Solar water disinfection, or SODIS, is a portable method for cleaning water that uses sunlight to make water safe to drink. Water contaminated with living germs, such as bacteria, viruses, and parasites, can be treated using this method. However, water polluted with harmful chemicals or heavy metals needs extra steps to become safe for drinking.

SODIS typically uses a mix of electricity from solar panels, heat from the sun, and ultraviolet light from sunlight. When electricity from solar panels is used, it creates a current that helps disinfect water by producing chemicals that destroy germs. Another method stores solar electricity in a battery and uses it at night or during low light to power a lamp that emits ultraviolet light to further clean the water.

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 or reflectors to focus sunlight, or with insulation to trap heat. Some systems treat water in batches, while others work continuously while sunlight is available. Water heated to less than 100 °C is called pasteurized water.

Ultraviolet light in sunlight can also kill germs in water. The SODIS method uses both UV light and heat from the sun to disinfect water using only sunlight and recycled plastic bottles. This method is free, effective, and used at the household level. It is supported by the World Health Organization as a reliable way to treat and store water safely. SODIS is already used in many developing countries, and educational materials about the process are available in many languages, including translations of the English version.

Process for household application

Guides for using SODIS at home explain the steps to follow. Clear, transparent PET water or soda bottles that are 2 liters or smaller and have only a few scratches on the surface are chosen. Glass bottles can also be used. All labels are removed, and the bottles are washed before first use. Water from possibly dirty sources is filled into the bottles, using the clearest water available. If the water is too cloudy (more than 30 NTU), it must be filtered or have particles removed before sunlight exposure. Filters can be made locally by stretching cloth over bottles with their bottoms cut off and turned upside down. To help oxygen mix better in the water, bottles should be filled three-quarters full, shaken for 20 seconds (with the cap on), then filled completely, recapped, and checked for clarity.

The filled bottles are placed in direct sunlight. Bottles heat faster and more intensely if placed on a sloped, sun-facing reflective metal surface. A metal roof with ridges or a slightly curved sheet of aluminum foil can increase the light inside the bottle. Overhanging structures or plants that block sunlight should be avoided, as they reduce both light and heating. After enough time, the treated water can be drunk directly from the bottle or poured into clean drinking cups. Storing water in the bottles reduces the risk of re-contamination. Refilling or storing water in other containers increases the chance of contamination.

The best areas to use SODIS are between 15°N and 35°N, and also between 15°S and 35°S. These areas receive high levels of sunlight, have little cloud cover or rain, and allow over 90% of sunlight to reach Earth as direct light. 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 people how to use SODIS is important to prevent confusion between PET bottles and other materials. Using SODIS without checking current hygiene practices or the rate of diarrhea may not solve other infection risks. Community trainers must be properly trained first.

Applications

SODIS is a useful way to treat water when fuel or cookers are not available or too expensive. Even when fuel is available, SODIS is often cheaper and better for the environment. However, SODIS can only be used if enough bottles are available and if the water is not too cloudy. If the water is very cloudy, SODIS alone is not enough. Extra filtering must be done first.

To check if water is too cloudy for SODIS, a simple test called the newspaper test can be used. For this test, a filled bottle is placed upright on a newspaper headline, and the user looks down through the bottle opening. If the letters on the headline are clear, the water is suitable for SODIS. If the letters are not clear, the water is likely too cloudy (more than 30 NTU), and pretreatment is needed.

SODIS could be used in 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 might be difficult in some situations.

Other methods for treating and storing water safely include chlorination, flocculation/disinfection, and various filtration methods. The best method depends on factors like effectiveness, other types of water pollution (such as turbidity or chemicals), cost, effort required, convenience, and user preference.

If water is very cloudy, SODIS alone is not enough. Extra filtering or flocculation must be done first to make the water clearer. Recent research shows that common table salt (NaCl) can help reduce turbidity for SODIS in some soil types. This could allow SODIS to be used in more areas with cloudy water at a lower cost.

SODIS can also be done using plastic bags. Bags may work better than bottles because they can reach higher temperatures, which speeds up treatment. Bags with a water layer of about 1 cm to 6 cm heat up faster than bottles and are more effective at killing Vibrio cholerae. This is likely because bags have a better surface area to volume ratio. In remote areas, bottles may be hard to find and expensive to transport. Bags can be packed more tightly and shipped more cheaply, making them a better option in some places. However, bags may give water a plastic taste, 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 household water treatment tools. Point-of-use means the water is treated in the same container it will be used from, reducing the risk of contamination later.

Cautions

If water bottles are not left in the sun long enough, the water 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 sun, the bottles need to stay in the sun longer.

Other things to think about include:

Health impact, diarrhea reduction

According to the World Health Organization, over 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 germs and harmful organisms from water very effectively. However, other diseases can also spread because of poor sanitation and hygiene. Studies on the use of SODIS show a 30 to 80 percent decrease in cases of diarrhea among people who use this method.

Research

The SODIS method was first found to be effective by Aftim Acra, who worked at the American University of Beirut in the early 1980s. Later research was done by Martin Wegelin’s group at the Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Kevin McGuigan’s group at the Royal College of Surgeons in Ireland (RCSI). Controlled experiments were led by Ronan Conroy from RCSI, working with Michael Elmore-Meegan.

A joint research project on SODIS involved the following groups:

• 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 study included areas in Zimbabwe, South Africa, and Kenya.

Other developments include a device that continuously disinfects water using sunlight and a special titanium dioxide film on glass cylinders, which stops bacteria from growing again after SODIS.

Research has shown that some low-cost materials can help SODIS work faster and more effectively in both sunny and cloudy weather. A 2008 study tested powdered seeds from five types of legumes (peas, beans, lentils, cowpeas, black lentils, soybeans, green peas, and peanuts). These seeds worked as well as commercial alum for clearing cloudy water, and they needed less of the material (1 gram per liter) to work quickly (7–25 minutes). The water’s hardness and pH remained unchanged. Later studies tested chestnuts, oak acorns, and Moringa oleifera (drumstick tree) for the same purpose.

Other research has explored using special materials called doped semiconductors to create more oxygen radicals under sunlight. Recently, scientists at Dublin City University developed 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 color to measure UV exposure.

In remote areas, wood smoke from fires used to boil water and cook makes lung disease worse. Studies show that people often skip boiling water because gathering wood is hard and wood is scarce. When given simple water treatment choices, people in remote parts of Africa prefer SODIS over boiling or other methods.

A simple solar water purifier for rural homes uses four layers of saree cloth and solar collectors to remove all coliform bacteria.

In July 2020, researchers announced a reusable aluminum surface that can purify water using sunlight to meet World Health Organization (WHO) and Environmental Protection Agency (EPA) standards for drinking water.

Promotion

The Swiss Federal Institute of Aquatic Science and Technology (EAWAG), through the Department of Water and Sanitation in Developing Countries (Sandec), helps manage SODIS promotion projects in 33 countries, including 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, among others, the SOLAQUA Foundation, several Lions Clubs, Rotary Clubs, Migros, and the Michel Comte Water Foundation.

SODIS has also been used in several communities in Brazil, including Prainha do Canto Verde, Beberibe west of Fortaleza. Villagers there using the SODIS method have been successful because temperatures can reach over 40 °C (104 °F), and there is little shade.

For public health workers helping communities find safe, affordable, and lasting water treatment methods, it is important to explain why clean water is essential for health and how to use these methods. While some communities have been hesitant to use SODIS and other household water treatment methods, sharing information about the health benefits of these methods can help increase their use.