The process of automatic purification of polluted water in due course of time by the stream itself which has been polluted is called as self purification process. Following are the force (factors) that affects self purification process:

• Dilution & dispersion: When the putresible organic matter is discharged into a large volume of water contained in the river stream it gets rapidly dispersed and diluted. Such action results into the decrement in pollution concentration.

• Sedimentation: Heavier settable solids settle at the river bed and hence near the outfall of river, the concentration of settleable solids decreases which affects the self purification process.

• Sunlight: Aquatic plants absorb sunlight and carries out the process of photosynthesis with the help of chlorophyll. Such process of photosynthesis supplies a valuable amount of O2 to the stream i.e. increases DO content of stream.

• Oxidation: The oxidation of the organic matter present in sewage effluent will start as soon as the sewage outfalls into the river water containing dissolved oxygen. The deficiency of O2 so created will be filled by atmospheric O2.

• Reduction: Reduction occurs due to hydrolysis of organic matter settled at the bottom either chemically or biologically. Anaerobic bacteria will help in splitting the complex organic constituents of sewage into liquids and gases and thus making the ways for their ultimate stabilization by oxidation.

Control measures of stream pollution:

• Administration of water pollution control should be in the hands of state or central government.

• Scientific techniques should be adopted for environmental control of catchment areas of rivers, ponds or streams.

• Industrial plants should be based on recycling operations as it helps prevent disposal of wastes into natural waters but also extraction of products from waste.

• Plants, trees and forests control pollution as they act as natural air conditioners.

• Trees are capable of reducing sulphur dioxide and nitric oxide pollutants and hence more tress should be planted.

• Laws, standards and practices should be established to prevent water pollution and these laws should be modified from time to time based on current requirements and technological advancements.