Water constitutes about 80% of the earth’s surface. It comprises all kinds of sources of water such as oceans, rivers, lakes, glaciers, polar ice caps and ground water. This part of earth which comprises water is called hydrosphere. Most of earth’s water (about 97%) is contained in oceans which is unfit for human consumption due to high salt content. World total supply of fresh water that also includes glaciers and polar ice caps is about 2.5% only.
On the average, man consumes about 2 L of water daily. About 70% of human body is water. Survival of plants is not possible without water. Water in abundance is required for domestic use, agricultural activities and industries. Human activities have polluted much of this limited quantity of water Rain water on its way-down to earth also brings number of air pollutants which mix with water on the ground and pollute it. 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.
MAJOR WATER POLLUTANTS AND THEIR SOURCES
The various water pollutants are as follows:
1.Sewage and other oxygen-demanding wastes.
2.Infectious or disease causing agents.
4.Synthetic organic chemicals.
5.Inorganic minerals and chemical compounds.
6.Suspended solids or sediments.
Let us now discuss these pollutants in brief.
1. Sewage and other Oxygen-demanding Wastes
Sewage from municipalities and other oxygen-demanding waste from industry and agriculture comprise of organic matter which undergoes oxidation in rivers under the action
of micro-organisms. If the amount of sewage and other waste is relatively small, the dissolved oxygen (DO) is sufficient to decompose it and the products formed, CO2, SO42-, NH4+ NO3- ions do not cause much of the pollution. If, however the amount of sewage and other wastes is heavy, then the dissolved oxygen (DO) is insufficient and oxidation of organic matter is incomplete. The incomplete oxidation results in the formation of amines, ethane, H2S, etc., which produce wormy smell. Thus, sewage and other oxygen-demanding wastes become water pollutants on two accounts:
- They give rise to compounds which have an extremely offensive odour.
- They deplete the dissolved oxygen (DO) from water which is rendered harmful because water devoid dissolved oxygen cannot sustain aquatic life. It affects or even kill fish and other aquatic living beings.
1. Infectious or Disease Causing Agents
These are the various pathogenic microorganisms 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.
2. Plant Nutrient
The presence of plant nutrients in lakes and slow-moving water supports high population of aquatic plants which on decay produce disagreeable odour. Besides this, these plants also deplete dissolve oxygen (DO) making the survival of aquatic life problematic.
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.
3. Synthetic Organic Chemicals
These include pesticides, detergents and other industrial chemicals. These chemicals when present in water can act as toxic poisons for plants, animals and humans. These chemicals enter the hydrosphere either by losses during their transport and usage or by accidental or intentional disposal of wastes
5. Inorganic Minerals and Chemical Compounds
These include various metals and metallic compounds released from human activities or from natural minerals. These pollutants enter the water bodies from municipal and industrial waste waters and mine runoff. The acid-rain consisting sulphate and nitrate ions make the water acidic. Most of these inorganic compounds particularly, those of heavy metals such as cadmium, mercury, chromium, lead and silver, etc., are toxic. These substances get attached to the tissues of aquatic organisms, produce physiological poisoning and are, therefore, capable of killing the living organisms in water bodies. Alkalies discharged by industries such as textiles tanneries, paper, etc., can also destroy aquatic life. The main constituents of mine drainage are sulphuric acid and iron compounds. These cause corrosion of metals and concrete and are fatal to fish.
In the recent years the levels of fluorides in the drinking water has considerably increased in different parts of India causing fluorosis (an incurable bone disease). The toxic elements which are commonly present in municipal an, industrial waste along with their adverse effect are given in Table 52.2.
Table 52.2. Toxic Elements Commonly Present in Municipal and Industrial Waste Waters
|Arsenic||Pesticides, chemical wastes, mining by-product.||Enzyme-inhibitor, carcinogenic|
|Beryllium||Nuclear power and space industries, coal||Toxic carcinogenic.|
|Boron||Industrial wastes, detergent formulations, coal.||Toxic to some plants.|
|Cadmium||Industrial discharge, metal plating, Ni-Cd batteries, mining waste||Causes high blood pressure, kidney ‘malfunctioning, anaemia, disorder of bone marrow.|
|Chromium||Metal plating industries.||Cr(VI) is carcinogenic|
|Zinc||Metal plating industries, industrial waste.||Toxic to plants.|
|Copper||Metal plating industries, mining.||Not very toxic to animals, toxic to plants and algae.|
|Fluorine||Natural geological sources, industrial waste.||Causes bone damage, mottled teeth.|
|Lead||Plumbing, mining, coal, gasoline.||Causes anaemia, kidney malfunctioning, nervous disorder.|
|Mercury||Pesticides, coal, industrial waste, mining.||Highly toxic.|
6. Suspended Solids or Sediments
Suspended solids in water are mainly sand, silt and minerals eroded from the land. Solid particles settle in reservoirs and dams and thus reduce their water storage capacity. The suspended particles in water bodies also block the sunlight required by bottom vegetation for the photosynthesis and thus reduce the availability of food to fish.
7. Radioactive Substances
Radioactive substances can be carried into water from nuclear power plants, wastes of uranium and thorium during their mining and refining processes and also from medical and scientific institutions which utilise radioactive materials. These substances may cause radioactivity in living organisms and produce harmful effects.
