wapi21.3 billion people worldwide use unsafe drinking water. Each year, 2 million children die of drinking contaminated water. In many countries with contaminated water, there is sufficient solar energy to heat water to a temperature that kills harmful micro-organisms, thus creating safe drinking water. This process is called “Solar Water Pasteurisation”.

Milk and water are pasteurised at approx. 71ºC during 15 seconds.

Water Pasteurisation Indicator (WAPI)

A Water Pasteurisation Indicator (WAPI) is a simple thermometer that indicates when water has reached pasteurisation temperature (65 to 70°C) and is safe to drink. The University of California designed the WAPI. It is a small clear plastic tube (we use a simple BIC ballpoint), partly filled with soybean wax or myverol (available at bakeries) that melts at approx. 70ºC. A stainless steel washer is placed around the tube as well as at both ends of the piece of nylon fishing line. The washers make sure that the WAPI stays at the bottom of the pan, while the wax is at the top. By hanging the WAPI in the water, the women only need to check when the wax has melted. At that point the water is safe to drink. The water does not need to be brought to boiling point.

The following diagram shows at what degrees the most common micro-organisms are killed.

Micro-organism Death at
Worms, protozoa 45°C
Bacteria 60°C
Hepatitis A virus 65°C


wapiHow to use the WAPI

Hang the WAPI in a black heat resistant glass jar, a kettle or a pan filled with water so that the small tube is in an upright position at the bottom of the pot. The wax must be at the top of the tube. Heat the water. The wax melts as the water temperature increases. At that point the water is pasteurised.

The water can be pasteurised at times when the CooKit is not being used for cooking. This requires planning and habit changes, topics that end-users must bear in mind.

Pasteurisation requirements

The CooKit is the most efficient way, but it can also be done on a wood-saving appliance. One needs a black (painted) glass pot with lid (only suitable for CooKit or Solar Box) or a black painted aluminium jug/kettle with lid.

Dr. Robert Metcalf has published about the WAPI on www.solarcookers.org

For more information on the WAPI:
“How to use CooKit and WAPI” and “How to manufacture CooKit and WAPI”.