February 13, 2009

Solar power kills bacteria in water

Scientists have improved solar water decontamination techniques in an attempt to reduce the spread of water borne diseases in developing countries.

Solar water disinfection is a simple way to kill bacteria in water. It is used by households in developing countries where safe drinking water is scarce. People fill plastic bottles with water and leave them in sunlight, where the UV radiation and increased water temperature kill the bacteria within six hours. But the method requires strong sunlight and can only treat limited volumes of water.

Water-filled plastic bottles for solar disinfection

Sunlight is used to disinfect water in plastic bottles but can only treat limited volumes


Kevin McGuigan from The Royal College of Surgeons in Ireland, Dublin, and colleagues investigated solar disinfection of Escherichia coli-contaminated water in large volume flow reactors. A pump circulated the water between a holding tank and a glass tube surrounded by solar collectors that focus the sun's energy onto the tube. They found that E. coli inactivation depends on the total dose of sunlight rather than the light's intensity. They also showed that the reactors can be ineffective because the bacteria receive an intermittent dose of radiation as they flow between the dark holding tank and the see-through tube. If the bacteria are not completely inactivated by the sunlight, the dark periods give them time to recover from the radiation damage, making them more resistant when reilluminated.

'For me, the major significance of the research is that these methods can be effective, but recalculating flow through solar disinfection reactors must be carefully designed in order to avoid the possibility of resistant sub-populations of pathogens remaining viable due to incomplete sunlight exposure,' says McGuigan.

"This work is an important contribution which points out potential advantages and limitations of solar disinfection"
- Cesar Pulgarin, Swiss Federal Institute of Technology, Lausanne, Switzerland
'This work is an important contribution which points out potential advantages and limitations of solar disinfection, depending on the type of solar photo reactor and operation mode,' comments Cesar Pulgarin, an expert in biological decontamination processes at the Swiss Federal Institute of Technology in Lausanne, Switzerland. 'It is also the first attempt that assesses the minimal UV dose required for complete bacterial inactivation by solar disinfection.'

The World Health Organization estimates that more than one billion people lack access to safe drinking water, resulting in millions of deaths each year from water-related diseases such as diarrhoea. McGuigan says he plans to introduce the flow reactor technology into developing countries, where he hopes it could provide emergency relief to communities affected by famine, flood and war.

Philippa Ross

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