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Water Pasteurization Frequently-Asked Questions

Dont you need to boil water for 20 minutes to sterilize it and make it safe to drink?

No, it is only necessary that you raise that water to 65 C and keep it around that temperature for 20 minutes 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 organisms are killed by pasteurizing the water at 65C?

Guardia, cryptosporidium, endameba, 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 65C.

How can water be tested in the field?

The best indicator of human or animal fecal contamination of water is the bacterium Escherichia coli, which is always present in human feces, 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 we have 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). I use the Colilert MPN tube, which is inoculated with 10 ml 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.

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.

Is there a kit available that contains everything needed for water testing in the field?

Yes Dr. Bob Metcalf, a microbiology professor at California State University at Sacramento has prepared such a kit. Dr. Metcalf can be contacted at rmetcalf@csus.edu. You can see a large collection of photographs from Dr. Metcalf's solar water pasteurization projects in Africa here.

This document is published on The Solar Cooking Archive at http://solarcooking.org/pasteurization/solar_water_pasteurization_faq.htm. For questions or comments, contact webmaster@solarcooking.org