SODIS
Practical Action
which is very resistant to environmental influences.
Water is often contaminated by raw sewage in open water bodies (lake, streams) or while
stored at home, through careless handling with unwashed hands.
How SODIS Works
SODIS is a simple, low-cost and
easily replicable solution to clean
drinking water. SODIS works
through a combination of heat and
pathogen-killing Ultra Violet
radiation (both through sunlight).
For this method to work well, the
exposure to sunlight should be at
least six hours, or until the water
reaches a temperature of 55°C.
Clear, plastic bottles are
used for SODIS application.
These bottles should be filled
Figure 2: Bottles are left for the day.
Photo: Practical Action / Zul.
with relatively clear water, since SODIS is not effective when the water turbidity is
high. Water turbidity should not be more than 30 NTU (Nephelometric Turbidity
Units). Bottles used for filling water should be transparent, clear bottles-not
coloured, not discoloured and not old, chapped bottles. Plastic bottles are used
becasue they let in more UV radiation than glass bottles, and are unbreakable, cheap
and easily replaceable.
The water in these bottles should be exposed to full sunlight for six hours, or for two
consecutive days during cloudy skies to ensure maximum benefit of the solar effect
on pathogens. The water is ready for consumption right after adequate sunlight
exposure. One must take care, however, to handle with clean hands so that there is
no secondary contamination of disinfected water.
How successful is SODIS?
Laboratory and field tests have shown that SODIS is 99.9% effective against pathogenic
micro-organisms found in water. This may not be as good as boiling which provides
pathogen-free sterile water or even pasteurisation. But, in terms of cost, affordability,
simplicity and sustainability, SODIS scores over these other methods. The success of SODIS
depends a lot on a number of local conditions such as availability of suitable water, suitable
containers and weather. But throughout the pilot tested countries, a large portion (81%) of
the samples gave a 99.9% disinfection rate.
SODIS reduces the incidence of infectious diarrhoea, dysentery, and also protects against
cholera.
Which micro-organisms is SODIS effective against?
The sun’s radiation is proven to be deadly to human pathogens, which are used to living in
the moist, humid, dark environment inside the body. Once they are discharged into the wider
environment, these pathogens are extremely sensitive to conditions outside. UV radiation can
be fatal to many such pathogens, while others are inactivated by maintaining a 50-57˚C heat
in the bottle for a given period of time.
4