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improved devices to pasteurize drinking water

in many poor countries drinking water is contaminated with a variety of germs and viruses. it is estimated that nearly 10,000 children die each day from dehydration caused by diarrhea, which in turn is caused by germs carried in drinking water. one way to provide clean drinking water is to heat-pasteurize the water, that is, to heat the water to 65 degrees c for 30 minutes, or 82 degrees c for a few seconds, or an intermediate temperature for an intermediate time. it is not necessary to boil the water as many people believe.

solar cookers international has been promoting the fact that water can be pasteurized by placing covered pots of water inside a solar box cooker. a pasteurization indicator available from sci can be used to determine if and when the water is pasteurized. while this technique is simple it is not very efficient, and produces at most about 8 liters of water per day with a standard kerr-cole solar cooker. a more efficient technique is used by pax world foundation, and uses 60 feet (18 m) of black tubing inside the same solar box cooker, with a thermostatic valve to allow water flow when the pasteurization temperature is reached. this technique can produce as much as 20 liters per day.

while these techniques work, they throw away the heat in the hot outgoing pasteurized water. if one could recover most of that energy one could decrease the amount of energy required to raise the incoming water to the pasteurization temperature. recently, several of us in the department of mechanical engineering at the university of california at berkeley have designed a device to do just that. it is designed to work with the pax unit and "recycles" the heat of the outgoing hot water, using this heat to preheat the water going into the solar box and decreases the amount of solar energy required to get to the pasteurization temperature by about 75%. this increases the amount of water that can be pasteurized fourfold, to about 80 liters per day. this device is shown in the drawing below.

for more detailed instructions, write to dale andreatta, u.c. berkeley, 5234 huntington ave., richmond, california 94804 usa

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