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SODIS is an acronym for solar disinfection. Its a cheap effective method for disinfecting water by using the heat and ultra violet rays of the sun. This method is used at the household level at the responsibility of the user.

 

SODIS solar water disinfection in action

 

Solar Water Disinfection (SODIS) in action.

In this method PET bottles are partially filled with water and shaken to oxygenate the water before filling them completely. The bottles are then left in the sun, preferably in a slanting position and (on a surface that heats quickly such as corrugated iron roofing) so as to allow maximum exposure to sunlight. Ultra violet rays in the sunlight interfere with the metabolism of and destroy the cell structure of bacteria. UV rays also react with oxygen in the water producing highly reactive forms of oxygen such as oxygen free radicals and hydrogen peroxide that are also believed to aid in destroying bacteria. Infra red radiation from the sun also heats the water making the process three times faster if the water is heated to above 50°C.

As far as time is concerned, the bottles are to be kept in the sun for 6 hours if sunny (less than 50% cloud cover) or for up to 2 days if it is cloudy (50-100% cloud cover but without rain). In situations where there is continuous rainfall, rainwater harvesting is considered more suitable for obtaining clean drinking water than the use of this technique. The most favorable locations for the use of this method are said to be between 15°N and 35°N and 15°S and 35°S as these regions have high levels of solar radiation coupled with relatively lower cloud cover. The second most favorable location is between 15°N and 15°S due to greater cloud cover and higher humidity which causes the solar radiation to be scattered to a greater extent.

PET soda/soft drink bottles are usually used for this process though they need to be relatively free of scratches and have to have their labels removed. Clear plastic bottles are to be preferred over colored variants. Also although glass bottles have been used, they are not recommended as they are at times thicker, reducing the penetration of solar radiation, also certain types of glass may inhibit the penetration of certain wavelengths of solar radiation. Polycarbonate bottles should also not be used as they block the penetration of all UVA and UVB rays which are necessary for the disinfection process.

 

PET Symbol

A bottle can be identified as a PET plastic bottle by the above mark which should be present somewhere on it's body.

Old PET bottles with many scratches and signs of wear reduce the efficacy of the technique and should not be used. PET bottles are also preferred because of their long relatively thin shape, as the deeper the rays have to penetrate into a body of water, the lower the efficacy of the UV rays. Care should also be taken to oxygenate the water by shaking a partially filled bottle of water as the level of oxygen also improves the speed of the process. Disinfected water should be drunk before storage as it has been found that storage in the dark may allow the few remaining bacteria to reproduce again. Solar disinfection is only useful against pathogens and cannot purify water that has been contaminated by chemicals.

The SODIS technique of water purification is now being widely practiced in the developing world and has been found to be a successful and cheap method of disinfecting water, lowering the rate of incidence and death from water borne diseases such as diarrhea.

 

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References -

http://www.sodis.ch/

http://www.sodis.ch/methode/forschung/publikationen/index_EN - A compilation of research on the usefulness of this technique, compiled by sodis.ch .