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Solar water disinfection, also known as solar water pasteurisation or SODIS, is a method of disinfecting drinking water using the heat and UV light (or sometimes just heat) of the sun. The water must be already clear, especially when using the sun's UV light. Unlike solar distillation, it is a very simple and Low cost way of disinfecting small volumes of water.

Radiation in the spectrum of UV-A (wavelength 320-400nm) and increased water temperature, both factors cause damage to the DNA of the Pathogens. The effects are also synergistic - the DNA being more prone to UV damage if the temperature is very high. If the water temperatures raises above 50°C, the disinfection process is three times faster, and much better results are obtained from the UV. Certain pathogens such as Giardia,^W however, are not easily killed by UV. For these, the heat is more important.^{[verification needed]}

Usability

• Keep in mind that SODIS is only intented as a disinfection method, and does not perform complete water purification; when the water is highly turbid, additional steps are necessary^[1] - for example adequate filtering before UV exposure.

SODIS: soda bottles and sunlight

Solar water disinfection, or SODIS^W, is a method to disinfect water using sunlight and PET bottles.

• Water from contaminated sources are filled into transparent water bottles. For oxygen saturation, bottles can be filled three quarters, then shaken for 20 seconds (with the cap on), then filled completely. Highly turbid water (turbidity higher than 30 NTU) must be filtered prior to exposure to the sunlight.
• Filled bottles are then exposed to the sun. Better temperature effects can be achieved if bottles are placed on a corrugated roof as compared to thatched roofs.
• The treated water can be consumed. The risk of re-contamination can be minimized if water is stored in the bottles. The water should be consumed directly from the bottle or poured into clean drinking cups. Re-filling and storage in other containers increases the risk of contamination.
  

Effectiveness can be increased by placing on a dark or reflective surface, e.g.

• a roof made of corrugated metal.
• or by painting half the bottle black, such that when laying down the clear half is facing the sky.

Main SODIS website: http://www.sodis.ch/index_EN

Using reflectors

Enhancement of Solar Water Pasteurization with Reflectors, Negar Safapourdagger and Robert H. Metcalf, 1998, Department of Biological Sciences, California State University Sacramento, Sacramento, California.

This uses simple cardboard based reflectors and a black jug. Heat is considered by the authors to be the more reliable mechanism for disinfection.

According to Village Earth, water heated in a normal flat plate collector does not boil..^[2] So to Pasteurize water and to cook at high temperature cannot use flat plate collectors, but must use more complicated concentrating collectors that require frequent adjustment to keep them pointed at the sun.

See also: CPCs

Using Salt to reduce turbidity so the SODIS method works

• See project page Decreasing turbidity to optimize solar water disinfection^[3] open access

History

Prof Aftim Acra of Lebanon researched this technique beginning in 1979. See Cleaning water with sunshine, Robert Bourgoing, IDRC Reports, April 1989. (or the original format with image, in PDF.) See also An Interview with Aftim Acra, Professor Emeritus of Environmental Sciences, Winter 2004.

Related projects

See also

• UV water treatment
• UV Water Purification (Mech425)

External links

• Wikipedia:Solar water disinfection
• Scientific Publications on Solar Water Disinfection at the SODIS website. Links to PDF format articles.
• Water testing & Pasteurization in Africa - A guide to water testing without access to a laboratory, and how to pasteurize contaminated water using sunshine and a simple solar cooker. Images with captions, by Bob Metcalf.
• WADI - Device for Solar WAter DIsinfection

References