A SUMMARY OF WATER PASTEURIZATION TECHNIQUES

Dale Andreatta, Ph. D., P. E.
andreatta@sealimited.com 

Much of this document is taken from: RECENT ADVANCES IN DEVICES FOR THE HEAT PASTEURIZATION OF DRINKING WATER IN THE DEVELOPING WORLD by Dale Andreatta, Derek T. Yegian, Lloyd Connelly, and Robert H. Metcalf, from the proceedings of the 29th Intersociety Energy Conversion Engineering Conference, American Institute of Aeronautics and Astronautics, Inc., 1994. See also the main article on Solar Water Pasteurization on the Solar Cooking Archive Wiki.

Introduction

Water quality and human health have been closely linked throughout history. However, it was not until the last quarter of the 19th century that pioneering work by Robert Koch and Louis Pasteur established the germ theory of infectious disease. With the understanding that fecal-borne bacteria, viruses, and protozoans were responsible for most water-borne diseases, it was possible to develop sanitation and water treatment practices which provided people with a safe water supply. In industrial countries safe water is now taken for granted.

In developing countries however, the burden of disease caused by contaminated water and a lack of sanitation continues to be staggering, particularly among young children. Diarrhea is caused by microbes entering the mouth, most often from contaminated water. According to the United Nations Children's Fund (UNICEF) diarrhea is the most common childhood disease in developing countries. Dehydration resulting from diarrhea is the leading cause of death in children under the age of five, annually killing an estimated five million children. Diarrhea is also the most common cause of child malnutrition, which can lead to death or permanently impaired mental and physical development.¹

UNICEF estimates that 60% of rural families and 23% of urban families in developing countries are without safe water. In some areas all water supplies may be contaminated.² If a water source is suspected of being unsafe, the most common recommendation is to boil the water.¹ This recommendation is seldom followed for several understandable reasons, the most important being the time and the amount of scarce fuel it would require.

Contrary to what many people believe, it is not necessary to boil water to make it safe to drink. Also contrary to what many people believe, it is usually not necessary to distill water to make it safe to drink. Heating water to 65º C (149º F) will kill all germs, viruses, and parasites.³ This process is called pasteurization and its use for milk is well known though milk requires slightly different time temperature combinations. One obvious problem that arises with pasteurization is the question of how to tell when and if the water has reached the right temperature. Solutions to this problem will be covered in the next section. Pasteurization will not help if water is brackish or chemically contaminated.

In this document we describe several pasteurization techniques applicable to developing countries. Pasteurization is not the only technique that can be used to make water safe to drink. Chlorination, ultra-violet disinfection, and the use of a properly constructed, properly maintained well are other ways of providing clean water that may be more appropriate, particularly if a large amount of water is needed. Conversely, if a relatively small amount of water is needed, pasteurization systems have the advantage of being able to be scaled down with a corresponding decrease in cost. As always, the selection of the right system should be based on local conditions.

This document describes techniques used to pasteurize water, but it is also necessary to educate people about the need for clean water and how to keep their water clean. Among many people in the developing world clean water is not perceived as being important. Also, since many people do not understand how germs are transmitted, many cased have been reported where people unthinkingly recontaminate their clean water by putting it into a contaminated container.

Basic Methods of Solar Water Pasteurization-Solar Cookers

A simple method of pasteurizing water is to simply put blackened containers of water in a solar box cooker, an insulated box made of wood, cardboard, plastic, or woven straw.³ A solar box cooker is sketched in Fig. 1. One popular type of solar box cooker is made of aluminized cardboard and has a solar collection area of about 58 cm by 48 cm (23 inches by 19 inches). It has a reflective lid that increases the sunlight collected. With this device a yield of 4 to 12 liters (1 to 3 gallons) per day is achieved in the field. Each person requires about 4 liters (1 gallon) of water per day, about half of which is for drinking and the other half is for dish washing and brushing one's teeth. The cost for this device is on the order of $20, US, depending on how easily available the basic materials are.

Figure 1: A solar box cooker being used to pasteurize water.

Other types of solar cookers can be used. A recent development in solar cookers is the solar panel cooker, which consists of reflective panels that concentrate sunlight on the food. The food is in an oven roasting bag to reduce heat loss. Replacing the food with a darkened container of water makes a solar water pasteurizer. While the cost of these panel cookers is low, not more than 2 liters of water can be pasteurized at a time, though in the right climate several batches per day can be pasteurized.

Regardless of the type of solar cooker used, a way of knowing that the water reached the pasteurization temperature is needed. An inexpensive device that does this was developed, and is shown in the Fig. 2. It is a plastic tube with both ends heated, pinched, and sealed, and with a particular type of soybean fat in one end that melts at 154º F. The tube itself is buoyant, but is weighted with a washer so it sinks to the bottom (coolest) part of the water, with the fat in the high end of the tube. If the fat is found in the low end of the tube at any time after, the water reached the proper temperature, even though the water may have since cooled down. A nylon string makes it easy to take the tube out without recontaminating the water. The tube is reused by flipping it over and sliding the string through the other way. This device works in any size water container, costs about $3, and is available from Solar Cookers International, 1919 21st St., Suite 101, Sacramento, California, 95811, (916) 455-4499, fax: (916) 455-4498. This device also works with fuel-heated water. Since heating the water to the pasteurization temperature rather than the boiling point reduces the energy required by at least 50%, the fuel savings offered by this simple device alone is considerable.

