Out at Welch Island we have a UV water purifier like one of these:
http://www.ultraviolet.com/water/sanitr01.htm
With the water in five to six months a year a UV bulb lasts for more that two years. There are two in line filters ahead of the UV unit, a sediment filter and a carbon filter. We change those filters twice a year.
The UV unit is more than 20 years old, no problems. We have the water tested once a year, in mid June, always comes back negative for bacteria.