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The sodis
Dowsing - is it science or magic?


Swiss rationality


CGWA retires hurt
Do you know the potential of rooftop water harvesting?


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Vol. 3                                      No. 6                          December 2001

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The sodis

Providing quality drinking water has always been a challenge in the developing countries. Though resource-poor, these countries are often rich in sunlight.

SODIS – Solar disinfection method uses the sun’s ultra violet radiation (UV) to improve the microbiological quality of drinking water. Synergies induced by radiation and thermal treatment have a significant effect on the die-off rate of microorganisms. The processes involved are indicated in the illustration (see Illustration: how to use Sodis) Sodis functioning depends on the following factors:

How to use sodis


Clean the bottle well

09_01.jpg Fill the 3/4 of the bottle
with water and cap it


Shake the bottle well

Keep the bottle on
black iron sheet for
minimum six hours 
in sun light for consumption

09_04.jpg * Don’t keep the bottle on thached roof
** The bottle should be kept in direct sun light


Weather and climate: The container needs to be exposed 6 hours to full sunlight. If the temperature raises above 50 degree celsius, the disinfection process is three times faster. The efficiency of the disinfection process can be increased by using half-blackened plastic bottles or by placing the plastic bottles on a black corrugated zinc sheet.

Water Turbidity: Suspended particles in the water reduce the penetration of solar radiation into the water and protect microorganisms from being irradiated. Sodis requires relatively clearer water with a turbidity less than 30 NTU (where NTU stands for Nephelometric turbidity unit and nephelometer is a modern commercial instrument used to measure turbidity)

Material: Various types of transparent plastic materials are good transmitters of light in the visible range of the solar spectrum. Plastic bottles made of poly ethylene terephtalate (PET) are preferred because they contain less UV- stabilisers than poly vinyl chloride (PVC) bottles .

Shape of containers: More the area of bottle exposed to sunlight, more would be efficiency in achieving disaffection.

Oxygen: Sodis is more efficient in water containing high levels of oxygen. Shaking the three-fourth filled containers for about 20 seconds before they are filled completely could increase oxygen levels. On reacting with this water sunlight produces highly reactive forms of oxygen. These reactive forms of oxygen kill the microorganisms.

Although, Sodis can neither treat turbid waters nor change the chemical quality of water, this method is ideal to disinfect small quantities of water used for consumption.

For further information:

Dowsing - science or magic?

Palapatti, a village in Tamil Nadu was starving for water. They had neither rains nor tubewells. Finally the villagers decided to take the help of Brother James Kimpton , a monk who has the rare ability of predicting the presence of groundwater in a particular area, with out any sophisticated instrument. He visited the site and successfully predicted water at a depth of seventy feet! Like Brother Kimpton, there are many who can locate underground water sources and can say how deep one should dig to tap them. In many villages of India this art is in practice.

The question however remains. What is this art all about? Known as dowsing, it is an ancient art of searching for hidden things (water, precious metals, etc). Reportedly, 80 per cent of people have this special gift - an ability to sense things not perceptible to others.

The basic dowsing tools include L-rods, pendulums, Y-rods commonly known as "Swiss" rods. When above water, the rods respond indicating the presence of water. It has been suggested that some emanation from below ground affect the diviner so that the muscles twitch and thus cause movement of the twig or other divining tool.

Does dowsing work? Thousands of dowsers practice their art every day in all parts of the world. There are large societies of dowsers in America and Europe. Of particular interest is a report by University of Munich physicist Hans-Dieter Betz, Water Dowsing in Arid Regions: Report on a ten-year German government project, covering over 2,000 successful examples from Zaire, Kenya and other countries.

What do you think about dowsing — is it science or magic ?

Send in your views to:
With inputs from: Lalit Gambhir
Email: lalit

Copyright CSE  Centre for Science and Environment