8. Thermal Discharges
Large amounts of water are used for cooling purposes in thermal and nuclear power plants. Water is also used as a coolant in many industries. Cooling water is thus discharged at a raised temperature. The increased temperatures have several adverse effects on water. The rates at which chemical reactions occur increase considerably. This results in faster
assimilation of water wastes and hence faster depletion of dissolved oxygen which affects aquatic life. The density and viscosity of water decrease with increase in temperature. This results in faster settling of suspended solids. The rate of evaporation also increases appreciably with increase in temperature. This results in greater wastage of water in the
form of its vapour and so on.
Oil and oil wastes enter rivers and other water bodies from different sources such as oil refineries, storage tanks, automobile waste oil, petrochemical plants and industrial effluents. Normal tanker operations and spillage from oil tankar accidents cause marine pollution and shore contamination.
Since oil is insoluble in water, it floats and spreads rapidly into a thin layer. The oil layer on the surface of water reduces the DO levels in water as oxygen transfer from atmosphere
is prevented. At sea, oil layer is responsible for the death of birds. The oil penetrates the bird feathers thereby affecting their insulation and buoyancy. The birds experience difficulty in floating and flying. Oil may be driven to shores through wind and tides where it is accumulated and pose aesthetic problems.
10. Industrial Wastes
Industrial wastes and effluents contaminate water ill the following respects:
i. Heavy Metals. Metals such as Cd, Pb and Hg may be present in industrial or mining waste. These metals can prove poisonous to humans. Cadmium and mercury can cause kidney damage, and lead poisoning can cause damage to the-kidneys, liver, brain and central nervous system. All of these metals are cumulative poisons because the body does not excrete them and their concentration builds up.
ii. Detergents and Fertilisers. These may contain phosphates as additives. The addition of phosphorus to water, in the form of the phosphate anion PO43-, encourages the formation of algae, which reduces the dissolved oxygen concentration of water. This process, known as eutrophication, impedes the development of higher life forms, such as fish.
iii. Acid-polluted Water (pH < 3). This is deadly to most forms of aquatic life. Water downstream from amine may be contaminated by acid mine drainage, the result of microbial oxidation of discarded waste material at the mine site. Acid mine water principally contains sulphuric acid produced by the oxidation of iron pyrites (FeS2). Industrial wastes and acid rain may also contribute to the acidity of natural waters.
iv. Polychlorinated Biphenyls (PCBs). These chemicals are recent additions to the list: of contaminants of water. PCBs are generally used as fluids in transformers and capacitors because of their high stabilities. PCBs are resistant to oxidation and their release into the environment causes skin disorders in humans. They are reported to be carcinogenic.
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International Standards for Drinking Water
The quality of water is of vital concern for mankind since it is directly linked with human welfare. There are some international standards for drinking water, which must always be obeyed if water is to be used for drinking purposes. These are:
Fluoride. Soluble fluoride is often added to drinking water to bring it up to a concentration of 1 ppm or 1 mg dm-3. This concentration is within agreed safety limits because it protects teeth against decay. However, concentrations of fluoride ions above 10 ppm (mg dm-3) are poisonous and are harmful to bones and teeth.
Lead. The limit for the concentration of lead ions in drinking water is 50 ppb (µg dm-3). If water is relatively acidic and read pipes are used for water transport, then the water is liable
to get contaminated with lead
pH of drinking water should be between 5.5 and ease in the pH of the water increases the solubility ions.
Other metals. The maximum recommended levels of metals in drinking water, set by the European Commission (EEC), are as follows. .
|Max Conc. (ppm)||
Sulphate is harmless at moderate levels, but excessive sulphate (>500 ppm) is thought to have a laxative effect.
Nitrate Excess nitrate in drinking water can lead to globinemia (‘blue-baby’ syndrome). It also may be stomach cancer, although this link has not been proved. The EEC have set a maximum limit of 50 ppm for the nitrate ion in drinking water. .
CONTROL OF WATER POLLUTION
Some of the steps which are helpful for the control of water pollution are being described as follows:
Some examples are:
1. Waste water treatment techniques can be applied before the polluted water enters a river, lake or pool. Available waste water treatment processes can be physical, chemical or biological. Physical processes comprise screening, sedimentation, flotation and filtration. Commonly used chemical processes are precipitation, coagulation and disinfection. The biological processes are biological filtration
and the activated sludge process. In particular cases, the
processes such as carbon adsorption, oxidation and reduction,
ion-exchange, reverse osmosis, electrodialysis, etc., are also used.
2. Septic tanks should be used for each house. This will
reduce the flow of municipal sewage and human excreta into
river, lake or pool.
3. Rivers, lakes, etc., should not be used for bathing and
washing purposes. In this way, water sources do not get
polluted with detergents and germs.
4. Too much use of pesticides which are not biodegrable should be avoided. These are highly toxic substances.
5. Efforts should be made to increase the use of low grade or polluted water. Some examples are: Treatment of domestic sewage for industrial cooling; reuse of water in mining and similar industries where the water availability is less. These efforts will save the fresh water to be polluted.