Wastewater Knowledge: Total Nitrogen (TN)

Total nitrogen is the total amount of all forms of inorganic and organic nitrogen in water, including inorganic nitrogen such as NO3−, NO2− and NH4+ and organic nitrogen such as proteins, amino acids and organic amines, calculated as milligrams of nitrogen per liter. It is commonly used to indicate the degree of nutrient pollution of a water body. [1]

Total nitrogen content in water is one of the important indicators of water quality. Its determination helps evaluate the pollution and self-purification status of a water body. When nitrogen and phosphorus in surface water exceed limits, microorganisms multiply massively and plankton grow vigorously, leading to eutrophication. [1]

Main determination methods

2. Gas-phase molecular absorption spectrometry: mainly applied in laboratories.

3. Ammonia nitrogen, nitrate and nitrite can also be measured separately and summed as the total nitrogen result; a typical application is Germany’s WTW.

In environmental surface-water and monitoring fields, alkaline potassium persulfate ultraviolet spectrophotometry and its optimized variants are the current mainstream methods.

This article mainly introduces the gas-phase molecular absorption spectrometry method. [2]

1. This standard applies to the determination of total nitrogen in surface water, reservoirs, lakes and rivers. Detection limit 0.050 mg/L; lower limit of determination 0.200 mg/L; upper limit of determination 100 mg/L.

GB 11894–89 Water quality — Determination of total nitrogen — Ultraviolet spectrophotometry.

1. Alkaline potassium persulfate UV spectrophotometry (GB 11894-89): brands such as UK RAIKING and China RuiQuan are mainstream products optimized on this standard.

Definitions

3.1 Gas-phase molecular absorption spectrometry: under specified analytical conditions, the analyte is converted into a gaseous molecule and introduced into the measurement system; the method determines its absorption of characteristic spectral lines.

4. In alkaline medium at 120°C–124°C, add potassium persulfate oxidant to oxidize ammonia, ammonium salts, nitrite and most organic nitrogen compounds in the sample into nitrate, then determine total nitrogen by gas-phase molecular absorption spectrometry in the form of nitrate nitrogen.

Emission limits

13. Total nitrogen is the general term for nitrate nitrogen, nitrite nitrogen, ammonia nitrogen and organic nitrogen, and is a main indicator of water-body eutrophication. According to the "Discharge Standard of Water Pollutants for Heterocyclic Pesticide Industry," in areas where environmental carrying capacity is weakening or environmental capacity is small, ecology is fragile and serious pollution is likely, existing and new enterprises must meet a special total-nitrogen discharge limit of 30 mg/L. In the draft revision of the "Discharge Standard of Water Pollutants for Synthetic Ammonia Industry," the total-nitrogen requirement is: existing enterprises from 2009-01-01 to 2010-06-30 shall meet 50 mg/L, and from 2010-07-01 shall meet 30 mg/L; new enterprises from 2008-07-01 shall meet 30 mg/L; and enterprises in special areas shall meet 20 mg/L.

Note: detailed reagent preparation, instrumentation and procedural steps are specified in the original Chinese national standards (GB 11894 and the gas-phase molecular absorption spectrometry standard referenced above).