# Drinking water/Solar disinfection

Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS is already applied in numerous developing countries. Educational pamphlets on the method are available in many languages, each equivalent to the English language version.

## Principle

Exposure to sunlight has been shown to deactivate diarrhea-causing organisms in polluted drinking water. Three effects of solar radiation are believed to contribute to the inactivation of pathogenic organisms:
* UV-A interferes directly with the metabolism and destroys cell structures of bacteria.
* UV-A (wavelength 320-400 nm) reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides), that are believed to also damage pathogens.
* Cumulative solar energy (including the infrared radiation component) heats the water. If the water temperatures rises above 50°C, the disinfection process is three times faster.
At a water temperature of about 30°C (86°F), a threshold solar radiation intensity of at least 500 W/m$^2$ (all spectral light) is required for about 5 hours for SODIS to be efficient. This dose contains energy of 555 Wh/m$^2$ in the range of UV-A and violet light, 350 nm-450 nm, corresponding to about 6 hours of mid-latitude (European) midday summer sunshine.
At water temperatures higher than 45°C (113°F), synergistic effects of UV radiation and temperature further enhance the disinfection efficiency.

## Process for household application

* Colourless, transparent PET water or pop bottles (2 litre or smaller size) with few surface scratches are chosen for use. The labels are removed and the bottles are washed before the first use.
* Water from contaminated sources are filled into the bottles. To improve oxygen saturation, bottles can be filled three quarters, shaken for 20 seconds (with the cap on), then filled completely and recapped. Very cloudy water with a turbidity higher than 30 NTU must be filtered prior to exposure to the sunlight.
* Filled bottles are then exposed to the sun. Bottles will heat faster and to higher temperatures if they are placed on a sloped sun-facing corrugated metal roof as compared to thatched roofs.
* The treated water can be consumed directly from the bottle or poured into clean drinking cups. The risk of re-contamination is minimized if the water is stored in the bottles. Refilling and storage in other containers increases the risk of contamination.
Suggested Treatment Schedule :
| Weather Conditions | Minimum Treatment Duration |
| sunny (less than 50% cloud cover) | 6 hours |
| cloudy (50-100% cloudy, little to no rain) | 2 days |
| continuous rainfall | unsatisfactory performance, use rainwater harvesting |

## Applications

SODIS is an effective method for treating water where fuel or cookers are unavailable or prohibitively expensive. Even where fuel is available, SODIS is a more economical and environmentally friendly option. The application of SODIS is limited if enough bottles are not available, or if the water is highly turbid.In fact, if the water is highly turbid, SODIS can not be used alone, additional filtering is then necessary.
In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles may be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, or iodine, or metal containers in which water can be boiled with fire. Additionally, in some circumstances, it may be difficult to guarantee that the water will be left in the sun for the necessary time.
Other methods for household water treatment and safe storage exist, e.g. chlorination, different filtration procedures or flocculation/disinfection. The selection of the adequate method should be based on the criteria of effectiveness, the co-occurrence of other types of pollution (turbidity, chemical pollutants), treatment costs, labor input and convenience, and the user’s preference.

## Cautions

If the water bottles are not left in the sun for the proper length of time, the water may not be safe to drink and could cause illness. If the sunlight is less strong, due to overcast weather or a less sunny climate, a longer exposure time in the sun is necessary.
The following issues should also be considered:
* Bottle material: Some glass or PVC materials may prevent ultraviolet light from reaching the water. Commercially available bottles made of PET are recommended. The handling is much more convenient in the case of PET bottles. Polycarbonate blocks all UVA and UVB rays, and therefore should not be used.
* Aging of plastic bottles: SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of wear reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.
* Shape of containers: the intensity of the UV radiation decreases rapidly with increasing water depth. At a water depth of 10 cm (4 inches) and moderate turbidity of 26 NTU, UV-A radiation is reduced to 50%. PET soft drink bottles are often easily available and thus most practical for the SODIS application.
* Oxygen: Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute in the destruction process of the microorganisms. Under normal conditions (rivers, creeks, wells, ponds, tap) water contains sufficient oxygen (more than 3 mg Oxygen per litre) and does not have to be aerated before the application of SODIS.
* Leaching of bottle material: There has been some concern over the question whether plastic drinking containers can release chemicals or toxic components into water, a process possibly accelerated by heat. The Swiss Federal Laboratories for Materials Testing and Research have examined the diffusion of adipates and phthalates (DEHA and DEHP) from new and reused PET-bottles in the water during solar exposure. The levels of concentrations found in the water after a solar exposure of 17 hours in 60°C water were far below WHO guidelines for drinking water and in the same magnitude as the concentrations of phthalate and adipate generally found in high quality tap water. Concerns about the general use of PET-bottles were also expressed after a report published by researchers from the University of Heidelberg on antimony being released from PET-bottles for soft drinks and mineral water stored over several months in supermarkets. However, the antimony concentrations found in the bottles are orders of magnitude below WHO and national guidelines for antimony concentrations in drinking water. Furthermore, SODIS water is not stored over such extended periods in the bottles.
* Regrowth of bacteria: Once removed from sunlight, remaining bacteria may again reproduce in the dark. A 2010 study showed that adding just 10 parts per million of hydrogen peroxide is effective in preventing the regrowth of wild Salmonella.
* Toxic Chemicals: The method does not remove toxic chemicals which may be present in the water. (factory waste, ...).

