Ultraviolet light has been used for years by surgeons to sterilize their equipment and is now being used in HVAC ductwork to purify air quality moving through the home. Unfortunately, we did not find Ultraviolet water treatment system to be cost effective in sterilizing residential swimming pool water.
UV is different than both chlorine and ozone in the way it sterilizes water. It does not kill bacteria, algae or viruses by oxidation. Rather, UV light neuters organics in the water so they cannot reproduce or grow. Then they are either oxidized by chlorine or filtered out of the water by the filter. So the addition of some oxidizer is necessary when using UV for pool water.
The key to UV effectiveness is the flow rate through the UV chamber. The flow rate affects the resident exposure time of the UV light. The water must be exposed to the UV light for a proper amount of time to sterilize the water efficiently. If it moves too quickly, there is not enough exposure time to purify the water properly. But if it moves too slowly, not enough water is purified to make a difference in a large body of water like a pool. The drinking water standard for UV effectiveness is 30 millijoules per square centimeter inside the chamber, and that requires properly regulated flow rates for pool water especially.
Using a UV system without a regulating bypass manifold is not going to give good results. We have combined UV with hydrogen peroxide and even with ozone systems on the same pool, but, in the end, found that UV systems on swimming pools are not cost effective since any savings are hypothetical and hard to prove.