The presence of pollutants of various types in water has been subject of great concern. The most common pollutants are biodegradable organic matter. Sewage is generated where ever humans live. Biodegradable wastes are also generated by innumerable industries.

The biodegradable pollutants constitute one of the greatest challenges to the environment. In addition to the industrial wastes, pollutants may also be found in all types of waste waters, e.g. Domestic waste, agricultural waste, pharmaceutical waste, tannery waste and chemical industry waste etc.

In order to control water pollution some engineering systems such as oxidation ponds, filter beds, septic tanks, water treatment plants have been used for removal of various pollutants from the sewage. It is needed now to have modern sewage treatment plants for removal of biodegradable as well as non-biodegradable pollutants so that pure water so obtained can be reused.

Water hyacinth has recently been used for purifying domestic and industrial waste water. The advantage of using this method is that the plant regenerates rapidly and has a tremendous capacity to accumulate heavy and radioactive metals. Apart from removal of pollutants, this plant has also been used as a new source of food, fertilizer and biogas.

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It is well known that synthetic herbicides, pesticides and fertilizers pose a challenge to natural degradative process, it has become essential to encourage an efficient and optimum use of organic manures. In recent times specific micro-organisms are being evolved by molecular breeding process to convert the chief ingredients of herbicides into carbon dioxide and chloride.

Certain bacteria are being used to get rid of cyanides and heavy metals from the polluted water. These bacterid absorb cyanides which are very toxic to fish and other aquatic life.

Amongst the several methods suggested for mitigating water pollution, it has been released that treatment of sewage is very important. The method which is being generally used for treatment of sewage involves three steps:

1. Preliminary Treatment:

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The principle objective of preliminary treatment are the removal of floating and suspended solid matter such as oil and grease if they are present in considerable quantities some physical methods which are commonly used are floatation, filteration and sedimentation and adsorption processes.

2. Secondary Treatment:

This involves biological method of purifying waste water where organic matters breakdown. In this method the organic matters are biologically degraded by different types of micro­organisms under controlled conditions.

The dissolved and colloidal organic matters are removed involving bacteria and other micro-organisms. Aeration is supplied to promote bacterial decomposition of organic compounds followed by chlorination to eliminate the bacteria.

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3. Tertiary Treatment:

The effluents of the secondary treatment are subjected to tertiary treatment. Thus, treatment is done in biological ponds in which waste water is allowed to stay for some time. The main objectives are as under:

(a) removal of bacteria

(b) removal of finely suspended solid

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(c) removal of dissolved inorganic solids

(d) removal of traces of organic substances.