Solar water disinfection, often referred to as SODIS, is a straightforward method for purifying water using sunlight. It improves drinking water quality by inactivating harmful pathogens like bacteria, viruses, and some protozoa, which cause waterborne illnesses. SODIS relies on transparent plastic bottles and natural sunlight. It is a low-cost, decentralized solution for making contaminated water safe to consume.
The Science Behind Solar Water Disinfection
The effectiveness of SODIS stems from two effects of solar radiation: ultraviolet-A (UV-A) radiation and increased water temperature. UV-A radiation, specifically wavelengths between 320 and 400 nanometers, damages the DNA of pathogenic microorganisms, preventing reproduction and illness. This damage also occurs through reactive oxygen species that disrupt microbial cell structures.
Elevated water temperatures contribute to the disinfection process. When water temperatures rise above 50°C (122°F), the inactivation of microbes accelerates. At these higher temperatures, proteins within microorganisms, which are necessary for their survival, become denatured. The combined action of UV-A light and heat creates a more potent germicidal effect than either factor alone.
Applying Solar Water Disinfection at Home
To apply SODIS, gather clear, colorless PET plastic bottles, commonly used for beverages (1 to 3 liters). Bottles must be clean and free of scratches, which can reduce UV light penetration.
Before filling, ensure water is clear. If turbid, meaning cloudy or murky, pre-filter through a cloth to achieve a turbidity below 30 NTU. Fill bottles with water, leaving a small air gap, and cap them securely.
Place bottles horizontally in direct sunlight for maximum exposure. A reflective surface, like corrugated iron, can enhance light absorption and temperature, speeding up disinfection. Ensure no shading obstructs sunlight.
Exposure time depends on weather. On clear, sunny days, six hours of direct sunlight is usually sufficient. If skies are cloudy, expose bottles for two consecutive days. Do not use SODIS during continuous heavy rainfall, as insufficient sunlight prevents purification.
Advantages and Important Considerations of SODIS
SODIS offers several advantages. It is low-cost and simple to implement, requiring minimal investment (around $0.63 per person per year). The method is environmentally friendly, relying solely on sunlight without chemicals or external energy sources. It is effective against a broad spectrum of waterborne pathogens, significantly improving the microbiological quality of drinking water.
Despite its benefits, SODIS has important considerations. Its effectiveness depends on consistent sunlight, making it less suitable during prolonged cloudy or rainy periods. Treatment time (6 hours to two days) means users need to plan for a continuous water supply.
SODIS treats small volumes (1 to 3 liters per bottle), often requiring many bottles for household needs. Water must have low turbidity; high cloudiness reduces UV light penetration and disinfection efficiency.
While SODIS targets biological contaminants, it does not remove chemical pollutants like arsenic, fluoride, or heavy metals. Storing water in the original treatment bottle reduces recontamination risk, but proper handling is still necessary to prevent reintroduction of pathogens.