A UV disinfectant system for treatment of water resident in a vessel. One or more UV lamps are mounted within the interior of the vessel so as to expose the water stored therein. The vessel may be, for example, a water tank, water heater, water filter or other component which holds a volume of water. A layer of photoreactive material may be formed on an inside surface of a wall of the vessel so as to be exposed to the UV radiation from the lamp. The photoreactive material may be tin dioxide, titanium dioxide, or other material that produces hydroxyl ions in response to UV exposure. The hydroxyl ions flow from the vessel into the piping and other components of the system so as to inhibit microbial action therein. The UV lamps may be mounted in quartz sleeves that surround the lamps for immersion in water and that are supported to protect the sleeves and lamps from vibration damage. The system may also be used to prevent freeze damage to the vessel and water system.