The SODIS procedure

The SODIS process, developed in Switzerland, represents an excellent alternative for mobile water disinfection, which is mainly used in developing countries, but also has other uses. How the interesting method works, and where its limits are, reveals our contribution.

Principle of the SODIS procedure

The abbreviation SODIS stands for Solar Water Disinfection. The process makes use of the existing even in natural sunlight UVA radiation advantage. This wavelength also has a pronounced germicidal effect, if it can act long enough. UVA radiation is also used in UV water disinfection.

However, as bottle and window glass is impermeable to UVA rays, commercially available PET bottles are used for the SODIS method. They allow the UV-A radiation to pass through. When using containers of up to three liters capacity, a sufficiently large penetration depth of the radiation is also ensured, so that the entire water surface can be uniformly irradiated and thus sterilized.

Prerequisite for the reliability of the effect is only a sufficiently long exposure time of the radiation to the water. In addition, other factors play a role:

  • Intensity of solar radiation
  • possible warming of the containers (temperatures above 50° C lead to a higher effectiveness of the method)
  • it must be clear (ie unclouded water without suspended matter) are used.
  • For the application, only PET containers are recommended that are not or only slightly scratched if possible

Practical use

Clear water is bottled in PET bottles and exposed to the sun. The exposure time is determined depending on the degree of cloudiness. At cloud densities below 50 percent, complete sterilization of the water is achieved after about 6 hours. However, if the degree of cloudiness in the sky is more than 50 percent, an exposure time of two days is necessary.

Shaking a three-quarters bottle before exposure to sunlight increases the oxygen content in the water. As a result, more free radicals are formed when exposed to the UV-A radiation, which in turn has a supporting effect on the sterilization.

Success of the methods

Since 2001, the method has been used and tested in more than 30 countries around the world, especially in developing countries. The decline in infections caused by diarrheal diseases and other diseases caused by microbial contaminated drinking water is on average between 30 and 80 percent, with a sharp decline especially in children.

The lower success rates in some countries or areas are almost always due to social factors. In many cases, despite intensive instruction, the method is not applied, not always or not correctly, because the benefits for people with very low levels of education are not always sufficiently clear and understandable.

It can also be considered successful that with the SODIS method many people have access to a reliable water disinfection method, even if no fuel is available or affordable for boiling water.

Other applications

The method is also quite suitable for trekkers and travelers when PET
For example, bottles may be exposed to intense sunlight on the backpack. As long as the exposure times are observed, one can assume a complete sterilization. This is also interesting with regard to often microbially contaminated bottled water in many poorer countries, since the filling methods are often extremely poor there.

Video Board: SODIS - Safe drinking water