This idea came after reading a article on the resurgence of Legionnaires' disease in France, where they say that you should let the hot water tap open several minutes (to flush the lines with water at more than 60 Celsius) when you want to use a building unoccupied for a long time.
I wondered if it exists a way to 'pasteurize' the water as it flows past instead of heating all the circuitry. For some reason, I though of microwaves rather than ultraviolet and this lead me to this variant of a tank-less water heater. It is not as efficient or powerful as a conventional electric heater, but it might be useful in cases where you want to avoid contact between the fluid and the heating element.