The Boil 3, Add 1 Method is a proposed method of water disinfection that provides the fuel savings of pasteurization (versus sterilization) without the need of a thermometer. This is achieved by replacing temperature measurements with simple volume measurements that anyone can perform.
Please note: This method needs to be validated by testing in the field.
Everybody knows that water contaminated by pathogens can be made safe to drink through boiling. Solar Cookers International co-founder Dr. Bob Metcalf, a microbiologist, confirmed in an article in APPLIED AND ENVIRONMENTAL MICROBIOLOGY (Feb. 1984)  that water could be made safe to drink using much lower pasteurization temperatures (65°C or 150°F), thus saving fuel.
Even though the fuel savings of pasteurization make it preferable to boiling, boiling is still commonly recommended because rising bubbles are a clear indication that boiling temperatures have been reached. In order to achieve the fuel savings of the lower pasteurization temperatures however, a thermometer is needed. The advantage of the Boil 3, Add 1 Method is that it can be spread as information, not needing any technology to put it to use.
Using the Boil 3, Add 1 Method
Bring 3 measures of water to a boil and then add 1 measure of cold water. A quick practical test at sea level shows that adding 1 measure of water at 1°C (33°F) (the coldest temperature where water is still liquid) to 3 measures of boiling water resulted in 4 measures of water at 78°C (173°F), which is well above the temperature where water is made safe to drink instantly. Mathematics predicts that the resultant temperature should have been 75°C (167°F), but the residual heat in the pot may have contributed to the higher experimental temperature.
Steps for use
- Select a container appreciably smaller than your cooking pot.
- Pour 3 containers full of water into the cooking pot.
- Heat the water to boiling.
- As soon as the water starts to boil, add 1 container full of cold water to the boiling water.
- If your altitude is below 2000 meters your water will be safe to drink immediately. Above 2000 meters, wait 15 minutes before drinking the water.
Sea level variation: Boil 5, Add 2
The normal "Boil 3, Add 1" method works at any any altitude up to 2000 meters since it takes into account the fact that water boils at a lower temperature at higher altitudes. At sea level, however, one can mix the water in a proportion of 5:2 instead of 3:1 and arrive at a temperature of 71.7°C. This variation allows for more water to be pasteurized at sea level with the simple rule of "Boil 5, Add 2."
- People using this method must be taught not to use the measuring cup as a drinking cup after the pot of water has cooled down.
- This method will not work unattended as does the Water Pasteurization Indicator.
Topics for research
The above method was designed to work even if the colder water is close to freezing and the boiling temperature is only 82.8°C (181°F), as it is at an altitude of 5000 meters. Design for the worst-case scenario is necessary for this method to work 100% of the time and to be easily communicated with others. However, humans are able to determine when water is lukewarm (body temperature) within a few degrees(as yet to be tested). This lukewarm water could be added in a proportion of 1:1 with boiling water and pasteurization might still be achieved (mathematics shows that the resulting temperature at sea level would be 68.5°C (155°F). At 5000 meters a ratio of 2:1 would result in water at 67.5°C (153.5°F).
- Excel spreadsheet for calculating the resulting temperature when mixing quantities of water a different temperatures