The primary usage for a UV light is to disinfect filtered water at a certain flow rate. The Bacteria, viruses, and other microorganisms are destroyed by the UV interfering with the DNA and RNA in the organisms' reproductive cycles. Ultraviolet light kills 99% of harmful bacteria in point-of-use filter systems and is FDA approved for disinfecting filtered water. The easily replaceable bulb is housed in a stainless steel tube and has a 9000 hour life. If your water source is known to be unsafe with microorganisms and you want to have extra protection, then you can add an UV as an additional stage to disinfect water.
The UV lamp stays on continuously. As the water flows around the lamp, it is irradiated by the ultraviolet light. Water flow is restricted to 2 GPM to allow time for the UV light to be effective. The lamp slides into a quartz tube which is sealed from the water and a purple color shows through the plastic flow fittings while it is working. A 6-watt transformer powers the UV bulb.