When sewage is discharged into a natural stream or river, the organic matter present in the sewage gets oxidized by bacteria and converted to simple, inoffensive, stable substances. In this process of oxidation the dissolved oxygen (DO) content of the river or stream water is utilized. Due to this deficiency of dissolved oxygen is created in river or stream water.
The deficiency of dissolved oxygen thus created in river or stream water is filled up by the absorption of atmospheric oxygen. Thus dissolved oxygen of river or stream water is consumed by sewage discharged into it and at the same time it is replenished by the atmosphere.
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This phenomena which occurs in all natural streams or rivers is known as self-purification of natural streams or rivers. It is thus seen that natural streams or rivers, polluted by sewage, are purified in natural course by the phenomenon of self-purification.
The rate of self-purification depends on various factors such as rate of re-aeration, type of organic matter present in sewage, temperature, velocity of flow, presence of available oxygen in receiving waters, sedimentation, etc.
The various actions involved in self-purification process are physical, chemical and biological in nature, and are as listed below:
(1) Dilution
(2) Oxidation
(3) Reduction
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(4) Sedimentation
(5) Action of sunlight
These actions are briefly explained below:
1. Dilution:
When sewage is discharged into a large volume of water flowing in a natural stream or river, it is dispersed and dilution takes place. Due to dilution the concentration of various constituents such as organic matter, BOD, suspend solids, etc., is reduced and thus the potential nuisance of sewage is also reduced.
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The concentration of any of these constituents in the resulting mixture of sewage and water flowing in stream or river is given by the following expression:
In the derivation of this expression conservation of mass and complete intermixing of sewage and water flowing in stream or river is assumed. Further this expression is applicable to dissolved oxygen content, BOD, suspended solids, and other characteristic contents of sewage.
When dilution factor (or dilution ratio) is quite high, large quantities of dissolved oxygen will be always available which will reduce the chances of putrefaction and pollutional effects. Thus because of dilution aerobic conditions will always exist. This will, however, not be so if dilution factor (or dilution ratio) is small, i.e., when large quantity of sewage is discharged into a small stream or river.
2. Oxidation:
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When sewage is discharged into a stream or river the organic matter present in the sewage is oxidized by aerobic bacteria utilizing dissolved oxygen of the water flowing in the stream or river. The deficiency of oxygen so created is filled up by atmospheric oxygen. The process of oxidation continues till the organic matter gets completely oxidized, the oxygen demand is then fully satisfied and the stream or river is said to have purified itself.
The stream or river heavily polluted by sewage gets purified in short time if it is capable of absorbing more oxygen rapidly as compared to the one which cannot absorb oxygen easily. Oxidation is the most important action responsible for effecting self-purification of stream or river polluted by sewage discharged into it.
3. Reduction:
The organic matter of sewage settled at the bottom is reduced to liquids and gases due to hydrolysis either chemically or biologically. Anaerobic bacteria split the complex organic matter or sewage into liquids and gases, and thus pave way for their stabilization by oxidation. Such reduction of complex organic matter assists the process of self-purification of stream or river polluted by sewage discharged into it.
4. Sedimentation:
The settleable solids contained in sewage drop down to the bottom of stream or river and are thus easily separated. Further the settleable solids are deposited in the form of sludge in which anaerobic decomposition may take place. The sedimentation in this way helps the process of self-purification of stream or river polluted by sewage discharged into it.
5. Action of Sunlight:
The sunlight has bleaching and stabilizing effects on bacteria. It also helps certain micro-organisms to derive energy from it, and through biological action convert themselves into food for other forms of life, and absorb carbon dioxide and release oxygen by a process known as photosynthesis.
Further sunlight acts as a disinfectant and stimulates the growth of algae which produce oxygen during daylight but utilize oxygen at night. Hence wherever there is algal growth water may be supersaturated with dissolved oxygen during daylight hours. However, during night anaerobic conditions exist. All such effects of sunlight assist in self-purification of stream or river polluted by sewage discharge into it.