Solar Cooking
Last edited: 10 September 2020      
Chad 2010 Karyn Ellis safe water

Refugees at the Touloum Refugee Camp use the sun to make their water safe to drink.

Contrary to common belief, biologically-contaminated water does not need to be boiled to make it safe to drink; it only needs to be brought to a temperature that is sufficient to kill all disease-causing organisms.[1] This is known as water pasteurization. Because the pasteurization temperature is much lower than the boiling temperature, pasteurization results in significant fuel savings when compared to boiling.

The dangers of unsafe drinking water cannot be overstated. The World Health Organization estimates that there are roughly 1.7 billion cases of diarrheal disease annually, resulting in the death of nearly 760,000 children under the age of five every year, making it the second leading cause of death in this age group. [2] Yet, in many of the most severely affected regions, sunshine is an abundant source of energy that can not only cook food but can also heat water to temperatures that kill harmful microbes, making water safe to drink.

It has been known since the late 1880s, when Louis Pasteur conducted groundbreaking research on bacteria, that heat can kill pathogenic (disease-causing) microbes. Most people know contaminated water can be made safe by boiling. What is not well known is that contaminated water can be pasteurized at temperatures well below boiling, as can milk, which is commonly pasteurized at 71 °C (160 °F) for 15 seconds.[1]

The chart below indicates the temperatures at which the most common waterborne pathogens are rapidly killed, thus resulting in at least 90 percent of the microbes becoming inactivated in one minute at the given temperature. (The 90 percent reduction is an indicator frequently used to express the heat sensitivity of various microbes.) Thus, five minutes at this temperature would cause at least a 99.999 percent (5 log) reduction in viable microbes capable of causing disease.

Microbe Killed Rapidly At
Worms, Protozoa cysts (Giardia, Cryptosporidium, Entamoeba)55 °C (131 °F)
Bacteria (V. cholerae, E. coli, Shigella, Salmonella typhi), Rotavirus60 °C (140 °F)
Hepatitis A virus65 °C (149 °F)
(Significant inactivation of these microbes actually starts at about 5 °C (41 °F) below these temperatures, although it may take a couple of minutes at the lower temperature to obtain 90 percent inactivation.)
Petrifilm E

E. coli bacteria (left plate) thrive in water sources, but are easily killed through solar water pasteurization (right plate) (Petrifilm)


GoSun Water Filtration1 04-20

GoSun water filtration system

North Star Solar Pasteurizer diagram, 8-8-17

The North Star Devices Solar Pastuerizer schematic, Photo credit: North Star Devices, Inc. 2017

