The goal of water purifying systems is to remove chemical and organic contaminants from drinking water. Some systems use mechanical filters, and in general this does not present any problems on Shabbat because filtering drinking water is not included in the prohibition of borair – separating waste from a desired substance.
Other purification systems work on a microbiological principle, using ultraviolet light to kill germs, and they are connected to an power source. Usually the light is turned on only when the water is flowing (detected by a flow sensor). The sensor usually turns the UV light on without the user being aware that anything special has happened.
Manufacturers who have obtained Shabbat approval from The Zomet Institute create a "Shabbat" status, where the sensor is disconnected and the UV light is on all the time, whether the water is flowing or not. On weekdays, the device is returned to normal operation.
When the light is on constantly, it is necessary to make sure that the nearby water will not be "cooked" by the heat of the bulb. Tests carried out in the laboratories of The Zomet Institute show that the water remains below a temperature of 40 degrees C, well below the temperature where a hand is scalded. Thus, the heat does not present any halachic problem.
For more details, contact The Zomet Institute.