UV treatment / Solar disinfection (SODIS)

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SODIS (Solar Water Disinfection Process) uses sunlight to inactivate harmful microorganisms in water almost completely. Transparent plastic (PET) or glass bottles filled with contaminated, but clear, water are exposed to the sun for six hours in sunny weather or two days in cloudy weather. If a temperature of 50 degrees Celsius is reached, an exposure time of one hour is sufficient.

Plastic bottles filled with contaminated water are placed in the sunlight

This technology was awarded a prize by SIMAVI and organisations such as SANDEC are promoting it in many developing countries. Solar disinfection is recommended by the World Health Oganization (WHO) as one viable option for drinking water treatment at household level.


Advantages Disadvantages
Very cheap.
Independent from energy sources other than sunlight.
Treated water is protected from re-contamination in the bottles.
The taste of treated water is fresh, not stale or otherwise altered. 		Cannot be used on days with continuous rainfall
Cannot be used to treat very turbid water (>30 NTU).
Bottles need to be replaced every 4-6 months.



Disinfection mechanism

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:

  • UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
  • UV-A (wavelength 320-400nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens.
  • Infrared radiation heats the water. If the water temperatures raises above 50°C, the disinfection process is three times faster.

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.

Guidelines for the application at household level

Pictograms SODIS.jpg

  • Water from contaminated sources are filled into transparent water bottles. For oxygen saturation, bottles can be filled three quarters, then shaken for 20 seconds (with the cap on), then filled completely. Highly turbid water (turbidity higher than 30 NTU (Nephelometric Turbidity Unit) must be filtered prior to exposure to the sunlight.
  • Filled bottles are then exposed to the sun. Better temperature effects can be achieved if bottles are placed on a corrugated roof as compared to thatched roofs.
  • The treated water can be consumed. The risk of re-contamination can be minimized if water is stored in the bottles. The water should be consumed directly from the bottle or poured into clean drinking cups. Re-filling and storage in other containers increases the risk of contamination.

Suggested treatment schedule

Weather Conditions Minimum Treatment Duration
Sunny
50% cloudy
50-100% cloudy
continuous rainfall
 6 hours
6 hours
2 days
unsatisfactory performance, use rainwater harvesting

Health impact, diarrhea reduction

The SODIS method (and other methods of household water treatment) can very effectively remove pathogenic contamination from drinking water. However, infectious diseases are also transmitted through other pathways such as direct person-to-person contact, food, or unhygienic living conditions. Lack of sanitation and hygiene practices exacerbate the problem. Studies on the reduction of diarrhea among SODIS users show reduction values of 30-80%

Worldwide application of SODIS

Numbers: Used by more than 1,000,000 in Asia, Africa and Latin America.

Worldwide application of SODIS in projects coordinated by Eawag

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.

Costs

Very low, re-use of plastic bottles. According to a comparative study on the cost-effectiveness of different household water treatment systems, SODIS is the least expensive method with an annual mean cost of US$0.63 per person.

Applying conditions

If the water bottles are not exposed to sunlight for the proper length of time, the water may not be safe to drink and could cause illness. If the intensity of sunlight is reduced, i.e. due to overcast weather, a longer exposure time is necessary.

The following issues should also be considered:

  • Bottle material: Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of PET (Polyethylene terephthalate) are recommended. The handling is much more convenient in the case of PET bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used. Glass also blocks UV rays and therefore would be ineffective.
  • Aging of plastic bottles: SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of wear reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.
  • Shape of Containers: the intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10cm and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and thus most practical for the SODIS application.
  • Oxygen: Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application of SODIS.
  • Leaching of bottle material: There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused PET-bottles in the water during solar exposure. The levels of concentrations found in the water after a solar exposure of 17 hours in 60°C water were far below World Health Organization (WHO) guidelines for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high quality tap water.
    Concerns about the general use of PET-bottles were also expressed after a report published by researchers from the University of Heidelberg on antimony being released from PET-bottles for soft drinks and mineral water stored over several months in supermarkets. However, the antimony concentrations found in the bottles are orders of magnitude below WHO [1] and national guidelines for antimony concentrations in drinking water. Furthermore, SODIS water is not stored over such extended periods in the bottles.

Games (Dutch)

Game Solar Disinfection

SODIS Manual

Although SODIS is very easy to do, SODIS has a pamphlet with more specific information.
A more extensive manual is available online in different languages.

Movies

External links

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