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

by Sarah Zhang in Health & Medicine, Discover Magazine

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.

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