The various types of water pollutants are classified under following heads

Atmospheric water condenses and falls to earth as rain, snow or some other form of precipitation. Once on the earth’s surface, water flows into streams, lakes and eventually the oceans, or percolates through the soil and into aquifers that eventually discharge into surface waters.

Through evaporation from surface waters or by evapotranspiration from plants, water molecules return to the atmosphere to repeat the cycle. Although the movement through some parts of the cycle may be relatively rapid complete recycling of ground water must often be measured in geologic time.

In evaporation state water is nearly pure. Additional impurities are added as the liquid water travels through the remainder of the hydrologic cycle and comes into contact with materials in the air and on or beneath the surface of earth. Human activities contribute further impurities in the form of industrial and domestic wastes, agrochemicals and other contaminants. Ultimately, these impure waters will complete the hydrological cycle and return to the atmosphere as relatively pure water moleules.

However, it is water quality in the intermediate stage, which is of greatest concern because it is the quality at this stage that will affect human use of the water. The impurities accumulated may be in both dissolved and suspended from throughout the hydrological cycle.

Dissolved materials consist of molecules or ions that are held by the molecular structure of water. Suspended materials consist of particles larger than molecular size that are supported by buoyant and viscous forces within the water.

The various types of water pollutants are classified under following heads:

1. Organic Pollutants :

Organic pollutants are of several types:

(i) Oxygen Demanding Wastes:

These include domestic and animal sewage, biodegradable organic compounds and industrial wastes from food processing plants, paper and pulp, tanneries as well as agricultural run -off. These wastes are degraded and decomposed by bacterial activity in dissolved oxygen. Thus, depletion of dissolved oxygen occurs which is harmful to aquatic organisms.

(ii) Disease Causing Wastes:

These include pathogenic micro-organisms. These microbes consisting of bacteria and virus can lead to water borne diseases in human.

(iii) Synthetic Organic Compounds:

These are man-made materials like synthetic pesticide, insecticides, detergents, synthetic fibres, plasticisers, plastics etc. All of these are potentially toxic to plants, animals and humans.

(iv) Sewage and Agricultural run-off:

Sewage and run-off from agricultural lands which may stimulate the growth of algae and other aquatic weed in water. The D.O. is decreased and degradation of water quality occurs.

2. Radioactive Materials :

These originate from:

(i) mining and processing of ores.

(ii) increased used of radioactive isotopes in various fields.

(iii) Nuclear materials from nuclear power plants and nuclear reactors.

(iv) Radioactive materials from testing and use of nuclear weapons.

These radioactive materials can be toxic to life.

3. Inorganic Pollutants :

These are mineral acids, inorganic salts, finely divided metals, trace elements, cyanides, sulphates, nitrates complexes of metals, organometallic compounds. Algal growth in water and metal toxicity in aquatic ecosystem are influenced on them. Among the trace elements, metals such as Hg, Pb, As, Sb and Se, are toxic. They have a great affinity for sulphur and attack – SH groups in enzyme.