The pathways through which nitrogen-containing substances enter the water environment are mainly natural processes and human activities.
The natural sources and processes of nitrogen-containing substances entering the water environment mainly include precipitation and dust reduction, non-urban runoff and biological nitrogen fixation.
Human activities are also an important source of nitrogen in the water environment, including untreated or treated urban and industrial wastewater, various leachates and surface runoff.
Synthetic chemical fertilizers are the main source of nitrogen nutrients in water. Most of the nitrogen compounds that are not used by crops are mostly brought into groundwater and surface water by farmland drainage and surface runoff. With the development of industries such as petroleum, chemical, food and pharmaceutical, and the continuous improvement of people’s living standards, the content of ammonia nitrogen in urban domestic sewage and landfill leachate has risen sharply.
In recent years, with the development of the economy, more and more emissions of nitrogenous pollutants have caused great harm to the environment. Nitrogen exists in wastewater in the form of organic nitrogen, ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3–N) and nitrite nitrogen (NO2–N).