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Solar Water Treatment

Table of contents

Solar water treatment uses the energy from the sun to purify water for drinking and household purposes. There are many forms of solar water treatment, as the sun’s energy can be used in many different ways. It has become more and more common for water treatment plants to use solar energy to power their treatment plants. Solar water treatment, however, is usually low-technology solutions that work to capture the heat and energy from the sun to make water cleaner and healthier for human use and consumption. Solar water treatment is particularly beneficial in areas that do not have access to safe sustainable electricity because electrification can be used to sanitize water in different ways. Some types of solar water treatment are used in developed countries and urban areas, but mostly these technologies are intended for rural communities that do not have water infrastructure and that often do not have electricity.

There are four main types of solar water treatment: solar water disinfection (SODIS), solar distillation, solar water pasteurization, and solar water treatment systems. Some of these technologies have been around for a very long time, but most are new adaptations to the concept of solar energy. These technologies are quite simple and easy to understand, usually require low financial input, and are proven effective.

Solar water disinfection


Solar water disinfection (SODIS) is a low-technology, simple process of purifying water using the power of the sun’s UV rays and solar radiation. SODIS technology was first introduced in 1980 by Aftim Acra et al. from the American University of Beirut1 . The initiative to spread SODIS in developing countries began with the Swiss Federal Institute of Science and Technology (Eawag) in 1991.2 Currently, the technical team at Eawag helps support projects in 24 countries through partner organizations in order to ensure more safe drinking water worldwide.3

The method for SODIS involves clear PET (polyethyleneteraphtalate) bottles and a location that is exposed to the sun, most commonly the roof top of a residence.4 The size of the bottles should be 2 liters or less, and the water is filled in the bottles to leave 2.5 cm of space in order to permit enough oxygen for the process, then shake the bottle of water to fully oxygenate the water5 The process involves exposure to the UV-rays and temperature, the higher the temperature the water reaches in the bottle the faster the disinfection process. If it is sunny or up to 50% cloudy, the water is disinfected in 6 hours of exposure. If it is 100% cloudy then the bottles need to be exposed for 2 days.6 If water temperature goes above 50 ͦ the disinfection process is complete in 1 hour.7 After this time period, the water is disinfected and ready to drink. These simple calculations of sunlight and temperature make this method of water purification very accessible.

SODIS kills many water borne pathogens including bacteria, viruses, and the giardia and cryptosporidia parasites8 . These pathogens are related to many health issues including cholera, typhus, diarrhea, polio, hepatitis, stomach cramps, fever, among others.9 Most likely, UVA-rays kill these pathogens most by damaging the respiratory system through radiation, but this is still in question.10 Because the UVA-rays play such a vital role in disinfection, the process happens even when the temperature is cool. The process is ineffective, however, during periods of heavy rainfall and many recommend rainwater harvest and filtration during this time.11 Furthermore, it is important that the water is relatively clear in order for the rays to penetrate and disable the functions of the pathogens12 Waters of higher turbidity must be filtered before SODIS treatment to ensure proper purification.13


Solar water distillation


Solar water distillation


Solar water distillation uses a solar still to condense pure water vapor and settle out harmful substances to make clean, pure drinking water. The history of solar water distillation dates back to over 2000 years ago when the system was used to produce salt from distilling sea water.14 The earliest documented large scale still was built in 1872 in Las Salinas, Chile to supply a mining community with drinking water15 . Before this project there is documented history of smaller scale still from back to 1551 when it was used by “Arab alchemists”.16 Today, there are many solar still systems all around the world ranging from small-scale individual systems to solar still large-scale plants in rural and urban settings.17

This process can be used when water is brackish (containing dissolved salts), contains harmful bacteria, for settling out heavy metals, and even for desalination of sea water.18 During the process in the still, water vapor evaporates, which separates the pure water from the harmful dissolved matter, and then is condensed and forms purified liquid water.19 Solar stills mimic the natural process of rainwater formation.20 This system can be used in place of boiling water for sanitation and is especially useful in communities that collect rain water as their main source for drinking water.21 Solar stills therefore have further health benefits to communities that use wood-burning stoves or fires to boil water as wood-burning has adverse health effects.22 If the water is more severely polluted or has other toxins, the water should be filtered before being distilled.23 The production capabilities of a solar still vary depending on temperature and intensity of the sun’s rays. In the southwestern United States, average production is 2 liters/day in the winder and 6 liters/day in the summer.24

A solar still uses a glass panel on top of a dark colored (usually black) basin that holds the water. The radiation permeates the glass and the dark colored basin heats the water. The evaporated water vapor condenses on the glass panel because it is not as hot as the basin. The condensed water trickles down the glass into a bottle or other storage area because the glass is on an angle to direct the runoff. During this process, the impurities are left behind in the basin and the water is drinkable.25 Solar stills last for many years and are an easy, low-technology solution for water purification.