Figure 2: A Water Pasteurization Indicator. The indicator would sit at an angle in the bottom of a water container.

This device works anytime when water is pasteurized in batches regardless of the source of the heat. If one were burning fuel to pasteurize pots of water the pasteurization indicator would still be usable, as long as one didn't get the nylon string too close to the fire. Since heating the water to the pasteurization temperature rather than the boiling point reduces the amount of energy required by at least 50%, the fuel savings offered by this simple device is considerable.

Flow-Through Pasteurization Devices

In order to produce more water PAX World Service produced a flow-through unit which consists of 15 meters (50 feet) of black-painted tubing coiled within a standard solar box cooker. One end of this tubing is connected to a thermostatic valve and the other to a storage tank for the untreated water supply. This storage tank also contains a sand/gravel/charcoal filter that does the preliminary filtering. The small amount of water (about 1.5 liters) within the tubing allows rapid heating of the water to the valve's opening temperature of 83.5º C (182º F). This is well above the required temperature, but the valve is derived from a mass-produced automotive radiator thermostat valve, so there is a limited selection of opening temperatures. The thermostatic valve opens allowing the pasteurized water to drain out of the tubing and into a second storage vessel for treated water. As the treated water drains from the solar box cooker, contaminated water from the storage tank automatically refills the tubing. Once this cool water reaches the valve the valve shuts and the pasteurization process begins anew.

This flow-through device addresses several of the problems inherent in the batch processes. First, potable water becomes available throughout the day as new increments of treated water are added to the clean storage vessel. Second, this type of unit can adapt to variable solar conditions which takes the guesswork out of filling the jugs in a batch process. If the insolation increases the time required to pasteurize and release the water in the tubing decreases, thus supplying increments of treated water at a faster rate. If insolation decreases the residence time in the solar box cooker will increase, but it will still be pasteurized which may not be the case in a batch unit where the user overestimated the amount of water which could be treated for that day. This is also a totally automatic process, freeing time for other chores and decreasing the likelihood of an accident occurring when transferring water in and out of a batch unit. Field trials by PAX World Service and the Pakistan Council of Appropriate Technology have regularly shown yields of 16 to 24 liters per day (4 to 6 gallons per day). The cost of this device is on the order of $50, US.

Although this is a respectable increase, much more dramatic improvements can still be achieved by recycling the heat in the outgoing pasteurized water. Once the water has been pasteurized and released from the solar box cooker the energy in this water can be used to preheat the incoming water. This process is shown in Fig 3. Since the temperature of the water entering the solar box cooker is higher, it takes less time to finish the pasteurization process, allowing more water to be treated. Also, the flow resistance of the heat exchanger smoothes the flowrate of the water.

Figure 3: PAX -style water pasteurizer with heat exchanger. Typical temperatures are shown in degrees Fahrenheit.

A simple device which accomplishes this preheating is a counter-current heat exchanger. The hot water flows on one side of a metal plate, while on the other side of the plate cooler fluid flows in the opposite direction. The energy from the hot water is transferred to the cold water, thus preheating the incoming contaminated water by lowering the temperature of the outgoing pasteurized water.

There are many ways of building a counter-current heat exchanger. Both a tubular version and a flat version have been tested using various configurations and materials, with experimental results favoring the less expensive flat version, though the tubular version is easier to construct from purchased parts. The flat plate unit allows between 75% and 80% of the energy to be reused in preheating the incoming water, and roughly four to five times more water will be pasteurized over a flow-through unit without a heat exchanger. This corresponds to about 80 to 96 liters (20 to 24 gallons) of treated water per day, which is a ten to twelve-fold improvement over the original solar box cooker batch method. An additional benefit is that the chance of burns is greatly reduced because the outflowing water is much cooler reducing the burn hazard. The cost of the heat exchanger itself is on the order of $15 US, making the cost of the complete PAX system about $65. Thus for an increase in cost of about 15% the heat exchanger provides about a 400% increase in water output.

Other Sources of Heat

A heat exchanger can produce benefits with any source of heat, including the exhaust heat from an engine, a fire (that may be used to cook food at the same time,) and heat from other types of solar collectors. We have done some engineering analysis and generated an equation to determine the water output of a particular system of this type. 4 This analysis can also be used to determine the relative benefits of a better heat exchanger, vs. a bigger solar collector vs. a better insulated collector.

If one went with a flame-heated system one would require a short piece of metal tubing, a thermostatic valve with housing, and a heat exchanger. The total cost of this type of system would be about $30. At present we have not done any experiments in this area.

The Solar Puddle-A Low Cost Large Area Device

While many factors determine the usefulness of a water pasteurizer, an important figure of merit is the water delivered per unit cost. A device which is made only of low cost materials is being called a "solar puddle" and it is essentially a puddle in a greenhouse. One form of the solar puddle is sketched in Fig. 4, though many variations are possible.

