The goal of potable water disinfection is the destruction of pathogens (primarily bacteria) that can cause human disease. Disinfection is the final treatment process applied to a surface water supply before community use (following coagulation, sedimentation, and filtration) in most municipal water services. Many of the pathogens are already removed before disinfection by this multi-stage treatment process, allowing the chemical agents used in disinfection to be highly effective. Deep drinking wells are often not disinfected because underground environments, including the natural filtering of aquifers that occurs as groundwater moves through soils, are not conducive to pathogen survival. However, well water consumed via a community piping system or shallow wells subject to surface water infiltration should be disinfected.
By far the most widely used and cost-effective disinfecting agent is chlorine, although other methods of chemical disinfection, including ozone treatment, may be equally effective in killing pathogens. Chlorine kills pathogens such as bacteria, and, to a lesser understood and less quantifiable degree, viruses, by disrupting the chemical processes in their cells which are required for their basic life functions. A key attribute of chlorine that makes it desirable for drinking water disinfection is that it has good residual potency. This allows drinking water stored in utility pipes and tanks to remain protected from pathogens, as long as proper chlorine dosing has been applied.
Chlorine gas, when combined with water, will form hypochlorous acid and/or hypochlorite ions (the exact ratios of these two depends on water pH and temperature), as well as free hydrogen and chlorine. Hypochlorous acid and hypochlorite ions are important chemical compounds that disrupt and kill pathogens. An initial disinfection period known as contact time is required to ensure effectiveness. Hypochlorous acid and hypochlorite ions will be eliminated during the disinfection process by reactions with other chemicals in the drinking water. Appropriate dosing of chlorine gas will result in enough available residual chlorine compounds to continually protect the water supply after treatment, while it is being stored or distributed.
Chlorine residuals can also be introduced into the water supply through initial chemical compounds other than chlorine gas, including calcium or sodium hypochlorite. Each of these options for chlorination has different pros and cons, such as differences in the cost of chemical procurement, handling hazards, and dosing equipment installation and maintenance.
Chlorine disinfection of the world’s drinking water supplies has resulted in revolutionary trends in human health improvement. Water-borne bacteria such as cholera, which have resulted in untold human suffering for centuries, can be effectively eliminated as a concern in modern water supplies through proper disinfection.
See also: We Might Have A Drinking Problem