Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a cheap and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS is already applied in numerous developing countries.
Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:
At a water temperature of about 30°C (86°F), a threshold solar radiation intensity of at least 500 W/m2 (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m2 in the range of UV-A and violet light, 350nm-450nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.
At water temperatures higher than 45°C (113°F), synergetic effects of UV radiation and temperature further enhance the disinfection efficiency.
|Weather Conditions||Minimum Treatment Duration|
|50% cloudy||6 hours|
|50-100% cloudy||2 days|
|continuous rainfall||unsatisfactory performance, use rainwater harvesting|
SODIS is an effective method for treating water where fuel or cookers are unavailable or prohibitively expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS is limited if enough bottles are not available, or if the water is highly turbid.
In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles may be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, or iodine. Additionally, in some circumstances, it may be difficult to guarantee that the water will be left in the sun for the necessary time.
Other methods for household water treatment and safe storage exist, e.g. chlorination, different filtration procedures or flocculation/disinfection. The selection of the adequate method should be based on the criteria of effectiveness, the co-occurrence of other types of pollution (turbidity, chemical pollutants), treatment costs, labor input and convenience, and the user’s preference.
If the water bottles are not left in the sun for the proper length of time, the water may not be safe to drink and could cause illness. If the sunlight is less strong, due to overcast weather or a less sunny climate, a longer exposure time in the sun is necessary.
The following issues should also be considered:
It has been shown that the SODIS method (and other methods of household water treatment) can very effectively remove pathogenic contamination from the water. However, infectious diseases are also transmitted through other pathways, i.e. due to a general lack of sanitation and hygiene. Studies on the reduction of diarrhea among SODIS users show reduction values of 30-80%.
The effectiveness of the SODIS was first discovered by Professor Aftim Acra at the American University of Beirut in the early 1980s . Substantial follow-up research was conducted by the research groups of Martin Wegelin at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Dr Kevin McGuigan at the Royal College of Surgeons in Ireland. Clinical control trials were pioneered by Professor Ronan Conroy of the RCSI team in collaboration with Dr T Michael Elmore-Meegan.
Currently, a joint research project on SODIS is implemented by the following institutions:
Other developments include a continuous flow disinfection unit and solar disinfection with titanium dioxide film over glass cylinders which prevents the bacterial regrowth of coliforms after SODIS. Research has shown that a number of low-cost additives are capable of accelerating SODIS and that additives might make SODIS more rapid and effective in both sunny and cloudy weather, developments that could help make the technology more effective and acceptable to users. Another study showed that natural coagulants (seeds of five natural plant species--Vigna unguiculata, Phaseolus mungo, Glycine max, Pisum sativam, and Arachis hypogea--were evaluated for the removal of turbidity), were as effective as commercial alum and even superior for clarification because the optimum dosage was low.
The Swiss Federal Institute of Aquatic Science and Technology (Eawag), through the Department of Water and Sanitation in Developing Countries (Sandec), coordinates SODIS promotion projects in 33 countries including Bhutan, Bolivia, Burkina Faso, Cambodia, Cameroon, DR Congo, Ecuador, El Salvador, Ethiopia, Ghana, Guatemala, Guinea, Honduras, India, Indonesia, Kenya, Laos, Malawi, Mozambique, Nepal, Nicaragua, Pakistan, Perú, Philippines, Senegal, Sierra Leone, Sri Lanka, Togo, Uganda, Uzbekistan, Vietnam, Zambia, and Zimbabwe. Contact addresses and case studies of the projects coordinated by the Swiss Federal Institute of Aquatic Science and Technology (Eawag) are available at sodis.ch
SODIS has also been applied in several communities in Brazil, one of them being Prainha do Canto Verde north of Fortaleza. There, the villagers have been purifying their water with the SODIS method. It is quite successful, especially since the temperature during the day can go beyond the 40°C (100°F) and there is a limited amount of shade.