First off, any water you may get from a rain barrel, your pool or a nearby creek should be considered contaminated. This water you collect may be dirty, or contain visible particles. A quick trick to make it look better is to pour the water through a coffee filter or a clean cloth. This won't make it safe to drink, but it can improve the clarity of the water, which will help the following disinfection methods work better. And remember, if you are very sensitive to chemicals, you can always just boil your water for 10 minutes instead of chemically treating it.