Raw Water Storage:

This is regarded as a first stage in treatment as it involves a complex combination of physical, chemical and biological changes. Raw water storage has been regarded as an almost essential "first line of defense" against the transmission of water-borne disease. When water is stored in a reservoir, there is usually a great decrease in the number of bacteria of intestinal origin and the specific organisms of typhoid and other water-borne disease also disappear. To some extent, this effect is due to sedimentation but the bactericidal action of ultra-violet radiation and of light is of major importance near the surface and there are numerous biotic agencies, which play a major role in reducing the pollution of enteric micro-organisms. But prolonged storage of raw water sometimes causes growth in large numbers of various forms of algae, which increases difficulties in treatment.