Solar water disinfection

Solar water disinfection ( ‘ SoDis ‘ ) is a type of portable water purification that uses solar energy to make biologically-contaminated (eg bacteria, viruses, protozoa and worms) water safe to drink. Water contaminated with non-biological agents such as toxic chemicals or heavy metals require additional steps to make the water safe to drink.

Solar water disinfection is produced by solar photovoltaic panels, heat ( solar thermal ) and solar ultraviolet light collection.

Disinfection using the effects of electricity generated by electrolytic processes, which causes the production of electrolytic disinfection. A second approach uses stored solar electricity from a battery, and operates at night or at low light levels to power an ultraviolet lamp to perform secondary solar ultraviolet water disinfection.

Solar thermal water disinfection uses heat from the Sun to heat water to 70-100 ° C for a short period of time. A number of approaches exist here. Solar heat collectors can have lenses in front of them, or use reflectors. They can also use different levels of insulation or glazing. In addition, some solar thermal water disinfection processes are batch-based, while others (through-flow solar thermal disinfection) operate almost continuously while the sun shines. Water heated to temperatures below 100 ° C is generally referred to as Pasteurized water.

The ultraviolet part of sunlight can also kill pathogens in water. The SODIS method uses a combination of UV light and high temperature (solar thermal) for disinfecting water using only sunlightand PET plastic bottles. SODIS is a free and effective method for decentralized water treatment , and is generally recommended by the World Health Organization as a viable method for household water treatment and safe storage. [1] SODIS is already applied in many developing countries . [2] : 55 Educational pamphlets on the method are available in many languages,[3] each equivalent to the English-language version. [2]


Exposure to sunlight has been shown to inactivate diarrhea -causing organisms in polluted drinking water . The inactivation of pathogenic organisms is attributed to the UV-A (wavelength 320-400 nm) part of the sunlight, which reacts with oxygen dissolved in the water and produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides ) that damage pathogens, while it also interferes with metabolism and destroys bacterial cell structures; and the full band of solar energy (from infrared to UV) heats the water.

At a water temperature of about 30 ° C (86 ° F), a solar irradiance threshold 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, 350-450 nm, corresponding to 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 disinfection efficiency. Above 50 ° C (122 ° F), the bacterial count drops three times faster.

Process for household application

Guides for the household use of SODIS describe the process.

Coloredless, transparent PET water or gold pop bottles of 2 liter or smaller size with few surface scratches are selected for use. Glass bottles are also suitable. Any labels are removed and the bottles are washed before the first use. Water from possibly-contaminated sources is in the bottles, using the clearest water possible. Where the turbidity is higher than 30 NTU it is necessary to filter or precipitate out particulatesprior to exposure to the sunlight. Filters are locally made from stretched fabric over inverted bottles with the bottoms cut off. In order to improve oxygen saturation, the guides recommend that bottles be filled with three-quarters, shaken for 20 seconds (with the cap on), then filled completely, recapped, and checked for clarity.

The filled bottles are then exposed to the fullest possible sunlight. If they are placed on a sloped Sun-facing reflective metal surface. A corrugated metal roof (or, rather, it is compared to thatched roof). Overhanging structures or plants that shade the bottles must be avoided, as they reduce both illumination and heating. After sufficient time, 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 [4]
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

The most favorable regions for the SODIS method are located between 15 ° N and 35 ° N, and also 15 ° S and 35 ° S. [2] These regions have high levels of solar radiation, with over 90% of sunlight reaching the earth’s surface as direct radiation. [2] The second most favorable region between latitudes 15 ° N and 15 ° S. These regions have high levels of scattered radiation, with about 2500 hours of sunshine annually. [2]

Local education in the use of SODIS is important to avoid confusion between PET and other bottle materials. Applying SODIS without proper assessment of existing hygienic practices and diarrhea incidence. Community trainers must be trained first. [2]


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 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. [5]

A basic field test to determine if the water is too turbid for the SODIS method to work properly is the newspaper test. [3] For the newspaper test the user has to fill the empty bottle. If the letters are readable, the water can be used for the SODIS method. If the letters are not readable then the turbidity of the water can not be exceeded 30 NTU, and the water must be pretreated.

In theory, the method could be used in disaster relief or refugee camps. However, supplying bottles can be more difficult than providing equivalent disinfecting tablets containing chlorine, bromine, gold iodine. In addition, 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 (eg, chlorination) differ filtration procedures or flocculation / disinfection. The selection of the appropriate 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.

When the water is highly turbid, SODIS can not be used alone; Additional filtering or flocculation is then necessary to clarify the priority of SODIS treatment. [6] [7] Recent work has shown that common table salt (NaCl) is an effective flocculation agent for decreasing turbidity for the SODIS method in some types of soil. [8] This method could be used to increase the cost of SODIS could be used for low cost. [9]

SODIS may alternatively be implemented using plastic bags. SODIS bags have been found to be more effective than SODIS bottles. [10] SODIS bottles with a water layer of 1 cm to 6 cm SODIS bottles, and treat Vibrio cholerae more effectively. [10]It is assumed that this is because of the surface area to volume ratio in SODIS bags. In remote regions, they are not widely available and can not be shipped in a very small area. Bags can be packed more densely than bottles, and can be shipped to other countries. The disadvantages of using plastic bags are more often than not, they are more difficult to handle when they are watered, and they typically require that the water be transferred to a second container for drinking.

Another important benefit of water treatment is the use of water-related devices. [11] Point-of-use means that the water is treated in the same way to handle container, thus decreasing the risk of secondary water contamination.


