Under certain conditions, contaminated water can be made pure enough to disinfect cheaply by simply adding salt, according to a new study that holds promise for millions of people in developing countries who have little or no access to potable water.

The salt binds to contaminants in the water and clumps them together so they can be removed, after which the water is usually clear enough that it can undergo a low-tech, cheap and widely available cleansing method called solar water disinfection.

Lead author Brittney Dawney, who at the time of the research was an undergraduate engineering student at Queen's University in Kingston, Ont., and Joshua Pearce, a professor of materials science at Michigan Technological University, published the paper in the current issue of the Journal of Water, Sanitation and Hygiene for Development.

Their study looks at how to reduce the turbidity, or murkiness, of water contaminated with clay so that it can be disinfected with just the sun's rays. The researchers tested the effect of adding different amounts of table salt to a litre of clay-contaminated water.

They found that two grams of salt could remove up to 98 per cent of a particular type of clay called bentonite from a litre of water, though it was less effective with other kinds of clay.

That level of salt is too high for human drinking, however. Lower amounts of salt, between one and 1.5 grams, cut up to 95 per cent of the clay, still making it clear enough for solar disinfection.

The method works because the salt causes the clay particles to form clumps that sink to the bottom.

Cheap water-treatment method

The research is important because more than a billion people worldwide have no access to treated or clean water, according to the World Health Organization, leading to two million mostly child deaths every year from diarrhea and other sanitation-related illnesses.

One of the cheapest and easiest ways to treat microbe-infected water is simply to put it in one- or two-litre clear plastic bottles and leave it in the sun for upwards of six hours, ideally on a metal rooftop or other reflective surface. The method has been tested since the 1980s and is promoted by the WHO when other, more expensive options like ceramic filters and chlorine tablets aren't feasible.

But solar water disinfection, known as SODIS, doesn’t work when the water is too murky, because the contaminants block the sun's ultraviolet and infrared rays from killing pathogens. That makes it necessary to find a cheap, widely available way to extract those contaminants.

While the salt method worked well in the lab to remove one kind of clay, the researchers said more testing would be required in real-world conditions, as well as longer-term follow-up to see what effect the salted water might have on the health of people drinking it.