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Water Pollution

Water pollution is the process of contamination of water of natural or artificial water resources with harmful substances. Water is essential for life. Without water there would be no life. Quality of drinking water is very important for human welfare. The pollution of water by sewage has been linked to the spread of diseases such as cholera and typhoid fever. The major water pollutants are In addition to the below given pollutants, industrial wastes also contaminate water:-

Pollutant

Source

Microorganism

Domestic sewage

Organic wastes

Domestic sewage, animal waste, decaying animals, plants and discharge from food processing factories

Plant nutrients

Chemical fertilizers

Toxic heavy metals

Industries and chemical factories

Sediments

Erosion of soil by agriculture and strip mining

Pesticides

Chemical used for killing insects, fungi & weeds

Radioactive substances

Mining of Uranium containing minerals

Heat

Water used by industrial plants which is discharged as hot water

(i) Heavy Metals: Metals such as Cd, Pb & Hg are known as heavy metals. These may be present in industrial  or mining waste. These metals are highly toxic. Cadmium and mercury can cause damage to kidney, liver, brain and central nervous system. All of these metals are cumulative poisons, the body does not excrete them and their concentration builds up.

(ii) Detergents & Fertilizers: These may contain phosphate 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. The process is 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 a mine may be contaminated by acid mine drainage, the result of microbial oxidation of  discarded  waste material at the mine site. Acid mine water principally contain sulphuric acid produced by the oxidation of iron pyrites (FeS2). Industrial wastes and acid rain may also contribute to the acidity of natural water.

(iv) Polychlorinated biphenyls (PCBs):  These chemicals are relatively recent additions to the list of contaminants of water. Having high stabilities, PCBs find many applications, for example they are used as fluids in transformer capacitors. PCBs are reported to be carcinogen.

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 1mg dm-3. This concentration is within agreed safety limits and has been shown to protect teeth against decay.  High concentrations of fluoride are poisonous  and are harmful to bone and teeth at levels over 10 ppm.

Lead: : Drinking water gets contaminated with lead when lead pipes are used for transportation of water.The limit for the concentration for lead ions in drinking water is 50 ppb. If water is relatively acidic and lead precipitates are used for water transport, then the water is liable to get contaminated with lead.

pH:  The pH of drinking water should be between 5.5 and 9.5. A decrease in the pH of the water increases the solubility of metal ions.

Other Metals: The maximum recommended levels of common metals in drinking water are:

Metals

 Max. concentration (ppm or mg dm-3)

Zn

5

Fe

0.2

Mn

0.05

Cu

3

Cd

0.005

Al

0.2

Sulphate: Sulphate in 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 methemoglobinemia (blue-baby syndrome). It also may be linked to stomach cancer, although this link has not been proved. A maximum limit of 50 ppm for the nitrate ion in drinking water has been set.

  • Biochemical Oxygen Demand (BOD) & Chemical Oxygen Demand (COD)

?The polluted water may contain  large amount of inorganic and organic compounds. Some of these can be oxidised by dissolved oxygen in presence of microorganisms.

?Biochemical Oxygen Demand (BOD) : The measure of the total contamination caused by compounds which can be oxidised in the presence of microorganisms. The BOD is taken as a realistic measure of water quality – clean water would have a BOD value of less than 5 ppm whereas highly polluted river water could have a BOD value of 17 ppm or more.

A large amount of organic and inorganic compounds, however, are resistant to microbial oxidation. They, therefore, don’t contribute to the BOD, though their presence makes water unfit for consumption. The “BOD” is taken as a realistic measure of water quality. “Clean water”  would have a BOD value of less than 5 ppm whereas highly polluted water could have a BOD vale 17 ppm or more.

Chemical Oxygen Demand (COD) is the amount of oxygen required to oxidise and stabilize the organic and inorganic matter present in a certain volume of water sample.It is measured by using a chemical oxidising agent.

In COD determination, water sample is treated with a known quantity of an oxidising agent, usually potassium dichromate K2Cr2O7 in acidic medium. This reagent oxidise most of the polluting substances, including those which are resistance to microbial oxidation. The remaining K2Cr2O7 is determined by back titration with a suitable reducing agent like Mohr’s salt. From the concentration of K2Cr2O7 consumed, the amount of oxygen used in the oxidation may be calculated using following chemical equation.

K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O +3/2O2

The results are expressed in terms of amount of oxygen, in ppm, that would be required to oxidise the contaminants, this is called COD

You can also refer to  Organic Chemistry Revision Notes and IIT JEE Chemistry Syllabus

To read more, Buy study materials of Environmental Chemistry comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.

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