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Solar Water Disinfection

Developed in the 1980s, SODIS (solar disinfection) has been used to treat water, inexpensively, for an estimated, over two million people in over 28 developing countries.

The principle is simple. The use of plastic water bottles are filled with water of low turbidity and left out in the sun. The sun’s UV is then left to disinfect the water.

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http://solarcooking.wikia.com/wiki/SODIS

Trish Morrow reports: Earlier in the year (2007), on 25 August, engineering staff tested the SODIS method of water treatment.

Four bottles of contaminated water were collected from the same location on the shores of Lake Tanganyika. One bottle was painted black with blackboard paint, two bottles were left clear and one bottle was half painted with blackboard paint, the other half being clear. Three bottles were placed in the sun for approximately four hours and the fourth bottle (clear, not painted) was kept in the shade indoors during the entire time. The bottles were placed on the corrugated iron roof of our office, where they were in a nearly horizontal position. The half-black bottle was placed with the transparent side upwards. After the three bottles had been left in the sun for approximately four hours, all four samples were analysed using a Del Agua bacteriological test kit. This kit uses the membrane filtration method. Results were as follows: Transparent bottle left in sun had 11 fecal coliforms per 100ml, bottle painted black had 2 fecal coliforms per 100ml, the control (raw water from Lake Tanganyika) had 29 fecal coliforms per 100ml and the bottle which was half painted black had 0 fecal coliforms.

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For complete inactivation of spores or cysts, temperatures of 50 degrees Celsius or higher are required

Rather than me try and convince anyone of the effects of water pasteurization, I’ve included a brief excerpt from solarcooking.wikia.com

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http://solarcooking.wikia.com/wiki/Water_pasteurization

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.

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.

MicrobeKilled 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 (9°F) below these temperatures, although it may take a couple of minutes at the lower temperature to obtain 90 percent inactivation.)

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As one can see, from the chart above, the protozoa which would really affect us with water, are easily made inactive through pasteurization.

Solar Water Disinfection

During an interview (included link at end of article),  Bob Metcalf, a microbiologist with over 30 years of experience,  discussed the efficacy of water pasteurization, it’s effectiveness on bacteria, viruses and protozoa. Included in this interview, Dr. Metcalf also discusses bacteria in food and the efficacy of solar cooking

One can conclude, both from the available data and the interview, once the water reaches boiling, and no more,  pasteurization has occurred  and rendered water safe to drink; moreover, it was safe to drink at temperatures well below boiling. Bringing it to a boil is only a visual indicator of temperature, which can be discarded if one has a thermometer or a WAPI (Water Pasteurization Indicator) stick.

This information, in and of itself, is of significant importance to those who spend time in the outdoors, where fuel may be scarce, are in a survival situation, or where people are simply trying to make accurate fuel calculations for backpacking trips.

Interview with Dr. Bob Metcalf regarding water pasteurization http://solarcooking.org/media/broadcast/default.htm?metcalf.

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Ever since I can remember, I’ve loved the outdoors. My dad was an avid outdoorsman and always taking the family camping, fishing, and hunting. I reckon I must have been about 10 or 12 (circa 80s) when I went to Segovia, Spain and spent a summer there...

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