Nutrients are chemicals essential for the growth of living things. For plants there are sixteen elements required for their healthier growth.
They are carbon, hydrogen, oxygen, (obtained mainly through C02 and water from the soil), nitrogen, phosphorus, potassium, calcium, magnesium, sulphur and micronutrients such as iron, cobalt, manganese, boron, molybdenum and copper.
The nutrients can become a pollutant when their enrichment allows thick growth or blooms of aquatic weeds, especially algae that cover up water. This prevents the entry of sunlight into water bodies.
Aquatic plants, along with algae, thus die; the bacteria present in water now decompose all these dead plants and ultimately reduce the dissolved oxygen (DO) in water.
The decayed organic matter adds unwanted color, odour, taste and turbidity to the water and acceptability for domestic purpose is reduced.
The depletion of dissolved oxygen leads to the death of fishes, other aquatic animals and plants. The process of nutrient enrichment is known as eutrophication.
Among the sixteen elements, carbon, nitrogen and phosphorus are the most important nutrients which control the growth of algae blooms.
Carbon is available from the atmosphere as C02 and decaying of other organic matter cannot be controlled. It is nitrogen and phosphorus which can be controlled to check algae growth rates.
The major sources of nitrogen and phosphorus polluting water bodies are soaps and detergents, chemical fertilizers like urea, ammonium sulphate or nitrate, super phosphate, etc.
Besides eutrophication problems, if water contains enough nitrates (N03~) and is consumed by children, certain bacteria in the intestinal tract of infants can convert nitrates to highly toxic nitrites (N02~).
Hemoglobin has higher affinity for nitrites compared to oxygen, therefore it leads to replacement of oxygen by nitrites causing a bluish discoloration of the infant and is commonly referred to as ‘blue baby’ syndrome.
The phosphorus from chemical fertilizers as well as detergents helps algae growth, resulting in enormous blooms. In detergents, the phosphates are in the form of tripolyphosphate. This tripolyphosphate slowly reacts with water forming orthophosphate and is used by plants for their growth.