Ammonia nitrogen (commonly referred to as NH₃-N or ammoniacal nitrogen) is one of the most important pollutants in municipal wastewater. It comes mainly from domestic sewage, urine, food waste, and some industrial discharges. High concentrations of ammonia nitrogen can cause eutrophication, reduce dissolved oxygen levels, and be toxic to fish and other aquatic organisms. Therefore, effective ammonia nitrogen removal is a key requirement in municipal wastewater treatment plants worldwide.
Traditional biological processes such as nitrification and denitrification are widely used to remove ammonia nitrogen, but in many cases, chemical or physicochemical ammonia nitrogen removal agents (also called ammonia nitrogen removers) are added to improve efficiency, especially when wastewater has high ammonia loads, low temperatures, or limited space for biological treatment.
Common ammonia nitrogen removal agents and methods include:
- Magnesium-based precipitants (e.g., MgCl₂ + Na₂HPO₄ or MgO): These form struvite (MgNH₄PO₄·6H₂O), a slow-release fertilizer. This chemical precipitation method is known as the “struvite process” and is very effective for ammonia recovery.
- Chlorine-based oxidants (sodium hypochlorite, chlorine dioxide): They convert ammonia into nitrogen gas through breakpoint chlorination (2NH₃ + 3Cl₂ → N₂ + 6HCl).
- Zeolites and ion-exchange resins: Natural or modified zeolites selectively adsorb ammonium ions (NH₄⁺) and can be regenerated for repeated use.
- Metal salts (aluminum or iron salts): Used in chemical-enhanced primary treatment to co-precipitate ammonia with phosphate and organic matter.
- Advanced oxidation processes (AOPs): Combinations of ozone, UV, or H₂O₂ that indirectly help oxidize ammonia under specific conditions.
In modern municipal wastewater treatment plants, a combination of biological treatment and chemical ammonia nitrogen removers often achieves the best results. For example, adding magnesium and phosphate salts to anaerobic digester supernatant can recover more than 90% of ammonia as struvite while reducing the nitrogen load returned to the main treatment process.
With increasingly strict discharge standards (often below 1–5 mg/L NH₄-N), ammonia nitrogen removal agents have become essential tools to ensure reliable compliance, protect receiving waters, and in some cases recover valuable nutrients.
In summary, the proper selection and application of ammonia nitrogen removal agents significantly improve the performance and sustainability of municipal wastewater treatment, helping cities meet environmental regulations while turning a pollutant into a potential resource.
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