We catch rainwater in a portable pool and rain barrels for water security. This water is normally used for watering the gardens but in an emergency it could be boiled or treated. I would normally just boil water, use a SteriPEN, or use the SODIS method with a glass container. (I also have a portable reverse osmosis filter, which I have never used.) If I had to chemically treat some water, I would use Lugol’s Solution which I keep in my medical kit for treating wounds. (Iodine is not safe for everyone and shouldn’t be used for long term water treatment.) If I had to treat a large amount of water, I got a big pail of “Pool Shock”! The Pool Shock is also good for making a powerful bleach solution.
More about SODIS: http://akvopedia.org/wiki/UV_treatment_/_Solar_disinfection_%28SODIS%29.
The picture is of my old pool. We just replaced it with a pool about twice the size. Hey, if you know anyone in Kamloops, BC, Canada that wants a FREE pool, have them contact me.