Urban Skills > S.O.D.I.S: Solar Water Disinfection


Millions of people around the world become sick each year from drinking non-potable, contaminated water. Children and the elderly are the predominant victims of this global tragedy. An estimated 1.5 billion cases of diarrhea occur each year, resulting in the death of nearly 2 million children. Worldwide, about 1.3 billion people do not have access to safe drinking water. However, in many of these affected regions, a solution may be right outside their doors. Solar energy is an abundant, renewable and free source of energy that can both cook food and disinfect questionable water.

This new technology: SODIS.

The concept of Solar Water Disinfection was presented by UNICEF in 1984. A research team at EAWAG (Swiss Federal Institute for Aquatic science and Research)/SANDEC (Dept. of Water and Sanitation) embarked on comprehensive laboratory experiments in 1991 to assess the potential of utilizing solar UV rays as a method for the inactivation of waterborne bacteria and viruses. Laboratory research revealed a relationship in the neutralization of microorganisms through the combined use of UV-A radiation and increased water temperature. Field tests confirmed this effect which significantly enhances the potential of Solar Water Disinfection - SODIS.

Solar Water Disinfection (SODIS) is a simple, environmentally sustainable, low-cost solution for drinking water treatment on a household or community level for people consuming biologically contaminated wild water. SODIS uses solar energy to destroy pathogenic microorganisms causing water borne diseases improving the quality of drinking water. Pathogenic microorganisms are vulnerable to two effects of the sunlight: radiation in the spectrum of UV-A light (wavelength 320-400nm) and heat (+50 degrees C).

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The SODIS method is very easy to apply: A transparent PET bottle is cleaned with soap. Then, the bottle is filled with water and placed in full sunlight for at least 6 hours. Result: water has then been disinfected and is safe to be drunk.

Important points to consider when applying the SODIS method:

Material, color and shape of the container:

PET bottles are recommended for use in the application of the SODIS method because of their durability and ease of availability (soda or bottle h2o bottles). However, glass bottles and or plastic bags can also be used (i.e. freezer bags). PET bottles are usually clearly labeled as such (see bottom of bottle).

The containers must be clear, transparent and colorless. Labels should be removed and heavily scratched bottles should be avoided. Use bottle or containers not larger than 3 liters.

Water Clarity:

If the water is very cloudy or muddy, the effectiveness of the method is reduced. To determine whether the water is sufficiently clear, place the filled PET bottle on top of a newspaper headline. Look through the bottle from the neck at the top to the bottom. If the letters of the headline are readable, the water can be used. If the letters are not clear, the water must be filtered, pour through a coffee filter, t-shirt or bandana, until this test can be performed successfully.

Weather & Climate:

The SODIS method is not effective during extended periods of clouds or rain. On these days, it is recommended collecting rainwater. Most effective in arid regions of the world.

As a general rule:

  • If less than half of the sky is clouded over, 6 hours will be enough to completely disinfect the water.
  • If more than half of the sky is covered with clouds, the bottle must be placed in the sun for 2 consecutive days.
  • Avoid freezing.
Solar disinfection has positive and negative aspects:
  • Solar disinfection will kill most germs that cause intestinal problems and disease if exposed to the sun long  enough.
  • Solar water treatment does not change the taste of the water.
  • The SODIS method can also be applied with clear plastic bags. The bags have the advantage that they can be easily transported and stored in large quantities.
  • Solar disinfection is something people can do themselves with widely available inexpensive or free materials.
  • The SODIS method does not kill all waterborne pathogens, just those which cause diarrhea. However, studies have shown many other bacteria, viruses and protozoa are also neutralized in the process.
  • Solar disinfection has no residual effect, so improper storage can lead to re-contamination. Water treated by this method should be stored safely and used within a few days.
  • Solar disinfection takes more time than other methods and requires extended periods of solar radiation.
For further information on SODIS see their website:


 
 
 

 

 
 
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