 Solar water purification systems




Pasteurization, or the use of moderate heat or radiation to kill disease-causing microbes, can be achieved with solar cookers that trap the heat from the sun. When unclean water is put into a solar cooker and the temperature reaches 65 ͦ C-68 ͦ C for several minutes, 99.9% disease-causing pathogens in the water will be killed.26 Solar water pasteurization has been proven to kill bacteria, protozoa, viruses, and worms.27 To pasteurize water in a solar cooker, a dark covered metal, plastic or glass container with the contaminated water is placed inside the cooker.28 Some companies manufacture solar water pasteurizers which use the same properties of a solar cooker to trap the heat and produce radiation to continually heat the water. 29

Another alternative to the solar cooker is a solar puddle, a pit in the ground that has the same function as a solar cooker. The pit should be about 4 inches deep and 3 feet wide, lined with natural insulating materials (leaves, grass, straw, paper) and then covered with layers of clear and black plastic. The pit must be located in an area of high sun. There should be a trough at one side of the pit and a drain siphon to transfer the water from one plastic container to another. After the containers are covered and the sun is shining, the solar puddle will have the same radiation effect as a solar cooker and bring the water to pasteurization temperature.30

To make sure that the water reaches pasteurization temperature, Dr. Fred Barret developed the propotype for Water Pasteurization Indicator (WAPI) in 1998 that was used by Dale Andreatta to develop the current WAPI model. The WAPI is a polycarbonate tube filled with soy fat that melts at 69 ͦ C. This tube can be placed inside the water container that is being pasteurized to make reaching this temperature much easier.31 Solar water pasteurization is effective and can use many different forms of solar energy to sanitize water. The WAPI measuring system makes water pasteurization even more effective by ensuring the proper temperature is reached. Solar pasteurization depends heavily on temperature and is not necessarily appropriate for cooler climates and areas with low sunlight.

Solar water purification systems




Solar water purification systems are systems that integrate solar electricity and water purification. Solar panels generate power for a battery that pumps the water through a filtration and purification system. These systems are often mobile and used in disaster relief efforts32 , but can be stationary as well. Each manufacturer has their own system and capacity, but purification systems that use solar energy can come in many sizes- from small scale personal use to commercial to community supply.33 Solar water purification systems have been highly effective in disaster relief efforts and conflict zones when there is no access to traditional power supplies or clean water.34 They have also been effective in communities with little or no access to electricity and clean water, as these systems can also be integrated to supply electricity. This is not a direct solar water treatment system, rather indirect because the purification happens in a water filter as it is pumped through by solar electricity.


1. Robert Metcalf, "The Microbiology of Solar Water Pasteurization with Applications in East Africa,"  p. 2.  Accessed May 4, 2010.

2. Sodis.ch, “About Us” AND Mäusezahl, Daniel et al. 18 August 2009, “Solar Water Disinfection (SODIS) to Reduce Childhood Diarrhea in Rural Bolivia: A Cluster-Randomized Controlled Trial” Plos Hub for Clinical Trials, plosmedicine.org

3. Sodis.ch, “About Us”

4. Sodis.ch, “Method”

5. Waterforlife.org, “Solar Disinfection of Water” AND Center for Disease Control, January 2008. “Household Water Treatment Options in Developing Countries: Solar Disinfection (SODIS).”

6. Waterforlife.org, “Solar Disinfection of Water”

7. Ibid.

8. Sodis.ch, “Method”

9. Sodis.ch, “Microbiology”

10. Ibid.

11.  Waterforlife.org, “Solar Disinfection of Water”

12. Ibid.

13. Center for Disease Control (2008)

14. Practical Action: Technology Challenging Poverty. 2008. “Solar Distillation” scribd.com

15. Ibid.

16. Solaqua.com. 2008. “Solar Still Background”.

17. Ibid.

18. Practical Action: Technology Challenging Poverty.2008. “Solar Distillation” scribd.com.

19. Ibid.

20. Solaqua.com,2008. “Solar Still Basics”

21. Solar Mexico: Renewable Energy Futures for Rural Mexico, 2004.“Renewable Technologies” nonprofitpages.com.

22. Ibid.

23. Practical Action: Technology Challenging Poverty. 2008. “Solar Distillation”

24. Solaqua.com, “Still Water Production”

25. Solaqua.com, “Solar Still Basics” AND Practical Action: Technology Challenging Poverty, “Solar Distillation”

26. Metcalf, Robert. “The Microbiology of Solar Water Pasteurization, With Applications in East Africa.” p. 2. Katherinehamnett.com.

27. Safewatersystems.com, “What is Solar Water Pasteurization?”

28. Metcalf, Robert. “The Microbiology of Solar Water Pasteurization, With Applications in East Africa.” p. 2. Katherinehamnett.com.

29. Safewatersystems.com, “What is Solar Water Pasteurization?”

30. Rolla, Trudy C. 1998. “Sun and Water: An Overview of Water Treatment Devices.” Journal of Environmental Health Vol. 60.

31. Ibid.

32. World Water and Solar Technologies . 3 Feb 2010. “World Water and Solar Technologies Solar Water Purification System First Providing Clean Water in Haiti.” Prlog.org.

33. Aqua Sun International, 2007. aqua-sun-intl.com

34. World Water and Solar Technologies . 3 Feb 2010. “World Water and Solar Technologies Solar Water Purification System First Providing Clean Water in Haiti.” Prlog.org.



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