Descripción
In this video you will get some real information about the typical survival book water filter. And a lot of more useful info about purifying water.
This type of survival filters are good, but they do not purify contaminated water into clean drinking water, as some books and videos claims. But they can filter away a lot of impurities, making the water ready for disinfection by boiling, chemical treatment, or UV-treatment.
About UV-desinfection:
The SODIS method(solar disinfection) is ideal for treating water for drinking in developing countries. All it requires is sunlight and PET bottles. How does it work? Clear PET bottles are filled with the water and set out in the sun for 6 hours. The UV-A rays in sunlight kill germs such as viruses, bacteria and parasites (giardia and cryptosporidia). The method also works when air and water temperatures are low. The World Health Organisation (WHO), UNICEF, and the Red Cross therefore recommend the SODIS method as a way to treat drinking water in developing countries. (Text from www.sodis.ch)
I will still recommend to keep the bottle(s) longer than 6 hours to be sure.
I just want to make this clear, cause in worst case, people can die cause of the wrong information out there.