SODIS

Solar water disinfection (SODIS) is a simple, low-cost method for disinfecting water using sunlight and plastic bottles. It is particularly useful in developing countries where access to clean drinking water is limited. The process involves filling transparent plastic bottles with contaminated water and exposing them to direct sunlight for a period of time, typically six hours. The ultraviolet (UV) radiation from the sun, along with the increase in temperature, works to inactivate pathogens present in the water.

History

The concept of using sunlight to disinfect water has been known for centuries, but the modern SODIS method was developed in the 1980s by researchers at the Swiss Federal Institute of Aquatic Science and Technology (Eawag). The method gained international recognition in the 1990s and has since been promoted by various organizations, including the World Health Organization (WHO) and the United Nations Children's Fund (UNICEF).

Mechanism

SODIS relies on two main mechanisms to disinfect water:

1. Ultraviolet Radiation: UV-A radiation (wavelength 320-400 nm) from the sun penetrates the water and damages the DNA of microorganisms, preventing them from reproducing and causing disease.
2. Thermal Inactivation: The heat generated by sunlight can raise the temperature of the water, enhancing the disinfection process. Temperatures above 50°C can significantly reduce the time required for effective disinfection.

Procedure

The SODIS method involves the following steps:

1. Selection of Bottles: Use clear PET plastic bottles, as they allow UV radiation to penetrate effectively. Glass bottles can also be used, but they are heavier and more fragile. 2. Filling the Bottles: Fill the bottles with water, leaving a small air gap at the top to allow for expansion. 3. Exposure to Sunlight: Place the bottles horizontally on a reflective surface, such as a corrugated iron sheet, to maximize exposure to sunlight. Ensure that the bottles are exposed for at least six hours on a sunny day or up to two days if the weather is cloudy. 4. Cooling and Storage: After exposure, allow the water to cool before consumption. Store the treated water in a clean, covered container to prevent recontamination.

Effectiveness

SODIS is effective against a wide range of pathogens, including bacteria, viruses, and protozoa. However, it is less effective against chemical contaminants and turbid water. For highly turbid water, pre-treatment such as filtration or sedimentation is recommended.

Advantages

• Low cost and easy to implement.
• No need for chemical additives or complex equipment.
• Utilizes renewable solar energy.
• Can be used in remote areas without access to electricity.

Limitations

• Requires consistent sunlight, making it less effective in regions with frequent cloud cover.
• Not suitable for large volumes of water.
• Does not remove chemical pollutants.

Applications

SODIS is widely used in rural and peri-urban areas of developing countries. It is particularly beneficial in emergency situations where access to clean water is disrupted.

Research and Development

Ongoing research aims to improve the efficiency of SODIS by exploring the use of different materials for bottles, enhancing UV penetration, and combining SODIS with other water treatment methods.

Also see

• Water purification
• Ultraviolet germicidal irradiation
• Point-of-use water treatment
• Safe drinking water