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Safe Water from Sunshine

Dr. Bob MetcalfHow can Peace Corps volunteers and other expats living overseas in developing countries be certain their water is safe to drink?  How can they purify it if theyre not sure its safe?  Although most volunteers have a personal supply of chlorine tablets, many of their host country neighbors are not so fortunate.  Even if chlorine tablets are widely distributed as part of regional public health programs, are they really sustainable? Is there any guarantee that tablets will be available in the necessary quantities for years to come?  Might there be a more sustainable way to ensure safe drinking water using a free, renewable energy source?   Finally, is it really necessary to boil water to make it safe?


The answers to the last two questions, according to California State University, Sacramento microbiology professor Dr. Robert Metcalf are: Yes, there is a sustainable way to make water safe to drink using a solar cooker and a Water Pasteurization Indicator, (WAPI) and, No.  You do not need to boil water to make it safe to drink.  You need only to pasteurize it at 65C.  A small number of PCVs have researched solar cooker technology on the internet and have introduced it in their villages. As a former PCV in Paraguay, whose family cooked every day over a wood fire, this is a technology I wish I had known about


Dr. Metcalf challenges the widespread claim that water can only be made safe to drink by bringing it to a rapid boil for several minutes.  He approaches the topic of water pasteurization using well-established concepts of food microbiology. If it actually took several minutes of a roiling boil to pasteurize water, then what heat treatment would be required to pasteurize milk?   Since microbes survive better in milk than in water, and since the relatively heat-resistant bacteria that cause tuberculosis, need to be inactivated in milk, one might assume then that the dairy industry would have to boil milk for an extended period of time. But that is not the case. Most milk is flash pasteurized at 71.7C (161F) for 15 seconds and raw eggs are pasteurized at 60C (140F) in 3.5 minutes.


Water Pasteruization IndicatorsIn his work, Dr. Metcalf cites extensive literature on the times and temperatures needed to kill pathogens in foods. He reports that the cysts of protozoa, bacteria and rotavirus, which cause most water-borne disease are inactivated rapidly at 60C (140F), and inactivated very rapidly at 65C (149F).  He also notes that these cysts are sometimes able to resist treatment by chlorine, which is not always able to penetrate the cyst walls unlike 65C water which heats the interior of the cyst and denatures critical enzymes.


Although he endorses the safe water system approach of the Centers for Disease Control using Water Guard, and the use of Aquatabs, he has found that solar water pasteurization with the WAPI provides another option for people who do not have access to chlorine, or find the taste of chlorinated water objectionable. Furthermore, solar pasteurization is a sustainable method since the world will never run out of sunshine.  


To pasteurize water in 4-5 liter batches, Dr. Metcalf uses a panel solar cooker (the Cookit, which can be purchased on-line or constructed at minimal cost from scrap cardboard and aluminum foil) to heat the water.  A WAPI is suspended in the pot of water while it is being heated in the Cookit.  When the water reaches 65C, the wax plug at the top of the WAPI melts and falls to the bottom of the tube, indicating that the water has reached pasteurization temperature and it is now safe to drink.  Dr. Metcalf stresses that safe storage of the water in a clean container after pasteurization is of course essential to maintain its purity.  


AquaPakA small, portable manufactured device for solar pasteurization of water, which might be a useful addition to a PCV's emergency kit, is the Aquapak, which contains a built-in WAPI and can pasteurize up to five liters of water at a time.   For large-scale solar water pasteurization requirements, the Hawaii-based company Safe Water Systems  makes the Sunray 1000, which can solar pasteurize up to 1000 liters of water per day.   Safe Water Systems' homepage notes that, "A 1998 U.S. Dept. of Energy, National Renewable Energy Laboratory (NREL) study concluded that solar water pasteurization was superior in most regards to competing technologies, e.g., chemical, filter and reverse osmosis."


Dr. Metcalf has conducted microbiology tests in Sacramento, Kenya and Tanzania comparing the solar cooker/WAPI method of pasteurizing water with the SODIS (solar water disinfection) method in which water is placed in clear plastic bottles and exposed to sunshine for a recommended six hours. He found that the SODIS method was not always able to completely inactivate E. coli in contaminated water even after six hours. In addition, viruses and protozoan cysts are inactivated by sunshine at a much slower rate than are bacteria. Thus, sunshine inactivation of bacteria does not correlate with inactivation of viruses or cysts. He also notes that with the SODIS method there is no way to know for certain if and when pathogenic bacteria, viruses, and protozoa have been neutralized.


Mobile Microbiology LaboratoryFor the past six years, Dr. Metcalf has carried out extensive studies on water testing in the U.S., Kenya, and Tanzania. He needed to conduct water tests where there was no laboratory, and he selected commercially available materials used in the water and food industries to detect the indicator of fecal contamination, the bacterium Escherichia coli. He has assembled a simple-to-use, inexpensive water testing kit, called the Mobile Microbiology Laboratory, which fits inside a gallon-size Ziploc plastic bag.  Clear results from the two tests are obtained in 12-18 hours, and the results correlate with WHO guidelines for low, moderate, high, or very high risk of disease. He has taught unskilled villagers to use the kit to test their water supplies.  Not only do they determine if their water sources contain fecal microbes, but they also witness how an invisible bacterial cell can grow into a visible colony of a billion cells overnight. This leads to a profound understanding of how similar explosive growth of typhoid, cholera, or dysentery bacteria in their intestine can result in serious disease. Dr. Metcalf has posted a detailed power point presentation on water testing using the Mobile Microbiology Laboratory at the following website:  He can be reached at


Patricia McArdle, a staunch advocate of solar cooking, joined the board of Solar Household Energy and Solar Cookers International in 2007 to help promote the production of clean, safe, distributable, and renewable energy. Ms. McArdle has more than 30 years of international and public affairs experience.


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