A cheap and effective way to disinfect water in tropical countries

There are many ways to make water safe for drinking. One method that's starting to gain acceptance in tropical countries is solar water disinfection, or SODIS, because it is simple, practically free and uses low-tech, easy-to-find materials. The World Health Organisation, UNICEF, and the Red Cross recommend the SODIS method for treating drinking water in developing countries.
Pictogram showing the SODIS method
Pictogram showing the SODIS method by Samuel Luzi, Fundacion SODIS

How to apply the SODIS method to make safe drinking water

The process uses sunlight to disinfect water stored in common PET bottles which are used for soft-drinks. It works because sunlight contains radiation that kills pathogens including bacteria, viruses and parasites that cause diarrhea.
  1. Collect some colorless, transparent PET soft-drink bottles that will contain no more than 2 liters of liquid. Choose bottles that have few surface scratches and blemishes. Remove any labels and wash well with clean water. Sunlight may not penetrate the water adequately if the bottles are too large or heavily scratched.
  2. Fill the bottle three-quarters full with the water to be disinfected, screw on the cap and shake well for 20 seconds. After shaking, fill the bottle with more water but leave a space at the top for air, then screw on the lid once again. This step oxygenates the water which helps speed up the disinfection process.
  3. Expose the bottle to sunlight. A good place is on a sloped surface facing the sun, like a corrugated iron sheet roof. The length of exposure depends on your weather conditions.
  4. Once treated, the water should be stored in and consumed directly from the bottles to avoid re-contaminating the water.
Suggested exposure times
Weather conditions Minimum exposure duration
Sunny (less than 50% cloud cover) 6 hours
Cloudy (50-100% cloudy, with little or no rain) 2 days
Continuous rainfall The SODIS method cannot be reliably used to disinfect water

Important considerations

  • The SODIS method kills pathogens in the water but does not remove toxic chemicals.
  • Turbid (cloudy) water must first be filtered to remove the particles before being used to fill bottles.
  • Old bottles that are scratched or discolored should be replaced.
  • PET or PETE (polyethylene terephthalate) bottles should be used. The easiest way to find out if a bottle is made out of PET is to look for the recycling mark shown below. 1 is the resin identification code for PETE or PET.
    PET resin identification code: 1
    PET resin identification code stamp


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After a disaster, beware of diarrhea

The aftermath of a disaster can often kill more people than the disaster itself and one of the main risks is disease. Poor sanitation caused by the disruption often leads to outbreaks of acute diarrhea. We saw this during Typhoon Ondoy where diarrhea was one of the top killers in evacuation centers. It was also a huge problem in post-earthquake Haiti when a cholera epidemic infected 1,500 people in just a few days. Even during normal times, diarrhea is the 3rd leading cause of child illness and the 4th leading cause of deaths among children less than 5 years in the Philippines. Diarrhea kills through rapid dehydration and children are especially susceptible as they can succumb in a matter of hours. Nevertheless, deaths can be prevented by simply making sure that the patient drinks a lot of clean water with oral rehydration salts. Unfortunately, these are difficult to find after a disaster. Aid workers in Haiti were distressed to find that many were dying for want of something that costs so little.

Simple ways to safeguard your family

There's no reason why your family should suffer from an outbreak of diarrhea.
  • Stock up on Oral Rehydration Salts. These are available from Watsons and Mercury Drug for around Php10 to Php15 per sachet.
  • Stock up on antidiarrheal medications like Diatabs (Loperamide) which are less than Php30 for four capsules.
  • Make sure you have access to clean water for drinking and washing.
  • Eat cooked food or food washed well in clean water.
  • As much as possible, continue with your usual sanitation habits.
Oral Rehydration Salts and Diatabs
Oral Rehydration Salts and Diatabs

Household alternatives

If you don't have any commercially produced ORS at home, rehydrate.org suggests the following alternatives:
  • Breastmilk
  • Gruels (diluted mixtures of cooked cereals and water)
  • Carrot Soup
  • Rice water (congee)
  • Fresh fruit juice
  • Weak tea
  • Green coconut water (buko juice)
  • A home-made solution of salt, sugar and if possible, orange juice or mashed banana (see link for recipe and instructions)
Remember: make sure you first check with your pediatrician if these alternatives are suitable for your child.

Additional resources

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