Recent Advances in Solar Water
Pasteurization
Boiling isn't necessary to kill disease
microbes
The main purpose of solar cookers is to change sunlight into heat which is then used to
cook foods. We are all familiar with how successful solar cookers are at cooking and
baking a wide variety of foods. In this article I want to consider using the heat in solar
cookers for purposes other than cooking. My main focus will be solar water pasteurization,
which can complement solar cooking and address critical health problems in many developing
countries.
The majority of diseases in developing countries today are infectious diseases caused
by bacteria, viruses, and other microbes which are shed in human feces and polluted water
which people use for drinking or washing. When people drink the live microbes, they can
multiply, cause disease, and be shed in feces into water, continuing the cycle of disease
transmission.
Worldwide, unsafe water is a major problem. An estimated one billion people do not have
access to safe water. It is estimated that diarrheal diseases that result from
contaminated water kill about 2 million children and cause about 900 million episodes of
illness each year.
Boiling contaminated water
How can infectious microbes in water be killed to make the water safe to drink? In the
cities of developed countries this is often guaranteed by chlorination of water after it
has been filtered. In developing countries, however, city water systems are less reliable,
and water from streams, rivers and some wells may be contaminated with human feces and
pose a health threat. For the billion people who do not have safe water to drink, what
recommendation do public health officials offer? The only major recommendation is to boil
the water, sometimes for up to 10 minutes. It has been known since the time of Louis
Pasteur 130 years ago that heat of boiling is very effective at killing all microbes which
cause disease in milk and water.
If contaminated water could be made safe for drinking by boiling, why is boiling not
uniformly practiced? There seem to be five major reasons: 1) people do not believe in the
germ theory of disease, 2) it takes too long, 3) boiled water tastes bad, 4) fuel is often
limited or costly, 5) the heat and smoke are unpleasant.
Some examples of the cost of boiling water are worth mentioning. During the cholera
outbreak in Peru, the Ministry of Health urged all residents to boil drinking water for 10
minutes. The cost of doing this would amount to 29% of the average poor household income.
In Bangladesh, boiling drinking water would take 11% of the income of a family in the
lowest quartile. In Jakarta, Indonesia, more than $50 million is spent each year by
households for boiling water. It is estimated that in the city of Cebu in the Philippines,
population about 900,000, about half the families boil their drinking water, and the
proportion is actually higher for families that obtain their water from an unreliable
chlorinated piped supply. Because the quantities of fuel consumed for boiling water are so
large, approximately 1 kilogram of wood to boil 1 liter of water, and because firewood,
coal, and coke are often used for this purpose, an inadequate water supply system
significantly contributes to deforestation, urban air pollution, and other energy-related
environmental effects.
If wood, charcoal, or dung is used as fuel for boiling water, the smoke creates a
health hazard, as it does all the time with cooking. It is estimated that 400 to 700
million people, mainly women, suffer health problems from this indoor air pollution. As a
microbiologist, I have always been perplexed as to why boiling is recommended, when this
is heat far in excess of that which is necessary to kill infectious microbes in water. I
presume the reason boiling is recommended is to make sure that lethal temperatures have
been reached, since unless one has a thermometer it is difficult to tell what temperature
heated water has reached until a roaring boil is reached. Everyone is familiar with the
process of milk pasteurization. This is a heating process which is sufficient to kill the
most heat resistant disease causing microbes in milk, such as the bacteria which cause
tuberculosis, undulant fever, streptococcal infections and Salmonellosis. What
temperatures are used to pasteurize milk? Most milk is pasteurized at 71.7° C (161° F)
for only 15 seconds. Alternatively, 30 minutes at 62.8° C (145° F) can also pasteurize
milk. Some bacteria are heat resistant and can survive pasteurization, but these bacteria
do not cause disease in people. They can, however, spoil the milk, so pasteurized milk is
kept refrigerated.
