Causes of nitrogen pollution
Nutrient pollution is a big problem. It's widespread and costly to rectify. Excess nitrogen can enter water and atmospheric systems from a variety of sources and cause great harm. Nitrogen is a natural component of water and air environments - in fact, Nitrogen is the biggest component of the air we breathe forming 78%. Oxygen forms 21% and other gases make up the remaining 1%. In water, nitrogen is essential for normal growth of algae and aquatic plants. Because water can travel great distances, pollution in one place can create adverse impacts thousands of miles away. Streams, rivers, estuaries, coastal waters, bays, seas and oceans are all vulnerable.
Agricultural causes of nitrogen pollution
Animal manure, fertiliser applied to crops and field and soil erosion make agriculture one of the leading sources of nitrogen pollution.
Fertilisers make crops grow faster and bigger so that crop yields are increased. They are water-soluble minerals. They must be able to dissolve in water so that plants can absorb them through their roots.
Synthetic fertilisers are prepared in the lab by the reaction of an acid with an alkali.
Fertilisers provide plants with the essential chemical elements needed for growth particularly nitrogen, phosphorus and potassium.
Examples of fertilisers, their chemical formula [and the essential elements]:
- ammonium nitrate > NH4NO3 [nitrogen]
- ammonium sulfate > (NH4)2SO4 [nitrogen]
- ammonium phosphate > (NH4)3PO4 [nitrogen and phosphorus]
- potassium nitrate > KNO3 [potassium and nitrogen]
- urea > (NH2)2CO [nitrogen]
The world population is increasing all the time, so more food has to be produced. Without fertilisers the yields of crops would be reduced. However, fertiliser has a tendency to run off into surrounding water supplies.
Fertilisers increase crop production by replacing essential elements used by a previous crop or by boosting levels of such elements. Nitrogen in particular is needed to build plant proteins, increasing growth.
Fossil fuels and nitrogen pollution
Airborne nitrogen pollution affects not only the quality of the air we breathe, but also the land and the water.
Nitrogen is the most abundant element in the air and is essential to plant and animal life. Sources of nitrogen from human activities, such as electric power generation, industry, transportation and agriculture, can upset the natural balance of nitrogen in the environment.
When fossil fuels are burned, they release nitrogen oxides into the atmosphere, which contribute to the formation of smog and acid rain.
The most common nitrogen-related compounds emitted into the air by human activities are collectively referred to as nitrogen oxides. Ammonia is another nitrogen compound emitted to the air, primarily from agricultural activities, but also from fossil fuels.
Major sources of nitrogen oxide emissions include:
- cars and trucks
- coal-fired power plants
- large industrial operations
- ships and airplanes
The presence of excess nitrogen in the atmosphere in the form of nitrogen oxides or ammonia is deposited back onto land, where it washes into nearby water bodies.
These excess nutrients contribute to pollution, harmful algal blooms and oxygen-deprived aquatic zones. Excess ammonia and low pH* in these areas are toxic to aquatic organisms and affect their survival.
*pH: a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acid (0 the lowest value) and higher values more alkaline (14 the highest value). The pH is equal to −log10 c, where c is the hydrogen ion concentration in moles per litre. Moles measuring the concentration of a solute in solution.
Stormwater and wastewater nitrogen runoff
When precipitation occurs over urban areas, the water runs over surfaces which contain or harbour nitrogen and phosphorous pollution.
Sewer and sanitation infrastructure are tasked with processing significant volumes of waste. No system is 100% efficient and some nitrogen and phosphorous makes it through the filtration measures into waterways.
Nitrogen in wastewater mainly takes the form of ammonia, a chemical that is challenging to handle. Nitrogen is therefore normally removed in a biological process consisting of three steps – ammonification, nitrification and denitrification.
Ammonification. While traveling through sewer pipes, the majority of the nitrogen contained in raw sewage (urea and fecal material) is converted from organic-nitrogen to ammonia and ammonium through a process called hydrolysis (the chemical breakdown of a compound due to reaction with water).
Nitrification. The biological conversion of ammonium to nitrate nitrogen is called Nitrification. Nitrification is a two-step process. Bacteria known as Nitrosomonas convert ammonia and ammonium to nitrite. Next, bacteria called Nitrobacter finish the conversion of nitrite to nitrate.
Denitrification. The biological reduction of nitrate (NO3 ) to nitrogen gas (N2 ) by facultative heterotrophic bacteria is called Denitrification. “Heterotrophic” bacteria need a carbon source as food to live. “Facultative” bacteria can get their oxygen by taking dissolved oxygen out of the water or by taking it off of nitrate molecules.