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Solar Water Purification

Solar water purification, also known as solar water disinfection or SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage.

Exposure to sunlight has been shown to purify water by deactivating harmful organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms. 1) UV-A light interferes directly with the metabolism and destroys cell structures of bacteria. 2) UV-A light (wavelength 320-400 nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens. 3) Cumulative solar energy (including the infrared radiation component) heats the water. If the water temperatures rises above 50C, the disinfection process is three times faster.

At a water temperature of about 30 degrees Celsius (86F), a threshold solar irradiance of at least 500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains solar energy of 555 Wh/m2 in the range of UV-A and violet light, 350 nm-450 nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine. At water temperatures higher than 45 degrees Celsius (113F), synergistic effects of UV radiation and temperature further enhance the disinfection efficiency for solar water purification.

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