There are some different disease microbes found in water, but they are not unusually
heat resistant. The most common causes of water diseases, and their heat sensitivity, are
presented in Table 1. The most common causes of acute diarrhea among children in
developing countries are the bacteria Escherichia coli and Shigelia SD. and the Rotavirus
group of viruses. These are rapidly killed at temperatures of 60° C or greater.
Solar water pasteurization
As water heats in a solar cooker, temperatures of 56° C and above start killing
disease-causing microbes. A graduate student of mine, David Ciochetti, investigated this
for his master's thesis in 1983, and concluded that heating water to 66° C in a solar
cooker will provide enough heat to pasteurize the water and kill all disease causing
microbes. The fact that water can be made safe to drink by heating it to this lower
temperature—only 66° C—instead of 100° C (boiling) presents a real opportunity
for addressing contaminated water in developing countries.
Testing water for fecal contamination
How can one readily determine if the water from a well, pump, stream, etc. is safe to
drink? The common procedure is to test the water for bacterial indicators of fecal
pollution. There are two groups of indicators which are used. The first is the coliform
bacteria which are used as indicators in developed countries where water is chlorinated.
Coliform bacteria may come from feces or from plants. Among the coliform bacteria is the
second indicator, Escherichia coli. This bacterium is present in large numbers in human
feces (approximately 100,000,000 per gram of feces) and that of other mammals. This is the
main indicator used if water is not chlorinated. A water source containing 100 E. coli per
100 milliliters poses a substantial risk of disease.
The standard method of testing water for the presence of coliforms and E. coli requires
trained personnel and a good laboratory facility or field unit which are usually not
present in developing countries. Thus, water supplies are almost never tested.
A new approach to testing in developing
countries
In 1987, the
Colilert MPN Test
(CLT) was introduced as the first method which used a defined substrate technology to
simultaneously detect coliforms and E. coli. The CLT comes as dry chemicals in test tubes
containing two indicator nutrients: one for coliforms and one for E. coli. The CLT
involves adding 10 ml of water to a tube, shaking to dissolve the chemicals, and
incubating at body temperature for 24 hours. I prefer incubating tubes under my belt
against my body. At night I sleep on my back and use night clothes to hold the tubes
against my body.
If no coliform bacteria are present, the water will remain clear. However, if one or
more coliforms are present in the water, after 24 hours their growth will metabolize ONPG
and the water will change in color from clear to yellow (resembling urine). If E. coli is
among the coliform bacteria present, it will metabolize MUG and the tube will fluoresce
blue when a battery-operated, long-wave ultraviolet light shines on it, indicating a
serious health hazard. I have invited participants at solar box cooker workshops in Sierra
Leone, Mali, Mauritania, and Nepal to test their home water supplies with CLT. One hundred
and twenty participants brought in samples. In all four countries, whether the water was
from urban or rural areas, the majority of samples contained coliforms, and at least half
of these had E. coli present. Bacteriological testing of the ONPG and MUG positive tubes
brought back from Mali and Mauritania verified the presence of coliforms/E. coli in
approximately 95% of the samples. It is likely that soon the Colilert MPN test will be
modified so that the test for E. coli will not require an ultraviolet light, and the tube
will turn a different color than yellow if E coli is present. This will make the test less
expensive and easier to widely use in developing countries to assess water sources.
Effect of safe water on diarrhea in
children
What would be the effect if contaminated water could be made safe for drinking by
pasteurization or boiling? One estimate in the Philippines predicts that if families using
moderately contaminated wells (100 E. coli per 100 ml) were able to use a high-quality
water source, diarrhea among their children would be reduced by over 30%. Thus, if water
which caused a MUG (+) test were solar pasteurized so it would be clear, this would help
reduce the chance of diarrhea, especially in children.
