Water Pollution
Water Pollution
Water is essential for life. Without water there would be no
life. We usually take water as granted for its purity, but we must ensure the
quality of water. Pollution of water originates from human activities. Through
different paths, pollution reaches surface or ground water. Easily identified
source or lace of pollution is called as point source, e.g., municipal and
industrial discharge pipes when pollutants enter the water-source. Non point
sources of pollution are those where a source of pollution cannot be easily
identifies, e.g., agricultural runoff (from farm, animals and crop-lands), acid
rain, storm-water drainage (from streets, parking lots and lawns), etc. Below
table shows the major water pollutants and their sources.
Cause of Water Pollution
(i) Pathogens: The most serious water pollutants
are the disease causing agents called pathogens. Pathogens include bacteria and
other organisms that enter water from domestic sewage and animal excreta. Human
excreta contain bacteria such as Escherichia coli and Streptococcus faecalis which
cause gastrointestinal diseases.
(ii) Organic Wastes: The other major water
pollutant is organic matter such as leaves, grass, trash etc. They pollute
water as a consequence of runoff. Excessive phytoplankton growth within water
is also a cause of water Pollution. These wastes are biodegradable.
The large population of bacteria decomposes organic matter
present in water. They consume oxygen dissolved in water. The amount of oxygen
that water can hold in the solution is limited. In cold water, dissolved oxygen
(DO) can reach a concentration up to 10 ppm (parts per million), whereas oxygen
in air is about 2000,000 ppm. That is why even a moderate amount of organic
matter when decomposes in water can deplete the water of its dissolved oxygen.
The concentration of dissolved oxygen in water is very impotent for aquatic
life. If the concentration of dissolved oxygen of water is below 6 ppm, the
growth of fish gets inhibited. Oxygen reaches water either through atmosphere
or from the process of photosynthesis carried out by many aquatic green plants
during day light. However, during night, photosynthesis stops but the plants
continue to respire, resulting in reduction of dissolved oxygen. The dissolved
oxygen is also used by microorganisms to oxidize organic matter.
If too much of organic matter is added to water, all the
available oxygen is used up. This causes oxygen dependent aquatic life to die.
Thus, anaerobic bacteria (which do not require oxygen) begin to break down the
organic waste and produce chemicals that have a foul smell and are harmful to
human health. Aerobic (oxygen requiring) bacteria degrade these organic wastes
and keep the water depleted in dissolved oxygen.
Thus, the amount of oxygen required by bacteria to
break down the organic matter present in a certain volume of a sample of water
is called Biochemical Oxygen Demand (BOD). The amount of BOD in the water is a
measure of the amount of organic material in the water, in terms of how much
oxygen will be required to break it down biologically. Clean water would have
BOD value of less than 5 ppm whereas highly polluted water could have a BOD
value of 17 ppm or more.
(iii) Chemical Pollutants: As we know that water is an excellent
solvent, water soluble inorganic chemicals that include heavy metals such as
cadmium, mercury, nickel etc. constitute an important class of pollutants. All
these metals are dangerous to humans because our body cannot excrete them.
Over the time, it crosses the tolerance limit. These metals he can damage kidneys, central
nervous system, liver etc. acids (like sulphuric acid) from mine drainage and
salts from many different sources including raw salt used to melt snow and ice
in the colder climates (sodium and calcium chloride) are water soluble chemical
pollutants.
The organic chemicals are another group of substances that
are found in polluted water. Petroleum products pollute many sources of water
e.g., major oil spills in ocean. Other organic substances with serious impacts
are the pesticides that drift down from sprays or runoff from lands. Various
industrial chemicals like polychlorinated industrial chemicals like
polychlorinated biphenyls, (PCBs) which are used as cleansing solvent, detergents
and fertilizers add to the list of water pollutants. PCBs are suspected to be
carcinogenic. Nowadays most of the detergents available are biodegradable.
However, their use can create other problems. The bacteria responsible for
degrading biodegradable detergent feed on it and grow rapidly. While growing,
they may use up all the oxygen dissolved in water. The lack of oxygen kills all
other forms of aquatic life such as fish and plants. Fertilizers contain
phosphates as additives. The addition of phosphates in water enhances algae
growth. Such profuse growth of algae covers the water surface and reduces the
oxygen concentration in water. This leads to anaerobic conditions, commonly
with accumulation of abnoxious decay and animal death. This, bloom-infested
water inhibits the growth of other living organisms in the water body. This process
in which nutrient enriched water bodies support a dense plant population, which
kills animal life by depriving it of oxygen and results in subsequent loss of
bio-diversity in known as Eutrophication.
International standards for Drinking Water
The International standards for drinking water are given
below and they must be followed.
Fluoride: For drinking purposes, water should
be tested for fluoride ion concentration. Its deficiency in drinking water is
harmful to man and causes diseases such as tooth decay etc. soluble fluoride is
often added to drinking water to bring its concentration up to 1 ppm or 1 mg dm-3. The F- ions make the enamel on teeth much harder by converting
hydroxyapatite, [3(Ca3(PO4)2. Ca(OH)2]the enamel on the surface of the teeth, into much harder fluorapatite, [3(Ca3(PO4)2.CaF2 ] however, F- ion concentration above 2 ppm causes brown motting of teeth. At the
same time, excess fluoride (over 10 ppm) causes harmful effect bones and teeth,
as reported from some parts of Rajasthan.
Lead: Drinking water gets contaminated with
lead when lead pipes are used for transportation of water. The prescribed upper
limit concentration of lead in drinking water is about 50 ppm. Lead can damage
kidney, liver, reproductive system etc.
Sulphate: Excessive sulphate (500 ppm) in
drinking water causes laxative effect, otherwise at moderate levels it is harmless.
Nitrate: The maximum limit of nitrate in drinking water is
50 ppm. Excess nitrate in drinking water can cause disease such as
methemoglobinemia (‘blue body’ syndrome).
Other metals: the maximum concentration of
some common metals recommended in drinking water are given below table.
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