Major Water Pollutants

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Water constitutes nearly three-fourths of the Earth's surface and it composes a large proportion of all living plant and animal matter. Approximately 70% of human body is water. Man drinks about 2 liters of water daily and it is required in abundance for domestic use, agricultural activities and industries. The part of Earth that comprises of water is called hydrosphere. All kinds of sources of water including oceans, rivers, lakes, glaciers, polar ice caps and groundwater makes it. About 97% of Earth's water is contained in oceans and is unfit for human consumption due to high salt content. Only 2.5% of the worlds total supply of water constitutes fresh water, which also includes inaccessible polar ice caps and mountain glaciers.

Survival of flora and fauna is not possible without water. As it is our most vital natural resource in both biological and chemical reactions, water pollution is a serious problem.

Water pollution refers to the presence of any foreign substance (organic, inorganic, radioactive or biological) in water which produce harmful effect and decrease the usefulness of water.

Human waste was the first water pollution problem since communities often drank from the same river in which they disposed their wastes. Human activities during the last century have polluted much of this limited quantity of water. Rainwater on its way down to Earth also brings number of air pollutants, which mix with water on the ground and pollute it

Major Water Pollutants

Current water pollution issues can be divided into the following categories:

• Sewage and other oxygen-demanding wastes,

• Infectious or disease causing agents,

• Plant nutrients,

• Synthetic organic chemicals,

• Inorganic minerals and chemical compounds,

• Suspended solids or sediments,

• Radioactive substances,

• Thermal discharges,

• Oil,

• Detergents.

Sewage and other oxygen-demanding waste

Oxygen-consuming wastes comprise of raw sewage from municipalities and other wastes from paper mills, tanneries, food-processing plants and agriculture; these cause a proliferation of aerobic decomposers who consume dissolved oxygen (DO) in water more rapidly than it can be replaced by the atmosphere. Organic matter undergoes oxidation in rivers under the action of micro-organisms.

Biochemical oxygen demand (BOD) is a measure of demand for oxygen utilized by micro-organism, during oxidation of organic matter. It is defined as the amount of oxygen in milligram per liter or in ppm used by micro-organisms to degrade the organic matter. A high BOD value indicates more polluted water.

The dissolved oxygen in rivers and other water bodies is usually sufficient to decompose only small or treated discharge of such waste. The products formed during aerobic decomposition e.g. CO₂, SO₄^2-, NH₄⁺ or NO₃^- ions do not cause much pollution.

If, however, the amount of sewage and other wastes are heavy, then the dissolved oxygen (DO) is insufficient and oxidation of organic matter is incomplete. Incomplete oxidation results in the formation of amines, ethane, H₂S, etc., which produce wormy smell. It also depletes the dissolved oxygen (DO) in water to levels, which cannot sustain aquatic life. If the level of dissolved O₂ falls below 5 ppm (parts per million), fish begin to die.

When we treat our sewage with chlorine to kill bacteria, we also kill marine ecosystems with the chlorine itself, or with carcinogenic chlorinated hydrocarbon by products.

Infectious or disease causing agents

These are the various pathogenic micro-organisms which may enter the water along with sewage or other wastes. These microbes, mainly bacteria and viruses, can cause various diseases such as cholera, typhoid, dysentery, gastroenteritis, polio, hepatitis etc. in the water supply drawn from these sources. The bacterium found in sewage (E-coli) has been known to affect shellfish and salmon populations in USA and causes severe food poisoning and gastroenteritis.

Plant nutrients

The presence of plant nutrients in lakes and slow-moving waters supports high population of aquatic plants that on decay, deplete dissolved oxygen (DO) making the survival of aquatic life problematic. Besides this, these dead plants also produce disagreeable odor.

The enrichment of water by nutrients is known as 'eutrophication'. Lakes and slow-moving waters age through eutrophication and over periods of several millennia, they get converted into swamps and marshes.

Synthetic organic compounds

Pesticides, detergents, chemical dyes and other industrial chemicals and their waste constitute synthetic organic compounds. When present in water, these chemicals can act as toxic poisons for plants, animals and humans. Such chemicals enter the hydrosphere either by usage, by accidental or intentional disposal of wastes from manufacturing units and by losses during their transport. In the USA at the turn of the century, lobster die-off occurred on Long Island. It may be linked to the runoff of the pesticide malathion, used to eradicate the West Nile virus-carrying mosquitoes, since crustaceans are closely related to insects.

Some laundry detergents and fertilizers such as nitrogen, phosphorus and other nutrients cause an overgrowth of algae, bacteria, and aquatic plants. This also leads to 'eutrophication' and eventually the lake can no longer support life.