How can one determine if heated water has reached 65° C? In 1988, Dr. Fred Barrett
(USDA, retired) developed the prototype for the Water Pasteurization Indicator (WAPI). In
1992, Dale Andreatta, a graduate engineering student at the University of California,
Berkeley, developed the current WAPI. The WAPI is a polycarbonate tube, sealed at both
ends, partially filled with a soybean fat which melts at 69° C ("MYVEROL"
18-06K, Eastman Kodak Co., Kingsport, TN 37662). The WAPI is placed inside a water
container with the fat at the top of the tube. A washer will keep the WAPI on the bottom
of the container, which heats the slowest in a solar box cooker. If heat from the water
melts the fat, the fat will move to the bottom of the WAPI, indicating water has been
pasteurized. If the fat is still at the top of the tube, the water has not been
pasteurized. The WAPI is
reusable. After the fat cools and becomes solid on
the bottom, the fish line string is pulled to the other end and the washer slides to the
bottom, which places the fat at the top of the tube. Another pasteurization indicator has
been developed by Roland Saye which is based on expansion of a bi-metal disc which is
housed in a plastic container. This also shows promise and is in the early testing stages.
The WAPI could be useful immediately for people who currently boil water to make it
safe to drink. The WAPI will clearly indicate when a safe temperature has been reached,
and will save much fuel which currently is being wasted by excessive heating.
[Editor's note: Using Beeswax & Carnauba Wax to Indicate Temperature: In SBJ #15 we
discussed using beeswax, which melts at a relatively low 62º C, as an indicator of
pasteurization. We have now found that mixing a small amount of carnauba was with the
beeswax (~1:5 ratio) raises the melting temperature of the beeswax to 70º - 75º C.
Carnauba wax is a product of Brazil and can be bought in the US at woodworking supply
stores. Further testing needs to be done to confirm that the melting point remains the
same after repeated re-melting. Write to
webmaster@solarcooking.org
and we will send you a small amount of
carnauba wax to experiment with.]
Different strategies for solar water
pasteurization
The solar box cooker was first used to pasteurize water. David Ciochetti built a deep
dish- solar box cooker to hold several gallons of water. At this time of the year in
Sacramento, three gallons could be pasteurized on our typical sunny days.
Dale Andreatta and Derek Yegian of the University of California. Berkeley, have
developed creative ways to greatly increase the quantity of water which can be
pasteurized, as we will hear about at this conference.
I am also excited about the possibility of pasteurizing water using the simple solar
panel cookers. By enclosing a dark water container in a polyester bag to create an
insulating air space, and by using lots of reflectors to bounce light onto the jar, it is
possible to pasteurize useful amounts of water with a simple system. It takes about four
hours for me to pasteurize a gallon of water in the summer with the system I am using.
Solar panel cookers open up enormous possibilities for heating water not only for
pasteurization, but also for making coffee and tea, which are quite popular in some
developing countries. The heated water can also be kept hot for a long time by placing it
in its bag inside an insulated box. In the insulated container I use, a gallon of 80° C
water will be approximately 55° C after 14 hours. Water at a temperature of 55° C will
be about 40° C after 14 hours, ideal for washing/shaving in the morning.
I will close with some advice from the most famous microbiologist, who pioneered the
use of vaccinations in the 1890s: Louis Pasteur. When he was asked the secret of his
success, he responded that above all else, it was persistence. I will add that you need
good data to be persistent about, and we certainly have that with solar cookers; the work
in Sacramento, Bolivia, Nepal, Mali, Guatemala, and wherever else the sun shines.
Continued overuse of fuelwood is non-sustainable. We need to persist until the knowledge
we have spreads and becomes common knowledge worldwide.
For questions or comments contact Dr. Robert Metcalf at
rmetcalf@csus.edu.
Dr. Robert Metcalf
1324 43rd St.
Sacramento, California 95819 USA.
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Two
weeks ago, when I posted an article on prepper propane, I had no idea
that there was such a thirst for knowledge on the topic. Nor did I
expect my technical consultant, Chris Newman, to respond to each and
every comment asked not only here but also on 
.
Call it serendipity if you will but a couple of months ago when I was contacted by Chris Newman, inventor of the 
and a couple of utensils won’t add much more weight, and will give you
even more subsistence options. The tiny stove and a canister will pack,
mostly, inside the wok. Don’t forget to pre-season your new wok, as you
would with cast iron.
