Johan Kjeldahl developed in 1883 a method for the quantitative determination of nitrogen contained in organic substances and inorganic substances – ammonia NH3 and ammonium NH4+. He named the sum of all measured nitrogen TKN – total kjeldahl nitrogen. TKN plus Nitrate Nitrogen (NO3-), Nitrite Nitrogen (NO2-) and Nitrogen gas (N2) can be summarized in the nitrogen cycle which explains the conversion of nitrogen into multiple chemical forms as it circulates among atmosphere, terrestrial, and marine ecosystems. The conversion from ammonia to Nitrite and Nitrate is called nitrification and describes a 2-step oxidization process facilitated by heterotrophic bacteria. The conversion from Nitrate No3- to nitrogen gas N2 is called denitrification. Under the absence of oxygen certain microorganisms are forced to extract oxygen from Nitrate to convert it to Nitrogen gas.

TKN is a very important parameter for wastewater treatment. Although only ammonium is directly available for nitrification TKN should still be measured in the influent to the wastewater treatment plant as well because it also considers organic nitrogen. Organic nitrogen can convert to ammonium nitrogen and must therefore be included in the calculation otherwise an important contribution in ammonia loading to the biological reactor is missed out.

The transformation from organic nitrogen to ammonium is called ammonification. When a plant or animal dies or an animal expels waste, the initial form of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within the remains back into ammonium (NH4+). Therefore, its important to account for organic nitrogen and ammonium in the influent of a wastewater treatment plant.

Most permits in the US require to measure the Ammonia, Nitrate and Nitrite but not TKN in the effluent of wastewater treatment plants. Usual permit limits are less than 10ppm for Nitrate and Nitrite and less than 1ppm for ammonia.