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Key Points of
Solar Disinfection

The Effectiveness of Solar Disinfection

Bacteria, viruses, and other pathogens in water and food can cause diarrhea, stomach ache, fever, and death. These pathogens are too small to be seen, but can be detected with appropriate testing. Many of these pathogens are present in human and animal feces and can pass into water.

Solar disinfection, which relies on the ultraviolet (UVA) component of daylight, will inactivate bacteria, viruses, and some cysts. It has been proven effective against cholera, typhoid, dysentery, crytosporidium, giardia, many forms of diarrheal disease, and polio.

How to Use Solar Disinfection

The following are basic points of solar disinfection. They must be elaborated and adapted to local conditions.

1. Place water in clean plastic (PET or PETE plastic) or glass bottles with labels removed. Bottles should have a top and be clear but may have a slight bluish cast. Mineral water bottles are commonly used. The bottles should be one to two liters. Larger bottles have too much water for the light to penetrate.

2. Do not use water which is too turbid or cloudy. Large dark print placed next to or under the bottle should be legible when viewed through the bottle of water. If excessive turbidity is present, allow it to settle in a separate container and then pour the clear water off the top into a bottle. If this does not remove excessive turbidity, prefilter the water through clean sand or several layers of cloth.

3. Aerate the water to enhance disinfection by leaving about one inch of air at the top of the bottle, securing the cap, and then shaking the bottle for about 15 seconds. The air renders the bacteria and viruses more water soluble and thus more easily disinfected by the ultraviolet light.

4. It is best if the bottles are inclined to receive the most sunlight. A common method is to place them on a corrugated roof or a piece of corrugated roofing material away from animals. In warm climates it is beneficial to place the bottles on a blackened surface to warm them to 50 degrees Celsius in order to hasten the disinfection process. When this temperature cannot be reached, the bottles can be placed on a reflective surface to increase the efficiency of the disinfection process.

5. Expose the bottles for four to five hours in full mid-day sun or for a full day in cloudy weather. They should not be in the shade. Many people find it convenient to place a set of bottles outside for a full day for use the following day. After proper exposure, the bacteria and viruses in the water will be disinfected and safe to drink.

6. Replace scratched, damaged, or cloudy bottles with new bottles. A program for the distribution of new bottles and recycling of old bottles should be developed, especially if plastic bottles are used.

7. Where bottles are not available, water can be placed in clear plastic bags, shallow pans, or shallow "plastic puddles" made with plastic sheets or tarpaulins. If possible, water in pans or "plastic puddles" should be covered with clear plastic sheeting to protect it from bird droppings, insects, and other contamination.

8. Do not burn plastic. Burning plastic can be highly toxic.

Advantages and Limitations of Solar Disinfection

Solar disinfection is a simple, low cost method for disinfecting small quantities of drinking water. By relying on the ultraviolet (UVA) in daylight to disinfect bacteria, viruses, and other pathogens in the water, it does not require any chemicals and does not change the color or taste of water.

Solar disinfection does not require the availability or purchase of chemicals or equipment and is thus immediately accessible to millions of people throughout the world. It is appropriate and highly effective for use in emergencies and disasters and on an on-going basis in daily life where clean water is not available.

By helping reduce the incidence of diarrheal diseases, the use of solar disinfection can reduce human suffering, the economic cost of disease, and the additional burden on women of caring for sick children. Since it reduces the need for fuel and firewood, it has beneficial environmental effects.

Although solar disinfection disinfects bacteria and viruses, its effect on some cysts and worms has not been fully researched and should not be assumed. It does not remove pesticides or chemical contaminants. It cannot be used with very turbid or cloudy water.

Drinking water purified by solar disinfection during one part of the day does not protect against consumption of contaminated drinking water at other times. While promoting health, clean water should not be considered to be a "medicine".

Use of solar disinfection should be combined with good hygiene such as washing hands after using the toilet, after cleaning a baby, after handling animals and before eating in order to reduce the incidence of disease.

For further information about GRI's Sun Water program, to offer suggestions, to discuss ways of partnering with us, and to engage in a dialogue regarding solar disinfection, please contact Peter Moulton, Ph.D., Executive Director at <>

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  last modified January 2006