Sun, salt and lime sounds like the beginnings of a cocktail recipe, but for some, it could mean cleaner, life-sustaining water.
In many developing countries, the only source of water is contaminated with viruses and bacteria. In fact, the United Nations estimates that 1 in 6 people don't have access to enough fresh drinking water.
Pouring water into clear plastic bottles and placing them in the sun can kill disease causing organisms in about six hours. It's a simple and cheap method that's been around forever, and it helps. (Who says sun tea isn't safe?)
But there's a hitch – the water has to be clear enough for the sun's rays to penetrate – and much of the world's water supply is murky from the clay soils in riverbeds and lake bottoms that mix with the water. Enter the scientists.
"Basically, you need to be able to read a newspaper through it. That means it's clear enough for the UV radiation to penetrate and kill the pathogens. If you can't see through it, it just won't work," explains Joshua Pierce, associate professor of materials science and engineering at Michigan Tech.
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."
Perhaps not so intuitive to the average person, but it's a process that may be more familiar to makers of wine, where bentonite is commonly used to clarify wine of impurities.
Drinking salty water may not sound all that healthy — or tasty, but Pierce says compared to the health risks posed by pathogens in dirty water, the taste and the health effects of a little added salt are not a big deal. He says in most cases the amount of salt is less than what is found in Gatorade – and by most people's standards — drinkable.
And if the salty water isn't all that palatable – or you're in a rush - perhaps a twist of lime will do the trick. A team of researchers from the Johns Hopkins Bloomberg School of Public Health reports that adding lime juice to water before placing it in the sun removed detectable levels of harmful bacteria such as E. coli significantly faster than solar disinfection alone.
Kellogg Schwab, director of the Johns Hopkins University Global Water Program and one of the study's authors, explains that chemicals called psoralens present in limes and other fruits and vegetables have been known to kill pathogens in blood for some time, so why not water?
Schwab and medical student Alexander Harding discovered that adding a half a Persian lime to a two-liter bottle of water reduced the disinfection time in the sun from six hours to a half an hour. That's just about the same amount of time it takes to boil water, and much more energy efficient.
"It's a relatively simple add-on to a treatment that's already widely accepted," Schwab said.
Schwab says that Psoralens are present in varying degrees in a variety of citrus fruits – and are concentrated mostly in the skin. And although Persian limes may not be available everywhere, he doesn't rule out the possibility that other indigenous sources of psoralens could be used.
But the method is imperfect – while it was a very effective killer of the bacteria E. coli, the lime juice did not seem to have any effect on diarrhea-causing viruses like the norovirus. When in doubt, Schwab says, just leave it in the sun longer.