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The solar water disinfection (from its developers[1] called SODIS) is a water treatment process used at household level. It is a simple technology used to improve the microbiological quality of drinking water. It is a low-cost method as it can be applied with locally available resources such as sunlight and empty PET-bottles. It uses solar radiation to destroy pathogenic microorganisms which cause water borne diseases. So, it is a technology suitable in areas where the local population does not have access to safe drinking water. It is ideal to treat small quantities of water. At present, the SODIS method is used for the daily treatment of drinking water by about 2 Million users.
Contaminated water is filled into transparent plastic bottles and exposed to full sunlight for six hours. Sunlight is treating the contaminated water through two synergetic mechanisms: radiation in the spectrum of UV-A (wavelength 320-400nm) and increased water temperature. If the water temperatures raises above 50°C, the disinfection process is three times faster.
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Solar water disinfection requires sun radiation and temperature:
Suspended particles in the water reduce the penetration of solar radiation into the water and protect microorganisms from being irradiated. SODIS requires relatively clear water. In water a turbidity of 30 NTU or higher, the inactivation of pathogens through UV-A radiation may not be satisfactory, and the water should be filtered before being exposed to the sun.
Various types of transparent plastic materials are good transmitters of light in the UV and visible range of the solar spectrum. Plastic bottles made from PET (PolyEthylene Terephtalate) are preferred because they contain less UV-stabilisators than PVC (PolyVinylChloride) bottles. How to distinguish PET and PVC: Bottles of PVC often have a bluish gleam. If PVC is burnt, the smell of the smoke is pungent, whereas the smell of PET is sweet. Ageing of plastic bottles (due to mechanical scratches and due to photoproducts) leads to a reduction of UV transmittance which will reduce the efficiency. Heavily scratched or old, blind bottles should be replaced.
Shape of Containers: UV radiation is reduced by increasing water depth. At a water depth of 10cm and moderate turbidity, UV-A radiation is reduced to 50%. This means that PET bottles do not have the most efficient shape as they have a small area for sunlight exposure and have a water depth of 6-10cm. Containers with a larger exposed area per water volume would be more efficient. However, PET soft drink bottles are often easily available and thus more practical for application.
Also glass bottles can be used. But it is not possible to construct shallow, large containers using ordinary window glass, as window glass does not transmit UV-radiation adequately.
Oxygen plays an important role in killing the pathogens: Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms.
Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application.
Solar water disinfection is a simple method to improve the microbiological quality of drinking water. Various health assessments have shown that the risk of contracting diarrhoea significantly is reduced when people drinking untreated raw water switch to the consumption of treated water. Several health impact studies have been conducted in users’ communities. These assessments have shown that the health benefits of use are significant: diarrhoea incidence was reduced by 20 to 50%!
Local promoters are representatives from the local community, health workers, nurses, teachers who are responsible for the training of the local people. It is of great importance that respected local people, leaders and influential people from the community are chosen as promoters. These people have a great influence on the level of acceptance of project activities in the community and on the resulting level of behaviour change. Promoters themselves use solar water disinfection for the treatment of their drinking water. Sustainable practice cannot be established at field level with an isolated single information event! Very often the knowledge on the various transmission routes of pathogens causing diarrhoea and the awareness on the importance of treating water before consumption is missing and has to be established before the know-how on water treatment methods can be transmitted. To establish such awareness and to achieve the corresponding behaviour change is a slow and demanding process which involves a long and multi-facetted contact with the concerned population.
Research on SODIS was taken up by Sandec - Department Water and Sanitation in Developing Countries at the EAWAG (Swiss Federal Institute of Aquatic Science and Technology) in 1991. EAWAG is a Swiss competence centre for national and international water research. Thorough scientific investigation in the laboratory and in the field of developing countries has shown that SODIS is an effective method for the disinfection of water at household level.
SODIS is recommended by WHO (World Health Organisation) as water disinfection method at household level and is part of WHO’s international network to promote household water treatment and safe storage.