Ultraviolet Light (UV) has been around since the early 80s and is used in multiple water treatment applications, but is still gaining popularity in the pool industry. Today UV serves as an effective secondary sanitation option on many pools and spraygrounds around the country. In some states UV treatment is becoming a requirement. California, for example, requires spraygrounds to have UV treatment installed on any discharge line that sends water to spray features or fountains.
At high levels UV becomes beneficial in water treatment applications due to its ability to eliminate combined chlorines and kill chlorine resistant pathogens.
While UV does break down free chlorine, the good form of chlorine used for pool disinfection, it is still highly recommended due to its ability to eliminate chloramines. Lower levels of chloramines lessens the corrosive off-gassing which attributes to the corrosion of deck equipment, HVAC, and internal structures of some facilities. Additionally, chloramines are typically responsible for the odor and irritation experienced by athletes, patrons and staff. Eliminating chloramines creates a healthier and more enjoyable environment for facility users and staff.
UV systems are typically recognized as one of two types, low and high pressure. The internal gas pressure and the amount of electricity supplied to the lamp determines the UV wavelengths that will be emitted from the lamp.
The wavelength and dosage of a UV system are the important factors that determine what exactly the system will eliminate from water. The difference between a medium and low pressure UV system lies in the wavelength of the lamp. Low pressure systems provide just one wavelength which only eliminates monochloramines, the first form of chloramine. However, medium pressure systems, which provide multiple wavelengths, will combat all three forms of chloramine. There are arguments for and against low pressure systems, with some people believing that dichloramines and trichloramines, the second and third forms of chloramine, cannot form once monochloramine is eliminated. In reality, the UV system is incapable of eliminating all combined chlorine from the pool system, so 
dichloramine and tricloramine will inevitably form. Once it is understood that all three forms of chloramines will be present in the pool, the benefits of using a medium pressure UV unit, with multiple wavelengths, becomes obvious. Besides the wavelength of the lamp, it is also important to understand its dosage. The UV dosage determines which pathogens will be eliminated from the water. If the intent of the UV system is to kill pathogens and chloramines, the recommended dosage is 60mJ/CM2 . This dosage is the recognized minimum required to eliminate chloramines. On indoor facilities this is very important due to the chloramines existing in a contained environment where they can potentially cause problems. On outdoor facilities the need to kill chloramines isn’t as important, so the minimum required UV dosage becomes 40mJ/CM2 . This dosage will adequately kill all major pathogens.
Not only with UV systems kill pathogens and chlormaines, but they will also eventually provide cost savings. It is common for a UV units to reduce the amount of chlorine used in a pool by 10%-15%. This is because the UV system is disinfecting the water and eliminating some of the work that would be done by the free chlorine.