Introduction

One of the simplest and cheapest treatment methods for microbiologically contaminated water is SODIS - Solar Water Disinfection. SODIS uses solar radiation to destroy pathogens. It is used for small quantities of water and requires only a transparent plastic bottle and sun to make safe drinking water. In this method bottle is filled with contaminated water and is exposed to full sunlight for six hours. After this period of time water is free of pathogens and can be drunk.    

Girls using SODIS in Indonesia; source: www.sodis.ch

How does SODIS work?

SODIS needs solar energy to improve microbiological quality of water. The sun continuously emits energy in the form of radiation at wavelengths of ultraviolet, infrared and visible bands. Key factors for SODIS application are UV-A radiation and infrared radiation.

UV radiation lies in the shortest wavelengths band. This kind of radiation cannot be perceived by human eye. It is a damaging radiation that can cause detrimental effects to skin, eyes and living cells. Earth is protected from UV-B and UV-C light (radiation between 200-320nm) as this radiation is absorbed by the ozone layer. Nevertheless, the UV-A radiation reaches the Earth and its aggressiveness can be used to fight with human pathogens that threaten safety of drinking water.

Human pathogens are very sensitive to sunlight as their natural environment is gastrointestinal tract. UV-A light interacts with the DNA, nucleic acids and enzymes of the living cells changing their molecular structure and leading to cell death.

UV light reacts also with oxygen dissolved in the water and produces highly reactive forms of oxygen - oxygen free radicals and hydrogen peroxides that interfere with the cell structure and kill the pathogens.

Infrared radiation is the light of the wavelength beyond 700nm. This radiation cannot be seen by a human but it can be felt as a heat. It raises water temperature and kills microorganisms that are sensitive to heat. Water does not have to be boiled to be disinfected. Heating it up to 50-60�C for one hour is enough to obtain the same effect.

The table below represents the temperature and exposure time required to perish microorganisms.

Thermo resistance of microorganisms, source: SANDEC at EAWAG, October 2002: "Solar Water Disinfection - A Guide for the Application of SODIS"

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How to use SODIS?

Before SODIS application one should check if the climate and weather conditions are suitable for SODIS (see >> Weather and Climates needs for SODIS). Then one should get plastic PET-bottles of 1- 2 litre volume. It is recommended to have at least 4 bottles for each member of the family. Two bottles should be exposed to the sun while the other 2 bottles are ready for consumption. The bottles should be checked if they are water tight and suitable support for exposing the bottle should be chosen (for example CGI sheet). Before exposing to the sun water turbidity should be checked (which should be lower than 30 NTU). (see>> SODIS and turbidity) If all these conditions are fulfilled one can follow the steps below:

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Weather and climate needs for SODIS

SODIS cannot be used everywhere because solar radiation does not reach every area of the Earth in sufficient amount to treat water effectively.

The best locations for SODIS application are latitudes between 15�N and 35�N as well as 15�S and 35�S. These are semi -arid regions with the highest amount of solar radiation that reaches the Earth directly due to limited cloud cover and rainfall. It is estimated that these latitudes have less than 250 mm of rain and about 3000 hours of sunshine annually.

The second most favourable region for SODIS application lies between 15�N and 15�S latitudes. Here the conditions are a little bit worse as it is estimated that there are about 2500 hours of sunshine annually. Radiation is scattered due to high humidity and frequent cloud cover. Nevertheless, the location is still very good for SODIS application.

While thinking about SODIS application one should also consider seasonal and daily weather variations. During completely overcast days for example the intensity of UV-A can be reduced to even one third of the radiation intensity that occurs during cloudless day.

One should remember that weather conditions play very important role in making safe drinking water. For SODIS application:

• "the container needs to be exposed to the sun for 6 hours if the sky is bright or up to 50% cloudy
• the container needs to be exposed to the sun for 2 consecutive days if the sky is 100% cloudy,
• during days of continuous rainfall, SODIS does not perform satisfactorily. Rainwater harvesting is recommended during these days.
• if a water temperature of at least 50�C is reached, an exposure time of 1 hour is sufficient"*

* EAWAG/ SANDEC : "Solar Water Disinfection " How Does It Work?", section: "Key Factors of SODIS Application"

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SODIS and Turbidity

Efficiency of SODIS is reduced in turbid water. Suspended particles disturb radiation penetration and because of it microorganisms may not be enough irradiated. Thus, it is required to have water of less than 30 NTU (Nephelometric Turbidity Units) for SODIS disinfection.

