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Dirty Water via Shutterstock
World Water Day

Sunlight can make this dirty water clean...and save lives

Today is World Water Day…but clean tap water is a pipe dream for 780 million people across the globe.

DIRTY DRINKING WATER is a known death trap…but Irish scientists may have a solution.

Researchers from the RSCI can prove that simply exposing dirty water in a plastic bottle to sunlight can kill all but the most resistant bugs.

Professor Kevin McGuigan and his team have already shown that solar disinfection of water (SODIS for short) almost halves the rate of diarrhoea in Kenya, Cambodia and Uganda. Now the team are working on ways to scale up SODIS, upping the effort in the battle against waterborne disease, which kills about one child every 21 seconds.

Harnessing the Sun

Clean tap water is a pipe dream for 780 million people worldwide but SODIS is designed to be used in the home and only needs sunlight, patience and cheap reusable plastic bottles to work.

“You put whatever drinking water you have available in the bottle, place it in direct sunlight and then after a minimum of six hours the water should be safe to drink,” explains McGuigan.

Direct sunlight kills microbes in the water by a combination of heat and UV sterilisation, a process that is effective even when the sun is not ‘splitting the sky’.

“If it is cloudy, we recommend that you put the water out for two consecutive days,” says McGuigan, who has even shown that SODIS can work in Ireland.

During sunny days in June we have put bottles out on the windowsill here and we got complete inactivation of Eschericia coli after two hours, so if you get the sun you get a good effect.

When SODIS was trialled in villages in Cambodia and Kenya, the rate of diarrhoea fell by 40 to 50 per cent.

“We haven’t yet found an important waterborne pathogen that isn’t susceptible to SODIS,” says McGuigan. “Protozoans such as cryptosporidium are more resistant but it can still be accomplished; it just takes a good wallop of energy.”


The major benefit of SODIS is the low price tag but this brings its own unique problem.

“The simplicity is its biggest advantage and its biggest disadvantage,” explains McGuigan.

If people see plastic bottles lying in the ditches and you give them a plastic bottle as the solution to unsafe drinking water, there’s not a lot of psychological value placed with the technology.

Even though SODIS has been given the thumbs up by the World Health Organisation (WHO) for use in emergency settings, McGuigan maintains that “the biggest barrier is that people just don’t know about it”.

Another challenge is dissolved solids in the water. Current advice is that if a newspaper headline is still readable through a bottle of cloudy water standing on top, then SODI will still be effective – otherwise it needs to be filtered. In trials, SODIS had a surprise advantage over chlorination, a more widely used method of decontamination.

“The Cambodians thought SODIS was fantastic,” continues McGuigan.

They loved the simplicity and the taste of the water. They don’t like the taste of chlorine, neither do the Maasi people in Kenya.

Building on Success

McGuigan and his team are now working on ways to scale up and enhance the technique.

“There‘s a lot of work associated with SODIS because the most you can disinfect in one go is a two-litre bottle. If you have a family of five or six kids, it’s quite a burden of work.”

Working in collaboration with the Plataforma Solar de Almería in Spain, McGuigan and his team developed a half-pipe reflective cradle that concentrates and focuses sunlight to a 14-litre water bottle.

The system is being tested in Uganda by Rosemary Nalwanga, a PhD student in the Makerere Kampala University, who is co‑supervised by McGuigan and Dr Brid Quilty in DCU as part of the “Water is Life” project, funded by Irish Aid. While the price tag of the system (it cost $200 to make the prototype) puts it out of range of most households, it would be cost-effective in schools or on a community basis.

McGuigan has a curious end-goal for SODIS – he hopes that it become obsolete.

“The ultimate goal is a safe effective water source. It would be better off investing in pumps and better pump technology.”

However, there is caution. “It costs $3,000 to sink a borehole. SODIS is an effective treatment for almost no cost.”

Read: Ireland is to send €3 million more aid to the Philippines

Read: Here’s what Twitter thinks Ireland actually smells like

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