Skip to main content
Report this ad

See also:


All of us have heard about the terrible typhoon in the Philippines. What is the most important thing the survivors need first? If you answered water give yourself a congratulatory slap on the back. Next question. What item is found in great abundance all over the world? Plastic water bottles is the answer I’m looking for. Final question. What does the Philippines have practically every day year round ? Sunlite! Enter the remarkable process known as SODIS or Solar Water Disinfection.

Tests have shown that water placed in clear plastic bottles and exposed to direct sunlight will be purified. The sun’s UV rays kill bacteria, viruses, and parasites. Water put in a PET bottle (the kind most soda and water is bottled in) and left outside in the sun for six hours will kill 99.99 percent of the bacteria in it. The key ingredient is direct sunlight. SODIS can be effective year round between 35 degrees north and south latitudes.

The World Health Organization states that plastic bottles given the PET rating never exceed the limiting values for drinking water. The bottles must not be colored and glass bottles won’t work either because too much UV-A is blocked. Cloudy water should be filtered first by pouring it through a cloth to capture as much sediment as possible. My source for much of this information is from:

Further information below is from:

“Know your water source (as best you can)… if you believe it to be chemically toxic, don’t use it.Lay the bottle down in the sun. Do not stand them up. Ideally the bottles would by placed so that they face the the sun at a similar angle, to maximize the UV-A penetration.Even better… lie the bottles on a reflective surface to increase the UV-A exposure using direct and reflected sunlight. This is not necessary, however it would shorten the required time and ensure optimum UV-A exposure.If the sky is partly cloudy with only a few clouds, then 6 hours sunlight exposure will be enough. If the sky is half filled with clouds, or more, then 2 days will be required.

Note that the outdoor temperature does not matter, so long as the UV-A sunlight exposure has been 6 hours.

A clear plastic bottle filled with water, exposed to the sun for 6 hours will make the water safe to drink (see the caveat list). In fact, the effectiveness of terminating harmful bacteria is an amazing 5-Nines, that is, 99.999 percent!

List of germs that are terminated from UV-A sunlight exposure at 6 hours

  • Bacteria – Escherichia coli
  • Bacteria – Vibrio cholera
  • Bacteria – Salmonella
  • Bacteria – Shigella flexneri
  • Bacteria – Campylobacter jejuni
  • Bacteria – Yersinia enterocolitica
  • Virus – Rotavirus
  • Parasites – Giardia
  • Parasites – Cryptosporidium (needs 10 hours exposure)

List of caveats to UV-A sunlight water purification

  • This method kills germs. If the water is already contaminated with chemicals from pollution, the chemicals will remain.
  • If the water is cloudy and dirty, it should be filtered first to allow the UV-A rays to effectively penetrate into the water.
  • The plastic water bottle should be no bigger than 2 liters. In moderately cloudy water, UV-A will lose 50 percent effectiveness at a depth of 10 mm (about 0.5 inch), whereas UV-A will only lose 25 percent effectiveness at a depth of 10 mm in clear water. Just use a typical size soda bottle or water bottle.”

I think this information should be very useful to everyone. Who knows when the next disaster will strike cutting off safe drinking water supplies. Or who knows when you might find yourself short of safe drinking water when out on a long hiking trip. A little knowledge is a valuable thing.

Report this ad