Hat tip to SZ at P4P for finding this! http://www.sodis.ch/Text2002/T-TheMethod.htmSolar Water Disinfection
SODIS, Solar Water Disinfection, improves the microbiological quality of drinking water, using solar UV-A radiation and temperature to inactivate pathogens causing diarrhoea.
In 1991 EAWAG (The Swiss Federal Institute for Environmental Science and Technology) and SANDEC (EAWAG's Department of Water and Sanitation in Developing Countries) conducted extensive laboratory and field tests to develop and test the Solar Water Disinfection Process (SODIS). The laboratory tests conduced as well as the practical experience gathered during the application in the field revealed a simple, low-cost technology with a great potential to improve the health of those still without access to safe drinking water.
Since 1995, SANDEC is engaged in providing information, technical support and advice to local institutions in developing countries for the worldwide promotion and dissemination of the Solar Water Disinfection Process, SODIS. In the last 4 years we have been coordinating the promotion and dissemination of SODIS in more than 20 countries.
SODIS is used at household level under the responsibility of the user. Therefore EAWAG is not liable for any harm caused by a faulty or inadequate application of the water treatment process.Solar Water Disinfection - The method
The Solar Water Disinfection (SODIS) process is a simple technology used to improve the microbiological quality of drinking water. SODIS uses solar radiation to destroy pathogenic microorganisms which cause water borne diseases.
SODIS is ideal to treat small quantities of water. 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 50Â°C, the disinfection process is three times faster.How do I use SODIS? (in pictures)http://www.sodis.ch/Text2002/T-Howdoesitwork.htmKey factors of SODIS applicationWeather and Climate
SODIS requires sun radiation and temperature:
* => the container needs to be exposed to the sun for 6 hours if the sky is bright or up to 50% cloudy Water Turbidity
=> the container needs to be exposed to the sun for 2 consecutive days if the sky is 100% cloudy,
=> during days of continuous rainfall, SODIS does not perform satisfactorily. Rainwater harvesting is recommended during these days.
=> if a water temperature of at least 50Â°C is reached, an exposure time of 1 hour is sufficient
* The most favourable region for SODIS lies between latitudes 15Â°N/ S and 35Â°N/ S. These semi-arid regions are characterized by high solar radiation and limited cloud coverage and rainfall (3000 hours sunshine per year). The second most favourable region lies between the equator and latitude 15Â°N/ S, the scattered radiation in this region is quite high (2500 hours sunshine per year).
(-> see technical Notes Nr.5)
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 with a turbidity less than 30 NTU.Material and Shape of the Containers
* In water with higher turbidity than 30 NTU pathogens will have to be inactivated by the temperature rather than radiation (>50Â°C for at least an hour) or the water has to be filtered before being exposed to the sun.
* Water Turbidity Test: place the bottle, full with water, on the SODIS logo on top of a table in the shade and look through the bottle from top to bottom. Water turbidity is less than 30 NTU, if you can read the letters of the SODIS logo through the water.
(-> see technical Notes Nr.7)
* 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.Oxygen
* 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.
(-> see technical Notes Nr.2)
* Glass: Also glass bottles can be used for SODIS. But it is not possible to construct shallow, large containers using ordinary window glass, as window glass does not transmit UV-radiation adequately.
* Ageing of plastic bottles (due to mechanical scratches and due to photoproducts) leads to a reduction of UV transmittance which will reduce the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.
(-> see technical Notes Nr.3)
* Photoproducts: Sunlight does not only destroy pathogenic microorganisms found in the water but also transforms the plastic material into photoproducts. Laboratory and field tests showed that these photoproducts are generated at the outer surface of the bottles. No migration of photoproducts or additives (UV-stabilisators) into the water was observed.
* Migration of organic compounds: the migration of organic compounds from reused and new PET Bottles into the water was examined by a team of researchers from the EMPA (Swiss Federal Laboratories for Materials Testing and Research). Adipate and phtalate such as DEHA and DEHP were detected in very low concentrations - the level of concentrations found in the water of reused and new PET-bottles were in the same magnitude as the concentrations of phtalate and adipate generally found in high quality tap water.
* Shape of Containers: UV radiation is reduced by increasing water depth. At a water depth of 10cm and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. This means that PET bottles do not have the most efficient shape for SODIS 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 the SODIS application.
* 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.Limitations of SODIS
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 of SODIS.
* SODIS does not change the chemical water quality
* SODIS requires relatively clear water (turbidity less than 30 NTU)
* SODIS requires suitable weather conditions
* SODIS is not useful to treat large volumes of water
Copyright Â© of all Texts, Pictures, Graphics by SANDEC (Water & Sanitation in Developing Countries) at EAWAG (Swiss Federal Institute for Environmental Science and Technology), CH-8600 DÃ¼bendorf, Switzerland.
Permission is granted for reproduction of this material in whole or part, for education, scientific or development related purposes except those involving commercial sale, provided that full citation of the source is given and written request is submitted to SANDEC.