There’s a lot of misinformation going around about UV sterilization of water, particularly when it concerns the presence of cysts such as Giardia and Cryptosporidium. These two (and there are others) are quite hardy in a straight bleach solution in cyst form. Some sources claim that sunlight will render stream or lake water safe to consume, but these two nasty critters are immune to UV radiation at the exposure rates found in the natural environment.
However, when UV light is used at industrial strength, as in a Steri-Light UV sterilizer, the outcome is much better. While passing through this unit (gotta have one large enough to assure adequate contact time) the right frequency of UV will strip away or scramble the organism’s DNA and it can no longer reproduce in a host. It eventually dies without starting a colony.
This all presupposes that the raw water has been filtered to the point where sediments have been removed so the little nasties don’t have a place to hide from UV exposure. Bulbs last a year in continuous service, and the ballast on the Steri-Light unit informs the user when it’s time to replace the bulb. [Hint: Have several spare bulbs, and an alternative energy capability so you can run the unit]
This article gets into the nitty gritty on UV sterilization. http://www.watertreatmentguide.com/ultraviolet_systems.htm
Pic 1 is a SteriLIght in service treating stream water that has been pre-treated and filtered down to .35 micron. The water is crystal clear when entering the sterilizer.