The Solar Puddle

A new water pasteurization technique for large amounts of water

The lack of clean drinking water is a major health problem in the developing world. To reduce this health risk ways of producing clean water at an affordable cost are needed, and people need to be educated about germs and sanitation, lest they accidentally re-contaminate their clean drinking water. Recently, several of us at the University of California at Berkeley have attacked the first of these requirements. Previous issues of this newsletter have included stories about our water pasteurization indicator and our flow-through water pasteurizers based on a design by PAX World Service. In this article we describe a new low-cost device that pasteurizes water.

For those not familiar with the pasteurization process, if water is heated to 149 F (65 C) for about 6 minutes all the germs, viruses, and parasites that cause disease in humans are killed, including cholera and hepatitis A and B. This is similar to what is done with milk and other beverages. It is not necessary to boil the water as many people believe. Pasteurization is not the only way to decontaminate drinking water, but pasteurization is particularly easy to scale down so the initial cost is low.

The new device is called a solar puddle, and it is essentially a puddle in a greenhouse. One form of the solar puddle is sketched in the figure below, though many variations are possible.

One begins by digging a shallow pit about 4 inches deep. The test device was a "family-size" unit, about 3.5 feet by 3.5 feet, but the puddle could be made larger or smaller. If the puddle is made larger there is more water to pasteurize, but there is also proportionately more sunshine collected. The pit is filled with 2 to 4 inches of solid insulation. We used wadded paper, but straw, grass, leaves, or twigs could be used. This layer of insulation should be made flat, except for a low spot in one corner of the puddle. Put a layer of clear plastic and then a layer of black plastic over the insulation with the edges of the plastic extending up and out of the pit. Two layers are used in case one develops a small leak. We used inexpensive polyethylene from a hardware store, though special UV stabilized plastic would last longer. Put in some water and flatten out the insulation so that the water depth is even to within about 0.5 inch throughout the puddle, except in the trough which should be about 1 inch deeper than the rest. Put in more water so that the average depth is 1 to 3 inches depending on how much sunshine is expected. A pasteurization indicator (available from Solar Cookers International 916/444-6616) should go in this trough since this is where the coolest water will collect. Put a layer of clear plastic over the water, again with the edges extending beyond the edges extending beyond the edges of the pit. Form an insulating air gap by putting one or more spacers on top of the third layer of plastic (large wads of paper will do) and putting down a fourth layer of plastic, which must also be clear. The thickness of the air gap should be 2 inches or more. Pile dirt or rocks on the edges of the plastic sheets to hold them down. The puddle is drained by siphoning the water out, placing the siphon in the trough and holding it down by a rock or weight. If the bottom of the puddle is flat, well over 90% of the water can be siphoned out.

Once the puddle is built it would be used by adding water each day, either by folding back the top two layers of plastic in one corner and adding water by bucket, or by using a fill siphon. The fill siphon should NOT be the same siphon that is used to drain the puddle, as the fill siphon is re-contaminated each day, while the drain siphon MUST REMAIN CLEAN. Once in place the drain siphon should be left in place for the life of the puddle.

The only expensive materials used to make the puddle are a pasteurization indicator (about $2 for the size tested). All of these items are easily transportable, so the solar puddle might be an excellent option for a refugee camp if the expertise were available for setting them up.

Many tests were done in the spring and summer of this year in Berkeley, California. On days with good sunshine the required temperature was achieved even with 17 gallons of water (2 1/2 inch depth). About 1 gallon is the minimum daily requirement per person, for drinking, brushing one's teeth, and dish washing. With thinner water layers higher temperatures can be reached. With 6 gallons (1 inch depth) 176 F was achieved on one day.

The device seems to work even under conditions that are not ideal. Condensation in the top layer of plastic doesn't seem to be a problem, though if one gets a lot of condensation the top layer should be pulled back to let the condensation evaporate. Small holes in the top layers don't make much difference. The device works in wind, or if the bottom insulation is damp. Water temperature is uniform throughout the puddle to within 2 F.

After some months the top plastic layers weaken under the combined effects of sun and heat and have to be replaced, but this can be minimized by avoiding hot spots. Another option would be to use a grade of plastic that is more resistant to sunlight. The two bottom layers of plastic tend to form tiny tears unless one is very careful in handling them, (that is why there are two layers on the bottom). A tiny hole may let a little water through and dampen the solid insulation, but this is not a big problem.

There are many variations of the solar puddle. We've been able to put the top layer of plastic into a tent-like arrangement that sheds rain. This would be good in a place that gets frequent brief showers. Adding a second insulating layer of air makes the device work even better, though this adds the cost of an extra layer of plastic. As mentioned the device can cover a larger or smaller area if more or less water is desired. One could make a water heater by roughly tripling the amount of water so that the maximum temperature was only 120 F or so, and this water would stay warm well into the evening hours. This water wouldn't be pasteurized though. One could help solve the problem of dirty water vessels by putting drinking cups into the solar puddle and pasteurizing them along with the water. The solar puddle could possibly cook foods like rice on an emergency basis, perhaps in a refugee camp.