Solar water disinfection, or SODIS, is a way to clean water using sunlight. It makes water safe to drink by killing harmful living things like bacteria, viruses, and parasites. Water that has harmful chemicals or heavy metals needs more steps to become safe.
SODIS often uses a mix of solar electricity, solar heat, and solar ultraviolet light. Solar electricity from solar panels can power processes that use electricity to clean water, such as creating substances that destroy germs. Another method stores solar electricity in batteries to power ultraviolet lamps at night or in low light.
Solar thermal water disinfection uses the sun’s heat to warm water to 70–100 °C for a short time. This can be done with solar collectors that use lenses, reflectors, or insulation. Some methods heat water in batches, while others work continuously while sunlight is available. Water heated below 100 °C is called pasteurized water.
Sunlight also has ultraviolet light that can kill germs. The SODIS method uses sunlight and recycled plastic bottles to combine UV light with heat to clean water. It is a free and effective way to treat water at home and is recommended by the World Health Organization. SODIS is used in many developing countries, and guides explaining the method are available in many languages.
Process for household application
Instructions for using SODIS at home explain the steps involved. Clear, transparent PET water or soda bottles with a capacity of 2 liters or less and minimal surface scratches are chosen for use. Glass bottles are also acceptable. All labels are removed, and the bottles are washed before first use. Water from possibly contaminated sources is poured into the bottles, using the clearest water available. If the water is cloudy and has a turbidity higher than 30 NTU, it must be filtered or have particles removed before sunlight exposure. Simple filters can be made locally by stretching cloth over inverted bottles with their bottoms cut off. To increase oxygen in the water, the guidelines suggest filling the bottles three-quarters full, shaking them for 20 seconds with the cap on, then filling completely, closing them tightly, and checking for clarity.
The filled bottles are then placed in direct sunlight. Bottles heat faster and more intensely when placed on a sloped, sun-facing reflective metal surface. A corrugated metal roof or a slightly curved sheet of aluminum foil can increase the amount of light inside the bottle. Overhanging structures or plants that block sunlight should be avoided, as they reduce both light and heat. After enough time in the sun, the treated water can be drunk directly from the bottle or poured into clean drinking cups. Storing the water in the bottles reduces the risk of re-contamination. Refilling the water into other containers increases the chance of contamination.
The best areas for using SODIS are between 15°N and 35°N, and between 15°S and 35°S. These regions receive high levels of sunlight, with little cloud cover or rain, and over 90% of sunlight reaching Earth’s surface as direct radiation. The second-best areas are between 15°N and 15°S. These regions have high levels of scattered sunlight, with about 2,500 hours of sunshine each year due to high humidity and frequent clouds.
Teaching communities how to use SODIS is important to prevent confusion between PET bottles and other types of containers. Using SODIS without properly evaluating current hygiene habits and the rate of diarrhea may not address other infection sources. Trainers who teach others must first be trained themselves.
Applications
SODIS is a useful way to treat water when fuel or cooking tools are not available or too costly. Even when fuel is available, SODIS is often cheaper and better for the environment. However, SODIS works only if enough bottles are available and the water is not too cloudy. If the water is very cloudy, SODIS alone cannot make it safe, and extra filtering is needed first.
To check if water is too cloudy for SODIS, a simple test can be done. The user places a filled bottle upright on a newspaper headline and looks down through the bottle. If the letters on the headline are clear, the water is not too cloudy and can be treated with SODIS. If the letters are not clear, the water is likely too cloudy (more than 30 NTU), and it must be cleaned before using SODIS.
SODIS could be used in places like disaster areas or refugee camps. However, providing bottles might be harder than giving disinfecting tablets with chlorine, bromine, or iodine. Also, ensuring water stays in the sun long enough for SODIS to work might be difficult in some situations.
Other ways to treat and store water safely include chlorination, flocculation/disinfection, and filtration. The best method depends on factors like how well it works, other types of pollution (such as cloudiness or chemicals), cost, effort needed, convenience, and user preference.
If water is very cloudy, SODIS alone cannot be used. Extra filtering or flocculation must be done first to make the water clearer. Recent studies show that common table salt (NaCl) can help reduce cloudiness in some types of soil, making SODIS usable in more areas at lower costs.
SODIS can also be done with plastic bags. Bags may work better than bottles because they reach higher temperatures, which speeds up treatment. Bags with water layers between 1 cm and 6 cm heat up more easily and treat Vibrio cholerae more effectively. This is likely because bags have a better surface area to volume ratio. In remote areas, bottles are not always available and must be brought from cities, which can be expensive and inefficient. Bags can be packed more tightly and shipped at lower costs, making them a better choice for some communities. However, bags can sometimes make water smell like plastic, are harder to handle when full, and usually require transferring water to another container for drinking.
