Ammonium is an inorganic nitrogen compound which arises, for example, in small sewage treatment plants from the biological degradation of organic nitrogen compounds. Ammonium can be converted into nitrate by adding oxygen (nitrification). In the aquatic environment, nitrification causes high oxygen consumption. In addition, ammonia, as a fertilizer, promotes the growth of algae. In sewage technology, usually only the nitrogen content of the ammonium, called ammonium nitrogen (NH4-N), is indicated. In small sewage treatment plants of effluent class C there is no threshold value for ammonium nitrogen. From effluent class N the limit value is 10 mg / l. Since the process of nitrification is very temperature-sensitive, limit values apply only at a wastewater temperature above 12 ° C.