How to Purify Water Using the Sun

Water – fresh clean pure water – is the most precious commodity on earth. Worth more than gold. Worth more than silver. Water is the very essence of life. Without water, we cannot live. Without water, we perish. But in many parts of the world, clean drinking water is becoming more and more difficult to find.  And even in developed countries, there are situations where we might find our drinking water contaminated. There are many ways in which to purify water for drinking. Here is a little known method that might come in handy some day.

Water does not need to be boiled to be made safe for drinking. Most microbes and pathogens will be killed at 150°F (65°C) for just five minutes.

Solar radiation can purify water and make it safe to drink.  Solar energy from the sun can disinfect water from lakes and rivers. The UV rays from the sun can kill harmful bacteria , parasites, and viruses, given enough exposure.

• Bacteria
  • E coli
  • Cholera
  • Salmonella
  • Shigella
  • Campylobacter jejuni
  • Yersinia e=Enterocolitica
• Parasites
  • Giardia
  • Crytosporidium
  • Entamoeba
  • worms
• Viruses
  • Rotovirus

The World Health Organization (WHO) recommends the following low tech method of disinfecting water.  Using only sunlight and PET plastic bottles, the SODIS (solar disinfection) method uses a combination of UV sunlight and increased temperature to kill pathogens in the water.

How To Purify Water Using The Sun

• The water should be from a clean, well known water source, such as a lake or stream. Running water is best. Avoid standing water; smelly water; water with an oily look; and water near latrines and sewers.
• The water must not be chemically polluted. This method will kill bacteria, it will not remove chemicals.
• If the water has visible contaminants, such as dirt, mud, or debris, filter the water by pouring it through several layers of cloth to catch as much sediment and debris as possible.  Then any remaining sediment settle to the bottom and filter until the water is as clear as possible. The UV rays will not be able to penetrate through cloudy, dirty water.
• Use clean,  clear soda or water bottles no larger than 2 liters. The bottles need to be in good condition with no scratches or other visible signs of wear.  It is best to use a “PET” (PolyEthylene Terephthalate) bottle. Look for the recycle code #1 on the bottom.
• Colored plastic and glass bottles will block too much of the UV rays. Indeed, some glass will block the UV rays completely
• Fill the bottles 2/3 full of water and shake for 20 -30 seconds. This will saturate the water with oxygen, which will help to kill the germs. Then finish filling the bottles to the brim with water. Lay them down in the sun. Do not stand them up. Laying them down allows the UV rays to penetrate to maximum depth.
• Although not necessary, if possible lay the bottles on a reflective surface to optimize the UV exposure.
• If the sky is mostly sunny with only a few clouds, then 6 hours of sunlight will be enough. However, if the sky is more than 50% cloudy, then it will take 2  days to purify the water.  The ambient air temperature is not a factor, it is the strength of the UV sunlight that is significant.  If there is a question about the proper length of time because of the sky conditions, it is best to err on the side of caution and allow the full 2 days of exposure.
• While there is some concern that toxins may leach out of the plastic bottle during the process, the Swiss Federal Laboratories For Materials Testing and Research have examined this process and found that the levels of toxins from the bottles is far below the WHO guidelines.

For more information on how to purify water using the sun, please read Solar Water Disinfection  or Water Pasteurization from SCInet or Water Pasteurization in a Pinch.

A simple device known as a WAPI (WAter Pasteurization Indicator) can also be used to determine when the water has been heated long enough to be safe.

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