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Solar water disinfection

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Portable water purification powered by sunlight

Solar water disinfection (SODIS) application in Indonesia using clear polyethylene terephthalate (PET) plastic beverage bottles<br>Solar water disinfection , in short SODIS , is a type of portable water purification that uses solar energy to make biologically contaminated (e.g. bacteria, viruses, protozoa and worms) 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.[1]

Solar water disinfection is usually accomplished using some mix of electricity generated by photovoltaics panels (solar PV), heat (solar thermal), 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. They may also use varying levels of insulation or glazing. In addition, some solar thermal water disinfection processes are batch-based, while others (through-flow solar thermal disinfection) operate almost continuously while the sun shines. Water heated to temperatures below 100 °C is generally referred to as pasteurized water.

The ultraviolet part of sunlight can also kill pathogens in water. The SODIS method uses a combination of UV light and increased temperature (solar thermal) for disinfecting water using only sunlight and repurposed PET plastic bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage.[2] SODIS is already applied in numerous developing countries.[3]: 55  Educational pamphlets on the method are available in many languages,[4] each equivalent to the English-language version.[3]

Process for household application<br>[edit]

SODIS instructions for using solar water disinfection<br>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.<br>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.[citation needed]

Aluminum reflects ultraviolet well<br>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 a 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.

Suggested treatment schedule[5]

Weather conditions<br>Minimum treatment duration

Sunny (less than 50% cloud cover)<br>6 hours

Cloudy (50–100% cloudy, little to no rain)<br>2 days

Continuous rainfall<br>Unsatisfactory performance;use rainwater harvesting

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.[3] 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...