Ammonium (NH₄⁺) is an inorganic nitrogen compound.
An important source of ammonium is the degradation of protein compounds. These are supplied to wastewater treatment plants with the domestic and commercial wastewater.
In uncontaminated groundwater, ammonium is present in only very low concentrations or does not occur at all. Thus, ammonium serves as an important indicator of contamination in the water.
Ammonium itself is not directly toxic to living beings. However, it is in a pH-dependent equilibrium to ammonia:

NH₄⁺ + OH^- <-> NH₃ + H₂O.

The higher the pH, the more the balance shifts toward ammonia and the higher the toxicity.
In wastewater treatment, ammonia is conventionally degraded by microorganisms to nitrate. Alternatively, it may be partially converted to nitrite and then directly to elemental nitrogen in a deammonification process (EssDe^®).

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