page_title: Solar water disinfection
section_title: Process for household application
hierarchical_section_title: Solar water disinfection / Process for household application
caption_attribution_description: English: The picture illustrates the procedure to disinfect drinking water with the SODIS method (solar water disinfection)
context_page_description: Solar water disinfection, in short SODIS, is a type of portable water purification that uses solar energy to make biologically-contaminated water safe to drink. Water contaminated with non-biological agents such as toxic chemicals or heavy metals require additional steps to make the water safe to drink.
  Solar water disinfection is usually accomplished using some mix of electricity generated by photovoltaics panels, heat, and solar ultraviolet light collection.
  Solar disinfection using the effects of electricity generated by photovoltaics typically uses an electric current to deliver electrolytic processes which disinfect water, for example by generating oxidative free radicals which kill pathogens by damaging their chemical structure. A second approach uses stored solar electricity from a battery, and operates at night or at low light levels to power an ultraviolet lamp to perform secondary solar ultraviolet water disinfection.
  Solar thermal water disinfection uses heat from the sun to heat water to 70–100 °C for a short period of time. A number of approaches exist. Solar heat collectors can have lenses in front of them, or use reflectors.
context_section_description: Guides for the household use of SODIS describe the process.
  Colourless, transparent PET water or soda bottles of 2 litre or smaller size with few surface scratches are selected for use.  Glass bottles are also  suitable.  Any labels are removed and the bottles are washed before the first use. Water from possibly-contaminated sources is filled into the bottles, using the clearest water possible.  Where the turbidity is higher than 30 NTU it is necessary to filter or precipitate out particulates prior to exposure to the sunlight. 
  Filters are locally made from cloth stretched over inverted bottles with the bottoms cut off. In order to improve oxygen saturation, the guides recommend that bottles be filled three-quarters, shaken for 20 seconds (with the cap on), then filled completely, recapped, and checked for clarity.
   
  The filled bottles are then exposed to the fullest sunlight possible. Bottles will heat faster and hotter if they are placed on a sloped Sun-facing reflective metal surface. A corrugated metal roof (as compared to thatched roof) or a slightly curved sheet of aluminum foil increases the light inside the bottle. Overhanging structures or plants that shade the bottles must be avoided, as they reduce both illumination and heating. After sufficient time, the treated water can be consumed directly from the bottle or poured into clean drinking cups. The risk of re-contamination is minimized if the water is stored in the bottles. Refilling and storage in other containers increases the risk of contamination.
  The most favorable regions for application of the SODIS method are located between latitude 15°N and 35°N, and also 15°S and 35°S. These regions have high levels of solar radiation, with limited cloud cover and rainfall, and with over 90% of sunlight reaching the earth's surface as direct radiation. The second most favorable region lies between latitudes 15°N and 15°S. these regions have high levels of scattered radiation, with about 2500 hours of sunshine annually, due to high humidity and frequent cloud cover.
  Local education in the use of SODIS is important to avoid confusion between PET and other bottle materials. Applying SODIS without proper assessment (or with false assessment) of existing hygienic practices & diarrhea incidence may not address other routes of infection. Community trainers must themselves be trained first.
caption: SODIS instructions for using solar water disinfection
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