A key advantage of using SODIS bottles instead of bags or other methods is that bottles are point-of-use treatment tools. Point-of-use means the water is treated in the same container it will be used from, reducing the chance of contamination after treatment.
Cautions
If water bottles are not left in the sun for the correct amount of time, the water inside might not be safe to drink and could make people sick. If the sunlight is weaker, such as on cloudy days or in areas with less sunlight, the bottles need to stay in the sun for a longer time.
The following issues should also be considered:
Health impact, diarrhea reduction
The World Health Organization reports that more than two million people die each year from diseases caused by unsafe water, and one billion people do not have access to safe and clean drinking water. Research shows that the SODIS method (and other ways to treat water at home) can remove harmful germs from water very well. However, diseases can also spread through poor sanitation and lack of good hygiene. Studies on people who use SODIS show that it can reduce cases of diarrhea by 30% to 80%.
Research
The effectiveness of SODIS was first discovered by Aftim Acra, from the American University of Beirut, in the early 1980s. Later research was conducted by Martin Wegelin’s team at the Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Kevin McGuigan’s team at the Royal College of Surgeons in Ireland. Clinical trials were led by Ronan Conroy of the RCSI team, working with Michael Elmore-Meegan.
A joint research project on SODIS involved these institutions:
• Royal College of Surgeons in Ireland (RCSI), Ireland (led the project)
• University of Ulster (UU), United Kingdom
• CSIR Environmentek, South Africa; EAWAG, Switzerland
• Institute of Water and Sanitation Development (IWSD), Zimbabwe
• Plataforma Solar de Almería (CIEMAT-PSA), Spain
• University of Leicester (UL), United Kingdom
• International Commission for the Relief of Suffering and Starvation (ICROSS), Kenya
• University of Santiago de Compostela (USC), Spain
• Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Switzerland
The project studied SODIS in multiple countries, including Zimbabwe, South Africa, and Kenya.
Other developments include a continuous flow disinfection unit and solar disinfection using titanium dioxide film on glass cylinders, which helps prevent bacteria from growing again after SODIS.
Research has shown that some low-cost additives can speed up SODIS, making it work better in both sunny and cloudy weather. A 2008 study found that powdered seeds from five legumes—cowpea, black lentil, soybean, green pea, and peanut—worked as well as commercial alum to remove cloudiness from water. These seeds used less than 1 gram per liter, worked quickly, and did not change water hardness or pH. Later studies tested chestnuts, oak acorns, and Moringa oleifera for the same purpose.
Other research has explored using doped semiconductors to increase oxygen radical production under solar UV-A light. Recently, scientists at Dublin City University created a low-cost, printable UV dosimeter for SODIS that can be read with a mobile phone. The phone’s camera takes a picture of the sensor, and special software on the phone analyzes the sensor’s color to measure UV exposure.
In remote areas, wood smoke increases lung disease because people often build fires to boil water and cook. Studies found that boiling water is often skipped due to the difficulty of gathering scarce wood. When given simple water treatment options, people in remote parts of Africa preferred SODIS over boiling or other methods.
A simple solar water purifier for rural homes was developed using four layers of saree cloth and solar tubular collectors to remove all coliforms.
In July 2020, researchers reported creating a reusable aluminum surface that can clean water using sunlight to meet WHO and EPA standards for safe drinking water.
Promotion
The Swiss institute that studies water and technology, called EAWAG, works with a group named Sandec to help promote SODIS in 33 countries. These countries include Bhutan, Bolivia, Burkina Faso, Cambodia, Cameroon, DR Congo, Ecuador, El Salvador, Ethiopia, Ghana, Guatemala, Guinea, Honduras, India, Indonesia, Kenya, Laos, Malawi, Mozambique, Nepal, Nicaragua, Pakistan, Peru, Philippines, Senegal, Sierra Leone, Sri Lanka, Togo, Uganda, Uzbekistan, Vietnam, Zambia, and Zimbabwe.
SODIS projects are supported by groups such as the SOLAQUA Foundation, Lions Clubs, Rotary Clubs, Migros, and the Michel Comte Water Foundation.
SODIS has also been used in some communities in Brazil, including a village called Prainha do Canto Verde in Beberibe, which is west of Fortaleza. In this area, villagers have had success using SODIS because temperatures often reach over 40°C (104°F), and there is not much shade.
For public health workers helping communities find safe, affordable, and long-lasting ways to treat water, it is important to teach people how clean water affects health and disease prevention. They also need to explain how SODIS and other water treatment methods work. While some communities have been hesitant to use these methods, sharing information about their health benefits can help increase their use.