Solar water disinfection

SODIS (short for Solar Water Disinfection ) is a method to disinfect water and is based on the germicidal effect of UV -A radiation in sunlight. The WHO recommends SODIS as an effective method for treating water at the household level. SODIS is applied in many developing countries in an increasing number of households.

Action principle

The SODIS method uses the disinfectant effect of UV -A light (wavelength 320-400 nm) of solar radiation. At sufficiently long irradiation spread pathogens ( diarrhea, cholera, etc.) are largely destroyed. At temperatures above 50 ° C and a strong synergistic effect of UV -A radiation and heat can be observed, which further increases the efficiency of SODIS.

Application

PET beverage bottles are best suited for the application of SODIS in developing countries. PET and glass bottles are permeable as opposed to window glass for UV light. The diameter of a bottle with a capacity of max. 3 liters of approximately corresponds to the effective penetration depth of the UV-A radiation.

Microbially contaminated water is filled into PET bottles. For oxygen saturation, the bottle can be filled first three quarters, shaken for 20 seconds and then completely filled. The bottle is sealed and allowed to stand for six hours horizontally in direct sunlight. With an overcast sky, the bottles should remain exposed for two days.

Recommended duration of exposure of the SODIS bottles

In addition there are other methods for SODIS water treatment at household level. The boiling of water is an effective and widely used method, but is often associated with a large expenditure for the purchase of fuel. Chlorination and filtration also achieve good disinfection, complete sterilization of the water is thus not also achieved.

Methods for decentralized water treatment can play an important role for people who would remain at an accelerated expansion of the centralized water supply for years without access to clean drinking water. Selection of the appropriate method should be supported on criteria such as effectiveness, cost efficiency, the occurrence of specific types of pollution (eg chemical pollutants, salinity, turbidity), labor and the users' preferences.

Research and Development

The SODIS method was discovered by Professor Aftim Acra at the American University of Beirut and tested in Switzerland by Eawag (Water Research Institute of the ETH Domain ) for years.

Research on the effectiveness of SODIS, and especially clinical studies on the efficacy of the method are also performed at the Royal College of Surgeons in Ireland ( Ronan Conroy, Kevin McGuigan, Michael Elmore - Meegan T ).

The longtime director of the SODIS project at Eawag, Martin Wegelin, was awarded the 2006 prize for humanitarian services of the Swiss Red Cross.

Impact on the health of water users

The efficiency relating to the inactivation of SODIS diarrheal causing bacteria and viruses has been shown in many studies. Studies on the reduction of diarrhea by the administration of SODIS show a reducing effect of 30-80 %. The transmission of diarrheal diseases but can also be done through other channels in addition to the drinking water ( food, hands) and depends strongly on the sanitation and general hygiene from.

In a three year study of the Swiss Tropical Institute, the effectiveness of SODIS in Bolivia could not be proven statistically. The researchers attribute this to the poor and inconsistent application of SODIS. The study was published in PLoS Medicine.

Critical factors

The effectiveness of SODIS is critically dependent on the exposure time. If the water is exposed to sunlight for insufficient, the consumer may - as in untreated water - lead to sickness and diarrhea. The effectiveness of SODIS is also limited at high turbidity. At a turbidity of about 30 NTU, the water should be filtered prior to the application of SODIS. In developing countries, the availability of PET bottles is a critical factor for the application of SODIS.

The following factors should also be considered:

• Bottle Material: Certain types of glass (window glass), and PVC absorb UV -A- light, and are therefore not suitable for SODIS. Commercially available bottles made of PET or glass are recommended. The handling of PET bottles, however, is significantly more pleasant.
• Lifetime of PET bottles The performance of SODIS also depends on the state of the PET bottle. Heavily scratched bottles (eg by cleaning with sand) have a reduced permeability for UV -A light and should be replaced.
• Shape of the container: The intensity of UV -A radiation increases with the depth of penetration into the water sharply. At a depth of 10 cm (with a turbidity of 26 NTU) the intensity of UV -A radiation is still 50%. Commercially available PET bottles ( 0.5-2 liters) are best suited for SODIS.
• Oxygen: The presence of oxygen in the water is desirable in the SODIS process due to the formation of highly reactive free radicals by UV -A radiation. Oxygen also positively influenced the taste of water, as opposed to boiled water tastes oxygenated SODIS water, fresh '. Oxygen saturation can be achieved by shaking the filled 3/4 full bottle before exposure in the sunlight. Water from rivers, wells or water pipes usually contains enough oxygen and does not need to be ventilated.
• Release of chemicals from the bottle material: The possibility of a release of chemical substances from PET bottles into the water is sometimes represented as a potential health risk. One danger of antimony (a catalyst in PET polymerization) is, however, considered to be rather low. Recent studies show that come as critical factors for the increased diffusion of antimony from PET bottles a long storage and increased water temperature in question. The Eawag (Swiss Federal Institute for Aquatic Science and Technology) assumes, however, that "the storage for 6 hours and the temperature development of significantly less than 85 ° C must not exceed the 6 ppb from the study by Westerhoff et al. will cause. " This estimate is based on the experience that the maximum achievable temperature does not exceed 60 ° C when not covered and a maximum of 65 ° C in covered SODIS method.
• The also found in some studies, plasticizers ( phthalates and adipate ) do not come from the PET itself and can get themselves into the water or the PET most by cross-contamination. New studies find concentrations of plasticizers in water from PET bottles well below the WHO limits.

Worldwide application

The SODIS method is used since 2001 in more than 30 countries and tested, including in Ethiopia, Bhutan, Bolivia, Burkina Faso, Ecuador, El Salvador, Ghana, Guatemala, Guinea, Honduras, India, Indonesia, Cambodia, Cameroon, Kenya, Congo, Laos, Malawi, Mozambique, Nepal, Nicaragua, Pakistan, Peru, Philippines, Senegal, Sierra Leone, Zimbabwe, Sri Lanka, Tanzania, Togo, Uganda, Uzbekistan and Vietnam, and has achieved more than two million people. It has been shown that perpetrators of diarrhea contaminated water was drinkable after treatment without risks. In the test areas diarrhea, cholera, salmonella and similar diseases have declined markedly.

SODIS projects are, among others, funded by the SOLAQUA Foundation, various Lions Clubs, Rotary Clubs, Migros, and the Michel Comte Water Foundation.