Water is drawn out of the source and stored in natural or artificial reservoirs. Storage provides a reserve of water which further prevents pollution. As a result of storage, considerable amount of natural purification takes place. Purification can be physical, chemical and/or biological.
Physical: About 90 per cent of the suspended impurities settle down in 24 hours by gravity thus improving the quality. The water becomes clearer. This allows penetration of light, and reduces the work of the filters,
Chemical: Certain chemical changes also take place during storage. The aerobic bacteria oxidize the organic matter with the aid of dissolved oxygen. As a result amount of free ammonia is reduced and nitrates are increased.
Biological: Major benefit of storage of river water is reduction in total bacterial count by as much as 90 per cent in the first 5-7 days. The pathogenic organisms are gradually killed. The optimum total period of storage of river water is considered to be about 10-14 days. Long periods of storage, results in development of vegetative growths such as algae which impart a foul smell and awful colour to the water.