Total Kjeldahl Nitrogen (TKN)

Total Kjeldahl Nitrogen (TKN) is an analytical measure of the nitrogen in a sample as the sum of organic nitrogen and ammoniacal nitrogen (NH₃/NH₄⁺), named after the Kjeldahl digestion method used to determine it. It does not include nitrate and nitrite nitrogen — those are measured separately. TKN is a key effluent parameter for nitrogen pollution, with an inland-surface-water and marine-coastal discharge limit of 100 mg/L.

Nitrogen in effluent matters because it drives eutrophication — excess nutrients in a water body fuel algal blooms that, on dying and decomposing, strip oxygen and kill aquatic life. Ammoniacal nitrogen is additionally directly toxic to fish as free ammonia. TKN captures the reduced (organic and ammonia) forms of nitrogen, which together with nitrate make up the total nitrogen load a discharge contributes.

For recyclers, TKN is central to the CBG/biogas sector. Anaerobic digestion of nitrogen-rich feedstock (poultry litter, food waste, certain crop residues) produces digestate high in ammoniacal nitrogen, and the liquid fraction carries a substantial TKN load. This is a double-edged number: as a discharge it must be controlled to the 100 mg/L limit, but as a fertiliser input that same nitrogen is the digestate's value. Food-processing and other organic-waste pre-processing streams also carry TKN.

The practical implication is that a CBG operator should manage TKN according to the effluent's destination. For land application/fertigation, the high nitrogen is an asset to be used at agronomically appropriate rates (excess causes nutrient burn and groundwater nitrate). For water discharge, nitrogen must be removed by nitrification-denitrification or other treatment to meet the 100 mg/L TKN limit. The same nitrogen that is fertiliser value on land is a pollutant in a river, so the disposal route determines whether TKN is managed as a resource or removed as a contaminant.