SODIS: Inexpensive
Water Disinfection for Developing Countries
Ralph Coppola discussed low-cost methods
for water purification in "Wanderings
No. 95" (The Citizen Scientist, 6 May 2005).
These methods are for use in developing countries and
during emergency situations. Among the very best sites
that Ralph introduced is SODIS
(Solar Water Disinfection).
The SODIS web site points out the very
serious problem posed by drinking water that is contaminated
with pathogenic microbes: "At least one-third of the
population in developing countries has no access to
safe drinking water. The lack of adequate water supply
and sanitation facilities causes a serious health hazard
and exposes many to the risk of water-borne diseases.
There are about 4 billion cases of diarrhoea per year,
out of which 2.5 million cases end in death. Every 15
seconds a child dies of dehydration due to diarrhoea."
The approach to water purification
proposed by SODIS is both inexpensive and simple (Fig.
1). Clear plastic bottles made from standard polyethylene
terephthalate (PETE or PET) are filled with water and
exposed to natural sunlight for six hours (Figs. 2-4).
Four hours will work under normal conditions, but six
6 hours are recommended to be on the safe side. Various
modifications of this basic procedure may be used to
enhance the disinfection effect. For example, bottles
can be placed along the grooves in corrugated metal
to collect more sunlight.
The SODIS web site is well worth a
visit, for it demonstrates how professional scientists
in Switzerland took a simple idea proposed by field
studies in Africa and transformed it into a working
project. The Swiss scientists are at SANDEC
(Department of Water and Sanitation in Developing Countries)
of EAWAG (The Swiss
Federal Institute for Environmental Science and Technology).
They performed extensive laboratory and field tests
under carefully controlled conditions to develop
and test SODIS . The laboratory tests and the practical
experience acquired from the field tests led to the
development of easily understood protocols for implementing
SODIS .
Especially interesting is the fact
that SODIS disinfection is achieved by UV-A (320 nm
to about 400 nm) rays of sunlight that pass through
the plastic bottle. While the disinfecting property
of UV-A is much weaker than that of artificial UV-C
light (256 nm) that is used in sophisticated water treatment
plants, it can be made to work by providing a longer
exposure. On a cool day, the SODIS effect is obtained
after 6 hours of solar exposure. If the water temperatures
rises above 50°C, the disinfection process is three
times faster than otherwise. Shaking the bottles to
aerate the water enhances the disinfection process.
All of the UV-C and much of the UV-B
rays of sunlight are absorbed by the ozone layer. The
UV-B rays that penetrate the ozone layer have both beneficial
and negative effects. For example, UV-B synthesizes
previtamin D in mammalian skin, while excess exposure
to UV-B causes sun burn and can lead to skin cancer
.UV-B sunlight disinfects water much more efficiently
than UV-A. The plastic in PET water bottles effectively
absorbs most UV-B while transmitting most UV-A.
The SODIS project was so successful
that SANDEC has been proving information, technical
support and advice to a variety of local agencies and
institutions in more than 20 countries since 1995. It's
user-friendly web site is part of this work.
Citizen scientists interested in the
problem of water disinfection will learn much from the
SODIS web site. There
are also excellent ideas for students looking for good
science fair research ideas.
Regula Meierhofer of SANDEC contributed to this article.
Forrest M. Mims III 
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