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The Solar Puddle - A New Water Pasteurization Technique

Boiling Point
Issue 36 (1995) Solar Energy in the Home
Article	The Solar Puddle - A New Water Pasteurization Technique
Author	Dale Andreatta

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 recontaminate their clean drinking water. Recently, several of us at the University of California at Berkeley have attacked the first of these requirements. Information on pasteurization is available from Solar Cookers International, 1724 11th Street, Sacramento, California, USA 95814. In this article we describe a new low-cost device that pasteurizes water.

In pasteurization, water is heated to 149°F (65°C) for about six minutes, killing all the germs, viruses, and parasites that cause disease in humans, 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 it is particularly easy to scale down so that 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 shown in the figure below, though many variations are possible.

Begin by digging a shallow pit. The test puddle was a 'family-size' unit, about three and a half feet square, but it could be larger or smaller. The pit is filled with two to four 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 for the drain out syphon.

The level of the bottom of the puddle must be above the level of the surrounding ground to permit syphoning.

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 sheet 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 half an inch throughout the puddle, except in the trough which should be about one inch deeper than the rest. Put in more water so that the average depth is one to three inches depending on how much sunshine is expected. A pasteurization indicator (available from Solar Cookers International, Tel: +916 444 6616, should go in the trough since this is where the coolest water will collect.

A basic solar puddle. Horizontal proportions are shown compressed for clarity A puddle can also be built with wooden sides on top of a table or roof.

Put a layer of clear plastic over the water, again with 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 two inches or more.

Pile soil or stones 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 per cent 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 recontaminated 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), a siphon tube (about $1), and four sheets of plastic (about $2 for the size tested). All of these items are easily transportable, so solar puddles might be an excellent option for refugee camps if the expertise were available for setting them up. On days with good sunshine the required temperature was achieved even with 17 gallons of water (two and a half inch depth).

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 there is a lot of condensation the top layer should be pulled back to let it evaporate. Small holes in the top layers do not make much difference. The device works in wind, or if the bottom insulation is damp. The water temperature is uniform throughout the puddle to within 2°F ( 1 °C).

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 such as places that are exposed to the sun but not cooled by the water. Another option would be to use an acrylic grade of plastic that is stronger and more resistant to sunlight but is more expensive. The two bottom layers of plastic tend to get 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. It is also possible to build a puddle with wooden sides, on the ground, on a table top, or on the roof of a building. We have 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.

One could make a water heater by roughly tripling the amount of water so that the maximum temperature was only 120°F (50°C) but this water would stay warm well into the evening. This water would not be pasteurized.

One could help solve the problem of dirty 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 in emergencies, for example in a refugee camp.

[top] [end]Contents: Boiling Point 36: Solar Energy in the Home

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Parameters for a Solar Cooker Programme - The Sunstove Solar Box Cooker - Sunstoves in the Republic of South Africa - Gaining Ground in Solar Box Cooking in Kenya - Solar Cookers - A Cause Worth Promoting - Free Energy from the Sun - A Solar Box Cooker with a Reflecting Lining - The Solar Puddle - A New Water Pasteurization Technique - Renewable Energy - A World Bank View - ESMAP study points toward village-level management of woodfuel resources in Chad - Burning Charcoal Issues - A Dangerous Trade - Saving Wood by Burning Coal - Haitis Domestic Fuel Project - Coal briquetting and clays for Zambian stoves - Improving the three-stone fire - Comparative tests of solar box cookers - Parabolic Solar Reflector and Heat Storage Cooker - An Affordable Parabolic Solar Cooker

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