SODIS: How does it work?

How does it work?

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. The water has then been disinfected and can be drunk.

Important points to consider when applying the SODIS method

	Material, colour and shape of the bottle We recommend using PET bottles in the application of the SODIS method because they are light and do not break. They are also easy to obtain in many regions. However, glass bottles or special bags can also be used. PET bottles are usually labelled as such, but they are not called by the same name in all countries. Factsheet Glass vs. PET (EN, FR, ES) Factsheet Bottle vs. Bag (EN, FR, ES) Factsheet Use of Bags (EN, FR, ES) The bottles must be transparent and colourless. PET bottles often have a bluish tinge. This is not a problem. Heavily scratched bottles must be replaced. Factsheet Ageing of bottles (EN, FR, ES) The bottles must not hold more than 3 litres. Factsheet Depth of water (EN, FR, ES)
	Turbidity If the water is very turbid, the effectiveness of the method is reduced. It is very easy to determine whether the water is sufficiently clear: The filled PET bottle must be placed on top of a newspaper headline. Now one must look at the bottom of the bottle from the neck at the top and through the water. If the letters of the headline are readable, the water can be used. If the letters are not readable, the water must be filtered. This test corresponds to a turbidity of 30 NTU. Factsheet Turbidity (EN, FR, ES)
	Cloudiness Cloudiness affects the strength of solar radiation and thus also the effectiveness of the method. Rule of thumb: 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. Factsheet Cloudiness (EN, FR, ES) Rain The method does not work satisfactorily during lengthy periods of rain. On these days, we recommend collecting rainwater.
	Preventing recontamination The treated water should be kept in the bottle and drunk directly from the bottle, or poured into a cup or glass immediately before it is drunk. In this way, it is possible to prevent the treated water from becoming contaminated again.