text: what is nh3-n/tkn? it is some type of chemical used in water treatment definition for nh3-n ammonia nitrogen - an inorganic, dissolved form of nitrogen that can be found in water, but in the presence of oxygen will convert to nitrate.  ammonia (nh3-n)  an inorganic form of nitrogen, is contained in fertilizers, septic system effluent, and animal wastes. it is also a product of bacterial decomposition of organic matter. ammonia nh3-n (nh3-n) becomes a concern if high levels of the un-ionized form are present. in this form nh3-n can be toxic to aquatic organisms. the presence of un-ionized ammonia is a function of the nh3-n concentration, ph, and temperature. conversion of nh3-n to nitrite nitrogen by nitrification requires large quantities of oxygen which can kill aquatic organisms due to the lowered dissolved oxygen concentrations in water. nh3-n concentrations are reported in mg/l nh3-n.