What water can I use for the SODIS method?

The SODIS method needs relatively clear water. You can find out with a simple test whether the water is too turbid. The source of the water (well, surface water) does not matter.

Water that has been polluted with chemicals (poisons, fertilisers, etc.) must not be used. The SODIS method only kills germs. The chemical composition of the water remains unchanged.

Does the SODIS method change the taste of the water?

The SODIS method improves the quality of the water without changing its taste. The bottles are closed while they are being exposed to the sunlight, so the oxygen dissolved in the water cannot escape. The water still tastes fresh. On the other hand, if we boil water, the oxygen dissolved in the water escapes. This gives the boiled water an unpleasant aftertaste. The use of chlorine also makes the water taste less good.

Where should I place the bottles?

Lay the bottles on a clean surface in the sun, where no shadows will fall for the whole treatment time.

If possible, lay the bottles on a reflective surface, like a sheet of corrugated iron, and protect it from cooling by the wind. The reflection and higher temperature will speed up the disinfection process. However, this is not essential for the application. The bottles can be set down on any surface (wood, concrete, clay brick, etc.).

How long can I store the water that I have treated with the SODIS method?

If the bottle is kept unopened after treatment and stored in a cool, dark place, it can be stored for as long as you wish. The dead bacteria cannot multiply again. The only things that may grow are algae. However, these do not represent a health hazard.

How long can I use the same PET bottle for?

UV-A radiation must penetrate the bottle in order to kill the germs. Clear, unused bottles normally allow more than 60% of the UV-A light over 340 nm to pass through. Experiments have shown that older, used bottles allow less UV-A light to pass through them. Besides the ageing process of the bottle material, scratches on its surface will also mean that it allows less UV-A light through. So it is very important to handle the bottles carefully. We recommend replacing old bottles and bottles that are no longer transparent after about 6 to 12 months of daily use.

How can I tell the difference between a PET bottle and a PVC bottle?

Only PET bottles should be used for the SODIS method because PVC can be harmful to your health. PET and PVC bottles are normally marked accordingly. The labels can vary from country to country, though. If the bottles are not marked, you can only tell the difference between the bottles by setting fire to them.

PET burns quickly and easily when it is held in a flame. When it is taken out of the flame, the fire goes out slowly, or it may keep burning. The smoke smells sweet.

PVC does not burn easily. The material does not burn at all when not in the flame. PVC smoke smells acrid.

Is it dangerous to your health to use PET bottles?

Scientific studies have confirmed repeatedly that when the SODIS method is applied correctly there is no danger to health.

How can I improve the efficiency of the method?

If the temperature is increased, the efficiency of the SODIS method can be improved. The germs are killed more quickly. In addition, amoebas are also rendered harmless above a temperature of 50°C. To increase the temperature, lay the bottles on a reflective surface, like a sheet of corrugated iron, and shield them from cooling by the wind.

Old, scratched bottles reduce the effectiveness of the method. Therefore, the bottles should be replaced regularly. But with careful handling, they should be usable without problems for 6 to 12 months.

Turbidity in the water also reduces the efficiency of the method. This problem can be corrected easily by filtering the water.

Does the SODIS method kill all bacteria?

The SODIS method is used to kill germs in the water. While the bottle is being exposed to the sunlight, other, harmless bacteria and organisms that occur naturally in the environment can grow, for example algae or naturally occurring coliform bacteria. However, these organisms do not represent a threat to human health.

Up to what degree of contamination with faecal coli is the SODIS method effective?

Laboratory experiments have shown that when the method is used properly, extreme contamination levels of 100,000 E. coli per 100ml can be made harmless. These concentrations are much higher than are normally found in natural water sources (a few thousand E.coli/100ml water or less).

How can I prove that the SODIS method is effective?

To prove the existence of germs, expensive analytical procedures are often necessary. It is not always possible to use s methods. In such cases, the World Health Organisation (WHO) recommends using E. coli as an indicator organism (WHO, Guidelines for drinking water quality, 1993).

E. coli is suitable for use as an indicator of faecal pollution because it

  • is present in large numbers in human faecal matter.
  • cannot survive in natural bodies of water.
  • has a similar resistance and survival rate to water treatment methodsas other germs.

It is also possible to analyse E. coli under difficult conditions in the field, for example with the DelAgua field test kit or with Petrifilms from 3M.

It is not recommended to use total coliform bacteria as indicators, because there are harmless strains that occur in natural bodies of water and are much more resistant to UV-A radiation than the germs that harm humans. The total bacteria count is also a poor standard for evaluating the efficiency of the method. Some bacteria grow while the bottle is lying in the sun. However, since they are harmless to people, the water can still be drunk without any ill effects.

What mistakes do new users make most often?

  • Using green or brown bottles for the SODIS method;
    => these bottles absorb UV-A light. This is why only colourless, transparent bottles must be used for the SODIS application.
  • The bottles used are too big;
    => the bottles must not be able to hold more than 3 litres.
  • Bottles are placed upright;
    => the bottles must be laid horizontally in the sun. This increases the area exposed to the sunlight and reduces the depth of the water the light must penetrate. (With turbidity of 26 NTU, only half of the UV-A radiation penetrates farther than 10cm)
  • After the SODIS method has been applied, the treated water is poured into dirty containers, so the water is immediately contaminated again;
    => 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.

© SODIS 2020
Last update: 23.09.2009