  • March 2014: Solar Water disinfection by a Parabolic Trough Concentrator (PTC) - An innovative solar water pasteurizer was developed to directly heat the water by solar radiation using a “Parabolic Trough Concentrator” (PTC). The efficiency of drinking water pasteurization by using the PTC was studied with a combination of analytical methods including flow-cytometric determination of total cell concentration and enumeration of cells with damaged membranes before and after treatment. The results of this study suggest that the pasteurization temperature of 87 °C (189 °F) is able to inactivate bacterial cells in drinking water. The research was performed at EAWAG (Swiss Federal Institute of Aquatic Science and Technology, Switzerland) in cooperation with CeTAmb (University of Brescia, Italy) and ADA Onlus (Italy). Read more on the research paper published by R. Bigoni, S. Kotzsch, S. Sorlini and T. Egli on Journal of Cleaner Production:
  • March 2014: AquaPak Field test Results from Nigeria - Professor Oluwole Agbede of the University of Ibadan, Nigeria was provided with AquaPak solar water pasteurizers by Femi Odediran, retired UNICEF water and sanitation senior adviser. Water from a borehole at Fanawole, a well at Oju Oja, and a stream at Adeosun areas were collected and all contained biological pathogens. The AquaPak solar water pasteurizers, each containing a WAPI (water pasteurization indicator), were used to heat the water to 65 °C (149 °F), thus, eliminating the harmful pathogens. A complete copy of Prof. Abgede’s a report is here: Pasteurization of Water Using Solar AquaPak: A Case Study - Oluwole A. Agbede
  • March 2014: Henry Ford Innovation Institute contributes AquaPaks - The Henry Ford Innovation Institute has contributed AquaPak solar water pasteurizers to the World Health Student Organization at Wayne State School of Medicine. Medical students from the organization will be using the AquaPaks as they travel to Ecuador, Nicaragua, Peru, and other countries. Feedback from their use of the AquaPak in these countries will be posted on our website as it becomes available.
  • February 2014: Dale Andreatta, inventor of the WAPI, discusses the latest developments in water pasteurization at the ETHOS 2013 conference in Kirkland, Washington, USA. He describes three uses for low temperature (below boiling) solar-heated water, and three simple devices for producing that hot water. In Johnson and Bryden (2012) it is estimated that in a particular village in Mali, 22% of the domestic wood consumption went for water heating exclusive of cooking, while another 52% went for cooking. In a sunny climate the methods presented here would provide most of the energy required for water heating exclusive of cooking, and a substantial fraction of the energy needs for cooking. Read more at: Solar Thermal Energy for the Village - Dale Andreatta
  • November 2013: Petra Wadstrom wins the Polhems Prize for the Solvatten system - The Polhem Prize is awarded by the Swedish Association of Graduate Engineers for a high-level technological innovation or an ingenious solution to a technical problem. The innovation must present environmental benefits and be available and shown competitive on the open market. The jury states: "Petra Wadstrom's innovation Solvatten is low tech but non the less ingenious. It addresses a growing global problem to millions of people living in poverty and without access to safe water and clean energy".
  • November 2013: Study of water treatment in Kenya demonstrates the effectiveness of the WAPI - Bob Metcalf, research microbiologist, and FOTO worked together on a study to compare results of various water treatment chemicals to the WAPI water pasteurization indicator used with a CooKit solar cooker. One finding of note is that the WAPI, when used with the CooKit, was 100% effective in producing negative results for E.coli, while the WaterGuard (3 drops/liter) method was only effective in most cases. Read the study at: The Goal is Zero: A Strategy to Eliminate Water-bourne Disease in Lower Nyakach, Kenya
  • March 2012: Haiti Ministers launching Solvatten Project - On 13th February the Green Haiti Project was officially launched by the Haitian Government. The Minister of Health, together with the Ministers of Environment and Agriculture, officially announced that Solvatten is the "miracle product" that Haiti has been waiting for. The Green Haiti Product including Solvatten is addressing multiple issues such as cholera, deforestation and unemployment. The Government is promoting this initiative on a broad scale to international donors, aid agencies and NGOs in the country. "The Green Haiti Project is the way forward for Haiti. We can't see a more appropriate project for Haiti than this. The project is morally motivated, legally solid, technically ingenious, strategically positioned and politically manifested." Dr Florence D. Guiiaume. Minister of Health, Haiti.
  • September 2012: The Swedish Church and the aid agency Lutheran World Federation (LWF) are launching Solvatten in Kakuma Refugee Camp in Kenya. The goal of the project is to reduce environmental degradation by enabling people to use alternative sources of energy in the camp and in the host community that surrounds it. Supply of fire wood has been a problem in Kakuma region since the camp was set up in 1992 but it has escalated in the past two years, despite that the refugees are not allowed to harvest firewood directly from the bushes. This is attributed to the continuous massive influx of people into the camp leading to the environmental degradation through the harvesting of sticks from the available vegetation for firewood. Firewood has become very expensive and the UNHCR struggles to purchase and supply fuel to the growing population of refugees in the camp.
  • August 2012: The Eliodomestico Solar Still was designed by Gabriele Diamanti. The project won a Core77 Design Award for Social Impact.

Solar water pasteurization

Water pasteurization lethal graph


Solar cooks know how easy it is to heat foods in a solar cooker to temperatures well above 60 °C (140 °F). Knowing this, microbiologist Dr. Bob Metcalf and a graduate student of his in the early 1980s, David Ciochetti, studied solar water pasteurization for the latter's master's thesis. They found that when contaminated water was heated in a black jar in a solar box cooker, both bacteria and rotaviruses -- the main cause of severe diarrhea in children -- were inactivated by 60 °C (140 °F)[1]. In the paper published this work (Applied and Environmental Microbiology, Vol. 47:223-228, 1984) it was concluded that if contaminated water were heated to 65 °C (149 °F), all pathogenic microbes would be inactivated. This includes the hepatitis A virus, which has a 90 percent reduction after two minutes at 60 °C (140 °F).

Another student, Negar Safapour, found contaminated water could be pasteurized in a black metal or glass container in a CooKit (Solar Cookers International's simple solar cooker) without using the clear plastic bag that is required for cooking. In our experiments it took about three minutes for each 1 °C increase from 55 - 65 °C (131 - 149 °F) when heating 2 liters (0.5 gal.) of water, and four to five minutes for each 1 °C (2 °F) increase when heating 4 liters (1 gal.). Thus, water is at lethal temperatures for several minutes as it is solar heated to 65 °C (149 °F), and it remains in the lethal zone for many more minutes as the water slowly cools back down to 55 °C (131 °F).


