I used a UV pen when I went backwoods camping in Rocky Mountain National Park and didn't have any issues. I would fill up a big water bottle directly from a lak

I used a UV pen when I went backwoods camping in Rocky Mountain National Park and didn't have any issues. I would fill up a big water bottle directly from a lake or stream. You have to stir it in the water for about 60 seconds. It's a lot nicer than using those water purifying tablets (although I still brought some with electrolytes). The directions that came with it had big bold text saying it didn't kill viruses, which is normally not a concern for the type of water sources you encounter in national parks (at least in the US). Edit: Changed "UVP" to "UV Pen" to clarify.