Microorganisms can be found in raw water from rivers, lakes and groundwater. While not all microorganisms are harmful to human health, there are some that may cause diseases in humans. These are called pathogens. Pathogens present in water can be transmitted through a drinking water distribution system, causing disease in those who consume it. In order to combat waterborne diseases, different disinfection methods are used to inactivate pathogens.

Chlorination of wastewater for BOD reduction was practised as long as 1859 in England . A number of studies of BOD reduction by effluent chlorination where it was recognised as such were conducted the 1920s and 1930s . In the USA in the 1920s and 1930s and thereafter the chlorination of Sewage was in use for purposes of protecting water supplies, bathing beaches and shellfish layings situated at the sewer outfalls. Chlorination was reported to reduce Biochemical Oxygen Demand of sewage as well as to control odors.

Micro-organisms polluted swimming pools. Every swimmer adds 1.000.000 to 1.000.000.000 micro organisms to the water. The water itself contains microorganisms, as well. After oxidation a disinfectant must be added to the water to kill pathogenic micro organisms. Disinfectants used for swimming pool water disinfection must meet certain demands. They should be harmless and non-irritating to swimmers and attendants.

This is a practical technological option for developing countries because of low cost of chlorine gas here, A technology is based on laws of physical sciences and scientific principles which never get old or outdated. On the other hand, an effluent treatment plant (ETP) is basically a chemical process plant consisting of a number of unit operations and unit processes whose performances are governed by underlying principles of thermodynamics, transport processes, reaction kinetics, separation processes etc.