anoxic zone alone showed a COD removal of 55.1% at an influent COD of 426 mg/L, confirming that heterotrophic denitrifiers contributed to carbon degradation.

The authors suggest installing a sludge-recycle pump to circulate biomass between the anoxic and oxic zones to further improve COD removal efficiency.

conclusion

Shortcut nitrification combined with an A/O biological system is a highly effective method for treating melamine production wastewater. It achieves strong nitrogen and carbon removal, withstands high ammonia loading, and works reliably under real plant conditions.

For melamine manufacturers struggling with wastewater treatment, this process provides a practical, cost‑effective, and compliant solution.

Tech Blog

Denitrification and decarbonization of melamine production wastewater

Characteristics of Melamine Production Wastewater

Shortcut Nitrification: Principles and Advantages