Ultraviolet light is a relative newcomer for in-the-home purification of drinking water, but the process has been used for many years commercially and in municipal water supplies. UV units are also well-known to pool and spa owners, where they are used as an adjunct to filtration and chemical disinfection. Although UV is a valuable tool in the battle against waterborne illnesses, it does have a few shortcomings.
How UV Works
Light in the ultraviolet spectrum penetrates the cell walls of a waterborne pathogen, rendering it inactive by interfering with its DNA. UV light is effective against bacteria, viruses and nearly all other microorganisms found in water, including those resistant to chemical disinfection such as cryptosporidium.
Since direct, intense exposure is required for the UV radiation to work, water must flow close to the UV source. Without regular maintenance, scale and biological film can build up on UV components, interfering with system efficacy.
Hard water or water that is high in iron content can absorb the UV rays before they are able to deactivate microorganisms. Water that is turbid or has excessive organic matter can also interfere with UV transmittance.
No Residual Disinfection
Unlike chlorine, bromine and other chemical disinfectants, UV light leaves no residual in the water. Though this is a positive in terms of possible chemical impact on health, no residual also means no continued protection against contamination after water leaves a UV-only system.
Because a UV system leaves no residual, there is no easy way to test the effectiveness of the process like there is with a chemical-based system. This means a UV system must be closely monitored to ensure the proper irradiance and exposure dose is being delivered.
Ultraviolet disinfection for the home is perhaps best used as an addition to a two- or three-pronged attack. In swimming pools, for instance, water that is chlorinated and tested for excess calcium hardness is filtered to remove organic material and then passed through a UV unit. For drinking water, UV disinfection gives added protection to reverse osmosis or other filtration systems.
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