To Disinfect Water Cheaply, Just Add Sunlight (and Salt or Lime Juice)

By Sarah Zhang | May 16, 2012 10:41 am

The cheapest and easiest way to disinfect water? Sunlight. Just leave a clear glass or plastic bottle out in the sun for six hours. SODIS, or solar water disinfection, is an age-old method touted by the World Health Organization for areas where access to clean water is limited. UV rays in the sunlight tear apart the microbes to make water safe. Drink up!

SODIS is quite effective, but scientists have found two hacks that make the technique even better. One problem is that the water may be cloudy from sediment, which can be fixed with a dash of salt. NPR’s Salt blog explains:

Pierce and his colleagues discovered that by adding a little table salt to this murky water, they could get the particles of clay to stick together and settle to the bottom, making the water clear enough to purify using the solar disinfection method. They also found that the addition of salt works best for certain kinds of clay soils, namely bentonite, and not so well with others. But when they added a little bentonite along with salt to water that contained other types of clay soils, it worked just as well.

Pierce says the method works because bentonite clays have an electrostatic charge – which makes them attracted to the charged ions in the salt. When bentonite is mixed with other particles, they stick together, and the salt pulls everything out of the water.

“So basically you add dirt and salt, to make the water cleaner?” I asked him.

“Right,” said Pierce laughing, “It’s not exactly intuitive.”

Some lime juice, on the other hand, cuts down the amount of time necessary to disinfect a two liter bottle of water from six hours to just half an hour. Limes contain chemical compounds called psoralens, which have been shown to kill pathogens in blood and, now, also in water. Many fruits and vegetables, including citrus, have psoralens too, so the hack is not specific to limes. With common ingredients and some ingenuity, sunlight becomes an even more effective disinfectant.

[via NPR]

CATEGORIZED UNDER: Health & Medicine
MORE ABOUT: clean water, SODIS, solar, water
  • Dave F.

    When in need of clean water and only cloudy is available, I will be sure to ask for it with a lime and on the rocks (of salt). 😉

  • G Wilkins

    Does that mean adding lemons to your sun-tea while it’s brewing will negate the potential bacteria bloom that people are worrying about these days?

  • Michael A. Smith

    @G Wilkins – only if your sun-tea has salt and bentonite in it, and is not cloudy. Or are they saying you can use citrus *instead* of salt and bentonite?

  • tOM Trottier

    Pasteurising works too – raise it to 62C for a few seconds. Boiling is overkill.

  • Kevin N

    Sunlight is the best disinfectant. -Louis Brandeis

  • IW

    Any word on what chemicals are leeched into the water from the plastic bottle being exposed to sunlight?!

    Any word on what percentage of actual UV light makes it through the glass, if a glass bottle is used to avoid leeching of chemicals from the plastic ones?!

  • Mike

    @Michael A Smith – “lime juice, on the other hand, cuts down the amount of time necessary to disinfect a two liter bottle of water” – yes, instead of salt and bentonite. The last paragraph of the article explains how certain compounds contained in lime juice kill pathogens.

  • SC

    @IW, I expect if you’re in a position where you’re depending on disinfecting your water in a bottle with sunlight, you’re probably not in a position to be too picky about the possibility of a few chemicals from the bottle finding their way into the water as well. Thirst or dysentery will both kill you a lot faster than BPA will.

  • Jess

    yes they addressed in the video the concerns of this method. some resorted back to boiling because of the free radicals released by the plastic bottles. One man understated this by saying that we are constantly exposed to free radicals, but i personally would not drink these free radicals because why would i want to expose my body to these by drinking them….

  • Pippa

    It works even better if the bottles of water are placed on shiny corrugated steel, usually available anywhere as it’s use for roofing. Sorry if this was in the video – link here too slow to watch. (Will keep the link and see it when I get home!)


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