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Sari cloth filtering

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Last updated: 17 October 2014      

In Bangladesh, cholera is a threat, particularly around the spring and summer monsoons. Some of the cholera bacteria attach to much larger -- but still microscopic -- plants and animals, which could possibly be filtered out of water. This has led to the recommendation to filter water through 8-10 folds of sari cloth (see National Geographic, February, 2002, p. 20). In Meatu district, Dr. Bob Metcalf and Ms. Polinelli tested eight different cloths, folded eight and twelve times. As they expected, for all eight cloths, the E. coli bacteria in the filtrate was equal to the concentration before filtering. Unattached bacteria, and likely viruses, which are a fraction of the size of bacteria, go through folded cloth like air molecules go through a screen!

Dr. Metcalf has always considered the gold standard of microbe inactivation procedures to be: Will the person recommending a specific procedure actually drink heavily contaminated water which has been treated by the particular method? Dr. Metcalf will, and has many times, with solar water pasteurization using a CooKit and a WAPI. Certainly the folded sari method is dangerous. The SODIS method is effective at reducing viable bacteria. However, its ability to inactivate viruses, particularly the dangerous rotavirus, is less certain. And there is no inexpensive, easy device that indicates when SODIS-processed water is safe to drink.

Most of the people that Dr. Metcalf has talked to about solar water pasteurization oven the past 20 years are astonished to learn that water does not have to be boiled to make it safe microbiologically. The fact that the WHO, the Center for Disease Control (CDC), and the Peace Corps routinely recommend boiling suspect water -- sometimes for as long as ten minutes -- reinforces the myth that water needs to be heated drastically more than milk to kill pathogens. One reason for boil orders, Dave Ciochetti and Dr. Metcalf speculated 20 years ago, is that without a thermometer, it is not possible to judge the temperature of heated water. What the world needed was the WAPI, which Solar Cookers International now manufactures. The SODIS method includes recommendations to boost water temperatures to 55°C (131°F) if possible, which greatly increases killing of bacteria. What is not well known is that the alternative to direct sunshine to pasteurize water is not boiling, but modest temperatures in the 60°C range, which are easily obtained using simple solar cookers. When it becomes common knowledge that 60-65°C (140-149°F) will guarantee all pathogens are killed, and boiling isn't needed, the CooKit and WAPI approach should become much more prominent. (Note that solar water pasteurization requires a container other than plastic, which starts to melt near 60°C.)

Another major reason to promote the CooKit/WAPI method of solar water pasteurization is that in most of the places where water is unsafe, people also use fuel wood for cooking, as they do in Meatu district. Women have to spend hours collecting scarce firewood every two or three days, and then spend more hours tending a smoky fire. The SODIS method doesn't inform 2.5 billion people who depend on traditional fuels that with sunshine, there is an alternative to fire.

In advocating for the spread of solar cooking, the many thousands of solar cooks who read the Solar Cooker Review might find that solar water pasteurization will open doors that the cooking function alone hasn't been able to open. That has happened the last two years in Tanzania, and this past July in Nairobi, Kenya, when Christine and I worked with SCI's eastern Africa staff to hold a three-day presentation and workshop on water testing and solar water pasteurization. We look forward to expanding these efforts in the future.

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