Solar Cookers International's Water Pasteurization Indicators (WAPIs) shown in the solid wax state (left) and liquid wax state (right)

Since thermometers are not accessible to many people around the world, there is a need for a simple device that indicates when water has reached pasteurization temperatures. Fred Barrett, who was with the United States Department of Agriculture when Dr. Metcalf worked with him on a solar box cooker project in Sierra Leone in 1989, came up with the idea of using vegetable wax, with a melting point near 70 °C (158 °F), as an indicator. He built several models based on the idea of wax inside a plastic cylinder, and successfully used them to verify pasteurization conditions of contaminated water. In 1992, Dale Andreatta -- a Ph.D. candidate in mechanical engineering at the University of California, Berkeley at the time -- created the Water pasteurization indicator (WAPI) in its current form. The WAPI is a clear plastic tube partially filled with a soybean wax that melts at about 70°C (158°F). With the solid wax at the top end of the tube, the WAPI is placed in the bottom of a black container of water that is solar heated. If the wax melts and falls to the bottom of the tube, it ensures that water pasteurization conditions have been reached. The WAPI has a stainless steel washer around it to keep it at the bottom of the container, which is the coolest location when solar heating water.

Flow-through pasteurizer schematic

An automotive thermostat can be used to control the flow of water through a Flow-through water pasteurization device.

Since top water temperatures are often 2-5 °C (4-10 °C) hotter than bottom water temperatures, lower WAPI placement helps to further insure that pasteurization conditions have been achieved.

Spade WAPI


Another technique that has developed from seeking a simple solution to water pasteurization is the SODIS approach. Basically it involves leaving sealed transparent bottles of clear, but untreated, water in direct sunshine from two to four hours. The time will depend on the surfaces below the bottles, and the intensity of the sunshine. Often the bottles are placed on a flat corrugated metal roof. When the water reaches 65°C(150°F) it is suitable for drinking. As mentioned earlier, a need developed to find a simple device, a WAPI, to indicate when the water was safe. Several designs use a melting wax method. A recent version, called the SPADE WAPI, is designed to be fitted directly to the cap on a water bottle. After drilling a 0.6 cm (.25 in) hole through the cap. The slender clear tube, with wax at one end, is submerged into the bottle. Reaching a safe temperature, the wax runs to the bottom of the tube. A compact approach to providing water pasteurization using existing bottles.

During numerous trips to developing countries, Dr. Metcalf has had the opportunity to conduct numerous solar water pasteurization experiments. In Meatu district, Shinyanga region, Tanzania, water most often comes from open holes dug in the sand of dry riverbeds, and it is invariably contaminated. Indeed, the water that was provided in Dr. Metcalf's guesthouse was heavily contaminated. Bacteriological tests of the water during solar pasteurization repeatedly showed indicator bacteria (key bacteria whose presence indicates faecal contamination) becoming inactivated at temperatures just below 60 °C (140 °F). During all of Dr. Metcalf's fieldwork, he has heated his own drinking water in a solar cooker using a WAPI as a temperature indicator and he has had no intestinal problems.

Flow-through water pasteurization device

One type of flow-through water pasteurizer with heat exchanger. Typical temperatures are shown in degrees F.

This type of device can pasteurize large amounts of water since it makes use of a thermostatic valve that opens when water has reached pasteurization temperature. Often a heat exchanger is used to transfer the heat from the exiting water to the water that is entering the device.

Boil 3, Add 1 Method

The Boil 3 Add 1 Method provides a way of obtaining the fuel savings of pasteurization without the need for a thermometer. This method replaces temperature measurements with simple volume measurements that everyone can perform.

Main article: Boil 3, Add 1 Method

Pasteurizing water in a solar cooker

A standard solar cooker can be used to pasteurize water by simply replacing the food to be cooked with the water to be heated.

Other simple solar pasteurization devices

Water testing

Water testing kit

Portable Microbiology Laboratory water testing kit contains these individual components.

Dr. Bob Metcalf, a microbiology professor at California State University at Sacramento has prepared a portable water testing kit. Contact information for Dr. Metcalf can be found on his biography page.


Frequently-asked questions

How is pasteurization accomplished?

Traditional fuels can be used to pasteurize water, but on sunny days solar energy can be used as well. Simple solar cookers can pasteurize water for a family at a rate of about one liter per hour. Solar Cookers International's reusable water pasteurization indicator (WAPI) can be used to determine when water heated by solar or conventional means has been heated to a high enough temperature to make it safe.

Don’t you need to boil water for 20 minutes to sterilize it and make it safe to drink?

No, it is only necessary to heat water to 65 °C (149 °F) to pasteurize it.

What is the difference between sterilization and pasteurization?

Sterilization kills all of the organisms in the water, while pasteurization kills only those organisms that can cause harm to humans.

What common disease organisms are killed by pasteurizing water?

