8.4 Water Pollution

  Enviromental Chemistry

The 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 & typhoid fever. The given table lists the major water pollutants and their sources.

In addition, industrial waster also contaminate water.

(i) Heavy Metals

Metals such as Cd, Pb & 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.

(ii) Detergents & fertilizers

They may contain phosphates as additives. The addition of phosphorus to water in the form of the phosphate anion encourages the formation of algae which reduces the dissolved oxygen concentration of water. This process is known as eutrophication.

(iii) Acid polluted water (pH < 3)

This is deadly to most forms of aquatic life. Water down stream from a mine may be contaminated by acid mine drainage, the result of microbial oxidation of discarded waste material at the mine site. Industrial wastes and acid rain may also contribute to the acidity of natural waters.

(iv) Poly chlorinated biphenyls (PCBs)

These chemicals are relatively recent additions to the list of contaminants of water. Having high stabilities, PCB’s find many applications PCB’s are resistant to oxidation and their release into the environment causes skin disorders in humans. They are reported to be carcinogenic.

Importance of dissolved oxygen in water

The concentration of dissolved oxygen in water is highly important for the support of aquatic life. The lower is concentration of dissolved oxygen, the more polluted is the water sample. Oxygen gets into water through two sources:

(i) Dissolution of oxygen at the surface of water from the atmosphere.

(ii) From photosynthesis of aquatic plants.

Deoxygenation of water

The dissolved oxygen in water is consumed rapidly by microorganisms to oxidise organic matter of sewage.

Oxygen in water may be consumed by the bio – oxidation of the nitrogenous material and by the chemical or bio chemical oxidation of chemical reducing agents such as $F{e^{2 + }}$ ions or $SO_3^{2 - }$ ions.

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)

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

BOD, is a measure of the dissolved oxygen that would be needed by the microorganisms to oxidise these compounds. BOD, therefore is a 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.