UV stands for UltraViolet. Ultravoilet light is a form of light found just beyond the visible light spectrum. When UV light is shone on to certain chemicals it causes a chemical reaction. It is those reactions that are harnessed to make UV sanitise swimming pool water.
UV light messes with DNA. When shone on to bacteria or an algae spore it will attack the DNA within the cell and prevent the cell from reproducing. UV light will also inactivate chlorine resistant parasites such as Cryptosporidium and Giardia. The now infertile cells are not removed from the water so a good filtration system is still a necessity.
As well as attacking DNA UV light also breaks down chloramines and other organic pollutants. The ability to break down chloramines (also known as Combined Chlorine) is a very useful feature.
Unlike a Salt Water Chlorinator a pool UV system will not leave any residual of sanitiser in the water. A salt chlorinator generates and re-generates chlorine but a UV system only treats the water within its chambers so Chlorine must still be added to the pool water. Because the UV system is taking out the algae, bacteria and chloramines the residual level of chlorine can be quite low, 0.5ppm is sufficient.
A pool UV system is only one tool in the amoury of the pool owner. Good filtration and an adequate turn over are still required but with all 3 in place you can expect an excellent level of water quality.
Inside the unit there are lamps that produce UV light and the pool water is passed over the lamps as it flows through the unit. Different lamps produce different wavelengths of UV light and some wavelengths attack DNA and some attack chloramines. The lamps are protected from the pool water by a quartz glass sleeve. This sleeve can get clogged with contaminates and so a means of removing the contaminants and cleaning the quartz sleeve has to be built in to the design of the UV unit.
The lamps are defined as being Low pressure or Medium pressure. Low pressure lamps deal with bacteria and Medium pressure lamps deal with chloramines. The lamps have useful lifetime and then must be replaced. A typical lifetime is about 8,000 to 10,000 hours. Good pool UV systems will have an indicator to tell you when the lamp is near the end of its lifespan.