• When the sewage discharge into a natural body of water the receiving water gets polluted due to waste product present in the sewage effluent, but the condition do not remain same forever, because natural forces of purification such as dilution, sedimentation, oxidation, reduction in sunlight etc go on acting upon the pollutant element & bring back water into it is original conditions this automatic purification of polluted water in due course is called as self purification of streams.

• Minimum length required for stream for self purification is 5km.

• Factors affecting are: dilution, temperature, sunlight, rate of oxidation etc.

• Various natural forces on which these natural forces of purification depend:

• Temperature
• Turbulence
• Hydrograph such velocity
• Available dissolved oxygen,
• Rate of re-aeration.