Cheap and Effective Way of Disinfecting Drinking Water, SODIS Method

Fresh water is an increasingly scarce resource, even more so, access to clean, potable water for people is very limited. An estimated 1.1 billion people or one sixth of the world’s entire population lack a safe water supply through pipes, bore holes, dug wells or protected springs.

This is a problem that is likely to increase. As the population grows the available clean water becomes insufficient. The lack of access to ‘good quality’ water leads to a greatly increased risk of contracting water-borne diseases, commonly diarrhea, cholera, typhoid fever, hepatitis A, amoebic and bacillary dysentery. An alarming 4 billion cases of diarrhea are reported annually causing over 2 million deaths.

Treating water to improve its quality should be combined with health-promoting practices to make a lasting change in the public health of people in developing countries.

Disinfecting Water

Even a population with access to a pipe borne water supply have no guarantee of its quality. Although, much investment has gone into treating the water supply to make it safe, many water supply schemes are constrained by resources, lack of chemicals, lack of trained staff, laboratory facilities and the continuing demand for more water.
There is also a large percentage of people who depend on public taps or open water bodies, rivers or steams for water. This water carries a huge risk of possible microbiological contamination, especially from sewage and garbage.

Ensuring clean and safe water is a household responsibility, even when a home enjoys the convenience of tap water. Individual households have to ensure that the water they collect is safe and of adequate quality for drinking.

There are a number of water treatment measures can be adopted at household level to ensure clean, safe water. These are water storing, boiling, water pasteurization, filtration and disinfections with chlorine.

Each method has its limitations:

  • Storing water is the simplest method to improve quality. While this will clear sedimentation, it will only be partly effective in removing turbidity and fecal chloroform contamination.
  • Boiling water is the best method of ensuring sterile, disease-causing-pathogen-free water of high quality, but boiling water takes up a lot of energy; not everyone can afford it.
  • Pasteurization is achieved by heating water to lower-than-boiling point, often 70-75?C, but maintaining the heat for 10 minutes or so. This also requires high energy input.
  • Filtration will remove much of the solid matter and sediment. It is more difficult to remove micro-organisms, plus, they are costly to buy and install.
  • Chlorine will kill micro-organisms like bacteria and viruses but it is not so affective in fighting pathogenic parasites like Giardia, Cryptosporidium and Helminth eggs. Further, skilled application is necessary since chlorine is a corrosive substance. Treated water has a distinct taste – not to everyone’s liking.

SODIS – An Alternative Method

Moving away from conventional water treatment, SODIS provides a simple, low-cost, easily applicable means of ensuring clean, drinkable water. SODIS or solar water disinfections uses the sun’s radiation (specifically UV-A rays and heat) to destroy pathogenic micro organisms present in water. Its efficiency in killing disease-carrying protozoa depends on the water reaching a certain heat through exposure to sunlight. UV-A rays also drastically reduce many forms of bacteria that contaminate water. To achieve this, transparent plastic containers are filled with water and exposed to full sunlight for at least six hours.

These tests revealed that the combined use of UV-A (ultra violet-A) radiation and increased water temperature through exposure to sunlight can be 99.9% effective in destroying micro organisms in raw water.

Subsequent field tests and pilot demonstrations have proved the viability of the system in real life, its socio-cultural acceptance and affordability. Pilot demonstrations were done in many developing countries, which enjoy good sunlight throughout the day – Colombia, Bolivia, Burkina, Togo, Thailand and Indonesia.

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 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 colored, not discolored and not old, chapped bottles. Plastic bottles are used because 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.

Research on SODIS has shown that pathogens (bacteria, virus, yeast, and mould) are destroyed during the process.

How to do it?

  1. Check if the climate and weather conditions are suited for SODIS – basically there should be fairly bright weather with not more than 50% cloud cover.
  2. Collect plastic PET bottles of 1 to 1-1/2  liter volume. PET is preferred to PVC, because PVC can contain harmful additives and to glass, because glass blocks out some of the UV rays.
  3. Each family member should be assigned two bottles per day. So a family should have four plastic bottles per member – two for the day’s consumption and two exposed to the sun.
  4. Check if the screw cap is water tight and clean.
  5. Bottles should be laid out on a suitable heat-reflecting surface – roofing sheets, CGI (Corrugated Iron) sheets etc, in a clear spot on the roof or garden. The bottles should be exposed to direct sunlight for at least six hours.
  6. Check if the water is clear enough. If there is a lot of discoloration and sediment, the water has to be pre-treated (filtered) before SODIS.
  7. It is best to have a specific person responsible for exposing the SODIS bottles to the sun, and at least two members of each family must be trained in the correct application of SODIS, especially on the importance of maintaining sun exposure throughout the period.

How to increase the efficiency of SODIS?

There are certain practices that can increase the efficiency of SODIS. These have to do with increasing sunlight exposure, harnessing available sunlight better and quickening the heating-up process for SODIS water.

  • Putting black paint on half of the outer surface of the PET bottle and laying the bottle blackened-side downwards increases the rate of heating.
  • Placing the bottles on a reflective surface-aluminum foil etc – can dramatically increase the water heating rate.
  • A bucket lined with aluminum foil can act as a solar collector, increasing the efficiency of heating quite dramatically.
  • Filling the bottles 75% first and shaking the bottle for 20 seconds increases the oxygen in the water, which, combined with the UV radiation helps to fight pathogens. Fill the rest of the water after shaking the bottle.
  • Bottles should be placed horizontally and not upright, nor should the bottles be half full.
  • Always replace scratched and old bottles.


  • Weather dependency is the biggest limitation of SODIS. Since the application is quite weather dependent requiring bright sunlight, the process cannot be carried out during rainy days. It is recommended that during long rainy spells, the household should resort to boiling or pasteurizing the water.
  • Another limitation is that a large quantity of water cannot be heated up through this process. PET bottles larger than 2 litre volume should not be used.
  • SODIS water can be consumed by children, healthy adults and even the aged. But, care must be taken in the case of infants (less than 18 months), very sick children or adults, malnourished children and those suffering from reduced immunity (such as AIDS).For these categories, boiled and cooled water is recommended.

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