A Simple Solar Water Pasteurizer

Drinking water can be easily pasteurized using solar energy and a few simple materials. When water or milk is pasteurized the common disease-causing organisms are killed.

What you will need

• A cardboard box or other container such as a basket or wooden box. Container materials must have basic heat retaining qualities. Bricks and metal containers do not work well unless they are insulated.
• Aluminum foil to line the inside of the box and the flaps. While it is possible to pasteurize a small amount of water without aluminum foil, its use appreciably improves performance.
• A dark colored solar absorber plate made of sheet metal, cardboard, or wood. Metal works best to conduct the heat to the water containers.

  

• A solar "window" made of glass or plastic film over the top of the box.

  

• A reflector to bounce in additional sunlight.
• Dark or clear containers to hold water or food (since you can cook in this oven as well).

To pasteurize water, heat it in the solar box to at least 65 degrees C (150 F) and keep the water at that temperature or above for at least 30 minutes. If no thermometer is available, heat until bubbles are rising from the bottom steadily. Natural waxes, such as beeswax, can be used to indicate pasteurization temperature.

Solar conditions, weather conditions, latitude and box efficiency are all variables that affect the ability of solar boxes to pasteurize water. As a general guideline, 4 liters (~1 gallon) of water can be pasteurized in about 3 hours on a day with strong sunlight and the sun high in the sky. The plastic- or glass-covered opening should be at least 45 x 60 cm (18" x 24") and have a depth just taller than the water containers inside. Larger boxes can pasteurize more water, smaller less.

Pasteurization kills germs and disease-carrying organisms in drinking water including bacteria, rotaviruses, enteroviruses, and cysts commonly transmitted in contaminated water. Pasteurized water is not sterilized, however, and therefore should not be used for medical procedures. Pasteurization does not remove chemical contamination such as pesticides or industrial wastes.

For further information on solar pasteurization see the following documents:

• Recent Advances in Solar Water Pasteurization (Solar Box Journal #18)
• Interview with microbiologist Dr. Bob Metcalf
• The Solar Puddle Water Pasteurizer
• Improved Devices to Pasteurize Drinking Water (Solar Box Journal #16)
• Pasteurization of Naturally Contaminated Water with Solar Energy (Applied and Environmental Microbiology v. 47 no 2, 1984)

Solar box cookers, in use throughout the world, are also very effective for cooking food. For further information, contact