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Ultra-Violet Disinfections

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Principles of UV Disinfection

In a typical UV system, approximately 95 percent of the radiation passes through a special quartz glass sleeve and into the untreated water that flows in a thin film over the lamp. The glass sleeve keeps the lamp at an ideal temperature of 104 °F. UV radiation affects microorganisms by altering the DNA in the cells and impeding reproduction. UV treatment does not remove organisms from the water, it merely inactivates them. The effectiveness of this process is related to exposure time and lamp intensity as well as general water quality parameters. Since lamp intensity decreases over time with use, lamp replacement is a key maintenance consideration with UV disinfection. In addition, UV systems should be equipped with a warning device to alert the owner when lamp intensity falls below the germicidal range.

Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very effective disinfectant, although the disinfection can only occur inside the unit. There is no residual disinfection in the water to inactivate bacteria that may survive or may be introduced after the water passes by the light source. The percentage of microorganisms destroyed depends on the intensity of the UV light and the contact time. If material builds up on the glass sleeve, the light intensity and the effectiveness of treatment are reduced.

Either sediment filtration or activated carbon filtration should take place before water passes through the unit. Particulate matter, color, and turbidity affect the transmission of light to the microorganisms and must be removed for successful disinfection.

UV is often the last device in a treatment train (a series of treatment devices), following reverse osmosis, water softening, or filtration. The UV unit should be located as close as possible to the point-of-use since any part of the plumbing system could be contaminated with bacteria. It is recommended that the entire plumbing system be disinfected with chlorine prior to initial use of a UV system.