Purpose of Disinfection:
The purpose of disinfection is to make the water safe for human consumption. Water carries a host of dissolved as well as suspended materials. Among these suspended materials are microscopic organisms, many of which have the potential to produce disease in humans. Diseases transmitted through water supplies are referred to as water-borne diseases. Due to the growth of human populations, it would be very difficult to find a surface water supply that has not been contaminated by both man and animals to some degree. Therefore it is necessary to disinfect water in addition to the other treatments that have previously been discussed.

Pathogenic Organisms (Diseases) Transmitted By Water:
Bacteria: Salmonella (salmonellosis), Shigella (bacillary dysentery), Bacillus typhosus (typhoed fever), Salmonella paratyphi (paratyphoid), Vibrio cholerae (cholera). Viruses: Enterovirus: Polio Virus, Coxsackie Virus, Echo Virus. Andenovirus, Reovirus, Infectious Hepatitis. Intestinal Parasites: Entamoeba histolytica (amoebic dysentery), Giardia lamblia (giardiasis), Ascaris lumbricoides (giant roundworm), Cryptosporidium.

Reasons For Using Chlorine:
Disinfection of water supplies in the United States is almost always accomplished by using chlorine. Disinfection with chlorine, combined with the other surface water treatment processes has greatly reduced the incidence of water-borne disease among humans in the United States. It is this proven record and the familiarity with chlorine that makes chlorine the disinfecting agent used at most systems. There are three basic reasons that chlorine is usually the disinfectant of choice.

ADVANTAGES
1. Chlorine is the most cost-effective disinfectant available considering its disinfecting power.
2. Chlorine is easily obtained through a variety of sources.
3. Chlorine produces a disinfecting residual.

DISADVANTAGES
1. Chlorine must be used and handled very carefully to prevent serious hazards to operators and the public.
2. Chlorine can sometimes form trihalomethanes (THMs) in water supples. Concentrations of THMs above the maximum contaminant levels (MCLs) are suspected of causing cancer.