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 sunlight is needed, it is necessary to have a good weather.

The following issues should also be considered:

Bottle material
Some glass or PVC materials may prevent ultraviolet light from reaching the water. [13] Commercially available bottles made of PET are recommended. The handling is much more convenient in the box of PET bottles. Polycarbonate (resin identification code 7) blocks all UVA and UVB rays, and therefore should not be used. Bottles that are clear, green lemon / lime soda pop bottles.
Aging of plastic bottles
SODIS efficiency depends on the physical condition of the plastic bottles, with scratches and other signs of reducing the efficiency of SODIS. Heavily scratched or old, blind bottles should be replaced.
Shape of containers
The intensity of the UV radiation decreases 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 is the most practical application for the SODIS application.
Sunlight produces highly reactive forms of oxygen (oxygen free radicals and hydrogen peroxides) in the water. These reactive molecules contribute to the destruction process of microorganisms. Under normal conditions (rivers, creeks, wells, ponds, taps) water contains sufficient oxygen (more than 3 mg / L of oxygen) and does not have to be aerated before the application of SODIS.
Leaching of bottle material
There has been some concern about the question of whether or not it is possible for them to be toxic or not toxic. 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 17 hours in 60 ° C (140 ° F) water were far below WHOGuidelines for drinking water and the same magnitude as the concentrations of phthalate and adipate found in high-quality tap water. Concerns about the general use of PET-bottles were also expressed by researchers from the University of Heidelberg on the release of antimony from PETs. However, the antimony concentrations found in the bottles are orders of magnitude below WHO [14] and national guidelines for antimony concentrations in drinking water. [15] [16] [17] In addition , SODIS is not included in this policy.
Regrowth of bacteria
Once removed from sunlight, the remaining bacteria may still be reproduced 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 . [18]
Toxic chemicals
Solar water disinfection does not remove toxic chemicals that may be present in the water, such as factory waste.

Health impact, diarrhea reduction

According to the World Health Organization , more than two million people per year of preventable water-borne diseases, and one billion people. [19] [20]

It has been shown that the SODIS method (and other methods of household water treatment) can be effectively removed from the water. However, infectious diseases are also transmitted through other pathways, ie due to a general lack of sanitation and hygiene. Studies on the reduction of diarrhea among SODIS users show reduction values ​​of 30-80%. [21] [22] [23] [24]


The effectiveness of the SODIS was first discovered by Aftim Acra, of the American University of Beirut in the early 1980s. Follow-up Was Conducted By the research groups of Martin Wegelinat the Swiss Federal Institute of Aquatic Science and Technology (Eawag) and Kevin McGuigan at the Royal College of Surgeons in Ireland . Clinical control trials were pioneered by Ronan Conroy of the RCSI team in collaboration with Michael Elmore-Meegan . ICROSS

A joint research project on SODIS was 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 from Almería (CIEMAT-PSA), Spain
  • University of Leicester (UL), United Kingdom
  • The International Commission for the Relief of Suffering and Starvation ( ICROSS ), Kenya
  • University of Santiago de Compostela (USC), Spain
  • Swiss Federal Institute of Aquatic Sciences and Technology (Eawag), Switzerland

The project is a multi-country study in Zimbabwe , South Africa and Kenya .

Other developments include the development of a continuous flow disinfection unit [25] and solar disinfection with titanium dioxide film over glass cylinders, which prevents the bacterial regrowth of coliforms after SODIS. [26]

Research has shown that a number of low-cost additives are capable of accelerating SODIS and that additives might make more efficient and acceptable to users. [27] A 2008 study showed that powdered seeds of five natural vegetables (peas, beans and lentils) – Vigna unguiculata (cowpea), Phaseolus mung (black lentil), Glycine max (soybean), Pisum sativum (green pea), and Arachis hypogaea(peanut) -when evaluated as natural flocculants for the removal of turbidity, were as effective as alum and 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. [28] Later studies have used chestnuts , oak acorns, and Moringa oleifera (drumstick tree) for the same purpose. [29] [30]

Other researches have been used to increase the production of oxygen radicals under solar UV-A. [31] Recently, researchers at the National Center for Sensor Research and the Biomedical Diagnostics Institute at Dublin City University have developed an inexpensive printable UV dosimeter for SODIS applications that can be read using a mobile phone. [32] The camera of the phone is used to acquire an image of the sensor and custom software running on the phone analyzes the sensor color to provide a quantitative measurement of UV dose.

In isolated regions the effect of wood smoke increases lung disease. Research groups have found that boiling water is neglected due to the difficulty of gathering wood, which is scarce in many areas. When presented with basic household water treatment options, they are shown to be a preference for SODIS method or other basic water treatment methods.


The Swiss Federal Institute of Aquatic Science and Technology (EAWAG), 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. [33]

SODIS projects are funded by, among others, the SOLAQUA Foundation , [34] 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 , Beberibe west of Fortaleza . There has been a good deal of success with the SODIS method, since the temperature is more than 40 ° C (104 ° F) and there is a limited amount of shade. quote needed ]

One of the most important things to consider for public health workers reaching out to communities in need of affordable, cost effective, and sustainable water treatment methods is teaching the importance of water quality in the context of health promotion and disease prevention while educating about methods Themselves. Although it may be difficult to use these methods of treatment, it may be necessary to treat these conditions.

See also

  • Appropriate technology
  • Ultraviolet Germicidal Irradiation
  • Water Pasteurization Indicator


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