I have read about SODIS - after hearing about it here - but never tried it. It is reportedly only effective in latitudes around the equator (I'm too far north).

It would certainly be good to learn about SODIS, but I would want some quicker means of purification available if possible. Assuming you're talking emergency situations in hostile areas, I wouldn't count on sitting around in one place waiting for the SODIS method to work. I guess they don't use iodine anymore these days, which is what I used to use backpacking. But some other chemical means sounds like a better option for when you'll probably be on the move. I've never been in the military myself, but I assumed that water purification would be a topic that is taught. Along with providing you the means (chemicals) to do so while on the move.