If turbidity is higher than 30 NTU then water should be first pretreated. It can be done by storing water the day before and letting the particles to settle down during the night. Particles can be separated using filtration, using sand layer or cloth. Turbidity can be eliminated also by flocculation and sedimentation using aluminium sulphate or crushed Moringa oleifera seeds. If water turbidity cannot be removed by all methods described above it is recommended to pasteurize or boil water.

To make the measurements of turbidity easier for SODIS users there is a special method for turbidity evaluation. The instruction is below.

"Water turbidity Test", source: SANDEC at EAWAG: "Solar Water Disinfection - A Guide for the Application of SODIS"

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SODIS and Oxygen

Aerated water is much better disinfected than not aerated. It is because the sunshine in contact with oxygen produces highly reactive forms of oxygen - oxygen free radicals and hydrogen peroxides. Due to exposure of pathogens to these reactive forms of oxygen microbes are being oxidized. This process is called photo-oxidative disinfection.

Aeration of water is obtained by shaking bottles (3/4 filled with water) during the period of 20 seconds before filling it completely and exposing to sun. Especially stagnant water drawn from ponds, wells, cisterns should be aerated before treatment. Bottles should not, however, be shaken during the exposure time as such activity reduces the SODIS effectiveness.

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SODIS and Containers

When choosing the material for SODIS container one should remember that it should well transmit solar radiation to its content. Plastic bottles made from PET (PolyEthylene Terephtalate) and PVC (PolyVinylChloride) are both good transmitters of light in the UV-A and visible range of solar spectrum. Both materials, however, contain UV-stabilizers to protect from oxidation and UV radiation. PET bottles contain much less such additives than PVC bottles thus it is recommended to use PET containers for SODIS application.

The bottle should not also be coloured as such material do not transmit enough UV radiation.

Glass bottles may be better from plastic because they are heat resistant and there are no photoproducts. They are, however, heavier and more expensive. Moreover, glass bottles are not good transmitters for UV-A radiation. Transmission of UV light through glass is determined by iron oxide content and for instance an ordinary window glass of 2 mm thickness transmits almost no UV-A light. Only specific glasses like Pyrex, Corex, Vycor, Quartz transmit more UV light than the ordinary window glass.

Another factor that should be taken into consideration is the shape of the container. UV radiation mitigates with increasing water depth. It is recommended that containers for SODIS should not exceed a water depth of 10 cm. This condition is fulfilled by PET bottles if they are being horizontally exposed.

PET bottles have a small area for sunlight exposure. Containers with a larger exposing area per water volume would be more efficient but PET soft drink bottles are often easily available and thus more practical for the SODIS application.

SODIS bottles might be placed on a black surface which enhances the disinfection as it induces a temperature gradient which causes the water to circulate within the container thereby improving the inactivation efficiency.

In general, PET bottles are said to be the best choice for SODIS application. They are the best UV light transmitters, they have low weight, are cheap, available, relatively unbreakable, transparent, neutral in taste, chemically stable and are easy to handle. PET bottles can be closed, thereby the risk of recontamination of the purified water is reduced. The only disadvantage of these containers is that they have limited heat resistance (65�C) and scratches or other aging effects may limit their application. Transmittance may be reduced to mechanical scratches or due to photoproducts, thus heavily scratched or blind bottles should be replaced.

source: www.sodis,ch

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Place of exposure

SODIS bottles should be exposed in a place that is in a full-sun during the day like on the roof for instance. They may be also put on corrugated iron sheet supports that reflect the sunlight thus magnifying the activity of the solar radiation. One should be very careful and check whether the SODIS bottle is really exposed to the solar radiation or if the place of exposure is not partially in shadow during the day as it can be the key factor of SODIS failure.

"Corrugated iron sheets are the best bottle supports for SODIS; source: www.sodis,ch

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Other considerations

One should remember that SODIS does not produce sterile water. Some organisms like algae for example can easily adapt to SODIS conditions and may grow inside of a bottle. These organisms however are not dangerous to human health.

Not sterile water may pose a risk for very young children. When babies are about 6 months old, women start to complement breast-milk with other food. It is very critical moment and statistics from developing countries show high morbidity and death risk of couple-months old children due to diarrhoeal diseases. As SODIS does not make completely sterile water there is a risk of infection of very young and fragile babies. Thus it is much safer to use boiled water that is 100% sterile for children up to 18 months old.

Also boiled, not SODIS, water should be used for people severely ill, malnourished, with decreased immuno-deficiency (AIDS) and for those with gastro-intestinal abnormalities or chronic gastrointestinal illnesses.

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1. www.sodis.ch
2. SANDEC (Department Water and Sanitation in Developing Countries) at EAWAG (Swiss Federal Institute of Environmental Science and Technology ), October 2002: "Solar Water Disinfection - A Guide for the Application of SODIS"