## Health impact, diarrhea reduction

It has been shown that the SODIS method (and other methods of household water treatment) can very effectively remove pathogenic contamination from the water. However, infectious diseases are also transmitted through other pathways, i.e. due to a general lack of sanitation and hygiene. Studies on the reduction of diarrhea among SODIS users show reduction values of 30-80%.

## Research and development

The effectiveness of the SODIS was first discovered by Professor Aftim Acra at the American University of Beirut in the early 1980s [3]. Substantial follow-up research was conducted by the research groups of Martin Wegelin at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Dr Kevin McGuigan at the Royal College of Surgeons in Ireland. Clinical control trials were pioneered by Professor Ronan Conroy of the RCSI team in collaboration with Michael Elmore-Meegan.
Currently, a joint research project on SODIS is implemented by the following institutions:
* Royal College of Surgeons in Ireland (RCSI), Ireland (coordination)
* University of Ulster (UU), United Kingdom
* CSIR Environmentek, South Africa, Eawag, Switzerland
* The Institute of Water and Sanitation Development (IWSD), Zimbabwe
* Plataforma Solar de Almería (CIEMAT-PSA), Spain
* University of Leicester (UL), United Kingdom
* The International Commission for the Relief of Suffering & Starvation (ICROSS), Kenya
* University of Santiago de Compostela (USC), Spain
* Swiss Federal Institute of Aquatic Science and Technology (Eawag), Switzerland
The project has embarked on a multi-country study including study areas in Zimbabwe, South Africa and Kenya.
Other developments include the development of a continuous flow disinfection unit and solar disinfection with titanium dioxide film over glass cylinders which prevents the bacterial regrowth of coliforms after SODIS. Research has shown that a number of low-cost additives are capable of accelerating SODIS and that additives might make SODIS more rapid and effective in both sunny and cloudy weather, developments that could help make the technology more effective and acceptable to users. A 2008 study showed that natural coagulants (powdered seeds of five natural legumes (peas, beans and lentils) – Vigna unguiculata (cowpea), Phaseolus mungo (black lentil), Glycine max (soybean), Pisum sativum (green pea), and Arachis hypogaea (peanut) – were evaluated for the removal of turbidity), were as effective as commercial alum and even superior for clarification in that the optimum dosage was low (1 g/L), flocculation was rapid (7–25 minutes, depending on the seed used) and the water hardness and pH was essentially unaltered. Later studies have used chestnuts, oak acorns, and Moringa oleifera (drumstick tree) for the same purpose.

## Issues to consider

The following are some of the issues discussed in the literature:
* Local education in the use of SODIS is important to avoid confusion between PET and other bottle materials.
* Applying SODIS without proper assessment (or with false assessment) of existing hygienic practices & diarrhea incidence may not address other routes of infection. Community trainers need to themselves be trained first.
* When the water is highly turbid, SODIS can not be used alone, additional filtering or flocculation is then necessary to clarify the water prior to SODIS treatment.

## Worldwide application

The Swiss Federal Institute of Aquatic Science and Technology (Eawag), through the Department of Water and Sanitation in Developing Countries (Sandec), coordinates 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, Perú, Philippines, Senegal, Sierra Leone, Sri Lanka, Togo, Uganda, Uzbekistan, Vietnam, Zambia, and Zimbabwe. Contact addresses and case studies of the projects coordinated by the Swiss Federal Institute of Aquatic Science and Technology (Eawag) are available at sodis.ch.
SODIS projects are funded by, among others, the SOLAQUA Foundation, several Lions Clubs, Rotary Clubs, Migros, and the Michel Comte Water Foundation.
SODIS has also been applied in several communities in Brazil, one of them being Prainha do Canto Verde north of Fortaleza. There, the villagers have been purifying their water with the SODIS method. It is quite successful, especially since the temperature during the day can go beyond 40°C (100°F) and there is a limited amount of shade.