Solar Water Disinfection (SODIS) is a straightforward, low-cost method for purifying biologically contaminated water using only sunlight and transparent containers. The technique relies on the sun’s natural energy to inactivate waterborne pathogens. This makes SODIS an accessible solution for households lacking centralized water treatment or during emergencies. As a decentralized approach, individuals can treat small quantities of water at the point of use, significantly improving microbial quality. Global health organizations widely recommend this process for household water treatment.
How Sunlight Inactivates Pathogens
The disinfection mechanism of SODIS is based on the combined action of two distinct components of solar radiation: ultraviolet (UV) light and heat. The primary driver is the UV-A radiation, which has wavelengths between 320 and 400 nanometers. This radiation penetrates the water and causes photochemical damage to the DNA and RNA of pathogenic bacteria, viruses, and protozoa.
UV-A exposure also triggers the production of highly reactive oxygen species, such as hydrogen peroxides, within the water. These powerful molecules react with cellular structures, contributing an additional layer of oxidative damage that leads to the destruction of the microorganisms. This optical inactivation occurs even at ambient temperatures, but its effectiveness is significantly amplified by the sun’s thermal energy.
The secondary mechanism is the thermal effect, which comes from the infrared and visible light spectrum of the sun heating the water. When the water temperature rises above 50°C (122°F), the disinfection process is substantially accelerated. This heat causes the proteins within the microbial cells to denature, which halts their replication and effectively kills the pathogens. The combined effect of UV-A radiation and heat is synergistic, meaning the total inactivation is greater than the sum of the two effects.
Practical Steps for Disinfecting Water
Successfully executing the SODIS method begins with selecting the correct container, which should be a clear, colorless polyethylene terephthalate (PET) plastic bottle, typically two liters or smaller. Glass bottles are generally not used because they often block the UV radiation needed for the process. The bottles must be thoroughly cleaned, and any labels or residual glue should be removed to ensure maximum light penetration.
Next, the bottles are filled with the source water, leaving a small air gap at the top. It is beneficial to shake the bottle vigorously for about 30 seconds before sealing it, as this action oxygenates the water. The presence of dissolved oxygen is helpful because it enhances the production of the reactive oxygen species that aid in the inactivation process.
The filled bottles are then placed horizontally in a location that receives full, uninterrupted sunlight. Placing the bottles on a dark, reflective surface, such as corrugated iron or aluminum foil, significantly improves the process. The reflective surface maximizes solar radiation hitting the bottle and helps the water reach higher temperatures faster, accelerating disinfection. The bottles must remain in this position for the required exposure time.
Conditions for Successful Disinfection
The efficacy of solar water disinfection is highly dependent on a few specific environmental and water quality factors. Foremost among these is water clarity, which is measured by turbidity. The water must have a low turbidity level, ideally less than 30 Nephelometric Turbidity Units (NTU), for the process to be effective. Cloudiness in the water, caused by suspended particles, blocks the penetration of the UV-A light, drastically reducing the disinfection rate.
The required exposure time is determined by the weather conditions and solar intensity. Under conditions of full, bright sunlight, the water must be exposed for a minimum of six hours. If the sky is partially cloudy, or more than 50% covered by clouds, the recommended exposure time is extended to two consecutive days. However, if the water temperature within the bottle reaches 50°C or higher, the required exposure time can be safely reduced to just one hour due to the accelerated thermal inactivation.
It is important to understand the limitations of the SODIS method, as it is strictly a microbiological treatment. While highly effective against bacteria, viruses, and protozoa, it does not remove chemical contaminants. The process will not eliminate heavy metals, pesticides, or other toxic substances. Reuse of PET bottles for this purpose is generally safe, with no significant leaching of plastic components at the temperatures typically reached during the process.