Using the sun to disinfect water is a great way to purify water when you can’t make a fire but are in clear sunny area. Solar disinfection or SODIS is a method of using PET plastic bottles to purify water. The idea is you put water into the container and the radiation from the sun to break down the cellular structure of bacteria. This method is very popular in under-developed countries. This is a great way to purify water on the go.
The World Health Organization highly recommends this form of water purification.