Figure 4: A basic solar puddle. Horizontal dimensions are shown compressed for clarity. A puddle can alse be built with wooden sides on top of a table or roof.

One begins by digging a shallow pit about 10 cm (4 inches deep). The test device was a "family-size" unit, about 1 meter by 1 meter (3 1/2 feet by 3 1/2 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 at least 5 cm (2 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, which is marked "trough" in Fig 4. A layer of clear plastic and then a layer of black plastic goes 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. One would then put in some water and flatten out the insulation so that the water depth is even to within about 1 cm (1/2 inch) throughout the puddle, except in the trough which should be about 3 cm (1 inch) deeper than the rest. More water would be added so that the average depth is 2 to 7 cm (1 to 3 inches) depending on how much sunshine is expected. A pasteurization indicator should go in the trough since this is where the coolest water will collect. At this point the drain siphon should be installed. It should be at the lowest part of the trough so that the most water will be siphoned out before the siphon starts to draw air. The end of the siphon should be held solidly in place by a weight or by several rocks. A layer of clear plastic goes over the water, again with the edges extending beyond the edges of the pit. An insulating air gap is formed 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 5 cm (2 inches) or more. Finally, dirt or rocks are piled on the edges of the plastic sheets to hold them down. 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 2 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 ($2-$3), a siphon tube (about $1), and 4 sheets of plastic (about $2 for the size tested). Many tests were done in the spring and summer of 1994 in Berkeley, California. On days with good sunshine the required temperature was achieved even with 68 liters (17 gallons) of water corresponding to a depth of 62 mm (2 1/2 inches). With thinner water layers higher temperatures can be reached. With 24 liters (6 gallons) corresponding to a depth of 21 mm (1 inch) 80º C (176º F) was achieved on one day.

The solar puddle works 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. The water temperature is uniform throughout the puddle to within 1º C (2º 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 such as places that are exposed to the sun but not cooled by the water. 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 of very careful in handling them. This 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. The least expensive form of a solar puddle is built into the ground as in Fig. 4, but a puddle could be built with wooden sides on top of the ground, on a tabletop, or on a roof. 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. A larger puddle would have a higher initial cost, but a lower unit cost for the water, since the same drain line and water pasteurization indicator could be used. One could make a water heater by roughly tripling the amount of water so that the maximum temperature was only 50º C (120º 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.

Cost Summary

The table below shows an approximate cost summary of the basic methods of water pasteurization described in this document. The initial cost is the amount of money that needs to be spent to get the system running. The water produced per dollar of long term cost is based on a 5-year lifetime, and includes expected maintenance costs and replacement parts. In some cases, a), b), and c) in particular, the maintenance costs are small. For the solar puddle, cases e) and f), the replacement costs for the plastic layers that degrade in the sunlight make up the majority of the long-term cost.

The assumption used in these calculations are:

1. The fuel cost is $0.02 per liter of boiled water (cases a) and b)). This number comes from a recent issue of the Solar Cookers International newsletter, and is the amount of money that some people in the developing world are willing to pay for the fuel to boil drinking water.
2. Pasteurization indicators must be replaced twice in 5 years (cases b), c), e) and f)).
3. Thermostatic valves must be replaced once in 5 years (case d)).
4. For the solar puddle the top 2 layers of plastic are replaced every 3 months, while the bottom 2 layers are replaced every 6 months (cases e) and f)).

It can be seen that the systems using fuel have low initial cost but high long term cost. The pasteurization indicator is an inexpensive way of nearly doubling the water produced per unit of fuel, though the long term costs of such systems are still high due to the cost of the fuel. The solar puddle has low initial cost and low long term costs, but involves the work of replacing the plastic layers frequently.

Conclusion

In this document water pasteurization has been presented as a way of providing clean drinking water in developing countries. Several techniques for pasteurizing water have been presented here. Some of these methods are less expensive, some produce more water per day, and some are in the form of a compact device that is easy to ship and set up in the field. Pasteurization is only one way of providing clean water. The purpose of this document is not to say that pasteurization is the best way of providing drinking water or to say that one pasteurization technique is necessarily better than other. As always, the selection of a method for providing clean water should be based on local conditions, and the selection process should include a variety of social factors as well as the technical and cost factors explored here. Field experience shows that education is also necessary to achieve successful results with any water system.

References:

1. UNICEF, The State of the World's Children, 1988, Oxford University Press, pg. 3, 1988.

2. UNICEF, The State of the World's Children, 1989, Oxford University Press, pg. 48, 1989.

3. Ciochetti, D. A., and Metcalf, R. H., Pasteurization of Naturally Contaminated Water with Solar Energy, Applied and Environmental Microbiology, 47:223-228, 1984.

4. Recent Advances in Devices for the Heat Pasteurization of Drinking Water in the Developing World, Dale Andreatta, P. E., Derek T. Yegian, Lloyd Connelly, and Robert H. Metcalf, Proceedings of the 29th Intersociety Energy Conversion Engineering Conference, 1994.

See also Solar Water Pasteurizers Make Safe Drinking Water in Tanzania