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How to Purify Water with Sunlight

 Solar water disinfection is a name of purifying water by using solar energy to make biologically contaminated water into pure drinking water. Water having a non-biological agent like heavy metals or toxic chemicals need additional steps to make it pure.

Solar water disinfection is normally using some combination of electricity generated by heat, photovoltaic panels, and solar ultraviolet light collection.

Solar disinfection applies the effect of electricity powered by the photovoltaic’s normally uses electric flow to give electrolytic methods that disinfect water. If it generates oxidative free radicals, it kills pathogens by destroying the chemical structure.

The second way to use solar electricity taken from the battery, do its work at night or low levels to use an ultraviolet lamp to do secondary solar ultraviolet water disinfection.

Solar thermal water disinfection takes heat from the sun to absorb heat from water to 70-100 degree C for a short period. There are many approaches here. The solar heat collectors use various levels of insulation or glazing.

They lenses before them and use reflectors. Some of the solar thermal water disinfection systems are batch-based, while some others operate at the time of sun shines. We use heat below 100 degrees C that is called pasteurized water.

Stage of Sunlight purification

The only drawback of this method is that pic fails to remove non-biological substances like toxic chemicals and heavy metals.

It uses three main solar disinfections namely heat, electricity and ultraviolet radiation.

  • The electric form of solar water purification applies electricity generated by solar panels to filter water.
  • Solar thermal disinfection applies solar heat collectors to raise the water temperature to 70 to 100 degrees C. which can destroy many dangerous biological agents.
  • It ultraviolet disinfection of water applies sunlight along with plastic bottles.

Solar Water Disinfection (SODIS)

Solar Water Disinfection

The UV form of water filtration is mostly used all over the world and-and is called Solare Water Disinfection or SODIS for short.

The method is effective and simple, and anyone can use it. The system is recommended by the World Health Organization to treat water for household or storage.

Benefits of Solar Water Purification

Keep water under sunlight has many benefits. The benefits are included herewith:

  • UVA rays may activate the oxygen into water, form hydrogen peroxide and oxygen free radicals that kill pathogens.
  • The merit of keeping the bottle under sunlight is to speed up the disinfection progress.
  • The process is inexpensive and portable and only needs plastic bottles and sunlight.
  • The rays of UVA destroy the cell structure of pathogens.

Instructions for Using SODIS

The method of house application is very simple and needs minimal cost.

  • Acquire plastic bottles. For useful disinfection, use colorless PET bottles, free from surface scratches. Remove the labels and wash well before use it. PET is the number one container to use. Note: some polycarbonate and glass bottles resist UVA rays and unable to use.
  • Fill the bottle with water. Fill the bottle up to ¾ full and shake it for up to half a minutes or activate oxygen. Again fill the rest of the bottle and seal it with the cap.
  • Keep bottle under sunlight. Keep the bottle down on their side to get the full exposure of the sun. The schedule will depend on the amount of the sun exposure. Normal suggestion for duration: on a sunny day for six hours and cloudy day for two days and rainy day is not perfect for disinfection.
  • Storing or drinking water. You can drink water directly from the bottle or pour into a cup or glass when the suggested length of sun exposure is finished. You can also store water for more use in future.

Cautions and Limitations

If water is not sun exposed properly, it is not safe to drink. This type of water may cause illness. For overcast weather, if the sunlight is not strong enough, a longer time of exposure is essential. Then you need to consider the following issues:

Bottle material

Bottle material

PVC materials or some glass resist ultraviolet rays from reaching to water. So commonly used a bottle that is made of PET are suggested. The handling of the bottle is more convenient in the case of PET bottles. UVB rays, Polycarbonate blocks all UVA are not suggested to use.

A bottle having no color is suggested then the colored bottles. Suppose the green of some lime soda or lemon pop bottles.

Aging of plastic bottles

SODIS efficiency relies on the physical condition of the plastic bottle, up to scratches or the other sign of wear lessening the efficiency of SODIS. Old, heavily scratched and blind bottles need to replace.

Shape of containers

Shape of containers

The intensity of the UV radiation becomes lessen with the increase of water depth. If the depth of water is 10 cm and turbidity of 26 NTU, UV-A radiation reduces to 50%. Soft PET bottles are found easily and most appreciating for SODIS application.


Sunlight makes highly reactive forms of oxygen over water. The reactive molecules help to take part in the process of microorganisms. Under the general condition, water holds sufficient oxygen and need not be created before the application of SODIS.

Leaching of bottle material

Leaching of bottle material

There are some questions in people mind whether plastic drinking water containers remove toxic components or chemical from water or not. The Swiss Federal Laboratories for Materials Testing and Research tested diffusions of phthalates or adipates from reused and new PET bottles during solar exposure.

The result found in water after 17 hours is 60 degrees C; water is far below than WHO standard of drinking water. At the same way, the concentration of adipate and phthalate is higher in tap water.

Think about the PET bottles of water. A report published by the researchers from the University of Heidelberg released of antimony from the PET bottles for mineral and soft drinks stored for few months in the supermarket.

The standard of water is below the standard of WHO and guideline for antimony concentration for drinking water. So, SODIS water is not kept over extended periods inside bottles.

Regrowth of bacteria

If you remove it from sunlight, the rest of bacteria will reproduce again in the dark. This is the outcome of a study in 2010 that just ten parts per million of hydrogen peroxide is very active to prevent the regrowth of wild Salmonella.

Toxic chemicals

Solar water disinfection fails to remove toxic chemicals which are found in the water like factory waste.

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