SODIS method of water purification?

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sodis method

Solar Water Disinfection (SODIS) is a simple water treatment technology that can be used at household level. As a point-of-use water purification method, SODIS improves the microbiological quality of drinking water with solar radiation at almost zero investment and maintenance costs. Today, SODIS is one of the recommended household level water treatment technologies by the World Health Organisation (WHO).

The World Health Organisation (WHO), UNICEF, and the Red Cross therefore recommend the SODIS method as a way to treat drinking water in developing countries.

WHO, 2007:

Solar disinfection is an example of another measure with proven health impact that requires little capital investment on the part of end-users, and is thus appropriate for the very poor.

UNICEF, 2009:

UNICEF promotes a variety of treatment methods such as user-friendly filtration, simple solar water disinfection (SODIS) and home chlorination. These are all low-cost, effective and manageable at the household level.

Red Cross Prize, 2006:

The jury considers SODIS an impressive way of contributing by the simplest means to making water supplies better and safer, thereby reducing diarrhoea and other diseases like it, and mortality in developing countries.

SODIS process
UV-B light (280 to 320 nm) can directly be absorbed by DNA, leading to damage, but only 1% of natural solar light is of this type. Hence, sunlight used during the SODIS process consists mainly of UV-A light (320 to 400 nm), which makes 6% of the total solar light. This light can be absorbed by molecules, which are naturally present in the water. When these molecules (called photosensitizers) have absorbed the energy, they can pass it to the oxygen in the surrounding water, leading to the formation of reactive oxygen species (ROS). ROS include free radicals such as the superoxide anion as well as non-radicals such as hydrogen peroxide. ROS damage the membranes proteins, enhancing the oxidative stress of the cell and accelerating their aging process (BOSSHARD, 2009). But ROS can also lead to some DNA damage, such as single strand breaks and nucleic base modifications, which may be lethal and mutagenic. This type of pathogen inactivation Is called photooxidative stress. Photo- because it is the light which generates ROS, and oxidative stress, because these reactive oxygen species (ROS) lead to stress.

When the water temperature reaches more than 45°C, a synergistic effect of UV-A radiation (optical inactivation) and infrared light (thermal inactivation) is observed. At increased water temperatures, the exposure time needed to disinfect the water is reduced. But during cloudy weather conditions, the solar infrared light is reduced and bottles need to be exposed for 2 days to guarantee that the radiation dose received is sufficient to disinfect the water.

In contrast to infrared light (heat), UV-A radiation is only slightly attenuated by clouds. But turbidity in the water reduces the amount of UV radiation that penetrates the water. Therefore, the containers used for SODIS should be relatively flat (i.e. not exceed a depths of 10 cm). Water turbidity can easily be determined: Place the full bottle on a newspaper headline (the letters should have a size of about 1.5 cm) and look through the bottle from top to bottom. Water turbidity is less than 30 NTU if you can read the letters through the water. If not, the water must be filtered first so that SODIS will work.

Suitable containers
Suitable containers for SODIS are PET or glass bottles. Ordinary window glass is almost opaque to UV light and cannot be used to construct SODIS containers. The use of PVC bottles is not recommended. It is normally marked on the bottle whether it is PET or PVC, however, the labels vary from country to country. PET and PVC bottles are normally marked accordingly. The labels can vary from country to country, though. If the bottles are not marked, you can only tell the difference between the bottles by setting fire to them. PET burns quickly and easily when it is held in a flame. When it is taken out of the flame, the fire goes out slowly, or it may keep burning. The smoke smells sweet. PVC does not burn easily. The material does not burn at all when not in the flame. PVC smoke smells acrid. Heavily scratched or old PET bottle should be replaced regularly as the scratching makes them opaque.

Applicability

  •  SODIS is not a replacement of the access to safe drinking water, as it is required to reach the millennium development goals, but a simple and extremely low cost method to improve the microbial quality of drinking water at household levels. Large volumes cannot be provided with the use of this method.
  • SODIS can be used in both rural areas and urban slums, but PET bottles are generally more available in urban regions.
  • SODIS cannot treat water that is chemically polluted (e.g. arsenic, fluoride or industrial and agricultural organic contaminants). SODIS is very well adapted as a secondary treatment of water coming from an improved source (e.g. well water), which does not meet the microbial quality standards.
  • As SODIS requires a consistently sunny climate and solar irradiation, it is most suited for regions within latitudes of 15°N/ S and 35°N/ S. Turbidity can deteriorate the effect and if the latter is higher than 30 NTU, pre-filtration is required.
  • As SODIS is extremely low cost, it is adapted for population with little income. However it should not be sold as a method for the poor as this could lower the attractivity of SODIS as people don’t like to be labeled as ‘poor’.
  • To distribute PET bottles for free will not result in a sustainable application. The integrated promotion together with other household-level drinking water treatments (e.g. chlorination, filtration) to give people the option to make a choice, seems the most promising.
  • It is also recommended to combine SODIS with other interventions to improve the health, sanitationand hygiene situation. This approach allows also working in close collaboration with governmental or other large-scale interventions

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