Solar Disinfection of Water (SODIS): A case study from Kenya

In 1996, issue #53 of EDN carried a short article that described how to disinfect water by using ultraviolet (UV) rays from the sun to destroy harmful microorganisms. It was very simple: clear plastic bags filled with water were allowed to sit in the sun. In order for this technique to work, the water must be clear (not murky).

The April 2004 edition of Water Lines, vol. 22 no. 4 summarizes a case study conducted in rural Kenya, in which this technology, now known as SODIS, is found to be an excellent method of disinfecting water for the rural homestead.

The basic treatment consists of filling several clean, transparent plastic (‘PET’ or ‘PETE’ (1) – polyethylene terephthalate) water or soft drink bottles with water and exposing them to full sunlight for at least six hours. The process is more effective if the water is aerated by shaking, and the bottles are placed on corrugated metal sheeting (roofing material) or on the house roof and exposed to the sun’s rays. Smooth bottles that hold two liters or less are best. The type of plastic bottle is very important—make sure it is ‘PET’ or ‘PETE’!

The following steps are recommended for SODIS:

1. Wash the bottle well before filling it.
2. Fill the bottle ¾ full with water.
3. Shake the bottle 20 seconds to oxygenate it.
4. Fill the bottle fully and cap it.
5. Place the bottle in the direct sun for 6 hours.
6. The water is now ready for drinking.

Exposure time should include the hours from 9:00 a.m. to 3:00 p.m. If skies are clear to partly cloudy, 6 hours is recommended. If the sky is covered more than 50% with clouds, treatment for two consecutive days is recommended.

A household health survey found that Kenyan households using SODIS had a 3-fold reduction in water-borne disease incidences compared to households not using SODIS. In the experimental part of the work, researchers measured the amount of fecal coliform bacteria as an “indicator organism” of pathogenic contamination. Water treated with SODIS had a 99.9% reduction of fecal coliform bacteria. SODIS effectively kills the following disease-causing organisms: Escherichia coli, Vibrio cholerae, Enterococcus faecalis, Salmonella paratyphii, Salmonella typhii (cause of typhoid fever and food poisoning), bacteriophage F2, rotavirus and encephalomyocarditis virus, Candida, Geotrichum, Penicillium, and Aspergillus flavus and Aspergillus niger (two species of fungi that produce aflatoxin). SODIS is also effective against protozoa organisms, some of which can cause disease. See Table 2.

In the community, SODIS project users compared SODIS with other methods of water treatment and rated SODIS number one over boiling, chlorination and filtration techniques, taking into account costs, ease of use, effect on the environment and efficiency in disease control. The community found that the technique is easy to use, costs little, saves fuel wood, saves time, and improves health.

The article recommends SODIS be further promoted as an acceptable method of disinfecting water for household use, thereby reducing the risk of waterborne diseases.

Tests conducted by the Swiss Federal Institute for Environmental Science and Technology (EAWAG) showed that while pathogenic microorganisms were killed, some microorganisms not harmful to human health were present in SODIS water. See SODIS website and technical notes for further details.

For further information or to read the entire article, refer to the SODIS website: www.sodis.ch