Giardia, cryptosporidium, entamoeba, the eggs of worms, cholera, shigella, salmonella bacteria and those that cause typhoid, the enterotoxogenic strains of E. Coli, Hepatitis A, and also rotavirus which is a major cause of disease in children are all killed or inactivated at 65 °C (149 °F).

How can water be tested in the field?

WAPI Dadaab

At the Dadaab Refugee Camp in Kenya, villagers check their Water Pasteurization Indicators to determine whether the water has been heated to a temperature (65 °C (149 °F)) that makes it safe to drink.

To test whether water has been heated enough to make it safe to drink you can use a thermometer or a water pasteurization indicator.

To test a water source for human or animal fecal contamination, it is best to test for the bacterium Escherichia coli, which is always present in human faeces at a level of about one hundred million E. coli per gram. The presence of E. coli in water indicates recent fecal pollution and a public health threat. Water containing one E. coli per milliliter is considered heavily contaminated. In order to do world-class microbiology in developing countries where there is no lab, since 2000 Solar Cookers International has used two complementary tests extensively.

The first test is a presence/absence test using Colilert, the most widely used test in the water industry (IDEXX Laboratories, Westbrook, Maine). SCI uses the Colilert MPN tube, which is inoculated with 10 ml (0.3 oz.) of water, and incubated at body temperature for 10-24 hours. If the liquid in the tube turns yellow, and fluoresces blue when illuminated with a battery-operated, hand-held ultraviolet light, the presence of E. coli in the water sample is confirmed. If the tube remains clear, or is yellow but does not fluoresce blue under UV light, it indicates that there were no E. coli cells in the 10 ml sample, and there is a low risk of disease from the water.

Incubation using body temperature

Heat from your own body can be used as a incubator in the field.

The second test is a quantitative test using the E. coli count, Petrifilm (3M Microbiology Products, St. Paul, Minnesota), which is used extensively in the food industry. One milliliter of the water sample is added to the Petrifilm, which is incubated at body temperature for 10-24 hours. If E. coli is present in the water sample, it will develop into a blue colony surrounded by gas bubbles. By counting the number of blue colonies with gas, the number of E. coli in a milliter can be determined. One E. coli colony on a Petrifilm indicates heavily contaminated water and a high risk of disease, 10 or more E. coli on a Petrifilm indicates grossly contaminated water and a very high risk of disease.

Commercial water pasteurization devices


Audio and video

  • July 2016:


Lowe's Hardware sponsored a water purification/pasteurization concept event, and Sharon Clausson explained the easily accessible solar cooker approach

  • December 2014:

Solvatten Kakuma, Kenya

The Solvatten system being used in Kakuma, Kenya.

  • March 2013:

InStove Safe Drinking Water System

InStove's co-founder Damon Ogle and independent engineer Dale Andreatta talk about the development of an innovative water pasteurization system application in final development phase. The system works in conjunction with InStove's biomass fueled 100 liter stove.

  • August 2012:


Eliodomestico Solar Still

  • January 2010:
  • April 2009:

Solar Cooking-1

A stove powered by the sun is making a big difference in impoverished countries. Bob Metcalf is featured.

  • March 2009:

Solar Water Pasteurization

Video showing use of the AquaPak and the WAPI

  • January 2009:

Dale Andreatta speaking on water pasteurization at the 2009 ETHOS conference

Dr. Dale Andreatta speaking on water pasteurization at the 2009 ETHOS conference - Dale describes the development of the WAPI, a simple device for determining whether water has been heated to a temperature that makes it safe to drink. He also discusses flow-through pasteurization devices and strategies to pasteurize water with the waste heat from a wood-burning cookstove. See also: Water pasteurization

WAPI in use

Boniphace Luhende in Tanzania demonstrates solar water pasteurization using a CooKit and Water Pasteurization Indicator (WAPI).

  • December 2002: An interview with microbiologist and SCI-founder Dr. Bob Metcalf discussing solar water pasteurization and food safety. Listen here.


Main article: SODIS

This method of solar water disinfection uses the light of the sun instead of the heat of the sun to kill pathogens. Using this method, clear plastics bottles full of water are exposed to the sun for many hours.

Topics needing research

Nitinol as a temperature indicator

Nitinol (Nickel-Titanium) is a memory metal which can be designed with a temperature transition threshold. It can be designed with two memory shapes: straight for below 65°C and circular for above 65 °C (149 °F). Nitinol wire can be manufactured on a  mass scale to provide an accurate indicator. The wire can be submerged into the water without a glass enclosure. When the wire becomes circular, the water temperature will have reached the 65°C threshold.

See: Nitinol research report - Ohio State University

Articles in the media

See also

External links