A monovalent cation. Ammonium salts are very soluble and do not cause a RO scaling problem. The ammonium ion is the result of very soluble gaseous ammonia (NH3) being dissolved in water. Non-ionized ammonia ionizes in water to form the ammonium ion and hydroxide ion. The degree of ionization of ammonia to ammonium is dependent on pH, temperature, and the ionic strength of the solution. At higher pH the ammonia gas is prevalent, and being a gas, will not be rejected by a RO (similar to carbon dioxide gas). At lower pH the ammonium ion is prevalent and is rejected by a RO. Ammonia and ammonium exists in an equilibrium at varying relative concentrations in the general pH range of 7.2 to 11.5. Ammonium is typically not found in well water sources, having been converted by bacterial action in soils to the transitory nitrite (NO2) ion and then oxidized into the more prevalent nitrate ion. Ammonium is found in surface water sources at low levels (up to 1 ppm as the ion), the result of biological activity and the breakdown of organic nitrogen compounds. Surface sources can be contaminated with ammonium from septic systems, animal feed lot runoff, or agricultural runoff from fields fertilized with ammonia or urea.

        Ammonium is prevalent in municipal waste facilities with levels up to 20 ppm as the ion in the effluent, the result of high levels of organic nitrogen compound compounds and biological activity. Another source of ammonium is the result of adding ammonia to chlorine to form biocidal chloramines.